Eastern Tropical North Pacific

Wakeham, S.G., and E.A. Canuel. 1988. Organic geochemistry of particulate matter in the eastern tropical North Pacific Ocean Implications for particle dynamics. Journal of Marine Research 46 183-213. 

Nameroff TJ, Balistrieri LS, Murray JW (2002) Suboxic trace metal geochemistry in the eastern tropical North Pacific. Geochim Cosmochim Acta 66 1139-1158... 

Depth profiles from the eastern tropical North Pacific (Figure 24.8) show the effects of nitrogen metabolism under 02-deficient conditions. The thermocline is characterized by a sharp decline in O2 concentrations that coincides with increasing nitrate and phosphate concentrations. The oxycline is produced by the respiration of sinking POM under vertically stagnant conditions. Below the oxycline, in depths where O2 concentrations are suboxic, phosphate concentrations continue to increase, but at a slower rate. In contrast, nitrate concentrations decline and reach a mid-water minimum that coincides with a nitrite maximum. The latter is referred to as the secondary nitrite maximum. (At this site the primary nitrite maximum is located at 50 m.)... 

At this site in the eastern tropical North Pacific, denitrification is responsible fiar the midwater loss of nitrate and production of nitrite. The size of the secondary nitrite maximum is dependent on the relative rates of its production from NO3 and its loss via dissimilatory reduction to N2. The amount of nitrate lost to denitrification is shown as the difference between the measured nitrate and the calculated nitrate. The latter was estimated by multiplying the observed phosphate concentrations by the average nitrate-to-phosphate ratio in the three deepest samples (11.9 1.6pmolN/L). Note that the zone of denitrification is restricted to mid-depths, i.e., the depths of the OMZ at this site. 

Ward, B. B., and Zafiriou, O. C. (1988). Nitrification and nitric oxide in the oxygen minimum of the eastern tropical north pacific. Deep Sea Res. 35, 1127-1142. 

Cline, J.D., and Kaplan, I.R. (1975) Isotopic fractionation of dissolved nitrate during denitrification in the eastern tropical North Pacific Ocean. Mar. Chem. 3 271-299. 

Castro, C. G., Chavez, F. P., and CoUins, C. A. (2001). Role of the California undercurrent in the export of denitrified waters from the eastern tropical North Pacific. Global Biogeochem. Cycles 15(4), 819-830. 

Codispoti, L. A., and Richards, F. A. (1976). An analysis of the horizontal regime of denitrification in the eastern tropical North Pacific. Limnol. Oceanogr. 21(3), 379—388. 

N2O profiles from oceanic regions with suboxic zones such as the Arabian Sea and the eastern tropical North Pacific Ocean, which are sites of intense denitrification activities, generally show a two-peak structure (Fig. 2.3) N2O maxima are found at the upper and lower boundaries of the oxygen minimum zone (OMZ), whereas in the core of the suboxic zone, N2O concentrations are considerably depleted (Bange et ah, 2001b Cohen and Gordon, 1978). In anoxic water masses such as found in the central Baltic Sea, the Cariaco Basin, and Saanich Inlet, N2O concentrations are close to the detection limit or not detectable (Brettar and Rheinheimer, 1991 Cohen, 1978 Hashimoto et ah, 1983 Ronner, 1983 Walter et ah, 2006b). 

Figure 2.3 N2O profiles (in nmol L ) from oceanic regions with denitrifying intermediate waters (A) Central Arabian Sea (Bange et at, 2001b) and (B) Eastern tropical North Pacific Ocean (Bange et al., unpublished data). Dashed lines stand for dissolved O2 (concentration in pmol L divided by 10).

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. 

Cohen, Y., and Gordon, L. I. (1978). Nitrous oxide in the oxygen minimum of the eastern tropical North Pacific Evidence for its consumption during denitrification and possible mechanisms for its... 

Yoshinari, T., Altabet, M. A., Naqvi, S. W. A., Codispoti, L., Jayakumar, A., Kuhland, M., and Devol, A. (1997). Nitrogen and oxygen isotopic composition of N2O from suboxic waters of the eastern tropical North Pacific and the Arabian Sea — Measurements by continuous-flow isotope-ratio monitoring. Mar. Chem. 56(3—4), 253—264. 

Figure 5.3 Depth distribution of ammonia oxidation rate from four stations in the Eastern Tropical North Pacific. Data obtained from N-NH4 tracer incubations at simulated in situ light intensities. (FromWard and Zafiriou 1988)...

Sutka, R. L., Ostrom, N. E., Ostrom, P. H., and Phanikumar, M. S. (2004). Stable nitrogen isotope dynamics of dissolved nitrate in a transect from the North Pacific Subtropical Gyre to the Eastern Tropical North Pacific. Geochimica Et Cosmochimica Acta 68, 517—527. 

Figure 6.Z( Dissolved oxygen versus nitrite concentration for the waters of the ODZ (<20 tM O2) the eastern tropical North Pacific (upper, redrawn from Cline and Richards, 1972), the Arabian Sea (center, redrawn after Morrison et ai, 1999), and Saanich Inlet (lower, redrawn from Devol, 1975). Note, axes are different on each plot. Vertical lines denote possible denitrification thresholds for each area.

Figure 6.5 Dissolved oxygen on the 26.5 sigma theta surface. Note the three main pelagic water column ODZs, the eastern tropical North Pacific, the eastern tropical South Pacific and the Arabian Sea. Althouth oxygen levels are very low in the Bay of Bengal, this area does not appear to be a location of water column denitrification (Rao et al., 1994).

The first quantitative estimates of denitrification in the eastern tropical North Pacific were made by Codispoti and Richards (1976). Codispoti and Richards used apparent oxygen utilization, AOU, and phosphate data to stoichiometricaUy extrapolate back to the nitrate concentration present when a given water mass was previously at the surface. In this way they were able to develop a relationship between pNOj and sigma-f for waters of the ETNP-ODZ, from which they calculated nitrate deficit as outlined in Eq. (6.4). 

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 6.15 Profiles of del N of N2 and NO3 through the oxygen deficient water of the eastern tropical North Pacific (left) and Arahian Sea (right). The shaded area represents the oxygen deficient zone. Redrawn from Brandes eta/., 1998.

Figure 6.16 Solutions to a horizontal ventilation model of denitrification for waters of the eastern tropical North Pacific for d NOa vs J, the fraction of nitrate remaining, i.e., the measured nitrate minus the nitrate deficit. Nitrate deficit was calculated as NOa deficit) = M.8 x PO4—NO3, where NO3 and PO4 are the measured concentrations.The two equations describing the steady-state nitrate isotope distrihution are 0[ NO3]/0t—J[ N03] + 0 [ NO3]/0a and0[ NO3]/0t—aQ/[ N03] + ri0 pNO3]/0x where is the eddy diffusion coefficient in the x direction, J is the denitrification rate, a is the fraction factor, and Q is a N N ratio that makes the system non-linear. (See Voss et at, 2001 for solution details). Solutions for three different values of are given ( = [l-0(] x 1000).

Brandhorst, W. (1959). Nitrification and denitrification in the eastern tropical North Pacific. J. Cons, int. Explor. Mer. 25, 3—20. 

Codispoti, L. A. (1973). Denitrification in the Eastern Tropical North Pacific Ocean. Ph.D., Universtiy of Washington, Seattle. 

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