Variations Of The Air-sea CO₂ Flux And Dissolved Inorganic Carbon Inventory In The Yellow Sea And Its Controls

As a transitional zone between the land and ocean,continental shelf plays an important role in the global ocean carbon cycle.Meanwhile,the carbon source or sink states of some continental shelf are unclear due to their high spatial and temporal variability.Therefore,large number of CO2 studies in the coastal oceans are initiated.This paper further discusses the sea-air CO2 fluxes,carbon inventory and evaluates their regulatory mechanism,it not only enriches and perfects the research of carbon source and sink states in the continental shelf,but also provides a basis for understanding the carbon cycle process and carbon budget in the Yellow Sea under the global environment.Based upon 10 carbonate surveying cruises conducted between 2011 and 2018,we showed that monthly variation of dissolved oxygen and carbonate parameters in the Yellow sea,calculated the sea-air CO2 fluxes,the air-sea equilibration time,the dissolved inorganic carbon inventory in four different subregions on Chinese side of the Yellow Sea and the net community production in the North Yellow Sea,discussed the controlling processes of sea surface fCO2 and the major mechanism maintaining annually net CO2 uptake in the Yellow Sea.Below are the results obtained:(1)During our surveys,surface water fCO2 ranged from 172 to 654 μatm,while the air-equilibrated partial pressure of CO2(pCO2air)varied limitedly from 381 to 420μatm.Considering the cruise-specific pCO2air,the North Yellow Sea was CO2-supersaturated in July and from September to November,the western South Yellow Sea was CO2-supersaturated in April and from June to November,the central South Yellow Sea was CO2-supersaturated during July to September,and the southern South Yellow Sea was CO2-supersaturated during September to November.In the other months for given subregions,the survey-averaged fCO2 was lower than pCO2air.All the four subregions served as weak sinks of the atmospheric CO2,with sea-air CO2 flux values of-0.7±1.7 mol m-2 yr1 in the North Yellow Sea,-0.2±2.2 mol m-2 yr-1 in the western South Yellow Sea,-0.8±1.8 mol m-2 yr-1 in the central South Yellow Sea,and-0.1±2.0 mol m-2 yr1 in the southern South Yellow Sea.The Yellow Sea acted as a carbon sink annually with a flux of-0.5±1.9 mol m-2 yr-1,which equals an absorption of 1.1 × 1012 g C from the atmosphere in the four subregions.Our results suggested that the Yellow Sea represents one of the weakest CO2 absorbers among those CO2-sinking shelf areas in the world.(2)Multiple mechanisms controlled the seasonal variation of surface fCO2 in the Yellow Sea.Besides the universal effects of warming and cooling on surface fCO2,biological CO2 drawdown promoted CO2 absorption from the atmosphere in warm seasons,while the enhanced vertical mixing in autumn and early winter transported the earlier remineralization products(i.e.,CO2)from the subsurface layers to the surface,leading to the Yellow Sea releasing CO2 to the atmosphere in these transitional seasons.Moreover,air-sea re-equilibration also played an important role in modulating surface fCO2,especially in cold and windy seasons.Cross-shelf circulation likely transports the excess carbon out of the shelf,maintaining the annually net CO2 uptake over the Yellow Sea.(3)The average annual air-sea CO2 and O2 equilibrium time in the Yellow Sea were 82±34 d and 7±4 d,respectively.The CO2 exchange rate is slower than O2,which is one of the reasons for the deviation between the fCO2-DO%and Redfield ratio in the Yellow Sea from summer to early winter.Wind speed has a significant effect on the air-sea CO2 equilibrium time.(4)The average annual DIC inventory in the Yellow Sea is 1.9×1013 mol,which is significantly correlated with salinity inventory.During the algal bloom,DIC in the upper mixed layer was transformed into organic carbon through the primary production process,and then decomposed in subsurface waters,resulting in no significant change in DIC inventory of the whole column.The volume proportion of the Cold Water Mass in the North Yellow Sea exceeded that of DIC inventory,which has a certain accumulation effect on DIC.Therefore,after the disappearance of water stratification,the high content of DIC in the Cold Water Mass will be brought to the surface with strong vertical mixing,and then increasing surface fCO2.