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Understanding past rates of sea-level rise under different climatic conditions allows us to better project the nature of future sea-level rise that will affect coastal water resources. Uranium-thorium dating is a valuable technique that provides robust age constraints on fossil coral-based sea level reconstructions. Unfortunately, coral skeletal material is prone to alteration and must be carefully screened to ensure that the coral’s primary chemistry has been preserved. One commonly used screening method involves the initial 234U/238U activity ratio from when the coral was alive, which is representative of seawater 234U/238U in the waters where the coral grew. Due to the relatively long residence time of uranium in the ocean (~400,000) years, seawater 234U/238U is often assumed to be constant since the late Pleistocene, and coral U-Th ages whose initial 234U/238U varies significantly from the composition of modern seawater are considered altered and, therefore, rejected. Several recent studies, however, have demonstrated that 234U/238U may vary on glacial-interglacial timescales. Various hypotheses have been proposed, from changes in physical weathering rates to oxidation of excess 234U in coastal sediments during sea-level lowstands. Nevertheless, the cause of this variability is subject to debate.
Our study draws upon a compilation of coral U-series data to better constrain observed variability in coral initial 234U/238U. Observations from the coral record will be evaluated using a two-box model of ocean circulation to determine if changes to the ocean’s uranium isotope budget can explain changes to seawater 234U/238U over the last glacial cycle. It is essential that researchers constrain any natural variability in seawater uranium chemistry. If seawater 234U/238U does, in fact, vary over thousands of years then existing screening criteria may be rejecting unaltered corals. Such interpretations of U-series ages are needed to produce robust chronologies of past sea-level change. |