Air-sea CO_2 fluxes in the southern Yellow Sea: An examination of the continental shelf pump hypothesis

摘要

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 (2.9molCm-2yr-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 ~7.38TgC (1Tg=10~(12)g) to the atmosphere annually. Thus, the updated CO_2 uptake flux in the combined SYS and ECS is reduced to ~0.86molCm~(-2)yr~(-1). If this value is extrapolated globally following Tsunogai et al. (1999), the global continental shelf would be a sink of ~0.29PgCyr~(-1), instead of 1PgCyr~(-1) (1Pg=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.