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

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 bet -ter 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 860Ni values (-0.63%. to-0.09%.) may represent diagnostic sig-natures 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 remineraliza-tion of organic matter, and their relative enrichments in Cu and V and heavier 860Ni values (to +0.41%.) may be diagnostic of this alternative microbial pathway. This study reports for the first time a fraction-ation 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 860Ni compositions. Such geochemical signatures may prove useful in determining processes of pyrite formation in paleo-depositional systems and in identify-ing ancient methane emission events.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

Geochemical analysis of authigenic pyrite aggregates from the Upper Quaternary of the western Andaman Sea revealed that 34S-enriched pyrites may be produced through sulfate-driven anaerobic oxidation of methane, while pyrites with higher metamorphic degree and low 860Ni values may be formed through early diagenetic remineralization of organic matter. These findings are important for understanding pyrite formation processes in paleo-depositional systems and identifying ancient methane emission events.