Rapid Forward and Reverse Weathering Reactions Drive Cryptic Silica and Cation Cycling in Arctic Fjord Sediments

Abstract Early diagenetic forward and reverse weathering reactions play a significant role in controlling alkalinity fluxes and silica, alkali metal and alkaline earth metal cycling in coastal systems. In Kongsfjorden, Svalbard, the inputs of autochthonous biogenic debris (diatomaceous silica) and allochthonous lithogenic material of varying reactivity (dominated by clays, especially illite and chlorite, and primary aluminosilicates, mostly plagioclase) drive complex balances of diagenetic silicate reactions in sediments. The rapid dissolution of reactive silica results in the release of dissolved silica (Si d ) into pore‐waters and sustains elevated benthic Si d fluxes (−0.2 to −0.8 mmol m −2 d −1 ), which are on the upper end of values previously determined for Arctic environments. Increases with depth in pore‐water lithium (Li + ), potassium, magnesium, and barium concentrations within the top centimeters provided evidence for forward weathering of clays quickly upon burial. Due to the prevalent occurrence of forward weathering, the benthic net Li + flux was associated with a light isotope signal. Decreases in pore‐water rubidium concentrations with depth at the near‐glacier station, elevated ratios of the authigenically altered silica to the biogenic silica pool at all sites, and small increases of pore‐water δ 7 Li values with depth showed that reverse weathering also takes place. Anoxic incubation of diatom frustule probes provided further evidence for the neoformation of cation‐rich clays. The superposition of reverse and forward weathering results in cryptic silica and cation cycling that muted net benthic fluxes. In deeper sediments, changes in pore‐water solute patterns indicated an interconnected occurrence of reverse and forward weathering, potentially driven by reactive silica‐limitation.