ANOMALOUS CH4 AND NH4-OCEAN-RIDGE HYDROTHERMAL SYSTEM( CONCENTRATIONSAT AN UNSEDIMENTED MID)

SINCE the discovery in 1977 of sea-floor hydrothermal systems, the stu dy of the chemistry of the venting fluids has transformed our understa nding of the geochemical cycles that influence the composition of sea water and the ocean crust. With few exceptions (Guaymas basin being th e most notable), the vent systems studied so far are free of sedimenta ry influence and the chemistry of the fluids can be explained on the b asis of interactions between sea water and basalt. Such fluids typical ly contain low methane concentrations, ranging from 50 to 120 muM (ref s 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 pr eviously at any unsedimented mid-ocean ridge. The C-13/C-12 ratio of t his CH4 is the lowest yet found in any hydrothermal environment, imply ing an unusual source. We attribute these high CH4 and NH4+ concentrat ions to the decomposition of sub-sea-floor organic matter associated w ith sediments buried at an earlier stage of the ridge's evolution. The se data illustrate that the organic geochemistry of unsedimented ridge s may be more complex than suspected hitherto.