Coral barium/calcium record of mid-Holocene upwelling activity in New Caledonia, South-West Pacific

A variety of inimical factors, i.e. sea surface temperature, nutrient input, nature of substrates, were invoked previously to explain the retardation in reef settlement in New Caledonia during the Holocene. In order to clarify the respective role played by temperature and nutrient level in the growth of fringing reefs along the southwest coast of the island, trace element ratios, i.e. Sr/Ca and Ba/Ca, were used, respectively, as a paleothermometer and as a tracer of nutrient load from modern and mid-Holocene (about 6000 calendar years old) massive Porites coral colonies. The results obtained from the modern coral record indicate that, near the outer barrier reef, upwelling activity is controlled by southeast winds blowing in summer. By contrast, along the southwest coastline, the fringing reefs are subjected to barium input mainly derived from weathering of barite-rich rocks cropping out in the adjacent mainland.Comparison between the fossil and modern coral Sr/Ca and Ba/Ca records shed light on three different aspects of the mid-Holocene climate and coral reef evolution in New Caledonia: (1) the variability of paleo-sea surface temperature (SST) seems to have been similar to the modern one, although the island probably experienced SST about 1 °C higher than at present; (2) upwelling activity appears to have been greater than at present. Barium being supplied from the open sea to coastal zones by strong-wind generated upwelling currents, in relation to cooling events; (3) the onset of reef development in the southwest areas of New Caledonia was delayed due to the stronger upwelling of nutrient-enriched waters during the early–middle Holocene. The flowing of upwelled waters over the southwest coastal areas began at 8200 calendar years BP, and ceased at the time when the outer barrier reef crest reached present-day sea level, about 5500 to 5000 calendar years ago.