Pyritization may archive the influence of Sulfate-AOM and bioturbation activities in marine sulfidic shelf sediments,
Author
Sivan, K.
Mazumdar, A.
Peketi, A.
Sudheer, A. K.
Pillutla, S. P. K.
Mishra, Subhashree
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Organoclastic sulfate reduction and anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SO42--AOM) are key processes regulating organic matter degradation and methane (CH4) consumption in marine sediments. We analyzed C-S-Fe, Mo, and U geochemistry in a sediment core collected from the shallow, seasonally hypoxic shelf off the West Coast of India (WCI; Eastern Arabian Sea) at 28.5 mbsl to study the past influence of benthic and AOM activity. Porewater profiles showed high hydrogen sulfide (ΣH2S) concentrations and a shallow sulfate-methane transition zone (SMTZ; 143–303 cmbsf). The molybdenum/Uranium enrichment factor ratio (MoEF/UEF) indicates persistently sulfidic porewaters during sediment deposition, attributed to SO42--AOM. The high-resolution pyrite sulfur isotope (δ34SCRS) record reveals two sulfidization trends (Trend I and II), characterized by isotopic fluctuations. Depleted δ34SCRS values may be attributed to early diagenetic Fe-sulfidization under open system conditions at the sediment-water interface involving benthic reworking during normoxia. In contrast, enriched δ34SCRS values reflect diminished benthic activity under hypoxic conditions and pyrite precipitation from 34S-enriched ΣH2S diffusing upward from the SMTZ. Furthermore, increasing δ34SCRS values (Trend II) below 336.5 cm suggest overprinting of early-stage sulfidization by deep burial sulfidization linked to SMTZ deepening. Therefore, the observed δ34SCRS variability reflects sulfidization resulting from mixing of isotopically distinct pyrite produced under variable benthic activity and vertical CH4 flux. Although focused on the seasonally hypoxic shelf off WCI, the findings of this study provide broader implications for understanding the coupled influences of bioturbation, CH4 flux, and redox variability on sulfur cycling and trace-metal enrichment in shelf sediments globally.
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