The Sea-Air CO2 Fluxes in the Korean Marginal Seas and the Western North Pacific

Abstract

To elucidate the distribution of surface fCO2, its controlling factors and sea-air CO2 fluxes in the marginal sea and open ocean, surface fCO2 data were analyzed in the Ulleung Basin (UB) of the East Sea (ES), northern East China Sea (ECS), and western North Pacific (NP). Temperature, salinity, chlorophyll a (Chl-a), and surface CO2 fugacity (fCO2) were extensively investigated in the UB of the ES during four cruises. In spring, surface fCO2 showed large variations ranging from 260 to 356 μatm, which were considerably lower than the atmospheric CO2 levels. Surface fCO2 was highest (316 to 409 μatm) in summer. The central part of the study area was undersaturated with respect to atmospheric CO2, while the coastal and easternmost regions were oversaturated. In autumn, the entire study area was fairly undersaturated with respect to atmospheric CO2. In winter, surface fCO2 ranged from 303 to 371 μatm, similar to that in autumn, despite the much lower sea surface temperature. The seasonal variation in surface fCO2 could not be explained solely by seasonal changes in sea surface temperature and salinity. The vertical mixing, lateral transport, and sea-air CO2 exchange considerably influenced the seasonal variation in surface fCO2. The UB of the ES was a sink of atmospheric CO2 in spring, autumn, and winter, but a weak source of CO2 to the atmosphere in summer. The annual integrated sea-air CO2 flux in the UB of the ES was -2.47 ± 1.26 mol C m-2 yr-1, quite similar to a previous estimate (-2.2 mol C m-2 yr-1) in the south ES. This indicates that the UB of the ES acts as a strong sink of atmospheric CO2. Temperature, salinity, chlorophyll a, nitrate, and sea-air differences of CO2 fugacity (4fCO2) were extensively investigated in the northern ECS during seven research cruises from 2003 to 2009. The 4fCO2 showed large intraseasonal variation in spring and summer. In spring, the areal mean 4fCO2 was almost two times lower in April 2008 than in May 2004, probably associated with differences in sea surface temperature (SST). In summer, the areal mean 4fCO2 in August 2003 was also twice as large as that in July 2006. In addition, 4fCO2 exhibited large seasonal variation with positive values in autumn and negative values in other seasons. The positive 4fCO2 in autumn was ascribed to vertical mixing with CO2-enriched subsurface waters and relatively high SST in this season. The annually integrated sea-air CO2 flux in the northern ECS was -2.2 ± 2.1 mol C m-2 yr-1, indicating CO2 absorption from atmosphere to the sea. The CO2 influx in the ECS was twice that estimated for continental shelves worldwide, suggesting that the ECS acts as a strong sink of atmospheric CO2 compared to other continental shelves. Temperature, salinity, and surface fCO2 were measured in the western NP during five research cruises from 2006 to 2010. Temporal SST variations were heavily associated with Oceanic Nino Index (ONI). However, ONI was not the immediate cause of temporal variations of surface fCO2. Up to 65~80% of temporal variations of surface fCO2 could be explain by thermodynamic changes from SST and sea surface salinity (SSS) variability in half of all the observations. Biological activity might affect to reduce the surface fCO2 in May 2010, while it might rarely in September 2006 and June 2009. Though mixed layer depth (MLD) was deepening with reversal ONI, deep MLD could not lead to increase surface fCO2. Sea-air CO2 flux (over 0.5 mmol C m-2 day-1) induced decrements the variability of surface fCO2. The North Equatorial Current (NEC), southern area, acted as a source of CO2 to the atmosphere from May to September. In October, the NEC acted as a CO2 sink. The subtropical gyre, northern area, acted as a CO2 source from June to September. While it acted as a CO2 sink in May and October. The annual CO2 uptake rate would be 3.15 x 10-3 Pg C yr-1 and 2.96 x 10-3 Pg C yr-1 in the UB of the ES and the northern ECS, respectively. Those were 0.22% and 0.21% of annual global carbon uptake rate, while those were 0.033% and 0.034% of global ocean area. In the western NP, the annual CO2 uptake rate would be 8.03 x 10-2 Pg C yr-1, 5.6% of global carbon uptake rate whereas 8.1% of area (Takahashi et al., 2009). The Korean marginal seas, the UB of the ES and northern ECS, are powerful atmospheric CO2 sinks about seven times as strong as open ocean, the western NP.