Sediment Characteristics And Its Environmental Significance Of Hemudu Site Since Holocene

In the study of global change, people pay more and more attention to their own survival between the environment and geographical ties, especially since the Holocene sea-level changes, affecting the coastal areas of human life. Hemudu site is located in the middle and lower reaches of the Yangtze River in the south wing of Yao River Valley by sea-level change due to climate change, drought and flood influence is sensitive, human beings need to understand future climate and sea-level change and the possible impact of natural disasters to human society, aimed at understanding the past global changes and predict future trends and to serve the human society’s sustainable development. This study through the Hemudu near HMD1501 and HMD502 two drilling holes in the sediment of natural sedimentary layer as the research object, to sediment lithology, grain size and pollen, ditch dinoflagellates, foraminifera and Ca, Mn, Ti environment proxies for research means, reveals the Yao River Valley since the Holocene vegetation and climate change, sea level change and sediment formation process. Sediment lithology, grain size characteristics, chemical elements, pollen, marine dinoflagellate, freshwater algae and foraminifera lining records show that:1) according to the deposition of Hemudu site two drilling lithology analysis, performance characteristics are as follows, hole HMD1501 below 36.5m deposition layer lithology for gray- green grey silty clay and fine sand interbed, clear horizontal bedding, humidity is big; 36.5-33.81 m, the lower part of the first hard clay layer, yellowish brown, core is relatively dry, the upper clayey silt; 33.81-4.1m, sediment lithology is grey brown silty clay, locally contain bedding, multiple shells; 4.1-2.67 m, sedimentary lithology is gray and black silty clay, top and bottom were seen black peat layer; 2.67-0.5m, sedimentary lithologic layer is silty clay, see plant residues. HMD1502 hole 41.5 M. the following deposition layer lithology is yellowish brown with clay silty sand, occasionally gravel; 41.5-39.5m, sediment lithology is a hard clay layer, yellow brown, core is relatively dry; 39.5-4.3m, lower about 3M thick coarse sand gravel mixed, the upper is grey brown silty clay containing shell; 4.3-1.41 m, deposition layer lithology is silty clay and fine sand interbed, horizontal bedding clear.2) by Hemudu crossing characteristics of grain size of drilling site, following the meaning of the environment on the time scale, hole HMD1501 in 10~9KaB.P 33-26 m and overall size variation is relatively stable, the grain size is fine, sand content maximum 6%, 0% of the minimum value; mean grain size decreasing trend, the overall grain diameter became fine. Sedimentary facies of clay and a small amount of shells, suggesting that this period for marine environment, sea level fluctuations smaller, also continue to rise, sedimentary layer thickness increasing, water flooded area increased, reflecting the since the last deglaciation, climate warming, leading to the result of the melting of glaciers, about 9~8.1KaB.P 26-15.11 m. During this period, the characteristics of various size parameters occur large fluctuations. Indicated that the formation of this period of sediment environment is not stable, kinetic deposition also showed the trend of "increase decrease"; at the same time, embodied in marine power is larger, for sea level change situation, in pollen chapters combined with the particle size characteristics. About 8.1~7.0KaB.P. During this period, according to the characteristics of grain size changes generally can be divided into three sub periods, 15-14 m, sand content is relatively small, little change; 14-9.5m, sand content increased, the average particle diameter is larger; 9.5-4m stage, sand percent content decreased, flat homogeneous particle size becomes smaller. Show that the sediments are formed when the kinetic energy of the divided stage of change, in the early stages of the kinetic energy is relatively small, the marine environment should belong to a more stable period, after the ocean environmental fluctuations, deposition kinetic energy is greater than the upper, the final stage gradually stabilized in the marine environment, deposition kinetic energy gradually weakened. About 7000~5.8KaB.P, during this period, grain size characteristics showed that larger changes. The average particle size is displayed in the 2-1.8m(6317aBP) and 1.9m(5895aBP) the average particle size is relatively large, sand content and change greatly. This period shows that the sediment formation kinetic energy is relatively large, and the sedimentary environment by terrigenous sediments of great influence, climate relative to the previous stage drought. HMD1502 hole grain size characteristics display about 11~8.7KaB.P. During this period, size of average particle size slightly change, sand content fluctuations larger, granularity of the holistic characteristic of this period show that formation of sediments in the environment is relatively stable, there is a peak in 24.61 m at various granularity parameters. Then sedimentary environment of large kinetic energy. About 8.7~8KaB.P. During this period, the size of parameters characteristic changes acutely, overall size is relatively large, in clay and silt interbedded sediments mainly containing more shells. During this period, the marine environment is quite unstable, and the dynamics of sediment formation is relatively large. This is similar to the characteristics of HMD1501 holes in 8.8~8.1KaB.P sediments. About 8~7KaB.P, this period, the grain size of the parameters of the overall change is relatively small, the content of sand gradually decreases to zero, the average size of the overall trend is gradually reduced. The sediments are mainly clay, and contain shells. It shows that the marine sedimentary environment in this period is relatively stable, and the kinetic energy is relatively weak. About 7~5.5KaB.P, the average size of the particle size of this period appears to decrease first and then increase, and a peak value appears at 4.7m(6819aBP). Show that the sediment in the form of kinetic energy decreases first and then increases, sedimentary facies deposited by marine transition to Hunan deposition that low surface formed at about 7KaB.P, again the transgression, expand the lake surface sediments by the influence of marine environment greatly, then sink product environment affected by land.3) according to the HMD1501/1502 hole pollen records. During the early Holocene(10589-7644 cal.yr.B), vegetation community in study area belongs to the evergreen broad-leaved and coniferous mixed forest, pine and evergreen oaks were the dominant species, herbaceous community to the natural growth of Gramineae were the dominant species, in the study area just experienced late Pleistocene "Younger Dryas event, the climate began to rapidly warming humidification; middle Holocene 7644-6773 cal.yr.B.P. vegetation not needle and broad-leaved mixed forest, but broadleaf woody plants began to decline, especially a sharp decline in evergreen oak, herbaceous and fern began to increase, indicating that the forest or low-lying office has already started small scale distribution of some of the grass. Especially before construction of the group of dominant species of evergreen Quercus content dropped sharply, and herb content, especially with cultivation of gramineous plants(pollen grain diameter more than or equal to 35 m) content increased, which reflects human activities has already begun to influence vegetation community structure climate mild and humid. During the late Holocene 6773-3327 cal.yr.B.P. temperate zone broad-leaved components increase, like warm tropical and Subtropical Broadleaf Species continued to decline, hi dry herbaceous pollen increased sharply, reflecting in the further impact of human activities, then grassland vegetation also increases and in the coastal area began formed by some dominated by Cyperaceae swamps. The vegetation belongs to the mixed coniferous and broad-leaved forest and grass, and the climate became dry and cool.4) according to HMD1501/1502 hole dinoflagellates, foraminifera, Ca, Mn and other environmental proxies, the sea level variation of Holocene period as follows, about 11-8.8Ka B.P, sea level rise in the shock, the hole in the HMD1502 32.2m that the shell, after dating for 11.5KaB.P, Ca elements displayed on the first hard clay the content of Ca, began to increase; about 8.8-8.0KaB.P during the great sea level fluctuations, and continued to rise, according to the characteristics of pore size of HMD1501, the initial period of severe variation of particle size, particle size after tapering, marine dinoflagellate and foraminifera lining concentration reached its peak in 8.2KaB.P, shows that about 8.2KaB.P water depth the maximum value of HMD1502, the same hole is about 8.200 KaB.P in marine dinoflagellates and foraminifera lining concentration also reached the highest value; about 8.0-6.5KaB.P period, sea level decreased gradually, part of the region began in the oceanic environment change lagoon or lake environment, the hole HMD1501 at 4.1m began to form peat layer, the age of about 7KaB.P, Ca content to about 7KaB.P declined, to quickly reduce the 6.5KaB.P. Reduce HMD1502 hole Ca content in suddenly 7KaB.P and sediments are composed of clay gradually changed for silt and sediment grain size, showed that distance from the shore near.