Open AccessSubseafloor nitrogen transformations in diffuse hydrothermal vent fluids of the Juan de Fuca Ridge evidenced by the isotopic composition of nitrate and ammonium
Author(s) -
Bourbonnais Annie,
Lehmann Moritz F.,
Butterfield David A.,
Juniper S. Kim
Publication year - 2012
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2011gc003863
Subject(s) - nitrate , ammonium , isotopes of nitrogen , nitrogen , hydrothermal circulation , environmental chemistry , denitrification , stable isotope ratio , geology , mineralogy , chemistry , physics , organic chemistry , quantum mechanics , seismology
Little is known about dissolved inorganic nitrogen (DIN) transformations in hydrothermal vent (HV) fluids. Here, we present the first isotopic measurements of nitrate ( δ 15 N and δ 18 O) and ammonium ( δ 15 N) 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 δ 15 N of NH 4 + in high‐T fluids suggest different subsurface nitrogen (N) sources (deep‐sea nitrate versus organic sediments) for hydrothermally discharged ammonium. Low NH 4 + community N isotope effects (<3‰) for net NH 4 + consumption suggest an important contribution from gross ammonium regeneration in low‐T fluids. Elevation of HV nitrate 15 N/ 14 N and 18 O/ 16 O 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 ε k < 3‰). The low N isotope effects suggest that nitrate assimilation or denitrification occur in bacterial mats, and/or in situ production of low δ 15 N nitrate. A significantly stronger relative increase for nitrate δ 18 O than for δ 15 N was observed at many sites, resulting in marked deviations from the 1:1 relationship for nitrate δ 15 N versus δ 18 O 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.