Impact of Intra-Skeletal Calcite on the Preservation of Coral Geochemistry and Implications for Paleoclimate Reconstruction

The geochemistry of tropical coral skeletons is widely used in paleoclimate reconstructions. However, sub-aerially exposed corals may be affected by diagenesis, altering the aragonite skeleton through partial dissolution, or infilling of secondary minerals like calcite. We analyzed the impact of intra-skeletal calcite on the geochemistry (δ¹⁸O, Sr/Ca, Mg/Ca, Li/Mg, Li/Ca, U/Ca, B/Ca, Ba/Ca, and Mn/Ca) of a sub-aerially exposed Porites sp. coral. Each micro-milled coral sample was split into two aliquots for geochemistry and X-ray diffraction (XRD) analysis to quantify the direct impact of calcite on geochemistry. We modified the sample loading technique for XRD to detect low calcite levels (1%–2%; total uncertainty = 0.33%, 2σ) in small samples (∼7.5 mg). Calcite content ranged from 0% to 12.5%, with higher percentages coinciding with larger geochemical offsets. Sr/Ca, Li/Mg, Li/Ca, and δ¹⁸O-derived sea-surface temperature (SST) anomalies per 1% calcite were +0.43°C, +0.24°C, +0.11°C, and +0.008°C, respectively. A 3.6% calcite produces a Sr/Ca-SST signal commensurate with local SST seasonality (∼1.5°C), which we propose as the cut-off level for screening calcite diagenesis in paleo-temperature reconstructions. Inclusion of intra-skeletal calcite decreases B/Ca, Ba/Ca, and U/Ca values, and increases Mg/Ca values, and can therefore impact reconstructions of paleoclimate and the carbonate chemistry of the semi-isolated calcifying fluid in corals. This study emphasizes the importance of quantifying fine-scale calcite diagenesis to identify coral preservation levels and assure robust paleoclimate reconstructions.