Trace Metal-Organic Interactions In Sediments and the Preservation of Paleoredox Proxies

Knowledge of trace-metal partitioning in marine muds (e.g., Fe, Mo, U, Zn) is required to construct accurate models for geochemical cycles in the modern environment. Such perspectives are essential for the use of these metals in mudrocks as paleoceanographic proxies. Factors such as oxygen penetration depth/bottom-water redox, sedimentation rate, and supply of reactants, including dissolved sulfide, are known to affect metal sequestration in and remobilization from coastal sediments. In addition to these factors, the flux of organic carbon (OC) and the composition of this carbon are also a key in understanding the fates of these metals. Currently, however, there is little quantitative information on the mass balance of metal fixation in coastal sediments through direct interactions with OC, how OC remineralization affects this mass balance, and the specific nature of metal-OC coupling.

To address this issue, we have undertaken a field-based experiment to test the hypothesis that porewater redox conditions are recorded in the concentrations of trace metals bound in the sedimentary organic carbon (OC) pool. We have collected sediments from several modern anoxic/euxinic basins: the central Black Sea, the Orca Basin, the Cariaco Basin, and Effingham Inlet, Vancouver Island. These environments vary in terms of bottomwater oxygen and dissolved sulfide concentrations, and the sediments are characterized by variations in organic carbon and iron sulfide geochemistry. Preliminary results suggest that variations in the ratios of aromatic to saturate hydrocarbons may control the speciation of metals in the organic carbon pool, with differences that correlate to bottomwater redox. The results of this study will further refine conceptual models that govern the use of trace metals and organic carbon as paleoceanographic proxies and provide an initial framework to assess the potential use of specific organo-metallic biomarkers (such as metallo-proteins) in paleoenvironmental studies.