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238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal
Kipp, M.A.; Li, H.; Ellwood, M.J.; John, S.G.; Middag, R.; Adkins, J.F.; Tissot, F.L.H. (2022). 238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal. Geochim. Cosmochim. Acta 336: 231-248. https://dx.doi.org/10.1016/j.gca.2022.09.018
In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, meer
Peer reviewed article  

Beschikbaar in  Auteurs 

Author keywords
    Uranium; GEOTRACES; D. dianthus; Paleo-redox

Auteurs  Top 
  • Kipp, M.A.
  • Li, H.
  • Ellwood, M.J.
  • John, S.G.
  • Middag, R., meer
  • Adkins, J.F.
  • Tissot, F.L.H.

Abstract

    Uranium isotope ratios are widely utilized in paleoceanography . The 238U/235U ratio (expressed as δ238U)is leveraged as a proxy for the areal extent of anoxic seafloor, and the234U/238U ratio (expressed as δ234U sec) tracks riverine and estuarine inputs to the ocean, in addition to featuring prominently in U-series geochronology . Both of these ratios are thought to be recorded by biological carbonates precipitating from seawater, with corals serving as one of the most commonly-used archives of seawater U isotope ratios in the past. The utility of the U isotope proxy in biological carbonate archives relies not only on this faithful archiving of ambient seawater signatures, but also on the homogeneity of the seawater U isotope composition, which enables samples to be leveraged as proxy for the entire ocean.

    Here we revisit the foundational assumption of homogeneity of the marine U reservoir, and the capacity of deep-sea corals to record the U isotopic composition of ambient seawater. We begin by evaluating the analytical limits ofprecision and accuracy achievable for both δ238U and δ 234Usec analysis by MC-ICP-MS. We then report data for 45 seawater and 26 deep-sea coral samples from multiple sites aroundthe world. We find subtle δ238U and δ234U sec heterogeneity that correlates with U concentrations, which allows us to calculate new salinity-normalized global mean seawater values for δ238U (−0.379 ± 0.023 ‰) and δ234Usec (+145.55 ± 0.28 ‰). At each site, biological carbonates act as precisearchives of the seawater δ238U value. The same is true for δ 234Usec, with a few exceptions where samples appearto show vital effects that cause intra-sample 234U/ 238U re-partitioning. In sum, these observations support deep-sea corals as a robust archive of seawater U isotope ratios, but highlight the importance of utilizing multiple sample sites and replicateanalyses to overcome coral vital effects (for δ234U sec) and subtle marine U isotopic heterogeneity.


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