Dissolved carbon species in associated diffuse and focused flow hydrothermal vents at the Main Endeavour Field, Juan de Fuca Ridge: Phase equilibria and kinetic constraints

Despite significant advances in the understanding of hydrothermal processes at mid-ocean ridges, the linkage between focused flow and diffusively flowing vent fluids has remained elusive. Here, we report the distribution of dissolved carbon species in fluids issuing from associated low-, moderate-, and hightemperature vent systems, collected in 2005 at the Main Endeavour Field (MEF), Juan de Fuca Ridge. Excess CO_(aq) measured in the moderate-temperature diffuse flow fluids indicates redox disequilibria between CO_(aq) and CO_2(aq), suggesting hydrothermal circulation of short residence time in near-seafloor mixing zones. According to geochemical modeling, the chemical variability of MEF fluids is consistent with a combination of conductive cooling, reaction transport, and subsequent seawater mixing of a single source fluid. The similar distribution of C ₁-C₃ alkanes observed in diffuse and focused flow fluids likely indicates a minimal fingerprint of biological and methanogenic metabolism on organic fluxes, consistent with a possible short-lived hydrothermal near-seafloor circulation. Accordingly, dissolved carbon species in low-temperature vent fluids can serve as geochemical proxies to distinguish the extent of compositional change in subsurface microbial habitats in the MEF hydrothermal fluids and elsewhere. Part of this reflects conditions unique to the near-surface, diffuse flow environment, but the flux of components from deeperseated hydrothermal processes is important as well. In comparison with earlier measurements (1999 and 2000) of high-temperature vent fluids at MEF, the 2005 data set exhibits lower Li/Cl ratios and H_2(aq) concentrations indicative of higher fluid/rock mass ratios and more oxidizing and/or lower-temperature conditions, respectively.