绰墩农业遗址中存在中全新世水稻土的新证据*

前人研究认为,在长江下游地区绰墩农业遗址的全新世地层中存在2个古水稻土层,但到目前为止尚缺乏土壤化学和物理学方面的证据。为了寻找土壤学证据,在江苏省昆山市绰墩遗址P-01垂直剖面中连续采集了12个样品,分别测定了全氮(TN)含量、总有机碳 (TOC) 含量、粒度组成、质量磁化率以及粘土矿物组成。结果显示,从剖面底部(200cm)到100cm处和从57cm处到现代土壤表层,TOC,TN,TOC/TN比值、高岭石含量以及高岭石/伊利石(K/I)比值两次出现逐渐升高的趋势,而且它们的曲线形态基本一致。这些特点有可能说明P-01剖面中包含着两次成壤过程:第1次形成了古土壤层(160~100cm),第2次形成了现代水稻土(57cm~现代土壤表层)。但TOC和TN以及水稻植硅石资料不支持在深度57~42cm段存在古土壤的观点。从200cm到130cm处,磁化率和粘粒含量有逐渐降低的趋势,说明此处具有长期淹水的特征。现代水稻土的研究表明,水稻田排水过程中可能使部分粘粒被带走流失。因此,粘粒含量从200cm到130cm处逐渐降低的趋势可能与中全新世的人工排水有关。研究资料与考古学、孢粉学和植硅体资料共同说明,该古土壤层(160~100cm)很可能是古水稻土。古水稻土的确认将为农业考古和古环境变化研究提供有利的条件。

NEW EVIDENCE FOR THE PRESENCE OF MIDDLE HOLOCENE PADDY SOIL IN THE CHUODUN ARCHEOLOGICAL SITE

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.