Silicon isotope constraints on sources and utilization of silicic acid in the northern South China Sea

Abstract

The stable silicon isotopic composition (delta Si-30) of waters and diatoms has increasingly been used to investigate the biogeochemical cycling of Si in the major ocean basins. Here we present the first Si isotope data set from the northern South China Sea (NSCS), a large marginal sea system in the western North Pacific to examine sources and utilization of silicic acid (Si(OH)(4)). During two cruises in July-August 2009 (summer) and January 2010 (winter), samples for isotope measurements of dissolved Si(OH)(4) (delta Si-30(Si(OH)4)) and of biogenic silica (delta Si-30(BSi)) in suspended particles were collected along a transect perpendicular to the coast from the inner shelf to the deep-water slope, as well as at the South East Asian Time-series Study (SEATS) station located in the NSCS basin. Surface delta Si-30(Si(OH)4) generally increased from values similar to+2.3 parts per thousand on the inner shelf to similar to+2.8 parts per thousand above the deep basin, suggesting an increasing utilization of dissolved Si(OH)(4) reflecting the transition from eutrophic to oligotrophic conditions. The delta Si-30(BSi) values were systematically lower than the corresponding delta Si-30(Si(OH)4) in the euphotic zone (above 100 m) on the shelf and slope. In contrast at station SEATS in the NSCS basin, delta Si-30(BSi) signatures in both seasons were within error equal to delta Si-30(Si(OH)4) in the surface mixed layer (above 50 m) and delta Si-30(BSi) in waters below were significantly higher than the corresponding delta Si-30(Si(OH)4). By comparing the field data with the Si isotope fractionation revealed by the Rayleigh or the steady state models, we demonstrate the existence of variable Si(OH)(4) origins in different areas of the NSCS. Surface waters on the inner shelf were largely fed by nutrients from the Pearl River input. While the primary source of Si(OH)(4) for the euphotic zone on the outer shelf and slope was upwelling or vertical mixing from underlying waters, the Si(OH)(4) in the surface mixed layer of the NSCS basin might have originated from horizontal mixing with other highly fractionated surface waters. As a consequence, the Si isotope dynamics in the NSCS are largely controlled by variable biological fractionation of Si in waters from different sources with different initial Si isotopic compositions rather than any single source water. (C) 2012 Elsevier Ltd. All rights reserved.