Examining the limitations of 238U/235U in marine carbonates as a paleoredox proxy

Variations of 238U/235U in sedimentary carbonate rocks are being explored as a tool for reconstructing oceanic anoxia through time. However, the fidelity of this novel paleoredox proxy relies on characterization of uranium isotope geochemistry via laboratory experimental studies and field work in modern analog environmental settings. This dissertation systematically examines the fidelity of 238U/235U in sedimentary carbonate rocks as a paleoredox proxy focusing on the following issues: (1) U isotope fractionation during U incorporation into primary abiotic and biogenic calcium carbonates; (2) diagenetic effects on U isotope fractionation in modern shallow-water carbonate sediments; (3) the effects of anoxic depositional environments on 238U/235U in carbonate sediments.

Variable and positive shifts of 238U/235U were observed during U uptake by primary abiotic and biotic calcium carbonates, carbonate diagenesis, and anoxic deposition of carbonates. Previous CaCO3 coprecipitation experiments demonstrated a small but measurable U isotope fractionation of ~0.10 ‰ during U(VI) incorporation into abiotic calcium carbonates, with 238U preferentially incorporated into the precipitates (Chen et al., 2016). The magnitude of U isotope fractionation depended on aqueous U speciation, which is controlled by water chemistry, including pH, ionic strength, carbonate, and Ca2+ and Mg2+ concentrations. Based on this speciation-dependent isotope fractionation model, the estimated U isotope fractionation in abiotic calcium carbonates induced by secular changes in seawater chemistry through the Phanerozoic was predicted to be 0.11–0.23 ‰. A smaller and variable U isotope fractionation (0–0.09 ‰) was observed in primary biogenic calcium carbonates, which fractionated U isotopes in the same direction as abiotic calcium carbonates. Early diagenesis of modern shallow-water carbonate sediments from the Bahamas shifted δ238U values to be 0.270.14 ‰ (1 SD) higher than contemporaneous seawater. Also, carbonate sediments deposited under anoxic conditions in a redox-stratified lake—Fayetteville Green Lake, New York, USA— exhibited elevated δ238U values by 0.160.12 ‰ (1 SD) relative to surface water carbonates with significant enrichments in U.

The significant U isotope fractionation observed in these studies suggests the need to correct for the U isotopic offset between carbonate sediments and coeval seawater when using δ238U variations in ancient carbonate rocks to reconstruct changes in ocean anoxia. The U isotope fractionation in abiotic and biogenic primary carbonate precipitates, during carbonate diagenesis, and under anoxic depositional environments provide a preliminary guideline to calibrate 238U/235U in sedimentary carbonate rocks as a paleoredox proxy.