Controls of eustasy and diagenesis on the 238U/235U of carbonates and evolution of the seawater (234U/238U) during the last 1.4 Myr
Using a leaching protocol designed for the study of U isotopes in recent carbonates, we measured the U isotope composition, both ^(238)U/^(235)U and ^(234)U/^(238)U, of modern and ancient corals (n = 6), a limestone and a dolostone, as well as 43 shallow-water carbonate sediments from the ODP Leg 166 Site 1009 drill core, on the slope of the Bahamas platform. Although bulk corals record the seawater δ^(238)U value within ±0.02‰, differences of up to 0.30‰ in the δ^(238)U of individual leachates suggest a control of the coral structure and a more positive ^(238)U/^(235)U ratio in the centers of calcification. The drill core δ^(238)U data shows that the ^(238)U/^(235)U ratio of shallow-water carbonates is controlled mainly by (1) variations in sea-level through the mixing of different amounts of platform-derived sediments (with δ^(238)U ∼0.50–0.60‰ heavier than seawater) and pelagic sediments (with seawater-like δ^(238)U values), (2) authigenic U enrichment via pore-watercirculation and U reduction both on the platform and down to ∼5 m below the surface (mbsf) after deposition of the sediment, and, to a lesser extent, by (3) early diagenetic processes (i.e., carbonate dissolution and/or recrystallization) during sediment burial. The global effect of these processes leaves the δ^(238)U values of shallow-water carbonates offset relative to that of seawater by Δ_(Carbonates-SW) = +0.24 ± 0.06‰ (95% CI, including all samples). This shift can be used in seawater paleoredox reconstructions based on carbonates deposited on shallow-water platform, shelf and slope environments (i.e., most of the carbonate sedimentary record prior to the Mesozoic) to account for the average effect of carbonate diagenesis. Assuming that the ^(238)U/^(235)U ratio of carbonate platform sediments directly records the seawater ^(238)U/^(235)U ratio would underestimate the extent of ocean-seafloor anoxia by at least a factor 10. The rapid fluctuations in δ^(238)U values due to sea-level changes (i) is a factor that should be considered before interpreting δ^(238)U variations as reflecting changes in oceanic paleoredox conditions and (ii) reinforces the need for statistically meaningful data sets. The δ(^(234)U) data suggest that the (^(234)U/^(238)U) ratio of the seawater has remained within ∼20‰ of the modern seawater value during the last 1–1.4 Myr. Furthermore, we find that small-scale (1–15‰) variations in seawater δ(^(234)U) mirror sea-level changes during the penultimate glacial-interglacial period (∼140 to ∼200 ka), thus confirming the record of lower δ(^(234)U)_(SW) during periods of low sea-level stand and expanding it to at least the last two glacial-interglacial events (i.e., ∼0.23 Ma). Such fluctuations in δ(^(234)U)_(initial) values should be taken into account when screening carbonate sediments U-Th ages on the basis of the initial (^(234)U/^(238)U) ratios of the samples.