Denitrification zone

Measurement of the enzyme activity of the respiratory electron transport system (ETS activity) has also been used to estimate denitrification rates in the Arabian Sea (Naqvi and Shail a, 1993) and eastern tropical South Pacific (Codispoti and Packard, 1980) denitrification zones. To measure the ETS activity a crude enzyme extract is made by grinding filtered seawater samples in a buffered medium to liberate the enzymes, after which the ETS substrates NADT1+ and NADPT1+ along with tetrazo-hum salt are added as an artificial electron acceptor. Samples are then incubated for a... 

The isotopic fractionation is easily seen in 8 N03 and 8 N2 distributions in the major open-ocean denitrification zones (Altabet et al., 1999 Brandes et al., 1998 Cline and Kaplan, 1975). Typical open ocean values ofsub-euphotic zone nitrate are about 5%o (Lehmann et al., 2005 Sigman et al., 2000 Wu et al., 1997) but within the ODZ they climb to upwards of 15%o. Concomitant with this increase is a decrease in the 8 N2 from about 0.6%o to 0.2%o (Fig. 6.15). The large enrichment of N-N03 and the mirror image decrease in N-N2 is undoubtedly due to fractionation during denitrification. It is also possible to derive a fraction factor, , from the isotope distributions in the ODZ if one makes some assumption about the amount of nitrate that has been removed by denitrification, i.e., the nitrate deficit. For the eastern tropical North Pacific Brandes et al. (1998) assumed a Raleigh fraction mechanism and both open (advection-reaction) and closed (diffusion-reaction) systems to derive fractionation factors from the data, in Fig. 6.15. (Raleigh fractionation 8 N03 = where 8 N03 is the isotopic composition... 

Figure 14.8 Vertical profiles of properties at 19°N, 67°E in the Arabian Sea (all data except for N2 collected onTN039 cruise of US JGOFS on 1-2/10/1994). (A) O2 (circles) and NOs (triangles) (B) N2O (circles) and N02 (triangles) (C) NOs deficit according to Codispoti et al. (2001) (dots connected by the solid line), N according to Gruber and Sarmiento (2002) (small filled triangles connected by the dashed line), and excess N2 calculated from the N2/Ar ratio (larger unconnected symbols - crosses for data collected on two different cruises from this site and triangles for those from other stations also located within the denitrification zone).

Figure 14.11 N2/Ar ratio normalized by equilibrium saturation values within (circles) and outside (crosses) the denitrification zone. Note that stations outside the denitrification zone were located within a broad latitudinal band (29°S-8.5°N) (from Devol eta/., 2006b).

Sedimentary denitrification, in contrast, shows very little expression of the enzymatic isotope effect. Benthic flux experiments have shown that, even with high rates of sedimentary denitrification, minimal increase of <5 Nni ate is observed (Brandes and Devol, 1997, 2002 Lehmann et al., 2004). The common explanation is that nitrate is nearly completely consumed at the locus of sedimentary denitrification. Because no nitrate escapes the denitrification zones, there is no expression of the isotope effect. This view is supported by the observation of extreme enrichment... 

Fig. 6.14 Distributions of (a) 02 (pM) and (b) N02 (pM) in the upper kilometre along the track of TN039 cruise of US JGOFS. The inset in panel (b) shows the station locations and the geographical extent of the Arabian Sea denitrification zone as demarcated by the 0.5 pM N()2 contour (Naqvi, 1991).

Dissolved gases Dissolved Ni in equilibrium with atmospheric Ni at the surface has a d N of 0.6%o. The isotopic composition of dissolved Ni does not vary greatly in ocean profiles, except in zones of denitrification. Production of low-d N Ni in denitrification zones results in measured N2 d N as low as 0.2%o (Figure 8(a)). Since N2 is the main product of denitrification, its d N provides a test of the nitrate-based estimates of the isotope effect for this process. 

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