Marine Sulfur and Strontium Cycling During the Transition from a Cool to Warm Greenhouse in the Early Cretaceous

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This dissertation develops and applies coupled marine sulfur and strontium cycle modeling to the highly dynamic Early Cretaceous demonstrating the value of increased model constraint from linkage of biogeochemical cycles with shared forcing factors. A foundation for this work is first provided by review of relevant geochemical, sedimentological, and paleontological evidence for the climatic shift from a cool to warm greenhouse in the Early Cretaceous, along with information about the timing (Geologic Time Scale 2012) of emplacement for several large igneous provinces, and the deposition of South Atlantic basinal evaporites. The coupling of the strontium and sulfur cycles first relies on a new iterative method developed here for modeling the Phanerozoic composite marine radiogenic strontium isotope record. Unlike previous strontium cycle models, variable strontium isotope ratios for the total weathering and diagenetic input fluxes are used instead of assuming that modern ratios were constant throughout the Phanerozoic. Despite the dominance of the linked hydrothermal and weathering input fluxes driving changes in both isotope records, this coupling enables a more detailed examination of the sulfur cycle, indicating that a complex interplay of changes to multiple sulfur cycle parameters is necessary to reproduce the seawater sulfate sulfur isotope record of the Early Cretaceous. Coupled strontium-sulfur modeling enables clear distinction of the Aptian-Albian (~20 Ma) isotopically light sulfate ocean as an alternate state requiring changes to multiple sulfur cycle parameters instead of the persistence of a negative excursion. The geologically rapid return to more 34S enriched sulfate values following the newly observed minor negative excursion at the Albian-Cenomanian boundary is thought to be caused by fluctuations in the pyrite burial flux driven by sea level changes in a low sulfate ocean. The model results presented here also suggest a distinctly different range of values for the sulfur isotope composition of the weathering flux, weathered evaporite:pyrite ratios, pyrite burial fraction and integrated fractionation associated with pyrite burial compared to previous models as well as the importance of estimating major changes in concentration for both strontium and sulfur

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  • 02/14/2018
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