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Temporal and spatial variability of western Indian Ocean reefs: climatic and environmental record

In terms of the 1 to 1000 year time scale, coral growth in the southern part of the studied area (limit of the intertropical climatic belt (eg Réunion island) is slower than that reported from the northern part, closer to the equator (eg Mayotte, Seychelles). Variability of the growth rate in time is probably controlled mainly by local influences like sediment input and water exchange, rather than by large-scale climatic fluctuations. Growth rates show an increasing trend from sheltered locations to more exposed stations with a close connection to the open sea. Low growth rates of colonies in lagoons are probably caused by high sediment concentrations and reduced light intensity.

Despite the problems described in the literature using strontium/calcium ratios as a paleothermometer, our first results show that the strontium/calcium thermometer provides a powerful tool for paleoclimatology. In any case, a calibration to instrumentally recorded datasets is essential, as long as the distortions caused by biological processes are not fully understood.

In terms of the 1000 10,000 year time scale the reconstructed sea level curves inferred from reef growth curves are regarded as providing an acceptable picture of Holocene sea level behaviour throughout the southwestern Indian ocean. These curves reflect a rapid rise followed by a clear inflection when the sea-level rise decreased down to 1.1 mm/year. The stabilization of the sea level to its present level occurred about 3000 years ago and there is no evidence of higher sea level stand. Our results emphasize the need to take into account the state of preservation of coral samples with the advent of greatly enhanced interest in post-glacial climatic reconstruction based on coral records. Assessment of the degree of diagenetic alteration in coral skeletons should be a prerequisite to obtain the optimum reliability of climate information. Misinterpretations may be gained if the total amount of aragonite is the only criterion used as indicative of pristine preservation of coral skeletons; accordingly, suspicion may be aroused on reconstruction of past climates published recently when possible perturbation effects were not clearly identified or, furthermore, when no diagenetic study was conducted on the coral archives used.

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Université de Provence Aix-Marseille I
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