CONCLUSIONS

The analysis of stable sulfur isotope ratios in dissolved sulfate from interstitial waters recovered from sediments of the eastern equatorial Pacific and the Peru margin demonstrate that net dissimilatory reduction of sulfate by a deep biosphere takes place to different extents at all investigated sites, except for the open-ocean deep-sea Site 1231. Maximum ³⁴S values are also consistent with both modeled and measured sulfate reduction rates, which show decreasing rates with increases in water depth. Observed sulfur isotope discrimination indicates different degrees of openness of the sediment column with respect to dissolved sulfate. The dominant processes controlling sulfur isotope fractionation in dissolved sulfate are dissimilatory bacterial sulfate reduction and mixing with deep sulfate-rich brines (Fig. F4). Sulfate in these deep brines seems to be derived from seawater that was evaporated and already subjected to microbial sulfate reduction.

Therefore, the results from Leg 201 show that sulfur isotope fractionation in interstitial water sulfate is an extremely powerful tool in the evaluation of sinks, sources, and especially microbiological transformation reactions upon sulfur diagenesis by the deep biosphere in different types of marine sediments.