Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air‐Sea CO2 Exchange Rate Tracing

Abstract Δ 14 C values of the atmosphere and seawater dissolved inorganic carbon (DIC) were measured during a cruise in the South China Sea (SCS) in September 2015, in order to determine the 14 C flux and bomb 14 C‐based air‐sea CO 2 exchange rates for this region. The background atmospheric Δ 14 C value (13.8 ± 5.0‰) for the SCS during that period was lower than that (35.4 ± 3.4‰) of surface seawater (5 m) DIC, and a net transfer of 14 C from the sea to the atmosphere (7.4 ± 5.0 × 10 11 atoms m −2 yr −1 ) was determined at the wind speed of 5.2 ± 1.7 m s −1 . Seawater DIC Δ 14 C profiles showed the highest value (37.9 ± 3.7‰) at a depth of 100 m, a rapid decrease below that depth to −220.3 ± 3.2‰ at 1,500 m, and nearly constant values below 1,500 m. The average mean penetration depth of bomb 14 C was 585.5 ± 99.2 m, and a value of 8.2 ± 1.0 × 10 9 atoms cm −2 was obtained for the bomb 14 C inventory in this region. Based on this inventory, a long‐term (1954–2015) average air‐sea CO 2 exchange rate of 20.2 ± 2.8 mol m −2 yr −1 was traced for the SCS. Combined with the p CO 2 measurements in this region, a net CO 2 flux rate of 0.54 ± 0.08 mol m −2 yr −1 was yielded for the SCS, which is comparable to the cruise measured flux (0.44 ± 0.62 mol m −2 yr −1 ) obtained from a synthesis study (Li et al., 2020, https://doi.org/10.1016/j.pocean.2020.102272). Our study highlights the importance of continued atmospheric and seawater 14 C observations on determining the air‐sea flux in this region.