Nutrients, Productivity, and Redox Conditions during Greenhouse Extinctions in the Panthalassic Ocean
Schoepfer, Shane DeGennaro
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This study aims to better understand the series of events leading up to and following the Permian-Triassic mass extinction as it took place in the Panthalassic Ocean basin, with a particular focus on the oceanographic factors controlling nutrient cycles, redox conditions, and the ecology of the planktonic community. The first two chapters focus on the Opal Creek section, in Alberta, Canada, which records deposition in a slope setting at subtropical latitudes on the northwestern margin of Pangaea. The first chapter of this study uses organic carbon and nitrogen isotopes, as well as the abundant but homogenous conodont community, to infer the presence of a nutrient rich coastal upwelling system along the continental margin, with highly enriched nitrogen isotopes interpreted as reflecting denitrification in a well-developed water column oxygen minimum zone. Subsequent analysis using trace element proxies for redox and productivity, as well as declining nitrogen isotope values, suggest a substantial weakening of coastal upwelling in the earliest Triassic coinciding with the extinction of the benthic sponge fauna and an increase in the sedimentation rate. Marine primary productivity is among the most important variables influencing water column redox conditions and benthic community structure, however it is extremely difficult to estimate from ancient marine rocks. The final chapter of this study aims to develop quantitative tools for estimating paleoproductivity, based on parameters that can be measured or calculated for most marine sedimentary systems. This study uses a compilation of 93 Cenozoic (primarily Quaternary) marine cores for which the sedimentary bulk accumulation rate could be calculated. Accumulation rates of organic carbon, phosphorus and biogenic barium are compared to estimates of primary and export productivity in the modern ocean derived from carbon-14 uptake and satellite measurements of chlorophyll. The resulting equations relating proxy accumulation to productivity can be applied to a wide variety of ancient marine systems, allowing for more quantitative reconstructions of paleoproductvity and comparison to other geochemical proxies for nutrient availability or environmental stress.