Rapid Acidification And Its Influence Factors In The Arctic Ocean

The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state(?arag),a process known as Ocean Acidification(OA).This would be detrimental to marine organisms and ecosystems.Over the past two decades,global warming and climate change have caused rapid changes in the Arctic,especially in the western Arctic Ocean.These changes include rapid seaice retreat and increases in sea surface temperatures,Pacific water inflow,freshwater storage,primary production,and surface CO2 concentrations.Projected climate change processes are thought to amplify OA in the Arctic Ocean,making it more vulnerable to rapid chemical changes than any other ocean basin.The studies that report proposedmechanisms are based on limited data collected fromonly the slope and southern basins,two cruises,or the sea surface.The current status and historical changes of OA are unknown.On the basis of field data of DIC,TAlk and pH,this dissertation use ?arag and pH derived from various cruises to identify an unprecedented rate and scale of ocean acidification in the western Arctic Ocean and evaluate links to environmental and climate change over the past two decades.This dissertation also for the first time propose a hypothesis of "low-low-high" variation trend in sea surface water pCO2 along a decreasing ice-cover gradient across three different zones in the western Arctic Ocean under a rapid sea ice melting scenario in the summer.We frist report the upper ocean(top 300 m)seawater ?arag from marginal seas to the basins as far north as 88°N from five trans-western Arctic Ocean programs during 1994-2010.The research indicates that,between the 1990's and 2010,acidified waters expanded northward at least 5°,to 85 °N,and from the Chukchi slope off the coast of northwestern Alaska to just below the North Pole.Also,the depth of acidified waters was found to have increased,from approximately from 100 to 250 meters depth.The Arctic Ocean is the first open ocean where we see such a large-scale increase in acidification,more than two times faster than observed in the Pacific or Atlantic oceans.Tracer data and model simulations suggest that increased PacificWinter Water(PWW),driven by retreating sea ice and anomalous circulation patterns in the summer season,is primarily responsible for this OA expansionWe also present seawater pCO2,?arag and pH underway data from the 2008 Chinese Arctic Research Expedition(CHINARE2008)cruise along a transect in the Western Arctic Ocean from the Chukchi Sea towards 88°N in the northern Basin.We observed variability of pCO2 in surface water under different sea ice melting scenario.A hypothesis is raised about the variability of pCO2 in surface water under rapid sea ice melting scenario in the Arctic Ocean:"low" pCO2 in the heavily ice-covered zone,"low"pCO2 in the partially ice-covered zone,and "high" pCO2 in the ice-free zone.This "low-low-high" pCO2 variation trend could be attributed to different driving forces.we primary attributed the low pCO2 values in the northern heavily ice-covered Basin is affected by several processes,including mixing of various source waters,micro-biological CO2 fixation,ice-water gas exchange,temperature change,and the main reason need further investigation.The low pCO2 observed in the partially ice-covered northern Canada Basin primary due to biological CO2 fixation and CaCO3 dissolution consume CO2.Finally,the observed high pCO2 values,?arag<land low pH in the ice-free southern Canada Basin due to CO2 invasion from the atmosphere,sea surface water warming and sea-ice melting.Finally,to better understand the extent of ocean acidification in the Arctic Ocean,we present pH measurements from the 2010 Chinese Arctic Research Expedition cruise along a shelf-slope-basin transect from the Chukchi Sea shelf to the Chukchi Abyssal Plain(CAP)in the western Arctic Ocean.We observed considerably low pH values in the Chukchi Sea shelf bottom waters(30 m-bottom)and CAP subsurface waters(100-200 m).In the Chukchi Sea shelf bottom waters,the pH values were 7.89-8.12,corresponding to DO values of 273-343 ?mol kg-1(i.e.,83%-98%).In the CAP subsurface waters,the pH values were 7.85-7.98,corresponding to DO values of 271-310 p.mol kg-1(i.e.,76%-84%).Biogeochemical model simulations suggest that respiration/remineralization-derived CO2 induces very low pH values in the bottom water of the Chukchi Sea(shelf)and in the upper halocline layer(UHL)waters of the CAP(basin).The UHL waters receive CO2 from respired organic matter as water flushing over the shelf bottom.In summary,via the data collected by five trans-Arctic cruises over the last two decades and model simulations suggest that increased PWW,driven by rapid sea-ice retreat and an anomalous circulation pattern,is primarily responsible for the large scale "acidified" waters expansion,at a rate of 1.5%per year,If the trend continues we suggest that within the middle of this century it is likely the entire upper water of the western Arctic Ocean may become undersaturated with respect to the aragonite.