Seawater ²³⁴U/ ²³⁸U recorded by modern and fossil corals

Abstract

U-series dating of corals is a crucial tool for generating absolute chronologies of Late Quaternary sea-level change and calibrating the radiocarbon timescale. Unfortunately, coralline aragonite is susceptible to post-depositional alteration of its primary geochemistry. One screening technique used to identify unaltered corals relies on the back-calculation of initial ²³⁴U/²³⁸U activity (δ ²³⁴Ui) at the time of coral growth and implicitly assumes that seawater δ ²³⁴U has remained constant during the Late Quaternary. Here, we test this assumption using the most comprehensive compilation to date of coral U-series measurements. Unlike previous compilations, this study normalizes U-series measurements to the same decay constants and corrects for offsets in interlaboratory calibrations, thus reducing systematic biases between reported δ ²³⁴U values. Using this approach, we reassess (a) the value of modern seawater δ ²³⁴U, and (b) the evolution of seawater δ²³⁴U over the last deglaciation. Modern coral δ²³⁴U values (145.0 ± 1.5 ‰) agree with previous measurements of seawater and modern corals only once the data have been normalized. Additionally, fossil corals in the surface ocean display δ²³⁴Ui values that are ∼5 to 7 ‰ lower during the last glacial maximum regardless of site, taxon, or diagenetic setting. We conclude that physical weathering of U-bearing minerals exposed during ice sheet retreat drives the increase in δ²³⁴U observed in the oceans, a mechanism that is consistent with the interpretation of the seawater Pb-isotope signal over the same timescale.