Remote Sensing Of The Partial Pressure Of Aquatic CO₂ In The South China Sea

The oceans are the largest carbon pool in the earth system.The exchange of carbon dioxide(CO2)between the ocean and the atmosphere is of great significance to the research on marine carbon cycle.The study on carbon cycle in marginal seas is also an important part of global change research.The South China Sea(SCS)is one of the largest marginal seas in the world,composed of central basin and continental shelves.So far,few underway datasets of sea surface partial pressure of carbon dioxide(pCO2)in the SCS have been obtained,with most of them in the northern part.The application of remote sensing technology can achieve large-scale and long continuous observation of sea surface,thus conducive to the research on variation and mechanism of the sea surface pCO2 in SCS.In this thesis,the underway data of the SCS during 2004 to 2012 was firstly analyzed.Then we discussed the main control factors of pCO2 in different parts of the SCS.Based on the mechanistic semi-analytical algorithm of pCO2(MeSAA),we established a remote sensing algorithm for pCO2 in the SCS basin and the Pearl River plume.The spatial-temporal variations of the sea surface pCO2 were revealed.In addition,we analyzed the impact of cold eddy on sea surface pCO2 in summer and proposed a summertime pCO2 remote sensing algorithm for cold-eddy in the mid-western SCS.Our research contributes to the estimate of the surface pCO2 in the SCS and provides data support and mechanism understanding for the research of the carbon cycle in the SCS.The main conclusions are as follows:(1)The spatial-temporal variations of pCO2 in the SCS were of significant regional characteristics.The pCO2 of water masses with high temperature and low chlorophyll concentration in the SCS basin were mainly controlled by temperature.And the pCO2 in the northern continental shelf and southwest basin in the SCS were jointly controlled by temperature and biological effects.In addition,mixing processing between water masses always occurred in the mid-western basin in the SCS during summer and autumn,in Pearl River(PR)plume area and the east coast of Fujian province during autumn,and in the southeastern Hainan Island in spring.Significant changes in the pCO2 were taken place in the central basin in SCS,the PR plume area and the mid-western SCS where cold-eddies occurred.(2)The pCO2 in the SCS basin was controlled by temperature.We quantified the influence of thermodynamic process on pCO2 by selecting a stable water mass with negligible biology and mixing processes as the based reference,and then overlaied the thermodynamic effect on to establish the MeSAA algorithm.The pCO2 we have estimated was turned out to be consistent with the underway pCO2.It turned 'out that the pCO2,which was high in summer and low in winter,was significantly affected by the monsoon,and showed a northeast-southwest distribution in spring and autumn with an increasing trend.Sea basin of the SCS acted as a weak carbon sink in spring and weak carbon source in autumn.(3)In summer,pCO2 in the PR plume was controlled by the mixing processes and biological effects.Quantifying these effects,we built a MeSAA algorithm for thepCO2 in PR plume.Based on the nearshore conservative mixing of freshwater and seawater,salinity in PR plume could be derived from satellite adg(443nm)data.It was showed that the PR plume mainly diffused eastward and westward and the plume area has increased during 2003-2016.The summer mean pCO2 in the PR plume was about 325-350?atm,which was lower than atmospheric pCO2.In generally,the PR plume area was a weak carbon sink in summer.(4)The summertime pCO2 in the mid-western SCS basin where the cold-eddy occurred was controlled by temperature and biological effect.In order to build the MeSAA algorithm,we quantified the effects of temperature and biology.Results showed that when a cold-eddy occurred,the sea surface pCO2 was higher than 420?atm,and the ocean was a strong carbon source to the atmosphere.In general,based on the analyses of the pCO2 control mechanism in the SCS,this thesis established the MeSAA algorithms,which quantified the contributions of thermodynamic,biological and water mass mixing processes to sea surface pCO2,in the sea basin,the PR plume and the cold-eddy effected areas.And then,this thesis achieved the temporal-spatial distribution and annually,seasonally,and monthly variations of pCO2.This research further deepens the understanding of control mechanism of sea-air CO2 fluxes and inversion mechanisms for remote sensing in the SCS,laying a good foundation for further comprehensively quantitative estimation of carbon budgets in the SCS.