Untangling the diagenetic history of uranium isotopes in marine carbonates: A case study tracing the δ²³⁸U composition of late Silurian oceans using calcitic brachiopod shells

The U isotope composition of ancient marine carbonates has been used to determine the δ²³⁸U of past oceans based on the premise that the δ²³⁸U signatures of marine carbonates reflect the δ²³⁸U of seawater at the time of deposition. However, recent work from the Bahamas shows that diagenesis alters the δ²³⁸U signatures of bulk carbonates by effectively creating a positive and variable offset from seawater. Therefore, finding a sedimentary archive that reliably records and preserves the δ²³⁸U signal from seawater becomes fundamental. Calcitic brachiopod shells may provide such an archive, as their shell structure can endure the effects of diagenesis and, thus, their δ²³⁸U signatures can record the U isotope composition of the seawater in which they lived. To explore whether calcitic brachiopods can more robustly record seawater δ²³⁸U than bulk carbonates, we analyzed brachiopod shells and their host carbonate matrices from five different stratigraphic levels across the upper Silurian ‘Lau’ Carbon Isotope Excursion (CIE). Using the data from brachiopod shells, our results suggest that seawater δ²³⁸U values were relatively stable and lower than present day (δ²³⁸U = –0.58 ± 0.11‰, 2sd, n = 17), indicative of a more anoxic ocean state across the positive CIE. We also find that carbonate matrices display a distinctive positive δ²³⁸U offset from contemporaneous seawater that varies between beds. Some beds show a ca. +0.30‰ offset, similar to what is described in the Bahamas; whereas others display no offset from the inferred seawater U-isotope composition. We propose the variability of the observed δ²³⁸U offset to be a function of the degree of U(VI) exchange between anoxic porewaters and seawater. A diminished U(VI) exchange would define a closed-system style of diagenesis and mute the net isotope offset expressed between sediments and contemporaneous seawater. Some of the factors affecting the degree of U(VI) exchange include organic matter fluxes into the sediments and diagenetic fluid flow rates, which could vary due to relative sea-level changes. As a result, the observed δ²³⁸U variability of the analyzed carbonate matrix samples could be the result of the environmental changes caused by the sea-level fluctuations described for the studied stratigraphic interval. We conclude that calcitic brachiopod shells provide an important archive for evaluating the δ²³⁸U of ancient oceans and the style of diagenesis of U in marine carbonates.