Fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO42- and delta S-34 data from four sites drilled dur ing Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48 degree sN, where basement temperatures reach up to 62 degreesC. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow. which promotes bacterial sulfate reductio n below the sulfate minimum zone at elevated temperatures. Pore fluid delta S-34 data show the highest values (135 parts per thousand) yet found in th e marine environment. The bacterial sulfur isotopic fractionation factor, c c. is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield er roneous values for a by as much as 15 parts per thousand in diffusive and a dvective pore fluid regimes. Model results are consistent with alpha = 1.07 7 +/- 0.007 with no temperature effect over the range 1.8 to 62 degreesC an d no effect of sulfate reduction rate over the range 2 to 10 pmol cm(-3) d( -1). The reason for this large isotopic fractionation is unknown, but one d ifference with previous studies is the very low sulfate reduction rates rec orded, about two orders of magnitude lower than literature values that are in the range of mu mol cm(-3) d(-1) to tens of nmol cm(-3) d(-1). In genera l, the greatest S-34 depletions are associated with the lowest sulfate redu ction rates and vice versa, and it is possible that such extreme fractionat ion is a characteristic of open systems with low sulfate reduction rates. C opyright (C) 2001 Elsevier Science Ltd.