238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal

Uranium isotope ratios are widely utilized in paleoceanography. The 238U/235U ratio (expressed as ?238U) is leveraged a proxy for the areal extent of anoxic seafloor, and 234U/238U ?234Usec) tracks riverine estuarine inputs to ocean, addition featuring prominently U-series geochronology. Both these thought be recorded by biological carbonates precipitating from seawater, with corals serving one most commonly-used archives seawater U past. utility carbonate relies not only on this faithful archiving ambient signatures, but also homogeneity composition, which enables samples entire ocean. Here we revisit foundational assumption marine reservoir, capacity deep-sea record isotopic composition seawater. We begin evaluating analytical limits precision accuracy achievable both ?238U ?234Usec analysis MC-ICP-MS. then report data 45 26 coral multiple sites around world. find subtle heterogeneity that correlates concentrations, allows us calculate new salinity-normalized global mean values (?0.379 ± 0.023 ‰) (+145.55 0.28 ‰). At each site, act precise value. same true ?234Usec, few exceptions where appear show vital effects cause intra-sample re-partitioning. In sum, observations support robust archive ratios, highlight importance utilizing sample replicate analyses overcome (for heterogeneity.