Excess total organic carbon in the intermediate water of the South China Sea and its export to the North Pacific

Abstract

Depth profiles of total organic carbon (TOC) were measured in spring (2005) and winter (2006) in the South China Sea (SCS), the largest marginal sea adjacent to the North Western Pacific (NWP). Compared to TOC profiles in the NWP, excess TOC (3.2 ± 1.1 μmol kg−1) was revealed in the intermediate layer of the SCS at σθ ∼ 27.2–27.6 (∼1000–1500 m). Below the depth of 2000 m, TOC concentrations were identical between the SCS and the NWP. Based on a one‐dimensional steady state diffusion advection model constrained by potential temperature, we estimated a net TOC production rate of 0.12 ± 0.04 μmol kg−1 yr−1 to maintain this excess. A positive relationship between TOC and apparent oxygen utilization in the SCS deep water lent support to such a model‐derived TOC production. This excess TOC in the out‐flowing intermediate water may carry 3.1 ± 2.1 Tg C yr−1 of organic carbon out from the SCS and potentially into the deep open ocean. In light of the short residence time of the SCS deep water, the exported TOC was likely from the recently fixed organic carbon within the SCS. The export of such organic carbon, thereby less likely to return to the atmosphere may therefore contribute significantly to the carbon sequestration in the SCS.