| Abstrakt: |
Δ14C 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 14C flux and bomb 14C‐based air‐sea CO2exchange rates for this region. The background atmospheric Δ14C 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 14C from the sea to the atmosphere (7.4 ± 5.0 × 1011atoms m−2yr−1) was determined at the wind speed of 5.2 ± 1.7 m s−1. Seawater DIC Δ14C 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 14C was 585.5 ± 99.2 m, and a value of 8.2 ± 1.0 × 109atoms cm−2was obtained for the bomb 14C inventory in this region. Based on this inventory, a long‐term (1954–2015) average air‐sea CO2exchange rate of 20.2 ± 2.8 mol m−2yr−1was traced for the SCS. Combined with the pCO2measurements in this region, a net CO2flux rate of 0.54 ± 0.08 mol m−2yr−1was yielded for the SCS, which is comparable to the cruise measured flux (0.44 ± 0.62 mol m−2yr−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 14C observations on determining the air‐sea flux in this region. A large amount of radiocarbon (14C) was injected into the stratosphere due to the atmospheric bomb tests during the 1950s–1960s, resulting in the much higher level of 14C in the atmosphere than that in the ocean. The bomb‐14C enters the ocean and can be used to determine the long‐term average air‐sea CO2exchange rate. As the fossil fuel emissions increase, the 14C level in the atmosphere will be lower than that in the ocean. This reversal will cause a net transfer of 14C from the ocean to the atmosphere. Here, we report the 14C level in the atmosphere and seawater dissolved inorganic carbon in the South China Sea (SCS) to determine the 14C flux and air‐sea CO2exchange rates based on bomb‐14C in this region. A net transfer of 14C from the sea to the atmosphere is observed in this region. The net CO2flux rate for the SCS based on bomb‐14C is comparable to the flux obtained from the cruise measurements. This study indicates that the ocean is becoming a net source of atmospheric 14C and highlights the importance of continued ocean 14C observations on determining the air‐sea flux. The air‐sea Δ14C gradient reversal that Δ14C in the air was lower than that in the surface seawater was observed in the South China SeaThe South China Sea serves as a source of atmospheric 14C with a net transfer of 7.4 ± 5.0 × 1011atoms m−2yr−1in 2015Bomb 14C‐based air‐sea CO2exchange rate and net flux rate are respectively comparable to previous similar studies and cruise measurements The air‐sea Δ14C gradient reversal that Δ14C in the air was lower than that in the surface seawater was observed in the South China Sea The South China Sea serves as a source of atmospheric 14C with a net transfer of 7.4 ± 5.0 × 1011atoms m−2yr−1in 2015 Bomb 14C‐based air‐sea CO2exchange rate and net flux rate are respectively comparable to previous similar studies and cruise measurements |
Plný text