Composition of oceans

Table 25-11. an average composition of ocean water (disregarding DISSOLVED GASES)... 

Figure 6. Summary of existing Mo isotope data from natural samples. Isotopic composition of ocean source is based on mass balance (see text). Data are presented as 5 Mo and S Mo relative to the Rochester JMC standard (5 Mo 2/3 x 5 Mo). References ( ) McManus et al. 2002 (t) Siebert et al. 2003 ( ) Barling et ah 2001 ( ) Arnold et al. 2004. Molybdenite values of Wieser and DeLaeter (2003) are omitted because of standard normalization problems (see text). Data obtained by different research groups using different standards are cross-calibrated by comparing seawater 5 values.

Lithium is a conservative element in the ocean with a residence time of abont one million year. Its isotope composition is maintained by inputs of dissolved Li from rivers (average 5 Li + 23%c, Huh et al. 1998) and high-temperature hydrothermal fluids at ocean ridges at one hand and low temperature removal of Li into oceanic basalts and marine sediments at the other. Any variance in these sources and sinks thus should cause secular variations in the isotope composition of oceanic Li. And indeed in a first attempt Hoefs and Sywall (1997) interpreted Li isotope variations in well preserved carbonate shells as indicating secular variations of the oceanic Li-cycle. 

The lowest observed 5 B-values of around -30%c are for certain tonrma-lines (Chaussidon and Albarede 1992) and some non-marine evaporite seqnences (Swihart et al. 1986), whereas the most enriched HB-reservoir is given by brines from Australia and Israel (Dead Sea) which have 5 B-valnes of np to 60%c (Vengosh et al. 1991a, b). A very characteristic feature of boron geochemistry is the isotopic composition of ocean water with a constant 5 B-valne of 39.5%c (Spivack and Edmond 1987), which is abont 50%c heavier than average continental crnst value of -10 2%o (Chaussidon and Albarede 1992). Isotope variations of boron in some geological reservoirs are shown in Fig. 2.7. 

Thus, separate levels of the oceanic crust are simultaneously enriched and depleted in relative to normal mantle values because of reaction with sea water at different temperatures. Muehlenbachs and Clayton (1976) and Gregory and Taylor (1981) concluded that the 0 enrichments are balanced by the depletions which acts like a buffer for the oxygen isotope composition of ocean water. 

The isotopic composition of ocean water has been discussed in detail by Craig and Gordon (1965), and Broecker (1974). It is governed by fractionation during evaporation and sea-ice formation and by the isotope content of precipitation and runoff entering the ocean. 

Another important question concerning the isotopic composition of ocean water is how constant its isotopic composition has been throughout geological history. This remains an area of ongoing controversy in stable isotope geochemistry (see... 

The present ocean is depleted in by at least 6%c relative to the total reservoir of oxygen in the crust and mantle. Muehlenbachs and Clayton (1976) presented a model in which the isotopic composition of ocean water is held constant by two different processes (1) low temperature weathering of oceanic crust which depletes... 

Besides temperature, a variable isotopic composition of the ocean is another factor responsible for variations in foraminifera. A crucial control is salinity ocean waters with salinities greater than 3.5% have a higher 0 content, because 0 is preferentially depleted in the vapor phase during evaporation, whereas waters with salinities lower than 3.5% have a lower content due to dilution by fresh waters, especially meltwaters. The other factor which causes variations in the isotopic composition of ocean water is the volume of low- 0 ice present on the continents. As water is removed from the ocean during glacial periods, and temporarily stored on the continents as 0-depleted ice, the ratio of the global ocean increases... 

Lloyd MR (1967) Oxygen-18 composition of oceanic sulfate. Science 156 1228-1231 Lloyd MR (1968) Oxygen isotope behavior in the sulfate-water system. J Geophys Res 73 6099-6110... 

Rees CE, Jenkins WJ, Monster J (1978) The sulphur isotopic composition of ocean water sulphate. Geochim Cosmochim Acta 42 377-381... 

Chaussidon, M., Jambon, A. (1994) Boron content and isotopic composition of oceanic basalts Geochemical and cosmochemical implications. Earth Planet. Sci. Lett., 121, 277-91. 

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. 

The isotopic composition of water is expressed in comparison to the isotopic composition of ocean water. For this purpose an internationally agreed upon sample of ocean water has been selected, called Standard Mean Ocean Water (SMOW) (Craig, 1961a, 1961b). 

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. P., Sigman, D. M., McCorkle, D. C., Granger, J., Hoffmann, S., Cane, G., and BruneUe, B. G. (2007). The distribution of nitrate N-15/N-14 in marine sediments and the impact of benthic nitrogen loss on the isotopic composition of oceanic nitrate. Geochimica Et Cosmochimica Acta 71(22), 5384-5404. 

Haase K. M. (1996) The relationship between the age of the lithosphere and the composition of oceanic magmas constraints on partial melting, mantle sources and the thermal structure of the plates. Earth Planet. Sci. Lett. 144(1-2), 75-92. 

Figure 8 (a) Nb/U compositions of oceanic basalts through time showing little variation from Archean to the present, (b) Nb/Th compositions of oceanic basalts. Although a range of compositions between primitive mantle and depleted mantle compositions are apparent at all times periods, the data suggest a progressive increase in Nb/Th that has been linked to extraction of the continental cmst (see text), (source Condie, 2003). 

Plank T. and Langmuir C. H. (1992) Effects of the melting regime on the composition of oceanic cmst. J. Geophys. Res. 

Second, the original composition of oceanic crust plays an important role in determining chemical fluxes. Uncertainties in the original composition translate directly into uncertainties of chemical fluxes, and in many cases these differences are the main sources of errors. Both of these features are addressed in the following sections. 

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