Air-sea CO₂ Fluxes And Ocean Acidification At The Tip Of The Antarctic Peninsula

The Southern Ocean has relative low pH and saturation states of aragonite(?_arag)due to cold temperatures and upwelling of CO₂-enriched deep waters,which is also considered to be a more acidified waters in the world's ocean.The Antarctic Peninsula,as the closest edge of the Antarctic continent to Scotia sea,has the strongest combination of deep and subsurface and surface waters.It is also of great significance to the global thermohaline circulation and the feedback of climate change.Located at the intersection of Weddell-Scotia Confluence?WSC?,the TAP is northwardly affected by the Antarctic Circumpolar Current?ACC?of the Scotia Sea and southward by the Weddell Circulation?WG?.The Weddell Sea Deep Water?WDW?in the south spreads northwest to the Drake Passage and is intensively mixed with the ACC that flows eastward.Due to the geographical location of TAP and the special effects of surface layer and deep water mass circulation,the coupling of sea ice process,biological process and hydrological process makes the study of the ability of the sea to absorb atmospheric CO₂ and its influence on ocean acidification more complicated.Most of the previous studies focused on the surface circulation and bioproduction of TAP.Only a few studies have reported the effects of sea ice melting on the carbonation chemistry of surface seawater in WSC.There is no systematic study of the carbonate chemistry and its driving mechanisms in this sea area.Therefore,we examined the dynamics of the carbonate system and air-sea CO₂exchanges during 33^th Chinese National Antarctic Research Expedition?CHINARE?from December 2016 to January 2017.On the base of temperature,salinity,depth,total alkalinity?TA?,total dissolved inorganic carbon?DIC?and calculated pH and?_arag,we evaluated the status of ocean acidificationand the impact on marine calcareous organisms.The results of this paper are as follows:The northwest of TAP was mainly controlled by the upwelling of CO₂-enriched deep water?CDW?to the surface layer.The average value of surface seawater pCO₂was 415±21?atm,which was higher than the atmosphere.The average air-sea CO₂flux was 5±4 mmol C m^-22 d^-1 here,which was a source of atmospheric CO₂;The increase of summer productivity and the weakening of vertical mixing caused by the shelf water of the South Oakney Sea Station are the main controlling factors in the Northeastern TAP.As a result,the average value of surface seawater pCO₂ was335±17?atm,which was lower than the atmosphere.The average air-sea CO₂ flux was-35±28 mmol C m^-22 d^-1 here,which was also drived by a strong sink of atmospheric CO₂;Due to the winter residual water layer,the average value of south surface seawater pCO₂ of TAP was 305±27?atm,which was much lower than atmospheric pCO₂.The average air-sea CO₂ flux was-22±20 mol C m^-22 d^-1 here,which was a strong sink of atmospheric CO₂.In addition,the average air-sea CO₂ flux at the tip of the Antarctic Peninsula was-25±20 mmol C m^-22 d^-1,which showed strong CO₂ absorption capacity.The summer surface seawater acidification observed in the three sea areas at the tip of the Antarctic Peninsula had large regional differences from the surface to the deep layer.Among them,the average values of surface seawater pH and?_arag in the northwest TAP were about 8.01±0.03 and 1.30±0.07,and the surface seawater acidification was the most serious.The minimum values of pH and?_aragrag were observed in the CDW of the deep water(pH<7.92,?_arag<1.00).The average values of surface water pH and?_arag in the northeastern TAP were about 8.10±0.03 and1.64±0.12.The deep acidifying water(pH<7.95,?_arag<1.10)was also the CDW brought by ACC;The average values of surface water pH and_aragonite in the Southern TAP were about 8.12±0.05 and 1.57±0.17.The acidifying water(pH<7.93,?_arag<1.10)observed in the deep layer was the WDW caused by the Weddell Gyre.Based on the endmember mixed model of Glacier?Sea Ice?WW and CDW,It was estimated that the northwest TAP was mainly affected by the CO₂-enriched CDW upwelling,and the contribution of this process to the ocean acidification index parameters?85%?was much greater than other effects;The average values of surface seawater pH and?_arag in the Northeastern TAP were about 8.10±0.03 and 1.64±0.12,respectively.The melting ice effects reduced pH and?_arag by 0.03 and 0.12,respectively,and bio-productive contributed in an increase of 0.10 and 0.34 for pH and?_arag;The average values of surface seawater pH and?_arag in the Southern TAP were about 8.12±0.05 and 1.57±0.17,respectively.The melting ice effects reduced pH and?_arag by 0.08 and 0.25,respectively,and bio-productive contributed in an increase of 0.10 and 0.34 for pH and?_arag,which also effectively buffered the negative effects of sea ice melting.