Authigenic carbonate crust archives a protracted seepage event of deep-seated methane sources

Abstract

Cold-seep carbonates are valuable archives for tracing the temporal evolution of cold seep methane seepage events. Here, we present the first high-resolution, multiproxy record of methane seepage spanning 14.8 ± 1.1 to 3.5 ± 0.1 kyr preserved in a brecciated methane-derived carbonate crust recovered from a 1463 m deep cold seep site, Mannar Basin, on the east coast of India. Petrographic, mineralogical, and geochemical data reveal two distinct aragonite phases formed under contrasting seepage intensity and redox conditions. (i) Micritic aragonite clasts are composed primarily of aragonite (75–90%), with subordinate amounts of high-magnesian calcite and pyrite. (ii) Aragonite cements with >99% aragonite minerals. Strongly depleted δ13C values (−53.9 to −39.4‰ VPDB) of aragonite crust confirm a methane-derived carbon source, while δ13Corg values (−45.4 ± 6.6‰ VPDB) indicate incorporation of methanotrophic biomass input in carbonate matrix. Positive δ18O values (3.4–5.9‰ VPDB) in aragonite crust reflect 18O-enriched fluid source from clay-mineral dehydration generated from deeper overpressure strata and gas hydrate dissociation. We propose a three-stage evolutionary model for carbonate crust formation. (1) Diffusive methane seepage promotes sulfide enrichment and chemosynthetic colonization. (2) Advective methane seepage through chimney system promoted both micritic aragonite precipitation and gas hydrate formation close to the seabed. (3) Arrested methane seepage leading to a shallow hydrate dissociation, brecciation, dissolution and precipitation of aragonite cement. These results provide the first dated methane-seep carbonate archive from the Mannar Basin and reveal a dynamic interplay among methane advection, hydrate dynamics, and carbonate diagenesis along a passive continental margin.