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Stable Isotope Profiles of Tridacna maxima as Environmental Indicators

1989; Society for Sedimentary Geology; Volume: 4; Issue: 5 Linguagem: Inglês

10.2307/3514585

1938-5323

Christopher S. Romanek, Ethan L. Grossman,

Isotope Analysis in Ecology

To better understand the relationship between stable isotopic composition (13C/2C, 1801160) and symbiosis, ontogeny, and environment, detailed isotopic profiles were constructed for specimens of the giant clam Tridacna maxima and an encrusting bivalve (Chama cf savignyi) and coral (Montastrea spp.). Specimens were collected alive from depths of 1 m and 9 m within Rose Atoll (14?31' S, 168?1O' W) in the southwestPacific. Shells were sampled sequentially on the external shell layer along radial sections extending from the umbo to the outer shell margin, recording growth throughout the ontogeny of the organisms. Juvenile portions of the tridacnids accurately record seasonal water temperature fluctuations at the atoll. The amplitude of the 8QO profiles in tridacnids is greater in shallow specimens due to greater environmental variability. These shallower specimens have lower 8180 and 813C values compared to the deeper channel mouth specimen, probably because of higher temperatures and the greater influence of 13C-depleted carbon from oxidation of organic matter in sediments and from meteoric water. Ontogeny also influences the isotopic record of Tridacna. Once maturity is reached, growth is suppressed by high temperatures during summer. In addition, 813C values decline. Knowing the time of collection, we can correlate 8180 maxima and minima of different tridacnids, and of a tridacnid and its encrusting Chama. However, the shapes of the peaks often differ, suggesting that correlation of the 8180 profiles of fossil tridacnids would be difficult. No correlation is possible between the 813C records of live tridacnids. The similarity between the carbon isotopic composition of the symbiont-free bivalve Chama and the symbiont-bearing bivalve Tridacna it encrusts suggests that the presence of symbionts has no identifiable effect on the isotopic composition of Tridacna shells.