Closing the oceanic lithium budget by continental shelf sediment removal

Abstract

Seawater lithium isotopes (δ⁷Li_SW) are increasingly used as a proxy for continental silicate weathering and marine reverse weathering throughout Earth’s history. However, the mechanisms driving δ⁷Li_SW evolution remain uncertain, partly due to insufficient constraints on modern Li sources and sinks. Among these, sediment diagenesis has long been identified as an important Li sink, yet direct and quantitative evaluations of global authigenic Li sink remains limited. Here, we present porewater data to explore the controls on porewater Li distribution and to directly quantify Li removal and isotopic fractionation in shelf sediments. We find that Li removal associated with marine authigenic clay (MAAC) formation in shelf sediments is not limited by Si(OH)₄ because of its sufficient supply from the dissolution of terrigenous reactive aluminosilicates and biogenic silica. We show that irrigation-enhanced transport, coupled with active Li removal into authigenic clays in shelf sediments, result in a flux of 21±8 G mol year^-1, suggesting that Li removal associated with MAAC formation in shelf sediments is the dominant Li sink from seawater. We further constrain an isotopic fractionation (Δ_SW−MAAC=δ⁷Li_SW-δ⁷Li_MAAC) of 14.7–16.5 ‰ associated with this irrigation-enhanced shelf sediment sink. Our results highlight that authigenic Li removal into shelf sediments, complemented by a smaller contribution from seafloor basalt alteration, can closely balance the oceanic Li sources and have profound implications for understanding authigenic clay formation and Li cycle in the ancient ocean.