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，南极大陆上形成了大冰盖，从而开始了目前的“冰屋”气候制度。造成气候剧烈变化的机制尚不完全清楚，但全球碳循环的变化，导致大气二氧化碳水平的下降，是主要的候选因素。一项关键证据表明，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--从表层海洋落下的碳酸钙在水柱中完全溶解的水平。洋面表层冰盖深度的加深是地球历史上过去6500万年来冰盖面积最大的持续变化，被解释为深海的酸化。然而，计算机模型表明，气候变化的加深很可能与大陆边缘浅水可供碳酸盐沉积的面积减少有关，这是由于在EOT期间伴随着大片极地冰盖的突然堆积而导致的海平面下降。虽然人们已经很好地了解了EOT期间CCD加深、海洋冷却和冰川作用之间的关系，但这些变化之前的时间间隔还没有被详细研究。这主要是由于岩心记录中的时间断裂、岩心回收率差和/或许多区段缺乏碳酸盐。然而，一项新的记录汇编显示，在位于海洋更深层次的几个地点存在低碳酸盐浓度区间。这些低碳酸盐层段在大西洋和太平洋盆地中都是同时存在的，发生的事件可能只持续了100到20万年。根据这一证据，我们推测，这些碳酸盐溶解层位与CCD的短暂浅化或海洋酸化的瞬时间隔有关，就在先前公认的CCD加深之前。相对于EOT期间的气候变化，海洋酸化这一短暂事件的时间安排是这一事件的一个关键方面。这一事件发生在降温和冰化之前，其发生的时间表明，海洋酸化状态的变化不仅仅是对气候变化本身带来的全球碳循环变化的反应(例如，通过冰川作用导致的海平面下降)。此外，它在初始阶段的位置表明，海洋碳酸盐化学的变化可能为EOT气候变化的原因机制提供基本线索。鉴于这一“初始阶段”的重要性，我们建议通过从选定的深海岩心收集新的岩性、古生物和地球化学数据，对这一时间间隔进行全面研究。这项工作的目的将是在浅滩事件期间限制海洋酸化的程度和全球气候变化，并准确地确定事件的时间和持续时间。这些新的记录将给出整个EOT过程中发生的化学变化的更完整的图景，这是全面了解此时导致大规模气候变化的机制的关键方面。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启动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可能的情景或情景的组合，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.