Abstract:
It has been demonstrated that in near-shore, shallowly buried sediments hydrogen concentrations are controlled by the thermodynamics of hydrogen-consuming catabolic reactions (Hoehler et al. 1998). It has also been argued that abiogenic hydrogen may fuel deep subsurface microbial communities. During ODP Leg 190 we established and tested procedures for the measurement of hydrogen in deep-sea sediments recovered on the ODP drillship JOIDES Resolution. We analyzed samples collected from five sites in the Nankai Trough accretionary prism with water depths ranging from approximately 1,750 to 4,800 meters. These samples covered a sediment depth range of a few meters below the seafloor (mbsf) to 1,100 mbsf and spanned an estimated temperature range of 3 to 100+ or -20 degrees C. In samples that had in situ temperatures less than 20 degrees C, hydrogen concentrations were determined following the procedures of Lovely and Goodwin (1988) as modified by Hoehler et al. (1998). The method is based on the equilibration of the headspace hydrogen with the dissolved pore fluid hydrogen in intact sediment. In high temperature samples concentrations were determined by bulk extraction into the headspace at in situ temperatures, 100+ or -20 degrees C. Hydrogen concentrations ranged over 3 orders of magnitude. In samples less than 20 degrees C, concentrations were in the low nanomolar range, 1 to 16 nanomolar. Hydrogen concentrations in samples collected with in situ temperatures of 100 + or - 20 degrees C ranged from 3 to 8 micromolar. Preliminary thermodynamic calculations indicate that the free energy of reaction associated with hydrogen trophic methanogenesis fell within a narrow range, -27.5+ or -8.5 kcal/mole, independent of the large concentration difference between low and high temperature samples. These results are consistent with the hypothesis that hydrogen concentrations are driven to a minimum value determined by the free energy required for ATP production as has been argued for near-shore shallowly buried sediments. These results demonstrate that hydrogen concentrations in deeply buried open-ocean sediments with temperatures up to 100+ or -20 degrees C are controlled by the thermodynamics of hydrogen trophic metabolism despite total cell counts being below the detection limit ( approximately 0.0001 cells/cm3).