Suboxic trace metal geochemistry in the Eastern Tropical North Pacific
Suboxic trace metal geochemistry in the Eastern Tropical North Pacific
复制标题
DOI：
10.1016/s0016-7037(01)00843-2
复制
发表时间：
2002-04
影响因子：
5
通讯作者：
T. Nameroff;L. Balistrieri;J. Murray
中科院分区：
文献类型：
--
作者：
T. Nameroff;L. Balistrieri;J. Murray
我们分析了墨西哥马萨特兰附近东部热带北太平洋强氧最低区 (OMZ) 的水柱、沉降颗粒和沉积物（0 至 22 厘米）样品中的 Al、Ti、Fe、Mn、Cu、Ba、Cd、U、Mo、V 和 Re。目标是确定这些元素的地球化学如何受到低氧水柱条件的影响，以及沉积物是否具有独特的“低氧”地球化学特征。水柱的特征是 Mn 最大值，在 400 m 处达到~8 nmol kg−1。 Cu、Ba、Cd、Mo、Re、U 和 V 的浓度不受低 O2 条件的影响，与公海的浓度相当。下沉颗粒由碎屑成岩颗粒和生物成因非成岩颗粒组成。 Al、Ti 和 Fe 大部分（至少 79%）是成岩的。大约 75% 的 Mn 是非成岩的。大量（至少 58%）的 Cu、Ba、Cd 和 Mo 是非成岩的。整个大陆架和斜坡的沉积物地球化学各不相同。镉、铀和铼的峰值集中在 310 m 处，在核心顶部分别为 12.3 ppm、10.9 ppm 和 68.3 ppb。 OMZ 表面沉积物中 Mo（平均 6.8 ppm）和 V（平均 90 ppm）含量较高。顶部 10 厘米内的所有氧化还原敏感元素都会发生额外的下核富集。对于 U、Mo、V 和 Re，表面沉积物并不是金属富集的良好指标。沉积物的非成岩成分与下沉颗粒的比较表明，富含金属的浮游生物物质的直接输入对总成分有显着贡献，特别是对于镉、铀和钼。我们评估了Re/Mo和Cd/U比率作为氧化还原环境的示踪剂。铼和钼的浓度以及铼/钼的比率并不能得出一致的结论。 Re 和 Mo 的同时富集是缺氧沉积环境的指标。相反，高 Re/Mo 比率是低氧条件的指标。镉在表层沉积物中富集，而铀则在地核下富集相当多。 Cd 和 U 的浓度以及 Cd/U 比率不遵循热力学预测的模式。虽然水柱是低氧的，但这四种氧化还原敏感元素表明沉积物是缺氧的。古研究的意义在于，表明缺氧条件的微量金属沉积物特征不一定归因于缺氧水柱。
We analyzed Al, Ti, Fe, Mn, Cu, Ba, Cd, U, Mo, V, and Re in water column, settling particulate, and sediment (0 to 22 cm) samples from the intense oxygen minimum zone (OMZ) of the eastern tropical North Pacific near Mazatlán, Mexico. The goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique “suboxic” geochemical signature. The water column was characterized by a Mn maximum, reaching ∼8 nmol kg−1at 400 m. Concentrations of Cu, Ba, Cd, Mo, Re, U, and V were unaffected by the low O2conditions and were comparable to those of the open ocean. Sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin. Al, Ti, and Fe were mostly (at least 79%) lithogenic. About 75% of the Mn was nonlithogenic. Significant amounts (at least 58%) of Cu, Ba, Cd, and Mo were nonlithogenic. Sediment geochemistry varied across the continental shelf and slope. Cadmium, U, and Re have prominent maxima centered at 310 m, with 12.3 ppm, 10.9 ppm, and 68.3 ppb, respectively, at the core top. High values of Mo (averaging 6.8 ppm) and V (averaging 90 ppm) are seen in OMZ surface sediment. Additional down-core enrichment occurs for all redox-sensitive elements in the top 10 cm. For U, Mo, V, and Re, surface sediments are a poor indicator of metal enrichment. Comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition, especially for Cd, U, and Mo. We evaluated Re/Mo and Cd/U ratios as tracers for redox environments. Rhenium and Mo concentrations and Re/Mo ratios do not lead to consistent conclusions. Concurrent enrichments of Re and Mo are an indicator of an anoxic depositional environment. In contrast, high Re/Mo ratios are an indicator of suboxic conditions. Cadmium is enriched in surface sediments, while U has considerable down-core enrichment. The concentrations of Cd and U and the Cd/U ratio do not follow patterns predicted from thermodynamics. Though the water column is suboxic, these four redox-sensitive elements indicate that the sediments are anoxic. The implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column.