靖远黄土—古土壤色度变化特征分析及古气候意义

Chroma chracteristics in the loess-paleosol at Jingyuan section and its signification to paleocliamete

对黄土高原西北缘厚层黄土剖面色度指标研究后发现,在磁化率指标不能很好地记录气候变化及土壤发育的情况下,色度指标的引入能够很好地弥补这一古气候重建上的缺陷.在靖远地区,红度a~＊主要受土壤中赤铁矿质量分数变化的影响,随赤铁矿的增加而增大,同磁化率联合使用可以反映土壤发育程度,较好地解释气候的变化过程.黄度b~＊受到针铁矿质量分数的影响,此外还可能受到黄铁矿、沼铁矿,褐铁矿等铁矿物的影响,在一定程度上可以反映降水量的变化情况.与a~＊一同分析,可以反映土壤发育时期的水热条件.亮度L~＊主要受土壤中w（TOC）,w（CaCO_3）变化的影响,随w（TOC）的增加而减小,而随w（CaCO_3）的增加而增大.此外在CIELAB颜色系统中a~＊的增大会显著地使L~＊变小,L~＊主要反映了土壤发育时期的降水状况以及植被发育程度.在靖远地区,只有综合考虑色度、磁化率、粒度、w（CaCO_3）,w（TOC）才能够合理、可靠地重建气候变化的过程.

The chroma of thick layer loess-paleosol profile was studied in Jingyuan,which is located on the northwestern Loess Plateau and the results indicated that the introduction of chroma could compensate for the defects in paleoclimate reconstruction when magnetic susceptibility（MS） cannot record the process of climate change and soil development very well.In Jingyuan region,a~＊ value（psychometric red-green chromaticness） is mainly affected by hematite content,i.e.it increases with an increase in Hematite content in the soil.In addition,the combined method of a~＊ value and magnetic susceptibility can reflect the degree of soil development and well explain the process of climate change.b~＊ value（psychometric yellow-blue chromaticness） is primarily influenced by the content of Goethite,and probably is also affected by other forms of iron,such as pyrite, marsh ore,limonite,etc.To some extent,b~＊ value can reflect changes in precipitation.The conditions of water and precipitation during the process of soil development could be interpreted by using a value and b~＊ value together.As for L~＊ value（psychometric lightness）,its whitish color and blackish color are influenced by calcium carbonate and total organic carbon respectively.L~＊ declines with an increase in the total organic carbon content and increases with increasing carbonate content.Furthermore,a~＊ value influences V value so significantly that the blackish color of L~＊ value obviously decreases with an increase in a~＊ value in CIE 1976 color space.L~＊ value mainly reveals the conditions of precipitation and the development of vegetation during the process of soil development.In Jingyuan area,a reasonable and reliable process of climate change could be reconstructed by taking various facts（e.g.soil color,magnetic susceptibility,total organic carbon,calcium carbonate,grain size） into consideration.