Abstract
Using a leaching protocol designed for the study of U isotopes in recent carbonates, we measured the U isotope composition, both 238U/235U and 234U/238U, 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 δ238U value within ±0.02‰, differences of up to 0.30‰ in the δ238U of individual leachates suggest a control of the coral structure and a more positive 238U/235U ratio in the centers of calcification.
The drill core δ238U data shows that the 238U/235U 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 δ238U ∼0.50–0.60‰ heavier than seawater) and pelagic sediments (with seawater-like δ238U values), (2) authigenic U enrichment via pore-water circulation 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 δ238U values of shallow-water carbonates offset relative to that of seawater by ΔCarbonates-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 238U/235U ratio of carbonate platform sediments directly records the seawater 238U/235U ratio would underestimate the extent of ocean-seafloor anoxia by at least a factor 10. The rapid fluctuations in δ238U values due to sea-level changes (i) is a factor that should be considered before interpreting δ238U variations as reflecting changes in oceanic paleoredox conditions and (ii) reinforces the need for statistically meaningful data sets.
The δ(234U) data suggest that the (234U/238U) 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(234U) mirror sea-level changes during the penultimate glacial-interglacial period (∼140 to ∼200 ka), thus confirming the record of lower δ(234U)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(234U)initial values should be taken into account when screening carbonate sediments U-Th ages on the basis of the initial (234U/238U) ratios of the samples.
