PALEOCLIMATES AND MONSOON VARIATIONS INTERPRETED FROM MICROMORPHOGENIC FEATURES OF THE BAOJI PALEOSOLS, CHINA

A full climatic interpretation of the loess-paleosol sequence in China that covers the last 2.5 Ma nearly continuously has been restricted because the soils in the oldest loess unit - the Wucheng Loess - spanning about 1.2-2.5 Ma are not distinct enough to ensure accurate recognition and subdivision in the field. Recently, the authors investigated the Baoji loess-paleosol sequence in the southern-most part of the Loess Plateau where 37 paleosols were clearly defined, including those of the Wucheng Loess. Based on macro- and micromorphological observations, the major pedogenic processes involved in the formation of the Baoji paleosol are believed to be carbonate eluviation and illuviation, clay translocation, pseudogleization and rubification. The diagnostic B horizons of the 37 soils are classified into four types - Bw or weak Bt, moderate Bt, strong Bt and very strong Bt. The Holocene soil at Baoji contains a moderately developed Bt, which is interpreted to have formed under steppe-forest vegetation. Most of the soils with Bt horizons to the Holocene soil are interpreted to be developed under forest and/or steppe-forest environments. Primary carbonates in most of the loess beds have been leached with the loess beds acquiring weak to strong granular structures. The loess is subdivided into four types - massive, weakly weathered, moderately weathered and strongly weathered. They are believed to have mainly accumulated under steppe environments. Variations of monsoonal rain exhibit frequent and wide oscillations over the last 2.5 Ma documented by alternations of loess and paleosol units. The oscillatory pattern appears to be a function of the time interval considered.