To assess the chemistry of fluids from the Dorado Outcrop, a cool ridge-flank hydrothermal system, we used a variety of techniques including autonomous, time-series samplers (OsmoSamplers) and discrete (point) sampling using the DSV Alvin. We also collected and analyzed pore fluids from a series of short sediment cores in the vicinity of hydrothermal springs as well as longer gravity cores from the surrounding region. Concentrations of dissolved inorganic carbon are slightly elevated in the hydrothermal fluids relative to bottom seawater, and dissolved organic carbon concentrations are generally consistent with the bottom water value to being perhaps slightly elevated. Samples for the major nutrients show that silicic acid is significantly enriched within the venting fluids relative to the concentration in bottom seawater, whereas dissolved phosphate is depleted within these fluids. The δ13C of the DIC seems to vary with the dissolved P concentration from the discrete samples, with with more depleted δ13C values coinciding with lower P concentrations. Pore fluids from short cores collected using DSV Alvin are highly variable in their major nutrient concentrations. This variability is likely caused by spatial variations in the reactions occurring within the thin sediment package as well as because of variable exchange between the pore fluids and the underlying crustal fluids.
Our results suggest that cool, ridge flank hydrothermal systems are a net source of inorganic carbon to the oceans and that this carbon has a depleted isotope signature. These systems are also a source of silicic acid to the ocean and a net sink for dissolved phosphorus. Based on the results from this study as well as that of collaborating scientists, it appears that the reactions that produce these chemical signatures likely take place both within the crustal aquifer as well as within the overlying sediment package (C.G. Wheat et al., Nature Communications, manuscript in review).