Distinguishing between water column and sedimentary denitrification in the Santa Barbara Basin using the stable isotopes of nitrate

Abstract

[1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O 2 ] $ 3 mM), 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 O suggest that the lowerthan-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.

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

“…Some of these low-N* regions clearly reflect the outward transport of a denitrification (i.e., negative N*) signal from the suboxic zones [Sigman et al, 2003]. However, some of these O 2 -bearing regions of low N* may be driven by sedimentary denitrification, a possibility that our BS study highlights.…”

“…The latter could result from the remineralization of organic matter with a d 15 N lower than that of bottom water or from the interaction of the fractionating oxidation of ammonium and/or nitrite with recently recognized mechanisms of fixed N loss other than denitrification [e.g., Kuypers et al, 2003]. With the additional capability to measure the O isotopes in nitrate, these possibilities can be addressed [Lehmann et al, 2003[Lehmann et al, , 2004Sigman et al, 2003]. For example, if the deep nitrate deficit were generated by denitrification with an isotope effect of 25% but the resultant 15 N enrichment were completely countered by an input of low d 15 N N, the d 18 O would still be elevated by $3.6%, which is clearly not observed.…”

“…In marine sediments, nitrate is often completely consumed within the zone of denitrification, and sedimentary denitrification in a variety of environments has been shown to cause very little net isotope enrichment of oceanic NO 3 À Devol, 1997, 2002;Lehmann et al, 2004;Sigman et al, 2001]. This aspect of N isotope systematics provides a critical constraint on the relative importance of water column versus sedimentary denitrification on a global scale [Brandes and Devol, 2002] and in isolated basins [Sigman et al, 2003]. This constraint is further strengthened by coupled measurement of the 18 O/ 16 O of NO 3 À , which addresses the possibility of multiple reactions with opposing effects on the N isotopes [Lehmann et al, 2004;Sigman et al, 2005].…”

Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes

“…While the δ 15 N of the newly nitrified NO − 3 depends on the δ 15 N of the precursor molecule being nitrified, the O atom is mostly derived from water (with a δ 18 O of ∼ 0 ‰) with significant isotopic fractionation associated with O incorporation during NO − 2 and NH + 4 oxidation (Casciotti, 2002;Buchwald and Casciotti, 2010;. Therefore, any deviation from this 1:1 ratio in the field has been interpreted as evidence that NO − 3 regeneration is co-occurring with NO − 3 consumption (Sigman et al, 2005;Bourbonnais et al, 2009). NO − 2 oxidation is associated with an inverse N isotope effect (Casciotti, 2009), atypical of biogeochemical reactions, and can cause both lower and higher ratios for 18 ε : 15 ε compared to pure NO − 3 assimilation or denitrification, depending on the initial isotopic compositions of the NO − 2 and NO − 3 and the 18 O added back .…”

“…The depletion of NO 4 ) assuming Redfield stoichiometry and the accumulation of biogenic N 2 (when measured) is detected as anomalies in N 2 / Ar relative to saturation with atmosphere (Richards and Benson, 1961;Chang et al, 2010;Bourbonnais et al, 2015). NO (Cline and Kaplan, 1975;Brandes et al, 1998;Voss et al, 2001;Granger et al, 2004Granger et al, , 2008Sigman et al, 2005).…”