Corals isotopic composition

Peninsula, Papua New Guinea (after Edwards et al. 1993). Open elhpses are data for samples collected from outcrop closed elhpses are data for samples collected from drill core. All points plot along the initial 5234 j =150 contom, indicating that all samples have maintained a primary marine uranium isotopic composition, consistent with closed-system behavior. Relatively yonng samples such as these are more likely to satisfy the closed-system assumption whereas older corals snch as those depicted in Figure 13 are not as likely to satisfy this assnmption. 

Foraminifera are also important in marine geochemistry studies, and Li isotopes have been measured in the shells of a variety of these organisms. The first report of this kind (You and Chan 1996) gave data for four Pleistocene samples of H obliquiloculata 5 Li = +19.3 to +23.0 for two glacial period samples and +26.6 to +42.4 for two interglacial samples (data without blank correction). The species effect interpreted by Marriott et al. (2004) in corals has also been suggested for forams. In the study of Rosier et al. (2001), core-top (i.e., Holocene)P. obliquiloculata samples yielded isotopic compositions close to modem seawater (5 Li = +27.8 to +31.1), whereas samples of G. tumida from the same samples had values of up to +50.5. 

Lithium isotope data from carbonate shells of other marine invertebrates have been reported. Hoefs and Sywall (1997) determined isotopic compositions of seven different species of modem bivalves from the North Sea coast. These samples had a relatively small range in 5 Li (+15 to +21), which corresponds well to the seawater-carbonate offset from inorganic calcite and modem corals (Marriott et al. 2004). 

Because of its relatively long mean residence time, ocean water has a constant isotope composition. Corals are about l%c and foraminifera are about 4.5%o lighter than ocean water. Thus significant Mg isotope fractionations occur during biomineralization of carbonate secreting organisms which is larger than for Ca isotopes (see Section 2.11). 

Graphs of rA vs. r89 for several isotopes are shown in Figure 1. It is important to note that all the normalized isotopic ratios are equal to unity when the cesium-137/refractory ratio is unity. This fact implies quite clearly that the samples analyzed are representative of the complete particle population. The linearity of the rA values relative to r137 indicates that there are two and only two isotopic compositions present in the particle population and that the isotopic composition of any arbitrary particle sample is a linear combination of these. In the case of subsurface detonations where a third particle class is observed, the normalized isotopic ratios for the other isotopes are not equal to unity when the cesium-137 ratio is. A minimum conclusion is that for this particular Coral Island... 

Weil S.M., Buddemeir R.W., Smith S.V. and Kroopnick P.M. (1981) The stable isotopic composition of coral skeletons Control by environmental variables. Geochim. Cosmochim. Acta 45, 1147-1153. 

Risk, M. J., and Erdmann, M. V. (2000). Isotopic composition of nitrogen in stomatopod (crustacea) tissues as an indicator of human sewage impacts on Indonesian coral reefs. Mar. Pollut. Bull. 40, 50—58. 

Hemming N. G., Guilderson T. P., and Fairbanks R. G. (1998a) Seasonal variations in the boron isotopic composition of coral a productivity signal Global Biogeochem. Cycles 12(4), 581-586. 

Fig. 2.3.2. Comparison of the range in carbon isotopic composition for hermatypic corals, ahermatypic corals, and marine carbonate precipitated in isotopic equilibrium with the COj" — HCO5 — CO2 system of ambient seawater. All of the corals (111 hermatypes and 59 ahermatypes) were collected from shallow-water, reef surface environments at Heron Island, Australia. The length of the horizontal bars is equivalent to the mean one standard deviation (Weber, 1974).

Fig. 2.3.3. Dispersion of the carbon isotopic composition of the hermatypic scleractinian genera Acropora, Porites, and Montipora, all living in shallow-water, surface-reef environments at 18 different, widely-scattered localities. The length of the horizontal bar corresponds to the mean one standard deviation. Figures in parentheses indicate the number of individual coral specimens of the three genera analyzed (Weber, 1974).

Land, L.S., Lang, J.C. and Barnes, D.J., 1975. Extension rate A primary control on the isotopic composition of West Indian (Jamaican) scleractinian reef coral skeletons. Mar. Biol., 33 221—233. 

Erez, j. 1978. Vital effect on stable-isotope composition seen in foraminifera and coral skeletons. Nature, 273, 199-202. 

Reynaud, S., Hemming, N.G., Juillet-Leclerc. A., Gattu-so. J.P., 2004. Effect of pCO and temperature on the boron isotopic composition of a zooxanthellate coral Acropora sp. Coral Reefs 23 539-546. 

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Rollion-Bard C., Chaussidon M. and France lanord C. (2003) pH control on oxygen isotopic composition of symbiotic corals. Earth Planet. Sci. Lett. 215, 275-288. 

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