| The ocean, being as one of the largest carbon reservoir in the earth surface, could absorb atmospheric CO2 with the flux of-1.7±0.4 Pg C yr-1 annually. It plays a vital part in regulating the global carbon cycle and controlling the increasing atmospheric CO2 concentration. The continental shelf seas, or the marginal seas, which cover approximately 7% of the global ocean area, account for 14-30% of the oceanic primary productoin and serve as an essential component in the global material and energy budget. Consequently, the air-sea CO2 exchanging process in the continental shelf seas could exert profound influences on the regional carbon cycles and the variations of marine environment. Basing on the systematic study of air-sea CO2 exchange processes in the Yellow Sea(YS) and the East China Sea(ECS), the temporal and regional variations of carboante chemical paremeters as well as the key controlling processes for the air-sea CO2 exchange flux were obtained in this academic dissertation. A series of results and conclutions were presented as follows: 1. The Yellow Sea served as an overall source for atmospheric CO2 and could release CO2 into the atmospheric with the flux of 2.11×1012 g C yr-1 annually. The air-sea CO2 exchange processes in the YS were provided with significant temporal and regional variations. Temperature and phytoplankton priamry production were two of the most important controlling factors, which was responsible for the variations of oceanic CO2 system in the offshore region and nearshore region of YS, respectively.The surface partial pressure of CO2(pCO2) and air-sea CO2 exchange flux(FCO2) were provided with significant temporal and regional variations in the YS. In winter, the study area acted as an overall atmospheirc CO2 source with the flux of 2.15 mmol·m-2·d-1 and the distribution of pCO2 demonstrated obvious “North-South” diversity. On one hand, the south and center parts of YS(33-36° N, 120-125° E) were regional sinks for the atmospheric CO2 with pCO2 values generally lower than the atmospheric concentration(300-390 μatm). On the other hand, the north part of YS(36-38.5° N, 122-124.5° E) behaved as a CO2 source because the pCO2 content there was as high as 400-600 μatm.The YS could serve as a seasonal atmospheric CO2 sink only in spring. The air-sea CO2 flux in April 2011 and June 2013 was 3.38 and-2.87 mmol·m-2·d-1, respectively. The pCO2 in spring usually showed evident “Nearshore-Offshore” difference. The water in nearshore of YS, including the coastal area of Jiangsu and the southern part of YS were CO2-supersaturated, with pCO2 varying from 380 to 580 μatm. Conversely, the offshore region of YS was deficient in pCO2 and serves as typical CO2 sink.As the summertime arrival, the air-sea CO2 exchanging processes in YS began to experience the most dramatic variations. The YS in summer generally behaved as an obvious atmospheric CO2 source with the air-sea CO2 flux of 1.64, 1.94,-1.21 mmol·m-2·d-1 in July 2012, August 2011, and August 2013, respectively. The Jiangsu coastal region, which centered around 34° N, 121° E, could intensively release CO2 into the atmosphere with the air-sea CO2 exchange flux of 6-18 mmol·m-2·d-1 and consequently played predominent role in regulating the global FCO2 properties of YS in summer. It was necessary to mention that the area which could absorb CO2 from the atmosphere in summer was limited in the southeastern part of YS.The distribution patterns of pCO2 and FCO2 in autumn was similar to those in spring to some degree. The shallwed west coast of YS got fairly high p CO2 values(350-420 μatm), whereas the deep central part of YS possessed pCO2 lower than the atmosphere content(350-370 μatm). However, the crucial difference of carbonate features between spring and autumn rested with that the YS in autumn was a remarkable atmospheric CO2 source with the largest air-sea CO2 flux of the year 6.43 mmol·m-2·d-1.The control effects of temperature, phytoplankton production, water mixing and terrestrial input on the air-sea CO2 exchange flux in YS possessed significant regional discrepancies. In the nearshore region(depth<50 m), especially in coast of Shandong Peninsula and Jiangsu Shallow, the phytoplankton production, water mixing and terrestrial input play leading roles in deciding the oceanic CO2 system. However, the control of temperature on pCO2 was predominant in the offshore of YS. 2. The East China Sea(ECS) served as sinks for atmospheric CO2 in spring and summer, with the seasonal average air-sea CO2 flux of-9.34 and-5.49 mmol·m-2·d-1, respectively. In total, the ECS could absorb atmospheric CO2 as much as 1.60×1012 g C in these two seasons, which accounted for 23-37% of the annual CO2 sink budget. Obvious regional variations and seasonal transformations were obtained in the distribution patterns of pCO2 and FCO2 in the ECS. In general, the air-sea CO2 exchange prosesses in the ECS were controlled jointly by the combiantion effects of phytoplankton production, water mass mixing, and terrestial material loading.With the avergae air-sea CO2 exchange flux of-9.34 mmol·m-2·d-1, the ECS behaved as a rather intensive sink for atmospheric CO2 in sping(2011 April). The outer estuary of Changjiang and the region with water deeper than 50 m were the major area where CO2-absorption occurred, while the releasing of CO2 mainly took place in the northeast part of ECS and the coastal region of Zhejiang province. The situation in early summer(June) was quite different from that in April. On one hand, the Changjiang estuary changed into an atmospheric CO2 source. And on the other hand, the coastal region of Zhejiang turned into CO2 sink. Further, in midsummer(August), the distribution patterns of CO2 not only got obvious seasonal variations but had significant interannual discrepancy. In August of 2011, the CO2 sources mainly located in the outer estuary of Changjiang and the southern part of ECS(26-28° N), while the CO2 sink occurred in the central of ECS. One year later, in summer of 2012, CO2 source in the southern part of ECS extended to 30° N and CO2 source in the outer estuary of Changjiang shrunk to the west of 122.5° E. The CO2 sink in 2012, which covered an area smaller than that in 2011, was limited in the region north of 29 ° N.The biological sequestration of CO2, which was also known as the “biological pump”, was the predominant controlling factors for air-sea CO2 exchange process in spring and summer of ECS. This viewpoint could be verified by the significant negative relationships between pCO2 and Chl a acquired both in the entire ECS in April 2011 and in the inner shelf in June 2012. What’s more, the mixing of different water massed and the terrestrial material loading also exerted important influences on the carbonate dynamics of ECS. For example, the significant positve correlations between pCO2 and TA that observed in the Changjiang estuary during June 2013 and in the nearshore of ECS in August 2011 just indicated that the import of dissolved inorganic carbon was the major cause of CO2-releasing there.The Changjiang estuary varied significantly in seasonal distributions of pCO2 and the magnitudes of FCO2. Overall, the Changjiang estuary behaved as CO2 sink only in summer, while in the other seasons of the year, it was significant CO2 sources for the atmosphere. The saturation state of carbonate minerals(Ω) in the Changjiang estuary displayed obvious regional differences. The nearshore region usually possessed relative low Ω, whereas the offshore region was general high in Ω. Seasonally, Ω values in winter and autumn were lower than those in spring and summer. The river mouth was under-saturated with aragonite in the cold season. Temperature, salinity, and inorganic carbonate parameters were the related controllign factors for the saturation state of carbonate minerals in Changjiang estuary. |
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