NEW EVIDENCE FOR THE PRESENCE OF MIDDLE HOLOCENE PADDY SOIL IN THE CHUODUN ARCHEOLOGICAL SITE
绰墩农业遗址中存在中全新世水稻土的新证据*

Records from buried paleosol offer important clues to understand global and local change of natural environment in geological history. Little anthropologic paleosol which may contain valuable information on ancient human activities and their effect on environment have been found. Previous studies suggested that two layers of ancient paddy soils existed in Holocene strata at the lower Changjiang River delta region in China,but little evidence of soil chemical and physical data has been offered up to now. Twelve continuous samples from the profile P-01 in Chuodun archeological site,Kunshan City,Jiangsu Province,China,were collected and analyzed. Total nitrogen (TN) was determined by using Kjeldahl method. Total organic carbon (TOC),clay mineral compositions,magnetic susceptibility and particle sizes were analyzed by using element analyzer,X-ray diffraction (XRD),magnetic susceptibility meter,and laser-particle-size analyzer,respectively. XRD data showed that the distribution of kaolinite contents and kaolinite to illite (K/I) ratios had two peaks at the depth from 200cm (bottom) to 100cm and from 57cm to 0 (surface layer). Such a distribution of clay minerals was in consonance with the distribution of TOC and TN as well as rice phytoliths,suggesting that there were two soil-forming periods recorded in the P-01 profile. The lower one was the formation of paleosol layer,and the latter was the formation of modern paddy soil. However,the TOC,TN and rice phytolith data did not support the viewpoint of the existence of a paleosol layer at the depth of 57cm to 42cm. The decrease of clay contents and magnetic susceptibility in the depth from 200cm to 130cm implied that the paleosol was formed in a waterlogged condition. Previous research on modern paddy soil suggests that a fraction of soil clay might be drained out from surface soil with drainage water when growing paddy rice under traditional cultivation. Thereby,the decrease of clay contents in depth from 200cm to 130cm might be resulted from drainage in the Middle Holocene. Evidence from archaeology,palynology as well as phytolith implied that the paleosol is associated with ancient paddy soil. The wave-like distribution pattern of magnetic susceptibility in the depth from 130cm to 42cm might respond to historical climate changes,although the involved processes were not clear yet. Further study on paleosol formation and climate changes is thus needed.