Biological effects on uranium isotope fractionation (238U/235U) in primary biogenic carbonates

A Cenozoic record of seawater uranium in fossil corals

“…The most recent, high precision measurements of coral δ 238/235 U (n=11) yield an average of -0.37 ± 0.02‰ (2σ s.d.) (Chen et al 2018a;Chen et al 2018b;Tissot et al 2018). Modern corals measured in this study (n=5) are consistent with previous measurements with an average of -0.36 ± 0.06‰ (2σ s.d.).…”

“…Five of these fossil samples also deviate from the modern coral average beyond analytical uncertainty based on the reproducibility of the Fe-Mn standard Nod-A-1 (Fig. 3b, (Gothmann et al 2015), the magnitude of the offset in δ 238/235 U for these samples as compared with modern fossil corals is general consistent with the diagenetic offsets observed in modern carbonate sediments (Romaniello et al 2013;Chen et al 2018b;Tissot et al 2018). We note, however, that samples with heavy δ 238/235 U do not have anomalously high U/Ca as compared with samples of similar age (see supplementary figure S4).…”

“…Published measurements of modern coral δ 238/235 U span a range of values: ~ -0.37 to -0.5 ‰ with an average of -0.39 ± 0.06‰ (2σ s.d.) (Stirling et al, 2007;Weyer et al 2008;Tissot and Dauphas, 2015;Chen et al 2018a;Chen et al 2018b;Tissot et al 2018). The most recent, high precision measurements of coral δ 238/235 U (n=11) yield an average of -0.37 ± 0.02‰ (2σ s.d.)…”

“…. Recent work shows that diagenesis of carbonate sediments in the presence of reducing pore fluids can cause an increase in carbonate [U] and a ~0.25 ‰ shift in carbonate δ 238/235 U toward heavier values(Romaniello et al 2013;Chen et al 2018b;Tissot et al 2018) Chen et al (2018b). suggest that recrystallization of biogenic aragonite in seawater or marine pore fluids can also result in a shift toward heavier δ 238/235 U.…”

Examining pelagic carbonate-rich sediments as an archive for authigenic uranium and molybdenum isotopes using reductive cleaning and leaching experiments

“…Extensive work on U has shown that biogenic and non-biogenic platform carbonates can record an isotopic signature close to seawater, but often with a small positive offset that has been attributed either to fractionation during initial calcification or early-diagenetic processes (Andersen et al, 2017;Chen et al, 2018b;Romaniello et al, 2013;Stirling et al, 2007;Tissot et al, 2018;Weyer et al, 2008). To date,  238 U from modern pelagic carbonates sediments have not been reported, although ancient pelagic carbonates have been inferred to represent seawater (Clarkson et al, 2018).…”