Isotopic determination of the role and fate of volatile carbon in mid-ocean ridge hydrothermal circulation

Hydrothermal circulation at mid-ocean ridges can be considered to occur in three phases: (1) cold seawater is downwelled in the "recharge zone", (2) fluids interact with a heat source and newly emplaced rocks in the "root zone", and (3) fluids exit the seafloor at hydrothermal vents. Measurement of stable and radiocarbon isotopes of the primary hydrothermal volatiles (CO2, CH4 and H 2) help define the processes that modify the volatile carbon content of seawater during these three stages of hydrothermal circulation. Fluids from Lost City, an off-axis, low-temperature hydrothermal site, reveal an additional source of carbon.; A distinct trend between the stable carbon and radiocarbon isotope compositions of CO2 from Endeavour fluids indicates that seawater bicarbonate is removed during circulation in the recharge zone. Because seawater bicarbonate was previously assumed to be a component of hydrothermal fluid CO2, its removal has considerable impact on the interpretation of past and present hydrothermal CO2 data. Elevated concentrations of isotopically depleted CO2 from Endeavour directly following an eruptive event demonstrate the large magmatic contributions of CO2 to hydrothermal fluids in the root zone. The analysis of deltaD-H2 and deltaD-CH 4 from hydrothermal fluids of diverse origins illuminates the temperature of root zone equilibrium isotopic exchange reactions. The application of hydrogen isotope geothermometers reveals that at black-smoker vents, where temperatures are greater than 275°C, isotopic equilibrium between H2O-H 2 and CH4-H2 is established essentially up to the point of sampling. Intriguing information is stored in the hydrogen isotopes of low-temperature fluids venting at Lost City, indicating reaction temperatures >110°C, subsequent cooling and microbial utilization. Microbial cycling of methane is indicated in the shallow subsurface at 9°50'N East Pacific Rise. Diffuse fluids are ∼10‰ depleted in delta 13CH4 and ∼0.55‰ depleted in delta 13CO2 compared with their source high-temperature fluid. A numerical box model incorporating microbial production and consumption of methane accurately describes the isotope and concentration data. An alternate source of carbon to hydrothermal fluids is observed at Lost City. Methane stable and radiocarbon isotope data indicate that methane and low molecular weight hydrocarbons are produced by Fischer-Tropsch type reactions.