Aragonite remnants in calcite speleothems - to date or not to date?: a case study of speleothems from the Cradle of Humankind, South Africa

The Cradle of Humankind (locally referred to as the Cradle) is associated with the recovery of almost a third of the world’s early pre-human (hominin) fossils from a series of now heavily eroded cave sites. Dating of these hominin and associated faunal fossils has been conducted through U-Pb based chronology of speleothems bounding fossiliferous clastic cave sediments. The dolomitic bedrock of the Cradle prompts the precipitation of aragonite within speleothems. Through varying levels of diagenesis, these aragonite crystals are observed as remnants within calcite. These layers within the speleothems are also rich in U, up to two orders of magnitude more concentrated than surrounding layers, even on a cm scale, and as such have been specifically targeted for U-Pb dating (). However, the suitability of these diagenetic layers and quality of resulting age data has been questioned. Using 44 speleothem samples (flowstone and stalagmite) from 3 caves across the Cradle, we investigate the integrity of the U-Pb chronometer and applicability for future dating and palaeoclimatic and palaeoenvironmental reconstructions. Through petrography and trace element profiles, we identify conservative diagenesis and elevated uranium concentrations in most diagenetic layers with aragonite remnants. This petrographic-geochemical method acts as a screening and selecting tool for speleothems undergoing further analysis. Retention of anti-phase, step-like patterns of trace elements such as Sr and Mg, and high (>1 ug/g) U concentrations support early conservative diagenesis for the majority of the these speleothems and further enables identifying unsuitable samples based on their petrographic fabrics and noisy trace element profiles. Overall, our results suggest that these speleothems have remained geochemically ‘closed systems’, within minimal, likely early, diagenesis, which does not compromise the validity of the U-Pb ages.