Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes of pyrite

Abstract

In marine sediments, formation of pyrite (FeS2) is promoted by both organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM), and these two microbial pathways might yield differing patterns of sulfur and nickel isotopic fractionation and trace-element enrichment. To better understand these pathways, we analyzed the geochemistry of authigenic pyrite aggregates from the Upper Quaternary of the western Andaman Sea (International Ocean Discovery Program, Expedition 353, Site U1447A). 34S-enriched pyrites (to ∼+41‰) in Unit Ib are interpreted as products of SD-AOM, and their enrichments in Co and Ni and low δ60Ni values (−0.63‰ to −0.09‰) may represent diagnostic signatures of a reverse-methanogenesis microbial pathway. In contrast, 34S-depleted pyrites (∼–46‰ to –38‰) in both Units I and II are consistent with sulfide formation through early diagenetic remineralization of organic matter, and their relative enrichments in Cu and V and heavier δ60Ni values (to +0.41‰) may be diagnostic of this alternative microbial pathway. This study reports for the first time a fractionation of Ni isotopes linked to preferential uptake of isotopically light Ni by the enzyme Mcr (M reductase enzyme) in reverse methanogenesis and demonstrates that OSR- and SD-AOM-associated pyrites are potentially distinguishable on the basis of their δ60Ni compositions. Such geochemical signatures may prove useful in determining processes of pyrite formation in paleo-depositional systems and in identifying ancient methane emission events.