The Yellow Sea is a sink of atmospheric CO2, rather than the previously thought source

What is it about?

Evidence suggests that the continental shelf CO2 influx rate might have increased globally during the past several decades. It remains a challenge to quantify air–sea CO2 fluxes in continental shelf regions and to understand their variations. The Yellow Sea on the western margin of the North Pacific represents one of the largest continental shelf regions. In this study, we conducted 10 carbonate surveying cruises between 2011 and 2018 that revealed regional differences in seasonal variation of surface fugacity of CO2 (fCO2) and air–sea CO2 fluxes on the Chinese side of the Yellow Sea. Results suggested that among various continental shelf CO2 sinks, the Yellow Sea represents one of the weakest marine CO2 absorbers. Despite the regionality of the study, we integrated spring/summer biological drawdown, summer/autumn vertical mixing, and universal warming/cooling and air–sea re-equilibration into an explanation of the seasonal variation in fCO2. These insights are fundamental to understanding sea surface carbonate chemistry in the Yellow Sea, and might help elucidate the biogeochemical controls of air–sea CO2 fluxes in other temperate seas.

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Why is it important?

The Yellow Sea is categorized as one of the 64 largest marine ecosystems globally. However, the status of the air–sea CO2 flux in the Yellow Sea remains unresolved. Briefly, Xue et al. (2011, 2012) concluded that the Yellow Sea serves as a net source of atmospheric CO2, whereas Xu et al. (2016) and Choi et al. (2019) suggested that several parts of the Yellow Sea act as annual net sinks of atmospheric CO2. It is worth noting that all previously published research on the air–sea CO2 exchange in the Yellow Sea (Xue et al., 2011, 2012; Qu et al., 2014, 2015, 2017; Xu et al., 2016; Choi et al., 2019) was based on insufficient data sets, having either temporal or spatial limitations. This limited and unclear understanding is in striking contrast to that concerning the adjoining East China Sea (ECS), which has been well documented as an example of a continental shelf that acts as an annual net sink of atmospheric CO2. This lack of understanding regarding the Yellow Sea is even more notable given that the surface area of the Yellow Sea is also considerable (i.e., approximately half that of the ECS) and that the Yellow Sea was once regarded as an extension of the ECS (e.g., Tsunogai et al., 1999). In this study, a multiyear data set of the carbonate system, collected during 10 mapping cruises conducted between 2011 and 2018 were analyzed to investigate the spatial variations of surface water carbonate parameters on the Chinese side of the Yellow Sea in different months. By resolving the temporal and spatial variations in the air–sea CO2 exchange fluxes, we found that the Chinese side of the Yellow Sea on the whole serves as an annual weak sink of atmospheric CO2. The findings of this study, based on one of the most complete CO2 data sets obtained thus far in the Yellow Sea, in hope of providing unambiguous insight regarding the status of air–sea CO2 flux in this important region.

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