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Abstract
Dissolved hydrogen (H2) concentrations were measured in aquatic sediments from three locations where the terminal electron-accepting process shifted from sulfate (SO4) reduction to methane (CH4) production. Within the surface SO4 reduction zone H2 concentrations varied between 2 and 8 nM. As the dominant H2-consuming process changed to methanogenesis there was a sharp increase in the interstitial dissolved H2 concentration. Within the CH4 production zone, the concentrations of hydrogen ranged from 11 to 29 nM. This relationship of lower dissolved hydrogen concentrations associated with sulfate reduction and higher values associated with methane production is consistent with results from laboratory studies.
There was good agreement in the dissolved hydrogen concentration data within the sulfate reduction and methanogenic zones at all three sites using both in situ samplers and sediment extraction techniques. The measured hydrogen concentrations were slightly higher for field sites than observed in cores returned to the laboratory, yet these data were lower than reported by others. Differences in hydrogen concentrations observed at three field sites as compared to laboratory measurements could be a result of non-steady-state conditions in the natural sediments versus steady-state conditions simulated in the laboratory experiments.