Iron Oxide-Mediated Polymerization and Anoxic Release of Silicates from Freshwater Lake Sediments

Chemical weathering and biogenic silicate regeneration do not fully explain rapid benthic dissolved silicate (DSi) surges or the presence of polymeric species. This study investigates DSi distribution in freshwater lake pore water to identify alternative supply routes. Crucially, we distinguish this pathway from bio-silicification by demonstrating an abiotic, mineral-surface-induced polymerization pathway. DSi distribution strongly correlates with dissolved Fe and Mn. Under anoxia, the relative increase in dissolved metals contracts the practical gap with dissolved total silicate, supported by systematic shifts in standardized effect sizes (Fe: d = 0.35 to 0.21; Mn: d = 0.76 to 0.64). Furthermore, segmented regression analysis identified a statistically significant structural break at a distinct threshold of 665 µM (p < 0.01), beyond which the silicate accumulation rate sharply accelerates. By pooling datasets across divergent basins—robustly justified by size-independent standardized effect sizes (d = 0.220) and converged common language effect sizes (55–63%)—these findings provide compelling evidence for silicate polymerization mediated by iron oxides within lake sediments. We demonstrate that these internally synthesized polymers are released into the water column during benthic redox transitions, persisting as a stable fraction of the total DSi pool. Consequently, a substantial driver of DSi enrichment in anoxic benthos is the mobilization of silicates previously adsorbed onto metal oxides, a portion of which is sequestered in polymeric form. While these polymers diffuse into the hypolimnion alongside monomers, they remain biologically unavailable, representing a unique geochemical sink that constrains overall silicate bioavailability.