Abstract
Abstract. Based upon 14 field surveys conducted between 2003 and 2008, we showed that the seasonal pattern of sea surface partial pressure of CO2 (pCO2) and sea–air CO2 fluxes differed among four different physical–biogeochemical domains in the South China Sea (SCS) proper. The four domains were located between 7 and 23° N and 110 and 121° E, covering a surface area of 1344 × 103 km2 and accounting for ~ 54% of the SCS proper. In the area off the Pearl River estuary, relatively low pCO2 values of 320 to 390 μatm were observed in all four seasons and both the biological productivity and CO2 uptake were enhanced in summer in the Pearl River plume waters. In the northern SCS slope/basin area, a typical seasonal cycle of relatively high pCO2 in the warm seasons and relatively low pCO2 in the cold seasons was revealed. In the central/southern SCS area, moderately high sea surface pCO2 values of 360 to 425 μatm were observed throughout the year. In the area west of the Luzon Strait, a major exchange pathway between the SCS and the Pacific Ocean, pCO2 was particularly dynamic in winter, when northeast monsoon induced upwelling events and strong outgassing of CO2. These episodic events might have dominated the annual sea–air CO2 flux in this particular area. The estimate of annual sea–air CO2 fluxes showed that most areas of the SCS proper served as weak to moderate sources of the atmospheric CO2, with sea–air CO2 flux values of 0.46 ± 0.43 mol m−2 yr−1 in the northern SCS slope/basin, 1.37 ± 0.55 mol m−2 yr−1 in the central/southern SCS, and 1.21 ± 1.48 mol m−2 yr−1 in the area west of the Luzon Strait. However, the annual sea–air CO2 exchange was nearly in equilibrium (−0.44 ± 0.65 mol m−2 yr−1) in the area off the Pearl River estuary. Overall the four domains contributed (18 ± 10) × 1012 g C yr−1 to the atmospheric CO2.
Highlights
We focused on four selected physical–biogeochemical domains in the South China Sea (SCS) proper (Fig. 1, Table 1), where the spatial coverage of our observational data was satisfactory at a temporal resolution of seasonal levels (Fig. 2)
In the oligotrophic northern SCS, it has been reported that temperature is the most important factor influencing the seasonal variation of sea surface pressure of CO2 (pCO2) (Zhai et al, 2005a; Tseng et al, 2007)
According to Zhai et al (2005a), the SSTdriven pCO2 values in the northern SCS vary in the range (370 ± 20) × e0.0423(SST −26), where 370 ± 20 is comparable to the range of local atmospheric pCO2 at 10 m, 0.0423 ◦C−1 is the coefficient for the temperature effect of seawater pCO2 (Takahashi et al, 1993), and 26 is comparable to the annual average sea surface temperature (SST) in the offshore area of the northern SCS (Fig. 3a)
Summary
As an important component of global carbon cycling, coastal ocean carbon has received considerable attention during the past three decades (e.g., Walsh et al, 1981; Smith and Hollibaugh, 1993; Tsunogai et al, 1999; Borges et al, 2005; Cai et al, 2006; Chen and Borges, 2009; Laruelle et al, 2010; Liu et al, 2010; Borges, 2011; Cai, 2011; Dai et al, 2013). Recent estimates of global coastal ocean sea–air CO2 fluxes have converged to conclude that the coastal ocean is an atmospheric CO2 sink of 0.2 to 0.4 Pg C yr−1 (Pg = 1015 g; Cai et al, 2006; Chen and Borges, 2009; Laruelle et al, 2010; Liu et al, 2010; Borges, 2011; Cai, 2011; Dai et al, 2013) This current estimate is a significant change from the earlier speculation of up to 1 Pg C yr−1 (Tsunogai et al, 1999), but Published by Copernicus Publications on behalf of the European Geosciences Union. Adding even more complexity is that the coastal ocean is very often characterized by the highest spatial gradient in both physics and biogeochemistry, and has inherited complicated and differing physical–biogeochemical domains
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