Closing in on carbonate uranium isotopes as a paleo-redox proxy based on continuous records from Late Oligocene to Holocene island carbonates

Abstract

Understanding the fractionation of uranium isotopes (238U/235U, commonly denoted as δ238U) during carbonate diagenesis is crucial for quantitatively reconstructing the variations in global seafloor anoxic extent throughout Earth’s history using marine carbonates. While the existing diagenetic framework is primarily based on the Bahamian carbonate platform, its robustness and applicability need validation in other regions. The reliability of proxies for recognizing U isotope alteration during diagenesis remains inadequate due to the lack of comprehensive investigations into the influence of diagenetic stages, environments, and redox conditions on U isotope fractionation. In this study, island carbonate samples were continuously collected from two deep drill cores (∼600 m XK-1 core and ∼ 1000 m NK-1 core) in the South China Sea, spanning from the Late Oligocene to Holocene (ca. 25 Ma to present). These samples, which underwent intensive meteoric and marine diagenesis, were notably devoid of organic matter, providing an excellent opportunity to validate the Bahamian diagenetic framework and to explore new geochemical tools for recognizing U isotope alterations during diagenesis. The observed significant positive offset from contemporaneous seawater in the South China Sea (0.24 ‰ ± 0.24 ‰, 1SD, n = 152) is similar to that found in the Bahamas (0.24 ‰ ± 0.15 ‰, 1SD), indicating that U isotope fractionation during early diagenesis is likely consistent on a global scale. The δ238U values associated with syndepositional diagenesis in the South China Sea exhibit a uniform offset of 0.25 ‰ ± 0.07 ‰ (1SD), consistent with those observed in the Bahamas (0.26 ‰ ± 0.10 ‰, 1SD), suggesting uranium isotope fractionation during syndepositional diagenesis occurs extensively and consistently across different redox environments in shallow-water carbonates. However, the δ238U values undergo further alteration during post-depositional diagenesis, with these values during post-depositional marine diagenesis being mainly determined by the redox conditions of the diagenetic fluids. Based on the sequential enrichment of vanadium (V)–uranium (U) concentrations, we classified the redox conditions of the diagenetic fluids within marine diagenetic environments into upper suboxic and lower suboxic conditions in the South China Sea. The uranium isotope offset from contemporaneous seawater is nearly negligible under upper suboxic conditions but progressively increases by about 1 ‰ due to the authigenic U(IV) enrichment under lower suboxic conditions. However, the uranium isotope offset (0.18 ‰ ± 0.12 ‰, 1SD) during eogenetic meteoric diagenesis is primarily inherited from the earlier syndepositional diagenesis. Our new data from the South China Sea confirm a general diagenetic offset of 0.24 ‰ ± 0.24 ‰ (1SD) between carbonates and seawater, with specific offsets for individual samples determined by their diagenetic stages, environments, and redox conditions, which can be quantitatively evaluated through V–U sequential enrichments in carbonates