Abstract Estuarine systems are of key importance for the riverine input of silicon (Si) to the ocean, which is a limiting factor of diatom productivity in coastal areas. This study presents a field dataset of surface dissolved Si isotopic compositions (δ30SiSi(OH)4) obtained in the estuaries of three of the world’s largest rivers, the Amazon (ARE), Yangtze (YRE), and Pearl (PRE), which cover different climate zones. While δ30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a dominant control by water mass mixing, significantly increased δ30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30e of −1.0 ± 0.4‰ (Rayleigh model) or −1.6 ± 0.4‰ (steady state model). In addition, seasonal variability of Si isotope behavior in the YRE was observed by comparison to previous work and most likely resulted from changes in water residence time, temperature, and light level. Based on the 30e value obtained for the ARE, we estimate that the global average δ30SiSi(OH)4 entering the ocean is 0.2–0.3‰ higher than that of the rivers due to Si retention in estuaries. This systematic modification of riverine Si isotopic compositions during estuarine mixing, as well as the seasonality of Si isotope dynamics in single estuaries, needs to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.

中文翻译：

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大河口溶解硅同位素动力学

摘要 河口系统对于河流向海洋输入硅 (Si) 至关重要，这是沿海地区硅藻生产力的限制因素。本研究提供了在世界上三大河流亚马逊 (ARE)、扬子 (YRE) 和珍珠 (PRE) 的河口获得的表面溶解 Si 同位素组成 (δ30SiSi(OH)4) 的现场数据集，其中包括不同的气候带。虽然 δ30SiSi(OH)4 在 YRE 和 PRE 中表现保守，支持水团混合的主导控制，但在 ARE 中观察到由于硅藻对 Si(OH)4 的利用而显着增加的 δ30SiSi(OH)4 特征，并反映了 Si 同位素-1.0±0.4‰（瑞利模型）或-1.6±0.4‰（稳态模型）的富集因子30e。此外，通过与之前的工作进行比较，观察到 YRE 中 Si 同位素行为的季节性变化，很可能是由于水停留时间、温度和光照水平的变化造成的。根据 ARE 获得的 30e 值，我们估计由于 Si 在河口滞留，进入海洋的全球平均 δ30SiSi(OH)4 比河流高 0.2–0.3‰。需要考虑河口混合过程中河流 Si 同位素组成的这种系统修改，以及单个河口中 Si 同位素动力学的季节性，以更好地限制大河流入海口在海洋 Si 循环中的作用。我们估计，由于Si滞留在河口，全球进入海洋的δ30SiSi(OH)4平均值比河流高0.2-0.3‰。需要考虑河口混合过程中河流 Si 同位素组成的这种系统修改，以及单个河口中 Si 同位素动力学的季节性，以更好地限制大河流入海口在海洋 Si 循环中的作用。我们估计，由于Si滞留在河口，全球进入海洋的δ30SiSi(OH)4平均值比河流高0.2-0.3‰。需要考虑河口混合过程中河流 Si 同位素组成的这种系统修改，以及单个河口中 Si 同位素动力学的季节性，以更好地限制大河流入海口在海洋 Si 循环中的作用。