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

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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.