Lithium isotopes trace silicate weathering-driven authigenic carbonate formation in marine sediments

Abstract

Marine authigenic carbonates are considered a major long-term carbon sink, yet the role of silicate weathering in their formation remains unclear. Here we use lithium isotope compositions of authigenic carbonates from the Gulf of Mexico to trace the coupling between silicate weathering and carbonate authigenesis in marine sediment. In contrast to the positive δ7Licarb values in seep carbonates formed near the seafloor, carbonates originating from organoclastic sulphate reduction in deeper burial settings exhibit negative δ7Licarb values (−6.6‰ to −1.2‰). Calculated δ7Lifluid values (−4‰ to +1.4‰) in pore fluids closely match the δ7Lisilicate values of the silicate component in the rocks, suggesting congruent silicate weathering is approached. A positive correlation between δ7Lifluid and δ13Ccarb values indicates organoclastic sulphate reduction enhances silicate weathering, which together promote carbonate authigenesis in anoxic sediments. Our findings demonstrate that lithium isotopes are a valuable tool for tracing carbon–silicate interactions and reconstructing carbon cycling in anoxic sediments.