Multiple sulfur isotopes in methane seep carbonates track unsteady sulfur cycling during anaerobic methane oxidation

The anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR) is a major microbially-mediated methane consuming process in marine sediments including methane seeps. The AOM-SR can lead to the formation of methane-derived authigenic carbonates which entrap sulfide minerals (pyrite) and carbonate-associated sulfate (CAS). We studied the sulfur isotope compositions of the pyrite and CAS in seafloor methane-derived authigenic carbonate crust samples from the North Sea and Barents Sea which reflect the time-integrated metabolic activity of the AOM-SR community as well as the physical conditions under which those carbonates are formed. In these samples, pyrite exhibits delta S-34 values ranging from -23.4 parts per thousand to 14.8 parts per thousand and Delta S-33 values between -0.06 parts per thousand and 0.16 parts per thousand, whereas CAS is characterized by delta S-34 values ranging from 26.2 parts per thousand to 61.6 parts per thousand and Delta S-33 mostly between -0.05 parts per thousand and 0.07 parts per thousand. Such CAS sulfur isotope compositions are distinctly lower in delta S-34-Delta S-33 space from published porewater sulfate values from environments where the reduction of sulfate is mostly coupled to sedimentary organic matter oxidation. Mass-balance modelling suggests that (1) AOM-SR appears to cause rapid carbonate precipitation under high methane flux near or at the sediment-water interface and (2) that the precipitation of pyrite and carbonates are not necessarily synchronous. The sulfur isotopic composition of pyrite is interpreted to reflect more variable precipitating conditions of evolving sulfide with porewater connectivity, fluctuating methane fluxes and oxidative sulfur cycle. Taken together, the multiple isotopic compositions of pyrite and sulfate in methane-derived authigenic carbonates indicate protracted precipitation under conditions of non-steady state methane seepage activity. (C) 2019 Elsevier B.V. All rights reserved.