Zhang, Zhouling, Cao, Zhimian, Grasse, Patricia , Dai, Minhan, Gao, Lei, Kuhnert, Henning, Gledhill, Martha , Chiessi, Cristiano M., Doering, Kristin and Frank, Martin (2020) Dissolved silicon isotope dynamics in large river estuaries. Geochimica et Cosmochimica Acta, 273 . pp. 367-382. DOI 10.1016/j.gca.2020.01.028.

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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 (30SiSi(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 30SiSi(OH)4 behaved conservatively in the YRE and PRE supporting a
dominant control by water mass mixing, significantly increased 30SiSi(OH)4 signatures due to diatom utilization of Si(OH)4 were observed in the ARE and reflected a Si isotopic enrichment factor 30 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 30 value obtained for the ARE, we estimate that the global average 30SiSi(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.

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