Nature of sulfidization in shallow shelf sediments: Influence of organoclastic sulfate reduction and anaerobic oxidation of methane

Nature of sulfidization in shallow shelf sediments: Influence of organoclastic sulfate reduction and anaerobic oxidation of methane

Anaerobic oxidation of methane is an important biogeochemical process in marine sediments responsible for methane consumption, significantly influencing the atmospheric methane budget, the marine carbon cycle, and sediment pore fluid chemistry. Sulfate-driven anaerobic oxidation of methane (SO₄^2--AOM) and organoclastic sulfate reduction (OSR) in marine sediments commonly lead to the precipitation of authigenic pyrite with characteristic sulfur isotopic compositions. In the present study, we have investigated the nature of pyrite and C–Fe–S geochemistry in a sediment core collected from a water depth of ~30 m off the West Coast of India, Eastern Arabian Sea, which represents shallow shelf sediments rich in methane and characterized by high carbon sulfur burial rates.  Our goal was to assess the sulfidization patterns to understand the past variation in methane fluxes within these sediments. Porewater geochemical profiles provide evidence for the combined influence of OSR and AOM on the sediment fluid chemistry. The sediment core is characterized by a shallow sulfate-methane transition zone (SMTZ) between 263 and 303 cmbsf. The Chromium reducible sulfur  (CRS) content and sulfur isotopic composition of pyrite (δ³⁴S_CRS) shows high variability throughout the core, with the upper sedimentary layers (from sediment-water interface to 2.7 mbsf) characterized by relatively low CRS content (0.7 to 3.93 wt %) and low δ³⁴S_CRS values (-37.53 to -25.94 ‰ VCDT). This pattern is interpreted to reflect the dominance of OSR in shallow sediments. In the deeper sediment layers (below ~2.7 mbsf), CRS contents (1.9 to 10.2 wt %) are enriched and δ³⁴S_CRS values show an overall trend towards positive values, suggesting that sulfide minerals are primarily linked to SO₄^2--AOM. The enrichment trend in δ³⁴S_CRS values corresponds to zones affected by ΣHS^- diffusion from relict SMTZs. The evidence for paleo-SMTZs, indicated by enriched δ³⁴S_CRS values and the presence of large framboids, framboid clusters, and rod-like aggregates at multiple depths underscores episodic upward methane flux events. Future research should focus on high-resolution geophysical and geochemical investigations to elucidate the mechanisms driving methane migration, sulfidization variability, and their implications for global carbon and sulfur cycling in these coastal marine systems.