A Cenozoic record of seawater uranium in fossil corals

Abstract

We measured U/Ca ratios, 4 He concentrations, 234 U/ 238 U, and 238 U/ 235 U in a subset of well-preserved aragonitic scleractinian fossil corals previously described by Gothmann et al. (2015). Comparisons of measured fossil coral He/U ages with the stratigraphic age demonstrate that well-preserved coral aragonite retains most or all of its radiogenic He for 10's of millions of years. Such samples must be largely or entirely free of alteration, including neomorphism. Measurements of 234 U/ 238 U and 238 U/ 235 U further help to characterize the fidelity with which the original U concentration has been preserved. Analyses of fossil coral U/Ca show that the seawater U/Ca ratio rose by a factor of 4-5 between the Early Cenozoic and today. Possible explanations for the observed increase include (1) the stabilization of U in seawater due to an increase in seawater [CO 3 2-], and a resulting increase in UO 2-CO 3 complexation as originally suggested by Broecker (1971); (2) a decrease in the rate of low-temperature hydrothermal alteration from Early Cenozoic to present, leading to a diminished U sink and higher seawater [U]; or (3) a decrease in uranium removal in reducing sediments, again leading to higher seawater [U].

The Role of Diagenesis in Shaping the Geochemistry of the Marine Carbonate Record

“…The potential to alter the proxy diagenetically depends on the U/Ca of the reactive solid relative to the equilibrium U/Ca (6). Relative to modern seawater, the U/Ca of foraminiferal calcite varies between ∼0.003 and ∼0.8 μmol:mol (e.g., Russell et al 1994Russell et al , 2004Yu et al 2008;Keul et al 2013), while aragonitic coral U/Ca varies between 0.04 and 2.2 μmol:mol (e.g., Min et al 1995, Anagnostou et al 2011, Inoue et al 2011, Gothmann et al 2019. In carbonate sediments from the Great Bahama Bank, U/Ca generally ranges from 0.8 to 2.1 μmol:mol (with outliers as high as ∼5, assuming Ca content from pure calcite sediments; Chen et al 2018a, Tissot et al 2018, and authigenic aragonite and high-Mg calcite at submarine vents average ∼0.6 μmol:mol (assuming Ca content of 40 wt.%; Teichert et al 2003).…”

238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal

“…While the U proxy -both for geochronology and paleo-redox studies -has been calibrated in modern seawater and biological archives (Chen et al, 1986;Cheng et al, 2000a;Chen et al, 2018b;Tissot and Dauphas, 2015;Tissot et al, 2018), no study has thoroughly revisited these calibrations with high-precision methods for U isotope analysis. Thus, while the U proxy has been successfully used to track episodes of widespread marine anoxia in the distant past (e.g., Clarkson et al, 2018;Zhang et al, 2018), it has to-date not been utilized with sufficient precision to resolve subtle redox changes in more recent geologic history, i.e., the Cenozoic (Wang et al, 2016;Gothmann et al, 2019;Clarkson et al, 2021). Such applications could be particularly useful in the study of climate evolution in the Quaternary, as high-temporal-resolution biological archives (e.g., deep-sea corals; Robinson et al, 2007) should in theory enable such records to be generated.…”

The uranium isotopic record of shales and carbonates through geologic time

“…The U/Ca distribution coefficient is highly variable and has been estimated based on culture experiments (e.g., Russell et al 1994), co-precipitation experiments of inorganic aragonite and calcite (e.g., Meece and Benninger 1993;DeCarlo et al 2015), studies of natural corals (e.g., Swart and Hubbard 1982;Gothmann et al 2019), measurements of other biogenic carbonates (e.g., Russell et al 1994;Keul et al 2013) and studies of coexisting porewater and carbonate in drill cores (Teichert et al 2003;Maher et al 2006). Although the average distribution coefficient through Earth's history might be expected to vary depending on proportion of aragonite precipitating from seawater, we find no clear correlation between the U concentration of carbonates and the inferred Mg/Ca ratio of coeval seawater (Halevy and Bachan 2017;Hardie 1996Hardie , 2003, suggesting minimal influence of primary carbonate mineralogy on U enrichment in the sedimentary carbonate record (see Fig.…”