Distribution Characteristics Of The Carbonate System Along 150°E Section In The Northwest Pacific Ocean In Autumn And The Effect Of Ocean Alkalization On Carbon Sink Enhancement

The Pacific Ocean is the largest ocean in the world,which plays an important role in the marine biogeochemical cycle of carbon and other chemical elements.As a part of the Pacific Ocean,it is important to study the distribution of the parameters and the influencing factors in the northwest Pacific Ocean.Field observation cruise in the northwest Pacific Ocean was carried out in November 2019 along the 150°E section(from 40°N to 13°N).Two parameters of the carbonate system of seawater,i.e.,pH and total alkalinity(TA)were determined.The other two carbonate system parameters,i.e.,dissolved inorganic carbon(DIC)and partial pressure of carbon dioxide(pCO2)were calculated using CO2SYS software combining with the in-situ temperature and salinity.The distribution features of the carbonate system were studied and the effect of temperature on pH and CaCO3 saturation level(?arag)were discussed.The distribution characteristics of pH and ?arag along the 150° E section at the end of this century were also predicted.In addition,experiments of adding alkaline minerals of different mass ratios(0.5‰ and 1.0‰)to seawater were carried out at four sampling stations.The effects of adding basic minerals with silicate as the main component on the parameters of the seawater carbonate system and nutrient index were investigated.The results are as follows.DIC and TA ranged from 1915 to 2014 ?mol/kg and 2243 to 2291 ?mol/kg,respectively;DIC in general decreased with decreasing latitude,but no clear latitudinal gradient was found for TA.pH on the total scale(pHT)was in the range of 8.044-8.110,decreasing with the decrease of latitude.?arag increased with the decrease of latitude in the range of 2.61-3.88.pCO2 values increased with the decrease of latitude and were all below the atmosphere pCO2 level,ranging from 332 to 387 ?atm.Air-sea pCO2 gradient(?pCO2)was in the range of 25-80 ?atm and decreased with the decrease of latitude.The distributions of pH,?arag and pCO2 along the survey area showed a strong regularity,among which ?arag and pCO2 increased with decreasing latitude,while pHT decreased with decreasing latitude.The distributions of TA and DIC may be affected by salinity,so the regularity was weak.After the salinity is normalized to 35,the nDIC and nTA of the two stations in the northernmost part of the survey area were higher than those of the other stations.By the end of this century,the predicted values of pHT and ?arag in the surface waters of this survey area would decrease by 0.3 and 1.2,respectively,compared with the present values.Larger declines were found in higher pHT and ?arag regions,resulting in the differences along the meridional gradient becoming smaller for both pHT and ?arag.The results of alkaline mineral addition experiments at four stations showed that the TA,pH and DIC of seawater increased to varying degrees,while pCO2 decreased.The increase of TA and pH was the most obvious.The average increment of TA was 76 and 127 ?mol/kg,respectively,and the increment of pHT was 0.082 and 0.11 8,respectively,with the addition mass ratio of 0.5‰ and?1.0‰.In addition,the addition of alkaline minerals has a certain influence on the nutrient parameters such as Si,P,NO2-and NO3-in seawater,with a large increase of the Si.The amount of NO2-and NO3-increases rapidly in the early stage and decreases gradually in the later stage,and the amount of P decrease to different degrees.The increase of TA and pH and the decrease of pCO2 indicate the feasibility of mitigating ocean acidification and enhancing ocean carbon sink through ocean alkalization.