Summertime Carbonate System Of The Bering Sea And Chukchi Sea

The Bering Sea and the Chukchi Sea are the marginal seas of the sub-Arctic and Arctic respectively.Due to the wide continental shelf and high productivity,they have become strong CO₂ sink areas.However,in recent decades,with the impact of global climate change,the marine environment and carbonate system of the two seas have also undergone significant changes.Water masses warming,sea-ice loss,and the northward movement of biological communities have contributed to CO₂ absorption capacity and increased ocean acidification in the Bering Sea.At the same time,the CO₂ sink trend of the Chukchi Sea is extremely uncertain.The inflow of the Pacific Ocean not only provides a large amount of nutrients for the Arctic Ocean in summer,but also brings more acidified waters to the Chukchi Sea,accelerating the acidification of the Arctic Ocean.Thus,ocean acidification poses a threat to the sub-Arctic and Arctic ecological environment.Therefore,the study of long-term trend of pCO₂ and ocean acidification in the Bering Sea is of great significance.We examined the dynamics of the carbonate system and air-sea CO₂ exchanges during 10^th Chinese National Arctic Research Expedition(CHINARE)from August to September 2019.Based on pCO₂,DIC,TA,dissolved oxygen,nutrients(NO₃^-,PO₄^3-)and calculated?_arag,we compared the pCO₂and air-sea CO₂ fluxes between the Bering Sea and Chukchi Sea.Then,we evaluated the status of ocean acidification and its driving mechanisms in the Bering Sea.In addition,based on the 20-year high-precision pCO₂ navigation data of the CHINARE1999?2019,the decadal variation of pCO₂ in the Bering Sea in summer and its main regulatory factors have been revealed.The main points of this study are as follows:In the summer of 2019,the Bering Sea was the strong CO₂ sink of atmospheric pCO₂ as a whole.The air-sea CO₂ flux was about-8.3±1.4 mmol m^-2 d^-1,and the net carbon sink was 23.3 TgC.There were significant regional differences in sea surface pCO₂ and air-sea CO₂ flux.The carbon sink capacity showed that the Bering Sea shelf and continental slope area were higher than the ocean basin area.In high-nutrient,low-chlorophyll(HNLC)basins,the CO₂ absorption was weaker than that of shelf and slope potentially.The eastern area close to the Alaska was affected by the riverine which has high pCO₂.As a result,the eastern area carbon sink capacity was weaker than that of the west.Non-thermodynamic factors such as biological processes and river input were the dominant factors that regulate the distribution of pCO₂ and CO₂ sink in the Bering Sea in 2019.From 1999 to 2019,sea surface pCO₂ growth rate of the Bering Sea was1.5 times of the atmospheric pCO₂ in summer.The growth rate of sea surface pCO₂ in the basin and slope area was higher than atmospheric pCO₂,while the growth rate of pCO₂ in the shelf area was lower than atmospheric pCO₂.As a result,the carbon sink capacity was weakening in the basin and slope,while the shelf was increasing.Thermodynamic factors were the dominant factor affecting the decadal changes of pCO₂ in the Bering Sea.In a cold-warm year cycle from 2008 to 2019,the sea surface temperature increased by about 2?from cold years to warm years.Simultaneously,?pCO₂ showed an increasing trend.The summer ocean acidification in the Bering Sea of 2019 showed obvious regional differences.In the basin,?_arag began to appear unsaturated at 100 m.Due to the remineralization of organic matter and the input of anthropogenic CO₂ at the depth of 2000 m,?_arag reached an extremely low value of 0.4.Ocean acidification of the shelf and slope area were not obvious.?_arag unsaturation occurs only in the bottom layer and shelf affected by river input.The western shelf of the Bering Strait,which the entire water column?_arag unsaturated showed the most serious ocean acidification.Natural processes and human activities affect?_arag of the Bering Sea in summer.Based on the seawater-river-other freshwater mixed model,we calculated that the river input could reduce the?_arag in the upper 20 m by about 0.47,and the air-sea CO₂ exchange made?_arag reduced only about 0.04.However,high bio-productivity increased?_arag by 0.81in summer,which was enough to offset the decrease in river water input and air-sea CO₂ exchange.As a result,bio-productivity slowed down the ocean acidification process of the upper water body of the Bering Sea in summer.At the same time,ocean acidification in the Bering Sea shelf area is the result of the accumulation of anthropogenic CO₂ accumulated in the Bering Sea shelf.In the summer of 2019,the Chukchi Sea was a strong CO₂ sink area.The average air-sea CO₂ flux was-12.1±2 mmol m^-2 d^-1,and the summer net CO₂ sink was about9.7 TgC.In the shelf south of 72°N,strong CO₂ sink was mainly controlled by strong biological productivity that the average net community productivity was 137 mg C m^-2d^-1.However,in the shelf north of 72°N where average net community productivity was only 54.5 mg C m^-2 d^-1,the weaker CO₂ sink was mainly affected by physical mixing.With the northward movement of the glacial line,the Chukchi Sea CO₂ sink capacity has increased.Compared with the Chukchi Sea,the Bering Sea?pCO₂ value remained relatively stable,while the Chukchi Sea?pCO₂ value was more negative.Coupled with the dilution of the melting ice water,the Chukchi Sea CO₂ sink became stronger.