The last interglacial high sea level in the granitic Seychelles, Indian ocean

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Les îles Gambier et l'atoll de Temöe (Polynésie française) : anciennes lignes de rivage et comportement géodynamique

Cette étude, menée aux îles Gambier et à l'atoll de Temöe, a permis de déterminer, grâce aux analyses pétrographiques et aux datations radiométriques, les variations du niveau marin au cours du Quaternair

Late glacial to post glacial sea levels in the Western Indian Ocean

Late glacial to post glacial sea-level changes provide direct evidence of the progress of melting of large ice sheets during the last deglaciation but, although the correlation between ice and ocean volumes is incontrovertible, the causal link is commonly obscured. Local effects including tectonics, isostatic and hydroisostatic responses and equatorial ocean-syphoning impose additional signals that hide the true picture. A detailed regional study of the Western Indian Ocean based on the analysis of drill cores carried out through modern reefs, in combination with observations and sampling of reef foreslopes, and investigations of outcrops provides a comprehensive data base. Sites from a range of tectonic settings include the microcontinental margins of Madagascar, the granitic Seychelles, and the isolated volcanic islands of Réunion, Mauritius and the Comoros in which the effects of subsidence can be shown to be small. These cover a range of latitudes, and comparisons with adjacent sites on continental margins allow the construction of sea-level curves that closely reflect the eustatic response and disengage this from the effects of other mechanisms. The Mayotte foreslope in the Comoro Islands provides the first coral reef record of sea-level change during the early deglaciation in the Indian Ocean (110–115 m below present sea level between 18,000 and 17,000 yr BP). Two distinctive reef terraces, at 90 and 60 m water depth are dated at 13,600 yr BP and partly attributed to the Younger Dryas period (12,700–11,600 cal yr BP). Reef drowning at around 13,500 yr BP may correspond to Meltwater Pulse 1A, and although there were surges in the rate of sea-level rise, most notably between 11,950 and 11,350 yr BP, there is little evidence to support a well-defined Meltwater Pulse 1B. Reconstructed Holocene sea-level curves are in good agreement and reflect a rapid sea-level rise of about 6 mm yr−1 between 10,000 and 7500 yr BP, followed by a clear inflection around 7500 yr BP when the rate fell to 1.1 mm yr−1. Modern reefs started to grow 8000–9000 years ago. In the post-glacial period the rate of sea-level rise was 1–1.5 mm yr−1 before stabilization at its present level 3000–2500 years ago. Curves for the 10,000–6000 yr−1 BP interval correspond closely with those predicted by theoretical models but lie below these in the subsequent period. In particular, and with the exception of the margins of the Madagascar microcontinent influenced by hydroisostatic processes, they do not reflect predicted higher sea-level stands during the late Holocene

Effects of calcification patterns on the oxygen isotope composition of the skeleton of the scleractinian coral Acropora formosa

International audienceOxygen isotope ratios were measured along the growth axis of branches of the scleractinian coral Acropora formosa collected at 2 and 12 metre depths at Yonge reef (Northern Great Barrier Reef, Australia). Measurements were made between two reference points separated by a distance corresponding to a growth period of six months, from mid-winter to mid-summer. For each of the two reference points, information concerning the environmental parameters controlling the development of the coral colonies was collected in situ. The variability of the isotopic values recorded from coral skeletons grown in identical physical and chemical conditions cannot be ascribed to technical problems, but may rather be induced by calcification mechanisms. In order to define the influence of calcification processes on oxygen isotopic composition, a simple mathematical model is developed, simulating the behaviour of oxygen isotopes. According to Gladfelter's studies (1982, 1983, 1984), we infer that the main factors affecting the isotopic ratio of skeletal aragonite in the scleractinian coral Acropora genus are on the one hand the initial quantity of aragonite deposited at the apical part of the branch and consequently the relative amount of primary and secondary aragonite infilling residual pores during coral growth, and on the other hand, the duration of the secondary aragonite infilling. Comparisons between the measured and the calculated isotopic profiles reveal that differences in calcification processes account for isotopic discrepancies encountered in the different colonies analysed. This study stresses that care must be taken when using the oxygen isotope composition of coral skeleton as a paleoenvironmental proxy

Seasonal variation of primary productivity and skeletal delta13C and delta18O in the zooxanthellate scleractinian coral Acropora formosa

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