Methane seepage alters local diagenetic conditions, influencing the coupled biogeochemical cycles of sulfur, carbon, and redox-sensitive elements such as uranium (U) in the sediment. This study investigates the U-isotopic compositions of carbonate-rich sediments and seep carbonates (Site ROV1-5A) in the Haima Cold Seep of the South China Sea and of non-seep marine sediments (sites DS12 and DS388). Terrigenous debris dominates U sources in non-seep marine sediments (S234U, -27%o to +26%o; U/Al, 0.17 mg/kg/wt.%), consistent with upper continental crustal (UCC) signatures. In contrast, seep carbonates (S234U, +105%o to +135%o; U/Al, 4.3 mg/kg/ wt.%) and carbonate-rich sediments (S234U, +38%o to +107%o; U/Al, 1.3 mg/kg/wt.%) exhibit elevated S234U values and U/Al ratios, indicating authigenic U enrichment. Positive correlations between U and carbonate contents suggest that carbonate is the main sedimentary host of authigenic U in methane-seep environments. In comparison to the UCC and non-seep marine sediments (average S238U = -0.27%o), carbonate-rich sediments exhibit 238U enrichment (-0.16%o to +0.01%o) and seep carbonates 238U depletion (-0.45%o to -0.32%o). Seep carbonates, which lack pyrite and exhibit moderately elevated (Mo/U) values (average 1.9), form under anoxic, non-sulfidic conditions. In contrast, carbonate-rich sediments, which are rich in pyrite and exhibit high (Mo/U) values (average 7.6), form under sulfidic conditions. We infer that the alteration of the redox environment resulting from anaerobic oxidation of methane and the abiotic reduction of U(VI) driven by Fe(II) are the primary factors responsible for the observed differences in S238U. Our results indicate that U isotopes are a sensitive proxy for local redox conditions in seep environments. The observed isotopic shifts provide insights into how methane seepage, through processes like AOM and associated redox changes, orchestrates the coupling of multi-elemental cycles and oceanic redox changes.
