Predicting benthic fluxes of silicic acid from deep‐sea sediments

Abstract

Benthic fluxes of silicic acid are driven by the dissolution of biogenic opal deposited at the seafloor. The fluxes vary over several orders of magnitude and correlate inversely with the mass ratio of lithogenic matter to biogenic opal of the sediments. The role of the lithogenic, or detrital, sediment fraction was studied in batch reactors, by monitoring the release of silicic acid from mixtures of a well‐characterized biosiliceous ooze from the Southern Ocean and model lithogenic materials (kaolinite or basalt). The rate constant of silicic acid production and the saturation silicic acid concentration decreased systematically with increasing detrital to opal mass ratio of the mixtures. Both effects, which are attributed to the release of soluble aluminum from lithogenic mineral phases, were incorporated into an early diagenetic model to calculate diffusive benthic silicic acid fluxes from deep‐sea sediments. The apparent silica dissolution rate constant, the saturation (or asymptotic) pore water silicic acid concentration, and the concentration of biogenic opal were the critical parameters determining the model‐derived benthic fluxes. Predicted benthic fluxes of silicic acid were compared to values reported for deep‐sea sediments with known detrital to opal ratio and asymptotic pore water concentration. When taking into account the dependence of pore water silicic acid build up on the detrital sediment fraction, good agreement was found between predicted and measured benthic fluxes, over a wide range of oceanic areas and sediment composition, from opal rich to opal poor sediments. The model provides a means to estimate the benthic flux of silicic acid for deep‐sea sediments for which the asymptotic silicic acid concentration and the composition of the uppermost sediment are known.