RECORDS OF LACUSTRINE SEDIMENTOLOGY AND POLLEN-CHARCOAL ASSEMBLAGES RESPONDING TO CLIMATE CHANGE AND HUMAN ACTIVITIES IN ZHONGZHOUZI OXBOW LAKE,HUBEI PROVINCE FOR ABOUT 70 YEARS
According to the meteorological data and pollen-charcoal record of deposits in Zhongzhouzi Oxbow Lake, Hubei Province which is located in the middle reach of the Yangtze River, this paper discussed the study on the response of pollen-charcoal to vegetation and climate change as well as human activities since the last 100 years, combined with chronological sequence, grain size characteristics and historical records or the existing research of documents. The length of sample drilling (29°48'02"N, 112°45'14"E) is 89cm from the center of the lake, and each of layer is 1 cm, Zhongzhouzi Oxbow lake still accept lakes sedimentary which from the upper Yangtze River in flood season, so its sediment contains the lake and fluvial facies sedimentary characteristics, and it shows the changes of the lake,the environmental even human activities. The corresponding relationship between five pollencharcoal zones of sediments and environment evolution and Man-land relationship (make iron and steel in 1958, returning land to lakes in 1998 and artificial afforestation in 1982) has been established. Pollen-charcoal assemblage shows: climate characterized warm to cold, wet to dry by cycle from 1940 A.D. to 2012 A.D.; the reduction of arbor pollen, increasing of the crop pollen and charcoal from 1954A.D. to 1961 A.D., which related to human's deforestation,the enhancements of arable land and incineration activities; the content of aquatic herb,fern and spore is a reflection of water intrusion or flood events from 1961 A.D. and 1967 A.D.; crop pollen content increased from 1967A.D. to 1982A.D. due to the policy of returning farmland to lakes or forests, so the area of wetlands increased; maple pollen was significantly increased along with the decrease of line pollen, it shows that maple became the dominant species after the plantation from 1982A.D. to 1997A.D.; the rapid increasing of aquatic herb and charcoal which greater than 125mum from 1997A.D. to 2012A.D. responsed to the phenomenon that arable land has been replaced by a large number of fish ponds and wetland area. The conclusion shows that: (1) pollen combination has a good response to climate change. Climate was dry in the mid 1960s, the contents of Pinus, Quercus, Ulmaceae, Liquidambar and Artemisia were on the rise, but the crop pollen decreased, meanwhile, aquatic plant pollen and fern spores showed lower constants corresponded to the climatic characteristics in this stage; measuring temperature and precipitation data showed a rising trend from the last century to the beginning of 21st century, the constants of Castanopsis spach,Hamamelidaceae, Gramineae and Brassicaceae were high. (2) In the last 1950s, the tree pollen decreased significantly, which on account of the steelmaking and deforestation; the constants of land area and crop pollen went up,as well as the charcoal concentration, it might due to the risen of the population in the mid of the 1960s. Jianli county implemented the policy of returning farmland to lakes, forests, resulting in Quercus, Pterocarya and Ulmaceae increased, coupled with the gradually spread and perfection of agricultural irrigation systems and crops pollen (Oryza sativa, Zea mays, Brassica campestris) contents rising rapidly. The explosive growth of pollen concentration in the last century, might be associated with land-use changes in the area.