Dissolved Silicon Isotopic Compositions in the East China Sea: Water Mass Mixing Versus Biological Fractionation

Abstract:

The stable silicon isotopic composition of dissolved silicic acid (δ³⁰Si_Si(OH)4) has increasingly been used to investigate the biogeochemical cycling of Si in the ocean. Here we present the first seawater Si isotope data set from the East China Sea (ECS), a large and productive marginal sea system in the western North Pacific, to examine sources and utilization of Si(OH)₄. During two cruises in summer (August 2009) and in winter (December 2009-January 2010), samples for δ³⁰Si_Si(OH)4 analyses were collected along a cross-shelf transect, extending from near the mouth of the Changjiang River estuary to the slope area open to the Kuroshio Current (KC). In summer, the lowest surface δ³⁰Si_Si(OH)4 coincided with the highest Si(OH)₄ concentrations observed nearest to the shore in the Changjiang Diluted Water (CDW). During advection on the ECS inner shelf, surface δ³⁰Si_Si(OH)4 increased rapidly from ~2.1‰ to ~3.1‰ while Si(OH)₄ became highly depleted, indicating significant biological utilization and fractionation of Si isotopes from the CDW. Such fractionation was also seen from the vertical profiles characterized by a general decrease of δ³⁰Si_Si(OH)4 and an increase of Si(OH)₄ with depth. In winter however, values of both parameters were nearly constant throughout the water column on the ECS shelf. The enhanced intrusion of the KC and vertical mixing dominated the distribution of the seawater δ³⁰Si_Si(OH)4 signatures. Significant seasonal differences in δ³⁰Si_Si(OH)4 were detected in the surface waters beyond the CDW region on the ECS shelf, where the winter values were ~1.0‰ lower than those in summer. Given that Si(OH)₄ in these surface waters were primarily sourced from the Kuroshio subsurface water during both seasons, such seasonality suggested higher biological consumption and fractionation in summer than in winter.