Climatic Environment Change Over 9 KaBP: Evidence From Jiangling Area, Jianghan Plain, China

Jianghan Plain is typical flood plain, composed of much of interfluvial lowlands, which become interfluvial lowlands lake or marsh after storing water. Quarternary sediments of plain areas are results of alternate fluvial and lacustrine evolvement, and bed phase and flooded and lacustrine facies sedimentary deposits occur alternately, and these reflect environmental evolvements of alternate fluvial and lacustrine variances in Jianghan Plain over Quarternary. Therefore, the fluvial and lacustrine sediments are good geologic body in the research on climatic and environmental changes since Quaternary in JianghanPlain. The fluvial and lacustrine sediments 6 m thick from Jiangling profile in Jianghan Plain offer continuous and complete climatic and environmental records over 9 kaBP in study region. Through abundant climatic carriers, included total organic carbon, total nitrogen, C/N ratio, organic carbon stable isotope, grain size and pollen, the article reconstruct the climatic and environmental evolvements since 9 kaBP at Jianghan Plain, and also carry out the comparison of sea and land climate, of regional climate, of long-range climate. Comparison with East China areas, such as the lower reaches and delta of the Yangtze River, The researches on climatic and environmental changes since middle-late Holocene, in the Middle reaches of Changjiang River, and especially in Jianghan Plain, are relatively weak. At one time, Holocene, which is important time interval in joining geological period to historical period, its climatic evolvement course and rule play an important roles to forecast coming global climatic changes. And so, the research is not only of significant value to global climate change research, but also to human living environment and social sustainable development. With the help of theoretical analysis and correlation of each other, the significance of each climatic indicator is basically found. The organic index, comprised of TOC,TN,C/N ratio,δ13C, is analyzed in the paper. The content of TOC and TN is in relation to vegetational cover in drainage area and import of terrestrial organic matter and lacustrine biomass. When climate is warm and humid, lacustrine productivity and biomass is massive, and that, vegetational cover in drainage area is high, and terrestrial organic matter come into lacustrine basin in large amounts with surface runoff, therefore, TOC and TN value show itself upward trend. However, when climate is dry and cold, lacustrine productivity and input of terrestrial of organic matter come into downtrend, as a result, content of TOC and TN reduce. Therefore, rise of TOC and TN value indicate rise of temperature, however, reduce represent reduce of temperature. The ratio of C to N can offer information about the origins of organic substances, and is an effective method of identifying the resource of organic substances. The high value of C/N reflect that contribution of terrestrial organic matter is biggish, and the increase of surfare runoff and regional precipitation. The content of TOC and TN in peat bed, 311 to 386 cm position in Jiangling profile, is relatively high, however, other is relatively low. Theδ13C value in Jiangling section is lower, and has an little fluctuation range, and theδ13C value in 311 to 386 cm section is an evident low value, and moreover is in reverse relation to TOC and TN value. Therefore, the lowerδ13C value correspond to warmer climate, and vice versa. It is topographic feature of underlaying surface of interfluvial lowland that be of great advantage to terrestrial coarse grain substances, eroded from surrounding highland by surface cutoff, deposit into the lowland lake. The variation of precipitation at lake basin drainage area, dominating developmental degree of surface cutoff, is main factor of controling grain size of sediments from daya lake for Jiangling section. On the basic of the complex comparison among environmental index from lacustrine sediments in Jiangling section, the article results show that coarse particle size correspond to humid climate when precipitation is high, at the same time, fine particle size correspond to arid climate when precipitation is low. The climatic significances of alluvial deposit grain size is primarily discussed. If changes of grain size of offchannel alluvial deposit are mainly dominated by flood frequency and levee break times, the grain-size changes may be in relation to climatic changes. By now, the increase of coarse grain content may indicate dry and cold climate, but the increase of fine grain content probably reflect warm and humid climate. In such a case, there are fourteen floodwater event records between 311 to 0 cm period of time. Through comparison of grain size characters between fluvial alluvium in 0 to 311 cm section and lacustrine bog sediments in 311 to 386 cm section, it is found that there are obvious difference in grain size characteristic between the two kind of sediments, and they can be distinguished with the use of distribution scatter diagram, and grain size images, and Q1-Md-Q3 grain fineness. The seven thousand one hundred forty two pollen have been in fifty two samples at Jiangling section, and respectively belong to one hundred forty five family and genus. The total features of pollen are that pollen species is relatively plentiful, and that dominant species isn't very obvious, and that xylophyta and herb and fern all are existent. The volume of xylophyta pollen is between 17.54%～59.4%, with an average of 36.38%, and herb pollen is 16.26%～66.67%, 35.92%, and fern pollen is 0～56.14%, 27.7%. Based on pollen concentration and percentage content, and combined with geochemical parameter and lithology change, the section is divided into three spore belt and six subzone. The spore assemblage, since 9 kaBP, show that the vegetation appearance of the study region mainly is subtropical grassland vegetation with sparse conifer and broadleaf. Pinus pollen is main needle leaved plant, and broadleaf plant primarily comprise Betula,Ulmus,Juglans,Quercus,Q.gilliana,Castanea/Castanopsis, and herb plant is mainly comprised of Gramineae,Cyperaceae and Artemisia, and fern plant consist of Psilamonoletes and Adiantum. Based on the above statement, four main phases of climate fluctuation were shown in the past 9000 years: 9.0—6.0kaB.P. , warm and wet; 6.0—4.6kaB.P. , cold and aridity; 4.6—2.5kaB.P. ,warm and wet; 2.5kaB.P.—present, cold. The climatic changes over 9kaBP in Jiangling area are coincident with large numbers of research information from domestic, moreover, many obvious climatic events are in good agreement with many international study result. It is showed that climatic change in Jiangling region is characterized by globality. There are eighteen floodwater records in the last 2870 aBP in Jiangling area. Based on the power spectrum analysis of paleo-floodwater records, it is found that the floodwater events is of cycle of 26,36,43,52,79,110a, and but 43,79,110a periods is most evident. The periods is the response to solar activity cycle.