Controls of eustasy and diagenesis on the U-238/U-235 of carbonates and evolution of the seawater (U-234/U-238) 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 U-238/U-235 and U-234/U-238, 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 delta U-238 value within +/- 0.02 parts per thousand, differences of up to 0.30 parts per thousand in the delta U-238 of individual leachates suggest a control of the coral structure and a more positive U-238/U-235 ratio in the centers of calcification. The drill core delta U-238 data shows that the U-238/U-235 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 delta U-238 similar to 0.50-0.60 parts per thousand heavier than seawater) and pelagic sediments (with seawater-like delta U-238 values), (2) authigenic U enrichment via pore-water circulation and U reduction both on the platform and down to similar 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 delta U-238 values of shallow-water carbonates offset relative to that of seawater by Delta(Carbonates-SW) = +0.24 +/- 0.06 parts per thousand (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 U-238/U-235 ratio of carbonate platform sediments directly records the seawater U-238/U-235 ratio would underestimate the extent of ocean-seafloor anoxia by at least a factor 10. The rapid fluctuations in delta U-238 values due to sea-level changes (i) is a factor that should be considered before interpreting delta U-238 variations as reflecting changes in oceanic paleoredox conditions and (ii) reinforces the need for statistically meaningful data sets. The delta(U-234) data suggest that the (U-234/U-238) ratio of the seawater has remained within similar to 20 parts per thousand of the modern seawater value during the last 1-1.4 Myr. Furthermore, we find that small-scale (1-15 parts per thousand) variations in seawater delta(U-234) mirror sea-level changes during the penultimate glacial-interglacial period (similar to 140 to similar to 200 ka), thus confirming the record of lower delta(U-234)(sw) during periods of low sea-level stand and expanding it to at least the last two glacial-interglacial events (i.e., similar to 0.23 Ma). Such fluctuations in delta(U-234)(initial) values should be taken into account when screening carbonate sediments U-Th ages on the basis of the initial (U-234/U-238) ratios of the samples. (C) 2018 Elsevier Ltd. All rights reserved.