Dissolved silicon isotopic compositions in the East China Sea: Water mass mixing vs. biological fractionation

Abstract

We present the first set of dissolved silicon isotope data in seawater (δ30SiSi(OH)4) from the East China Sea, a large and productive marginal sea significantly influenced by the Kuroshio Current and freshwater inputs from the Changjiang (Yangtze River). In summer (August 2009), the lowest surface δ30SiSi(OH)4 signatures of +2.1‰ corresponding to the highest Si(OH)4 concentrations (∼30.0 μmol L-1) were observed nearshore in Changjiang Diluted Water. During advection on the East China Sea inner shelf, surface δ30SiSi(OH)4 increased rapidly to +3.2‰ while Si(OH)4 became depleted, indicating increasing biological utilization of the Si(OH)4 originating from the Changjiang Diluted Water. This is also reflected in the water column profiles characterized by a general decrease of δ30SiSi(OH)4 and an increase of Si(OH)4 with depth on the East China Sea mid-shelf and slope. In winter (December 2009-January 2010), however, the δ30SiSi(OH)4 was nearly constant at +1.9‰ throughout the water column on the East China Sea shelf beyond the nearshore, which was a consequence of enhanced vertical mixing of the Kuroshio subsurface water. Horizontal admixture of Kuroshio surface water, which is highly fractionated in Si isotopes, was observed only beyond the shelf break. Significant seasonal differences in δ30SiSi(OH)4 were detected in the surface waters beyond the Changjiang Diluted Water-influenced region on the East China Sea shelf, where the winter values were ∼1.0‰ lower than those in summer, despite the same primary Si(OH)4 supply from the Kuroshio subsurface water during both seasons. This demonstrates significantly higher biological consumption and utilization of Si(OH)4 in summer than in winter.