238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (?238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent global oceanic anoxia in deep time. Multiple studies have examined whether primary can directly capture seawater ?238U and measurements recent ancient preserve U isotope signatures. Here we assess role diagenesis altering signatures sediments that calcitic mineralogy at Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid warming deoxygenation ?56 million years ago. Although abiotic biogenic (aragonite calcite) small offsets (<0.1‰) relative modern seawater, diagenetic alteration Bahamian shallow-water platform carbonate predominantly aragonitic resulted significantly larger (up 0.6‰). Since concentration aragonite is least one order magnitude higher than calcite (>1 ppm vs. <0.1 ppm), should be even more susceptible aragonite. We find strong evidence this effect analysis PETM from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations ?0.69 +0.71‰ around boundary but consistently heavier (between ?0.14 +0.47‰) outside interval. lighter values were interpreted result operation Mn oxide shuttle. are most likely caused by authigenic reductive accumulation U(IV) pore waters below sediment-water interface. found tend increase increasing U/Ca. This relationship well-explained model simply assumes addition during cements containing isotopically U(IV). work confirms expectations amount cementation compared aragonite, variations would dominantly reflect local redox state depositional early environments. It essential identify original mineralogy, history, constrain deposition environments sedimentary rocks when applying for