Hydrogen distributions in marine sediments1,2

Abstract

Hydrogen concentrations have been measured in sulfate-reducing sediments of a British Columbian fjord and in Skan Bay, Alaska, as well as in the hemipclagic sediments of the eastern north tropical Pacific off the Mexican coast. In the sediments of both the Mexican shelf and Canadian fjord, hydrogen levels were lowest near the surface, increased with depth and reached a maximum pore-water concentration of about 25-35 nM before sulfate was totally depleted. Deeper in the sediments, H, levels decreased again. In Skan Bay sediments, concentrations of pore-water hydrogen increased almost linearly with depth to a maximum level of 60 nM. Measured rates of hydrogen production in Skan Bay sediments varied little with depth (about 200 nM d-l), and in Skan Bay the hydrogen pool had a turnover time of, at most, several hours. Model-predicted net rates of hydrogen production in Skan Bay are not significantly different than zero. These results are consistent with the hypothesis of interspecies hydrogen transfer between hydrogen-producing and -consuming bacterial populations, which predicts low ambient hydrogen concentrations and tight coupling between production and consumption.

Towards constraining H2 concentration in subseafloor sediment: A proposal for combined analysis by two distinct approaches

“…5). The absence of above-LoD m [H 2 ] EXT in all but one sample from organic-lean sediments at Site GeoB 12815 agrees with previous findings (Table 1; Novelli et al, 1987;D'Hondt et al, 2009).…”

“…Except for the deepest site GeoB 12815, the other four sites have the following features in common: the [H 2 ] EXT values are usually below or close to the LoD m in the near-surface sediments, reach a subsurface maximum of 80-240 nmol L À1 , and decrease with depth to below or around the LoD m . A similar downcore distribution of [H 2 ] EXT with maximum values of up to 60 nmol L À1 has been reported by Novelli et al (1987). Along our transect, maximum values of [H 2 ] EXT decline with decreasing TOC contents, but the depths of the subsurface maxima appear to be independent of water depth and TOC content.…”

“…(1) Extraction methods suffer from the presence of background H 2 which gives rise to high blanks and limits of detection (for definition of terms see Section 3.1.1). Novelli et al (1987) reported a blank H 2 level of 10-14 nmol L À1 , whereas the method of D'Hondt et al (2009) had a limit of detection (LoD) of 2-229 nmol L À1 . Given such a blank and LoD, the methods cannot always unambiguously detect thermodynamically controlled low concentrations of H 2 (Table 1), and [H 2 ] EXT in marine sediments can be expected to be largely below the LoD if concentrations were in fact governed by close coupling of microbial syntrophy.…”

“…Theoretically, these two hypotheses can be tested when the headspace equilibration method is combined with direct analysis of pore To date, no method is available for reliable direct determination of H 2 concentrations in pore-water. Most published methods for determining [H 2 ] EXT involve an extraction step in which a sediment slurry is equilibrated with a H 2 -free headspace (e.g., Conrad et al, 1985;Novelli et al, 1987;D'Hondt et al, 2009). This approach avoids incubation, which may generate both biological and chemical artifacts.…”

Acetogenesis from CO2 in an anoxic marine sediment

“…The first two and last two samples of the experiment (when hydrogen concentrations were at steady state) were analyzed by the headspace equilibration technique of Lovley and Goodwin (1988) as modified by Hoehler et al (1994). During the intermediate portion of the experiment (days 16-20), potentially rapid changes in pore-water H 2 concentrations necessitated the use of an alternative methodology based on that of Novelli et al (1987). An aliquot of whole sediment (10 ml) was rapidly dispensed into a nitrogenflushed vial containing 5 ml of 2 M NaOH.…”