Diagenetic and Mineralogical Effects in Clumped Isotope Thermometry and Application to Last Interglacial Climate.

[摘要] This dissertation focuses on how mineralogical differences affect original clumped isotope (Δ47) values and how they are changed through diagenesis. A primary motivation for this work was to understand the geologic conditions under which the clumped isotope thermometer can be applied with confidence to paleoclimatic problems. To place empirical constraints on the conditions under which Δ47 alteration can occur, samples from a ~4500 m long drill core from Andros Island, Bahamas were analyzed. These limestones and dolomites formed under near-surface temperature conditions, but for samples below ~1.3 km in depth, calculated Δ47 temperatures increase by ~10° C. This indicates a shift from preserved near-surface temperatures to diagenetically modified values. Importantly, this shift is not accompanied by changes in common diagenetic indicators, and thus raises the possibility of solid-state clumped isotope alteration occurring at much lower temperatures than previously thought. Similar Δ47 temperatures recorded by each carbonate phase suggest that fine-grained dolomites and calcites are equally viable materials for recording surface temperature conditions, but both are also equally susceptible to alteration of their primary clumped isotope abundances when buried.A challenge remaining in clumped isotope thermometry was the lack of an empirical calibration fordolomite, which had been theoretically predicted to differ from established calibrations. Here, analysis of a set of synthetic and natural dolomites formed at known temperatures results in a calibration line statistically indistinguishable from calcite clumped isotope calibrations. This supports the idea of a universal calibration for carbonate clumped isotope thermometry and enables new investigations into conditions of dolomite formation.Finally, the clumped isotope thermometer can be applied to key intervals for investigating paleoclimate and testing paleoclimate questions. To determine sea surface temperatures and water composition in Bermuda during the onset of Last Interglacial sea level rise, conventional and clumped isotope measurements offossil shells were conducted. These results suggest meltwater influence and temperatures as much as 10° C colder than modern, requiring explanations that allow for major changes inNorth Atlantic surface ocean circulation. They also illustrate extreme sensitivity of Bermudian climate to broad-scale climate and ocean circulation changes.

[发布机构] University of Michigan

[效力级别] carbonate geochemistry

[关键词] clumped isotopes;carbonate geochemistry;paleoclimate;diagenesis;dolomite;Last Interglacial;Geology and Earth Sciences;Science;Earth and Environmental Sciences