末次冰期-间冰期旋回朝那黄土颜色特征及古气候意义

Characteristics of Color in Chaona Section and Its Paleoclimatic Significance During the Last Glacial-interglacial Cycle

对黄土高原中部朝那剖面末次冰期-间冰期旋回黄土-古土壤序列色度指标与磁化率对比研究后发现,亮度L*值与Hm(赤铁矿)和Gt(针铁矿)的比值相关性较好,在S0和L1阶段其与Hm和Gt的比值呈正相关,在S1阶段为负相关。红度a*与Hm和Gt的比值有较强的正相关关系,反映土壤发育时期的水热条件。而黄度b*与针铁矿的相关性较好,与低频磁化率χlf使用可以更好地反映土壤的发育程度。色度指标在变化频率和幅度上较磁化率大,能很好地识别S1阶段弱黄土-古土壤层。

The loess-paleosol sequences on the Chinese Loess Plateau（CLP） record the evolution of East Asian Monsoon. Soil color, a sensitive proxy of paleoclimate, has long been used for soil identification and qualitative determination of soil characteristics. Meanwhile it is also used as an indicator of field soil physics, chemical and biological properties as well as of occurrence of soil process. Here we focus on the characteristics and significance of soil color of Chaona loess-paleosol Sequence in CLP since the last Interglacial period. The study area is located near the town of Chaona（35°7′N,107°12′E） in the central CLP. The loess deposit of sequence is 175 m thick and contains 33 paleosol units. The paleosol within the loess is brownish or reddish with substantial clay skins developed and carbnate nodules are scattered commonly below the soils. Our attention has been given to studies of the last glacial-interglacial cycle and the overlying Holocene paleosol（S0）. The last glacial loess（L1） consists of two primary loess units（L1LL1and L1LL2）, separated by a weak soil complex（L1SS）. And the last interglacial paleosol（S1） can be further divided into three subunits, including two moderate developed paleosol complexes（S1SS1and S1SS2） and a loess horizon（S1LL）. We compared chromatic indexes with magnetic susceptibility. The results indicated that lightness L＊ can be good correlation with the ratio of Hm and Gt that reflects the precipitation. Lightness L＊ has a positive correlation with the ratio of Hm and Gt in since the last glacial period, but there is obvious negative correlation between lightness L＊ and the ratio of Hm and Gt during the last interglacial period. Meanwhile we find that a positive correlation exists between a＊ and the ratio of Hm and Gt in the section, which reflects hydrothermal conditions in the soil development period.The value of b＊ primarily influenced by the content of Gt can be used together with χlfin order to reproduce the development degree of the soil. In contrast to magnetic susceptibility, chroma indexes from S1 paleosol are characterized by high-frequency and high-amplitude variations, which also documents the weak loess-paleosol sequences during the last interglacial period（S1）. Furthermore, a＊ and b＊ are the major factors that influences L＊ value significantly. Therefore, Chroma index, a proxy that documents the sensitive and great evidence of climate variations, can be used to develop the complementary study with magnetic susceptibility, which can reconstruct the process of paleoclimate reasonably and reliably.