@article {ISI:000299990700001,
	title = {{Subseafloor nitrogen transformations in diffuse hydrothermal vent fluids of the Juan de Fuca Ridge evidenced by the isotopic composition of nitrate and ammonium}},
	journal = {GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS},
	volume = {13},
	year = {2012},
	month = {feb},
	type = {Article},
	abstract = {Little is known about dissolved inorganic nitrogen (DIN) transformations in hydrothermal vent (HV) fluids. Here, we present the first isotopic measurements of nitrate (delta N-15 and delta O-18) and ammonium (delta N-15) 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 delta N-15 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 ({\textless}3 parts per thousand) for net NH4+ consumption suggest an important contribution from gross ammonium regeneration in low-T fluids. Elevation of HV nitrate N-15/N-14 and O-18/O-16 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 ((15)epsilon(k) {\textless} 3 parts per thousand). The low N isotope effects suggest that nitrate assimilation or denitrification occur in bacterial mats, and/or in situ production of low delta N-15 nitrate. A significantly stronger relative increase for nitrate delta O-18 than for delta N-15 was observed at many sites, resulting in marked deviations from the 1:1 relationship for nitrate delta N-15 versus delta O-18 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 N-2 fixation). Our isotope data confirm the role of subsurface microbial communities in modulating hydrothermal fluxes to the deep ocean.},
	keywords = {HOV Alvin (Human Occupied Vehicle), ROV Jason (Remotely Operated Vehicle)},
	issn = {1525-2027},
	doi = {10.1029/2011GC003863},
	author = {Bourbonnais, Annie and Lehmann, Moritz F and Butterfield, David A and Juniper, S Kim}
}
