Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering
Fractionation of rhenium isotopes in the Mackenzie River basin during oxidative weathering
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DOI：
10.1016/j.epsl.2021.117131
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发表时间：
2021-11
影响因子：
5.3
通讯作者：
M. Dellinger;R. Hilton;G. Nowell
中科院分区：
文献类型：
--
作者：
M. Dellinger;R. Hilton;G. Nowell
铼（Re）是一种微量元素，其氧化还原化学性质使其成为追踪一系列地球化学过程的理想候选元素。本文首次报道了河流沉积物中Re同位素的测量结果（δ 187 Re），以评估化学风化对大陆尺度Re同位素的影响。在加拿大西北部的麦肯齐河及其主要北极支流的水、悬浮沉积物和底沙中测量了δ 187 Re。我们发现，河流沃茨的δ 187 Re（相对于NIST SRM 989）范围为− 0.05‰至+ 0.07‰，通常高于相应的河流沉积物（− 0.25‰至+ 0.01‰）。结果表明，河流沉积物中δ 187 Re的变化范围（<0.30‰）受物源-基岩同位素变异（物源）和现代氧化风化作用的综合控制。在校正基岩变异性后，固体的δ 187 Re似乎与氧化风化相关的Re贫化量呈正相关。这种相关性以及河水和沉积物之间δ 187 Re的偏移可以通过具有高δ 187 Re的活性相（即岩石有机碳、硫化物矿物）的优先氧化来解释，但也可能是氧化过程中的分馏或次生风化作用的影响。总体而言，我们发现麦肯齐河的盆地平均基岩δ 187 Re（δ-0.05 ‰）和溶解δ 187 Re（δ-0.01 ‰）均低于大西洋海水的δ 187 Re（+0.12 ‰）。这些观测结果为未来限制海水Re同位素质量平衡的工作提供了动力，并评估了其δ 187 Re值随时间推移的长期变化潜力，这可能为追踪地球表面当前和过去的氧化还原过程提供额外的同位素代理。
Rhenium (Re) is a trace element whose redox chemistry makes it an ideal candidate to trace a range of geochemical processes. Here, we report the first rhenium isotopic measurements (δ 187 Re) from river-borne materials to assess the influence of chemical weathering on Re isotopes at continental scale. The δ 187 Re was measured in water, suspended sediments and bedloads from the Mackenzie River and its main Arctic tributaries in Northwestern Canada. We find that the δ 187 Re (relative to NIST SRM 989) of river waters ranges from− 0.05‰ to+ 0.07‰, which is generally higher than the corresponding river sediment (− 0.25‰ to+ 0.01‰). We show that the range of δ 187 Re in river sediments (∼ 0.30‰) is controlled by a combination of source bedrock isotopic variability (provenance) and modern oxidative weathering processes. After correcting for bedrock variability, the δ 187 Re of solids appear to be positively correlated with the amount of Re depletion related to oxidative weathering. This correlation, and the offset in δ 187 Re between river water and sediment, can be explained by preferential oxidation of reactive phases with high δ 187 Re (ie rock organic carbon, sulfide minerals), but could also result from fractionation during oxidation or the influence of secondary weathering processes. Overall, we find that both basin-average bedrock δ 187 Re (∼− 0.05‰) and dissolved δ 187 Re (∼− 0.01‰) in the Mackenzie River are lower than the δ 187 Re of Atlantic seawater (+ 0.12‰). These observations provide impetus for future work to constrain the Re isotope mass balance of seawater, and assess the potential for secular shifts in its δ 187 Re values over time, which could provide an additional isotopic proxy to trace current and past redox processes at Earth's Surface.