Constrains on the Uranium Isotopic Composition of Seawater and Implications for Coral U/Th Geochronology

Abstract:

Coral U-series geochronology is an important tool for calibrating records of sea level change during the late Quaternary and coral ¹⁴C dates for the radiocarbon timescale. However, coralline aragonite is highly susceptible to diagenesis, and samples must be carefully screened to ensure a specimen is unaltered. One method used to accept or reject U-series ages is the initial coral ²³⁴U/²³⁸U activity ratio, which reflects the ²³⁴U/²³⁸U activity of seawater at the time of coral skeleton formation. Due to the long residence time of uranium in the ocean (~400,000 years), researchers often assume that seawater ²³⁴U/²³⁸U has remained constant throughout the late Pleistocene. Coral specimens whose U-series ages yield an initial ²³⁴U/²³⁸U value that is significantly different than modern seawater are considered altered. Several studies have demonstrated that coral initial ²³⁴U/²³⁸U and, hence, seawater ²³⁴U/²³⁸U may have varied significantly on glacial-interglacial timescales, but the cause of this variability is subject to debate.

To evaluate the pattern and mechanisms of ²³⁴U/²³⁸U variability in seawater over the last glacial cycle, we draw upon a compilation of U-series measurements of shallow and deep water corals to better define the observed variability. Observed trends from the coral record will be assessed using a simple two-box model of the ocean to determine how changes to the ocean’s uranium isotope budget during glacial cycles can explain shifts in seawater ²³⁴U/²³⁸U. An improved understanding the evolution of seawater ²³⁴U/²³⁸U composition will enable more robust interpretations of both closed-system and open-system ages for corals. Such interpretations of U-series ages are essential to the development of robust chronologies for climate and sea level change and for improving the calibration of the radiocarbon timescale.