中东亚干旱半干旱区C3植物δ13 C值的分布 及其对气候的响应*

植物化石和土壤中的有机质碳同位素指标常用来反映古气候的变化,然而碳同位素这个指标在特定地区反映气候的定量关系缺乏检验。研究剖面选择自中国的秦岭(34°14'24″N,106°55'30″E)到蒙古人民共和国北部,接近贝加尔湖地区(51°35'08″N, 100°45'49″E)的研究剖面线,选择了3种C₃植物(Artemisia scoparia, Ajania achilleides 和 Artemisia frigida),在剖面线上沿南北方向上每隔4'到5'采取一个样点,共选取161个C₃植物茎叶样品进行了δ¹³ C值测定。同时收集了剖面线附近气象站的降水、气温等资料,用插值方法得到每个采样点的气温、降水数据。分析表明:C₃植物的δ¹³ C值分布范围为-30 ‰ ~-22 ‰ ,其平均值为-26.81 ‰ ,该平均值较全球C₃植物δ¹³ C平均值偏正。通过对比C₃植物δ¹³ C与年均温、年均降水量、生长季节的干燥度等随纬度的变化规律,发现C₃植物δ¹³ C、年均降水量、生长季节的干燥度有非常一致的变化趋势,而C₃植物δ¹³ C和年均温不具有一致性。通过一元回归分析也同样发现C₃植物δ¹³ C与年均降水量呈线性负相关关系(y=-0.0077x-24.838,n=161,R²=0.4418,p=0.01),与生长季节的干燥度呈线性正相关关系(y=0.7328x-28.806,n=161,R²=0.3685,p=0.01),而与年均温度没有明显的相关关系(y=-0.0461x-26.756,n=161,R²=0.0232,p=0.01)。在本研究区C₃植物δ¹³ C对年均降水量和生长季节的干燥度响应十分显著,而对温度的响应不明显。研究区具有明显的降水和温度的梯度分布特征,是验证植物碳同位素与气候关系的理想场所,而土壤中的有机质碳同位素与其地面上的植物碳同位素息息相关。研究也说明,在本研究区或其他气候植物组合相似的地区可以利用古土壤中的有机质碳同位素来定量或半定量地反映古气候的变化。

THE DISTRIBUTION OF δ13 C VALUE OF C3 PLANT AND ITS RESPONSE TO CLIMATE IN ARID AND SEMIARID CENTRAL EAST ASIA

Carbon isotope of plant fossils and bulk organic matter in soil have been widely used to infer past climatic changes. Carbon isotope ratio of soil is consistent with that of plant community. To exam the relationship between isotopic signals and specific climatic factors we carried out a field observation and data collection along a sampling transect starting from 34°14'24″N and 106°55'30″E in the northern slope of the Qinling Mountain near Baoji (China) and ending at 51°35'08″N and 100°45'49″E at Hanhayn Huryee near Mongolia-Russian border. Three species of C₃ shrubs (Artemisia scoparia, Ajania achilleides and Artemisia frigida) were selected to be sampled approximately every 5 latitudinal minutes along the sampling transect. 161 samples in sum were collected in the summer of 2002. Carbon isotope of CO₂ was measured on MAT 252 Mass Spectrometer in the MOE key laboratory of the Western Environmental System (Lanzhou University).Precipitation and air temperature data at weather stations close to the transect were obtained. Insert value method is used to get precipitation and temperature in every sampling spot. The main results show that the δ¹³ C value of C₃ plants varies from -30 ‰ to -22 ‰ with the average value of -26.81 ‰ . The average value deflects positively than the average δ¹³ C value of C₃ plants throughout the world. Along the latitude, an adverse trend between δ¹³ C value of C₃ plants and annual precipitation and a consistent trend between δ¹³ C value of C₃ plants and growing-season aridity were found, although no obvious similar trends between δ¹³ C value of C₃ plants and annual temperature was observed. Quantitative analysis also shows that there is a strong correlation coefficient between δ¹³ C value of C₃ plants and annual precipitation (y=-0.0077x-24.838, n=161, R²=0.4418, p=0.01) and the correlation coefficient between δ¹³ C value of C₃ plants and growth-season aridity is also strong (y=0.7328x-28.806, n=161, R²=0.3685, p=0.01), while there is little relationship between δ¹³ C value of C₃ plants and annual temperature (y=-0.0461x-26.756, n=161,R²=0.0232, p=0.01). The strong relationships between δ¹³ C value of C₃ plants and annual precipitation and growth-season aridity obtained from this study show that the study transect is an ideal place to explore the relationships between plant isotopic signals and climatic factors. They also indicate that paleoclimates might be quantitatively retrieved using isotopic signals in paleosol in our study zone and other similar areas.