Evolution of rare earth element and εNd compositions of Gulf of Mexico seawater during interaction with Mississippi River sediment
Evolution of rare earth element and εNd compositions of Gulf of Mexico seawater during interaction with Mississippi River sediment
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墨西哥湾海水与密西西比河沉积物相互作用过程中稀土元素和μNd成分的演变
DOI：
10.1016/j.gca.2022.08.024
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发表时间：
2022
影响因子：
5
通讯作者：
Johannesson, Karen H.
中科院分区：
文献类型：
--
作者：
Adebayo, Segun B.;Cui, Minming;Williams, Thomas J.;Martin, Ellen;Johannesson, Karen H.
为评价墨西哥湾（GOM）海水与密西西比河沉积物相互作用对体系中溶解稀土元素（REE）浓度和钕同位素组成（εNd）的影响，进行了为期270 d的封闭系统间歇反应实验。与以往研究玄武岩和沉积物对海洋稀土元素和εNd影响的研究不同，本研究重点研究了北美大陆风化作用下硅质沉积物对大陆边缘海水稀土元素和εNd的地球化学反应。我们的研究结果表明，在实验开始的前33天，密西西比河岩性沉积物不稳定相的溶解导致溶解稀土浓度增加了约100倍。在实验的第33天至270天之间，次生矿物降水似乎降低了稀土浓度，尽管海水稀土浓度与初始值相比仍然升高。涉及溶解和沉淀的双向元素转移导致实验结束时（即第270天）溶解稀土浓度净增加24±12（平均±1σ）倍。实验33天后观察到的溶解稀土浓度最大值代表了河流沉积物中操作定义的“可交换”部分中约0.37%的稀土含量的动员。活性造岩组分的εNd值分别为- 9.77和- 9.95，与GOM值（- 9.81±0.36）相近。由于εNd值之间的相似性，整个实验过程中海水Nd同位素值的变化很小（平均±标准差，反应海水εNd为- 9.87±0.17）。最高的REE浓度与最高的辐射成因εNd相吻合（-9.65±0.23;day 33），表明GOM的REE浓度和εNd组成可能受到系统中沉积物通量的缓冲。我们的研究结果与之前有关玄武岩和/或玄武岩组成的沉积物的研究结果相当，因为它们表明，硅质、河流沉积物在海洋环境中对稀土元素的动员具有高度活性。实验结果进一步表明，“边界交换”在影响以大型河流系统为主的大陆边缘海水的εNd中发挥重要作用，尽管边界交换的影响将在环境海水和河流沉积物具有不同Nd同位素组成的地方最为深刻（例如玄武岩或前寒武纪盾质物质）。结果表明，墨西哥湾海水的εNd值在很大程度上受密西西比河向盆地输送的岩性沉积控制。
A closed-system batch reaction experiment was conducted for 270 days to evaluate the effects of interaction between Gulf of Mexico (GOM) seawater and Mississippi River sediments on the system’s dissolved rare earth elements (REE) concentrations and neodymium isotopic compositions (εNd). This study specifically focuses on geochemical reactions involving silicic sediments derived from weathering of the North American continent as they affect the REEs and εNd of seawater along continental margins, in contrast to previous studies that investigated the influence of basaltic rocks and sediments on REEs and εNd in the ocean. Our results show that the dissolution of labile phases of lithogenic Mississippi River sediments leads to an approximately 100-fold increase in dissolved REE concentrations within the first 33 days of the experiment. Secondary mineral precipitation appears to lower the REE concentrations between days 33 and 270 of the experiment, although seawater REE concentrations remain elevated compared to initial values. The two-way elemental transfer involving dissolution and precipitation results in a net increase by a factor of 24 ± 12 (mean ± 1σ) in the dissolved REE concentrations by the end of the experiment (i.e., day 270). The dissolved REE concentration maxima observed after 33 days of the experiment represent the mobilization of approximately 0.37 % of the REE content of the operationally defined “exchangeable” fraction of the riverine sediments. The εNd values of the reactive lithogenic components were −9.77 and −9.95, which are similar to the GOM value of −9.81 ± 0.36. Because of the similarity between εNd values, changes in the seawater Nd isotope value throughout the experiment were subtle (mean ± std, reacted seawater εNd of −9.87 ± 0.17). The highest REE concentrations coincided with the most radiogenic εNd (-9.65 ± 0.23; day 33), which suggests that REE concentrations and εNd compositions of the GOM may be buffered by fluxes from sediments in the system. Our results are comparable to previous studies involving basaltic rocks and/or sediments of basaltic composition in that they demonstrate that silicic, river sediments are highly reactive in marine environments with regard to REE mobilization. The experimental results further suggest that “boundary exchange” plays an important role in influencing the εNd of seawater along continental margins dominated by large river systems, although the impacts of boundary exchange will be most profound where ambient seawater and river sediments have distinct Nd isotopic compositions (e.g., basaltic, or Precambrian shield material). Finally, our results indicate that the εNd value of GOM seawater is largely controlled by the lithogenic sediment delivered to the basin by the Mississippi River.
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DOI：
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发表时间：
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期刊：
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