Dissolved Silicon Isotope Dynamics in Large River Estuaries

Zhimian Cao1, Zhouling Zhang1, Patricia Grasse2, Minhan Dai1, Lei Gao3, Henning Kuhnert4, Martha Gledhill5, Cristiano M Chiessi6, Kristin Doering7 and Martin Frank5, (1)State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China, (2)GEOMAR Helmholtz Centre for Ocean Research Kiel, German Centre for Integrative Biodiversity Research (iDiv), Kiel, Germany, (3)State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, China, (4)Department of Geosciences & MARUM University of Bremen, Bremen, Germany, (5)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (6)University of São Paulo, School of Arts, Sciences and Humanities, São Paulo, Brazil, (7)Department of Oceanography, Dalhousie University, Halifax, NS, Canada
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 data set 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 30ε of −1.0±0.4‰ (Rayleigh model) or −1.6±0.4‰ (steady state model). The δ30SiSi(OH)4 value of the river water endmember was heavier in the YRE (1.8±0.2‰) than in the ARE (1.2±0.2‰) and PRE (1.4±0.2‰), presumably due to stronger fractionation in the Yangtze River catchment induced by a combined effect of weathering, biological utilization, and anthropogenic activities as suggested previously. 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 30ε value obtained for the ARE, we estimate that the global average δ30SiSi(OH)4 entering the ocean is 0.1-0.3‰ 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, need to be taken into account for better constraining the role of large river estuaries in the oceanic Si cycle.