Abstract Novel metal isotope systematics are increasingly used to understand environmental change in geological history. On a global scale, the isotopic budgets of these metals respond to a range of environmental processes, allowing them to trace complex changes in the global climate system and carbon cycle. In particular, uranium (U) and molybdenum (Mo) isotopes are useful tools for quantifying the global extent of oceanic anoxia and euxinia respectively. The oceanic signature of these metals is recorded in contemporaneous marine sediments. Whilst, traditionally, organic-rich anoxic ‘black shales’ have provided a useful archive of these metals, carbonate sediments are increasingly being used as a passive recorder of ocean chemistry. The majority of published U and Mo isotope studies come from shallow water platform environments. By contrast, pelagic carbonate sediments are an under-explored archive for these metals, yet are widely available for important periods of Earth history. Despite their advantages, carbonates are a complex archive, containing multiple ‘contaminant’ components such as Mn-oxides, organic matter and detrital minerals. Each of these phases can have different metal concentrations and isotopic signatures, giving the potential to distort or bias the true oceanic signature recorded by the carbonate. Reductive cleaning procedures and selective leaching protocols can be used to avoid these contaminant phases, and are tested here on modern and ancient samples to judge their efficacy in isolating a ‘carbonate-bound fraction’. To this end, leaching experiments were performed using different concentration acetic acid, HCl and HNO3, on reductively cleaned and uncleaned sample pairs. The data demonstrate that Mn-oxide coatings and exchangeable phases have a large impact on the Mo isotopic signature (δ98Mo) of carbonates, even when weak leaching techniques are used to preferentially dissolve them. Furthermore, detrital sources of Mo are also easy to liberate with different leaching protocols, and exert a significant control on leachate isotopic composition. The leaching studies identify that the pelagic carbonate end-member has a relatively high δ98Mo, but the precise relationship to seawater compositions remains unclear. For U, significant contributions from non‑carbonate phases can clearly be identified in higher concentration leaching acids using U/Ca ratios. However, U isotopes (δ238U) show no resolvable difference with different leaching procedures and are not affected by reductive cleaning. This result probably reflects (a) the low potential for leaching refractory residual detrital U phases (e.g., zircon) that contain the majority of U in the sample and (b) the low U inventories of Mn oxides versus those of Mo. Instead, leaching likely extracts U that is mineralogically bound in carbonates and authigenic clays, which share a common isotopic signature. These new data suggest that U incorporation into pelagic carbonates may be dominated by adsorption, and be offset from seawater by ~−0.15‰, in a similar manner to that seen for clays.

中文翻译：

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使用还原清洗和浸出实验检查富含碳酸盐的远洋沉积物作为自生铀和钼同位素的档案

摘要 新的金属同位素系统学越来越多地用于了解地质历史中的环境变化。在全球范围内，这些金属的同位素收支响应一系列环境过程，使它们能够追踪全球气候系统和碳循环的复杂变化。特别是，铀 (U) 和钼 (Mo) 同位素是分别量化全球海洋缺氧和euxinia 范围的有用工具。这些金属的海洋特征记录在同期的海洋沉积物中。虽然传统上富含有机物的缺氧“黑色页岩”提供了这些金属的有用档案，但碳酸盐沉积物正越来越多地被用作海洋化学的被动记录器。大多数已发表的 U 和 Mo 同位素研究来自浅水平台环境。相比之下，远洋碳酸盐沉积物是这些金属的未充分探索的档案，但在地球历史的重要时期广泛可用。尽管有其优势，碳酸盐是一个复杂的档案，包含多种“污染物”成分，如锰氧化物、有机物和碎屑矿物。这些相中的每一个都可以具有不同的金属浓度和同位素特征，从而有可能扭曲或偏向碳酸盐记录的真实海洋特征。还原性清洁程序和选择性浸出方案可用于避免这些污染物相，并在现代和古代样品上进行测试，以判断它们在分离“碳酸盐结合部分”方面的功效。为此，使用不同浓度的乙酸、HCl 和 HNO3 进行浸出实验，在还原清洁和未清洁的样品对上。数据表明，即使使用弱浸出技术优先溶解碳酸盐，Mn 氧化物涂层和可交换相对碳酸盐的 Mo 同位素特征 (δ98Mo) 也有很大影响。此外，Mo的碎屑源也很容易通过不同的浸出方案释放，并对浸出液同位素组成产生显着控制。浸出研究表明，远洋碳酸盐端元具有相对较高的 δ98Mo，但与海水成分的确切关系仍不清楚。对于 U，可以使用 U/Ca 比率在更高浓度的浸出酸中清楚地确定来自非碳酸盐相的显着贡献。然而，U 同位素 (δ238U) 在不同的浸出程序中没有显示出可分辨的差异，并且不受还原清洗的影响。该结果可能反映了 (a) 浸出样品中含有大部分 U 的难熔残余碎屑 U 相（例如锆石）的可能性较低，以及 (b) Mn 氧化物的 U 库存相对于 Mo 的库存较低。相反，浸出可能提取的 U 在矿物学上结合在碳酸盐和自生粘土中，它们具有共同的同位素特征。这些新数据表明，U 掺入到远洋碳酸盐中可能以吸附为主，并以与粘土类似的方式从海水中抵消 ~-0.15‰。锆石），样品中含有大部分 U 和 (b) Mn 氧化物的 U 存量比 Mo 的低。 相反，浸出可能提取出矿物结合在碳酸盐和自生粘土中的 U，它们具有共同的同位素特征。这些新数据表明，U 掺入到远洋碳酸盐中可能以吸附为主，并以与粘土类似的方式从海水中抵消 ~-0.15‰。锆石），样品中含有大部分 U 和 (b) Mn 氧化物的 U 存量比 Mo 的低。 相反，浸出可能提取出矿物结合在碳酸盐和自生粘土中的 U，它们具有共同的同位素特征。这些新数据表明，U 掺入到远洋碳酸盐中可能以吸附为主，并以与粘土类似的方式从海水中抵消 ~-0.15‰。