Collaborative Research: Testing Hypotheses of Global Warming during Three Major Mass Extinctions

大规模灭绝是地质上短暂的事件，在此期间，大部分现存物种灭绝，并对生物圈、环境和全球地球化学循环产生深远影响。大型天体对地球的撞击被认为是造成白垩纪末期生物大灭绝的唯一或主要原因，人们对这一事件的理解最为透彻。在地质记录中，撞击通常被广泛怀疑是导致生物大灭绝的原因。或者，大规模灭绝可能是地球环境内源性变化的结果，包括大气温室气体和/或火山气溶胶的积累，大气氧含量的变化，或生物圈有毒硫化物化合物的产生。这些假设的“杀伤机制”在表面和海洋温度变化的预测时间、幅度和分布方面各不相同。因此，高分辨率的温度记录可以区分这些相互竞争的假设，并有可能扩大我们对伴随这些重大事件的地球表面变化的理解的深度。我们建议通过三次主要的大灭绝事件来研究大气和海洋表面的温度历史，使用一种新的基于碳酸盐矿物晶格中13C和18O相互“聚集”成键的地热测量技术。我们的目标是获得二叠纪/三叠纪和三叠纪/侏罗纪边界的大陆土壤碳酸盐和浅海碳酸盐化石生长温度的第一个定量（约2摄氏度）和时间分辨记录，并用于比较，跨越更好理解的白垩纪/古近纪边界的记录。考虑到气候变化在大灭绝的假设原因中的突出地位，以及在这些关键时期定量古温学的缺乏，我们相信这些数据可能会导致我们对大灭绝原因的理解取得突破。我们方法的关键在于，我们将使用的新型碳酸盐团块同位素温度计基于均相平衡（一种涉及单相组分的反应），因此可以严格限制温度，而不需要任何有关碳酸盐生长的水的同位素组成的信息。因为我们可以用这个温度计独立地约束温度，我们也将能够解释古土壤碳酸盐和海洋化石的deltao - 18值，作为它们生长的大气和海水的deltao - 18的代用物（也就是说，因为我们将能够在一个独立已知的温度下“分离”碳酸盐-水的分馏）。在土壤碳酸盐的情况下，我们将进一步通过基于deltaC13测量估计大气的二氧化碳分压来补充我们的测温数据。这项研究是加州理工学院和华盛顿大学的合作成果。该项目的更广泛影响是，它将为加州理工学院的一名博士生和三名本科生提供暑期研究奖学金。埃勒还将利用这笔赠款帮助高中生暑期实习。我们期望这项研究能为通过结合“团块同位素”测温和常规稳定同位素研究重建过去气候提供一种新的、广泛适用的方法模型，从而有助于地球化学仪器和方法的发展。最后，我们的结果将说明气候变化和灭绝之间的关系，因此与预测现代全球变暖的可能后果有关。

Mass extinctions are geologically brief episodes during which a large fraction of extant species became extinct, and have profound consequences for the biosphere, environment and global geochemical cycles. Impact of a large body with the earth is recognized as the singular or dominant cause of the best understood mass extinction at the end of the Cretaceous, and impacts generally are widely suspected of driving mass extinctions deeper in the geological record. Alternatively, mass extinctions might be consequences of endogenous changes in earth's environment, including build-up of atmospheric greenhouse gases and/or volcanic aerosols, variations in atmospheric oxygen content, or biospheric production of toxic sulfide compounds. These hypothesized 'kill mechanisms' differ from one another in the predicted timing, magnitude and distribution of surface and ocean temperature change. Therefore, high-resolution temperature records could discriminate among these competing hypotheses, and potentially expand the depth of our understanding of changes in the earth's surface that accompanied these seminal events.We propose to examine the temperature history of the atmosphere and surface ocean through three major mass-extinction events using a new geothermometric technique based on 'clumping' of 13C and 18O into bonds with each other in the carbonate mineral lattice. Our goal is to obtain the first quantitative (about 2 degrees C) and time-resolved records of growth temperatures of continental soil carbonates and shallow marine carbonate fossils across the Permian/Triassic and Triassic/Jurassic boundaries, and, for comparison, a record across the better-understood Cretaceous/Paleogene boundary. Given the prominence of climate change in hypothesized causes of mass extinctions and the dearth of quantitative paleothermometry at these key times, we believe these data could lead to a breakthrough in our understanding of the causes of mass extinctions.The key to our approach is that the new carbonate clumped-isotope thermometer we will use is based on a homogeneous equilibrium (a reaction involving components of a single phase) and so can rigorously constrain temperature without any information about the isotopic composition of water from which carbonate grew. Because we can use this thermometer to independently constrain temperature, we will also be able to interpret the deltaO18 values of paleosol carbonates and marine fossils as proxies for the deltaO18 of meteoric and ocean waters from which they grew (i.e., because we will be able to 'tease apart' the carbonate-water fractionation at an independently known temperature). In the case of soil carbonates, we will further supplement our thermometric data by estimating pCO2 of the atmosphere based on deltaC13 measurements.This research is a collaborative effort between Caltech and the University of Washington. Broader impacts of this project are that it will support one Ph.D student and three summer research fellowships for undergraduate students at Caltech. Eiler will also use this grant to help support high school student summer internships. We anticipate that this study will provide a model for a new and widely applicable approach to reconstructing past climates through combined 'clumped isotope' thermometry and conventional stable isotope studies, and thus will contribute to the development of instruments and methods for geochemistry. Finally, our results will speak to the relationship between climate change and extinction, and thus has relevance for predicting the possible consequences of modern global warming.