Open AccessDistinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate
Author(s) -
Sigman D. M.,
Robinson R.,
Knapp A. N.,
van Geen A.,
McCorkle D. C.,
Brandes J. A.,
Thunell R. C.
Publication year - 2003
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2002gc000384
Subject(s) - nitrate , denitrification , geology , water column , structural basin , sedimentary rock , sediment , fractionation , geochemistry , environmental chemistry , stable isotope ratio , hydrology (agriculture) , oceanography , nitrogen , chemistry , geomorphology , physics , geotechnical engineering , organic chemistry , quantum mechanics
Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O 2 ] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15 N/ 14 N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15 N/ 14 N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18 O/ 16 O suggest that the lower‐than‐expected 15 N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low‐ 15 N/ 14 N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.