Controlling Processes To The Seasonal And Spatial Dynamics Of The Carbonate System In The Northern South China Sea

The coastal ocean consists of several distinct but tightly connected ecosystems that include estuaries,tidal wetlands and the continental shelf.Carbon cycling in the coastal waters that connect terrestrial with oceanic systems is acknowledged to be a major component of global carbon cycles and budgets.Synthesis studies suggest that the CO2 flux in the coastal ocean on a global scale ranges from 0.19 to 0.4 Pg C y-1,which is about 10-20%of the global ocean CO2 sink.Carbon fluxes within and between coastal subsystems,and their alteration by climate and anthropogenic changes,are substantial.In coastal oceans,the carbonate system is modulated by a suite of coupled physical and biogeochemical processes,such as riverine discharge,coastal upwelling,terrestrial input,typhoon,submarine groundwater discharge and interaction with the open seas,the complex hydrology and biogeochemistry shape the carbon dynamics.It is therefore essential to understand,and accurately account for,the factors regulating these fluxes and how they affect the ocean and global carbon budgets.The South China Sea(SCS)is the largest semi-enclosed marginal sea in China.In this study,based on large-scale surveys conducted during all four seasons from 2009-2012,we investigated the carbonate systems on the northern SCS(NSCS)shelf and in the northwestern SCS(NWSCS).Furthermore,in order to validate the Ocean Dominated Marginal(OceMar)conceptual model in other marginal systems,the carbonate dynamics in Arabian Sea was further examined.As a result:1)The carbonate systems on the NSCS shelf featured much higher variations in both seasonality and spatiality on its inner-shelf(<40 m)as compared to the areas on the mid-outer shelf(>40 m).The most notable forcing on the mid-outer shelf include the intrusion of Kuroshio water leading to high surface salinity and high total alkalinity(TA)in winter,the impact of which is however limited to the northeastern part of the NSCS.The Pearl River Plume(PRP),a prominent feature in summer also has profound impact on the carbonate system on the mid-outer shelf.On the inner-shelf,the carbonate system was much more dynamic,featuring complex modulations by coastal upwelling and submarine groundwater discharge(SGD)associated with relatively high dissolved inorganic carbon(DIC)and TA in summer,and the China Coastal Current(CCC)of high DIC in winter,spring and fall.In addition,the influences of coastal plume water(CPW)from local rivers were identifiable on the inner-shelf in winter,spring and summer.Such distinction between inner-shelf and mid-outer shelf in the dynamics of DIC,the partial pressure of CO2(pCO2)and saturation state index of aragonite(Ωarag)is also obvious.On the mid-outer shelf,the salinity normalized DIC(nDIC)fluctuated seasonally between 1974±9 and 2001±9 μmol kg-1.The decline of nDIC from winter to spring and spring to summer mainly results from CO2 out-gassing,while the increase in nDIC from summer to fall and from fall to winter is due to entertainment of the carbon-enriched subsurface water.The pCO2 increases from a minimum of 344±9 μatm in winter to a maximum of 387± 14 μatm in spring,which is in phase with temperature changes and the fluctuations of nDIC.The Qarag ranged 3.28-3.68 with the highest value in summer but lowest value in winter,which is consistent with the seasonal cycles of the nDIC.Nearshore on the inner-shelf influenced by the CCC water in winter and the mid-outer shelf influenced by the PRP in summer,the spatial dynamics of sea surface pCO2 and Ωarag are modulated by both temperature and the water mass mixing between CCC,PRP,and shelf waters.Here,the high biological uptake sustained by nutrients in the CCC and PRP drawdown the pCO2 and augmented the Ωarag,while the CO2 sequestration enhanced the sea surface pCO2 but drawdown the Ωarag.In 2009,the water column at nearshore stations off the northeastern Hainan Island(HNEU)was influenced by CPW and coastal upwelling on the inner-shelf.Elevated DIC and TA concentrations were observed in CPW-and upwelling influenced-waters at HNEU,attributable to both SGD and biological uptake and organic matter(OM)remineralization.In surface waters,mediated by SGD and the high biological production,net DIC and TA additions in CPW-influenced water were estimated as 25.3 ±19.2 and 27.0±19.1 μmol kg-1,respectively with minor changes in seawater pCO2,pH and Ωarag.In subsurface waters,SGD contributed to a DIC and TA addition of 14.5± 19.3 and 12.9±14.5 μmol kg-1,respectively,with minor changes in pH and Ωarag.While OM remineralization contributed to an addition of 18.0 ± 14.2 μmol DIC kg-1,it drew down the pH andΩarag by 0.06 ± 0.04 and 0.32 ± 0.27,respectively.In 2012,SGD was not detected while high biological production dominated carbonate dynamics in CPW-influenced water at HNEU with a net DIC consumption of 70.9 μmol kg-1 on the inner-shelf,which drew down sea surface pCO2 by 101 μatm and increased pH and Ωarag by 0.13 and 0.79,respectively.2)The Arabian Sea is a large marginal sea of Indian Ocean.Driven by the seasonal transition of Asian monsoon,the hydrogeological setting in Arabian Sea shows obvious seasonal and spatial variability.In Arabian Sea,the upper waters in southeastern areas was featured low salinity(35.7-36.2)and high temperature(26.4-28.2 ℃)in winter and spring,which was influenced by the North Equatorial Current(NEC).In summer,the surface water in coastal area and the Findlater Jet axis was featured low salinity(～35.8-36.1)and high pCO2(>450 μatm),suggesting the influence of the coastal upwelling which was however decreased in fall.Among all the four different sub-regions,the distribution patterns of carbonate parameters on the Indian shelf were rather uniform,suggesting a steady state situation.We thus diagnosed the CO2 flux with the OceMar conceptual model on the Indian shelf.As a result,the diagnosed CO2 fluxes(394±7 μatm in winter,343±8 μatm in spring,367±4 μatm in summer and 375 ±4 μatm in fall)were consistent with the field observations.Our result suggested that,rather different from the Arbain Sea where the carbonate dynamics was dominated by the coastal upwelling in summer and the NEC in winter and spring without the influences of freshwater discharge and SGD,the carbonate system on the NSCS was much more complex.Diagnosed result suggested that,controlled by the temperature effect and the seasonal changes of mixed layer depth(MLD)(including the water mass mixing when the MLD is getting deeper and the stratification of water column in summer),the Arabian Sea is a CO2 source in the whole year,while the SCS is a CO2 sink in winter and a CO2 source in spring,summer and fall.Overall,both the Arabian Sea and SCS are marginal systems that followed the OceMar concept,where the carbonate system in surface water was controlled by the temperature effect and the seasonal changes of MLD mediated by the seasonal cycles of Asian monsoon.In spring and summer,the water column was stratified,which prevented the up transportation of carbon-and nutrient-enriched subsurface water,thus,low npCO2 and low biological production were observed in surface.In fall and winter,the MLD is getting deeper,the water mass mixing and biological production were largely strengthened.More carbon-and nutrient-enriched subsurface water was transported to surface,thus,leveled up the sea surface npCO2 and sustained high biological production.