中东亚地区表土碳酸盐含量和pH值的空间变化及其气候代用指标意义

Geographic variation of carbonate content and pH in surface soil in East Central Asia:Significance as climate proxies

黄土-古土壤以及湖泊沉积物的碳酸盐含量作为常用的气候指标,被人们广泛地应用于干旱、半干旱地区的古气候研究,然而碳酸盐含量的气候意义却缺乏足够的验证。中东亚干旱、半干旱区覆盖我国西北部和蒙古国全境,区域内温度和降水等具有显著的空间分布特征,这为验证包括碳酸盐矿物含量在内的地球化学代用指标的气候意义提供了理想的研究场所。为此,选取了一条横跨该区不同气候带的采样线,对采样线上表土碳酸盐含量、pH值以及气象站的记录资料行了分析和对比。研究发现:(1)表土碳酸盐含量的分布特征沿着纬度呈现出南高北低的总体趋势,在北纬34.50°～42.50°的广大黄土、类黄土地区,碳酸盐含量较高,而北纬42.5°以北地区碳酸盐含量则相对较少,局部地区由于土壤母质为碳酸盐岩而具有较高的碳酸盐含量;(2)表土碳酸盐含量与pH值的非线性拟合特征反映出土壤碳酸盐矿物含量与pH值之间的化学平衡,两者在0.01水平上呈显著的正相关关系(r=0.51)。酸性土壤地区的碳酸盐体系难处于饱和状态,碳酸盐易于淋失含量低,此时,碳酸盐矿物含量不宜作为气候指标,而碱性土壤地区的碳酸盐矿物含量变化与气候变化关系密切;(3)高温、干燥的气候环境使降水被迅速蒸发,加剧了碳酸盐类物质的表聚作用,使黄土、类黄土地区表土碳酸盐含量与降水量、温度都呈现出了一定的正相关关系;(4)黄土、类黄土地区表土pH值与多年平均降水量关系密切(pH>7,R2=0.90),可以作为我国黄土高原地区过去大气降水量变化的代用指标。

Carbonate content in loess sequences or lake sediments has been widely used to infer past climate changes. Its climatic significance, however, need to be verified. The arid and semiarid East Central Asia （ECA）, which comprises north China and Mongolia, has distinct climatic changes. It provides an ideal background for probing the relationship between geochemical parameters and climate changes. The carbonate content and pH value of the surface soils are measured from a variety of geographic units along a transect from central China to northmost Mongolia. The results show that the carbonate content of surface soil in the southern part is higher than that in the northern part along the transect. Carbonate content of soil in loess plateau or loess-like sediments with latitude between 34. 5°N and 42. 5°N is higher than that in the other regions （〉 42.5°N）. But if the parent rock of soil in northern Mongolia is carbonate rock, it will have high carbonate content too. The relationship between carbonate content and pH value of surface soil is nonlinear, and it also reveals the chemistry equilibrium of CaCO3-H2O-CO2 system. The Pearson correlation coefficient between carbonate content and pH value was r = 0.51 （P = 0.01 ）. The nonlinear correlation shows that the carbonate content cannot be used as a climate proxy in acid soil regions,because the soil carbonate was eluviated over. But it had a close relationship with climate variation in alkaline soil regions. Effect of high temperature and arid climate on evaporation of water, speeds up the carbonate accumulation rate of surface soil. As a temperature in loess plateau result, carbonate content is positively correlated or loess-like sediment region. The analysis of soil pH on loess with precipitation and and loess-like sediment region reveals that there is a significant linear correlation （R^2 =0. 90） between soil pH and the annual mean precipitation when pH 〉 7. Therefore, soil pH value can indicate the annual mean precipitation changes sensitively, and may be possibly served as another useful proxy index of climate.