Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals

Fossil coral is an excellent archive to extend the radiocarbon ( ¹⁴C) calibration beyond tree-ring records; however, published coral data older than 26,000 years before present are too disparate for practical calibration. We propose an explanation for these discrepancies: trace amounts of secondary calcite and organic matter in fossil corals shift the ¹⁴C ages toward younger ages. In a series of acid-leaching experiments to simulate the standard cleaning procedure prior to ¹⁴C analysis, we measure up to a 300% enrichment of diagenetic calcite in corals due to the solubility differences between calcite and aragonite. In model calculations, we show the ¹⁴C age offsets produced by acid leaching could be hundreds to thousands of years when typical amounts of calcite are present in samples. We demonstrate the necessity and our ability to detect <0.2% calcite in aragonite by X-ray diffraction and apply "<0.2% calcite" as our a priori criterion for our coral samples. The estimated age offsets of samples with <0.2% calcite fall within the reported analytical uncertainty of ¹⁴C dating. In addition, we present ¹⁴C results from an extended hydrogen peroxide (H₂O₂) pretreatment for coral samples to remove organic matter. Our data show that treated samples yield consistent or older ¹⁴C dates compared to non-treated samples. These rigorous but necessary screening and cleaning techniques provide precise, reproducible and accurate radiocarbon calibration data.