Hydrothermal flow through the Mariana Mounds: Dissolution of amorphous silica and degradation of organic matter on a mid-ocean ridge flank

Abstract

We apply one-dimensional advection-diffusion-reaction equations to interpret amorphous silica, dissolved silica, organic carbon, and dissolved nitrate concentrations from the Mariana Mounds ridgeflank hydrothermal system as a means of elucidating the effects of convectively driven porewater flow on the dissolution of amorphous silica and degradation of organic carbon in sediments. We develop these mass transfer equations into models that predict an increase in the dissolution of amorphous silica with increasing porewater upwelling velocity, rate of reaction, and difference between the concentration of dissolved silica in basement from the steady-state dissolved silica concentration in the sediment column. One prediction of models for the degradation of organic matter is the depth of oxygen penetration, and thus the depth at which dissolved, reduced chemical species will oxidize and precipitate. The models can also be used to determine the porewater upwelling velocity. Our dissolution and degradation models are applied to two mid-ocean ridge-flank hydrothermal systems, the Mariana Mounds and the Galapagos Mounds, which have different basement fluid compositions. In the Mariana Mounds, seawater enters basement through faults and outcrops and the resultant basement fluid has a similar composition to that of bottom seawater. As this fluid upwells through the sediment, it enhances the rate of dissolution and degradation. In contrast, seawater enters the basement in the southern section of the Galapagos Mounds by downwelling through the sediment column, resulting in a basement fluid that is saturated with amorphous silica and reducing. As this fluid upwells through the sediment, it limits dissolution and degradation. The amounts of amorphous silica dissolution and organic matter degradation in ridge-flank hydrothermal systems are greater on an areal basis than those that would occur in nonhydrothermal settings. Fluid flow through the sediments of these hydrothermal systems may alter indicators of paleoproductivity and impede the formation of cherty deposits, thereby yielding a potential indicator of ancient ridge-flank hydrothermal systems. Ridge-flank hydrothermal systems also may introduce old dissolved organic carbon and large organic molecules to the deep ocean.