Abstract

We sampled low temperature (<15 °C) hydrothermal fluids that discharge from the Dorado Outcrop on the eastern flank of the Cocos Ridge. Our sampling techniques included discrete sample collection using DSV Alvin and autonomous time-series samplers deployed using RSV Jason II. The sampled fluids are enriched in dissolved inorganic carbon (DIC) by ∼0.10 mM and have a δ¹³C_DICthat is on average between 0.2 and 0.5‰ lower than the surrounding bottom seawater. Assuming that the measured DIC enrichment is representative of low temperature hydrothermal systems, the magnitude of the DIC source to the ocean would be 1 × 10¹² mol C/y, which is roughly the same magnitude as the high temperature hydrothermal source, but is more than a factor of three smaller than the estimated rate of carbon removal via carbonate precipitation within the ocean crust. Based on an isotope balance of the discharging fluids, which considers added sources of both basalt-derived inorganic and marine-derived organic carbon, the net DIC carbon isotope signature of vent fluids is most consistent with a primary carbon source from seawater (95.9%), plus a component from the weathering of basalt (3.4%) with a δ¹³C value of −6‰, and a component from organic matter degradation (0.7%), with a δ¹³C value of −22‰. This particular balance places the upper limit of organic carbon respiration at ∼0.3 × 10¹² mol C/y; however, if our DIC input estimate is too high, then the isotope balance requires a larger organic carbon component, which is not consistent with the dissolved oxygen and nitrate data. Although low temperature hydrothermal systems are often thought to be important locations for carbonate precipitation, there is little evidence for current carbonate precipitation at Dorado Outcrop. Similar trends in DIC are observed at North Pond, another low temperature (<15 °C) ridge flank hydrothermal system. These data suggest that much of the current ridge flank discharge is a source of DIC to the ocean.