利用多年冻土区湖相沉积物中埋藏植物稳定碳同位素组成重建大气CO2浓度

沉水植物碳同位素分馏同水中溶解无机碳浓度有一定的关系，因而可以通过青藏高原多年冻土区的湖相沉积物中埋藏沉水植物－－龙须眼子菜（Potamogeton pectinatus)植物屑的碳同位素组成重建该地大气CO2浓度的变化情形，研究结果表明，该地在9．17－6．77ka BP间，大气CO2浓度是整个研究时间段中最低的，其后在6．77－4．56ka BP时期大气CO2浓度增加，在4．56－2．17ka BP之间，大气CO2浓度是整个研究时间内CO2浓度最高的阶段，植物屑的碳同位素组成反映了溶解无机碳浓度的变化，从而可用以重建大气CO2浓度的变化情况。

The Records of Atmospheric CO2 Derived from the Stable Carbon Isotopic Composition of Buried Plant Tissues in Perennial Frozen Lacustrine Sediments

The stable carbon isotopic fractionation in submerged plant is dependent upon the concentration of dissolved inorganic carbon (DIC) which equilibrates with atmosphere CO2 by water-gas exchange and indirectly provides an approach to rebuild the past atmospheric CO2 concentration. The Ngoring Lake is the largest outflow freshwater 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 C. 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－Potamnogeton pectinatus by identification. The stable carbon isotope composition (δ13C) is calculated from stable carbon isotope ratio measured from a carbon dioxide by FINNIGAN MAT251 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 800C for 2 h. The study shows that the relationship betweenδ13C of Potamogeton pectinatus and the concentration of dissolved CO2 CO2e] can be described as Equation (5). The concentration of dissolved CO2 CO2e] is calculated as CO2e ] = a × Pco , where Pco2 is at-mospheric CO2 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). δco. 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 CO2 fluctuations reconstructed from stable carbon isotopic composition of the samples show that the atmospheric CO2 concentration had an increasing tendency from9. 16 to 2. 17 ka BP, and in the same time three main stages of low, middle and high atmospheric CO2 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.