Novel Paleoclimate and Paleoenvironmental Applications of Stable and Radiogenic Isotope and Elemental Geochemistry from the Holocene through the Cretaceous

isotope geochemistry;paleoclimate;mercury;carbonate clumped isotopes;radiogenic isotopes;Late Cretaceous K-Pg extinction;Geology and Earth Sciences;Science;Earth and Environmental Sciences

Meyer, KyleBlum, Joel D ;

University of Michigan

关键词: isotope geochemistry;    paleoclimate;    mercury;    carbonate clumped isotopes;    radiogenic isotopes;    Late Cretaceous K-Pg extinction;    Geology and Earth Sciences;    Science;    Earth and Environmental Sciences;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/144022/meyerkw_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Anthropogenic influence on modern climate and the environment is unambiguous and bears profound implications for agriculture, water availability, and natural resource management. These influences are global in extent and require critical examinations of past climatic and environmental perturbations in the geological record to predict the magnitude of anticipated changes (e.g. surface temperature increase) that will impact humanity in the future. This dissertation explores the use of existing and novel geochemical proxies for environmental information ranging from atmospheric circulation patterns of the Pleistocene/Holocene to coastal marine temperatures and volcanism of the Late Cretaceous. The initial focus of this research has centered on the application of carbonate clumped isotope (Δ47) paleothermometry to marine mollusk fossils in order to reconstruct marine temperatures preceding, during, and after the Cretaceous-Paleogene (K-Pg) extinction. The K-Pg boundary interval serves as an important analogue to modern and future climate projections, with atmospheric CO2 concentrations estimated at ~400 to 1100 ppm (compared to an annual average of 402.8 ppm in 2016) and a world lacking significant continental ice sheets. The K-Pg boundary is also known for pervasive terrestrial and marine extinctions including the demise of dinosaurian groups. These methods revealed Δ47-derived coastal marine temperatures from the ancient Mississippi Embayment and Atlantic Coast ranging from 7 to 25 °C, which compare closely to modern values in the range of 10 to 29 °C. The similarity between modern and reconstructed paleotemperatures suggests that the surface temperatures required at the subtropics for an unglaciated world may be closer to those observed in the modern than originally believed and implies a shallow equator-to-pole thermal gradient. Cold paleotemperatures along the Atlantic Coast near New Jersey (3 to 14 °C) may provide evidence for the presence of the Gulf Stream current, and subsequent North Atlantic return flow as early as the Campanian/Maastrichtian. The temperature ranges reconstructed for these sites also compare well to marine temperatures determined using other proxy methods, with the exception of the TEX86 organic geochemical temperature proxy, which may be seasonally biased towards summer temperatures. Coupled with the temperatures reconstructed from Δ47 values, we have developed a unique proxy for environmental mercury concentrations, [Hg], in the past using the same mollusk fossils from before, during, and after the K-Pg boundary interval. This work represents the first deep-time application of [Hg] records in biogenic carbonates. We have measured both [Hg] and Δ47 values from globally-distributed specimens across a latitudinal range from 70 °N to 67 °S. We found covariation between elevated marine temperatures (~7 to 10 °C excursions) and peak [Hg] between 17 to 42 ng g-1 (relative to a background of ~0 to 5 ng g-1) immediately prior to the K-Pg extinction and coinciding with the onset of the main eruptive phase of the Deccan Traps Large Igneous Province (LIP). Evidence of a global signal of volcanogenic emissions (e.g. CO2 and gaseous elemental Hg) and changing climate prior to the K-Pg boundary suggest causality and serve to clarify the role of the Deccan Traps in terms of extinction patterns. This work also reveals the potential of [Hg] in biominerals at other critical intervals of the geological record in addressing questions relating to LIP volcanism and environmental mercury cycling.