The partial pressure of carbon dioxide and air–sea fluxes in the northern South China Sea in spring, summer and autumn

Abstract

The distribution of pCO₂ in the surface waters of the northern South China Sea (NSCS) was examined in the summer of 2000, the spring of 2001 and the late fall of 2002. For the offshore region >100 km away from the coastline, surface water pCO₂ varied within a range of 360–450 μatm during all the surveys. Nevertheless, they were generally higher than atmospheric pCO₂. Sea–air ΔpCO₂ ranged in 50–100 μatm in the summer, 0–50 μatm in the spring and 0–90 μatm in the late fall. Average sea-to-air CO₂ flux was 7 mmol CO₂ m⁻² day⁻¹ in the summer and 1–3 mmol CO₂ m⁻² day⁻¹ in the spring and fall. Nearshore pCO₂ showed a very dynamic pattern likely associated with the regional hydrodynamic settings, yet again pCO₂ in the surface water overall exceeded the air pCO₂. Data from this study thus suggests that the NSCS served as a source of atmospheric CO₂. Seasonal variations of the pCO₂ in the NSCS outer shelf and slope surface waters were significantly influenced by sea surface temperature.

Introduction

Continental shelves and slopes comprise only ∼7% of the world's ocean surface area, yet they play a disproportionally important role in the global oceanic carbon cycle (Walsh et al., 1981, Rabouille et al., 2001, Chen, 2003). At present it remains difficult to reliably assess the source/sink terms and the associated air–sea CO₂ fluxes of the global ocean margin due primarily to the lack of pCO₂ field data with high spatial and temporal resolution in this complex regime (Fasham et al., 2001). Based upon annually integrated data obtained from the East China Sea (ESC) (Chen and Wang, 1999, Tsunogai et al., 1999, Wang et al., 2000) and the North Atlantic European shelf, Frankignoulle and Borges (2001) argued that middle- and high-latitude continental shelves in the Northern Hemisphere are a net sink of atmospheric CO₂. Sea–air fluxes in these shelves were estimated in the range from −0.68 mol m⁻² year⁻¹ off New Jersey (Boehme et al., 1998) and −0.9 mol m⁻² year⁻¹ in the Baltic Sea (Thomas and Schneider, 1999), to −2.9 mol m⁻² year⁻¹ in the ESC (Tsunogai et al., 1999, Wang et al., 2000) and in the North Atlantic European shelf (Frankignoulle and Borges, 2001). Liu et al. (2000) hypothesized that the world continental margins as a whole would be a weak CO₂ sink, ∼0.1 Gt C year⁻¹ as indicated by the net downward flux between atmosphere and margins. Extrapolated from seasonal field observations in a Northern European shelf sea, the North Sea, Thomas et al. (2004) reported an enhanced uptake of CO₂ by global coastal and marginal seas as ∼20% of the world ocean's uptake of anthropogenic CO₂, i.e. ∼0.4 Gt C year⁻¹. We contend that reliable global margin flux estimates may have to take into account the potential latitudinal difference between world margins (Cai and Dai, 2004). It is important to realize that most of the presently studied shelves are located at middle-latitude associated with a higher level of biological productivity. As revealed in open ocean regimes, the relative importance of the overall biological effect decreases from high latitude to low latitude. For example, the relative importance ratios of temperature to biological effects on surface water pCO₂ were typically 0.02 for the Ross Sea at 76°30′S, 0.9 for the North Pacific at 50 °N and 2.7 for the Sargasso Sea at 32°50′N (Takahashi et al., 2002). Therefore, while higher latitude shelves have been reported to behave as an atmospheric CO₂ sink, low latitude ocean margins may behave differently (Cai et al., 2003, Cai and Dai, 2004). Thus, a better understanding of the fluxes associated with the tropical and subtropical shelf waters is required to better constrain the overall source/sink terms of marginal seas.

Located between the equator and 23 °N, and characterized by a tropical and subtropical climate, the South China Sea (SCS) is the world's second largest marginal sea with a deep semi-closed basin and wide continental shelves to the northwest and south (Chen et al., 2001, Liu et al., 2002). This marginal sea is fed by two world major rivers (the Mekong and the Pearl Rivers) and some smaller rivers featuring tropical and subtropical drainage basins. Thus it provides an interesting contrast to most other CO₂ flux studies at marginal seas. Thus far, direct measurements of pCO₂ in this region were limited to a survey along the eastern boundary of the SCS in September 1994 (Rehder and Suess, 2001) and two surveys in the Taiwan Strait (north to the SCS, Fig. 1a) in August 1994 and February 1995 (Zhang et al., 2000). pCO₂ from the above two studies were measured using semi-continuous or discrete systems based on gas chromatography. Data from Rehder and Suess (2001) suggest that, when SST reached its maximum in the late summer, the SCS was a moderate source of atmospheric CO₂ with sea–air CO₂ fluxes of 0–1.9 mmol m⁻² day⁻¹ in the basin and 0.3–5.5 mmol m⁻² day⁻¹ in the southern shelf. Zhang et al. (2000) reported that the southern Taiwan Strait was a weak source in summer and a sink in winter, with the sea–air fluxes of ∼0.1 and ∼−8 mmol m⁻² day⁻¹, respectively. In addition, Chen and Huang (1995) estimated that the SCS contained ∼0.43 Gt C of anthropogenic CO₂ with a limited penetration depth of ∼500 m based upon seawater carbonate system measurements in the northeastern SCS. In summary, sea–air CO₂ flux data are at paucity and results from current studies are far from conclusive whether the SCS acts as a source or sink for atmospheric CO₂.

We report here the distribution of pCO₂ in the northern SCS (NSCS) based upon underway determinations during 3 cruises that encompass spring, summer and late fall. These data represent one of the most complete pCO₂ data sets obtained thus far for China Marginal Seas, and provide an important case study which implies low latitude marginal sea may act a source of atmospheric CO₂.