Abstract
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 (Chapter 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).
Highlights
Introduction and BackgroundSalinity is used to resolve ocean density structure and the spreading of deep water masses (Munk 1950; Stommel and Arons 1959)
During the Last Glacial Maximum (LGM), between 19-26 ka (Clark et al 2009; Peltier and Fairbanks 2006; Yokoyama et al 2000), atmospheric CO2 concentrations were approximately 30% lower than preindustrial levels (Mix, Bard, and Schneider 2001; Petit et al 1999), global average sea level was 13 0 meters lower, and average seawater was approximately 3.6% saltier (Clark et al 2009; Clark and Mix 2002; Peltier and Fairbanks 2006)
Adkins et al(2002) interpreted porewater Cl- and δ18O using one-dimensional diffusion modeling (e.g. Schrag et al.1992) at several sites in the Atlantic. They achieved average uncertainties for LGM salinity (±0.1 g/kg, 0.3 %) and temperature based on oxygen isotopes (± 0.53 C) a few orders of magnitude larger than the precision with which we can resolve these parameters in the modern deep ocean, identifying the largest source of error as the necessary storage of porewater samples between collection and analysis (Adkins and Schrag 2003)
Summary
Introduction and BackgroundSalinity is used to resolve ocean density structure and the spreading of deep water masses (Munk 1950; Stommel and Arons 1959). Bottom water chloride concentration and its uncertainty is determined as the average and standard deviation of the 100 boundary conditions, [Cl-](0,t), associated with the porewater chloride best fits for the measured profile of interest.
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