翦知湣
, 党皓文
同济大学海洋地质国家重点实验室,上海 200092
Jian Zhimin, Dang Haowen
中图分类号: P756.5
文献标识码: A
文章编号: 1001-8166(2017)12-1267-10
收稿日期: 2017-10-30
修回日期: 2017-11-26
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:Jian Zhimin(1966-), male, Changde City, Hu’nan Province, Professor. Research areas include marine geology.E-mail:jian@tongji.edu.cn
作者简介:翦知湣(1966-),男,湖南常德人,教授,主要从事海洋地质学研究.E-mail:jian@tongji.edu.cn
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摘要
2013年启动的国际大洋发现计划(IODP)针对当前大气温室气体浓度急剧升高和全球变暖的气候变化现状,提出全球气候对CO2增高的响应、冰盖和海平面对全球变暖的响应、中—低纬水文循环的变化机制以及海洋碳化学体系的缓冲能力等4个科学挑战。截至2017年8月已经完成的8个IODP气候变化主题航次聚焦于亚洲—太平洋—印度洋区域的季风过程和西太平洋暖池的新生代演变,着重探索轨道—千年尺度上亚洲季风系统的变化特征和主导机制,以及构造时间尺度上亚洲季风与青藏高原隆升和剥蚀的动力联系。未来2年IODP将瞄准南半球高纬的冰盖、海冰、洋流和碳循环等气候因子,重点考察新生代西南极冰盖和海冰变化、白垩纪和古近纪南大洋的海洋环流和碳循环等。因此,IODP旨在深入探索以亚洲季风和西太平洋暖池为代表的热带海洋气候过程和以西南极冰盖为代表的高纬气候因子在多种时间尺度上的演变,为认识当前气候变化、预测未来气候趋势提供自然变化的科学依据。中国的优势在于全球季风概念和热带驱动假说方面的研究,特别是巽他陆架的气候效应。
关键词：
Abstract
Aiming at the current climate status, i.e., drastic rise of atmospheric greenhouse gases and the apparent trend of global warming, the International Ocean Discovery Program (IODP), launched in 2013, proposed four scientific challenges, including the response of global climate to CO2 rise, the feedback of ice-sheet and sea-level to global warming, the dynamics of the mid- and low-latitude hydro-cycle, and the mechanism of the marine carbon-chemical buffering system. By August 2017, eight IODP expeditions of climate-related themes were implemented, focusing on the Neogene evolution of the monsoon system over Asia-Pacific-Indian and the West Pacific Warm Pool, with specific interests in the variabilities and mechanisms of the Asian Monsoon system on orbital-to millennial-scales, as well as the connections between Asian Monsoon and the uplift/weathering of the Tibetan Plateau on tectonic time scale. The planned IODP expeditions in the forthcoming two years will explore the Southern high-latitude climate histories of West Antarctic ice in the Cenozoic, and Southern Ocean currents and carbon cycle in the Cretaceous-Paleogene. In sum, during the current phase of IODP (2013-2023), our knowledge about the marine climate system would be greatly advanced via deciphering the past changes in tropical processes of Asian Monsoon and West Pacific Warm Pool, as well as in high-latitude factors of the West Antarctic ice. A better scientific background of natural variability would be provided, accordingly, for predicting the future tendency in climate change. In this context, China’s strategic directions include the global monsoon concept, the tropical forcing hypothesis, and in particular the climate effect of the Sunda Shelf.
Keywords：
大洋钻探可以为研究气候和海洋变化提供不同时间分辨率和地质历史长度的连续海洋沉积记录,从而用以重建地质历史时期生物圈、水圈、大气圈以及岩石圈之间的生物地球化学循环、通量和相互作用。大洋钻探研究揭示过去100 Ma以来地球气候经历过剧烈变化,且只有大洋钻探的岩芯记录能够展示过去长时间尺度上气候与海洋自然变化的图景,特别是对于地质历史中曾出现过的大气CO2浓度和全球温度远高于现代水平的时期。这些古代记录是理解现代气候变化和预测未来气候前景的关键之一。
气候与海洋变化作为2013年启动的国际大洋发现计划(International Ocean Discovery Program, IODP)的四大核心课题之一,在当前人为碳排放剧烈增加、全球变暖严峻威胁人类社会的情势下,提出了未来10年大洋钻探气候与海洋变化方面的4个关键科学挑战[1]:①地球气候系统对大气CO2增高如何反映?②冰盖和海平面对于全球变暖作何反应?③降水分布由什么控制?比如季风和厄尔尼诺的降水是受什么控制的?④大洋化学成分发生变动后,是如何恢复的?
其中,“科学挑战一”将改善我们对“全球气候敏感度”(包括大气CO2浓度持续较高以及碳循环发生快速、剧烈扰动的2种情景)的估计,确定大气CO2浓度升高导致的冰盖和海冰损失幅度及极地升温的放大机制,理解较高大气CO2浓度状况下热带温度、上升流和厄尔尼诺的变化模式,判定更好用于预测未来气候变化的数值模型中的关键物理、化学过程。“科学挑战二”聚焦于解答“长期温暖气候背景下冰盖的动力行为”,在远离冰盖的珊瑚礁区开展钻探研究,检验陆地冰量的变化量及其对海平面变化的贡献;在极地近冰盖海区钻探,可以直接考察冰盖涨缩的历史。“科学挑战三”着重解答全球水文循环的核心要素——季风和ENSO的地质演变,包括在大陆边缘钻探研究降雨在陆地和海洋的变化在开放大洋钻探研究海表温度梯度的变化,通过区域对比建立水文循环的动力机制以及探索构造活动和气候变化的联系。“科学挑战四”关注海洋吸收CO2导致的海洋酸化,通过研究过去长时间尺度上海洋生物对酸化和缺氧的应对,来了解大洋海水缓冲超量输入CO2的能力。
IODP的2013—2023年科学计划有效地指导了大洋钻探研究的主要方向。2013年至2017年7月IODP已经执行完成17个航次,其中以气候变化为主题的航次有8个,主要在亚洲—太平洋—印度洋的热带—亚热带区域(图1),集中探索新生代以来亚洲季风、印度—太平洋暖池为代表的气候演变热带过程。这其中,有6个航次以季风为主题(IODP346“东亚季风”,IODP353“印度季风”,IODP354“孟加拉扇”,IODP355“阿拉伯海季风”和IODP359“马尔代夫季风和海平面”,以及IODP356“澳大利亚季风”)[2,3,4,5,6,7],2个航次直击热带气候变化的核心区——“印度尼西亚穿越流”(IODP356)[7]和“西太平洋暖池”(IODP363)[8],另有1个以重要的中—低纬度洋流和气候变化为主题(IODP361“南非气候——阿古拉斯海流末次冰盛期的密度剖面”)[9](表1)。因此,这些航次集中对应于IODP新10年科学计划书提出的“科学挑战三:全球水文循环”和“科学挑战一:海水温度与大气CO2”,并且涵盖了其他2个科学挑战的相关内容,例如,热带地区的海平面变化、热带海洋生物对海水化学扰动的响应等。
图1 2013—2019年IODP气候变化主题的航次分布示意图
数字表示IODP航次编号,红色表示截至2017年8月已完成航次,绿色表示2017—2019年计划航次;方框示对应航次主要钻探区域范围
Fig.1 Location of the IODP expeditions on climate change for 2013-2019
Numbers denote the expedition number. Red color indicates the expeditions implemented before August 2017, while green color indicates the expeditions scheduled for 2017-2019. Squares show the drilling areas of the corresponding expeditions
表1 2013—2017年已完成的IODP气候变化主题航次
Table 1 The IODP expeditions on climate change implemented during 2013-2017
| IODP 航次 | 航次主题 | 钻探区域 | IODP站位 | 航次时间 | 取芯 总长/m | 首席科学家 |
|---|---|---|---|---|---|---|
| 363 | 西太平洋暖池 | 澳大利亚西北岸外,巴布亚新几内亚东部岸外,卡洛琳海 | U1482~U1490 | 2016年 10~12月 | 6 956 | Rosenthal Y Holbourn A |
| 361 | 非洲南部气候 | 南非岸外 | U1474~U1479 | 2016年 1~3月 | 5 176 | Hall I Hemming S |
| 359 | 马尔代夫季风 和海平面 | 马尔代夫 | U1465~U1472 | 2015年 9~11月 | 3 097 | Betzler C Eberli G |
| 356 | 印度尼西亚穿越流 | 澳大利亚西部—西北岸外 | U1458~U1464 | 2015年 7~9月 | 5 185 | Gallagher S Fulthorpe C |
| 355 | 阿拉伯海季风 | 东阿拉伯海(印度扇) | U1456~U1457 | 2015年 3~5月 | 1 722 | Pandey D Clift P |
| 354 | 孟加拉扇 | 孟加拉扇 | U1449~U1455 | 2015年 1~3月 | 1 727 | France-Lanord C Spiess V |
| 353 | 印度季风 | 孟加拉湾 | U1443~U1448 | 2014年11月至 2015年1月 | 4 280 | Clemens S Kuhnt W |
| 346 | 亚洲季风 | 日本海和中国东海 | U1422~U1429 | 2013年 7~9月 | 6 135 | Tada R Murray R W |
近4年来已经完成的IODP航次中,有6个是以季风为主题,分别从亚洲东部大陆边缘(IODP346日本海和中国东海)、南亚次大陆岸外海域(IO-DP353和354孟加拉湾,IODP355东阿拉伯海,IODP359马尔代夫)和澳大利亚西北岸外(IODP356),追寻新生代以来多个时间尺度上亚洲—澳洲季风的演化历史,及其与青藏高原山脉隆升和剥蚀、洋流变迁以及太阳辐射量、全球冰量、大气温室气体浓度和跨赤道潜热传输等多种驱动机制之间的动力联系。
亚洲季风是最强的区域季风体系,包含南亚季风(或称印度季风)、东亚季风和西北太平洋等3个次级(亚)季风系统,其中青藏高原分隔开东亚季风和南亚季风并对亚洲季风体系具有决定性的动力影响[10]。因此,上述6个季风主题航次的一个共同任务就是去追索青藏高原对亚洲季风的作用,科学目标包括:①青藏高原隆升与亚洲季风起源;②青藏山脉剥蚀风化的主控因素(气候还是构造)和气候效应(对季风、碳循环的影响等);③亚洲季风的驱动因素、轨道和千年尺度变化特征、对重大气候事件的响应;④东亚—南亚季风系统的“风雨”(风应力和区域降雨分布)联系,及亚洲季风的全球地位[2,3,4,5,6,7]。
IODP346航次在日本海和中国东海共钻探7个站位、取芯总长超过6 000 m,日本海的5个站位最老钻到约12 Ma,东海的2个站位(U1428和U1429)取得约350 ka以来的高分辨率(40~50 cm/ka)沉积记录[2]。日本海沉积物中显著的明—暗互层甚至纹层可能反映了东亚大陆东亚夏季风降雨强度的变化。沉积物的明—暗层变化在约2.6 Ma开始出现,并在约1.2 Ma之后更为频繁和显著,可能反映轨道—千年尺度的东亚季风变化在这2个时间段开始显著增强[2]。
IODP353和354航次连续在孟加拉湾和安达曼海的13个站位钻探,分别取得总长4 280 m和1 727 m的沉积物岩芯。353航次在孟加拉湾最南端(U1443站)获得了约72 Ma以来的印度洋沉积物岩芯,在孟加拉湾中心、印度东部岸外和安达曼海分别钻取了最老约16 Ma的沉积物岩芯[3],获得了研究构造至百年时间尺度的印度季风演变的深海沉积记录。354航次沿约8°N在孟加拉沉积扇上钻取了7个站位,取得了自始新世以来孟加拉扇的沉积记录,特别是中新世—更新世的连续沉积序列和精细剖面结构[4]。在印度以西的阿拉伯海东部,IODP355航次在印度沉积扇的2个站位分别钻进到海底以下约1 000 m,直达海盆基底的火成岩,取得约10 Ma以来的沉积片段(在井深100~150 m以下出现大量浊流和沉积间断)和约2 Ma以来的连续完整沉积记录[5]。这3个IODP航次,分别从印度次大陆的东、西两侧钻取新生代以来的沉积记录,旨在通过研究深海沉积扇的沉积结构,区分构造活动和气候变化对山脉剥蚀风化的效应,梳理亚洲季风降雨和风场的耦合关系,并分析季风环流对不同驱动机制的响应敏感度,详细探索亚洲季风与青藏高原山脉(喜马拉雅、喀喇昆仑和兴都库什等)的剥蚀和风化的动力联系。
此外,IODP359航次在马尔代夫内海8个站位钻取了晚渐新世以来碳酸盐台地沉积历史,用以重建热带印度洋海平面变化,以及受季风驱动的印度洋海流和碳酸盐台地的演化[6]。IODP356航次在澳大利亚西部和西北岸外钻探的9个边缘海站位,可以为重建中新世以来澳洲季风和澳大利亚内陆干旱化的变化提供海洋沉积记录[7]。
作为全球表层海水温度最高的区域,印度—太平洋暖池是地球大气最重要的水汽和热量源区,同时也是大气对流和强降雨中心。因此,暖池区海表温度和上层海洋热容量决定Hadley和Walker环流对流上升的强度和主要位置,从而影响行星尺度的大气环流和热带水文循环。热带太平洋—印度洋的温跃层和洋流也与暖池区的海—气过程紧密耦合,而洋流的变迁调节着洋盆内和洋盆之间的水热循环。
IODP363航次在印度洋—太平洋暖池的核心区,从帝汶海到巴布亚新几内亚东岸外和卡洛琳海总共钻取了9个站位,共计取芯约7 000 m,取得澳大利亚西北岸外晚中新世以来、西南热带太平洋边缘晚更新世以来以及西太平洋暖池北部晚渐新世/早中新世以来的连续海洋沉积记录[8]。这些站位的沉积速率普遍较高(最高60~75 cm/ka,巴布亚新几内亚岸外U1484和U1485站),可以满足轨道到百年尺度的高分辨率古海洋和古气候研究[8],因此为继续探索亚轨道尺度上暖池区海水温度和海洋环流变化及其与全球季风系统、热带辐合带(Inter-Tropical Convergence Zone, ITCZ)移动、类厄尔尼诺—南方涛动(ENSO)气候态演化等气候过程的联系提供了高质量的材料。
大洋温盐环流将全球大洋联系在一起,进行全大洋的物质和能量交换。其中,印度尼西亚穿越流是中低纬度太平洋—印度洋之间的唯一通道。在印度尼西亚穿越流出口位置,IODP363(U1482和U1483站)和IODP356(U1461~U1464站)航次分别钻取晚中新世以来以及上新世—更新世的陆坡和陆架沉积记录[7,8]。借助海水温度和盐度、水团性质、陆源沉积物的矿物和地化组成等指标重建,这些沉积记录可以用于研究构造和轨道时间尺度上印度尼西亚穿越流——利文流(Leeuwin Current)的变迁,以及陆架生物礁系统发育的变化。
当前,印度尼西亚穿越流汇入印度洋北赤道流之后形成阿加勒斯流(Aguhlas Current),在南非岸外将暖而咸的印度洋海水输入南大西洋。因此,印尼穿越流和阿加勒斯流对于低纬—高纬之间和三大洋盆之间的水、热、盐的交换具有重要作用。IODP361航次在非洲南部岸外的6个站位开展钻探,其中4个站位钻进到中新世(6~7 Ma),2个近河口站位取得末次冰期旋回(U1477,沉积速率约为100 cm/ka)及约4 Ma以来(U1478)的高分辨率沉积记录[9]。这些站位全部位于印度洋—大西洋海道的关键位置,主要目标就是研究上新世以来阿加勒斯流的古海洋学变化,并探讨高纬/低纬过程、季风、大西洋经向翻转流以及非洲陆地气候等与阿加勒斯流的联系。特别地,U1477站的高分辨率记录为详细探索末次冰期以来阿加勒斯流在百年及更长时间尺度上的变化提供了可能[9]。
未来2年已列入实施计划的IODP气候变化主题航次集中在中—高纬度南大洋(图1),核心的科学问题就是西南极冰盖和环南极洋流的演化历史,主要对应于IODP新10年科学计划书提出的“科学挑战二:冰盖与高纬气候”。其中,IODP374和379航次将分别在罗斯海和阿蒙森海进行钻探,探索西南极冰盖新生代以来的变化;IODP369,371和378航次将分别在澳大利亚西南岸外、塔斯曼海和亚极地南太平洋开展钻探,旨在研究白垩纪以来南半球气候以及环南极洋流的变迁;IODP382航次则将在南美岸外斯科舍海作业,目标是研究中新世以来环南极洋流和海冰的变化(表2)[11,12,13,14,15,16]。
表2 2017—2019年已列入计划的IODP气候变化主题航次
Table 2 The IODP expeditions on climate change scheduled for 2017-2019
| IODP 航次 | 航次主题 | 钻探区域 | 计划 站位数量 | 航次时间 | 首席科学家 | 最深钻探 目标年龄 |
|---|---|---|---|---|---|---|
| 371 | 塔斯曼前沿俯冲和 古近纪气候 | 塔斯曼海 | 6 | 2017年7~9月 | Sutherland R Dickens G | 新生代 |
| 369 | 澳大利亚白垩纪气 候和构造 | 西南澳大利亚岸外 | 7 | 2017年9~11月 | Huber B Hobbs R | 三叠纪 |
| 374 | 罗斯海西南极冰盖 历史 | 罗斯海 | 6 | 2018年1~3月 | McKay R De Santis L | 早中新世 |
| 378 | 南太平洋古近纪 气候 | 亚极地西南太平洋 | 9 | 2018年10~12月 | Thomas D Röhl U | 古近纪 |
| 379 | 阿蒙森海西南极 冰盖历史 | 阿蒙森海 | 6 | 2019年1~3月 | Gohl K Wellner J | 晚白垩纪 |
| 382 | 冰山排泄通道古海洋学和南福克兰陆坡堆积体 | 亚极地西南大西洋 | 6 | 2019年3~5月 | Weber M (待确定) | 中中新世 |
| 383 | 太平洋南极绕极流 动力学 | 亚极地南太平洋 | 6 | 2019年5~7月 | (待确定) (待确定) | / |
环南极洋流和西南极冰盖是南半球高纬海洋—气候系统最重要的2个因子。现代环南极洋流是三大洋盆的唯一直接通道,并且驱动了全球的深海洋流系统。在古近纪,由于澳大利亚和南美洲仍然与南极洲相连,因此当时的环南极洋流分隔成太平洋和印度洋—大西洋2段[17]。西南极冰盖大量坐落于海洋之上,因此其对气候变化最为敏感。当海洋储热和大气CO2浓度上升,西南极冰盖最有可能崩溃,造成海平面的剧烈上升[18]。
未来2年的IODP气候变化主题航次,共同的核心科学问题就是环南极洋流的变化。预计取得从晚中生代(IODP369和379)和早新生代(IODP371和378)直到更新世—全新世的沉积记录,可能的重大研究成果包括:①白垩纪印度洋段环南极洋流的变化;②德雷克海道、塔斯曼海道打开以及印度尼西亚海道的关闭对环南极洋流的作用;③白垩纪到始新世的暖室期环南极洋流特征及其对全球大洋环流和碳循环的贡献。以往大洋钻探在南大洋的研究,主要关注新生代以来的古海洋变化,对于从白垩纪暖室期(大气CO2浓度和全球温度远高于现代水平的时期)以来环南极洋流的演变知之甚少,而这正是未来2年IODP气候变化主题航次的重点,将有助于认识当前全球变暖背景下高纬海区气候变化的作用及其趋势。
另外,传统的气候演变“米兰科维奇”过分强调北半球高纬地区的作用,对于两极冰盖演变的研究也侧重北半球,而南极冰盖演变的研究,不仅时间不连续,地理覆盖面也明显不足,从而对南极冰盖的时空演变尚缺乏全面的认识。因此,针对南极冰盖变化这一课题,IODP374和379航次将在西南极2个最大的海盆——罗斯海和阿蒙森海进行钻探,直接追溯西南极以海洋为基底的冰盖在新生代以来的演化,探索在比现代更暖、温室气体浓度更高的历史时期西南极冰盖的动力变化及其在区域和全球范围内的海洋和气候效应[11,12]。IODP382航次的作业区域位于南美洲岸外,是南极冰山向南大洋排泄的重要通路,其主要研究目标为中新世以来重大气候变化时期南极冰山排泄的状况、变化和动力机制[16]。
气候演变的低纬过程和高低纬度联系是IODP气候与海洋变化研究的重要内容。南太平洋中纬度海区是南大洋与热带太平洋联系的中间环节,而由于太平洋碳酸盐保存很差,古海洋学研究一直偏弱。通过前期大量的地质地球物理调查,在南太平洋—澳洲沿岸发现了保存最老可达三叠纪的一系列沉积地层,自然成为大洋钻探的主要目标之一。IODP369,371和378航次将在澳大利亚东、西两侧和南太平洋钻进到三叠纪的沉积地层,其有关气候变化的主要科学目标在于研究晚中生代—早新生代南大洋的异常高温状况以及向极地的热量输送、深海缺氧事件和海洋碳化学等古海洋学内容[13,14,15]。白垩纪到始新世的暖室期,南大洋如何维持高温?环流状况如何?海洋—大气碳循环如何演变?诸如此类问题的回答对于我们预测未来全球变暖的可能后果具有现实意义。
新生代以来,印度与亚洲碰撞、特提斯洋关闭、图尔盖海消失等,决定了亚洲季风系统的演化;而印度尼西亚海道的逐渐关闭,则改造了太平洋—印度洋之间的海水流通,影响印度和澳大利亚的季风气候[19]。其中一个最关键的要素是作为地球表面“第三极”的青藏高原。亚洲季风的3个子系统——南亚(印度)、东亚和西北太平洋季风区——分布在青藏高原周围,青藏高原的存在是印度季风强盛、东亚季风形成的必要条件[10,19,20]。其一,随着高原隆升,海—陆之间的热量差异加剧,可以强化季风环流[10];另一方面,季风气候过程加速高原山脉的剥蚀和风化,又影响着高原山脉的隆升和演化[21,22]。因此,造山运动和季风气候之间的关系并非单向、线性,而是相互作用。
探讨青藏高原与亚洲季风的相互关系,一个突破点在于找到亚洲季风气候的起始时间。秦安黄土剖面[23]、南海沉积记录[24]、中国新生代植被带演化[25]等证据表明,东亚季风系统的形成不会晚于早中新世初期。而印度洋的沉积记录指示印度季风在7~8 Ma之后才开始强化[26,27]。现有对东亚和南亚季风气候起始时间的分歧,很大程度上是因为对印度季风长尺度演化的认识不足。例如,最近就有人提出,印度季风在7~8 Ma增强这一认识,依据在于发现印度洋沉积序列中偏好富营养水体(上升流)的有孔虫属种含量在7~8 Ma显著增加,但是这一现象首要原因可能在于印度洋Owen Ridge抬升,造成钙质化石保存状况改善,因此并不能拿来说明季风的变化[28]。
因此,IODP353,354和355航次集中在印度次大陆外围海区钻探新生代以来的海底扇沉积记录,通过详细调查孟加拉扇和印度扇的沉积历史,旨在建立新生代以来青藏高原山脉的风化剥蚀—搬运—沉积的演化历史,从而有望更好地认识印度季风的起始时间,并检验造山运动与季风气候、地表和海洋过程之间的耦合关系。其中,上新世之前(当时的大气CO2浓度和全球冰量等边界条件与上新世—更新世的基本不同)的高分辨率“深时(deep-time)”季风记录是研究的关键,将有助于认识现代季风系统什么时候、又是如何建立的。
岁差周期是轨道尺度季风演变的最主要特征[10,20],表现为南、北半球接收到的太阳辐射量的季节分配呈现半球间反相位的周期性变化。近20年来,来自全球各个季风区的高分辨率古气候记录,包括洞穴石笋氧同位素、湖泊和海洋沉积层中的陆源输入通量、海洋风驱上升流区的古生产力以及冰芯气泡的甲烷浓度和氧气氧同位素等,都显示出季风气候演变的岁差周期性,且具有南、北半球间反相位的特征[10,20]。
季风的岁差周期特征,有力地证明了太阳辐射量对气候演变的驱动,可以通过以季风为代表的热带海洋—气候过程直接作用于中—低纬度地区,而无需经由高纬地区的冰盖涨缩间接传导。笼统而言,全球夏季风体系的一致性可以用ITCZ机制来解释。ITCZ代表了气候赤道,热带近地面风向近赤道的地表温度最高地区辐合并形成上升对流,造成对流云和强降雨;太阳辐射量对地表的加热提供了对流所需的动力;因此太阳辐射加热的核心区随季节、海陆分布以及岁差周期的变化就会造成ITCZ的相对移动和强度变化,从而导致了季风气候的岁差周期变化。
然而,全球季风在岁差周期上受太阳辐射量调控的一致性,并不能涵盖季风演变的全貌。例如,印度洋季风上升流指标反映的印度季风滞后岁差周期约8 ka[29],且印度洋夏季风记录同时表现岁差和斜率周期[30],黄土—古土壤堆积序列也展现出亚洲季风的冰期旋回和斜率周期特征[31]。在亚轨道的千—百年时间尺度上,无论是东亚或者西非的季风记录都显示出与格陵兰冰芯记录的同步性和相似性,可能说明季风降雨可以通过大气过程(ITCZ的南—北移动、西风急流的位置和强度等)与北大西洋的冰盖动力变化和冰融水倾泻事件紧密联系[32,33]。季风在不同区域的差异表明了各个季风区的特殊性,这是由各个季风区的海陆和地形格局决定的。而区域季风在不同时间尺度上变化的差异性,说明即使由太阳辐射量驱动,但是地球气候系统内部反馈机制也在控制季风变化,核心的要素包括全球冰量、海平面升降及由此导致的海陆分布格局变化、大气温室气体浓度、半球间的潜热输送以及跨区域的水汽输送等。因此,当下和未来研究轨道和亚轨道尺度季风演变的核心任务就在于发展和验证各种多学科的季风替代性指标,重建东亚、南亚、东南非和澳大利亚等区域季风气候系统在晚新生代(主要上新世—更新世以来)的轨道—亚轨道尺度演变特征,厘清这些区域气候系统的内部反馈机制。
以北大西洋高纬区为中心的现行冰期—间冰期气候演化模式,近年来受到严重挑战。日益增多的发现表明热带海区,尤其是西太平洋暖池区,作为地球表面接受辐射能的中心,在全球气候变迁中起着关键作用。因此,低纬区西太平洋暖池和高纬区北大西洋在气候演变过程中都具有重要的地位。探索热带在地球气候系统演变中的作用,涉及到研究热带海区的物理过程,也涉及到海洋微生物对大洋生产力变化的影响等诸多科学问题,如西太平洋暖池的物理、生物地球化学过程及其在全球气候环境中的作用,暖池在轨道尺度至年代际上的变化,及其与赤道太平洋海气耦合体系、季风气候系统的关系等[34]。
由于距离两极冰盖最远,暖池区表层海温的冰期—间冰期变化与冰盖的直接关系较弱,可能主要由大气温室效应驱动[35]。同时,在一定程度上,暖池区的表层海温和上层水体热结构也受太阳辐射量的影响,例如全新世以来暖池区的降温[36]以及温跃层温度的变化[37]都指示了岁差尺度上辐射量变化的控制。控制暖池区海洋热储量和热结构的动力枢纽在于海洋的次表层,温跃层环流圈联通了热带—亚热带的上层海洋[38]。当前,暖池区蒸发—降雨过程在过去亚轨道、轨道和更长时间尺度上的变化,仍有相当多的未知。许多第四纪降雨指标记录揭示西太平洋近赤道地区的轨道尺度降雨变化也由岁差主导、冰期旋回特征不显著。然而,菲律宾最南端和巴布亚新几内亚岸外的降雨记录分别显示与北半球和南半球夏季辐射量同步的特征,可以联系于各自半球的夏季风及ITCZ的南北移动[39,40]。此外,暖池区的海陆分布格局也对大气对流上升的位置和强度有直接影响,例如模拟结果表明末次冰期时由于海平面下降、暖池区大面积陆架出露,大气对流上升中心就被迫迁移到太平洋更偏东的海水上空[41]。
因此,要更好地了解暖池区的演变及其气候效应,需要利用时间分辨率相对较高的沉积材料,更精确地重建暖池区表层海温、次表层海水温度和热结构以及区域蒸发—降雨的变化,从而检验大气温室气体的气候敏感度,探讨赤道—亚热带—亚极地海洋之间的动力联系,追溯暖池区大气对流上升的变迁及其区域和全球气候影响。IODP356和363航次在印度尼西亚海域、帝汶海、巴布亚新几内亚岸外、卡洛琳海等暖池的核心和边缘区开展钻探取芯,可以更完整地重建西太平洋暖池晚新生代的演化历史,其高时间分辨率研究记录可以直接与冰芯和石笋记录对比,从而服务于研究千—百年尺度西太平洋暖池的变化及其与全球季风和极地冰盖的动力联系。
中国的气候与海洋变化研究,最大的优势来源于独特的地理位置:背靠世界最大的大陆——欧亚大陆和世界第三极——青藏高原,同时面对着最大的大洋——太平洋及西北太平洋的一系列边缘海。控制东亚地区气候的核心要素正是亚洲季风。因此,中国在古气候和古海洋研究领域的研究重点理应在于亚洲大陆边缘的海陆相互作用,以及以全球季风和西太平洋暖池为代表的低纬过程演变。
季风是当今地球上范围最大的低纬区气候系统,是ITCZ季节性迁移的结果。季风演变贯穿着整个地质时期,主要以2万年周期的氧同位素序列,以及40~50万年周期的碳同位素序列为特征[10]。最近几年,有关全球季风的地质演变成为国际学术争论的热点。全球季风可以通过夏季风的降水和化学风化等过程影响从陆地到海洋的物质输送,从而又影响海洋的生产力和碳循环。特别是,全球季风与热带太平洋的海洋—气候系统等低纬过程紧密相关,是影响全球水循环的主要低纬过程,一旦找到全球季风的替代性标志,就可用来与冰盖消长作为高纬过程的标志相并列,共同组成气候变化的两大动力机制。
南海大洋钻探ODP184航次和深海国家重点基础研究发展计划(973)项目研究首次发现碳循环的偏心率长周期,提出了气候演变的冰盖驱动和热带驱动的“双重驱动”假说[42],并将气候演变热带驱动落实在“全球季风”演变的新概念[10]上,使得低纬全球季风演变的研究与冰盖消长一样,已成为国际过去全球变化(Past Global Changes,PAGES)的新命题。以新10年IODP为例,过去4年里有7个航次的主题与地质尺度的全球季风和热带过程演变相关。
然而,气候的热带驱动是一个有争议的、还没有成为学术界主流意识的命题。经过近10年的探索,已经初步认识到“热带驱动”可能的突破点,关键在于如何在地球表层穿越大气、海洋、生物和岩石圈探索不同时间尺度(构造尺度、轨道尺度、海洋尺度)上的水循环和碳循环及其相互关系。问题的解决要求从现代过程观测到数值模拟、再到地质记录验证的一系列研究工作,具体说,可以从以下3个方面着手:①全球季风与水文循环;②海洋碳储库的长周期变化;③高低纬相互作用与次表层水[34]。其中,全球季风的水循环是关键,尤其要注重建立和验证季风的替代性指标[10]。另外,深海生物地球化学过程在海洋气候变化中的作用日益凸现,而其中微生物及其与生物地球化学的关系,涉及众多的海洋碳、氮等与气候变化相关的物质循环,从海洋碳储库的长周期入手,也是当前“古今结合”全球季风碳循环的研究前沿[42]。气候环境演变研究是我国在大洋钻探领域最具有国际竞争力的方面,尤其是将南海深海记录与现代深海过程的研究结合起来,将有助于揭示全球季风与气候演变热带驱动在全球气候变迁中的作用,促进我国地球科学的古今结合、向跨圈层跨学科的方向发展。
地球表层的第四纪地理变化,高纬区是两极冰盖的消长,而低纬区唯一的变化就是巽他陆架的出没。南海南部巽他陆架也称为“亚洲大浅滩”,面积1.8×106 km2,是极地以外最大的陆架区。由于近几百年来构造稳定、雨量丰沛、沉积迅速,特别是有坡度极小的泥质沉积区,保存了丰富的地层记录。全球有3个大陆横跨热带,其中南美和非洲的低纬都没有广阔的陆架,海陆分布在冰期旋回中相对稳定;只有地处“海洋大陆(maritime continent)”的巽他陆架在冰期时出露水面,构成冰期里热带地区全球最大的地理变化(图2)。加以东南亚是地球表面能量流、物质流和基因流的中心,巽他陆架的地理变化可能是热带过程影响全球的主要源泉,可惜至今在古环境研究中缺乏重视。
图2 世界3个大陆的赤道河系
Fig.2 The equatorial river systems in the world’s three continents
为此,中国IODP专家咨询委员会提出在2020年左右通过国际合作,用租船方式由我国组织实施国际的 IODP 航次,实现南海“巽他陆架”的科学钻探。巽他陆架大洋钻探的科学目标,可以从海平面、河系和植被演变等3个方面入手。巽他陆架是北冰洋以外最宽的陆架,其大洋钻探将有可能首次取得全球海平面变化的直接证据,提供几百万年来冰期旋回中海平面变化幅度的历史;同时通过地球化学和矿物学分析,将能找到沉积物的源区,可以为探索东南亚构造与河系的演变,再造南海南部水流途径的变化提供依据;特别是,通过再造东南亚热带植被的演变历史, 将揭示低纬陆地生物圈和碳储库在冰期旋回中的变化。
总之,近年来古海洋和古气候研究的发展,已经进入地球系统研究的新阶段。我国科学家应当发挥自己的优势,充分利用IODP航次提供的契机,在全球季风地质演变和气候演变热带驱动的理论研究上取得突破;同时,在南海南部巽他陆架的海平面变化及其对全球碳循环影响的研究上掌握科学主导权。
The authors have declared that no competing interests exist.
| [1] |
Illuminating Earth’s Past, Present and Future, the Science Plan for the International Ocean Discovery Program 2013-2023 [M]. |
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Integrated Ocean Drilling Program Expedition 346 Preliminary Report: Asian Monsoon-Onset and Evolution of Millennial-scale Variability of Asian Monsoon and Its Possible Relation with Himalaya and Tibetan Plateau uplift[R] . |
| [3] |
International Ocean Discovery Program Expedition 353 Preliminary Report: Indian Monsoon Rainfall[R] . |
| [4] |
International Ocean Discovery Program Expedition 354 Preliminary Report: Bengal Fan-Neogene and Late Paleogene Record of Himalayan Orogeny and Climate: A Transect Across the Middle Bengal Fan[R] . |
| [5] |
International Ocean Discovery Program Expedition 355 Preliminary Report: Arabian Sea Monsoon-Deep Sea Drilling in the Arabian Sea: Constraining Tectonic-monsoon Interactions in South Asia[R] . |
| [6] |
International Ocean Discovery Program Expedition 359 Preliminary Report: Maldives Monsoon and Sea Level[R] . |
| [7] |
International Ocean Discovery Program Expedition 356 Preliminary Report: Indonesian Throughflow—A 5 Million Year History of the Indonesian Throughflow, Australian Monsoon, and Subsidence on the Northwest Shelf of Australia[R] . |
| [8] |
International Ocean Discovery Program Expedition 363 Preliminary Report: Western Pacific Warm Pool-Neogene and Quaternary records of Western Pacific Warm Pool Paleoceanography[R] . |
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International Ocean Discovery Program Expedition 361 Preliminary Report: South African Climates (Agulhas LGM Density Profile)[R] . |
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The global monsoon across time scales: Mechanisms and outstanding issues [J].
The present paper addresses driving mechanisms of global monsoon (GM) variability and outstanding issues in GM science. This is the second synthesis of the PAGES GM Working Group following the first synthesis “The Global Monsoon across Time Scales: coherent variability of regional monsoons” published in 2014 (Climate of the Past, 10, 2007–2052). Here we introduce the GM as a planetary scale circulation system and give a brief accounting of why it exhibits regional structure. The primary driver of the GM is solar insolation, and the specific features in the underlying surface, such as land-sea distribution, topography, and oceanic circulations, are mainly responsible for the differences among regional monsoon systems. We then analyze the monsoon formation mechanisms, together with the major processes that drive monsoon variations at various timescales, including external forcings and internal feedbacks. On long time scales external forcings often induce variability on a global scale, whereas short-term variability in regional monsoon systems is usually caused by internal feedbacks within the climate system. Finally, a number of debatable issues are discussed, with an emphasis on time scales beyond the instrumental record. These include the dual nature of the monsoon as wind and rain, the meaning of oxygen isotope in hydrological cycle, in particular of speleothem δ 18 O, the role of ice-sheet in monsoon variations, etc. In general, the GM as a system comprises a hierarchy of regional and local monsoons with different level of similarity, but all show coherent variability driven by a common solar forcing. The GM concept, therefore, is by no means to replace or depreciate research on the regional monsoons, but helps to dissect the mechanism and controlling factors of the monsoon variability at various temporal-spatial scales.
|
| [11] |
Expedition 374 Scientific Prospectus: Ross Sea West Antarctic Ice Sheet History[R] . |
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IODP Proposal 839-Full (Amundsen Sea Ice Sheet History): Development and Sensitivity of the West Antarctic Ice Sheet Tested from Drill Records of the Amundsen Sea Embayment[R] . |
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Expedition 369 Scientific Prospectus: Australia Cretaceous Climate and Tectonics[R] . |
| [14] |
Expedition 371 Scientific Prospectus: Tasman Frontier Subduction Initiation and Paleogene Climate[R] . |
| [15] |
IODP Proposal 567-Full (South Pacific Paleogene Climate): Paleogene South Pacific APC Transect: Heat Transport and Water Column Structure During an Extreme Warm Climate[R] . |
| [16] |
IODP Proposal 902-Full (Iceberg Alley Paleoceanography): Late Neogene Reconstruction of Ice-sheet, Atmosphere, and Ocean Dynamics in Iceberg Alley[R] . |
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Eocene circulation of the Southern Ocean: Was Antarctica kept warm by subtropical waters? [J].
[1] Near the Eocene's close (09080434 million years ago), the climate system underwent one of the largest shifts in Earth's history: Antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed. Previous studies conjectured that poleward penetration of warm, subtropical currents, the East Australian Current (EAC) in particular, caused Eocene Antarctic warmth. Late Eocene opening of an ocean gateway between Australia and Antarctica was conjectured to have disrupted the EAC, cooled Antarctica, and allowed ice sheets to develop. Here we reconstruct Eocene paleoceanographic circulation in the Tasmanian region, using (1) biogeographical distributions of phytoplankton, including data from recently drilled Ocean Drilling Program Leg 189 sites and (2) fully coupled climate model simulations. We find that the EAC did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern. Our results do not support changes in 090008thermal isolation090009 as the primary driver of the Eocene-Oligocene climatic transition.
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| [18] |
Climate Change 2014 Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [M]. |
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Global monsoon in a geological perspective [J].全球季风的地质演变 [J].
技术的进步和科学的积累, 使得对季风的认识已经从区域性现象拓展为全球性系统. 在300多年来, 季风只看成是一种“巨型的海陆风”, 而现在遥感和直接观测的资料支持新的假说, 把季风看作是热带辐合带(ITCZ)季节性迁移的表现, 因而是一个环球系统. 作为低纬区的气候现 象, 季风存在于除南极以外的各大洲, 而且贯穿至少6亿年来显生宙的地质历史. 系统研究全球季风在时间和空间里变化的时机已经成熟. 地质证据表明: 在构造尺度即10<sup>6</sup>~10<sup>8</sup>年的时间尺度上, 全球季风受大陆分合的“威尔逊旋回”调控, 合成“超级大陆”时出现“超级季风”, 大陆分解后季风减弱. 在轨道尺度即10<sup>4</sup>~10<sup>5</sup>年时间尺度上, 全球季风受地球运行轨道几何形态的变化即轨道周期调控, 呈现2万年的岁差周期以及10万年、尤其是40万年的偏心率周期. 在千年及更短的时间尺度上, 全球季风受太阳活动周期等多种因素的调控. 全球季风演变的周期性, 是地球表层系统以及人类生存环境变化的一项基本因素, 其40万年偏心率长周期被比喻为地球系统的“心跳”, 季风的岁差周期导致4千年前一系列亚洲古文明的衰落, 1千年前太阳活动的周期曾导致美洲玛雅文明的毁灭. 因此, 古气候研究不能只注意冰盖消长的高纬过程, 季风、厄尔尼诺等低纬过程在地质历史上更具有普遍性; 全球季风的研究是我国地学界有希望做出突破性贡献的领域.
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| [20] |
The global monsoon across timescales: Coherent variability of regional monsoons [J]. |
| [21] |
Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation [J].
Recent interpretations of Himalayan-Tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the Tibetan plateau, and that ductile extrusion of high-grade metamorphic rocks between coeval normal- and thrust-sense shear zones can explain exhumation of the Greater Himalayan sequence. Here we use coupled thermal-mechanical numerical models to show that these two processes-channel flow and ductile extrusion-may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low-viscosity material. Our models provide an internally self-consistent explanation for many observed features of the Himalayan-Tibetan system.
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| [22] |
The influence of climate on the tectonic evolution of mountain belts [J].
Simple physical arguments, analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate. However, obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging. Spatial correlations between intense precipitation or glaciation and zones of rapi...
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| [23] |
Onset of Asian desertification by 22 Myr ago inferred from loess deposits in China [J].
姝he initial desertification in the Asian interior is thought to be one of the most prominent climate changes in the Northern Hemisphere during the Cenozoic era.But the dating of this transition is uncertain,partly because desert sediments are usually scattered,discontinuous and difficult to date.Here we report nearly continuous aeolian deposits covering the interval from 22 to 6.2 million years ago,on the basis of palaeomagnetic measurements and fossil evidence.A total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils.This new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of Asia by the early Miocene epoch,at least 14 million years earlier than previously thought.Regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in Asia.
|
| [24] |
Changes in terrestrial ecosystem since 30 Ma in East Asia: Stable isotope evidence from black carbon in the South China Sea [J]. |
| [25] |
How old is the Asian monsoon system? Palaeobotanical records from China [J].
The recent discovery of monsoon records in early Miocene raised a question of the time when the East Asian monsoon system initiated. A distinguishing feature of the modern monsoon system is its geographic distribution which disturbs the zonal pattern indigenous to the planetary climate system, and the appearance of the monsoonal climate pattern in the geological records should signify the onset of the monsoon system. Here we present the results of a compilation of palaeobotanical and lithological data from 125 sites over China, that has revealed two completely different patterns of climate zones: the Palaeogene pattern with a broad belt of aridity stretched across China from west to east, and the Neogene pattern with the arid zone restricted to northwest of China which has persisted until today. The reorganization of the climate system around the Oligocene/Miocene boundary provides evidence for the establishment of the modern East Asian monsoon. Since then, the Neogene has witnessed significant variations of the monsoon system, including enhancement of aridity and monsoon intensity at about 15-13 My, around 8 My and 3 My. The new data do not support the onset of the Asian monsoon system around 8 My. Rather, the new data led to a hypothesis that the transition to the monsoon climate system in East Asia occurred in the latest Oligocene.
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| [26] |
Onset of monsoonal related upwelling in the western Arabian Sea [ |
| [27] |
Sensitivity of the Indian monsoon to forcing parameters and implications for its evolution [J].
General-circulation-model simulations used to estimate the sensitivity of the Indian monsoon to changes in orbital parameters, the orography of Tibet—Himalaya, atmospheric C0 2 concentration and the extent of glacial-age surface boundary conditions show that increased elevations and increased summer solar radiation are most effective in strengthening the monsoon. Strong monsoons (similar to today's) can be induced by strong solar forcing only when the elevation is at least half that of today. These conditions may have been attained in the late Miocene.
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| [28] |
The Owen Ridge uplift in the Arabian Sea: Implications for the sedimentary record of Indian monsoon in late Miocene [J].
61Identification of a Late Miocene episode of deformation in the Owen Basin.61The age of uplift of the Owen Ridge is assessed at 658.5Ma.61The uplift of the Owen Ridge accounts for an increase in G. bulloides abundances.61A new hypothesis for the origin of the Siwalik environmental change (Pakistan).
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| [29] |
Forcing mechanisms of the Indian Ocean monsoon [J].
Sediments in the Arabian Sea provide biological, biogeochemical and lithogenic evidence of past changes in the Indian Ocean summer monsoon winds. For the past 350,000 years, this system has been externally forced by cyclical changes in solar radiation, and internally phase-locked to the transport of latent heat from the southern subtropical Indian Ocean to the Tibetan Plateau. In contrast to the results of general circulation models, these geological data suggest that the climate change associated with variability in global ice volume is not a primary factor in determining the strength and timing of the monsoon winds.
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| [30] |
A 350,000-year summer-monsoon multiproxy stack from the Owen Ridge, Northern Arabian Sea [J].
Five summer-monsoon proxies from the Northern Arabian Sea are combined using stacking and principal components analysis (PCA) to create two very similar multi-proxy records of summer-monsoon variability. The five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer-monsoon indicators (e.g. preservation, dissolution, diagenesis, sediment reworking). As such, stacking and PCA average out non-monsoon variance, yielding a more pure monsoon signal. These stacked and PCA records (hereafter summer-monsoon stack and summer-monsoon factor) allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands; these two parameters are less reliable when estimated from individual proxy records. In fact, the summer-monsoon factor (SMF) accounts for only 33% of the total variance in the five records, suggesting that relative amplitude variations in each individual proxy time series are influenced by non-monsoon processes. The summer-monsoon stack (SMS) and SMF are spectrally very similar, dominated by variance in the 41-k.y. (obliquity) and 23-k.y. (precession) bands; there is very little variance at the 100-k.y. (eccentricity) band associated with large-scale changes in global ice-volume. Indeed, equally strong monsoons occur in both glacial and interglacial intervals. Within the 23-k.y. precession cycle, monsoon maxima fall at 61121° relative to precession minima (June 21 perihelion, maximum Northern Hemisphere (NH) summer insolation). This phase falls midway between δ 18 O minima (6178°) and December 21 perihelion (61180°) indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band: (1) sensible heating of the Asian Plateau which is maximized at times of ice-volume minima (6178°), and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of December 21 perihelion (61180°). The seasonal cycle at December 21 perihelion is characterized by warm Southern Hemisphere (SH) summers followed by cold SH winters, a combination that preconditions the ocean to export latent heat during the boreal summer-monsoon season. Summer-monsoon winds transport this latent heat into Asia where it is released during precipitation, enhancing the Asian monsoon low. Within the 41-k.y. obliquity cycle, monsoon maxima are in phase with obliquity maxima. This indicates that two mechanisms, quite similar to those in the precession band, influence the timing of strong summer-monsoons in the obliquity band: (1) sensible heating of the Asian Plateau but with no ice-volume delay, and (2) latent heat export from the southern subtropical Indian Ocean which is maximized at times of obliquity maxima. Again, the seasonal cycle at obliquity maximum is characterized by warm SH summers followed by cold SH winters, ideal for maximizing latent heat export during the boreal summer monsoon.
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| [31] |
Astronomical timescale and palaeoclimatic implication of stacked 3.6-Ma monsoon records from the Chinese Loess Plateau [J].
Magnetic susceptibility and grain size records from two continuous red-clay and loess–palaeosol sequences on the Chinese Loess Plateau have been generated to investigate the evolution and variability of the East Asian monsoon (EAM) during the late Pliocene and Pleistocene. Tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess–palaeosol sequence (0–2.602Ma), and an extended age model for the upper red-clay formation (2.6–3.602Ma). This timescale indicates older ages for a number of the magnetic polarity boundaries, consistent with lock-in depth offsets in loess sequences. Good site-to-site correlation enables generation of stacked grain size and susceptibility records spanning the last 3.602Myr. These records indicate that monsoon evolution since the late Pliocene can be subdivided into three phases: 0–1.25, 1.25–2.72 and 2.72–3.402Ma; each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon (magnetic susceptibility) and winter monsoon (grain size). Spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity (410- and 100-kyr), obliquity (41-kyr) and precession (23- and 19-kyr) bands over the past 3.402Myr, implying a non-linear response of the long-term EAM evolution to orbital and glacial forcing.
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| [32] |
Millennial-and orbital-scale changes in the East Asian monsoon over the past 224,000 years [J].
Abstract High-resolution speleothem records from China have provided insights into the factors that control the strength of the East Asian monsoon. Our understanding of these factors remains incomplete, however, owing to gaps in the record of monsoon history over the past two interglacial-glacial cycles. In particular, missing sections have hampered our ability to test ideas about orbital-scale controls on the monsoon, the causes of millennial-scale events and relationships between changes in the monsoon and climate in other regions. Here we present an absolute-dated oxygen isotope record from Sanbao cave, central China, that completes a Chinese-cave-based record of the strength of the East Asian monsoon that covers the past 224,000 years. The record is dominated by 23,000-year-long cycles that are synchronous within dating errors with summer insolation at 65 degrees N (ref. 10), supporting the idea that tropical/subtropical monsoons respond dominantly and directly to changes in Northern Hemisphere summer insolation on orbital timescales. The cycles are punctuated by millennial-scale strong-summer-monsoon events (Chinese interstadials), and the new record allows us to identify the complete series of these events over the past two interglacial-glacial cycles. Their duration decreases and their frequency increases during glacial build-up in both the last and penultimate glacial periods, indicating that ice sheet size affects their character and pacing. The ages of the events are exceptionally well constrained and may thus serve as benchmarks for correlating and calibrating climate records.
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| [33] |
155,000 years of West African monsoon and ocean thermal evolution [J].
A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial-interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.
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| [34] |
Ocean carbon cycle and tropical forcing of climate evolution [J].大洋碳循环与气候演变的热带驱动 [J].
<font face="Verdana">20世纪气候演变研究的最大突破，在于地球轨道变化驱动冰期旋回的米兰柯维奇理论。然而近年来学术界对热带过程和大气CO2浓度变化的研究进展，暴露了传统的轨道驱动理论存在着对低纬区和碳循环在全球气候系统中作用估计不足的严重缺陷。国家重点基础研究发展计划项目“大洋碳循环与气候演变的热带驱动”拟以南海与西太平洋暖池的深海记录为依据，进行全球性对比和跨越地球圈层的探索，通过观测分析结果与数值模拟的结合、地质记录与现代过程的结合，检验和论证大洋碳储库长周期变化机制的假说，对于不同时间尺度上低纬过程如何通过碳循环在全球气候环境演变中的作用，实现理论上的突破。同时简要介绍了该项目的目的、科学意义、关键科学问题及预期目标等。<br /></font>
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| [35] |
The 100 000-yr cycle in tropical SST, greenhouse forcing, and climate sensitivity [J].
The key scientific uncertainty in the global warming debate is the equilibrium climate sensitivity. Coupled atmosphere-ocean general circulation models predict a wide range of equilibrium climate sensitivities, with a consequently large spread of societal implications. Comparison of models with instrumental data has not been able to reduce the uncertainty in climate sensitivity. An alternative way to gauge equilibrium climate sensitivity is to use paleoclimatic data. Two recent advances, the development and application of proxy recorders of tropical sea surface temperature (SST) and the synchronization of the deep-sea and Antarctic ice-core time scales, make it possible to directly relate past changes in tropical SST to atmospheric carbon dioxide (CO2) levels. The strong correspondence of a proxy SST record from the eastern equatorial Pacific and the Vostok CO2 record suggests that varying atmospheric carbon dioxide is the dominant control on tropical climate on orbital time scales. This effect is especially pronounced at the 100 000-yr cycle. Calibration of the CO2 influence via tropical SST variability indicates a tropical climate sensitivity of 4.400°-5.600°C (errors estimated at 00± 1.000°C) for a doubling of atmospheric CO2 concentration. This result suggests that the equilibrium response of tropical climate to atmospheric CO2 changes is likely to be similar to the upper end of available global predictions from coupled models.
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| [36] |
Holocene evolution of the Indonesian throughflow and the western Pacific warm pool [J].
High sea surface temperatures in the western Pacific warm pool fuel atmospheric convection and influence tropical climate. This region also hosts the Indonesian throughflow, the network of currents through which surface and thermocline waters are transported from the western equatorial Pacific Ocean into the Indian Ocean. Here we show, using records of the delta O-18 and Mg/Ca of planktonic foraminifera from eight sediment cores, that from about 10,000 to 7,000 years ago, sea surface temperatures in the western sector of the western Pacific warm pool were about 0.5 degrees C higher than during pre-industrial times. We also find that about 9,500 years ago, when the South China and Indonesian seas were connected by rising sea level, surface waters in the Makassar Strait became relatively fresher. We suggest that the permanent reduction of surface salinity initiated the enhanced flow at lower, thermocline depths seen in the modern Indonesian throughflow. However, the uniformly warm sea surface temperatures found upstream and downstream of the Indonesian throughflow indicate that the early Holocene warmth in this region was not directly related to reduced heat transport by the throughflow that may have resulted from surface freshening of the Makassar Strait. Instead, we propose that the elevated temperatures were the result of a westward shift or expansion of the boundaries of the western Pacific warm pool.
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| [37] |
Decoupled Holocene variability in surface and thermocline water temperatures of the Indo-Pacific Warm Pool [J].
The Holocene variability in sea surface and thermocline water temperatures (SST and TWT) in the Indo-Pacific Warm Pool (IPWP) has been reconstructed by planktonic foraminiferal Mg/Ca from sediments of the western tropical Philippine Sea. Afterward the Younger Dryas interval (YD), SST warmed gradually till 0010 ka and remained approximately constant afterwards, but TWT rose more rapidly to a peak between 0012 and 0010 ka and then declined by 001.5°C through the Holocene. The trend of TWT closely followed the boreal summer insolation and could be correlated to tropical climate changes represented by southward movement of the Inter-tropical Convergence Zone (ITCZ) and related changes in East Asian monsoons.
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| [38] |
Interdecadal climate fluctuations that depend on exchanges between the tropics and extratropics [J].
The unexpected and prolonged persistence of warm conditions over the tropical Pacific during the early 1990s can be attributed to an interdecadal climate fluctuation that involves changes in the properties of the equatorial thermocline arising as a result of an influx of water with anomalous temperatures from higher latitudes. The influx affects equatorial sea-surface temperatures and hence the tropical and extratropical winds that in turn affect the influx. A simple model demonstrates that these processes can give rise to continual interdecadal oscillations.
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| [39] |
Monsoon variability and deep oceanic circulation in the western equatorial Pacific over the last climatic cycle: Insights from sedimentary magnetic properties and sortable silt [J].
[1] Magnetic and grain size properties of a sediment core located in the western equatorial Pacific, off the southeastern tip of the Philippine island of Mindanao, are presented in an effort to reconstruct past changes in the East Asian Monsoon and deep ocean circulation during the last 160 kyrs. The sedimentary concentration of magnetic particles, interpreted to reflect past changes in runoff from Mindanao, varies almost in antiphase with Northern Hemisphere insolation. This suggests that precipitation was lower in the western equatorial Pacific region during boreal insolation maxima and thus corroborates model results showing opposing trends in precipitation between land and the marine realm there. Variations in the grain size distribution of the inorganic sediment fraction, as recorded by both the sortable silt mean size and the magnetic grain size, provide a monitor of changes in sediment reworking by bottom currents. The close correlation of this proxy of bottom current strength and the benthic 脦麓18O record from the same site implies a tight coupling between deep water flow, most likely Antarctic Intermediate Water (AAIW), and global climate.
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| [40] |
The precession phase of hydrological variability in the Western Pacific Warm Pool during the past 400 ka [J].
The low-latitude hydrological cycle is a key climate parameter on different timescales, as it contributes to various feedback processes. Modelling studies suggest that the interhemispheric insolation contrast is the major factor controlling the cycle, although the influence of glacial conditions and the phase relationships relative to insolation forcing remain undetermined. In this work, we studied precipitation variability over Papua New Guinea (PNG, 3°S) for the past 40002ka using terrigenous fractions transported by the Sepik River to the Western Pacific Warm Pool (WPWP). A multi-decadal to centennial resolution of the elemental content was obtained using X-ray fluorescence scanning of a marine sediment core using an age model based on 14C dates and benthic foraminiferal δ 18O. Indicators of the coarse river particulate fraction (bulk and CaCO 3-free basis Ti concentrations, the log intensity ratios of Ti/K and Ti/Ca) displayed a dominant 2302ka periodicity without a clear glacial–interglacial trend. Our precipitation records showed a tight relationship with local summer insolation (3°S, January) with time-dependent lag of 0 to 402ka. They were generally in anti-phase for U–Th dated Chinese speleothem δ 18O records. Based on an analogy to modern climate, we propose that precipitation over PNG was primarily determined by interhemispheric insolation contrast, and the contribution of austral fall/winter precipitation added second-order variability that formed the lags. For the last four climate cycles, the WPWP hydrological cycle was closely associated with the eastern Asian monsoon, and the influence of glacial conditions on the low-latitude hydrological cycle was estimated to be limited.
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| [41] |
The effect of sea level on glacial Indo-Pacific climate [J].
The Indo-Pacific Warm Pool – the Earth’s largest body of warm water and main source ofheat and moisture to the global atmosphere – plays a prominent role in tropical and globalclimate change. The physical mechanisms driving changes in the warm pool over glacial-interglacial timescales are largely unknown. Here we show that during the Last GlacialMaximum (LGM) changes in global sea level influenced tropical climate by exposing theSunda Shelf and altering the Walker Circulation. Our result is based on a synthesis of marine and terrestrial proxies sensitive to hydroclimate and a multi-model ensemble of climatesimulations. The proxy data suggest drying throughout the warm pool, and wetter conditions in the western Indian and Pacific oceans. Only one model out of twelve simulates asimilar pattern of hydroclimate change, as measured by the Cohen’s statistic. According to this model, weakened convection over the warm pool in response to exposure of theSunda Shelf drives the proxy-inferred hydrological changes. Our study demonstrates thaton glacial-interglacial timescales, ice sheets exert a first order influence on tropical climatethrough changes in global sea level.
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| [42] |
Obscuring of long eccentricity cyclicity in Pleistocene oceanic carbon isotope records [J].
Long eccentricity (400-kyr) cycles in carbon isotope records from the Pacific and Atlantic oceans and the Mediterranean sea of the past 5.002Ma are compared. All records show maximum δ δ 13 C values ( δ 13 Cmax) at eccentricity minima during the Pliocene, but this relationship obscured in the Pleistocene after 65021.602Ma in particular for the open ocean deep-water δ 13 C records. Since a clear anti-phase relationship was set up between oceanic δ 18 O and δ 13 C in the 100-kyr band from this time, we attribute the obscured 400-kyr signal to a major change in the oceanic carbon reservoir probably associated with restructure of the Southern Ocean. A similar change occurred in the Miocene at 13.902Ma when the 400-kyr cyclicity in δ 13 C records flattened out together with a drastic cooling and Antarctic ice-sheet expansion. A remarkable exception is the Mediterranean surface water δ 13 C record, which remained paced by the long-term eccentricity cycle throughout the Pliocene and Pleistocene, suggesting a low-latitude climatic origin of the 400-kyr signal that is independent of glacial–interglacial forcing. Since the Earth is currently passing through an eccentricity minimum, it is crucial to understand the nature of the δ 13 Cmax events.
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