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.

地球气候中最深刻的变化之一发生在3400万年前。海洋迅速冷却，大陆在南极大陆上发展，因此开始了当前的“冰屋”气候状态。引起这种气候变化的急剧变化的机制尚未完全了解，但是碳循环的全球变化，导致大气二氧化碳水平的减少是主要的候选者。一个关键的证据表明，在始新世 - 新世过渡（EOT）期间碳循环的干扰是方解石补偿深度（CCD）的全球加深 - 水柱中碳酸钙从表面海洋下雨的水平完全溶解。 CCD在EOT上的加深代表了地球历史的最后6500万年CCD的最大持续变化，并被解释为深海的去酸化。但是，计算机模型表明，CCD的加深最有可能与沿大陆边缘的浅水区域可用区域的减少有关，这是由于海平面降低伴随着EOT期间大型极性冰原的突然堆积。虽然在EOT期间，CCD加深，海洋冷却和冰川化之间的关系是相当理解的，但直接在这些变化之前的时间间隔尚未详细研究。这主要是由于核心记录中的时间中断，核心恢复差和/或在许多部分中缺乏碳酸盐。然而，新的记录汇编揭示了位于海洋更深层次的几个地点的低碳酸盐浓度间隔。在大西洋和太平洋盆地中，这些低碳酸盐间隔是同时的，在一次可能只持续100至20万年的事件中。基于这些证据，我们假设这些碳酸盐溶解范围与CCD的短暂浅滩或海洋酸化的短暂间隔有关，直到先前认识到的CCD加深之前。相对于EOT期间的气候变化，短暂的海洋酸化发作的时机是事件的关键方面。在冷却和冰河之前，发生的时间紧接发生，此事件的时间表明，海洋酸化状态的变化不仅是对气候变化本身带来的碳周期变化的响应（例如，通过冰川而导致的海平面降低）。此外，其在开始阶段的位置表明，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的线索。鉴于这个“起始阶段”的重要性，我们通过收集来自此时间间隔的选定深海核心的新岩性，古生物学和地球化学数据的新岩性，古生物学和地球化学数据进行了全面研究。这项工作将旨在限制浅滩事件期间CCD的海洋酸化和全球变化的大小，并精确地确定事件的时间和持续时间。这些新记录将使整个EOT发生的化学变化更完整地了解，这是完全理解此时引起大规模气候变化的机制的关键方面。通过EOT更好地了解变化的是海洋碳酸盐化学，我们计划使用计算机模型来研究可能导致这些变化的机制，并引发了EOT期间发生的气候的主要变化。这项工作将寻求确定最可能在模型模拟中最紧密地重现地质观察的情况。