牛富俊
, 王玮
Niu Fujun, Wang Wei
中图分类号: P343.3;P642.14
文献标识码: A
文章编号: 1001-8166(2018)04-0335-08
收稿日期: 2018-01-15
修回日期: 2018-03-3
网络出版日期: 2018-04-20
版权声明: 2018 地球科学进展 编辑部
基金资助:
作者简介:
First author:Niu Fujun(1970-), male, Huining County, Gansu Province, Professor. Research areas include permafrost engineering and thawing hazards.E-mail:niufujun@lzb.ac.cn
作者简介:牛富俊(1970-), 男, 甘肃会宁人, 研究员, 主要从事寒区岩土工程与热融灾害方面的研究.E-mail:niufujun@lzb.ac.cn
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摘要
结合国际冻土研究的热点问题和青藏高原脆弱生态环境可持续发展的需求,针对多年冻土退化过程中趋于加剧的热喀斯特现象,及其因融穿冻土、造成区域地下水位的改变而诱发的生态环境影响,在国家自然科学基金重点项目支持下开展“青藏高原多年冻土区热喀斯特湖环境及水文学效应”研究。项目主要通过遥感分析及野外调查,分析气候变化和工程活动影响下的青藏工程走廊内热喀斯湖时空分布规律,评价其生态环境效应;选取热喀斯湖广泛发育区域,调查其发育条件、规模和几何分布特点,揭示典型热喀斯特湖影响因素变化、水热状况等,通过水文学、同位素示踪试验等阐明热喀斯特湖与地下水之间的转换关系;结合抽水疏干试验进行热喀斯特湖对区域地下水位影响分析,并通过数值模型及模拟,依热喀斯特湖发育不同阶段、规模,分析其对区域冻土、水文条件及生态环境的影响。研究成果将有助于区域性冻土生态环境演化准确评价及趋势预测,以及深入理解诸如江河源多年冻土区水文状况演化影响因素,及其相应的生态环境响应机制。
关键词：
Abstract
Thermokarst lake is the most visible morphologic landscape developing during the process of permafrost degradation, and it is still an international hot topic in permafrost research. The climate warming, and the consequent degradation of the permafrost on the Qinghai-Tibet Plateau aggravate thermokarst lake development. The permafrost is normally considered as an aquiclude, and the permafrost degradation, especially when the permafrost is completely thawed by a thermokarst lake, might influence regional ground water. Therefore, a research program focusing on environmental and hydrological effects of thermokarst lakes in permafrost regions of the Qinghai-Tibet Plateau was started and supported by the National Natural Science Foundation of China. The work proposed by the application includes: To analysis the spatial and temporal distribution rule of thermokarst lakes in the Qinghai-Tibet Engineering Corridor (QTEC) under the climate change and engineering activities, and to evaluate the ecological environment effects through remote sensing and field investigation; to reveal the main factors influencing a typical thermokarst lake and its hydrothermal condition, and to elucidate the conversion relationship between the thermokarst lake and the groundwater with hydrological and isotope tracer tests; to make an analysis of the influences of different lake stage and size on regional permafrost, hydrological conditions and ecological environment through numerical simulation and statistical modelling, considering the relationships between the thermokarst lake and the ground water level. The research results will help to accurately assess regional permafrost ecological environment evolution and trend prediction, and to reasonably understand the impact factors of the permafrost hydrological evolution and its response mechanism to the ecological environment in the river source regions of the Qinghai-Tibet Plateau. In this paper, the research status analysis, the main research contents, research objectives and prospects were introduced so as to provide some references for related researchers and engineers.
Keywords：
青藏高原平均海拔4 500 m以上,年平均气温为-26 ℃,素有“世界第三极”之称,是我国多年冻土的代表,也是世界中、低纬度(32°~36°N)地区高海拔多年冻土的代表。高原冻土面积约为150×104 km2,约占我国冻土面积的70%[1]。全球气候变暖、青藏高原普遍升温已是不争的事实,青藏高原是全球变化的“驱动器”和“放大器”[2],其升温将高于全球平均值。气候变化对多年冻土的影响方面,以表征多年冻土特征主要参数之一的活动层厚度为例,1980—2010年,青藏公路沿线天然地表环境下活动层厚度增加速度约为1.33 cm/a,1995年以来受人类活动扰动较大的青藏公路/铁路工程走廊带活动层厚度的平均增加速度可达7.5 cm/a[3],而工程走廊两侧受人类活动扰动较小的区域,活动层增厚速率的平均值为3.6 cm/a[4]。因此,气候变化及工程活动影响下的青藏高原多年冻土处于退化过程。而根据2015年7月中国科学院青藏高原研究所的报告《西藏高原环境变化科学评估》,相对于1980—1999年气候平均值,2030—2049年青藏高原大部分地区年平均地面气温的升温幅度为1.4~2.2 ℃。在这种气温升高的影响下,未来青藏高原冻土将大幅度退化[5,6]:在年增温0.02 ℃情形下,50年后多年冻土面积比现在缩小约8.8%;如果升温率为0.052 ℃/a,50年后高原多年冻土将退化13.5%,100年后退化达46%。虽然这些预测存在很多不确定性,但青藏高原冻土在未来30~50年内的退化趋势将很明显,正如施雅风院士曾经指出:“21世纪可以说是青藏高原多年冻土处于融化过程的世纪”。
多年冻土退化必然导致区域性冻土环境的改变,其中最为显著的表现是以热喀斯特湖为代表的热融现象的广泛发育(图1)。以北半球的多年冻土为例,近期发表在Nature Communications和Nature Climate Change上的研究成果表明约20%的冻土区域存在潜在的热喀斯特发育趋势[7],占全球海岸带34%的北极地区海岸陆地,因热喀斯特及海水冲刷引起的坍塌后退速率最大可达25 m/a[8]。而在整个青藏高原,1970—1990年湖泊面积和数量呈现轻微增加的趋势[9],但之后至2010年呈显著增加趋势。导致这一变化的原因在于降水增加和冰川退缩,但多年冻土退化是否也起到一定作用尚不清楚。实际上,根据本课题组近期在有限区域开展的热喀斯特湖调查及历史遥感资料分析,多年冻土区热喀斯特湖呈现数量和面积明显增加趋势[10],这一趋势与世界上其他多年冻土区热喀斯特湖的发育趋势明显不同[11],从而造就了其对多年冻土环境及区域水文学效应的不同。在青藏高原,热喀斯特湖是冻土退化过程中突出而具代表性的现象,其发育程度和演化趋势与当前高原冻土工程建设、稳定性维护和生态环境建设等活动密切相关。
热喀斯特问题一直是国际冻土研究中独特的内容,在高纬度多年冻土区,研究工作主要包括热喀斯特湖发育过程、水文学效应、冻土环境效应及温室气体排放[12]。在对多年冻土的影响方面,热喀斯特湖是最为直接的热源。根据本课题组野外监测结合沉积物测年,青藏高原多年冻土区腹地的北麓河盆地发育的一个热喀斯特湖(BLH-A),在热喀斯特湖发育的890年间,多年冻土经历着从厚到薄再到融穿的过程,不同深度的地温也经历着缓慢增温的过程[13]。热喀斯特湖的侧向热流也使得周围多年冻土环境发生了较大变化,主要表现为上限下移、冻土温度升高、下限抬升;越靠近热喀斯特湖,多年冻土上限越深,年平均地温越高,下限越浅;越远离热喀斯特湖,多年冻土上限越浅,年平均地温越低,下限越深。此外,课题组近期较为系统的分析表明,热喀斯特湖的发育也会影响到周边土体水文学特性的变化[14]。受研究条件限制,目前仅分析了热喀斯特湖对多年冻土的影响,而基于水分补给、水循环的生态环境影响研究十分欠缺。
图1 发育在不同多年冻土区的热融湖塘现象
(a)俄罗斯雅库茨克地区;(b)加拿大育空地区;(c)青藏高原北麓河盆地;(d)东北大兴安岭
Fig.1 Thermokarst lakes in different permafrost regions
(a)Thermokarst lake developed in Yakutsk, Russia; (b)Yukon, Canada; (c)Beiluhe basin of the Qinghai-Tibet Plateau, China; (d)Daxinganling, Northeast China
在青藏高原工程环境研究中,早期仅将热喀斯特湖作为工程选线中一种不良冻土现象对待。近年来本课题组开展了相关监测工作,在发现冻土融穿、边岸冻土退化严重等现象后,初步实现了典型热喀斯特湖工程影响及青藏工程走廊内的发育敏感性评价[15]。在未来青藏高速公路、输油管道等建设逐步纳入规划,以及在高原江河源区生态环境保护需求日益突出的情况下,以热喀斯特湖为重点的热融灾害,客观上要求研究工作趋于区域化和系统化,并强调其与冻土环境、地表地下水及工程之间的相互作用研究,为区域工程和资源开发、环境协调发展提供科技支撑。
青藏高原江河源区生态环境状况、演化及其影响因素一直是不同研究方向学者们都十分关心的问题。一些研究结论认为,江河源区生态环境恶化趋势加重,草原退化和土地沙漠化问题日趋严重,鄂陵湖、扎陵湖水位下降,黄河径流量逐年减少,生态环境形势严峻[16,17]。但近期研究认为气候变化总体以变暖变湿为特征,青藏高原水体发生了显著变化,湖泊水量以每年80亿t的速率增加,导致湖泊水位以平均每年0.14 m的速率上升[18]。诚然江河源区环境及水量的变化影响因素众多且关系复杂,但若忽视了蕴含丰富地下冰的多年冻土与其他因素之间的关系,对于区域性水文状况演化及其相应的生态环境的分析则有所不足。
高原多年冻土区脆弱生态环境的突出特点是其与冻土及其反复冻融过程的密切联系。多年冻土层是维持地下水位的重要条件,冻土层作为隔水底板,其存在和发展与区域地下水位、湖泊、沼泽湿地变化紧密联系。冻土厚度自上而下逐渐变薄,会导致地下水位下降;更为严重的是,若出现融穿,地表水将不可避免地向地下水转换和流失,引发高寒草甸、沼泽草甸植被的退化,沙漠化、荒漠化趋势加剧。一直以来,对青藏高原多年冻土区的生态环境变化评价及预测仅局限于植被状况评估和植被恢复预测等[19]。但从根本上来说,要解决或者缓解冻土区的生态环境变化难题,必须立足于揭示水循环与补给机制的冻土区土壤、生物以及物质能量等各系统组分相互关系、系统稳定性及演变规律等基础之上。然而,目前在冻土对人类活动或全球气候变化响应的理解方面,我们对冻土区的各系统组分及其之间的相互关系、演变规律及其机制等所知甚少。因此,针对热喀斯特湖与外界物质平衡与水力联系这2个核心关键科学问题,通过物质平衡关系及其过程影响分析,开展冻土区热喀斯特湖环境及水文学效应研究,将有助于理解多年冻土区区域性水文状况演化,及其相应的生态环境响应机制,进而深入理解江河源多年冻土区生态环境演化及其长期影响。
冻土退化过程中,地下冰的消融会导致地表发生沉降。在高含冰量冻土区,这种地表沉降伴随着大量冰体的融化往往会形成热喀斯特湖。近年来,国际上有关热喀斯特湖的研究工作主要包括热喀斯特湖对冻土区地貌的重塑作用[20]、对区域生态、水文环境的影响[21]以及碳排放[8]。这些研究主要集中在北半球的北极地区和亚北极地区。在中国,关于热喀斯特湖对青藏高原工程设施的影响早在30年前就已经被提及,但较为成熟的研究工作至今仍很匮乏。最近牛富俊团队主要针对热喀斯特湖的分布特征、发展过程、易发敏感性,对多年冻土以及工程设施的热影响等方面开展了一些研究工作,但有关热喀斯特湖的生态、水文效应方面的研究还基本空白。
在热喀斯特湖形成过程及其影响因素研究方面,通过调查发现通常是一些外部营力而导致高含冰量多年冻土的融化而形成,这些外部营力包括森林大火、气候变暖[22]以及人类活动[23]。由于积水区域的反射率低,蓄热能力较高,因此太阳辐射会加速积水区域及其下伏多年冻土温度的升高。热喀斯特湖一旦形成,湖水的热对流过程将会导致其下部多年冻土持续融化并最终形成融区[23]。Burn[24]指出湖底融区的形成与湖水的深度以及冬季冰层的最大厚度密切相关。如果水深大于最大冰层厚度,湖底的平均温度将大于0 ℃,湖底的融区会常年存在;如果水深小于最大冰层厚度,湖底将不会形成融区[25]。此外,地下冰含量的多少对热喀斯特湖深度也有很大影响。West等[26]的模型分析结果表明在大量厚层地下冰存在的区域,热喀斯特湖的最大深度会达到20 m以上。热喀斯特湖的侧向扩张过程主要受湖岸下部冻土含冰量的制约[27],此外,湖岸的扩张还受夏季风向、波浪的搬用、相邻湖塘的吞并、沙丘、积雪以及地形等因素的影响[28]。
热喀斯特湖的发展过程比较复杂,从其形成至干涸经历诸多阶段。Soloviev[29]于1973年指出热喀斯特湖的发展经历了4个阶段,分别是:①多边形冰楔的融化;②积水;③多年冻土完全融化及热喀斯特湖形成;④湖塘干涸、湖盆内冻胀丘形成。Morgenstern等[30]于2011年将热喀斯特湖的发展分了5个阶段,分别是:①多边形的形成;②热喀斯特湖的侧向和垂向发展阶段;③湖底沉积物和融区的完全形成;④湖塘排干;⑤原有湖盆中冻胀丘的形成。但是,青藏高原热喀斯特湖的发展阶段不同于以上2种模式。基于对青藏高原北麓河区域典型热喀斯特湖的调查和监测,本研究团队指出热喀斯特湖的发展主要包括4个阶段:①起始阶段:水体的集聚并向深部土体传热;②发展阶段:多年冻土的融化导致湖塘垂直向和水平向扩张;③稳定阶段:热喀斯特湖垂直向扩张停止,湖岸坍塌速率降低;④恢复阶段:热喀斯特湖的排干以及湖盆新的冻土形成。无论何种阶段划分方案,可以看出,热喀斯特湖的形成与发展都与多年冻土的退化与进化关系密切,也伴随着地表水和地下水之间的转换,因这2个过程及关系的存在,自然会影响到周边及区域性环境与水文条件。
区域性热喀斯特湖的发育是一个动态过程,基于经过地形矫正的航片及遥感影像,诸多研究表明北半球多年冻土区热喀斯特湖的数量和面积均发生了显著的变化。在北极和亚北极地区,热喀斯特湖正在逐渐减少或消失[31]。Kokelj等[32]研究认为热喀斯特湖的排干主要受湖水水量平衡以及其他一些外部因子的影响,如冰楔融化、排泄通道的形成、冲沟对湖塘的侵蚀、河流以及相邻湖塘形成的开孔排泄以及海岸侵蚀。在不连续多年冻土区,湖水的排干主要是由融区的渗作用引起的。大多数研究表明高纬度多年冻土区的热喀斯特湖面积和数量都在减少,如Smith 等[31]研究发现, 1973—1998年西伯利亚515 000 km2区域内面积大于40 hm2的热喀斯特湖的数量减小了11%,面积减小了6%;Marsh等[33]发现加拿大北极和西部地区的41个湖塘在1950—2000年消失了。Carroll 等[34]基于中分辨率的MODIS数据,研究发现加拿大热喀斯特湖的面积在2000—2009年减小了6 700 km2。Morgenstern 等[30]在勒拿河三角洲的研究发现,热喀斯特湖的数量明显少于干湖盆的数量,表明该区域的热喀斯特湖在过去几十年内存在减少的趋势。
相对于发现热喀斯特湖减少或消失的研究成果,国际上关于多年冻土区热喀斯特湖扩张的报道较少。Jones等[35]基于高分辨率遥感影像发现阿拉斯加西沃德半岛的北部大于0.1 hm2的水体的数量从1950—1951年到2006—2007年增加了10.7%。不同于其他大部分多年冻土区的是,在青藏高原多年冻土区,牛富俊团队发现青藏高原工程走廊带的热喀斯特湖有明显的增加趋势。此外,基于1969年的航片和2010年的遥感影像,研究发现青藏铁路沿线楚玛尔河至北麓河段沿线10 km的范围内,热喀斯特湖的数量增加了867个,面积增加了1.7×106 m2;而北麓河盆地2 500 km2范围内的湖塘1969—2010年数量增加了534个,面积增加了4.1×106 m2[10]。尽管目前还没有关于青藏高原大范围内热喀斯特湖变化的研究,但该区域热喀斯特湖的湖底大多为渗透率较低的泥岩地层,可能起到防止湖水通过底部排泄而消失的作用。因此,随着多年冻土的逐年退化和降水的逐渐增加,青藏高原热喀斯特湖的面积和数量将会进一步增加。
热喀斯特湖的形成对区域冻土环境产生较大影响,尤其是融区的形成和发展通常会引起冻土地温及热交换过程的变化。此外,热喀斯特湖的形成还会对湖塘周围及下部土层的化学、物理性质产生重要影响[36,37],而且热喀斯特湖周围的高含冰量沉积物的融化也会反过来影响湖水的化学性质[38]。冻土通常被认为是相对的不透水层,因此其退化将会对区域地下水产生影响。热喀斯特湖底部多年冻土的全部融化将提供连接地表水和地下水的通道。当地表水通过热喀斯特湖下部的通道汇入到地下水中,近地表的水位将会减小并可能导致沙漠化的产生[22](图2)。热喀斯特过程能够快速而且广泛地改变寒区陆地景观格局:在俄罗斯西伯利亚的雅库茨克多年冻土区,热喀斯特湖的发育导致湖岸坍塌后退,使得大量的森林树木被侵入水中。研究者长期的监测结果表明,该地区热喀斯特湖的湖岸后退每年达到3 m以上[39]。因此,预测、控制热喀斯特过程成为对寒区科学发展的一个重要挑战。
已有研究成果表明,由于热喀斯特湖对冻土环境与冻土工程以及寒区水文的较大影响,多年来除中国外的几个冻土大国都相继开展了较为深入的热喀斯特湖研究,尤其近期开始涉及到湖水来源及水量平衡的分析[40,41]。我国青藏高原多年冻土以高温高含冰量为特征,其抵抗气候及人类活动影响的能力弱(敏感性强),而未来无论趋于转暖的气候变化还是趋于加剧的工程活动,都将影响到多年冻土的进一步退化[7]及相应热喀斯特现象的发育。
总之,热喀斯特湖研究具有气候反馈、环境影响及水文条件评价等各个方面的意义,也是国际冻土界研究的重点议题,但我国在该领域的研究相对迟缓。尽管热喀斯特湖对冻土、水文环境及工程的影响是明显、直接而又长期的,但目前尚无一项有关热喀斯特湖与高原冻土区域水文之间关系的研究结论。
图2 热喀斯特湖融穿引起的周边生态环境退化(据参考文献[
Fig.2 Ecosystem degradation caused by a thermokarst lake in permafrost region(modified after reference[22])
针对青藏高原气候转暖、人类工程活动加剧的背景,考虑到青藏高原未来重大工程建设与环境协调发展需求,本项目拟从面上结合遥感解译与对比、系统调查青藏工程走廊多年冻土区热喀斯特湖发育状况及其环境影响,从区域上进行系统监测;从点上通过野外小型典型热喀斯特湖抽水疏干、地下水位监测及同位素示踪测定等工作,分析典型区域地表水(包括热喀斯特湖)与地下水、地下冰之间的转换关系,以及水文条件变化后的短期环境要素变化,最终阐明青藏高原多年冻土区热喀斯特湖的发育状况、趋势及其生态环境及水文学效应。具体的研究工作包括以下3个方面:
(1) 青藏工程走廊内热喀斯特湖发育的时空演化规律研究。收集整理高原已有的不同时期勘察、调查、监测和遥感资料(最早1969年),分析不同地貌单元、冻土环境中热喀斯特湖分布特征和发育强度,结合发育敏感性、沉积物测年、区域气候条件及变化评价结果等,阐明工程走廊内热喀斯湖发育影响因素及时空演化规律。重点突出以下几个方面:青藏工程走廊内热喀斯特湖区域性变化及发育现状;青藏工程走廊内典型热喀斯特湖发育过程;青藏工程走廊内热喀斯特湖发育的主要影响因素。
(2) 热喀斯特湖与多年冻土间相互作用及环境效应研究。主要以北麓河盆地区域为主,基于现场调查及相关监测,建立不同地表扰动及构造活动影响下热喀斯特湖发育及演化模型,分析不同冻土地温及含冰条件下热喀斯特湖与多年冻土间的相互作用模式,结合区域降水、蒸发及风速条件,阐明并评价热喀斯特湖不同演化阶段中的冻土环境效应。重点突出以下2个方面:揭示湖塘水体、冻土、生态环境之间的影响关系和互馈机制;揭示热喀斯特湖对区域环境的影响过程和机理,包括对冻土环境的热融过程、对湖岸植物生境的影响过程、对周围土壤的冻融侵蚀过程、以及对区域水环境的影响特征。
(3)多年冻土区热喀斯特湖水文学效应评价与预测研究。基于前期曾经开展的多年冻土区热喀斯特湖监测研究积累,结合气象监测资料,通过降水、湖水及地下冰同位素检测,建立热喀斯特湖水热平衡模型,以及建立典型湖塘水热耦合的水文地质数值模型。补充活动层含水量、地下水位、冻土地温监测,结合湖塘沉积物成分及湖塘水来源(降水、地表径流、地下水、地下冰融化)分析,利用热喀斯特湖水热平衡模型和水文地质模型,评价热喀斯特湖水体与周边区域地下水之间相互影响,及其对热喀斯特湖演化的控制作用。结合气候变化趋势预测及区域工程活动历史评价,预测热喀斯特湖发育趋势及其长期区域水文学效应。重点突出以下几个方面:热喀斯特湖物质平衡过程中多年冻土的作用与贡献;热喀斯特湖发育不同阶段与周边土体地下水间的相互影响机制及程度;考虑热喀斯特湖影响下的区域地下水位变化及其生态环境效应。
项目拟通过系统调查、试验与数值模拟,从机制方面阐明热喀斯特湖发育规律、演化过程及其环境及水文学影响,为未来区域工程规划、建设及生态文明及其协调发展提供理论支撑。项目的具体研究目标包括:基于对多年冻土变化的气候背景及工程活动影响分析,阐明青藏工程走廊内热喀斯特湖时空演化规律,以及多年冻土区热喀斯特湖发生、发展过程及机理;评价和预测其对区域冻土及水文环境的影响。
热喀斯特湖的研究在国际冻土界是一项热点议题,因其是多年冻土排放温室气体的直接通道,且其与区域地下水位关系密切,从而反馈到气候变化并影响到区域生态环境的演变。我国对于青藏高原热喀斯特湖的研究以分散、点状及以现象描述性为主,或者只是分析了其影响因素的统计关系,缺乏系统、定性与定量相结合的物理模型角度的分析。未来的研究工作除了开展冻土区热喀斯特湖环境及水文学效应研究工作以外,还应将热喀斯特湖与区域碳循环、水循环过程相结合,开展热喀斯特湖气候反馈及水资源平衡方面的研究工作。在研究方法方面应充分结合高分辨遥感数据及涡动观测数据,开展整个青藏高原乃至北半球范围内热喀斯特湖的时空演化及温室气体排放定量评估等研究。
The authors have declared that no competing interests exist.
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Thermokarst lake, hydrological flow and water balance indicators of permafrost change in Western Siberia [J].
Permafrost, mainly of discontinuous type, that underlies the tundra and taiga landscapes of the Nadym and Pur river basins in northwestern Siberia has been warming during the recent decades. A mosaic of thermokarst lakes and wetlands dominates this area. In this study we tested the hypothesis chain that permafrost thawing changes thermokarst lake area and number, and is then also reflected in and detectable through other associated hydrological changes. Based on indications from previous studies, the other hydrological changes in a basin were expected to be decreasing intra-annual runoff variability (quantified by decreasing maximum and increasing minimum runoff) and systematically decreasing water storage. To test this hypothesis chain, we mapped thermokarst lake changes using remote sensing analysis and analyzed both climate (temperature and precipitation) and water flow and balance changes using available monthly data records. This was done for the whole Nadym and Pur river basins and a smaller sub-basin of the former (denoted 7129) with comparable data availability as the whole river basins. The results for the 7129 sub-basin show all the indicators (thermokarst lake and other hydrological) changing consistently, as could be expected in response to permafrost thawing that alters the connections between surface and subsurface waters, and leads to overall decreases in water (including ground ice) storage within a basin. Over the Nadym and Pur basins, the relative area influenced by similar permafrost thawing and associated lake and hydrological effects appears (yet) too small to be clearly and systematically reflected in the basin-average indicators for these large basins.
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| [22] |
Shrinking thermokarst ponds and groundwater dynamics in discontinuous permafrost near Council, Alaska [J].
Abstract The purpose of this study was to characterize the geomorphological processes controlling the dynamics of ponds and to identify and characterize groundwater infiltration and surface water dynamics for a tundra terrain located in discontinuous permafrost near Council, Alaska. Thermokarst processes and permafrost degradation were studied, focusing upon the interaction between surface and groundwater systems via an open talik. Synthetic aperture radar (SAR) data were used for classification of terrain units and surface water properties, while historical aerial photographs and satellite images (IKONOS) were used for assessment of pond shrinking and recent thermokarst progression. Geophysical surveys (ground penetrating radar and DC resistivity) were conducted to detect permafrost thickness and talik formations. Temperature boreholes and hydrological observation wells were monitored throughout the year and provided ground truth for validation of remotely-sensed data and geophysical surveys. Field and laboratory analyses enabled quantitative determination of subsurface hydrologic and thermal properties. We found many areas where alluvium deposits and ice-wedge polygonal terrain had developed thermokarst features within the last 20 years. Thermokarst ponds located over ice-wedge terrain have decreased in surface area since at least the early 20th Century. Small thermokarst features initially developed into tundra ponds perched over permafrost in response to some local disturbance to the surface. These thermokarst ponds grew larger and initiated large taliks that completely penetrated the permafrost. These taliks allowed internal drainage throughout the year causing the ponds to shrink under recent climatic conditions. Shrinking pond surface areas may become a common feature in the discontinuous permafrost regions as a consequence of warming climate and thawing permafrost. Copyright 2003 John Wiley & Sons, Ltd.
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| [23] |
Thermal regime of a thermokarst lake and its influence on permafrost, Beiluhe Basin, Qinghai-Tibet Plateau [J]. |
| [24] |
Lake-bottom thermal regimes, western Arctic coast, Canada [J].
Lake-bottom temperatures have been measured for several years at two lakes with littoral terraces on north-central Richards Island, a residual pond of the Illisarvik experimental drained lake site, and a taiga lake near Inuvik. The tundra lakes possess distinct thermal regimes: in (1) the deep central pools; (2) shallows where winter ice may reach bottom; and (3) on littoral terraces, where water depth is less than 1 m. In summer, the tundra lakes are uniformly well mixed and reach similar lake-bottom temperatures at all depths. In winter, conditions vary, depending on the proximity of the ice cover to lake bottom. The annual mean lake-bottom temperatures have been about 3°C in the deep central pools, 0°C in the shallow pools, and -2°C on the terraces of the tundra lakes. For the taiga lake, where late-winter ice cover reaches only about half the thickness of the two tundra lakes, annual lake-bottom temperatures follow the same pattern as in the central pools of the tundra lakes, but the mean temperature is over 5°C. If the thermal regime of the taiga lake is an analogue for tundra conditions following climate warming, then the width of lakes with through taliks on Richards Island may decline by between 20 and 100 m. At equilibrium, about 45% of the lakes and 20% of the surface area of Richards Island may then be underlain by taliks that penetrate permafrost. Copyright 08 2005 John Wiley & Sons, Ltd.
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| [25] |
Numerical simulation of permafrost thermal regime and talik development under shallow thermokarst lakes on the Alaskan Arctic Coastal Plain [J].
[1] Thaw lakes are one of the most obvious manifestations of the hydrological system at work in the tundra regions of the Alaskan Arctic Coastal Plain, but the extent of the role of thaw lakes in Arctic land-atmosphere interactions and feedback has yet to be fully understood. This study uses a two-dimensional heat transfer model with phase change under a cylindrical coordinate system to simulate the long-term influence of shallow thaw lakes on the thermal regime of permafrost and talik development on the Alaskan Arctic Coastal Plain. On the basis of previous studies of permafrost and thaw lakes at Barrow, Alaska, a series of simulation cases was conducted using different combinations of long-term mean lake bottom temperature and lake depth. The simulated results indicate that shallow thaw lakes are a significant heat source to permafrost and talik. For a thaw lake with a long-term mean lake bottom temperature of greater than 0.000°C a talik forms under the thaw lake. The maximum talik thicknesses (vertical distance from the ground surface to the permafrost surface) are 28.0, 43.0, and 53.2 m 3000 years after the formation of a shallow thaw lake with long-term mean lake bottom temperatures of 1.000°, 2.000°, and 3.000°C, respectively. Talik development rate is very high in the first several years after a thaw lake formation and decreases gradually with time. No talik forms below a thaw lake with a long-term mean lake bottom temperature equal to or lower than 0.000°C, but the temperature of permafrost below the thaw lake increases with time. Three thousand years after the formation of a thaw lake with a long-term mean lake bottom temperature of greater than or equal to 0908082.000°C, ground temperature increases of more than 0.500°C occur as far as 300 m from the lake shore and as deep as about 400 m below the ground surface. It is concluded that variation of long-term mean lake bottom temperature has a significant influence on permafrost thermal regime and talik development. Continued monitoring for thaw lake bottom temperature and ground temperature under shallow thaw lakes is needed to further improve the simulation.
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| [26] |
Time-dependent morphology of thaw lakes and taliks in deep and shallow ground ice [J].
[1] The shape and depth of thaw lake basins depends on lake age, on whether the talik (thaw bulb) is at steady state, and on the distribution of ice in the ground. To investigate implications of this broad hypothesis, we use a numerical model of conductive heat transfer, phase change, and thaw subsidence of ice-rich sediment beneath a lake in cross section. For ground thermal properties with lake temperatures and dimensions consistent with measurements, modeled talik depth approximately increases with except in lakes in deep ground ice which deepen more rapidly because of consolidation on thawing. In deep ground ice environments, basins achieve depths of 09090820 m in 0909085 ka. Expanding lakes with disequilibrium taliks have deep basins with broad, 100+ m wide, inclined margins. In shallow ground ice settings (either original epigenetic ice or in permafrost that has reformed in drained basins), lakes are <3 m deep and flat bottomed. Unaffected by preexisting topography and ground ice variations, first-generation lakes in deep ground ice are rounder and grow larger in area than later-generation lakes. These predictions are consistent with GPS, sonar, and remote sensing measurements of bathymetry and plan view shape of first- and later-generation lakes in substrates with deep syngenetic ground ice (Pleistocene loess, northern Seward Peninsula, Alaska) and shallow epigenetic ice (Yukon Arctic coastal plain).
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| [27] |
Origin and polycyclic behaviour of thaw slumps, Mackenzie Delta region [J].
In tundra uplands east of the Mackenzie Delta, retrogressive thaw slumps up to several hectares in area typically develop around lakes. Ground temperatures increase in terrain affected by slumping due to the high thermal conductivity of exposed mineral soils and deep snow accumulation in winter. Mean annual temperatures at the top of permafrost were several degrees warmer in thaw slumps (-0.1°C to -2.2°C) than beneath adjacent undisturbed tundra (-6.1°C to -6.7°C). Simulations using a two-dimensional thermal model showed that the thermal disturbance caused by thaw slumping adjacent to tundra lakes can lead to rapid near-surface lateral talik expansion. Talik growth into ice-rich materials is likely to cause lake-bottom subsidence and rejuvenation of shoreline slumping. The observed association of thaw slumps with tundra lakes, the absence of active slumps on the shores of drained lakes where permafrost is aggradational and depressions in the lake bottom adjacent to thaw slumps provide empirical evidence that thermal disturbance, talik enlargement and thawing of subadjacent ice-rich permafrost can drive the polycyclic behaviour (initiation and growth of slump within an area previously affected by slumping) of lakeside thaw slumps. Copyright 08 2009 John Wiley & Sons, Ltd. and Her Majesty the Queen in right of Canada.
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| [28] |
The origin of oriented lakes: Evidence from the Bolivian Amazon [J].
61Cores were taken across the margins of 3 oriented and rectangular lakes in Bolivia61Stratigraphic markers indicate that these lakes are not tectonic61Radiocarbon ages show that lakes formed in the mid to late Holocene61Wind action is the most likely explanation for their shape and orientation
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| [29] |
Thermokarst phenomena and landforms due to frost heaving in Central Yakutia [J].
react-text: 441 Thermokarst (thaw) lakes emit methane (CH4) to the atmosphere formed from thawed permafrost organic matter (OM), but the relative magnitude of CH4 production in surface lake sediments vs. deeper thawed permafrost horizons is not well understood. We assessed anaerobic CH4 production potentials from various depths along a 590 cm long lake sediment core that captured the entire sediment... /react-text react-text: 442 /react-text [Show full abstract]
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| [30] |
Spatial analyses of thermokarst lakes and basins in Yedoma landscapes of the Lena Delta [J]. |
| [31] |
MacDonald G M, et al. Disappearing Arctic lakes [J]. |
| [32] |
Advances in thermokarst research [J]. |
| [33] |
Changes in thaw lake drainage in the Western Canadian Arctic from 1950 to 2000 [J].
Abstract The permafrost of the Western Canadian Arctic has a very high ground ice content. As a result, the vast number of thaw lakes in this area are very sensitive to a changing climate. With thaw lakes prone to either increases in area due to thermokarst processes, or complete drainage in less than one day due to melting of channels through ice-rich permafrost. After a lake drains, it leaves a topographic basin that is often termed a Drained Thaw Lake Basin (DTLB). An analysis of aerial photographs and topographic maps showed that 41 lakes drained in the study area between 1950 and 2000, for a rate of slightly less than one lake per year. The rate of drainage over three time periods (1950–1973, 1973–1985, 1985–2000), decreased from over 1 lake/year to approximately 0·3 lake/year. The reason for this decrease is not known, but it is hypothesized that it is related to the effect of a warming climate. There is a large spatial variation in DTLBs, with higher number of drained lakes in physiographic areas with poor drainage. It is likely that this variation is related to variations in ground ice. Although previous studies have suggested that lakes drain during periods of high water level, it is likely that a combination of a warm summer, a resulting deep active layer, and a moderately high lake level were responsible for the drainage of a lake in the study area during the summer of 1989. Although this study has documented changes in the rate of lake drainage over a 50-year period, there is a need for further research to better understand the complex interactions between climate, geomorphology, and hydrology responsible for this change, and to further consider the potential hazard rapid lake drainage poses to future industrial or resource development in the area. Copyright 08 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada. The contributions of P. Marsh, M. Russell, H. Haywood and C. Onclin belong to the Crown in right of Canada and are reproduced with the permission of Environment Canada.
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| [34] |
Shrinking lakes of the Arctic: Spatial relationships and trajectory of change [J].
Over the past 3 decades the Arctic has seen substantial warming. Previous local to regional scale studies have shown a considerable reduction in the size of lakes in this region. The subsequent exposure of carbon- and methane-rich sediments and the direct changes in surface albedo feed back into the drivers of regional and global climate change. Understanding and quantifying changes in the Arctic is a critical component of climate modeling due to the cooling effect of the Arctic on the global climate. The current work utilizes global satellite data from the Moderate Resolution Imaging Spectro-radiometer (MODIS) instrument to investigate changes in lakes across Canada between 2000 and 2009. The results show a net reduction of more than 6,700 kmin the surface area of water in lakes across Canada. Modest gains in the southern regions are offset by larger losses in surface area farther north. Additionally, spatial analysis shows that the lakes showing change are clustered in groups. This suggests that local variability may play a role in the observed changes. Further work is needed to extend the analysis to the circumpolar Arctic.
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| [35] |
116: G00M03 .DOI:10.1029/2011JG001666. |
| [36] |
Climatic warming and the degradation of warm permafrost [J]. |
| [37] |
Ecosystem and human health assessment to define environmental management strategies: The case of long-term human impacts on an Arctic lake [J].
There are rich deposits of mineral and fossil natural resources in the Arctic, which make this region very attractive for extracting industries. Their operations have immediate and vast consequences for ecological systems, which are particularly vulnerable in this region. We are developing a management strategy for Arctic watersheds impacted by industrial production. The case study is Lake Imandra watershed (Murmansk oblast, Russia) that has exceptionally high levels of economic development and large numbers of people living there. We track the impacts of toxic pollution on ecosystem health and then 鈥 human health. Three periods are identified: (a) natural, pre-industrial state; (b) disturbed, under rapid economic development; and (c) partial recovery, during recent economic meltdown. The ecosystem is shown to transform into a qualitatively new state, which is still different from the original natural state, even after toxic loadings have substantially decreased. Fish disease where analyzed to produce and integral evaluation of ecosystem health. Accumulation of heavy metals in fish is correlated with etiology of many diseases. Dose鈥揺ffect relationships are between integral water quality indices and ecosystem health indicators clearly demonstrates that existing water quality standards adopted in Russia are inadequate for Arctic regions. Health was also poor for people drinking water from the Lake. Transport of heavy metals from drinking water, into human organs, and their effect on liver and kidney diseases shows the close connection between ecosystem and human health. A management system is outlined that is based on feedback from indices of ecosystem and human health and control over economic production and/or the amount of toxic loading produced. We argue that prospects for implementation of such a system are quite bleak at this time, and that more likely we will see a continued depopulation of these Northern regions.
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| [38] |
Sedimentology and geochemistry of thermokarst ponds in discontinuous permafrost, subarctic Quebec, Canada [J].
No abstract is available for this article.
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| [39] |
Thermokarst phenomena and landforms due to frost heaving in Central Yakutia [J].
react-text: 441 Thermokarst (thaw) lakes emit methane (CH4) to the atmosphere formed from thawed permafrost organic matter (OM), but the relative magnitude of CH4 production in surface lake sediments vs. deeper thawed permafrost horizons is not well understood. We assessed anaerobic CH4 production potentials from various depths along a 590 cm long lake sediment core that captured the entire sediment... /react-text react-text: 442 /react-text [Show full abstract]
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| [40] |
Landscape-gradient assessment of thermokarst lake hydrology using water isotope tracers [J].
Thermokarst lakes are widespread in arctic and subarctic regions. In subarctic Qu茅bec (Nunavik), they have grown in number and size since the mid-20 th century. Recent studies have identified that these lakes are important sources of greenhouse gases. This is mainly due to the supply of catchment-derived dissolved organic carbon that generates anoxic conditions leading to methane production. To assess the potential role of climate-driven changes in hydrological processes to influence greenhouse-gas emissions, we utilized water isotope tracers to characterize the water balance of thermokarst lakes in Nunavik during three consecutive mid- to late summer seasons (2012-2014). Lake distribution stretches from shrub-tundra overlying discontinuous permafrost in the north to spruce-lichen woodland with sporadic permafrost in the south. Calculation of lake-specific input water isotope compositions (未 I ) and lake-specific evaporation-to-inflow (E/I) ratios based on an isotope-mass balance model reveal a narrow hydrological gradient regardless of diversity in regional landscape characteristics. Nearly all lakes sampled were predominantly fed by rainfall and/or permafrost meltwater, which suppressed the effects of evaporative loss. Only a few lakes in one of the southern sampling locations, which overly highly degraded sporadic permafrost terrain, appear to be susceptible to evaporative lake-level drawdown. We attribute this lake hydrological resiliency to the strong maritime climate in coastal regions of Nunavik. Predicted climate-driven increases in precipitation and permafrost degradation will likely contribute to persistence and expansion of thermokarst lakes throughout the region. If coupled with an increase in terrestrial carbon inputs to thermokarst lakes from surface runoff, conditions favorable for mineralization and emission of methane, these water bodies may become even more important sources of greenhouse gases.
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| [41] |
. A synthesis of thermokarst lake water balance in high-latitude regions of North America from isotope tracers [J]. |
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