Multi-isotope investigation of paleozoic climate an environment: a new approach using redox sensitive stable isotope tracers

在过去的5.4亿年里，海平面比今天高得多，导致古大陆的大片区域被淹没。如果大陆在赤道附近发生淹没，就会沉积石灰岩。海洋早已从大陆撤退，但石灰岩依然存在。在石灰岩内部有同位素指纹形式的地球化学线索，我们可以用这些线索来了解地球过去的环境和气候的性质。通过在古代海洋石灰岩的地层序列中进行测量，我们可以跟踪地球环境和气候随时间的演变。同位素指纹告诉我们古代的碳循环和温室效应，海平面，以及陆缘海域的海水循环模式，海水的温度和盐度。这种类型的信息为我们提供了关于气候与海平面环流模式或气候与过去大陆风化速率之间关系的思考和模型。这与关于人为二氧化碳对当代全球变暖的影响的辩论有关。我的工作重点是完善对碳同位素随时间变化的记录的解释，开发古代碳循环的模型，并推进像钕这样的同位素示踪剂的应用，我发现这种示踪剂与过去的海平面变化有关。新的研究将大大增加这一同位素工具的储备；包括首次作为古代海水中溶解氧水平的替代指标--铀和铬的稳定同位素变化。将首先研究铀和铬同位素在现代环境中的行为。这将涉及对海水、沿海分层水，甚至是底层没有溶解氧的现代分层湖泊进行采样。这些现代研究将描述现代系统中铬和铀同位素分馏的范围，以及控制同位素分馏的过程。这些发现将有助于利用古代海洋沉积物中的铀和铬同位素指纹作为跟踪地球浅海和海洋中氧气水平随时间变化的工具。

In the past 540 million years, sea level has been much higher than today, causing large areas of the paleocontinents to be submerged. If submergence occurred when a continent was near the equator, limestones were deposited. The seas have long since retreated from the continents, but the limestones remain. Inside the limestones there are geochemical clues in the form of isotopic fingerprints that we can use to understand the nature of the Earth's past environment and climate. By making our measurements in stratigraphic successions of ancient marine limestones we can track the evolution of the Earth's environment and climate over time. The isotopic fingerprints tell us about the ancient carbon cycle and greenhouse effect, sea level, and seawater circulation patterns in epicontinental seas, temperature, and salinity of marine waters. This type of information informs our thinking and models on the relationship between climate and sea level circulation patterns, or climate and continental weathering rates in the past. And it bears on the debate regarding the impact of anthropogenic carbon dioxide on contemporary global warming. My work is focused on refining interpretations of carbon isotope records through time, developing models of ancient carbon cycling, and advancing the application of isotope tracers like neodymium, which I have found to be related to sea level changes in the past. New research will add significantly to this repertoire of isotopic tools; including, for the first time, a proxy indicator of dissolved oxygen levels in ancient seawater-stable isotope variations of uranium and chromium. The behavior of uranium and chromium isotopes will be first studied in modern environments. This will involve sampling ocean waters, stratified waters in coastal seas, and even a modern stratified lake with no dissolved oxygen in the bottom waters. These modern studies will characterize the range of chromium and uranium isotope fractionation in modern systems, and the processes that control the isotope fractionations. These findings will inform on the utility of uranium and chromium isotope fingerprints in ancient marine sediments as tools for tracking changing oxygen levels in the earth's shallow seas and oceans over time.