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The Records of Atmospheric CO_2 Derived from the Stable Carbon Isotopic Composition of Buried Plant Tissues in Perennial Frozen Lacustrine Sediments

LIN Qing, WANG Shao ling, ZHAO Lin (State Key Laboratory of Frozen Soil Engineering, CAREERI, CAS, Lanzhou Gansu 730000,China)  
The stable carbon isotopic fractionation in submerged plant is dependent upon the concentration of dissolved inorganic carbon (DIC) which equilibrates with atmosphere CO 2 by water gas exchange and indirectly provides an approach to rebuild the past atmospheric CO 2 concentration. The Ngoring Lake is the largest outflow fresh water lake in the source region of the Yellow River and has well developed aquatic vegetation of submersed plants. Due to low air temperature ( -4.5 ℃ in average), the lacustrine sediments are perennial frozen below the depth of 1 05 m, at which the temperature is -0.5 ℃. The plant tissues in the sediments are well preserved and can be separated. In this study, the well preserved plant tissues were separated from 2 m thick perennial frozen lacustrine sediments which consist of fine silty clays. These tissues were apparently formed under relatively uniform water condition and mainly came from the same species of submerged plant? 玃otamogeton pectinatus by identification. The stable carbon isotope composition (δ 13 C) is calculated from stable carbon isotope ratio measured from a carbon dioxide by FINNIGAN MAT-251 mass spectrometer. The carbon dioxide is produced by combustion of an aliquot of plant tissues with a little pre cleaned cupric oxide and platinum filament in an evacuated fine quartz tube at 800℃ for 2 h. The study shows that the relationship between δ 13 C of Potamogeton pectinatus and the concentration of dissolved CO 2 [CO 2e ] can be described as Equation (5). The concentration of dissolved CO 2 [CO 2e ] is calculated as [CO 2e ] = a×p CO 2 , where p CO 2 is atmospheric CO 2 concentration and a is the solubility parameter which can be induced by the absolute temperature of water from ln a=(58 0931+90 5069× (100/T) + 22 294× ln (T /100).δ CO 2e can be calculated using the equilibrium isotope fractionation and mixing model among the inorganic carbon species as a function of temperature, carbon isotope composition and ratio of different sources. The water temperature calculation is consulted with the water temperature-time curve of the Lake Qinghai. The atmospheric CO 2 fluctuations reconstructed from stable carbon isotopic composition of the samples show that the atmospheric CO 2 concentration had an increasing tendency from 9 16 to 2 17 ka BP, and in the same time three main stages of low, middle and high atmospheric CO 2 concentration are distinguished, which corresponded respectively to the time of 9 16~6 77 ka BP, 6 77~4 56 ka BP and 4 56~2 17 ka BP.
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