Multiple Sulfur Isotopes of Iron Sulfides From Thick Greigite‐Bearing Sediments Indicate Anaerobic Oxidation and Possible Leakages of Coastal Methane.

Magnetic greigite may be a valuable indicator for methane emissions in the geological past, if its formation pathway and diagenetic environment can be unambiguously defined. Here, we investigate sulfur isotopic compositions of iron sulfides and ferrous iron concentrations of thick greigite‐bearing sediments (TGBSs) in the South Yellow Sea, a shallow marginal sea with strong methane emissions. For the first time, isotopically heavy iron sulfides (up to 28.7‰ in δ34S and 0.19‰ in Δ33S) and enrichments of ferrous iron in the TGBSs are observed. We interpret the data as indicating synchronic occurrences of anaerobic oxidation of methane coupled to sulfate and iron reductions, which occur in coastal methanic zones with limited sulfate availability and therefore probably imply leakages of methane. Consequently, we suggest that greigite is a promising geological indicator for tracing methane liberated from coastal sediments, accounting for ∼60% of the currently rising global marine methane budget. Plain Language Summary: Greigite (Fe3S4) is an easily detected ferromagnetic mineral and extensively observed in shallow coastal sediments, where strong methane emissions to the atmosphere prevail. It probably forms in a relatively sulfur‐poor environment associated with the consumption of methane by sulfate reduction. Here, we found a unique sulfur isotope fingerprinting in iron sulfides and enrichment of ferrous iron in these greigite‐bearing sediments, likely defining a methanic condition associated with potential coastal methane leakages. Our study highlights the intrinsic linkage between methane and greigite formation, supports that greigite may be a potential indicator for quantifying the global natural methane budget, and therefore has paleoenvironmental implications for geological global warming events. Key Points: Isotopically heavy sulfides (both δ34S and Δ33S) and enrichment of ferrous iron are synchronically observed in greigite‐bearing sedimentsPyrite with elevated Δ33S in thick greigite‐bearing sediments likely formed under a methanic conditionGreigite is a potentially useful tracer for methane leakages from coastal sediments [ABSTRACT FROM AUTHOR]