EVALUATION OF ELEMENT/CA RATIOS AS A SCREENING TECHNIQUE TO QUANTIFY THE AMOUNT OF DIAGENETIC CALCITE IN FOSSIL CORALS

Fossil corals are widely used as a proxy for past sea level position. Past changes in sea level, in turn, are useful for understanding the dynamics between changes in temperature, ice sheet volume, and sea level. Aragonitic corals may undergo alteration to calcite via diagenesis. This process complicates accurate isotopic dating of the samples. X-ray diffraction (XRD) is used to determine the extent of diagenesis by determining the presence and amount of calcite.

Here, we examine a possible alternative method for determining polymorphism in coral samples, which involves analyzing elemental ratios of Mg/Ca and Sr/Ca and comparing them to known percentages of calcite. The advantage of this approach is that it is quicker and less intensive than quantitative XRD procedures. The aim is to assess whether a trend exists between the percentage of calcite in a sample and the quantity of Mg and Sr substituted into the crystalline structure. X-ray diffraction was used to determine the percentage of calcite in 20 samples collected in the Seychelles in the west Indian Ocean. Inductively- Coupled Plasma – Mass Spectrometry (ICP-MS) was used to measure element/Ca ratios in each sample. Initial results indicate that multiple trend lines exist in the dataset that correspond to the presence of low-Mg calcite or high-Mg calcite. Thus, in samples that may contain a mixture of the two, element/Ca ratios on their own cannot reliably be used to infer the percent calcite. However, in a marine environment where the corals have not been subaerially exposed, this approach may have more promise.