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Abstract

Uranium isotopes (238U/235U) are commonly utilised in marine carbonates to reconstruct the redox evolution of ancient oceans and the atmosphere. However, questions remain concerning U isotope fractionation during carbonate precipitation and/or early sedimentary diagenesis. Moreover, the stromatolitic nature of many Precambrian limestones and dolostones brings further questions about U isotope fractionation during stromatolite formation due to microbial activity. Thus, it is unclear whether stromatolites are reliable archives for the U isotope composition of seawater. Here we measure the 238U/235U ratio of modern stromatolites from the hypersaline Shark Bay, Western Australia, a key study area that represents the largest modern example of stromatolite growth globally. A component-specific approach was utilised to analyse the uppermost crusts of modern stromatolites, older material from the deeper stromatolite laminae and the substrate upon which the stromatolites are forming. Our interpretations of δ238U data are aided by δ234U, δ18O, δ13C and trace element measurements, including some mineralogy and total organic carbon determinations for selected samples.

Modern aragonitic crusts of stromatolites from the subtidal and intertidal zones around Shark Bay exhibited a narrow range of δ238U (−0.30 to −0.33‰), corresponding to a small offset of ca. +0.07‰ from seawater. This range of δ238U values overlaps with that of other primary carbonate precipitates in shallow marine environments such as corals but with a slightly higher mean δ238U. We investigated if this offset may be associated with the coprecipitation of U with aragonite from hypersaline seawater in Shark Bay, which has a higher proportion of dissolved U present as Ca2UO2(CO3)3 compared to open seawater. However, this measured offset is much smaller than that predicted using a speciation-dependent isotope fractionation model. Sub-recent material in the deeper stromatolite laminae exhibited higher and more variable δ238U (up to −0.15‰) compared to the modern aragonitic crusts of stromatolites. The strong inverse correlation between δ238U and the total organic carbon content (r = −0.83) and positive correlation with Mn concentrations (r = 0.91) suggest that 238U was preferentially reduced in the deeper laminae in association with organic carbon remineralisation under reducing conditions. The average δ238U offset of sub-recent stromatolite laminae from modern seawater was +0.13 ± 0.11‰ (1 s.d.), which is slightly lower than, but within uncertainty of, the offset measured for bulk carbonates from the Bahamas (+0.27 ± 0.14‰; 1 s.d.). These results demonstrate that stromatolites are valuable archives of global seawater δ238U, even in hypersaline restricted basins such as Shark Bay, but an offset to account for syndepositional diagenesis is likely required to accurately reconstruct seawater δ238U using ancient stromatolites.