方家松
, 李江燕
Fang Jiasong, Li Jiangyan
中图分类号: P715.5
文献标识码: A
文章编号: 1001-8166(2017)12-1297-10
通讯作者:
收稿日期: 2017-10-16
修回日期: 2017-11-28
网络出版日期: 2017-12-20
版权声明: 2017 地球科学进展 编辑部
基金资助:
作者简介:
First author:Fang Jiasong(1961-),male, Honghu City, Hubei Province, Professor. Research areas include marine microbiology and biogeochemistry.E-mail:jsfang@shou.edu.cn
作者简介:方家松(1961-),男,湖北洪湖人,教授,主要从事海洋微生物学和生物地球化学研究.E-mail:jsfang@shou.edu.cn
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摘要
过去50年深海钻探计划(DSDP)、大洋钻探计划(ODP)及综合大洋钻探计划(IODP)等国际间综合深海钻探计划的实施,使我们对地球和海洋的认识取得了显著进步。海底井控观测装置(CORK)的研发和应用是上述深海钻探计划带给我们的最宝贵财富之一。目前地球科学和海洋科学已由过去的间断性观测模式升级为现在的连续性原位观测模式,海底CORK观测系统为地球科学、海洋科学与生命科学领域的科学家对海底洋壳复杂而又相互关联的深部过程进行数秒至数十年时间尺度的研究提供了新手段和新机遇。通过对海底CORK观测系统的发展演变、在ODP和IODP航次中的使用以及在此过程中获得的科学经验和教训进行总结,对CORK系统如何应用于我国洋壳地质学、水文学、微生物学及生物地球化学过程研究提出看法。
关键词：
Abstract
In the past 50 years, we have witnessed remarkable progress in our understanding of the Earth and ocean system, as a result of the internationally integrated deep ocean drilling programs, the Deep Sea Drilling Program (DSDP), the Ocean Drilling Program (ODP), and the Integrated Ocean Drilling Program (IODP). One of the legacies of the deep ocean drilling programs is the development and applications of the CORK, Circulation Obviation Retrofit Kit. Earth and ocean sciences have been shifting from a traditional discontinuous, expeditionary mode toward a mode of sustained in situ observations today. The seafloor CORK observatories offer Earth, ocean and life scientists new opportunities to study multiple, interrelated deep marine subsurface processes, over time scales ranging from seconds to decades. Here, we first provided a concise examination of the development history of the CORKs, then described the first installations of ODP CORKs, the evolution of different models of CORK, and finally, summarized the scientific lessons learned in the installation and operation effort of the CORKs. In the end, we offered our perspectives on using CORKs to study geological, hydrogeological, microbiological, and biogeochemical processes in the deep marine subsurface biosphere, particularly pertaining to China’s efforts in establishing and enhancing its deep-sea and deep-biosphere research and monitoring programs.
Keywords：
洋壳含水层是地球上最大和最为连续的含水层,其含水量与冰盖和冰川的含水量相当,它构成了所谓的“海底之下的大洋”[1,2],同时也是深部生物圈的主要组成部分[3](图1)。但是海底是“漏”的[5]:海水在海底沉积物和洋壳玄武岩中流动,并在洋壳内的断层、裂隙以及其他可渗透性通道中活跃地进行循环,由此形成了海洋与洋壳间的流体、热量、溶质和生物物质交换和再分配[6,7,8,9];每200 ka全部海水在这种洋壳大规模动态通道系统中完成一次循环[10]。因此,研究洋壳水文地质学、地壳深部生物圈以及海水、岩石与生物之间的相互作用,进而探索海底资源的形成与分布、元素循环及全球气候变化,具有重大理论与实际意义。
深海钻探计划(Deep-Sea Drilling Program,DSDP,1968—1983年)、大洋钻探计划(Ocean Drilling Program,ODP,1983—2003年)和综合(国际)大洋钻探计划(Integrated/International Ocean Drilling/Discovery Program,IODP,2003—2013年;2013—2023年)开展的50年是地球科学特别是海洋科学极富成效和创造力的50年。科学家能在全球范围内定期取回海底沉积物芯和玄武质洋壳岩芯样品,这为地球科学和生命科学带来了前所未有的思维变革。这期间还涌现出了诸多新的理论,包括板块构造学说的发现、全球气候变化的阐明、海底热液口及其所支撑的生物群(化能合成)的解密等;尤其引人瞩目的是,在这期间我们已经认识到在深部生物圈(包括大洋沉积物以及结晶洋壳)中还存在一个巨大的海底生物群[11]。海底井控观测装置(Circulation Obviation Retrofit Kit,CORK)技术的发展,使其已广泛应用于洋壳水文地质学、微生物学及生物地球化学等的长期研究中,该装置可实现对洋壳温度、压力、流体通量及微生物群落的实时观测和原位实验。本文首先对CORK的发展历程做简要回顾,其次对CORK首次在ODP和IODP航次中的布放以及不同类型CORK的变革和发展进行阐述,同时总结在CORK布放和操作过程中获得的经验教训,最后,对CORK如何应用于洋壳水文地质学、微生物学及生物地球化学研究提出自己的观点。
大洋科学钻探使地球科学取得了突破性进展,此外,深海科学钻探还在专业钻探和取芯工具及技术的研发应用方面为我们积累了非凡财富。这些特殊的工具和技术有助于科研人员获取不同环境条件下的高质量岩芯或沉积物芯样品,同时还可实施海底原位长期系列观测,CORK的开发利用便是其中最好的范例之一,我们称之为“深海钻探计划之子”。
深海钻探开始的最初20年,大多数DSDP和ODP钻孔在取样后均被遗弃,仅1%~3%的钻孔配备了用于后期进行仪器设备布放的再入圆锥筒(reentry cone)[12]。即便如此,科学家依然能够获得沉积物芯或岩芯样品,并在对这些未封闭的DSDP和ODP钻孔中渗透层与洋底海水间立面流热状态进行监测和解译方面积累了丰富经验[12,13,14,15]。然而,通过深海钻探仅能获取钻探点的洋壳样品,而洋壳地质、地热、水文、微生物生态和生物地球化学过程是区域性和全球尺度的,这些过程不但改变着洋壳成分,也改变着洋壳含水层中循环流体的化学成分[16]。因此,遗弃这些钻孔是科学界的重大损失[17]。研究发现,钻孔中的流体可通过沉积物渗透到玄武质洋壳或基底,进而对洋壳地热、水文以及微生物系统产生严重扰动,同时也导致了流体、溶质和微生物在海底沉积物与基底及其上的海水间的活跃交换[3,15]。很显然,将海底钻孔封闭起来是非常必要的,这样一方面可以阻止上述交换过程的发生,恢复海底地热和水文系统平衡,另一方面,也使我们的洋壳原位观测(温度、压力、水文参数等)成为可能。此外,为了更好地研究洋壳水文系统,获取空间上和时间上连续的观测数据也十分必要。
1989年,当Carson,Becker 和Davis在一张餐巾纸上绘出CORK的最初草图时,“海底井控观测装置”的概念也应运而生(图2),这是深海海底钻探在取样和观测技术上的革命性发展。这张草图作为插图被用在上述作者1990年分别向加拿大地质调查局和美国国家自然科学基金会提交的2份项目申请书中,这2个项目是他们申请资助在1991年首次布放CORK。1991年夏天,ODP139航次在Middle Valley扩张中心成功完成了最早的2个CORK的布放[18]。
CORK是Circulation Obviation Retrofit Kit的首字母缩写,由ODP执行主管Glen Foss提出[15]。最初CORK的结构和功能均与红酒瓶的软木塞有某种程度相似。“CO”是“封闭”钻孔的意思,意即对钻孔进行控制,阻止洋壳地层与海底海水间发生流体交换;如果钻入洋壳水文地质条件活跃地层的钻孔没有封闭,这种交换的发生是可以预期的。“RK”是“(重新)进入”的意思,指不论是近期的新钻孔还是20年前的老钻孔,通过再入圆锥筒均可再入钻孔并安装仪器设备进行科学实验和观测[15]。
图3展示了不同型号CORK装置的基本结构,其主要组成包括:①再入圆锥筒与套管系统(Reentry Cone and Casing system,RECC);②CORK主体,用于封闭钻孔顶部的套管;③长期数据记录仪与传感器链。RECC系统是永久性海底装置,最多可装配4个相互嵌套的套管,起着支撑海底松散沉积物、使观测设备到达深部地层的作用。为提高钻孔长期观测、采样及数据收集的质量,RECC系统允许反复进行钻孔加深及观测设备的布放。由此,我们可根据需要将CORK配置为初始CORK,ACORK或其他类型CORK。 此外,为阻止钻孔与海水或洋壳地层间的水和物质交换,还需对钻孔深部套管进行水泥封闭。
过去几十年间,已有数十个CORK被布放于不同海域的海底,科学家通过这些装置获得了大量研究数据,研制海底观测设备的技术手段也在此过程中不断提升。随着科技进步与观测系统应用范围的扩大,CORK已在初始CORK研发的基础上衍生出了另外4个新的版本[15,19,20],简述如下:
初始CORK:这是最初的单封闭型CORK,是由再入圆锥筒形成的单一的、在垂向上无分隔的钻孔密封装置,温、压监测器及数据记录仪连接在被再入圆锥筒密封的钻孔中;该装置通过再入圆锥筒上的一个阀门进行流体采集,主要应用于洋壳水文观测(图3a)。ODP实施过程中已有14套该类观测装置布放于被沉积物覆盖的年轻洋壳或海底俯冲带中[15]。
图1 全球海底水文地质系统活动特征示意图(据参考文献[
Fig.1 A scheme showing characteristic marine hydrogeologic systems that may occur throughout the global ocean(modified after reference[4])
图2 Carson B,Becker K和Davis E于1989年在餐巾纸上绘制的最早的海底井控观测装置概念示意图[
Fig.2 The early conceptual sketch of the instrumented borehole seal, drawn on a dinner napkin by B. Carson, K. Becker, and E. Davis in 1989[
然而,科学家很快意识到,这类海底钻孔密封装置获得的监测数据为整个钻孔不同层位的平均值,无法对不同深度地层进行分层研究。于是,1998年开始,4种新型CORK相继问世——ACORK,CORK Ⅱ,有缆型CORK和L-CORK。
ACORK:改进型(advanced)CORK(图3b)。用多个分隔器(packer)在单一钻孔内将所钻洋壳在垂向上分隔成互不连通的观测层位[11,15,20],套管外的传感器链穿过分隔器,将不同层位获取的数据传输至装置顶部。套管中央还可单独携带一条传感器链。2套ACORK已于2001年布放于Nankai海槽增生楔地层中(ODP196航次)[21]。
CORK Ⅱ:是一类增加了地震/地层应变量监测器的钻孔观测系统。如图3c所示,CORK Ⅱ通常使用直径较小的分隔器、透流窗(screen)和脐带取样管(umbilical),它们被安装在直径11.43 cm的套管上,而11.43 cm套管则悬挂在IODP标准40.64~27.3 cm的套管内[12]。目前已有2套CORK Ⅱ布放于哥斯达黎加俯冲边界上 (ODP205航次)[22]。
有缆型CORK(Wireline CORK,图3d):是一种有缆绳连接的多分隔观测系统[22],通过常规科考船上的专控运载工具在可以多次进入的钻孔中进行布放[23]。这类多功能多分层观测系统配备有透流窗[24]、电热调节器链[25]、长期流体采样器[26]、原位微生物实验设备[27] 或应变监测仪[28]。2套有缆型CORK已于2001年布放在哥斯达黎加裂谷南部的新生洋脊上[22]。
图3 CORK观测系统工程示意图(据参考文献[
(a)初始CORK;(b)改进型CORK;(c)CORK II;(d)有缆型CORK;(e)L-CORK
Fig.3 Engineering schematic of the CORKs (modified after reference[15])
(a) The original CORK; (b) ACORK; (c) CORK II; (d) Wireline CORK; (e) L-CORK
L-CORK(Lateral CORK)和天才井塞(genius plug):L-CORK由CORK II发展而来,因其使用更为方便且更具确定性和保障性而被称为天才井塞(图3e)。其主要有5个方面的改进[9,29]:①增加了一个与CORK主体上端管道连接的直径10.2 cm的球阀,由此便可安装感应流量计,从而提供一个与钻孔内传感器和采集器相接的新通道。②增加了一个顶部阀门,用于避免出现IODP301航次所遇到的安装顶部井塞的问题[9]。③增加了备用充气型和遇水膨胀型分隔器,强化了对钻孔的密封。膨胀型分隔器长1.5 m,材质为FREECAP FSC11聚合体。④CORK布放机制的改变:将钻孔中所有渗透压流体采样器(OsmoSampler)置于带孔的、有环氧树脂涂层的钻铤中,从而避免因暴露在套管外而被埋藏。安装钻铤的另外2个好处是:增加套管的强度(ODP 195和206航次、IODP301航次CORK安装的失败可能与此有关)[9];其内外所涂的环氧树脂可减少微生物学和地球化学研究过程中的污染问题[27]。⑤增设了耦合器系统和相关的新阀门,可通过脐带取样管采集钻孔内流体。IODP327航次于2012年在胡安·德富卡洋脊(Juan de Fuca Ridge)1362A和1362B站位布放了L-CORK,用于研究地壳演化、洋壳水文地质学、地球化学循环、微生物生态和生物地球化学过程 (图4)[9,10]。
上述分布在不同海域、时间上可进行长期连续观测的CORK在科学研究中具有十分重要的应用价值[17],可以帮助我们:①获取可靠的孔隙水压力数据;②通过测定地层对潮汐和海底地震载荷的响应,取得洋壳地层弹性与水文特性资料[24,30];③通过大范围、长期连续观测,获取如地震、滑塌、深部海底紊流等瞬时事件记录,并通过数据来分析该类事件发生前的环境变化[24,31];④找到与幕式流体流动或地震前兆有关的温度异常[25,32];⑤获取流体化学成分及地层渗透率实时变化数据[32];⑥通过直接注入特定化学成分的流体或采集深部流体,对海底地层中微生物群落及其生命过程进行研究[27,33]。Becker等[15]对可以安装在CORK上的各类实验设备进行了总结。
自CORK首次在ODP139和146航次布放成功后,截止2011年,ODP和IODP已先后布放了28个CORK观测站用于开展海底探测和研究工作(图5)[16]。这些CORK观测系统主要用于研究固体地球和地球物理[34]、洋壳水文地质学(包括示踪实验[29] 和区域水文学响应[35]、流体地球化学和流体迁移[8,16,36~39])及深部生物圈微生物学和生物地球化学[27,40~42]。CORK 布放和研究最多的区域是胡安·德富卡洋脊(图5)。IODP327航次在此洋脊东侧的U1362B钻孔基底上部注入示踪剂,并进行了单孔和跨孔示踪实验(图4)[10]。 Geoffrey等[16]通过在此洋脊进行连续4年的钻孔流体采样,研究了海水—玄武岩—微生物间的相互作用。他们将CORK安装在穿过262 m厚沉积物和108 m厚玄武岩的基底上,通过研究该洋脊东侧洋壳海水的补给排泄揭示了该区域的热力学、水文学和地球化学过程以及微生物代谢。
图4 IODP327航次在胡安·德富卡洋脊东侧进行的跨钻孔示踪实验(据参考文献[
(a)CORK观测站位置及跨钻孔示踪实验的海底地形图:示踪剂溶液注入U1362B钻孔中,利用整合在CORK观测系统中的自动取样器在U1301A,U1301B,U1362A,1026B和U1362B钻孔中对示踪剂的到达进行观测,蓝色虚线箭头指示推断流体在基底流动的趋势方向;内插图中的蓝色方框指示了主图的分布区域;主图等高间隔为10 m,红色轮廓表示基底露头;(b) 有限时间内注入示踪剂导致羽流形成的示意图;(c) 所注入示踪剂的穿透曲线;(d) U1362B钻孔示踪剂注入几何简图
Fig.4 Cross-hole tracer experiment at the eastern flank of the Juan de Fuca Ridge during IODP Expedition 327 (modified after reference[10])
(a) Bathymetric map illustrating the locations of borehole observatories at the site. Tracer solution was injected in Hole U1362B. Tracer arrival was monitored with automated samplers integrated with CORK observatories in Holes U1301A, U1301B, U1362A, and 1026B and also in Hole U1362B. Blue dashed arrows indicates general direction of inferred fluid flow in basement. Blue rectangle in the insert shows the area of themain figure. Contour interval = 10 m, red contours = basement outcrops; (b) Tracer is injected for a finite time, leading to formation of a plume; (c) Breakthrough curves of the injected tracer; (d) Schematic illustration of tracer injection geometry in Hole U1362B
Becker等[15]对这些年在ODP 和IODP的CORK布放与使用过程中获得的教训和经验进行了总结:
(1)CORK密封性和管道系统质量:CORK系统密封性的好坏是能否正确观察到瞬时及平均压力状态的关键。CORK的密封部位以及不同部位的管道(套管与地层之间、分隔器周围或钻孔附近没有封闭的辅助钻孔)均有可能产生渗露,这将会导致压力损失。要尽最大努力确保整个装置完全无泄漏。系统封闭性测试可通过热液系统观测来实现。
(2)系统顺应性:填充于CORK套管中的流体、ACORK与CORKⅡ取样管的压缩率以及分隔器或密封装置的顺应性均可导致压力信号失真。因此,进行系统设计时需尽量缩小液压管直径并清除其中的空气或其他游离气体。目前尚不清楚分隔器对系统稳定性的影响,但其可能导致低渗透率地层中的高频信号失真。
图5 位于洋壳流体流动区域观测站点的CORK布放位置分布图[
JdF. 胡安·德富卡洋脊东侧(ODP168航次、IODP301和327航次); MV. Middle Valley(ODP139航次);Van和Or. Cascadia边缘(ODP146和IODP328航次);CR. 哥斯达黎加(ODP205航次);Bar. 巴巴多斯海岭(ODP156航次);NP. North Pond(ODP174B和IODP336航次);Nan.日本南海海槽(ODP196航次、IODP319和332航次);SCh.海山岛南部(ODP195航次)
Fig.5 Locations of CORK deployments, these observatories are located in regions where fluid flows through the oceanic crust[
JdF: Eastern flank of Juan de Fuca Ridge (Leg 168 and Expeditions 301 and 327); MV: Middle Valley (Leg 139); Van and Or: Cascadia margin (Leg 146 and Expedition 328); CR: Costa Rica (Leg 205); Bar: Barbados Ridge (Leg 156); NP: North Pond (Leg 174B and Expedition 336); Nan: Nankai Trough (Leg 196 and Expeditions 319 and 332); SCh: South Chamorro Seamount (Leg 195)
(3)采样及系统压力损耗:采集流体样品时,除非将采样器密封在取样地层中,否则利用CORK系统采集流体样品通常都会导致系统渗漏。在具有多个分隔器的CORK系统中,海底流体采样可通过一个小口径通道进行,通过对该通道进行适当平衡维护以使流体保持一定流速,这样可将采样器与海底间的转换时间最大限度缩短到可以接受的程度,但也不能无限度缩短,否则会造成压力丢失或热结构失真。渗漏的存在给低渗透率海底地层研究提出了巨大挑战。
(4)海水的侵入及扰动:在打钻过程以及钻孔完成而未密封之前,低温、高密度海水侵入地层会对地层造成较大扰动,高渗透率地层尤其如此。这种非原生流体会影响温度和压力的测量、置换地层水并影响原始地层流体样品的采集。选择自然的超疏水站点有助于规避上述问题,尽可能缩短钻孔开始钻入地层与钻孔密封之间的时间也可减弱上述影响,还可通过桥塞封堵或分隔器密封主套管底部周边来将上述影响最小化。
没有海底观测网的建立所提供的连续时间序列观测系统,将很难研究很多地球构造动力学体系。同样,空间上分布的CORK观测网对重建钻孔内海底地层原位状态平衡、了解洋壳水文地质状况及过程也是必不可少的。海底观测网的建立还有可能在全球地质灾害评估及监测方面发挥关键作用,这些灾害大多与沿大陆边缘的孕震区有关。此外,微生物学、生态学及生物地球化学研究也可能大多受益于这些依赖于空间和时间过程的时间序列观测网。比如,最近对深部生物圈的研究已经表明,洋壳中存在不同的微生物种群:古菌、细菌及其休眠芽孢[43,44,45,46,47,48,49]。因此,海底CORK观测系统可为地球科学和生命科学领域的研究者们提供独特的新手段和新机遇,以探究洋壳复杂的,与地质、水文、化学、生物及地球化学有关的过程,这些过程的时间尺度可从数秒到数十年,空间尺度可从海底表层沉积物到洋壳深部;同时,通过CORK观测网,科学家还可进行区域和流域尺度过程的相关对比研究。海底CORK观测网将为我们提供上述各种过程区域分布的、位置固定的时间序列观测,这将是对地球科学和生命科学研究中系统研究法的重要贡献。
首个长期海底观测站布放于胡安·德富卡洋脊(ODP139航次),持续14个月的压力记录显示,在洋脊渗透性极强的上基底中存在着滞留时间极短的高通量侧向流体活动[50,51]。首个跨钻孔观测系统(ODP168航次)和首个三维CORK观测系统(IODP301航次)同样展布于胡安·德富卡洋脊,它们继续向人们展示了高通量侧向流体及其高渗透性特性,它们还记录了与地震活动和潮汐有关的瞬变流[35]。根据以往CORK布放和使用已取得的经验和教训,我们推荐2种适合在西太平洋边缘(包括我国在内)建立CORK观测系统的模式:独立式CORK观测系统和电缆连接式CORK观测系统。
(1)独立式CORK观测系统(Stand-Alone CORKs, SACOs):一套CORK装置可满足独立海底观测站所需的所有基本要素,包括长期不间断进行海底地球动力学和水文学测量的稳定工作平台、在海底洋壳进行微生物学实验的实验室。然而,由于SACOs需要配备独立的动力和数据装置,这使得其与地面进行双向通讯和状态参数传输的能力受限,其实时或近实时数据的传输量也受到限制。高速光通讯调制解调器可为CORK获得的数据实施水下远程通讯传输[52]。水下光通讯调制解调器可为我们提供200 m范围内10 Mbps的高速数据传输,在该距离内的ROVs或AUVs无需直接连线或进行设备收回便可进行通讯[52]。
(2)电缆连接式CORK观测系统(Cabled CORK Observatories,CCOs):该观测系统通过光电连接器和电缆2种控制方式实现海底与陆地间的实时数据传输,从而帮助我们获得连续不间断的海底时间序列数据。CCOs相当于固定在海底的实验室,它不间断地产出实时数据,还可进行交互式原位实验,甚至还具有监测海底事件并作出响应的能力。我们可对通过电缆网与陆地相连的传感器包进行标准化,从而测量海底的关键化学和物理参数(温度、压力、氧气浓度和pH值等)。
海底CORK观测网的建立,为科学家对海底生物圈进行数秒到数十年时间尺度的地质学、地热学、水文学、化学、微生物学与生物地球化学过程基础研究提供了手段,这是地球科学、海洋科学以及生命科学获得新发现、取得重大进展的重要基础。这些不同学科领域取得的发现和进展,已在诸如地震等自然灾害评估与减防、深部海底微生物在全球微生物地球化学循环与全球气候变化中的作用、矿物及生物资源的形成与分布、宇宙生命的形成及演化等诸多方面带来了重大影响。例如,深部生物圈可能代表着地球上最大的生态环境聚集地,它向海底和陆地表面下方延伸达数千米范围,其体积比地球洋盆的体积至少高出2个数量级,可能蕴藏着地球上巨大的生物量[53]。深部生物圈微生物对地球上的生物地球化学循环具有关键性的影响,包括碳循环、能量流动、物质循环和气候变化等[3]。由于深部生物圈中蕴育着大量人们尚未探知的生物地球化学过程,因而使得针对深部微生物系统的动态变化过程的调查成为地球科学和生命科学研究最具生命力的领域之一。CORK观测站的建立为采集深部生物圈流体(水、气体)样品、进行微生物原位培养和观测,进而研究深部生物圈微生物介导的生物地球化学过程提供了前所未有的崭新平台。利用我国自己的资源和人材构建海底CORK观测系统,还将为国家培养一大批高精尖海洋科技人才,为我国海洋科学飞速发展、尽快跻身世界海洋强国行列奠定坚实基础。
The authors have declared that no competing interests exist.
| [1] |
The heat flow through oceanic and continental crust and the heat loss of the Earth [J].
Simple thermal models based on the creation and cooling of the lithosphere can account for the observed subsidence of the ocean floor and the measured decreased in heat flow with age. In well-sedimented areas, where there is little loss of heat due to hydrothermal circulation, the surface heat flow decays uniformly from values in excess of 6 0008cal/cm0005 s (250 mW/m0005), for crust younger than 4 Ma (4 m.y. B.P.), to close to 1.1 0008cal/cm0005 s (46 mW/m0005) through crust between 120 and 140 Ma. After 200 Ma the heat flow is predicted to reach an equilibrium value of 0.9 0008cal/cm0005 s (38 mW/m0005). The surface heat flow on continents is controlled by many phenomena. On the time scale of geological periods the most important of these are the last orogenic event, the distribution of heat-producing elements, and erosion. To better understand the effects of age, each continent is separated into four provinces on the basis of radiometric dates. Reflecting the preponderance of Precambrian crust, two of these provinces cover the Archean to the middle Proterozoic, and the third covers the late Proterozoic to the Mesozoic. The mean heat flow decreases from a value of 1.84 0008cal/cm0005 s (77 mW/m0005) for the youngest province to a constant value of 1.1 0008cal/cm0005 s (46 mW/m0005) after 800 Ma. The nonradiogenic component of the surface heat flow decays to a constant value of between 0.65 and 0.5 0008cal/cm0005 s (25 and 21 mW/m0005) within 200090009400 Ma. Using theoretical models, we compute the heat loss through the oceans to be 727 0103 1010 cal/s (30.4 0103 1012 W). The comparison between the theoretical and measured values allows an estimate of 241 0103 1010 cal/s (10.1 0103 1012 W) for the heat lost owing to hydrothermal circulation. We show that the heat flow through the marginal basins follows the same relation as that for crust created at a midocean spreading center. These basins have a corresponding heat loss of 71 0103 1010 cal/s (3.0 0103 1012 W). The heat loss through the continents is calculated from the observations and is 208 0103 1010 cal/s (8.8 0103 1012 W). Our estimate of the value for the shelves is 67 0103 1010 cal/s (2.8 0103 1012 W). The total heat loss of the earth is 1002 0103 1010 cal/s (42.0 0103 1012 W), of which 70% is through the deep oceans and marginal basins and 30% through the continents and continental shelves. The creation of lithosphere accounts for just under 90% of the heat lost through the oceans and hence about 60% of the worldwide heat loss. Convective processes, which include plate creation and orogeny on continents, dissipate two thirds of the heat lost by the earth. Conduction through the lithosphere is responsible for 20%, and the rest is lost by the radioactive decay of the continental and oceanic crust.We place bounds of between 0.6 and 0.9 0008cal/cm0005 s (25 and 38 mW/m0005) for the mantle heat flow beneath an ocean at equilibrium and between 0.40 and 0.75 0008cal/cm0005 s (17 and 31 mW/m0005) for the heat flow beneath an old stable continent. The computed range of geotherms for an equilibrium ocean overlaps the range of stable continental geotherms below a depth of 100 km. The mantle heat flow beneath a continent decays with a thermal time constant similar to that of the oceanic lithosphere. The continental basins subside with the same time constant. These observations are evidence that there is no detectable difference between the thermal structure of an equilibrium ocean and that of an old continent. Thus the concept of the lithosphere as a combination of a mechanical and a thermal boundary layer can be applied to both oceans and continents. We evaluate the constraints placed on models based on this concept by seismological observations. In the absence of compelling evidence to the contrary we favor these models because they provide a single explanation for the thermal structure of the lithosphere beneath an equilibrium ocean and a stable continent.
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| [2] |
Fluxes of fluid and heat from the oceanic crustal reservoir [J].
Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40掳C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.
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| [3] |
Explore the deep biosphere [J]. |
| [4] |
Hydrogeology program planning group final report [J].
ABSTRACT JOIDES established the Hydrogeology Program Planning Group (PPG) at the end of 1999. The overall goal of this PPG was to define and prioritize the main problems in submarine hydrogeology. The group met three times during 2000 and 2001, leading to the production of this report. In the main body of the report, we first outline the important hydrogeologic questions. We II then provide an overview of the fundamental principles of fluid flow in coupled geologic processes. Next we review the methodologies in hydrogeologic studies focusing on modeling and hydrogeologic testing. Finally, we make six recommendations for addressing the scientific issues and suggest strategies for implementing the recommendations.
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| [5] |
Opening a time tunnel through the Earth system to understand its history [J].穿凿地球系统的时间隧道 [J]. |
| [6] |
Constraints on hydrothermal heat flux through the oceanic lithosphere from global heat flow [J].
A significant discrepancy exists between the heat flow measured at the seafloor and the higher values predicted by thermal models of the cooling lithosphere. This discrepancy is generally interpreted as indicating that the upper oceanic crust is cooled significantly by hydrothermal circulation. The magnitude of this heat flow discrepancy is the primary datum used to estimate the volume of hydrothermal flow, and the variation in the discrepancy with lithospheric age is the primary constraint on how the hydrothermal flux is divided between near-ridge and off-ridge environments. The resulting estimates are important for investigation of both the thermal structure of the lithosphere and the chemistry of the oceans. We reevaluate the magnitude and age variation of the discrepancy using a global heat flow data set substantially larger than in earlier studies, and the GDHI (Global Depth and Heat Flow) model that better predicts the heat flow. We estimate that of the predicted global oceanic heat flux of 32 x 10(exp 12) W, 34% (11 x 10(exp 12) W) occurs by hydrothermal flow. Approximately 30% of the hydrothermal heat flux occurs in crust younger than 1 Ma, so the majority of this flux is off-ridge. These hydrothermal heat flux estimates are upper bounds, because heat flow measurements require sediment at the site and so are made preferentially at topographic lows, where heat flow may be depressed. Because the water temperature for the near-ridge flow exceeds that for the off-ridge flow, the near-ridge water flow will be even a smaller fraction of the total water flow. As a result, in estimating fluxes from geochemical data, use of the high water temperatures appropriate for the ridge axis may significantly overestimate the heat flux for an assumed water flux or underestimate the water flux for an assumed heat flux. Our data also permit improved estimates of the 'sealing' age, defined as the age where the observed heat flow approximately equals that predicted, suggesting that hydrothermal heat transfer has largely ceased. Although earlier studies suggested major differences in sealing ages for different ocean basins, we find that the sealing ages for the Atlantic, Pacific, and Indian oceans are similar and consistent with the sealing age for the entire data set, 65 +/- 10 Ma. The previous inference of a young (approximately 20 Ma) sealing age for the Pacific appears to have biased downward several previous estimates of the global hydrothermal flux. The heat flow data also provide indirect evidence for the mechanism by which the hydrothermal heat flux becomes small, which has often been ascribed to isolation of the igneous crust from seawater due to the hydraulic conductivity of the intervening sediment. We find, however, that even the least sedimented sites show the systematic increase of the ratio of observed to predicted heat flow with age, although the more sedimented sites have a younger sealing age. Moreover, the heat flow discrepancy persists at heavily sedimented sites until approximately 50 Ma. It thus appears that approximately 100-200 m of sediment is neither necessary nor sufficient to stop hydrothermal heat transfer. We therefore conclude that the age of the crust is the primary control on the fraction of heat transported by hydrothermal flow and that sediment thickness has a lesser effect. <This inference is consistent with models in which hydrothermal flow decreases with age due to reduced crustal porosity and hence permeability.
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| [7] |
CORK-II: Long-term monitoring of fluid chemistry, fluxes, and hydrology in instrumented boreholes at the Costa Rica subduction zone [J].
Two boreholes were drilled on the Costa Rica subduction zone to study the geochemical fluxes and related processes associated with sedi- ment compaction, dewatering, and alteration. The holes were outfitted with modified CORKs (CORK-IIs) that include instruments capable of fluid sampling and measuring flow rates, temperature, and pressure. Fluids are sampled continuously within the d茅collement zone and in the uppermost oceanic crust with long-term OsmoSamplers for both dissolved ions and gases. The major advantage of the CORK-II is that samples and data can be retrieved without disrupting the pressurized horizons by temporarily opening them to hydrostatic pressures during instrument exchange. This paper describes the concepts, design, and deployment of CORK-IIs in Holes 1253A and 1255A.
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| [8] |
Continuous chemical monitoring with osmotically pumped water samplers: OsmoSampler design and applications [J].
Abstract Long-term osmotically pumped fluid samplers, or OsmoSamplers, were developed to reliably and autonomously collect continuous small-volume water samples for monitoring aqueous environments in remote locations for up to several years. OsmoSamplers provide sequential milliliter-size samples that, when analyzed, yield high-resolution time-series for a wide range of dissolved components. These instruments fill an important niche that has not been addressed by automated samplers or in situ analyzers. OsmoSamplers can be customized for flow rate and duration for addressing numerous scientific questions. The samplers consist of an osmotic pump that continuously pulls fluid sample into a long small-bore tube. They are thus extremely simple, reliable, and require neither electrical power nor moving parts. Upon recovery, fluid samples are extracted from subsections of the sample tubing, where each subsection integrates a discrete time interval. The time-stamped subsamples are then analyzed for chemical species of interest. Sample smearing due to static and dynamic diffusion and mixing is kept to a minimum by the use of small-bore tubing (0.5 to 1.2 mm inside diameter) and low flow rates (0.1 to 12 mL d -1). Theory and laboratory experiments show that sample smearing is not significantly greater than that calculated from static diffusion alone. OsmoSamplers have been tested in the laboratory and deployed at sea. Results from laboratory tests and field deployments illustrate initial results and potential applications. 漏 2004, by the American Society of Limnology and Oceanography, Inc.
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| [9] |
Fluid sampling from oceanic borehole observatories: Design and methods for CORK activities (1990-2010) [ |
| [10] |
Design, deployment, and status of borehole observatory systems used for single-hole and cross-hole experiments, IODP Expedition 327, eastern flank of Juan de Fuca Ridge [J].
Plans for ongoing and future experiments . . 13 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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| [11] |
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| [12] |
ODP/IODP ‘CORK’ Long-term subseafloor hydrogeological observatories [ |
| [13] |
Herzen R P V, Erickson A J, |
| [14] |
Herzen R P V,
We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2-m.y.-old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 19000±10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350090009370 m, indicating that ocean bottom water was flowing down the hole into the upper 090804100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 60000900097000 1/h in late 1979 to about 1500 1/h 2 years later; altogether over 500103106 kg of seawater has been drawn into the basement. We estimate a permeability of 090906601031009080814 m2 for the reservoir in the upper 090804100 m of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a).
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| [15] |
A review of CORK designs and operations during the Ocean Drilling Program [J].DOI URL [本文引用: 12] 摘要
Abstract This paper provides a review of 1989-2003 designs and operations of the 20 Circulation Obviation Retrofit Kit (CORK) long-term subseafloor hydrogeological observatories installed in 18 holes during the Ocean Drilling Program (ODP). The basic configura- tions of the four models of CORKs developed during the ODP pe- riod are summarized: the original single-seal CORK (14 installa- tions in 12 holes, 1991-2001) and three multilevel models, including the Advanced CORK or ACORK (2 installations, 2001), a wireline instrumented multipacker system or wireline CORK (2 installations, 2001), and the CORK-II (2 installations, 2002). The evolution of the scientific instrumentation installed in ODP CORKs and the history of postinstallation submersible operations are described. This instrumentation was provided by scientists with support of national ODP research funding, which also sup- ported the extensive submersible time devoted to postinstallation data downloads and instrument servicing. Although the purpose of this paper does not include a review of CORK scientific results, we offer some comments on scientific lessons learned during the ODP CORK effort. We describe the funding and engineering sup- port structure that held for the ODP CORK installations and close with some comments on the importance of engineering support for the Integrated Ocean Drilling Program goals involving long- term borehole observatories. We also provide a complete bibliog- raphy of CORK-related literature through 2004 and all of the data sets in digital form collected through 2003 from the six ODP CORK installations installed in either 1991 or 1996 near the Juan de Fuca Ridge, of which all but one are still in service.
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| [16] |
Subseafloor seawater-basalt-microbe reactions: Continuous sampling of borehole fluids in a ridge flank environment [J].
[1] Integrated Ocean Drilling Program (IODP) Hole 1301A was drilled, cased, and instrumented with a long-term, subseafloor observatory (CORK) on the eastern flank of the Juan de Fuca Ridge in summer 2004. This borehole is located 1 km south of ODP Hole 1026B and 5 km north of Baby Bare outcrop. Hole 1301A penetrates 262 m of sediment and 108 m of the uppermost 3.5 Ma basaltic basement in an area of warm (64C) hydrothermal circulation. The borehole was instrumented, and those instruments were recovered 4 years later. Here we report chemical data from two continuous fluid samplers (OsmoSamplers) and temperature recording tools that monitored changes in the state of borehole (formation) fluids. These changes document the effects of drilling, fluid overpressure and flow, seawater-basalt interactions, and microbial metababolic activity. Initially, bottom seawater flowed into the borehole through a leak between concentric CORK casing strings. Eventually, the direction of flow reversed, and warm, altered formation fluid flowed into the borehole and discharged at the seafloor. This reversal occurred during 1 week in September 2007, 3 years after drilling operations ceased. The composition of the formation fluid around Hole 1301A generally lies within bounds defined by springs on Baby Bare outcrop (to the south) and fluids that discharged from Hole 1026B (to the north); deviations likely result from reactions with drilling products. Simple conservative mixing of two end-member fluids reveals reactions occurring within the crust, including nitrate reduction presumably by denitrifying microbes. The observed changes in borehole fluid composition provide the foundation for a conceptual model of chemical and microbial change during recharge of a warm ridge-flank hydrothermal system. This model can be tested through future scientific ocean drilling experiments.
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| [17] |
Simple, affordable and sustainable borehole observatories for complex monitoring objectives [J].
Around 20 years ago, the scientific community started to use borehole observatories, so-called CORKs or Circulation Obviation Retrofit Kits, which are installed inside submarine boreholes, and which allow the re-establishment and monitoring of in situ conditions. From the first CORKs which allowed only rudimentary fluid pressure and temperature measurements, the instruments evolved to multi-functional and multi-level subseafloor laboratories, including, for example, long-term fluid sampling devices, in situ microbiological experiments or strainmeter. Nonetheless, most boreholes are still left uninstrumented, which is a major loss for the scientific community. In-stallation of CORKs usually requires a drillship and subsequent ROV assignments for data download and instru-ment maintenance, which is a major logistic and financial effort. Moreover, the increasing complexity of the CORK systems increased not only the expenses but led also to longer installation times and a higher sensitivity of the in-struments to environmental constraints. Here, we present three types of Mini-CORKs, which evolved back to more simple systems yet providing a wide range of possible in situ measurements. As a regional example the Nankai Trough is chosen, where repeated subduction thrust earthquakes with M8+ occurred. The area has been investigated by several drilling campaigns of the DSDP, ODP and IODP, where boreholes were already instrumented by different CORKs. Unfortunately, some of the more complex systems showed incomplete functionality, and moreover, the increased ship time forced IODP to rely on third party funds for the observatories. Consequently, the need for more affordable CORKs arose, which may be satisfied by the systems presented here. The first type, the so-called SmartPlug, provides two pressure transducers and four temperature sensors, and monitors a hydrostatic reference section and an isolated zone of interest. It was already installed at the Nankai Trough accretionary prism during IODP Exp. 319 and successfully recovered during IODP Exp. 332, both cruises being part of NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment). The 15-months long data showed transients related to the arrival of seismic waves, storms and can further be used for detection of seismogenic strain events. Moreover, based on tidal signals in the pressure data, it was possible to make assumptions regarding the elastic properties of the surrounding formation. The SmartPlug was exchanged by an enhanced version, the GeniusPlug, which provides additional fluid sampling devices and microbiological experiments during the monitoring period. Its recovery is planned for 2013. Going one step further in simplicity, a Mini-CORK has recently developed especially designed for the portable seafloor drill rig MeBo (MARUM, Univ. Bremen, Germany), which can be installed without a drillship and which, due to its telemetric unit, makes costly recovery operations obsolete. The MeBo can be operated from any re-search vessel and allows coring to a depth of 70 m, which may be followed by instrumentation of the borehole with the MeBo-CORK. Two designs are available: the first design allows in situ measurement of pressure and temperature solely, whereas the second design consists of a seafloor unit including additional mission specific sensors (osmo-samlers for geochemistry and microbiology, etc.). A first field test for the MeBo-CORKs into mud volcanoes in the Kumano forearc basin is envisaged for summer 2012 to complement IODP project NanTroSEIZE.
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| [18] |
CORK: A hydrologic seal and downhole observatory for deep-ocean boreholes [J].
ABSTRACT A new tool has been developed and successfully deployed that provides a means by which cased reentry holes drilled by the Ocean Drilling Program can be hydrologically sealed and instrumented. The seal prevents flow of water into or out of a hole that would otherwise cause severe thermal and chemical disturbance to the formation drilled. The seal is capable of withstanding both positive and negative differential pressures that may be present in hydrothermally or tectonically active environments, and can be removed for later drilling operations. The instrumentation, developed for initial deployments during Ocean Drilling Program Leg 139, was designed to monitor the formation temperatures and pressures for up to 2 years during and following the recovery from drilling disturbances. A recording gauge measures absolute pressure in the borehole below the seal. A sensor string containing 10 thermistors and a fluid-sampling tube hang in the hole below the recording package. The sampling tube extends through the seal to a port, allowing the differential pressure to be measured and fluids to be tapped from deep within the hole. Data recovery and fluid sampling can be done via submersible or remotely operated vehicle. Two units were deployed in the sediment-filled Middle Valley rift of the northern Juan de Fuca Ridge. The first was installed in Hole 857D, which was drilled to a total depth of 936 meters below seafloor, through 470 m of turbidite sediment and into a highly permeable sediment-sill complex that is inferred to be a sediment-sealed hydrothermal "reservoir." The second was deployed in Hole 858G, which was drilled to a total depth of 433 meters below seafloor into a buried volcanic edifice that underlies a hydrothermal vent field. Data were recovered successfully from both holes about three weeks after deployment. Temperatures and pressures recorded in Hole 858G show that formation conditions are severely affected by drilling and by downward fluid flow into the formation through a nearby uncased exploratory hole that was not adequately sealed with cement. The differential pressure across the seal in Hole 858G was initially -0.24 MPa, and at the time of the data recovery was becoming increasingly negative. Temperatures and pressures recorded in Hole 857D appear to be recovering slowly from drilling disturbances. Initially, the differential pressure across the seal in this hole was -1.10 MPa; at the time of the data recovery, the differential had reduced to -0.55 MPa. The instruments will continue to monitor formation pressures and temperatures once per hour for 2 years.
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| [19] |
Overview of Ocean Drilling Program engineering tools and hardware [J]. |
| [20] |
New insights into the hydrogeology of the oceanic crust through long-term monitoring [J].
The hydrogeology of the oceanic crust influences numerous global processes and properties, including the thermal evolution of oceanic lithosphere, crustal alteration and the chemistry of crustal fluids, the nature and significance of subseafloor microbial ecosystems, tectonic and volcanic characteristics of active margins, and the creation of hydrate and ore deposits on and below the seafloor. Understanding these processes and properties has been a fundamental goal for scientific ocean drilling for over three decades, but progress has been limited in many cases by a vexing conundrum: drilling into the seafloor often causes open exchange of formation fluids and ocean bottom waters when coring penetrates through sediments and into permeable oceanic basement (e.g., Hyndman et al., 1976; Becker et al., 1983). Such open exchange strongly perturbs in situ conditions and limits validity of borehole measurements to resolve the natural state. These perturbations have greatly limited our ability to quantify fundamental physical, chemical, and microbiological parameters that control and are controlled by fluid flow within the oceanic crust.
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| [21] |
Explanatory notes [J]. |
| [22] |
Temperature and video logs from the upper oceanic crust, Holes 504B and 896A, Costa Rica Rift flank: Implications for the permeability of upper oceanic crust [J].
In 2001, we revisited thickly sedimented 5.9 Ma crust on the southern flank of the Costa Rica Rift for wireline re-entry of two important ocean crustal boreholes, Holes 504B and 896A, more than 8 years after they were last drilled in 1993. Here we report borehole temperatures measured in both holes within casing through the sediment sections and then into open hole in uppermost basement, as well as a video log from the upper basement section of Hole 896A. Since it first penetrated into oceanic basement in 1979, Hole 504B has been known for downhole flow of ocean bottom water into uppermost basement that was initially strong (≈100 m/h) but then waned; our temperature data indicate a very slow lingering downflow at 0.4 m/h. The pressure differential driving this slow flow was determined to be 11–12 kPa from pressure data acquired when the hole was sealed by our wireline installation of a long-term hydrological observatory. The combination of the flow rate and the pressure differential constrains an estimate for the average permeability of the upper basement section in Hole 504B of 1–5×10 6114 m 2 , a value similar to but slightly less than past determinations. In Hole 896A, which is located 651 km away on a sediment-covered basement high, the temperature log indicated uphole flow of formation fluids at an average temperature of 57.8 °C and at a total rate of 12 m/h through casing; it also showed that at least three zones in uppermost basement produce fluids of different temperatures that contribute to this total flow. Although the associated pressure differential could not be measured in Hole 896A, estimates of average permeability of the section with the producing zones can be derived by assuming a differential of 6520 kPa similar to those measured in other ridge-flank sites in basement highs also known to produce formation fluids; estimated permeability values for uppermost basement in Hole 896A are on the order of 1–4×10 6113 m 2 , again consistent with past packer determinations. The video log in Hole 896A provides unprecedented visual images that document the discrete nature of the permeability of the producing zones. It also suggests an abundance of bacterial floc within the hole that may be either flushed from the formation by the producing fluids or blooming within the hole in response to nutrients advected by the producing fluids.
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| [23] |
First ocean-research-ship-supported fly-in re-entry to a deep ocean drill hole [J]. |
| [24] |
A discrete episode of seismic and aseismic deformation of the Nankai trough subduction zone accretionary prism and incoming Philippine Sea plate [J].
New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10 6 yr 1) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr 1) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.
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| [25] |
Transient formation fluid pressures and temperatures in the Costa Rica forearc prism and subducting oceanic basement: CORK monitoring at ODP Sites 1253 and 1255 [J].
Seafloor and formation-fluid pressure data from two Ocean Drilling Program (ODP) borehole hydrologic observatories installed at the toe of the subduction-zone prism off Costa Rica provide new information about the average and transient state of this non-accretionary prism. Data collected to date span a 16-month period from the time of installation during ODP Leg 205 in late 2002 to the most recent submersible site visit in March 2004. Pressure monitoring is part of a larger coordinated effort involving temperature monitoring and continuous fluid sampling within the formation and at the seafloor. The holes are positioned 800 m apart and monitoring points include two in igneous basement just seaward of the prism toe, one in the decollement that separates the underthrust sediments of the incoming Cocos plate from the Costa Rica prism, and one in the overthrust-prism sediments. Response of formation-fluid pressure to oceanographic loading at the seafloor constrains the framework compressibility of basement (ca . 1.1–1.3 × 10 61 10 Pa 61 1) and the prism and decollement sediments (ca . 4–7 × 10 61 9 Pa 61 1). Values are equivalent to ones determined elsewhere in similar sections. Once effects of seafloor loading are removed, pressures at both basement levels are seen to be steady, nearly identical, and less than but very close to hydrostatic (61 6 kPa). This state probably reflects the local hydrothermal regime of the oceanic crust, not the hydrologic regime of the consolidating subduction complex, and is consistent with basement being highly permeable and hydrologically well connected to distant igneous outcrops where free exchange of water between the crust and the ocean can occur. To what depth in the subduction zone high basement permeability persists is not known, but until permeability is reduced by alteration or mechanical fracture closure, basement must serve to provide a drainage path for water expelled from the consolidating underthrust sedimentary section. The decollement and overlying prism are observed to be superhydrostatic, although not highly so during this phase of observation. Pressures (expressed as the pore pressure ratio) range from λ 68 ≈ 0.25 at the decollement early in the monitoring period to ≈ 0.1 in the overlying prism at the end of the monitoring period. The cause of the initially elevated pressures is not known. If generated by contractional strain, elevated pressures appear not to be maintained for long periods of time at these lithologic/structural levels. The cause of the decline in pressure is also not known; it may be the consequence of strain relaxation or hydrologic drainage. No observations were made in the underthrust sediments, where greater hydrologic isolation may allow higher average pressures and transient pressures of greater amplitude and persistence. Two minor transients were observed at the decollement- and prism-monitoring levels that correlate with deformational events that occurred during a GPS monitoring experiment on the Nicoya Peninsula. One of these is inferred by Protti et al. [M. Protti, T. Gonzalez, T. Kato, T. Iinuma, S. Miyazaki, K. Obana, Y. Kaneda, P. LaFemina, T. Dixon, S. Schwartz, A creep event on the shallow interface of the Nicoya Peninsula, Costa Rica seismogenic zone, EOS, Trans. Am. Geophys. Union, Fall Meeting Program with Abstracts, 85 (2004) F1378; M. Protti, P. LaFemina, V. Gonzalez, T.H. Dixon, S.Y. Schwartz, T. Kato, T. Iinuma, S. Miyazaki, K. Obana, Y. Kaneda, A possible slow slip event within the seismogenic zone, Nicoya peninsula, Costa Rica, Geophys. Res. Lett. (submitted for publication)] to have propagated some 60 km to the northeast across the peninsula over the course of 2–3 weeks. The pressure transients at the ODP drill sites, located roughly 60 km offshore, began on May 24 and October 12, 2003, also 2–3 weeks after the initiation of the GPS-recorded Nicoya strain events at the coast. Propagation of dislocations updip (offshore) as well as downdip along the subduction thrust may be the cause of these transients.
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| [26] |
Long-term hydrogeochemical records in the oceanic basement and forearc prism at the Costa Rica subduction zone [J].
Two sealed borehole hydrologic observatories (CORKs) were installed in two active hydrogeochemical systems at the Costa Rica subduction zone to investigate the relationship between tectonics, fluid flow, and fluid composition. The observatories were deployed during Ocean Drilling Program (ODP) Leg 205 at Site 1253, ~020.2km seaward of the trench, in the upper igneous basement, and at Site 1255, ~020.5km landward of the trench, in the décollement. Downhole instrumentation was designed to monitor formation fluid flow rates, composition, pressure, and temperature. The two-year records collected by this interdisciplinary effort constitute the first co-registered hydrological, chemical, and physical dataset from a subduction zone, providing critical information on the average and transient state of the subduction thrust and upper igneous basement. The continuous records at ODP Site 1253 show that the uppermost igneous basement is highly permeable hosting an average fluid flow rate of 0.3m/yr, and indicate that the fluid sampled in the basement is a mixture between seawater (~0250%) and a subduction zone fluid originating within the forearc (~0250%). These results suggest that the uppermost basement serves as an efficient pathway for fluid expelled from the forearc that should be considered in models of subduction zone hydrogeology and deformation. Three transients in fluid flow rates were observed along the décollement at ODP Site 1255, two of which coincided with stepwise increases in formation pressure. These two transients are the result of aseismic slip dislocations that propagated up-dip from the seismogenic zone over the course of ~022weeks terminating before reaching ODP Site 1255 and the trench. The nature and temporal behavior of strain and the associated hydrological response during these slow slip events may be an analog for the response of the seaward part of the subduction prism during or soon after large subduction zone earthquakes.
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| [27] |
Colonization of subsurface microbial observatories deployed in young ocean crust [J].
Abstract Oceanic crust comprises the largest hydrogeologic reservoir on Earth, containing fluids in thermodynamic disequilibrium with the basaltic crust. Little is known about microbial ecosystems that inhabit this vast realm and exploit chemically favorable conditions for metabolic activities. Crustal samples recovered from ocean drilling operations are often compromised for microbiological assays, hampering efforts to resolve the extent and functioning of a subsurface biosphere. We report results from the first in situ experimental observatory systems that have been used to study subseafloor life. Experiments deployed for 4 years in young (3.5Ma) basaltic crust on the eastern flank of the Juan de Fuca Ridge record a dynamic, post-drilling response of crustal microbial ecosystems to changing physical and chemical conditions. Twisted stalks exhibiting a biogenic iron oxyhydroxide signature coated the surface of mineral substrates in the observatories; these are biosignatures indicating colonization by iron oxidizing bacteria during an initial phase of cool, oxic, iron-rich conditions following observatory installation. Following thermal and chemical recovery to warmer, reducing conditions, the in situ microbial structure in the observatory shifted, becoming representative of natural conditions in regional crustal fluids. Firmicutes, metabolic potential of which is unknown but may involve N or S cycling, dominated the post-rebound bacterial community. The archaeal community exhibited an extremely low diversity. Our experiment documented in situ conditions within a natural hydrological system that can pervade over millennia, exemplifying the power of observatory experiments for exploring the subsurface basaltic biosphere, the largest but most poorly understood biotope on Earth.
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| [28] |
NanTroSEIZE Stage 2: Riserless observatory [J].[本文引用: 1] 摘要
ABSTRACT One primary objective of the Nankai Trough Seismogenic Zone Experiment (NanTro- SEIZE) complex drilling project is to drill and instrument a series of holes across the seismogenic subduction system offshore the Kii Peninsula, Japan. Integrated Ocean Drilling Program (IODP) Expedition 332 followed up on initial observatory opera- tions begun during IODP Expedition 319 in 2009. This expedition focused mainly on engineering work, including (1) retrieval of a temporary observatory instrument in- stalled during Expedition 319 at IODP Site C0010, which penetrates the shallow egasplay- fault in the midforearc, and installation of a new suite of temporary sen- sors; (2) deployment of an upgraded temporary observatory at Site C0010; and (3) in- stallation of a permanent observatory at IODP Site C0002 in the outer Kumano Basin, at the location of planned future deep riser drilling. Expedition 332 began on 25 October 2010 and ended on 11 December 2010. During the first few weeks, the focus was on exchanging the SmartPlug temporary observa- tory with an upgraded GeniusPlug, both attached to a retrievable casing packer above the screened megasplay fault zone at Site C0010. The SmartPlug recovery was success- ful despite the strong Kuroshio Current, which can be attributed to an efficient reduc- tion of vortex-induced vibration (VIV) on the drill string by attaching ropes. Times series data recovered from the self-contained instrument include seafloor and forma- tion pressure as well as four independent temperature records from the fault zone and the overlying seafloor reference. Tentative analysis of the data proves the effective seal of the bridge plug; dampened pressure amplitudes in the tight, slightly overpressured formation; and identification of prominent earthquake and tsunami events in the 15 month record (23 August 2009-7 November 2010). The SmartPlug was replaced with a GeniusPlug, which is similar in geometry and equally self-contained but with an ad- dendum that hosts an OsmoSampler for collecting fluids for geochemical analysis and a flow-through osmotic colonization system for microbiological study. The sys- tem was installed at a depth that placed the addendum in the center of the 22 m wider screened cased section across the megasplay fault. At Site C0002, somewhat upslope of Site C0010, a new hole was drilled with logging while drilling (LWD) and cased for placement of a long-term borehole monitoring system. The monitoring system comprises a circulation obviation refrofit kit (CORK) assembly with a hydrogeological unit measuring pressure at four depth levels as well as a broadband seismometer, volumetric strainmeter, tiltmeter, geophones, and a thermistor string. The key goals include pore pressure monitoring in the upper accretionary prism (Unit IV), a series of measurements in the homogeneous sediments of Unit III (strain, tilt, seismicity, and pressure) in the transition zone, and temperature and pressure monitoring in the overlying Kumano Basin sediments of Unit II. The string of the CORK assembly had a total length of 965 m and was carefully secured during deployment with centralizers, bands, and straps to withstand the strong cur- rent. VIV was minimized using ropes, and acceleration was monitored during deploy- ment. The lower portion of the assembly is isolated against the overlying ocean body by a swellable packer at 746 meters below seafloor (mbsf). Part of the instrument string below was cemented (~780-935 mbsf) to couple the strainmeter and seismom- eter to the formation/casing. The CORK head was revisited prior and after cementing for system tests of the borehole instruments using the remotely operated vehicle (ROV), and all of these experiments were successful. The CORK was left with mini- mum battery power after the expedition and will be revisited in spring 2011 by ROV to connect an additional seafloor unit for power and data storage. In winter 2011/ 2012, the unit is anticipated to be connected to the real-time seafloor cabled network Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET).
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| [29] |
Preparation and injection of fluid tracers during IODP Expedition 327, eastern flank of Juan de Fuca Ridge [J].
Acknowledgments. . . . . . . . . . . . . . . . . . . . . . 10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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| [30] |
Theory for the propagation of tidally induced pore pressure variations in layered subseafloor formations [J].
Tidally induced pore pressure variations below the seafloor depend on the elastic moduli and transport properties of the pore fluid and formation. Hence observations of pore pressure variations, in conjunction with model predictions, can provide important constraints on these formation properties. In this paper, we study the propagation of tidally induced pore pressure variations in a layered poroelastic medium. We derive an analytic solution and use the solution to investigate the effects of various parameters, in particular, the bulk modulus of the formation, the bulk modulus of the pore fluid, and the formation permeability. Specific examples are considered that include the typical continuous depth variation of properties that occurs through normal sediment consolidation and imbedded layers of contrasting properties. Diffusive propagation of tidal pressure variations from the seafloor depends on the hydraulic diffusivity. The depth limit of diffusive propagation scales with the inverse square root of permeability and the period of the signal; for typical fine-grained marine sediments the depth scale at tidal frequencies is only a few meters. Any internal contrast in elastic properties, due to the presence of free gas for example, can give rise to large instantaneous pressure changes across a layer boundary, which in turn can induce diffusive propagation of signals above and below the interface. Long-term pressure records from sealed deep-ocean boreholes that included tidal signals are considered in light of the model results. In Ocean Drilling Project (ODP) Hole 857D on the Juan de Fuca Ridge, the observed attenuation of the seafloor tidal signal to 15% is consistent with the relatively low compressibility of the hydrothermally indurated section intersected by the open part of the borehole and with the high compressibility of the hot formation fluid. In ODP Hole 892B in the Cascadia accretionary prism, the attenuation to 55% and several degree phase lead of the formation tidal signal are probably the result of the open part of the hole being connected to an overlying interval bearing a few percent free gas via a high-permeability fault zone.
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| [31] |
Using ODP boreholes for studying sub-seafloor hydrogeology: Results from the first decade of CORK observations [J].
Résumé non disponible.
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| [32] |
Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone [J].
Continuous measurements of fluid flow were made over a six month period across the Nicoya Peninsula, Costa Rica (Pacific), convergent margin utilizing osmotically-driven fluid flow meters designed to quantify both inflow and outflow rates on the order of 6510 61 5 to 3 cm/d. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of 6530 km. Amplitudes of ground velocity recorded on collocated ocean bottom seismometers (OBS) increase during the three correlated flow events. The seismic signal has frequency characteristics that resemble volcanic and non-volcanic tremor. We hypothesize that repeated plate boundary slow slip events, potentially originating at the up dip limit of the seismogenic zone, generate the observed signals within the toe of the forearc. We propose a model in which the poro-elastic stress/strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBSs and the latter generates the flow transients recorded by the fluid flow meters. We suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible.
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| [33] |
Microbial activity in the marine deep biosphere: Progress and prospects [J].
The vast marine deep biosphere consists of microbial habitats within sediment, pore waters, upper basaltic crust and the fluids that circulate throughout it. A wide range of temperature, pressure, pH, and electron donor and acceptor conditions exists—all of which can combine to affect carbon and nutrient cycling and result in gradients on spatial scales ranging from millimeters to kilometers. Diverse and mostly uncharacterized microorganisms live in these habitats, and potentially play a role in mediating global scale biogeochemical processes. Quantifying the rates at which microbial activity in the subsurface occurs is a challenging endeavor, yet developing an understanding of these rates is essential to determine the impact of subsurface life on Earth's global biogeochemical cycles, and for understanding how microorganisms in these “extreme” environments survive (or even thrive). Here, we synthesize recent advances and discoveries pertaining to microbial activity in the marine deep subsurface, and we highlight topics about which there is still little understanding and suggest potential paths forward to address them. This publication is the result of a workshop held in August 2012 by the NSF-funded Center for Dark Energy Biosphere Investigations (C-DEBI) “theme team” on microbial activity (www.darkenergybiosphere.org).
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| [34] |
A geological and geophysical investigation of Baby Bare, locus of a ridge flank hydrothermal system in the Cascadia Basin [J].
Baby Bare is one of three small basement outcrops on the eastern, sedimentburied Juan de Fuca Ridge flank that have localized heat loss and fluid movement within 3.5 Ma oceanic crust. Low-temperature (25C) hydrothermal vents near the summit of Baby Bare represent the highest-temperature occurrence of off-axis hydrothermal activity found in oceanic crust older than 1 million years. This site has been investigated with seismic reflection profiling, towed-camera surveys, and an Alvin dive series that included heat flow measurements to document the detailed geological setting of these off-axis vents. A new geologic map based on visual observations of Baby Bare shows that the distribution of rock, sediment, and biota appears to be controlled by seafloor slope and elevation, while specific vent locations are controlled by faulting and occur only in areas of thin or no sediment cover. Alvin heat flow data indicate that conductive heat loss from the edifice is 4.5 times greater than that from the sediment-blanketed area around the outcrops. Although the outcrop is generally conical, seismic reflection profiles reveal that the sediment-buried portions of the edifice have an asymmetric morphology, strongly suggesting that Baby Bare is a volcano built upon a preexisting, fault-generated abyssal hill. This evidence, combined with previously published petrologic data and results of Ocean Drilling Program Leg 168 drilling, is consistent with the hypothesis that Baby Bare formed by off-axis volcanism rather than at the adjacent ridge axis; sediment thickness and fossil assemblages indicate that it could be as young as 2.7 Ma. Off-axis volcanoes such as Baby Bare increase the overall roughness of basement topography and thus delay complete sediment burial during normal lithospheric aging, particularly in areas where near-axis sediment accumulation is rapid. Partially buried seamounts play an important role in focusing hydrothermal exchange between the oceans and young oceanic crust and, if Baby Bare is representative, may contribute as much as 85% of the heat flux from a sedimented ridge flank.
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| [35] |
Borehole-to-borehole hydrologic response across 2.4 km in the upper oceanic crust: Implications for crustal-scale properties [J].
[1] Subseafloor hydrologic observatories (CORKs) were installed in four boreholes in young seafloor on the eastern flank of the Juan de Fuca ridge to evaluate the hydrogeology of the upper oceanic crust. Two CORKs installed at Site 1301 were incompletely sealed, allowing cold bottom water to flow into basement at 20900095 L/s and causing a pressure perturbation in a preexisting sealed CORK at Site 1027, which was 2.4 km away. The pressure perturbation at Site 1027 is analyzed using conventional aquifer test methods, yielding transmissivity of T = 0.5 to 1.2 0103 100908082 m2/s and bulk permeability of k = 0.7 to 2 0103 1009080812 m2 in the upper 300 m of basement. Storativity (a parameter that includes fluid and aquifer compressibilities, porosity, and layer thickness) is S = 1 to 3 0103 100908083, corresponding to a crustal aquifer (matrix) compressibility of 0205m = 3 to 9 0103 1009080810 Pa0908081. The inferred basement permeability is consistent with, but at the low end of, permeabilities calculated from single-hole packer experiments at this and other young oceanic crustal sites; it is much less than values estimated from numerical models, analyses of formation response to tidal pressure oscillations, or pressure responses to coseismic strain events. The relatively low permeability indicated by the cross-hole response may result from the basement aquifer in this area being azimuthally anisotropic, with a preferential flow direction oriented subparallel to the abyssal hill topography and tectonic structural fabric created at the ridge axis; this hypothesis will be tested during future experiments.
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| [36] |
Fluid and geochemical transport through oceanic crust: A transect across the eastern flank of the Juan de Fuca Ridge [J].
The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80km transect across the eastern flank of the Juan de Fuca Ridge at 48–N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from ~16° to 63°C. Although thermal effects of advective heat exchange are only seen within 20km east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca 2+ and decreases in alkalinity, Mg 2+ , Na + , K + , SO 2- and δ 18 O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember 87 Sr/ 86 Sr ~ 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with 14 C ages less than 10,000yr and particle velocities of ~1–5m/yr, in agreement with thermally constrained volumetric flow rates through a ~600m thick permeable layer of ~10% porosity. Δ element /Δ heat extraction ratios are similar to values for ridge-crest hydrothermal systems.
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| [37] |
Dissolved inorganic carbon isotopic composition of low-temperature axial and ridge-flank hydrothermal fluids of the Juan de Fuca Ridge [J].
We analyzed stable carbon (δ13C) and radiocarbon (Δ14C) isotopes of ocean crustal fluid samples from two low-temperature environments on and near the Juan de Fuca Ridge (JDFR), a seafloor spreading center in the northeastern Pacific Ocean. The major goals of this work were to resolve relative dissolved inorganic carbon (DIC) sources and removal processes, and characterize the isotopic signatures of DIC vented to the overlying ocean. DIC was isolated from diffuse vents on the Main Endeavour Field (MEF), on zero age seafloor, and from two ridge-flank sites located 10002km to the east, on 3.502Ma seafloor; the Baby Bare outcrop and Ocean Drilling Program (ODP) Hole 1026B. Low-temperature MEF fluids were enriched in DIC (3.13 to 5.5102mmol kg611) relative to background seawater (2.602mmol kg611), and their Δ14C and δ13C values are consistent with simple two-endmember mixing of pure high-temperature (≥30002°C) hydrothermal fluid and bottom seawater (secondary recharge). These data suggest that no major sedimentary or biological DIC sources are present, however our results do not preclude a minor sedimentary influence on low-temperature MEF vent fluids. DIC Δ14C and δ13C values of ridge-flank fluids from this area are consistent with an off-axis recharge source, followed by water–rock interaction at moderate temperatures (60–7002°C) during flow through basement. An observed offset in radiocarbon ages between fluids from Baby Bare outcrop and Hole 1026B (65110002yr) is consistent with crustal flow from south to north, at rates similar to those inferred from other geochemical and thermal tracers. The ridge-flank hydrothermal fluids are strongly depleted in DIC and δ13C relative to bottom seawater, suggesting more extensive carbon removal in this setting (655.7×101202mol C yr611) than has been previously suggested. DIC isotopic depletion is consistent with carbonate vein precipitation in conjunction with a minor addition of CO2 from basalt vesicles, and suggests that ridge-flank systems may be an important sink for seawater inorganic carbon, and comprise an important global reservoir of isotopically depleted and “pre-aged” DIC.
|
| [38] |
Cross-hole tracer experiment reveals rapid fluid flow and low effective porosity in the upper oceanic crust [J].URL 摘要
Numerous field, laboratory, and modeling studies have explored the flows of fluid, heat, and solutes during seafloor hydrothermal circulation, but it has been challenging to determine transport rates and flow directions within natural systems. Here we present results from the first cross-hole tracer experiment in the upper oceanic crust, using four subseafloor borehole observatories equipped with autonomous samplers to track the transport of a dissolved tracer (sulfur hexafluoride, SF 6 ) injected into a ridge-flank hydrothermal system. During the first three years after tracer injection, SF 6 was transported both north and south through the basaltic aquifer. The observed tracer transport rate of 2-3 m/day is orders of magnitude greater than bulk rates of flow inferred from thermal and chemical observations and calculated with coupled fluid-heat flow simulations. Taken together, these results suggest that the effective porosity of the upper volcanic crust through which much tracer was transported is <1%, with fluid flowing rapidly along a few well-connected channels. This is consistent with the heterogeneous (layered, faulted, and/or fractured) nature of the volcanic upper oceanic crust.
|
| [39] |
Cool seafoor hydrothermal springs reveal global geochemical fluxes [J]. |
| [40] |
Microbial life in ridge flank crustal fluids [J].
To determine the microbial community diversity within old oceanic crust, a novel sampling strategy was used to collect crustal fluids at Baby Bare Seamount, a 3.5 Ma old outcrop located in the north-east Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Stainless steel probes were driven directly into the igneous ocean crust to obtain samples of ridge flank crustal fluids. Genetic signatures and enrichment cultures of microorganisms demonstrate that these crustal fluids host a microbial community composed of species indigenous to the subseafloor, including anaerobic thermophiles, and species from other deep-sea habitats, such as seawater and sediments. Evidence using molecular techniques indicates the presence of a relatively small but active microbial population, dominated by bacteria. The microbial community diversity found in the crustal fluids may indicate habitat variability in old oceanic crust, with inputs of nutrients from seawater, sediment pore-water fluids and possibly hydrothermal sources. This report further supports the presence of an indigenous microbial community in ridge flank crustal fluids and advances our understanding of the potential physiological and phylogenetic diversity of this community.
|
| [41] |
Hydrothermal recharge and discharge guided by basement outcrops on 0.7~3.6 Ma seafloor east of the Juan de Fuca Ridge: Observations and numerical models [J].
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| [42] |
Novel microbial assemblages inhabiting crustal fluids within mid-ocean ridge flank subsurface basalt [J].
Although little is known regarding microbial life within our planet's rock-hosted deep subseafloor biosphere, boreholes drilled through deep ocean sediment and into the underlying basaltic crust provide invaluable windows of access that have been used previously to document the presence of microorganisms within fluids percolating through the deep ocean crust. In this study, the analysis of 1.7 million small subunit ribosomal RNA genes amplified and sequenced from marine sediment, bottom seawater and basalt-hosted deep subseafloor fluids that span multiple years and locations on the Juan de Fuca Ridge flank was used to quantitatively delineate a subseafloor microbiome comprised of distinct bacteria and archaea. Hot, anoxic crustal fluids tapped by newly installed seafloor sampling observatories at boreholes U1362A and U1362B contained abundant bacterial lineages of phylogenetically uniqueNitrospirae,Aminicenantes,CalescamantesandChloroflexi. Although less abundant, the domainArchaeawas dominated by unique, uncultivated lineages of marine benthic group E, the Terrestrial Hot Spring Crenarchaeotic Group, theBathyarchaeotaand relatives of cultivated, sulfate-reducingArchaeoglobi. Consistent with recent geochemical measurements and bioenergetic predictions, the potential importance of methane cycling and sulfate reduction were imprinted within the basalt-hosted deep subseafloor crustal fluid microbial community. This unique window of access to the deep ocean subsurface basement reveals a microbial landscape that exhibits previously undetected spatial heterogeneity.
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| [43] |
Spore dipicolinic acid contents used for estimating the number of endospores in sediments [J].
Endospores are heat-resistant bacterial resting stages that can remain viable for long periods of time and may thus accumulate in sediments as a function of sediment age. The number of spores in sediments has only rarely been quantified, because of methodological problems, and consequently little is known about the quantitative contribution of endospores to the total number of prokaryotic cells. We here report on a protocol to determine the number of endospores in sediments and cultures. The method is based on the fluorimetric determination of dipicolinic acid (DPA), a spore core-specific compound, after reaction with terbium chloride. The concentration of DPA in natural samples is converted into endospore numbers using endospore-forming pure cultures as standards. Quenching of the fluorescence by sediment constituents and background fluorescence due to humic substances hampered direct determination of DPA in sediments. To overcome those interferences, DPA was extracted using ethyl acetate prior to fluorimetric measurements of DPA concentrations. The first results indicated that endospore numbers obtained with this method are orders of magnitude higher than numbers obtained by cultivation after pasteurization. In one of the explored sediment cores, endospores accounted for 3% of all stainable prokaryotic cells.
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| [44] |
In situ enrichment of ocean crust microbes on igneous minerals and glasses using an osmotic flow-through device [J].
The Integrated Ocean Drilling Program (IODP) Hole 1301A on the eastern flank of Juan de Fuca Ridge was used in the first long-term deployment of microbial enrichment flow cells using osmotically driven pumps in a subseafloor borehole. Three novel osmotically driven colonization systems with unidirectional flow were deployed in the borehole and incubated for 4 years to determine the microbial colonization preferences for 12 minerals and glasses present in igneous rocks. Following recovery of the colonization systems, we measured cell density on the minerals and glasses by fluorescent staining and direct counting and found some significant differences between mineral samples. We also determined the abundance of mesophilic and thermophilic culturable organotrophs grown on marine R2A medium and identified isolates by partial 16S or 18S rDNA sequencing. We found that nine distinct phylotypes of culturable mesophilic oligotrophs were present on the minerals and glasses and that eight of the nine can reduce nitrate and oxidize iron. Fe(II)-rich olivine minerals had the highest density of total countable cells and culturable organotrophic mesophiles, as well as the only culturable organotrophic thermophiles. These results suggest that olivine (a common igneous mineral) in seawater-recharged ocean crust is capable of supporting microbial communities, that iron oxidation and nitrate reduction may be important physiological characteristics of ocean crust microbes, and that heterogeneously distributed minerals in marine igneous rocks likely influence the distribution of microbial communities in the ocean crust.
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| [45] |
Endospore abundance, microbial growth and necromass turnover in deep sub-seafloor sediment [J].
Two decades of scientific ocean drilling have demonstrated widespread microbial life in deep sub-seafloor sediment, and surprisingly high microbial-cell numbers. Despite the ubiquity of life in the deep biosphere, the large community sizes and the low energy fluxes in this vast buried ecosystem are not yet understood. It is not known whether organisms of the deep biosphere are specifically adapted to extremely low energy fluxes or whether most of the observed cells are in a dormant, spore-like state. Here we apply a new approach--the D:L-amino-acid model--to quantify the distributions and turnover times of living microbial biomass, endospores and microbial necromass, as well as to determine their role in the sub-seafloor carbon budget. The approach combines sensitive analyses of unique bacterial markers (muramic acid and D-amino acids) and the bacterial endospore marker, dipicolinic acid, with racemization dynamics of stereo-isomeric amino acids. Endospores are as abundant as vegetative cells and microbial activity is extremely low, leading to microbial biomass turnover times of hundreds to thousands of years. We infer from model calculations that biomass production is sustained by organic carbon deposited from the surface photosynthetic world millions of years ago and that microbial necromass is recycled over timescales of hundreds of thousands of years.
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| [46] |
Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor [J].
Microbial life inhabits deeply buried marine sediments, but the extent of this vast ecosystem remains poorly constrained. Here we provide evidence for the existence of microbial communities in ~40 to 60C sediment associated with lignite coal beds at ~1.5 to 2.5 km below the seafloor in the Pacific Ocean off Japan. Microbial methanogenesis was indicated by the isotopic compositions of methane and carbon dioxide, biomarkers, cultivation data, and gas compositions. Concentrations of indigenous microbial cells below 1.5 km ranged from <10 to ~10(4) cells cm(-3). Peak concentrations occurred in lignite layers, where communities differed markedly from shallower subseafloor communities and instead resembled organotrophic communities in forest soils. This suggests that terrigenous sediments retain indigenous community members tens of millions of years after burial in the seabed.
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| [47] |
LaRowe D E. Ocean sediments—An enormous but underappreciated microbial habitat [J].
react-text: 89 The shallow-sea vents, heated sediments and geothermal wells at Vulcano are home to a diverse composition of heat-loving Archaea and Bacteria. Their potential energy sources include hundreds of organic and inorganic redox reactions that yield up to ~120 kJ per mole electrons transferred. The dominant biogeochemical processes in the anoxic sediments appear to be fermentation and sulfate reduction. /react-text react-text: 90 /react-text
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| [48] |
A combination of extreme environmental conditions favor the prevalence of endospore-forming firmicutes [J].
Environmental conditions unsuitable for microbial growth are the rule rather than the exception in most habitats. In response to this, microorganisms have developed various strategies to withstand environmental conditions that limit active growth. Endospore-forming Firmicutes (EFF) deploy a myriad of survival strategies in order to resist adverse conditions. Like many bacterial groups, they can form biofilms and detect nutrient scarcity through chemotaxis. Moreover, within this paraphyletic group of Firmicutes, ecophysiological optima are diverse. Nonetheless, a response to adversity that delimits this group is the formation of wet-heat resistant spores. These strategies are energetically demanding and therefore might affect the biological success of EFF. Therefore, we hypothesize that abundance and diversity of EFF should be maximized in those environments in which the benefits of these survival strategies offsets the energetic cost. In order to address this hypothesis, geothermal and mineral springs and drillings were selected because in these environments of steep physicochemical gradients, diversified survival strategies may become a successful strategy.We collected 71 samples from geothermal and mineral environments characterized by none (null), single or multiple limiting environmental factors (temperature, pH, UV radiation, and specific mineral composition). To measure success, we quantified EFF gene copy numbers (GCN;spo0Agene) in relation to total bacterial GCN (16S rRNA gene), as well as the contribution of EFF to community composition. The quantification showed that relative GCN for EFF reached up to 20% at sites characterized by multiple limiting environmental factors, whereas it corresponded to less than 1% at sites with one or no limiting environmental factor. Pyrosequencing of the 16S rRNA gene supports a higher contribution of EFF at sites with multiple limiting factors. Community composition suggested a combination of phylotypes for which active growth could be expected, and phylotypes that are most likely in the state of endospores, in all the sites. In summary, our results suggest that diversified survival strategies, including sporulation and metabolic adaptations, explain the biological success of EFF in geothermal and natural springs, and that multiple extreme environmental factors favor the prevalence of EFF.
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| [49] |
Predominance of viable spore-forming piezophilic bacteria in high-pressure enrichment cultures from ~1.5 to 2.4 km-deep coal-bearing sediments below the ocean floor [J].
Phylogenetically diverse microorganisms have been observed in marine subsurface sediments down to ~2.5 km below the seafloor (kmbsf). However, very little is known about the pressure-adapted and/or pressure-loving microorganisms, the so called piezophiles, in the deep subseafloor biosphere, despite that pressure directly affects microbial physiology, metabolism and biogeochemical processes of carbon and other elements in situ. In this study, we studied taxonomic compositions of microbial communities in high-pressure cultivated sediment, obtained during the Integrated Ocean Drilling Program (IODP) Expedition 337 off the Shimokita Peninsula, Japan. Analysis of 16S rRNA gene-tagged sequences showed that members of spore-forming bacteria within Firmicutes and Actinobacteira were predominantly detected in all enrichment cultures from ~1.5 to 2.4 km-deep sediment samples, with the sequence frequency followed by members of Proteobacteria, Acidobacteria, and Bacteroidetes. To further study the physiology of the deep subseafloor sedimentary piezophilic bacteria, we isolated and characterized two bacterial strains, 19R1-5 and 29R7-12, from 1.9 and 2.4 km-deep sediment samples, respectively. The isolates were both low G+C content, gram-positive, endospore-forming and facultative anaerobic piezophilic bacteria, closely related to Virgibacillus pantothenticus and Bacillus subtilis within the phylum Firmicutes, respectively. The optimal pressure and temperature conditions for growth were 20 MPa and 42oC for strain 19R1-5, and 10 MPa and 43oC for strain 29R7-12. Bacterial (endo)spores were observed in both the enrichment and pure cultures examined, suggesting that these piezophilic members were derived from microbial communities buried in the ~20 million-year-old coal-bearing sediments after the long-term survival as spores and that the deep biosphere may host more abundant gram-positive spore-forming bacteria and their spores than hitherto recognized.
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| [50] |
Observations of natural-state fluid pressures and temperatures in young oceanic crust and inferences regarding hydrothermal circulation [J].
Four boreholes, drilled a few tens of meters into igneous basement on the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, were sealed and instrumented for long-term monitoring to observe the hydrologic state of young sediment-sealed oceanic crust. The thermal regime is dominated by the effects of rapid fluid circulation in uppermost igneous basement driven by very small non-hydrostatic pressure gradients. Upper basement temperatures are uniform laterally between pairs of holes over distances of hundreds of meters to kilometers. In the case of two holes drilled into a sediment-buried basement ridge and adjacent valley, basement temperatures differ by less than 2 K despite the 2.2 km lateral separation of the sites and the 2.5:1 contrast in sediment cover thickness. Under conductive conditions, upper basement temperatures would differ by roughly 50 K. By comparison with modeling results, the observed degree of isothermality suggests a fluid flux of at least 10 616 m s 611 (30 m yr 611 ), and an effective permeability in the range of 10 6110 –10 619 m 2 in the uppermost igneous crust. The pressure difference available to drive this rapid flux between the ridge and valley, estimated by comparing the observed pressures via the isothermal upper basement hydrostat that is inferred to connect the two sites, is small (≈2 kPa) and also suggests high permeability. Relative to the hydrostats defined by the local conductive sediment geotherms, substantial super-hydrostatic pressure (+18 kPa) is present within the buried basement ridge, and sub-hydrostatic pressure is present in the adjacent valley (6126 kPa). Such pressure differentials are the direct consequence of the advection-dominated thermal regime and small pressure losses in high-permeability basement, and are available to drive fluid seepage through sediment sections vertically up above and horizontally away from buried ridges, and down above valleys. No constraints are provided by any of the observations on the depth in the crust to which thermally or chemically significant flow might extend, although just as in the overlying sediments, the pattern of deep flow may be affected by the near-isothermal and near-hydrostatic conditions present in the permeable uppermost crustal section.
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| [51] |
Scientific and technical design and deployment of long-term subseafloor observatories for hydrogeologic and related experiments, IODP Expedition 301, eastern flank of Juan de Fuca Ridge [J].
Post-Expedition 301 CORK operations . . . . . . 11 Prospects for future experiments . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
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| [52] |
A high-speed optical modem communication system for CORK seafloor observatories [ |
| [53] |
The deep subsurface biosphere in igneous ocean crust: Frontier habitats for microbiological exploration [J].
We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of selected ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "provinces." The basis for this framework is three governing conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. We present a brief overview of subseafloor conditions, within the context of these three characteristics, for five field sites where microbial studies have been done, are underway, or have been proposed. Technical challenges remain and likely will limit progress in studies of microbial ridge flank ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible from work focused at a small number of sites. A characterization framework such that as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of oceanic ridge flanks.
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