Stable Isotopes in Unstable Times: A geochemical investigation of the end Cretaceous mass extinction
Tobin, Thomas Steven
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This primary goal of this dissertation is to increase understanding of the end Cretaceous (or Cretaceous - Paleogene or K-Pg) mass extinction through the use of light stable isotope geochemistry. These studies attempt to examine any climatic and environmental changes that occurred around the K-Pg boundary, and might have contributed to the K-Pg mass extinction, specifically by examining isotopic records at high stratigraphic resolution around the boundary. Studies are completed in two field areas, the Antarctic Peninsula and eastern Montana, USA, both of which preserve the K-Pg boundary. While these works, like most scientific studies, lead to further questions that warrant investigation and confirmation, they generally support the idea the end Cretaceous mass extinction was more complicated than a simple asteroid strike. This dissertation is comprised of five scientific chapters as well as short introductory and concluding sections. The introduction explains the background and context behind each study, and the process by which I ultimately worked with a wide variety of co-authors to complete the various projects. The conclusion begins the process of examining the differences and similarities of each study, and explores further avenues of research to test some of the proposed hypotheses or reconcile potentially contradictory data. Four of the chapters are written as scientific manuscripts, while a fifth chapter details the work done as part of my astrobiology rotation. Chapter 1 (Tobin et al., 2011) outlines the discovery of analytical errors in the typical process of measuring carbonate stable isotopes (δ<super>13</super>C and δ<super>18</super>O) on small sample sizes of powder. Carbonate material is typically prepared using one of two methods, either by drilling using a high speed drill or micromill, or by crushing a sample using a mortar and pestle. Drilling produces a finer grain size of material, which is consequently more prone to being altered in its δ<super>18</super>O value during a typical automated measurement process, while the sample is waiting in the queue to be analyzed. This chapter outlines the specific parameters under which this phenomenon occurs, and describes a correction procedure, though we encourage every lab to develop their own correction scheme. Chapter 2 (Tobin and Ward, submitted) is the second of two papers (in order of analysis and publication, but first presented here) that analyze and interpret δ<super>13</super>C and δ<super>18</super>O values from molluscan shell carbonate collected on the Antarctic Peninsula, though a small amount of fossil shell material was also used in Chapter 1. In this study, we examine trends in δ<super>13</super>C for ammonites and other benthic mollusks using our own collections with added samples coming from collections currently housed at the Paleontological Research Institute (PRI). In both collections we find a notable offset in δ<super>13</super>C between ammonites and benthic mollusks, though good correspondence in δ<super>18</super>O. Ultimately, the best interpretation of this pattern is that ammonites are incorporating more respired CO<sub>2</sub> into their shell material, potentially from a higher metabolic rate. A more active lifestyle could potentially have increased the susceptibility of ammonites to an event like bolide impact at the end of the Cretaceous. Chapter 3 (Tobin et al., 2012) also looks at isotopic records, in this case focusing on δ<super>18</super>O values, for fossil mollusks from Antarctica. Paleotemperature can generally be inferred from δ<super>18</super>O values if the δ<super>18</super>O of the water from which it came can be estimated reliably, as is generally thought to be the case for seawater during the Cretaceous. We generate a time series of δ<super>18</super>O across the K-Pg boundary. We also generated a magnetostratigraphic record for the section, as well as paleobiological data in the same stratigraphic context. Statistical analysis revealed two extinction events, one at the peak of warming from the paleotemperature record (50 meters below the K-Pg boundary), the other simultaneous with the iridium anomaly indicating the asteroid strike. The warming events are also correlated (using magnetostratigraphy) with the timing of Deccan Traps volcanism, which could potentially generate warming via CO<sub>2</sub> emission. The evidence is most consistent multiple causes contributing to the end Cretaceous mass extinction over a short geological interval. The plausibility of the Deccan - warming link is explored in Chapter 5, though it is ultimately inconclusive due to the uncertainty in total volumes of CO<sub>2</sub> emitted during this event. Chapter 4 (Tobin et al., 2014) performs a similar analysis to that in Chapter 3, but on freshwater mollusks from eastern Montana. Because the paleo-depositional setting was fluvial/lacustrine, traditional δ<super>18</super>O paleotemperature reconstruction is not a useful tool. Carbonate clumped isotope paleothermometry, while more challenging analytically, avoids this problem, and was used to generate a temperature record across the K-Pg boundary. A cooling trend in summer temperatures was identified in the last ~30 meters of the Cretaceous (though bivalve nutrient stress could plausibly explain the pattern as well). This pattern occurs over the same stratigraphic interval that vertebrate paleontologists have identified biodiversity changes, and could plausibly be related. As with Chapter 3, this study is most consistent with a multiple cause mass extinction.