The relation between temperature and silica benthic exchange rates and implications for near-seabed formation of diagenetic opal

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types—siliciclastic and carbonate—were used to explain the variability of biogenic silica dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with biogenic opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic silica is hosted in surficial sediments mostly composed of biogenic carbonates. This association is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution (also its reprecipitation as opal-CT) and clay formation; thus strongly reducing the retarding influence of clays on biogenic opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for biogenic opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (< 4 Ma) near seabed in the Antarctic Site 751, the relationship between silica solubility and surface-area variability in time is a more critical control. Since silica solubility and surface area decrease with time, the < 4 Ma elapsed time aged opal-A to the point that changes in specific surface area caused minor effects on solubility, allowing for formation of opal-CT at low temperature settings near the seabed.