Eastern tropical Pacific Ocean, oxygen

Fig. 18-1 Benthic foraminiferal oxygen isotope record from 3477 m water depth in the eastern tropical Pacific ocean from Ocean Drilling Program site 677 (Shackleton et al, 1990). ratios are expressed in the S...

At 20 °C, K = 10 - and so water of pH=8.1 in equilibrium with atmospheric O2 (p02 — 0.21 atm) has pe = 12.5. This conforms to surface conditions, but the pe decreases as the O2 content diminishes with depth. The oxygen minimum is particularly well developed beneath the highly productive surface waters of the eastern tropical Pacific Ocean, where there is a large flux of organic material to depth and subsequently considerable oxidation. The O2 becomes sufficiently depleted i.e., hypoxia) that the resulting low redox conditions causes NOs to be reduced to N02 - Aeolian transport of nitrate to Chesapeake Bay can lead to low O2 conditions. Similarly, intermittent hypoxia develops in parts of the Gulf of Mexico due to the riverine transport of nutrients derived from agricultural uses in the Mississippi catchment. 

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). 

LipschrJtz, F., Wofsy, S. C., Ward, B. B., Codispoti, L. A., Friedrich, G. J. W., and Elkins, J. W. (1990). Bacterial transformations of inorganic nitrogen in the oxygen-deficient waters of the Eastern Tropical South Pacific Ocean. Deep-Sea Research 37, 1513—1541. 

Cline, J. D., and Richards, F. A. (1972). Low oxygen concentrations and nitrate reduction in the eastern tropical North Pacific ocean. Limnol. Oceanogr. 17, 885—900. 

Voss, M., Dippner, J. W., and Montoya, J. (2001). Nitrogen isotope patterns in the oxygen-deficient waters of the eastern Tropical North Pacific ocean. Deep Sea Res. 48, 1905—1921. 

Wakeham S.G. (1987) Steroid geochemistry in the oxygen minimum zone of the eastern tropical North Pacific Ocean. Geochim. Cosmochim.Acta 51, 3051—69. 

Until very recently there were no data available reporting the concentration of Os in sea water. Since 1996, however, there have been several independent studies that focused on this problem making Os the PGE whose marine chemistry has been most extensively studied. Although the recent studies agree that deep water Os concentration is roughly 50-60 fmol kg (Table 2), the vertical distribution of Os in the water column is still open to debate (Figure 3). Results from analyses of samples from the Indian Ocean led to the conclusion that Os behaves conservatively in sea water. A separate study in the Eastern Tropical North Pacific reported a 30% depletion in Os concentration within the core of the oxygen minimum... 

---