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

地球气候最深刻的变化之一发生在3400万年前。海洋迅速冷却，南极大陆上形成了巨大的冰盖，从而开始了目前的“冰窖”气候状态。引起这种剧烈气候变化的机制尚不完全清楚，但全球碳循环的变化，导致大气中二氧化碳水平的下降，是一个主要的候选原因。始新世-渐新世过渡（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——从海洋表面降下的碳酸钙完全溶解的水柱水平。东第三纪带CCD的加深代表了地球历史上6500万年以来CCD最大的持续变化，并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中可用于碳酸盐沉积的面积减少有关，这是由于海平面下降导致的，而海平面下降伴随着大极地冰盖在EOT期间的突然积聚。虽然在东第三纪期间，CCD加深、海洋冷却和冰川作用之间的关系相当好理解，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断裂，岩心采收率低，以及/或许多剖面缺乏碳酸盐。然而，一项新的记录汇编显示，在位于海洋深处的几个地点，存在一段低碳酸盐浓度的间隔。这些低碳酸盐层段在大西洋和太平洋盆地中是同时期的，在一个可能只持续10到20万年的事件中。根据这一证据，我们假设这些碳酸盐溶解层与CCD的短暂浅滩化或海洋酸化的短暂间隔有关，该间隔紧接在先前认识到的CCD加深之前。与EOT期间的气候变化相关的短暂海洋酸化事件的时间是该事件的一个关键方面。这一事件发生的时间紧接在冷却和冰期之前，表明海洋酸化状态的变化不仅仅是对气候变化本身（例如，由于冰期导致的海平面降低）所带来的全球碳循环变化的反应。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。考虑到这一“起始阶段”的重要性，我们建议通过收集跨越这一时间间隔的精选深海岩心的新的岩性、古生物学和地球化学数据，对这一区间进行全面研究。这项工作将旨在限制浅滩事件期间海洋酸化的程度和CCD的全球变化，以及精确确定事件的时间和持续时间。这些新记录将对整个EOT时期发生的化学变化提供一个更完整的图景，这是充分理解导致当时大规模气候变化的机制的关键方面。通过EOT对海洋碳酸盐化学变化有了更好的了解，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，并启动EOT期间发生的主要气候变化。这项工作将寻求确定最可能的情景或情景组合，最接近地再现模型模拟中的地质观测。

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.