Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

Geochemical proxies measured in the carbonate skeleton of tropical coral Porites sp. have commonly been used to reconstruct sea surface temperature (SST) and more recently seawater pH. Nevertheless, both reconstructed SST and pH depend on the preservation state of the skeleton, here made of aragonite; i.e., diagenetic processes and its related effects should be limited. In this study, we report on the impact of the presence of intra-skeletal calcite on the skeleton geochemistry of a live-collected Porites sp.. The Porites skeleton preservation state was analyzed using X-ray diffraction and scanning electron microscopy. Sr/Ca, Mg/Ca, U/Ca, Ba/Ca, Li/Mg, and B/Ca ratios were measured at a monthly and yearly resolution using solution ICP-MS and multi-collector ICP-MS. The δ11B signatures and the calcite percentages were acquired at a yearly timescale. The coral colony presents two parts, one with less than 3% calcite (referred to as “no-calcite” skeleton), the other one, corresponding to the skeleton formed during the last 4 yrs. of growth, with calcite percentages varying from 13 to 32% (referred to as “with calcite” skeleton). This intra-skeletal calcite replaces partly or completely numerous center of calcification (COCs). All geochemical tracers investigated are significantly impacted by the presence of calcite. The reconstructed SST decreases by about 0.1°C per calcite-percent as inferred from the Sr/Ca ratio. Such impact reaches up to 0.26°C per calcite-percent for temperature deduced from the Li/Mg ratio. So, less than 5% of such intra-skeletal calcite does not prevent SST reconstructions using Sr/Ca ratio, but the percentage and type of calcite have to be determined before fine SST interpretation. Seawater pH reconstruction inferred from boron isotopes drop by about -0.011 pH-unit per calcite-percent. Such sensitivity to calcite presence is particularly dramatic for fine paleo-pH reconstructions. Here we suggest that after being brought to shallow waters following a cyclone, the studied coral was seasonally subjected to rainfall-related water freshening that could have mimicked a vadose environment like can be encountered on raised fossil coral reefs. Nevertheless, the process of calcite precipitation remains to be determined.

Lazareth, C. E., Soares-Pereira C., Douville E., Brahmi C., Dissard D., Cornec F. L., Thil F., Gonzalez-Roubaud C., Caquineau S. & Cabioch G., in press. Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process. Geochimica et Cosmochimica Acta. Article (subscription required).

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