Ocean is the largest carbon reservoir on the earth's surface, approximately 50 times of atmospheric carbon pool and 20 times of the terrestrial carbon pool. The air-sea CO₂ flux is the key to understand the role of ocean on global carbon budget.

A large amount of ¹⁴C was injected into the stratosphere due to the atmospheric bomb tests during the 1950s–1960s, and subsequently it entered the ocean. The penetrated bomb-¹⁴C can be used as a tracer to constrain the air-sea CO₂ flux.

Recently, scientists from Institute of Earth Environment, Chinese Academy of Sciences traced the air-sea CO₂ flux through the Δ¹⁴C observation in the South China Sea, the largest sea in China. They measured the Δ¹⁴C levels in the atmosphere and seawater dissolved inorganic carbon in this region, compared the current and pre-bomb Δ¹⁴C depth profiles to determine the mean penetration depths and the inventories of bomb-¹⁴C, and then obtained the air-sea CO₂ exchange rates as well as the CO₂ flux rates combined with the CO₂ partial pressure between the ocean surface and the atmosphere.

They obtained a long-term average air-sea CO₂ exchange rate of 20.2±2.8 mol m-2 yr-1, and found that the bomb-¹⁴C traced air-sea CO₂ flux rate (0.54±0.08 mol m-2 yr-1) was comparable to the measured average flux (0.44±0.62 mol m-2 yr-1) by 47 cruise surveys in the South China Sea (Li et al., 2020). Additionally, they observed an air-sea Δ¹⁴C gradient reversal, that is, Δ¹⁴C in the air was lower than that in the surface seawater mainly due to the fossil fuel emissions, and then determined the ¹⁴C flux in this region.

Their study showed the ability of bomb-¹⁴C to determine the air-sea CO₂ flux in the South China Sea, and was published in Journal of Geophysical Research: Biogeosciences on Mar. 7.

In addition to the cruise surveys, satellite remote sensing and tower eddy covariance measurement, more various means are needed to accurately understand the size of oceanic carbon sinks in China, and then to provide credible data for the realization of Carbon Neutrality in China as early as possible. The first author, Prof. NIU Zhenchuan suggested a high resolution of temporal-spatial Δ¹⁴C observation there in future.

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn