National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2012

Subseafloor nitrogen transformations in diffuse hydrothermal vent fluids of the Juan de Fuca Ridge evidenced by the isotopic composition of nitrate and ammonium

Bourbonnais, A., M.F. Lehmann, D.A. Butterfield, and S.K. Juniper

Geochem. Geophys. Geosyst., 13, Q02T01, doi: 10.1029/2011GC003863 (2012)

Little is known about dissolved inorganic nitrogen (DIN) transformations in hydrothermal vent (HV) fluids. Here, we present the first isotopic measurements of nitrate (δ15N and δ18O) and ammonium (δ15N) from three HV fields on the Juan de Fuca ridge (NE-Pacific). The dominant process that drives DIN concentration variations in low-T diffuse fluids is water mass mixing below the seafloor, with no effect on the DIN isotope ratios. Strong inter-site variations in the concentration and δ15N of NH4+ in high-T fluids suggest different subsurface nitrogen (N) sources (deep-sea nitrate versus organic sediments) for hydrothermally discharged ammonium. Low NH4+ community N isotope effects (<3‰) for net NH4+ consumption suggest an important contribution from gross ammonium regeneration in low-T fluids. Elevation of HV nitrate 15N/14N and 18O/16O over deep-sea mean isotope values at some sites, concomitant with decreased nitrate concentrations, indicate assimilatory or dissimilatory nitrate consumption by bacteria in the subsurface, with relatively low community N isotope effects (15εk < 3‰). The low N isotope effects suggest that nitrate assimilation or denitrification occur in bacterial mats, and/or in situ production of low δ15N nitrate. A significantly stronger relative increase for nitrate δ18O than for δ15N was observed at many sites, resulting in marked deviations from the 1:1 relationship for nitrate δ15N versus δ18O that is expected for nitrate reduction in marine settings. Simple box-model calculation show that the observed un-coupling of N and O nitrate isotope ratios is consistent with nitrate regeneration by either nitrite reoxidation and/or partial nitrification of hydrothermal ammonium (possibly originating from N2 fixation). Our isotope data confirm the role of subsurface microbial communities in modulating hydrothermal fluxes to the deep ocean.

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