Hydrothermalism in the Tyrrhenian Sea: Inorganic and microbial sulfur cycling as revealed by geochemical and multiple sulfur isotope data

The Palinuro volcanic complex and the Panarea hydrothermal field, both located in the Tyrrhenian Sea (Italy), are associated with island arc magmatism and characterized by polymetallic sulfide mineralization. Dissolved sulfide concentrations, pH, and Eh measured in porewaters at both sites reveal a variable hydrothermal influence on porewater chemistry. Multiple sulfur isotopic measurements for disseminated sulfides (CRS: chromium reducible sulfur) extracted from sediments at Palinuro yielded a broad range in delta S-34 range between -29.8 and + 10.2 parts per thousand. and Delta S-33 values between + 0.015 and + 0.134 parts per thousand. In contrast, sediments at Panarea exhibit a much smaller range in delta S-34(CRS) with less negative values between -11.3 and -1.8 parts per thousand. The sulfur isotope signatures are interpreted to reflect a mixture between hydrothermal and biogenic sulfide, with a more substantial biogenic contribution at Panarea. Multiple sulfur isotope measurements were performed on sulfides and elemental sulfur from drill core material from the Palinuro massive sulfide complex. delta S-34 and Delta S-33 values for pyrite between -32.8 and -1.1 parts per thousand, and between -0.012 to +0.042 parts per thousand. respectively, as well as for elemental sulfur with delta S-34 and Delta S-33 values between -26.7 and -2.1 parts per thousand and between + 0.035 and + 0.109 parts per thousand. respectively, point to a microbial origin for much of the sulfide and elemental sulfur studied. Moreover, data suggest a coupling of bacterial sulfate reduction, sulfide oxidation and sulfur disproportionation. In addition, delta S-34 values for barite between + 25.0 and + 63.6 parts per thousand are also in agreement with high microbial turnover of sulfate at Palinuro. Although a magmatic SO2 contribution towards the formation of the Palinuro massive sulfide complex is very likely, the activity of different sulfur utilizing microorganisms played a fundamental role during its formation. Thus, porewater and multiple sulfur isotope data reveal differences in the hydrothermal activity at Palinuro and Panarea drill sites and underline the importance of microbial communities for the origin of massive sulfide mineralizations in the hydrothermal subsurface. (C) 2010 Elsevier B.V. All rights reserved.