Substantial incorporation of isotopically heavy reduced U species into marine carbonate sediments

The uranium (U) isotope ratio (238U/235U, reported as 8238U) in marine carbonates is an important proxy for reconstructing changes in past oceanic redox conditions, based on the essential assumption that U in carbonates originates from dissolved U(VI) in seawater. However, diagenetic processes may induce the addition of isotopically heavy U(IV) into the carbonates, complicating the application of the U isotope proxy. So far, the valence states of trace amounts of U and its relation with U isotope composition in marine carbonate sediments have not been directly studied. In this study, we have calibrated an ion-exchange chromatographic method to separate U (IV) from U(VI) in geological carbonate samples (e.g., modern and fossil corals, stalagmites, cold seep carbonates) and quantified their contents by mass spectrometry. Our study confirms that modern coral carbonates are faithful archives of U(VI) sourced from seawater. Surprisingly, drill core samples from a modern coral carbonate platform, which bear no sign of alteration inferred from conventional diagenetic proxies (e.g., Mn/Sr), show significant positive correlation between U(IV) fraction and bulk carbonate 8238U, suggesting U(IV) is present in the marine carbonates and drives 8238U toward higher values than that of seawater. We therefore suggest that coupled valence and isotope analyses of U in marine carbonates could provide critical constraints toward reliable reconstruction of marine redox evolution in the Earth's history.

Automated summary

The uranium isotope ratio in marine carbonates is an important tool for reconstructing past oceanic redox conditions. However, diagenetic processes may complicate this proxy by introducing isotopically heavy uranium into the carbonates. This study quantifies the valence states of trace amounts of uranium in geological carbonate samples and finds that modern coral carbonates faithfully record uranium from seawater. Surprisingly, drill core samples from a modern coral carbonate platform show a significant positive correlation between a specific valence state of uranium and the uranium isotope ratio, suggesting the presence of this valence state in marine carbonates and its influence on the uranium isotope ratio. The study suggests that coupled valence and isotope analyses of uranium in marine carbonates could provide critical constraints for reconstructing marine redox evolution.