Summertime Carbon Dioxide System In The Bering Sea And Western Arctic Ocean

The Arctic region is one of the most sensitive regions to the global change in the world.The rapid change occurring in the Arctic Ocean,such as sea ice dramatically thinning and shrinking,which were induced by global warming,are extremely remarkable and are focused by more and more global warming researchers.As an important part of schedule during the 4^th International Polar Year(IPY), which were held every 50 years normally,the 3^rd Chinese National Arctic Research Expedition(CHINARE 2008) was carried out from July to September in 2008.The partial pressure of CO₂(pCO₂) in the air and in the surface sea water were observed along the cruise track,and quantity numbers of sea water samples were taken for CO₂ system parameters measurements(viz.pH,total alkalinity(TA),total dissolved inorganic carbon(DIC)).121 stations in total and 1166 samples were taken for each parameter,including 35 and 86 stations in Bering Sea and the western Arctic Ocean respectively.The distributions of CO₂ system parameters in the Bering Sea and the western Arctic Ocean were all clarified,and the according controlling factors were discussed. The flux of CO₂ from air to sea and its trend of changing in the future were estimated in the Arctic Ocean under the rapid changes,which could give us the better knowledge of the response and its feedback of the Arctic to the global change.Results can be mainly summarized as the followings:1,Spatial-tempo distributions of pCO₂ in the surface sea water in the Bering Sea in summertime were described as the followings:central Bering shelf＜Bering Basin＜northeast Bering shelf＜Bering strait upwelling,where pCO₂ in the central Bering shelf was 213±34μatm(148～301μatm),pCO₂ in the Bering basin was 297±51μatm(178～374μatm),pCO₂ in the northeast Bering shelf influenced by river plume was 332±30μatm (277～387μatm),and Bering strait upwelling had pCO₂ values of 476±68μatm(302～562μatm) because of high pCO₂ in the bottom water.Bering Sea acts as a net sink for atmospheric CO₂ in summer generally. 2,Analyzing pCO₂ controlling factors for different areas of Bering Sea indicated that pCO₂ northward decrease in the Bering basin were attributed to temperature cooling and mainly driven by solubility pump, while low pCO₂ in Bering shelf was driven by biological pump as a result of high primary production.However,northeast Bering shelf received much river runoff discharge which was commonly supersaturated in pCO₂,so pCO₂ of this region was mainly influenced by water masses mixing and net balance of photosynthetic CO₂ fixation and respiration of organic material(OM).In the upwelling of Bering Strait,pCO₂ was controlled by hydrological processes,and acted as a strong CO₂ source though its primary production and chlorophyll were both high.In other regions of Bering Strait,pCO₂ was driven by different water masses and their mixing.3,The ranges of summertime DIC and TA in the Bering Sea surface water were 1827～2145μmol kg^-1,2097～2244μmol kg^-1,respectively.TA in Bering Sea surface water was mainly influenced by fresh water input,but DIC showed complicated distribution driven by many factors.Our results suggested TA in the upper water column of Bering Sea was mainly driven by conservative mixing with fresher water,and no obvious CaCO₃ dissolution/precipitation were detected.When removed the effect of dilution by normalized DIC and TA to salinity as 35,the nDIC showed a good relationship to the nTA,with a value ofΔnTA/ΔnDIC approximate to the Radfield ratio,and implicated that DIC in the upper water column of Bering Sea were mainly controlled by biological factors.4,Using the average wind speed and average pCO₂,the air-sea CO₂ fluxes in Bering shelf-slope and Bering basin were estimated,-12.3 mmol m^-2d^-1 and -7.6 mmol m^-2d^-1,respectively,then the annual uptake of Bering Sea was～30Tg C. 5,The summertime pCO₂ distribution in the western Arctic Ocean was: Chukchi Sea＜Canada Basin sea-ice zone＜Mendeleyev Ridge＜Canada Basin ice-free zone.The whole western Arctic Ocean was a CO₂ sink in summer.Summertime Chukchi Sea shelf pCO₂ was very low,varying from 132μatm to 301μatm,and average was 200μatm.However,pCO₂ increased sharply from shelf 218μatm to slope 367μatm.High rates of seasonal primary and net community production during the brief, summertime exposure of nutrient-laden surface waters of Pacific Ocean origin maintained the summertime Chukchi Sea shelf low pCO₂ and a perennial oceanic CO₂ sink.Relative high pCO₂(303～363μatm) was detected in the slope-basin area around 75°N and 155°W,and was attributed to a combination of OM remineralization generated earlier in this season and riverine OM decomposition.Except for area around 75°N and 155°W,Canada Basin pCO₂ was not high,ranging from 230μatm to 303μatm,and was a result of CO₂ biological removal by ice algae and phytoplankton,the shelf to basin outflow of low pCO₂ water from the Chukchi Sea shelf horizontally exports into the Canada Basin,and northward transit cooling.6.The ranges of summertime TA,nTA,DIC and nDIC in the western Arctic Ocean surface water were 1757～2229μmol kg^-1,2383～2722μmol kg^-1,1681～2034μmol,2119～2600μmol kg^-1,respectively. The TA in the Canada Basin and Chukchi shelf-slope,as well as the DIC in the Canada Basin and the Chnkchi slope,have a good relationship with salinity,illustrating a mixing process together with sea-ice melt water and river runoff.ΔnTA/ΔnDIC was equal to 1 in the slope and basin, suggested a conservative mixing without net biological uptake of CO₂.In contrast,the DIC had an obvious biological removal in Chukchi shelf at the salinity of 29 to 33.However,it seems like conservative at both ends of relative low and high salinity.7,The air-sea CO₂ fluxes in the Chukchi shelf and the slope were -16.5,-6.7 mmol m^-2 d^-1,respectively,then an annual uptake～10.9T g was calculated.The Canada Basin was divided into 4 parts based on sea-ice condition and pCO₂ values:ice-free zone,floating ice zone,pack ice zone and Mendeleyev Ridge,and each air-sea CO₂ fluxes were -3.6,-5.2,-0.5 and -7.0 mmol m^-2 d^-1,respectively.