Program
 
General Session 2: Marine & estuarine biogeochemistry
 
 
 
Poster
Diagnosing controls of inter-seasonal changes of surface sea water pCO2 in the northern South China Sea basin
GS2-32-S
Wei Yang* , State Key Laboratory of Marine Environmental Science, Xiamen University, Xiang’an District, Xiamen 361102, China
Zhimian Cao, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiang’an District, Xiamen 361102, China
Xianghui Guo, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiang’an District, Xiamen 361102, China
Minhan Dai, State Key Laboratory of Marine Environmental Science, Xiamen University, Xiang’an District, Xiamen 361102, China
Presenter Email: yangwei@stu.xmu.edu.cn

The South China Sea (SCS) is the largest marginal sea of the Pacific Ocean. In our previous work, the seasonal variations of sea-air CO2 fluxes were studied based on multiple-year underway measurements showing that the SCS is source to atmospheric CO2 in most of the seasons except in winter. This study examined how temperature, biological productivity and air-sea CO2 exchange affect the inter-seasonal changes of seas surface of the partial pressure of CO2(pCO2).

We selected a study area (18N-20N, 114E-118E) in the northern SCS basin for our diagnosis of the seasonal variations. Data of the pCO2 and other parameters were collected from four different cruises conducted in spring (2011), summer (2009), fall (2010) and winter (2010). We considered that the pCO2 inter-seasonal variation is controlled by the following four processes: temperature, air-sea exchange, biological activity, and a residual term that is mainly ascribed to the combined effects of physical mixing among different water masses through advective and diffusive transport. We adopted the temperature dependent coefficient proposed by Takahashi to calculate the inter-seasonal changes in pCO2 modulated by temperature. The inter-seasonal changes associated with air-sea exchange were estimated based on the air-sea CO2 flux and the total amount of dissolved inorganic carbon (DIC) in the mixed layer. The fractional contribution by biological activity was estimated based on the net community production (NCP) and DIC.

From winter to spring, the total pCO2 variation was 60±17 μatm, most of which was contributed by the sea surface temperature (SST) (from 24.17℃ in winter to 28.56℃ in spring) change that leveled up the pCO2 by 72±11 μatm. Air-sea exchange and biological activity drew down pCO2 by 20 μatm and 15 μatm, respectively. The residual terms was 23±20 μatm. From spring to summer, the total pCO2 variation was -28±23 μatm and the inter-seasonal change in temperature could only level up the pCO2 by 6±23 μatm. Air-sea exchange contributed -40 μatm due to the fact that the SCS was as a source of atmospheric CO2 in summer. Biological activity brought down the pCO2 by -32 μatm (net biological production). Other processes augmented the pCO2 by 38±33 μatm. From summer to fall, the total pCO2 variation was -14±27 μatm. Temperature effect drew down pCO2 by -47±23 μatm due to the decline in SST from 28.88℃ in summer to 25.79℃ in fall. The decrease in biological productivity resulted in an increase in pCO2 by 55 μatm and other processes drew down pCO2 by -32±35 μatm. From fall to winter, the total change in pCO2 was -18±22 μatm. Temperature effect contributed -24±29 μatm of pCO2 drawdown while the air-sea exchange leveled up the pCO2 by 37μatm due that the SCS was a sink of atmospheric CO2 in winter.