Precision measurement of δ²³⁴U in annually banded tropical corals

Abstract. Tropical corals preserve geochemical ²³⁸U/²³⁴U ratios that provide valuable records of past seawater uranium isotope compositions. Variations in the coral skeletons and thus seawater are likely indicative of freshwater contributions from submerged groundwater discharge and river runoff, or reflecting coral diagenesis. Advances in multi-collector ICP-MS allow precise determinations of ²³⁸U/²³⁴U ratios, enabling the reconstruction of subtle (typically >1 ‰) environmental changes in these marine records. In this study, we evaluate the reliability of coral-based δ²³⁴U records across multiple genera, sampling strategies, and intra-skeletal variability. Analyses of reference material NBS-CRM-112A demonstrate reproducibility within ±0.4 ‰. Replicate sampling across coral structures and colonies indicates measurable intra-band heterogeneity (±0.6 ‰), although local hydrodynamics and submarine groundwater discharge can introduce small inter-colony offsets. No species-dependent isotope fractionation was detected, underscoring the robustness of δ²³⁴U as a geochemical proxy. These findings demonstrate that coral skeletons provide reliable archives of up to sub-annual δ²³⁴U_sw for detecting subtle climatic and hydrological signals.