On The Inorganic Carbon Metabolism In The South China Sea And Its Role In The Carbon Cycle

Both inorganic carbon metabolism (calcification/dissolution) and the organic carbon metabolism (photosynthesis/respiration) are two primary biological processes modulating the ocean carbon cycle. While the photosynthesis/respiration drives a net flux of fixed organic carbon from the surface to the deep ocean, the net CaCO₃ production counteracts the organic carbon export by shifting the carbonate system toward a higher CO₂ concentration. This research is attempting to for the first time, examine the relative importance of these two processes in the world's largest tropic-subtropical marginal sea - the South China Sea (SCS), based on our measurements of particulate inorganic and organic carbon (PIC and POC), dissolved calcium ion (Ca²⁺) and carbonate system parameters including partial pressure of CO₂ (pCO₂), total alkalinity (TAlk) and dissolved inorganic carbon (DIC).Our results showed that, in autumn, surface PIC concentration in the northern SCS (NSCS) basin, which is oligotrophic and permanently stratified, ranged from 0.14 to 0.24μmol C L^-1, nearly one order of magnitude lower than POC. In the wintertime, however, SCS is characterized by enhanced wind-induced mixing, resulting in the relatively higher primary production as compared to other seasons. For example, in December 2006, we observed that, PIC in the euphotic layer of the deep-sea zone off Luzon was approximately 1μmol C L^-1, which was one magnitude higher than the NSCS basin, and the PIC/POC concentration ratio was as high as 1:2. PIC in the shallow water of the western SCS off Vietnam was much higher, nearly 2μmol C L^-1, and the average PIC/POC concentration ratio was higher than 1:2, suggesting strong calcification in this region. In the northern shelf and slope region, affected by the terrestrial inputs, the PIC content was 3 to 4 times more than in the NSCS basin, with the highest PIC concentration close to 0.8μmol C L^-1. PIC in the surface waters of the Xisha coral reef system was comparable to POC, which was on average >1μmol C L^-1 and the highest among all the study areas.It is also demonstrated that, the ratio of calcification to photosynthesis in the NSCS basin and the Xisha coral reef system was -1:14 and 1:4-1:3, respectively, indicating the organic carbon production was dominant in the basin area but the CaCO₃ production was a much more important component in the reef system. Using an ocean biogeochemical-transport box model, we estimated the CaCO₃ to organic carbon export ratio of the top 100 m water in the basin area to be -0.12, agreed well with the measured PIC/POC concentration ratio of this area.Vertical profiles of Ca²⁺ in the NSCS basin showed that excess Ca (defined as the difference between the measured Ca²⁺ concentration and that calculated from salinity) existed in the water below the depth of 500 m. Similar to the scenario in open ocean region, this obvious excess Ca might come from the CaCO₃ dissolution under the control of both thermodynamics and the proton flux. However, unlike excess Ca existing in the mid- and deep water in the deep-sea zone off Luzon, there was no obvious excess Ca in the western SCS. In the northern shelf and slope region, while Ca²⁺ distribution in surface waters was controlled by the mixing process of the Pearl River plume and the SCS water, excess Ca in the slope area was also observable and its magnitude was close to the NSCS basin. Ca²⁺ depletion happened in some time during the time-series observation at the reef flat, which again suggests that calcification was substantial in this typical coral reef system.In summary, while the inorganic carbon metabolism in the NSCS basin had a similar function with that in the open ocean, CaCO₃ and Ca²⁺ behaviors in other regions were more or less different, due to among others, the effects of vertical mixing, terrestrial inputs and biological activities. The most dynamic behavior was observed in the coral reef system, in which calcification resulted in the highest PIC and lowest Ca²⁺ concentrations, and had a potential effect on the community metabolism and the carbon cycle system.