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Diagenesis and Geochemistry of Late Quaternary Porites Corals: Implications for Palaeoenvironmental Reconstructions H.V. McGregor and M.K.Gagan

Diagenesis and Geochemistry of Late Quaternary Porites Corals: Implications for Palaeoenvironmental Reconstructions

H.V. McGregor and M.K.Gagan

Coral proxy records of hydrological balance and sea surface temperature, using Sr/Ca and oxygen-isotope ratios, have become important tools in palaeoclimate reconstruction. However, there have been few systematic investigations of the potential impact of post-depositional alteration of coral skeletons on geochemical tracers commonly used in such reconstructions. In order to address this, we analysed Sr/Ca, oxygen (delta18O), and carbon (delta13C) isotope ratios in sub-aerially exposed, diagenetically altered mid-Holocene Porites corals from Muschu Island, Papua New Guinea. Thin-section analysis of the coral skeletons reveals a predictable sequence of vadose-zone diagenesis, which progresses from initial leaching of coralline aragonite with fine calcite overgrowths, to calcite void filling and calcitic neomorphic replacement of the coral skeleton. Calcite percentages were determined by X-ray diffraction.

The results show that coral Sr/Ca ratios are particularly sensitive to calcite diagenesis (Figure 3). Pure diagenetic calcite yields a very low Sr/Ca ratio of 0.0021 (Figure 3C), reflecting the Sr/Ca ratio of dissolving carbonate phases and the low Sr/Ca partition coefficient for calcite. The delta18O value of 8.1 (Figure 3B) for pure calcite is also much lower than that for coral aragonite precipitated from seawater, due to calcite precipitation from 18O-depleted equatorial rainfall in the vadose zone. Decreases in delta13C values are more variable among secondary calcite samples (1.4 to 10.5, Figure 3A) and reflect the concentration of 13C-depleted organic material in the soil cover adjacent to each location.

 

These results indicate that secondary calcite contamination of fossil coral skeletons will have a much greater impact on palaeotemperature reconstructions derived from coral Sr/Ca, compared to those derived from delta18O (Figure 3D). Using conventional Sr/Ca-temperature relations, coral palaeotemperature reconstructions may be consistently biased toward warmer apparent temperatures at the rate of ~1°C per 1% calcite added to the skeleton. On the other hand, calcite diagenesis does not cause large shifts in apparent temperature based on delta18O. Although diagenesis of coralline aragonite could severely alter palaeoclimate proxies, fine-scale sampling, textural analysis, and geochemical screening techniques can be imposed to ensure that it does not impede the reconstruction of tropical palaeoclimates.