Spatiotemporal Variations Of Carbon Dioxide And Their Controls In The South China Sea

Carbon dioxide(CO₂)is one of the most important greenhouse gases.It plays a vital role in the climate system.As a major CO₂ reservoir,ocean has absorbed a large amount of anthropogenic CO₂ since the industrial revolution,modulating thereby the atmospheric CO₂ concentrations.Ocean margins are the most dynamic regime with multiple scale interactions among the land,ocean and atmosphere.Cabon cycling in ocean margins is highly complicated,representing an important component of the global carbon cycle.The distribution and dynamics of CO₂ in the surface seawater are regulated by various physical and biogeochemical processes,while the understanding of these controls in different spatiotemporal scales is still limited.Precisely quantifying the air-sea CO₂ fluxes and their controlling mechanism are,therefor,crucial to better understanding the carbon cycle in marginal seas and the role of coastal oceans playing in the global climate change.China seas,including primarily the East China Sea(ECS)and the South China Sea(SCS),consist of one of the largest marginal sea systems of the world.Previous studies have shown that the ECS is a net sink and the SCS is a net source of the atmospheric CO₂,whereas the source/sink nature varies among different domains and seasons in both seas.However,the controlling processes and mechanisms leading to these spatiotemporal variations are yet to be fully understood.In the context of the global climate change,in particular of the global warming hiatus since 1998,more attentions are also needed to be paid into the response and feedback of air-sea CO₂ exchange in the China seas in the future.This dissertation focuses on the SCS,the largest marginal sea of the northern Pacific Ocean.Based on multiple years observation on board 25 cruises covering both the nearshore,shelf,slope,and basin area of the SCS and the adjacent western Pacific between 2003 to 2014,this dissertation sought to synthesize the dynamics of the partial pressure of CO₂(pCO₂)in the surface seawater,the air-sea CO₂ fluxes,and the controlling processes.Four processes potentially influencing the surface pCO₂ in the SCS including the temperature effect,biological activities,water mass mixing and airsea CO₂ exchange were discussed.In addtion,this dissertation examined the interannual variations of surface pCO₂ and the air-sea CO₂ fluxes in the South East Asia Time-Series Station(SEATS)located in the deep basin of the northern SCS.These data were compared with those obtained in the open ocean and the global atmospheric pCO₂ variability aiming to better understand the future change in CO₂ source/sink nature of the SCS.Major findings of this dissertation are as of below:1.Both surface seawater temperature(SST)and surface seawater salinity(SSS)displayed significant spatiotemporal variations in the SCS.SST was overall higher in the south than in the north and higher in summer than in winter.SSS was higher in the central region than in the north and south.The highest SSS value were generally observed in the shelf and slope area of the northern SCS,while the lowest one were observed in the nearshore of the northern SCS.SSS in the deep basin ranged bwteen those in the two regions.In contrast to the seasonal variations of SST,the SSS was lower in summer than in winter.The intra-and inter-seasonal variations of both SST and SSS were more distinguished in the nearshore than in the deep basin.2.The spatial distribution of surface seawaterpCO₂ in the SCS generally followed that of SST,which was higher in the south than in the north.The deep basin was overall a CO₂ source with surface pCO₂ higher than the atmospheric pCO₂;Surface pCO₂ in the shelf and slope area was was close to the atmospheric pCO₂ suggesting that it was nearly in equilibrium;The nearshore area was overall a CO₂ sink with surface pCO₂ lower than than the atmospheric pCO₂.On the other hand,seasonal variations of surface seawater pCO₂ in the slope and basin area were similar to those of SST.While the lowest value was observed in winter,surface pCO₂ increased rapidly in spring,reached the highest value in summer and decreased generally in autumn;On the northern shelf,the lowest surface pCO₂ was also observed in winter,whereas the highest value was frequently observed in late spring;In some nearshore areas significantly affected by biological activities,surface seawater pCO₂ could be low in summer,resulting in a bimodal pattern showing lower pCO₂ in winter and summer and higher pCO₂ in spring and autumn.The intra-and inter-seasonal variations of surface seawater pCO₂ were also more distinguished in the nearshore than in the deep basin.The SCS was a net CO₂ source during a whole year,which released 0.017 Gt C per year to the atmosphere.3.The dominating control of spatial varitations of surface seawater pCO₂ and airsea CO₂ fluxes in the SCS was SST,while physical mixing and biological activities played changing roles in different regimes.The intra-seasonal variations of surface pCO₂ were primarily controlled by physical mixing and biological activities.The dominating factors controlling inter-seasonal pCO₂ variations were SST in the slope and basin area,the coupling between SST and physical mixing on the northern shelf and the interaction of SST,physical mixing and biological activities on the nearshore area,respectively.Air-sea CO₂ exchange,however,moderately buffered the influence of other three factors on the surface seawater pCO₂.4.In the context of the global warming hiatus,surface seawater pCO₂ at SEATS increased at a rate of 1.42±1.14 ?atm per year from 2000 to 2014,which mostly likely resulted from the exchange with atmospheric CO₂ showing similar increase.In the near future,the deep basin of the northern SCS,as suggested by the scenario at SEATS,will keep being a source of CO₂ to the atmosphere.In summary,via systematical investigation of the spatiotemporal variation of surface pCO₂ and their controls in the SCS,this dissertation confirms that the SCS is a net source of atmospheric CO₂ releasing 0.017 Gt carbon per year.Meanwhile,the surface seawater pCO₂ in the deep basin of the northern SCS have been increasing in pace with the atmospheric CO₂,suggesting that the CO₂ source nature of the SCS will not be inversed in the near future.