Hydrogen Cycling and Its Relationship to Carbon Remineralization in Anoxic Marine Environments

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Hydrogen is a Key intermediate in the anaerobic diagenesis of organic carbon. Hydrogen partial pressures are important in controlling fermentation pathways, and theoretical arguments suggest hydrogen production rates are high. In addition, competition for hydrogen can be instrumental in determining the relative importance of sulfate reduction and methanogenesis (and possibly iron reduction) in sediments. H2 is also an important precursor for methane in methanogenic sediments (both freshwater and marine). However, relatively few data are available which permit detailed discussion of the relationship between hy>rogen geochemistry and carbon diagenesis in natural environments. Scranton has studied the distribution and cycling of hydrogen and acetate in anaerobic sediments for several years and has found that hydrogen turnover in these systems is very rapid. Production and consumption rates appear to be virtually identical (no net release or uptake of hydrogen) and appear to be closely coupled spatially (occurring in "clusters") in the sediments. Measured rates of "apparent" hydrogen production, however, are much lower than would have been predicted from acetate production rates measured simultaneously, suggesting that artifacts may remain in the measurements of production rates. Scranton proposes to estimate in situ hydrogen production rates using a new approach with very thin layers of sediment, to determine the relationship between a number of fatty acids and hydrogen concentrations in the sediments, and to extend our data set to include the relatively simpler methanogenic systems.