Volcanic ash alteration triggers active sedimentary lithium cycling: Insights from lithium isotopic compositions of pore fluids and sediments in the Hikurangi subduction zone

Volcanic ash alteration is deemed a crucial diagenetic process that can impact element budgets in the ocean. In this study, we aim to evaluate the role of volcanic ash alteration in controlling sedimentary lithium (Li) cycling through Li isotope analyses of pore fluids and sequentially leached fractions of bulk sediments (i.e., exchangeable Li, carbonate-bound Li, and silicate-bound Li) of three sites drilled along a transect from the incoming to the upper plates of the Hikurangi margin, New Zealand, during IODP Expedition 375. The downcore trends of Li concentration and isotopes in the Quaternary hemipelagic sediments of the three study sites reflect multiple concurrent diagenetic processes, including Li desorption via ion exchange with ammonium, volcanic ash dissolution, and authigenic clay formation. Reaction-transport modeling results yield higher rates of Li release by ion exchange and volcanic ash dissolution than those of Li consumption by authigenic clay formation at both Sites U1518 and U1520, leading to a net increase of Li concentration up to 252 μM and lower δ⁷Li (10.4–23.5 ‰) than seawater. At Site U1519, authigenic clay formation exerts a larger role in regulating sedimentary Li budget than ion exchange and ash dissolution. Notably, the drastic increase in Li concentrations and the decrease in δ⁷Li values at the boundary of Units III/IV of Site U1520 are attributed to the extensive autochthonous volcanic ash alteration, rather than Li transport by fluid flow. Similar Li geochemical profiles have also been observed in other ash-bearing sediments worldwide, e.g., Site 918 drilled in the Irminger Basin, southeast Greenland, Site 1040 in the Costa Rica subduction zone, and Site 808 in the Nankai Trough. Additional reaction-transport simulations of Li data show that volcanic ash dissolution rates outcompete clay authigenesis rates at sites that have experienced rapid sediment deposition (0.05–0.14 cm yr⁻¹); in contrast, clay authigenesis dominated the pore-fluid Li budget at Site 919 where a low sedimentation rate (7.1 × 10^–3 cm yr⁻¹) was estimated. Based on these observations, we surmise that the rapid depositional system with sufficient volcanic ash supply can act as a source of isotopically light Li, in contrast to the general perception that volcanic ash alteration represents a major sink for oceanic Li. This previously unrecognized Li source may be important in the marine Li cycle.