ANOMALOUS CH4 AND NH4+ CONCENTRATIONS AT AN UNSEDIMENTED MID-OCEAN-RIDGE HYDROTHERMAL SYSTEM

SINCE the discovery in 1977 of sea-floor hydrothermal systems, the study of the chemistry of the venting fluids has transformed our understanding of the geochemical cycles that influence the composition of sea water and the ocean crust. With few exceptions (Guaymas basin being the most notable), the vent systems studied so far are free of sedimentary influence and the chemistry of the fluids can be explained on the basis of interactions between sea water and basalt. Such fluids typically contain low methane concentrations, ranging from 50 to 120 muM (refs 1-7), and ammonium concentrations less than 10 muM (ref. 8). Here we report CH4 and NH4+ concentrations from the Endeavour segment of the Juan de Fuca Ridge which are many times greater than those measured previously at any unsedimented mid-ocean ridge. The C-13/C-12 ratio of this CH4 is the lowest yet found in any hydrothermal environment, implying an unusual source. We attribute these high CH4 and NH4+ concentrations to the decomposition of sub-sea-floor organic matter associated with sediments buried at an earlier stage of the ridge's evolution. These data illustrate that the organic geochemistry of unsedimented ridges may be more complex than suspected hitherto.