The Younger Dryas period, about 12 000 years ago, was marked by a sharp cooling event in the Northern Hemisphere. Temperatures there fell by between 2 and 10°C. The East Antarctic in contrast experienced an episode of warming. Data have up to now been insufficient or too inconclusive to enable palaeoclimatologists to track this climatic event in the southern temperate regions and the tropics. An IRD researcher campaign took a 2 m drill core sample from the isle of Espiritu Santo, Vanuatu, found to contain a giant fossil coral of a single species, Diploastrea heliopora, well preserved in a condition of growth. The specimen age was estimated at between 12 449 and 11 719 calendar years, a span covering nearly the entire Younger Dryas. This unique fossil provides clear evidence of the spatial signature this major climatic cooling event left in the tropics.
Mineral skeleton growth of these corals is a steady few millimetres per year over many centuries, which offers a precise record of ancient environmental conditions. Fossil skeleton concentrations in chemical elements such as strontium or oxygen isotopes indicate the sea surface temperature (SST) on which they depended when the corals were alive. Corals of the genus Porites, which which grow by about 1 cm per year, are the type most used as paleothermometers, but the Diploastrea used in this study have the advantage of growing more slowly. Moreover, there is only one species of this marker, Diploastrea heliopora, which eliminates any inter-specific differences, always a source of uncertainty.
The IRD researchers, working with Australian and American colleagues (1), first compared SSTs, data obtained by way of Sr/Ca ratio analyses in the Diploastrea and modern Porites originating respectively from New Caledonia and Indonesia. Similarity of the figures obtained both validated the use of Diploastrea as a palaeothermometer and allowed calibration of the data acquired from this coral in modern times, before applying it as a palaeoclimatic marker on fossil forms from Vanuatu.
Marie Guillaume | alfa
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