Sulfur isotope composition of ODP Site 801 in the western Pacific Ocean

Alteration of the oceanic crust commonly yields secondary pyrite that is depleted in 34S relative to igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures of biological fractionation, previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration.

Here, we report results for multiple S-isotope (32S, 33S, 34S) compositions of altered basalts at ODP Site 801 in the western Pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ ion-microprobe analyses of secondary pyrite in basement rocks show a large range of delta34S values, between -45 per mil and 1 per mil , whereas bulk rock delta34S analyses yield a more restricted range of -15.8 to 0.9 per mil .