Revisiting the Seasonal Variations of Sea-Air CO2 Fluxes in the Northern East China Sea

Dongseon Kim,Jeonghee Shim,Cheol-Ho Kim,Sang-Hwa Choi,Kyung-Hee Kim

doi:10.3319/tao.2012.12.06.01(oc)

Abstract

Temperature, salinity, chlorophyll a (Chl-a), nitrate, and sea-air differences of CO2 partial pressure (ΔpCO2) were extensively investigated in the northern East China Sea (ECS) during seven research cruises from 2003 to 2009. The ΔpCO2 showed large intraseasonal variation in the spring and summer. In spring, the areal mean ΔpCO2 in May 2004 was almost half of that in April 2008, probably associated with differences in sea surface temperature (SST). In summer, the areal mean ΔpCO2 in August 2003 was also twice as large as that in July 2006. In addition, ΔpCO2 exhibited large seasonal variation with positive values in autumn and negative values in other seasons. The positive ΔpCO2 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 m-2 yr-1, indicating CO2 absorption from atmosphere to the sea, which was more than two times lower than the previous estimate (Shim et al. 2007) reported for the same region. This large difference was presumably responsible for the underestimation of winter CO2 influx by Shim et al. (2007) and the large interannual variation of CO2 flux. 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.

Highlights

Continental margins cover only about 7% of the world’s ocean surface area, they play a major role in oceanic carbon cycling, receiving larger nutrient supplies from riverine input and sustaining higher biological production because of coastal upwelling (Chen and Borges 2009)
Surface nitrate concentrations were highest in the westernmost part of the study area, and decreased gradually eastward; concentrations in the eastern part of the study area were lower than 1.0 μmol L-1 (Fig. 2)
Kim et al (2009) suggested that the high surface nitrate concentrations in the northern East China Sea (ECS) during spring resulted from vertical mixing, which brought large supplies of nitrate from deep water

Summary

Introduction

Continental margins cover only about 7% of the world’s ocean surface area, they play a major role in oceanic carbon cycling, receiving larger nutrient supplies from riverine input and sustaining higher biological production because of coastal upwelling (Chen and Borges 2009). Chen and Borges (2009) synthesized worldwide measurements of the partial pressure of CO2 (pCO2) at the continental shelves and suggested that the temperate and high-latitude shelves are undersaturated with respect to atmospheric CO2 in all seasons, while low-latitude shelves are oversaturated. The East China Sea (ECS) is the third largest marginal sea in the world and includes a large area of shallow continental shelf with enormous freshwater inputs from the Changjiang. The ECS is one of the most productive marginal seas in the world due to the large nutrient supply from the Changjiang and upwelling (Wong et al 2000; Gong et al 2003). Measurements of the surface pCO2 and other carbon parameters have been carried out in the ECS since the late

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