深海沉积与全球变化研究的前缘与挑战

doi:10.13745/j.esf.sf.2022.6.28

摘要/Abstract

摘要：

海洋正在经历变暖和酸化等人类活动引发的全球变化的影响,而深海沉积储存着地球演变历史时期由自然因素驱动过去全球变化的详细档案,通过探究其现今和过去全球变化过程,能够揭示全球变化的特征和规律,为预测未来变化提供依据。近年来在该领域的突出研究进展,是针对社会选择的未来排放轨迹,在深海记录中都能够找到相应的类似情形,用于评估未来地球系统各种变化的过程和后果。其中,以Dansgaard-Oeschger变化为代表的千年尺度事件、以厄尔尼诺-南方涛动(ENSO)和北大西洋涛动(NAO)为代表的十年尺度气候变化事件,是最接近现今地球变暖的快速气候变化场景。地球系统的发展轨迹目前正处于人类排放温室气体的“热室地球”路径的起点上,如果地球超过了这个“临界点”,它将沿着一条不可逆的道路进入“热室地球”状态,另一种路径则是通向“稳定地球”状态。深海沉积档案中的类似情形能够为社会选择未来排放的轨迹提供重要参考。全球变化研究面临的重大挑战是重新认识其关键过程的理论机制。以海洋变暖和酸化影响硅藻和颗石藻的海洋生物泵过程为例,传统知识认为酸化有利于硅藻建造,但最新的围隔实验研究却发现酸化大幅减少全球硅藻输出;传统知识认为酸化导致海洋生物钙化危机,但近期针对中生代大洋缺氧事件的黑色页岩研究,发现颗石藻的碳酸钙输出在海洋酸化期间大幅增加。这些颠覆性的认识严重挑战了传统全球变化某些关键过程的理论体系。

关键词: 深海沉积, 全球变化, 海洋变暖, 海洋酸化, 快速气候变化, 临界点

Abstract:

Present-day oceans are experiencing the effects of global change such as warming and acidification caused by human activities. Deep-sea sediments store a detailed archive of past global changes driven by nature during Earth’s evolution. By exploring today’s and past global change processes, we can reveal the characteristics and laws of global change and provide a basis for predicting future changes. The outstanding research progresses made in recent years were the discovery in the deep-sea of a variety of analogous scenarios that humans, when choosing future greenhouse gas emission trajectories, can reference to for evaluating future changes—their processes and consequences —in the Earth system. Among such scenarios, millennium-scale climate events, represented by the Dansgaard-Oeschger oscillation, and decadal-scale climate events, represented by the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO), are the closest rapid climate change analogues to today’s global warming. The Earth system is currently at the beginning of a “Hothouse Earth” emission trajectory. If the system passes the “tipping points”, it will follow an irreversible path to a “Hothouse Earth” state; whereas an alternative path may lead to a “Stabilized Earth” state. Analogous scenarios in the deep-sea sedimentary archive can provide societies with valuable information in choosing future emission trajectories. The major challenges for researchers are to fully understand the mechanisms of the key processes of global change. Taking the example of ocean warming and acidification’s influence on diatom and coccolithophore functioning as marine biological pump. The conventional knowledge has suggested that acidification is beneficial to diatom formation; but recent mesocosm experiments have found that global diatom output is significantly reduced. Likewise, the suggestion that acidification leads to calcification crisis in marine life has also been proven wrong, as recent black shale studies of Mesozoic oceanic anoxic events showed that calcium carbonate output from coccolithophores increased significantly during ocean acidification. The above examples demonstrate that conventional knowledges of the key processes of global change are under serious challenge.

Key words: deep-sea sediments, global change, ocean warming, ocean acidification, rapid climate change, tipping points

中图分类号:

刘志飞, 陈建芳, 石学法. 深海沉积与全球变化研究的前缘与挑战[J]. 地学前缘, 2022, 29(4): 1-9.

LIU Zhifei, CHEN Jianfang, SHI Xuefa. Deep-sea sediments and global change: Research frontiers and challenges[J]. Earth Science Frontiers, 2022, 29(4): 1-9.

图/表 8

图1 深海在地球表层的分布根据全球水深和高程数据库[5]绘制,红线显示水深超过2 000 m的范围。

Fig.1 Distribution of deep-sea regions on the Earth’s surface (modified from [5]) showing outlined areas (red line) with water depths exceeding 2000

图2 白垩纪大洋缺氧事件与现今人类世海洋变暖和酸化的场景对比(引自文献[6]) A—白垩纪大洋缺氧事件:火山喷发释放的温室气体引发海洋变暖、酸化和缺氧;B—人类世场景:人类活动释放的温室气体引发海洋变暖、酸化和缺氧。

Fig.2 Comparison between a Cretaceous oceanic anoxic event (A) and present Anthropocene ocean warming andacidification scenarios (B). Adapted from [6].

图3 深海记录的过去55 Ma以来大气CO2演化与2300年大气CO2预测场景(引自文献[6,17-18]) IPCC评估从“高排放”到“大幅减排”提供多种场景,但未来的排放轨迹取决于社会的选择。深海沉积档案提供了地球历史时期的“始新世气候适宜期”(+8 ℃)、“中新世气候适宜期”(+3~4 ℃)、“上新世暖期”(+2~3 ℃)和“全新世气候适宜期”(+0.7 ℃)等多种类似情形,从而为社会选择未来排放的轨迹提供重要参考。

Fig.3 Evolution of atmospheric CO2 recorded in deep-sea sediments over the past 55 Ma (left) and predicted atmospheric CO2 scenarios up to the year 2300 (right). Adapted from [6,17-18].

图4 东太平洋Santa Barbara盆地ODP893A钻孔晚第四纪浮游有孔虫氧同位素和生物扰动指数(引自文献[24]) A—格陵兰冰芯(GISP2)氧同位素;B—温跃层种浮游有孔虫N.pachyderma氧同位素;C—表层种浮游有孔虫G.bulloides氧同位素;D—浮游有孔虫N.pachyderma右旋盘的相对丰度;E—生物扰动指数。揭示东太平洋末次冰期Dansgaard-Oeschger千年尺度的气候变化事件,可以同格陵兰冰芯(GISP2)记录的气候事件对比,表明北大西洋高纬地区发生的气候事件与太平洋表层和中层水发生的变化存在直接联系。

Fig.4 Planktonic foraminiferal oxygen isotope and bioturbation index of Late Quaternary at ODP Hole 893A, Santa Barbara Basin, East Pacific. Adapted from [24].

图5 现代海洋表层平均温度(A)与1998年1月厄尔尼诺事件(B)和上新世类似厄尔尼诺状态(C)的海洋表层温度对比(引自文献[20,28])

Fig.5 Comparison of modern annual average sea surface temperature (A) with sea surface temperature during the January 1998 El Niño event (B) and Pliocene annual average sea surface temperature (C). Adapted from [20,28].

图6 海洋酸化影响下的全球硅酸垂向分布差异(引自文献[35]) 显示大西洋和太平洋加强的深海硅酸盐泵和南大洋硅酸盐圈捕作用,从而引发全球上层海洋硅藻减少。OA:海洋酸化;AABW:南极底层水;AAIW:南极中层水;CDW:绕极深层水;NADW:北大西洋深层水;NPDW:北太平洋深层水;SAMW:亚南极模态水。

Fig.6 Variation of global silicic acid distribution influenced by ocean acidification. Adapted from [35].

图7 大洋缺氧事件前后浮游植物向海底输出碳酸钙及钙质超微印痕化石的形成过程示意图(引自文献[37]) 显示碳酸钙输出在大洋缺氧事件发生期间大幅增加,而后期的沉积物酸性孔隙水造成了实体钙质超微化石的溶解。OAE:大洋缺氧事件;OMZ:最低含氧带。

Fig.7 Schematic diagram illustrating changes of phytoplankton export and formation of nannofossil imprints through an oceanic anoxic event. Adapted from [37].

图8 人类世地球系统的潜在发展轨迹(引自文献[6,39]) 显示地球系统在经历更新世/全新世冰期-间冰期旋回变化的稳定阶段后,在人为排放温室气体驱动下进入当前的人类世,达到未来发展的岔路口。一条发展轨迹是沿着不可逆转的道路进入“热室地球”状态,另一条轨迹是通向“稳定地球”状态。

Fig.8 Potential trajectories of the Earth system development in the Anthropocene. Adapted from [6,39].

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