238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal
238U, 235U and 234U in seawater and deep-sea corals: A high-precision reappraisal
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DOI：
10.1016/j.gca.2022.09.018
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发表时间：
2022-09
影响因子：
5
通讯作者：
M. Kipp;Haoyu Li;M. Ellwood;S. John;R. Middag;J. Adkins;F. Tissot
中科院分区：
文献类型：
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作者：
M. Kipp;Haoyu Li;M. Ellwood;S. John;R. Middag;J. Adkins;F. Tissot
铀同位素比值在古海洋学中有广泛的应用。238U/235U比率（表示为δ238U）被用作缺氧海底面积范围的代理，234u /238U比率（表示为δ234Usec）除了在u系列地质年代学中具有突出特点外，还跟踪了河流和河口对海洋的输入。这两个比值被认为是由海水中沉淀的生物碳酸盐记录的，珊瑚是过去最常用的海水U同位素比值档案之一。生物碳酸盐档案中U同位素代用不仅依赖于对环境海水特征的忠实存档，还依赖于海水U同位素组成的均匀性，这使得样本可以作为整个海洋的代用。本文回顾了海洋铀储层均匀性的基本假设，以及深海珊瑚记录环境海水铀同位素组成的能力。我们首先评估了MC-ICP-MS对δ238U和δ234Usecanalysis可达到的精密度和准确度的分析极限。然后，我们报告了来自世界各地多个地点的45个海水和26个深海珊瑚样本的数据。我们发现δ238U和δ234Usec的非均质性与U浓度相关，这使我们能够计算出新的盐度归一化的全球平均海水值δ238U（−0.379±0.023‰）和δ234Usec（+145.55±0.28‰）。在每个测点，生物碳酸盐作为海水δ238U值的精确档案。δ234Usec也是如此，只有少数例外，其中样本似乎显示出导致样本内部234u /238U重新分区的重要影响。总之，这些观测结果支持深海珊瑚作为海水U同位素比率的可靠档案，但强调了利用多个采样点和重复分析来克服珊瑚的重要影响（δ234Usec）和微妙的海洋U同位素异质性的重要性。
Uranium isotope ratios are widely utilized in paleoceanography. The238U/235U ratio (expressed as δ238U) is leveraged as a proxy for the areal extent of anoxic seafloor, and the234U/238U ratio (expressed as δ234Usec) 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 of precision and accuracy achievable for both δ238U and δ234Usecanalysis by MC-ICP-MS. We then report data for 45 seawater and 26 deep-sea coral samples from multiple sites around the world. We find subtle δ238U and δ234Usecheterogeneity 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 precise archives of the seawater δ238U value. The same is true for δ234Usec, with a few exceptions where samples appear to show vital effects that cause intra-sample234U/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 replicate analyses to overcome coral vital effects (for δ234Usec) and subtle marine U isotopic heterogeneity.