Pacific Ocean isotopic composition

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). 

This simple two component model for the Fe isotope composition of seawater does not consider the effects of the Fe isotope composition of dissolved Fe from rivers or from rain. Although the dissolved Fe fluxes are small (Fig. 19) the dissolved fluxes may have an important control on the overall Fe isotope composition of the oceans if they represent an Fe source that is preferentially added to the hydrogenous Fe budget that is ultimately sequestered into Fe-Mn nodules. In particular riverine components may be very important in the Pacific Ocean where a significant amount of Fe to the oceans can be delivered from rivers that drain oceanic islands (Sholkovitz et al. 1999). An additional uncertainty lies in how Fe from particulate matter is utilized in seawater. For example, does the solubilization of Fe from aerosol particles result in a significant Fe isotope fractionation, and does Fe speciation lead to Fe isotope fractionation ... 

Seawater. A half dozen analyses of Mo isotopes in the Pacific, Atlantic and Indian oceans, covering depths to -3000 m (Barling et al. 2001, Siebert et al. 2003), reveal two important facts. First, there is no detectable 5 Mo variation in the oceans with location or depth. Second, d Mo in the oceans is similar to the heaviest of euxinic sediments, and is heavier than in igneous rocks or ferromanganese sediments by -1.5%o and -2%o, respectively. A uniform isotopic composition in the oceans is consistent with the 10 -1 O " year ocean residence time. The explanation for the heavy isotopic composition is discussed further below. 

Rakestraw NM, Rudd DP, Dole M (1951) Isotopic composition of oxygen in air dissolved in Pacific Ocean water as a function of depth, J Am Chem Soc 73 2976... 

The seasonal cycle of CCN has also been shown to be correlated with that of cloud optical depth in one remote marine area (Boers et al., 1994), and the isotope composition of non-sea salt sulfate over remote regions of the southern Pacific Ocean has been shown to be consistent with a DMS source (Calhoun et al., 1991). 

The effects of anthropogenic sulfur dioxide on the remote marine atmosphere may be evident from rainwater studies by Chukhrov et al. (60) in which the isotopic composition of sulfur in rain was studied systematically at great distances from the continent. Rainwater sulfate ranged from +12.1 to +15.0 0/00 over the Atlantic and from +9.5 to +16.2 0/00 over the Pacific, with a one month average value of +13.3 0/00 for the two oceans. Their study included measurements of rainwater sulfate from a wide variety of continental areas and found that most inland 634S values were significantly lower than those over the oceans. The oceanic rainfall sulfate was most likely a mixture of the isotopically lower continental sulfate and the more enriched marine sulfate. 

Mn crusts grow slowly over millions of years, sequestering high concentrations of Pb and providing a long-term record of such changes (Christensen et al., 1997). The analysis of Fe-Mn crusts for Pb isotopic composition using laser ablation MC-ICP-MS has been employed to produce a record of Pb contributions to the central Pacific Ocean (Christensen et al., 1997). 

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... 

Hemond C., Devey C. W., and Chauvel C. (1994) Source compositions and melting processes in the Society and Austral plumes (South Pacific Ocean) element and isotope (Sr, Nd, Pb, Th) geochemistry. Chem. Geol. 115, 7-45. 

Wang X.-C. and Druffel E. R. M. (2001) Radiocarbon and stable carbon isotope compositions of organic compound classes in sediments from the NE Pacific and Southern Oceans. Mar. Chem. 73, 65-81. 

Lacan E. and Jeandel C. (2001) Tracing Papua New Guinea imprint on the central Equatorial Pacific Ocean using neodymium isotopic compositions and rare earth element patterns. Earth Planet. Sci. Lett. 186(3-4), 497-512. 

Woodhouse O. B., Ravizza G., FaUmer K. K., Statham P. J., and Peucker-Ehrenbrink B. (1999) Osmium in seawater vertical profiles of concentration and isotopic composition in the eastern Pacific Ocean. Earth Planet. Set Lett. 173(3), 223-233. 

Fluctuations that have been measured in the 5 G values of sedimentary organic matter over the Earth s history (e g. Schidlowski 1988) can thus be interpreted in terms of the productivity in the water coltrrrm and the availability of DIG in a particular geological time period. In a study of sediments from the central equatorial Pacific Ocean spanning the last 255,000 years it has been demonstrated that the carbon isotopic composition of fossil organic matter depends on the exchange between atmospheric and oceanic GO. Ghanges with time can then be used to estimate past atmospheric carbon dioxide concentratiorrs (Jasper et al. 1994). 

These observations enabled Hein et al. (2000) to prepare a schematic map of deep-ocean flow in the world ocean showing the characteristic changes in the Nd, Pb and Be isotopic composition of the ferromanganese crasts along the bottom water flow path (Fig. 11.22). Particular interest in this figure lies in comparing this schematic flow path with the actual flow path of Pacific deep water based on measurements of the A C age of the deep bottom waters as presented by Schlosser et al. (2001, Plate 5.8.17 Fig. 11.23). These data confirm that the ocean bottom water flow in the Pacific is counterclockwise with the oldest waters being found in the... 

Barfod DN, Ballentine CJ, Halliday AN, Fitton JG (1999) Noble gases in the Cameroon tine and the He, Ne, and Ar isotopic compositions of high p, (HDVtU) mantle. J Geophys Res 104 29509-29527 Batiza R (1982) Abundances, distribution and sizes of volcanoes in the Pacific Ocean and implications for the origin of non-hotspot volcanoes. Earth Planet Sci Lett 60 195-206 Becker TW, Kellogg JB, O Connell RJ (1999) Thermal constraints on the survival of primitive blobs in the lower mantle. Earth Planet Sci Lett 171 351-365... 

---