Testing and modelling a transient episode of ocean acidification prior to the Eocene-Oligocene onset of the Cenozoic 'ice house'

测试和模拟新生代“冰屋”始新世-渐新世开始之前短暂的海洋酸化事件

基本信息

批准号：
NE/G007500/1
负责人：
Caroline Lear
金额：
$ 7.1万
依托单位：
CARDIFF UNIVERSITY
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FG007500%2F1
关键词：
Testing modelling transient episode ocean

项目摘要

One of the most profound changes in Earth's climate occurred 34 million years ago. The oceans rapidly cooled and large ice sheets developed on the Antarctic continent, thus beginning the current 'icehouse' climate regime. The mechanisms that caused this dramatic change in climate are not fully understood, but global changes in the cycling of carbon, leading to a drawdown of atmospheric carbon dioxide levels, are a prime candidate. A key piece of evidence that points to a disturbance in the carbon cycle during the Eocene-Oligocene Transition (EOT) is a global deepening of the calcite compensation depth (CCD) - the level in the water column at which calcium carbonate raining down from the surface ocean is fully dissolved. The deepening of CCD at the EOT represents the largest sustained change in the CCD during the last 65 million years of the Earth history and is interpreted as de-acidification of the deep ocean. Computer models, however, indicate that the deepening of the CCD is most likely linked to the decrease in the area available for carbonate deposition in shallow waters along continental margins, resulting from the sea-level lowering that accompanied the sudden build-up of large polar ice sheets during the EOT. While the relationship between CCD deepening, ocean cooling, and glaciation during the EOT is reasonably well understood, the time interval that immediately precedes these changes has not been studied in detail. This is principally due to time breaks in the core records, poor core recovery, and/or lack of carbonate in many sections. Nevertheless, a new compilation of records reveals the presence of an interval of low carbonate concentration at several sites positioned in the deeper levels of the ocean. These low carbonate intervals are contemporaneous in both the Atlantic and Pacific basins, during an event perhaps lasting only 100 to 200 thousand years. Based on this evidence, we hypothesize that these carbonate dissolution horizons are related to a brief shoaling of the CCD, or a transient interval of ocean acidification, immediately preceding the previously recognized deepening of the CCD. The timing of the short-lived episode of ocean acidification relative to the climatic shifts during the EOT is a critical aspect of the event. Occurring immediately prior to cooling and glaciation, the timing of this event suggests that changes in the acidification state of the ocean are not solely a response to global changes in the carbon cycle brought about by the climatic changes themselves (e.g. through sea level lowering resulting from glaciation). Furthermore, its position in the initiation phase indicates that variation in the carbonate chemistry of the ocean may provide a fundamental clue to the causal mechanisms of climate change at the EOT. Given the importance of this 'initiation phase,' we propose a comprehensive study of this interval through the collection of new lithological, paleontological, and geochemical data from selected deep sea cores that span this time interval. This work will be aimed at constraining the magnitude of ocean acidification and global changes in the CCD during the shoaling event, as well precisely determining the timing and duration of the event. These new records will give a more complete picture of the chemical changes that took place through the entire EOT, which is a critical aspect to fully understanding the mechanisms that caused large-scale climatic changes at this time. With better understanding of changes is ocean carbonate chemistry through the EOT, we plan to use computer models to investigate mechanisms that may have caused these changes and initiated the major shifts in climate that occurred during the EOT. This work will seek to identify the most likely scenario, or combination of scenarios, that most closely reproduce the geological observations in the model simulations.

地球气候最深刻的变化之一发生在 3400 万年前。海洋迅速变冷，南极大陆形成大冰盖，从而开始了当前的“冰室”气候体系。造成气候剧烈变化的机制尚不完全清楚，但导致大气二氧化碳水平下降的全球碳循环变化是一个主要候选机制。始新世-渐新世过渡期 (EOT) 期间碳循环受到干扰的一个关键证据是方解石补偿深度 (CCD) 的全球加深，即碳酸钙从水柱中倾泻而下的水柱高度。表层海洋完全溶解。 EOT 的 CCD 加深代表了地球历史上过去 6500 万年中 CCD 最大的持续变化，被解释为深海的去酸化。然而，计算机模型表明，CCD 的加深很可能与大陆边缘浅水区可用于碳酸盐沉积的面积减少有关，这是由于伴随着大型极地突然形成的海平面下降而导致的。 EOT 期间的冰盖。虽然 EOT 期间 CCD 加深、海洋变冷和冰川作用之间的关系已得到相当充分的了解，但这些变化之前的时间间隔尚未得到详细研究。这主要是由于岩心记录的时间中断、岩心采收率差和/或许多部分缺乏碳酸盐造成的。然而，新的记录汇编显示，位于海洋较深处的几个地点存在一段低碳酸盐浓度区间。这些低碳酸盐区间在大西洋和太平洋盆地中是同时发生的，在一个可能仅持续 100 至 20 万年的事件期间。基于这一证据，我们假设这些碳酸盐溶解层与 CCD 的短暂浅滩或海洋酸化的短暂间隔有关，紧接在先前认识到的 CCD 加深之前。 EOT期间短暂的海洋酸化事件相对于气候变化的时间是该事件的一个关键方面。该事件发生在冷却和冰川作用之前，其时间表明海洋酸化状态的变化不仅仅是对气候变化本身带来的碳循环全球变化的反应（例如，由于气候变化导致海平面下降）冰川作用）。此外，它在起始阶段的位置表明海洋碳酸盐化学的变化可能为EOT气候变化的因果机制提供基本线索。考虑到这个“起始阶段”的重要性，我们建议通过从跨越该时间间隔的选定深海岩心收集新的岩性、古生物学和地球化学数据来对该间隔进行全面研究。这项工作的目的是限制浅滩事件期间海洋酸化的程度和 CCD 的全球变化，并精确确定该事件的时间和持续时间。这些新记录将更全面地描述整个EOT期间发生的化学变化，这是充分理解当时造成大规模气候变化机制的关键方面。随着通过 EOT 更好地了解海洋碳酸盐化学的变化，我们计划使用计算机模型来研究可能导致这些变化并引发 EOT 期间发生的气候重大变化的机制。这项工作将寻求确定最有可能的情景或情景组合，以最接近地再现模型模拟中的地质观测结果。