Seawater 234U/238U recorded by modern and fossil corals. (1st March 2018)

• Record Type: Journal Article
• Authors: Chutcharavan, Peter M.; Dutton, Andrea; Ellwood, Michael J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 224(2018)
• Page Start: 1
• Page End: 17
• Publication Date: 2018-03-01
• Subjects: U-series -- Corals; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2017.12.017
• Languages: English
• ISSNs: 0016-7037

Abstract

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 234 U/ 238 U activity (δ 234 Ui ) at the time of coral growth and implicitly assumes that seawater δ 234 U 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 234 U values. Using this approach, we reassess (a) the value of modern seawater δ 234 U, and (b) the evolution of seawater δ 234 U over the last deglaciation. Modern coral δ 234 U 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 δ 234 Ui values that are ∼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 δ 234 U observed in the oceans, a mechanism that is consistent with the interpretation of the seawater Pb-isotope signal over the same timescale.