Abstract The sedimentary concentration and stable isotope composition of molybdenum (Mo) is widely used as a proxy for paleo redox conditions in the marine environment. However, the behavior of Mo during early diagenesis is still not fully understood, which complicates the application of the Mo proxy in ancient continental margin environments. Here, we present Mo concentrations and isotope compositions of sediment and pore water samples from the Guaymas Basin in the Gulf of California. Our sample set covers a broad range of depositional environments, including sediments from within the eastern equatorial Pacific oxygen minimum zone (OMZ), from a semi-restricted oxic graben, and from near a hydrothermal vent-field. By investigating Mo cycling in these different settings, we provide new insights into different modes of Mo fixation and the associated isotope fractionation. Sediments from the OMZ have authigenic Mo concentrations (Moauth) between 3.3 and 17.2 µg/g and δ98Mo between +1.64 and +2.13‰. A linear decrease in pore water Mo concentrations to the depth where hydrogen sulfide accumulates, along with sedimentary authigenic δ98Mo values (δ98Moauth) close to seawater, indicate diffusion of Mo from the bottom water into the sediment with little isotope fractionation during quantitative Mo removal. Sediments from the site with oxic bottom water within the basin reveal Moauth concentrations ranging from 1.2 to 14.7 µg/g and δ98Moauth signatures between –1.39 to +2.07‰. Pore water Mo concentrations are generally higher than ambient bottom water concentrations and the light δ98Mo signatures of the pore waters between +0.50 and +0.80‰ and δ98Moauth signatures of the sediments indicate continuous Mo exchange between the pore water and Mn and Fe oxides during early diagenesis. Sediment samples from the vent field mainly consist of black smoker debris and are characterized by Moauth concentrations ranging from 8.6 to 33.2 µg/g and δ98Moauth values as high as +2.20‰. The relatively high Mo concentrations and seawater-like δ98Mo can be explained by near-quantitative Mo scavenging from hydrothermal solutions with little isotope fractionation at high temperatures. Comparison of our new data for the OMZ sediments in the Gulf of California with previously published data for sediments from the Peruvian OMZ highlights that Mo isotope compositions in this kind of setting strongly depend on how Mo is delivered to the sediment. If diffusive Mo delivery from the bottom water into the sediment significantly contributes to Mo accumulation in the solid phase, as is the case in the Guaymas Basin, sedimentary Moauth concentrations are relatively low but the isotope values are close to the δ98Mo signal of seawater. If Mo is exclusively delivered by particles, like on the Peruvian shelf, much higher sedimentary Moauth concentrations can be attained. In the latter case, Moauth isotope values will be lighter because the sediments preserve the isotopic offset that was generated during adsorption or uptake of Mo by particles. Our findings de-emphasize the role of dissolved Mo speciation in pore waters but highlight the importance of the mode of Mo delivery for the Mo concentration and isotope composition preserved in the paleo-record.

中文翻译：

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颗粒与扩散钼供应机制对大陆边缘沉积物钼同位素组成的影响

摘要 钼 (Mo) 的沉积浓度和稳定同位素组成被广泛用作海洋环境中古氧化还原条件的代表。然而，Mo在早期成岩作用中的行为仍不完全清楚，这使得Mo代理在古代大陆边缘环境中的应用变得复杂。在这里，我们展示了加利福尼亚湾瓜伊马斯盆地沉积物和孔隙水样品的 Mo 浓度和同位素组成。我们的样本集涵盖了广泛的沉积环境，包括来自东赤道太平洋最低氧区 (OMZ) 内的沉积物、来自半限制性好氧地堑以及来自热液喷口场附近的沉积物。通过调查这些不同环境下的 Mo 骑行，我们对不同的 Mo 固定模式和相关的同位素分馏提供了新的见解。来自 OMZ 的沉积物的自生 Mo 浓度 (Moauth) 介于 3.3 和 17.2 µg/g 之间，δ98Mo 介于 +1.64 和 +2.13‰ 之间。孔隙水 Mo 浓度线性下降到硫化氢积聚的深度，以及靠近海水的沉积自生 δ98Mo 值 (δ98Moauth)，表明 Mo 从底部水中扩散到沉积物中，在定量 Mo 去除过程中几乎没有同位素分馏。来自盆地内含氧底水的地点的沉积物显示 Moauth 浓度范围为 1.2 至 14.7 µg/g，δ98Moauth 特征值介于 –1.39 至 +2.07‰ 之间。孔隙水 Mo 浓度通常高于环境底水浓度，孔隙水的轻 δ98Mo 特征在 +0.50 和 +0.80‰之间，沉积物的 δ98Moauth 特征表明在早期成岩作用期间孔隙水与 Mn 和 Fe 氧化物之间存在连续的 Mo 交换. 来自喷口场的沉积物样本主要由黑烟器碎片组成，其特征是 Moauth 浓度范围为 8.6 至 33.2 µg/g，δ98Moauth 值高达 +2.20‰。相对较高的 Mo 浓度和类似海水的 δ98Mo 可以通过从高温下几乎没有同位素分馏的热液溶液中清除近定量的 Mo 来解释。我们将加利福尼亚湾 OMZ 沉积物的新数据与之前发布的秘鲁 OMZ 沉积物数据进行比较，突出显示这种环境中的 Mo 同位素组成在很大程度上取决于 Mo 是如何输送到沉积物中的。如果从底水中扩散到沉积物中的 Mo 显着促进固相中的 Mo 积累，如在瓜伊马斯盆地的情况，则沉积 Moauth 浓度相对较低，但同位素值接近海水的 δ98Mo 信号。如果 Mo 完全由颗粒输送，例如在秘鲁大陆架上，则可以获得更高的沉积 Moauth 浓度。在后一种情况下，Moauth 同位素值会更轻，因为沉积物保留了在粒子吸附或吸收 Mo 期间产生的同位素偏移量。