Authigenic Clays Versus Carbonate Formation as Products of Marine Silicate Weathering in the Input Sequence to the Sumatra Subduction Zone

We use geochemical and petrographic data from anoxic sequences of the Nicobar Fan to document extensive marine silicate weathering (MSiW) in the input sediment of the Sumatra subduction zone and the conditions that result in authigenic minerals originating from this reaction: precipitation of authigenic carbonate—which sequesters carbon—and formation of authigenic clay—which releases CO2. Increase in 87Sr/86Sr in pore fluids from International Ocean Discovery Program Expedition 362 (Site U1480 to 0.71376 and Site U1481 to 0.71296) reveals a radiogenic strontium contribution from alteration of the Himalayan continental sediment that dominates the Nicobar Fan. Peaks in the dissolved strontium isotope data coincide with zones of methane presence, consistent with MSiW reactions driven by CO2 generation during methanogenesis. Later‐stage fan sequences from 24 to 400 mbsf (meters below seafloor) contain only minor carbonate with 87Sr/86Sr ratios that deviate only slightly from the co‐eval seawater values (0.70920–0.70930); geochemical data in this zone point to a contribution of authigenic clay formation. In contrast, microscopy and elemental mapping of the carbonate‐cemented zones in the earliest fan deposits (>780 mbsf) show replacement of feldspars and dense minerals by carbonate, which ranges in volume from a few percent of the grain to near total grain obliteration. This deeper authigenic carbonate is significantly enriched in radiogenic 87Sr (0.71136–0.71328). Thus, MSiW leads to distinct products, likely in response to a weathering‐derived supply of silica in the younger setting versus calcium enrichment via diffusion from oceanic basement in the older sequence. Plain Language Summary Unstable minerals in sediments shed from the continents continue to undergo weathering in the early stages of burial, which is more pronounced in zones where methane generation increases the sediment acidity. Materials derived from dissolution of continent‐derived silicates may appear as newly formed (authigenic) silicates (mostly clay minerals) and also contribute to the nucleation and composition of authigenic carbonates. The authigenic mineral reactions documented here in the input sediments of the Sumatra subduction zone provide constraints on global elemental budgets and the characteristics of materials before they are subducted. Key Points Continent‐derived minerals shed from the Himalayas undergo weathering in anoxic sequences of the Nicobar Fan Co‐occurring peaks of