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

Abstract

AbstractΔ14C 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 CO2 exchange 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 × 1011 atoms m−2 yr−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 × 109 atoms cm−2 was obtained for the bomb 14C inventory in this region. Based on this inventory, a long‐term (1954–2015) average air‐sea CO2 exchange rate of 20.2 ± 2.8 mol m−2 yr−1 was traced for the SCS. Combined with the pCO2 measurements in this region, a net CO2 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 14C observations on determining the air‐sea flux in this region.