Air–sea CO 2 fluxes in the southern Yellow Sea: An examination of the continental shelf pump hypothesis

Abstract

The southern Yellow Sea (SYS), located to the north of the East China Sea (ECS), was considered part of the ECS when Tsunogai et al. (1999) proposed the “continental shelf pump” (CSP) hypothesis. However, the original CSP carbon dioxide (CO 2) uptake flux (2.9 mol C m −2 yr −1) appears to have been overestimated, primarily due to the differences between the SYS and the ECS in terms of their CO 2 system. In this paper, we estimated air–sea CO 2 fluxes in the SYS using the surface water partial pressure of CO 2 ( pCO 2) measured in winter, spring, and summer, as well as that estimated in fall via the relationship of pCO 2 with salinity, temperature, and chlorophyll a. The results indicate that overall, the entire investigated area was a net source of atmospheric CO 2 during summer, winter, and fall, whereas it was a net sink during spring. Spatially, the nearshore area was almost a permanent CO 2 source, while the central SYS shifted from being a CO 2 sink in spring to a source in the other seasons of the year. Overall, the SYS is a net source of atmospheric CO 2 on an annual scale, releasing ∼7.38 Tg C (1 Tg=10 12 g) to the atmosphere annually. Thus, the updated CO 2 uptake flux in the combined SYS and ECS is reduced to ∼0.86 mol C m −2 yr −1. If this value is extrapolated globally following Tsunogai et al. (1999), the global continental shelf would be a sink of ∼0.29 Pg C yr −1, instead of 1 Pg C yr −1 (1 Pg=10 15 g). The SYS as a net annual source of atmospheric CO 2 is in sharp contrast to most mid- and high-latitude continental shelves, which are CO 2 sinks. We argue that unlike the ECS and the North Sea where carbon on the shelf could be exported to the open ocean, the SYS lacks the physical conditions required by the CSP to transport carbon off the shelf effectively. The global validity of the CSP theory is thus questionable.