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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]

Although sea‐level is the primary control over sedimentary architectures on continental shelves, deciphering such changes is still challenging, except for the easily recognized transgression‐regression cycles. This problem becomes more prominent in sedimentary units with relatively homogenous and fine‐grained lithologies that have been deposited over short periods. Here, we focus on a dominantly clayey silt section of a sediment core from the outer shelf of the East China Sea. Detailed rock magnetic and electron microscopy analyses indicate that authigenic greigite (Fe3S4) is the major magnetic component in an almost 4‐m‐thick interval that was deposited during the late Marine Isotope Stage (MIS) 3, while pyrite (FeS2) is enriched in the underlying middle MIS 3 sediments. According to microfossil analyses, the identified greigite formed in a prodelta setting that was affected by cold coastal currents after the middle MIS 3, when the Taiwan Warm Current and its associated upwelling were prevailing within a middle‐outer shelf setting favorable for pyrite formation. Large‐amplitude regression occurred subsequently during the MIS 2, limiting sulfate supply and favoring the greigite preservation. Our results, along with the previous studies, demonstrate that both the material and environmental conditions that are crucial to iron sulfide survival have been largely controlled by sea‐level change. Moreover, we make a preliminary proposition that a water depth range of 30–60 m is more likely to lead to the enrichment of greigite on continental shelves. We, therefore, provide a promising avenue for the assessment of sea‐level change on continental shelves over orbital and even suborbital timescales. Key Points: Greigite was identified as the major magnetic mineral in almost 4‐m‐thick late MIS 3 sediments from the outer shelf of the East China SeaAuthigenic greigite and pyrite have great potential to indicate sea‐level change on continental shelves even on suborbital timescalesWater depths of 30–60 m are likely to be more suitable for extensive formation and preservation of greigite in continental shelf sediments [ABSTRACT FROM AUTHOR]