HYDROTHERMAL FLOW-THROUGH THE MARIANA MOUNDS - DISSOLUTION OF AMORPHOUS SILICA AND DEGRADATION OF ORGANIC-MATTER ON A MIDOCEAN RIDGE FLANK

We apply one-dimensional advection-diffusion-reaction equations to int erpret amorphous silica, dissolved silica, organic carbon, and dissolv ed nitrate concentrations from the Mariana Mounds ridge-flank hydrothe rmal system as a means of elucidating the effects of convectively driv en porewater flow on the dissolution of amorphous silica and degradati on of organic carbon in sediments. We develop these mass transfer equa tions into models that predict an increase in the dissolution of amorp hous silica with increasing porewater upwelling velocity, rate of reac tion, and difference between the concentration of dissolved silica in basement from the steady-state dissolved silica concentration in the s ediment column. One prediction of models for the degradation of organi c matter is the depth of oxygen penetration, and thus the depth at whi ch dissolved, reduced chemical species will oxidize and precipitate. T he models can also be used to determine the porewater upwelling veloci ty. Our dissolution and degradation models are applied to two mid-ocea n ridge-flank hydrothermal systems, the Mariana Mounds and the Galapag os Mounds, which have different basement fluid compositions. In the Ma riana Mounds, seawater enters basement through faults and outcrops and the resultant basement fluid has a similar composition to that of bot tom 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 downwe lling 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 amou nts of amorphous silica dissolution and organic matter degradation in ridge-flank hydrothermal systems are greater on an areal basis than th ose that would occur in nonhydrothermal settings. Fluid flow through t he sediments of these hydrothermal systems may alter indicators of pal eoproductivity and impede the formation of cherty deposits, thereby yi elding a potential indicator of ancient ridge-flank hydrothermal syste ms. Ridge-flank hydrothermal systems also may introduce old dissolved organic carbon and large organic molecules to the deep ocean.