Measurements of trace metal abundances and isotopes in carbonate sediments, Exuma, Bahamas

In order to validate the use of 238U/235U as a paleoredox proxy in carbonates, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1 ppm) than in primary precipitates (avg. 1.5 ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H2S, and thus indirectly to organic matter content. d238/235U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, d238/235U of bulk sediments from the push cores was 0.2-0.4 per mil heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and 238U/235U ratios. We also report the occurrence of dolomite in a tidal pond core which contains low 234U/238U and 238U/235U ratios and discuss the possibility that the dolomitization process may result in sediments depleted in 238U. From this initial exploration, it is clear that 238U/235U variations in ancient carbonate sediments could be driven by changes in global average seawater, by spatial and temporal variations in the local deposition environment, or subsequent diagenesis. To cope with such effects, proxies for syndepositional pore water redox conditions (e.g., organic matter content, iron speciation, and trace metal distributions) and careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of 238U/235U data from ancient carbonate sediments.为验证²³⁸U/²³⁵U作为碳酸盐岩中古氧化还原代用指标（paleoredox proxy）的适用性，我们研究了巴哈马浅海碳酸盐沉积物中铀同位素的赋存过程与早期成岩演化特征。本研究的样品集包含多种原生沉淀物，它们代表了地质历史时期重要的各类碳酸盐生产生物及组分：石珊瑚、钙绿藻、钙红藻、鲕粒以及软体动物。此外，我们在不同沉积环境中采集了4根短活塞岩芯（push cores），以评估早期成岩作用与孔隙水化学性质对块状碳酸盐岩铀同位素组成的影响。研究发现，块状碳酸盐沉积物的铀浓度（平均4.1 ppm）远高于原生沉淀物（平均1.5 ppm）；几乎所有样本中，块状沉积物的最低铀浓度均不低于原生沉淀物的最高铀浓度，这与早期成岩阶段还原态四价铀（U(IV)）的自生富集现象一致。该过程的强度似乎受孔隙水中硫化氢（H₂S）含量调控，进而间接反映有机质丰度。所有原生沉淀物的δ²³⁸U/²³⁵U值均与海水同位素值极为接近，表明这类碳酸盐组分可用于重建海水铀地球化学的演化变化。然而，活塞岩芯中块状沉积物的δ²³⁸U/²³⁵U值较海水（及原生沉淀物）偏高0.2‰~0.4‰。上述结果表明，碳酸盐沉积物中普遍存在的开放体系硫化孔隙水环境下的铀自生富集作用，会显著影响块状沉积物的铀浓度与²³⁸U/²³⁵U比值。我们还报道了潮坪岩芯中白云岩（dolomite）的产出特征，该类白云岩具有较低的²³⁴U/²³⁸U与²³⁸U/²³⁵U比值，并讨论了白云岩化作用可能导致沉积物贫化²³⁸U的可能性。基于本次初步探索，可以明确古碳酸盐沉积物中的²³⁸U/²³⁵U比值变化，可由全球平均海水的地球化学变化、局部沉积环境的时空差异，或是后续成岩作用所驱动；为避免对古碳酸盐沉积物中²³⁸U/²³⁵U数据的错误解释，需采用沉积同期孔隙水氧化还原条件的代用指标（如有机质含量、铁形态分析与痕量金属分布），并审慎考量沉积后可能发生的蚀变作用。