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

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 U-234/U-238 activity (delta U-234(i)) at the time of coral growth and implicitly assumes that seawater delta U-234 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 delta U-234 values. Using this approach, we reassess (a) the value of modern seawater delta U-234, and (b) the evolution of seawater delta U-234 over the last deglaciation. Modern coral delta U-234 values (145.0 +/- 1.5 parts per thousand) agree with previous measurements of seawater and modern corals only once the data have been normalized. Additionally, fossil corals in the surface ocean display delta U-234(i) values that are similar to 5-7% 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 delta U-234 observed in the oceans, a mechanism that is consistent with the interpretation of the seawater Pb-isotope signal over the same timescale. (C) 2017 Elsevier Ltd. All rights reserved.