| Summary: | Sedimentary porewater chemical and isotopic profiles contain complex records of past bottom water composition which are overprinted by in situ biotic and abiotic reactions. We developed a density-based method to determined relic deep water salinities and applied it to map global-scale water mass properties and distributions in the deep northwest Atlantic at four previously unsampled sites. Paleosalinities determined by density have higher precision and accuracy than previously published results and confirm the northward expansion of southern deep water and a reversal in the Atlantic’s bottom water meridional salinity gradient during the Last Glacial Maximum (Chapters 1 & 2). Nitrogen isotopic composition profiles of deeply buried porewaters are a unique dataset used to distinguish between biotic and abiotic nitrogen reactions under conditions that approach the canonical temperature and pressure limits of life. We attribute observed variations in nitrogen concentrations and isotopic compositions of porewater and sediment to a variety of microbially mediated processes including assimilation, ammonification, sulfate reducing ammonia oxidation, accretion, dissolution, and illitization (Chapter 3).
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