Seawater lithium (Li) isotope records are widely used to reconstruct silicate weathering, ocean-crust interactions, and long-term carbon cycling. Their interpretation, however, depends on a complete understanding of the sources and sinks that governing the marine Li cycle. Seawater Li may also be removed by adsorption onto particles during settling and early burial, forming a potentially important but poorly quantified particle-bound sedimentary sink. At methane seeps, methane-driven early diagenesis may remobilize this adsorbed Li, making the otherwise cryptic reservoir observable. The released fraction can then place a lower-bound constraint on the magnitude of this particle-bound sink.
    Here, we investigate pore-water Li concentrations and isotopic compositions in methane-affected sediments from the Haima seep area of the South China Sea. One core exhibits a pronounced near-surface maximum in dissolved Li, whereas chloride remains nearly constant. This decoupling argues against substantial Li input from deep fluids and instead indicates local release within the early-diagenetic reaction zone. The upward diffusive Li flux at this site is substantially higher than at a nearby background site. Concentration and isotope mass balance yields an effective released endmember of δ⁷Li = 15–18‰, isotopically lighter than seawater and consistent with the remobilization of seawater-derived Li previously retained on exchange sites, particle surfaces, or weakly bound secondary phases.
    The excess return flux therefore provides a lower-bound constraint on seawater Li previously captured by particles during early burial. Scaling the excess return flux using modern sediment accumulation rates yields a burial-equivalent flux of 0.3–3.4 Gmol Li yr^-1, equivalent to approximately 3–36% of modern riverine Li input. These results reveal an overlooked particle-bound pathway that couples seawater Li removal with early-diagenetic recycling. 

Keywords: Lithium isotopes; particle-bound lithium; early diagenesis; marine lithium sink