Abstract Molybdenum (Mo) and uranium (U) contents in sedimentary archives are often used to reconstruct past changes in seafloor oxygenation. However, their sequestration processes are as yet poorly constrained in low-salinity coastal waters, which often suffer from anthropogenic eutrophication but only mild oxygen depletion. Due to the consequent lack of robust long-term paleo-redox reconstructions in such settings often characterized by a shallow front of dissolved sulfide accumulation within the sediment pore waters, inadequate understanding of the long-term drivers behind oxygen loss impedes cost-effective mitigation of this environmental problem. Here, we investigate the mechanisms of Mo and U sequestration in an oxic, low-salinity coastal setting in the northern Baltic Sea where anthropogenic eutrophication over the 20th century has resulted in formation of a shallow sulfate-methane transition zone (SMTZ) in the sediment column of this brackish-water basin. Our results demonstrate remarkably similar patterns for authigenic Mo and U sequestration, whereby the depth and intensity of the SMTZ exerts a first-order control on their solid-phase uptake. Sequential extraction analysis suggests that a large part of the authigenic Mo pool is hosted by refractory Fe–S phases such as pyrite and nanoscale FeMoS4, implying that the Fe-sulfide pathway is the dominating process of authigenic Mo scavenging. However, we also observe a pool of extremely labile Mo deep within the SMTZ, which might record an intermediate phase in authigenic Mo sequestration and/or partial switch to the organic matter (OM) pathway at low dissolved Fe levels. Authigenic U resides in acid-extractable and refractory phases, likely reflecting uptake into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Similarly to Mo, authigenic U uptake is active at two fronts within the SMTZ, paralleled by increases in dissolved sulfide levels, suggesting coupling between sulfide production and U reduction. Our results imply that both Mo and U could provide viable proxies for mild bottom water deoxygenation in these settings, through the indirect link between seafloor oxygen conditions and the depth of SMTZ. Of these, Mo appears to more robustly capture variations in seafloor oxygen levels due to the significantly higher share of the authigenic pool. However, temporal resolution of these proxies is limited by the vertical offset between seafloor and the zone of authigenic uptake, and the superimposed character of the signal at a given depth due to vertical migrations of the SMTZ. These results have important implications for the use of Mo and U as paleo-redox proxies in other low-salinity coastal settings exposed to eutrophication.

中文翻译：

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硫酸盐-甲烷过渡带的深度和强度控制富营养化低盐度环境中沉积钼和铀的封存

摘要 沉积档案中的钼 (Mo) 和铀 (U) 含量通常用于重建过去海底氧合的变化。然而，它们的封存过程在低盐度沿海水域中受到的限制很少，沿海水域经常遭受人为富营养化，但只有轻度的氧气消耗。由于在这些环境中缺乏稳健的长期古氧化还原重建，通常以沉积物孔隙水中溶解的硫化物积累的浅层为特征，对氧气损失背后的长期驱动因素的了解不足阻碍了具有成本效益的缓解这个环境问题。在这里，我们研究了 Mo 和 U 在好氧中的封存机制，波罗的海北部的低盐度沿海环境，20 世纪的人为富营养化导致在该咸水盆地的沉积柱中形成浅层硫酸盐-甲烷过渡带 (SMTZ)。我们的结果表明自生 Mo 和 U 封存的模式非常相似，其中 SMTZ 的深度和强度对其固相吸收施加了一级控制。顺序提取分析表明，自生钼池的很大一部分由难熔的 Fe-S 相（如黄铁矿和纳米级 FeMoS4）承载，这意味着 Fe-硫化物途径是自生钼清除的主要过程。然而，我们也在 SMTZ 深处观察到了一个极其不稳定的 Mo 池，这可能会记录自生 Mo 封存的中间阶段和/或在低溶解 Fe 水平下部分转换为有机物 (OM) 途径。自生 U 存在于酸萃取相和难熔相中，可能分别反映了对结晶性差的单体 U(IV) 和结晶铀矿的吸收。与钼相似，自生 U 吸收在 SMTZ 内的两个前沿活跃，同时溶解硫化物水平增加，表明硫化物产生和 U 减少之间存在耦合。我们的结果意味着，通过海底氧气条件和 SMTZ 深度之间的间接联系，Mo 和 U 都可以为这些环境中的温和底水脱氧提供可行的代理。这些，由于自生池的份额显着增加，Mo 似乎更有效地捕获了海底氧气水平的变化。然而，这些代理的时间分辨率受到海底与自生吸收区之间的垂直偏移以及由于 SMTZ 的垂直迁移而导致的给定深度信号的叠加特征的限制。这些结果对于在其他暴露于富营养化的低盐度沿海环境中使用 Mo 和 U 作为古氧化还原代理具有重要意义。