Abstract Mass-dependent variations in selenium stable isotope ratios have recently been developed as a paleo-redox proxy. Since the reduction of selenium oxyanions occurs at a relatively high redox potential, this system holds promise for probing conditions relevant to the evolution and diversification of eukaryotic and animal life, which required substantial dissolved oxygen levels. Although several studies have identified selenium isotopic variability during oxygenation events in Earth’s distant past, we still have only a broad understanding of the mechanisms controlling this isotopic variability. This currently limits the robust interpretation of selenium isotope variability to first-order mechanisms driving large-magnitude changes. Here, we explore selenium isotope variability within and among Paleozoic black shales deposited on the North American craton that have been well-studied using a variety of other paleo-environmental proxies. Using this combined dataset, we attempt to unravel the controls on selenium abundance and isotope ratios in organic-rich ancient marine sedimentary rocks. We find that in the Late Pennsylvanian units, an estuarine nutrient trap on the Midcontinent Shelf enabled vigorous selenium recycling, leading to very high concentrations in sediments and enrichment of heavy isotopes in the aqueous selenium reservoir. In contrast, we find that among the Late Devonian units, differences in local basinal hydrography led to a gradient in selenium abundance and isotopic fractionation, with the more restricted basins depleting their selenium reservoirs and causing enrichment of heavy isotopes in the residual aqueous reservoir. In both of these case studies, the additional context provided by complementary paleo-environmental proxies was critical for distinguishing between possible drivers of selenium isotopic variability. When extending such studies to other paleo-environmental settings, we suggest that the continued use of complementary datasets will enable the most robust use of the selenium paleo-redox proxy. Moreover, further development of techniques for high-precision and phase-specific selenium isotope measurements will greatly improve the ability to deduce subtle redox fluctuations with this proxy.

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古生代黑色页岩中硒富集和同位素组成的盆地水文和氧化还原控制

摘要 硒稳定同位素比率的质量依赖性变化最近被开发为古氧化还原代理。由于硒氧阴离子的还原发生在相对较高的氧化还原电位下，因此该系统有望用于探测与真核生物和动物生命的进化和多样化相关的条件，这需要大量的溶解氧水平。尽管有几项研究已经确定了地球遥远过去氧化事件期间硒同位素的变异性，但我们对控制这种同位素变异性的机制仍然只有广泛的了解。目前，这将硒同位素变异性的稳健解释限制为驱动大规模变化的一级机制。这里，我们探索沉积在北美克拉通上的古生代黑色页岩内部和之间的硒同位素变异性，这些页岩已使用各种其他古环境代理进行了充分研究。使用这个组合数据集，我们试图解开对富含有机物的古代海洋沉积岩中硒丰度和同位素比率的控制。我们发现，在宾夕法尼亚晚期的单元中，中大陆陆架上的河口养分捕集器实现了强有力的硒循环，导致沉积物中非常高的浓度和硒水库中重同位素的富集。相比之下，我们发现在晚泥盆世单元中，局部盆地水文的差异导致硒丰度和同位素分馏的梯度，更受限制的盆地耗尽了它们的硒库，并导致残余水库中重同位素的富集。在这两个案例研究中，互补的古环境代理提供的额外背景对于区分硒同位素变异的可能驱动因素至关重要。当将此类研究扩展到其他古环境设置时，我们建议继续使用补充数据集将能够最有效地使用硒古氧化还原代理。此外，高精度和特定相位硒同位素测量技术的进一步发展将大大提高用该代理推断细微氧化还原波动的能力。互补的古环境代理提供的额外背景对于区分硒同位素变异的可能驱动因素至关重要。当将此类研究扩展到其他古环境设置时，我们建议继续使用补充数据集将能够最有效地使用硒古氧化还原代理。此外，高精度和特定相位硒同位素测量技术的进一步发展将大大提高用该代理推断细微氧化还原波动的能力。互补的古环境代理提供的额外背景对于区分硒同位素变异的可能驱动因素至关重要。当将此类研究扩展到其他古环境设置时，我们建议继续使用补充数据集将能够最有效地使用硒古氧化还原代理。此外，高精度和特定相位硒同位素测量技术的进一步发展将大大提高用该代理推断细微氧化还原波动的能力。我们建议继续使用补充数据集将能够最有效地使用硒古氧化还原代理。此外，高精度和特定相位硒同位素测量技术的进一步发展将大大提高用该代理推断细微氧化还原波动的能力。我们建议继续使用补充数据集将能够最有效地使用硒古氧化还原代理。此外，高精度和特定相位硒同位素测量技术的进一步发展将大大提高用该代理推断细微氧化还原波动的能力。