Nitrate isotopic composition

Sigman et al. [134] have described a bacterial method for measuring the isotopic composition of seawater nitrate at the natural-abundance level. The method is based on the analysis of nitrous oxide gas (N2O) produced quantitatively from nitrate by denitrifying bacteria. The classical denitrification pathway consists of the stepwise reduction of nitrate (NOp to nitrite (N02), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) ... 

Poth and Focht (1985) reported on a series of experiments with N. europaea in which [ NJnitrite or [ N]nitrate was mixed with NH, and the rates of formation and isotope compositions of nitrite and N2O were followed. The systems were either well oxygenated (shaken) or poorly oxygenated (static). When shaken, no N2O was produced by the cultures. With static cultures, N2O but not N2 was produced in the presence of nitrite but not nitrate, and nitrite was the preferred source of N for N2O. The observed N20/ N20 ratio was 0.25 rather... 

Sebilo, M., Billen, G, Grably, M., and Mariotti, A. (2003) Isotopic composition of nitrate-nitrogen as a marker of riparian and benthic denitrification at the scale of the whole Seine River system. Biogeochemistry 63 35-51. 

Sigman, D.M., Altabet, M.A., Michener, R., McCorkle, D.C., Fry, B., and Holmes, R.M. (1997) Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate An adaptation of the ammonia diffusion method. Mar. Chem. 57, 227-242. 

Brandes, J. A., Devol, A. H., Yoshinari, T., Jayakumar, D. A., and Naqvi, S. W. A. (1998). Isotopic composition of nitrate in the central Arabian Sea and eastern tropical North Pacific A tracer for mixing and nitrogen cycles. Limnol. Oceanogr. 43(7), 1680—1689. 

Knapp, A. N., Sigman, D. M., and Lipschultz, F. (2005). N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic time-series study site. Global Biogeochemical Cycles 19(1), GB1018. 

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... 

Casciotti, K. L., Sigman, D. M., Hastings, M. G., Bohlke, J. K., and Hilkert, A. (2002). Measurement of the oxygen isotopic composition of nitrate seawater and freshwater using the denitirifier method. 

Figure l6.l6 Time series of particulate nitrogen (PN) export and isotopic composition determined from 150-m floating sediment trap collections at Station ALOHA. (Top) PN flux (pmol N m d ). Error bars represent SE of three to six individual trap measurements. (Bottom) Stable nitrogen isotopic composition (c5 N) of trap-collected material (%o vs. air N2). Error bars, where shown, represent SE of two to three individual trap measurements. Dotted lines represent the isotopic compositions of exported PN expected when supported entirely hy nitrate (6.5°/oo) or entirely by dinitrogen gas (0 %o). The solid line indicates a decreasing trend determined hy linear regression. The trend break at the end of 2002 remains to he explained. Updated and revised from Dore et al. (2002). 

Nitrate is the largest pool of combined nitrogen in the ocean, with deep water concentrations around 20 to 30 pmol L in the Atlantic and up to 45 pmol in the Pacific. The isotopic composition of the NOs" pool is affected by a variety of processes that move N in and out of the ocean or its biota (Fig. 29.3), and subsurface N03 acts as a critical isotopic end member for biological production in the upper water column. Of the processes shown in Fig. 29.3, pelagic denitrification and N2-fixation are generally viewed as the major, long-term controls on the size and isotopic composition of the oceanic pool of NOs" (Brandes and Devol, 2002). 

Tanaka, T., and Saino, T. (2002). Modified method for the analysis of nitrogen isotopic composition of oceanic nitrate at low concentration. Oceanogr. 58, 539—546. 

Lehmann, M. F., Sigman, D. M., McCorkle, D. C., BruneUe, B. G., Hoffmann, S., Kienast, M., Cane, G., and Clement, J. (2005). Origin of the deep Bering Sea nitrate deficit Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate flrtxes. 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

It has been shown that atmospheric aerosol nitrate possesses one of the largest mass-independent oxygen isotopic compositions observed in nature... 

As Michalski and Thiemens (2000) have shown, aerosol nitrate possesses an extraordinarily large mass-independent isotopic composition. As for aerosol sulfate, this isotopic signature has already been shown to provide a new means to elucidate source and chemical transformation processes. This has proved to be an important technique by which the nitrate biogeochemical cycle may be understood further. For example, the massive mass-independent isotopic signature observed in Chilean desert nitrates uniquely reveals that these nitrates must be atmospherically derived since all other sources (by measurement) possess mass-dependent isotopic compositions. In addition, these measurements, coupled with contemporary aerosol nitrate measurements reveal that the oxygen isotopic signatures are stable on million year timescales. This is particularly valuable, as this permits measurement of nitrate in polar ice samples to examine paleo-variations in nitrate and in general, chemistry. As... 

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