Environmental evolution of gucheng lake for the past 4000 years revealed by a short core

Abstract

对固城湖6.2m深的现代沉积物柱状岩芯进行了放射性同位素、粒度、有机质含量、有机质σ¹³C和孢粉分析。沉积记录巾各项环境指标的变化表明, 4ka以来固城湖经历了高湖面—低湖面—高湖面的显著变化。气候变化是影响湖泊环境演化的重要自然因素, 但2.5ka以来人类活动对湖泊演化的影响显得更为明显。特别是2.5ka和1.1ka两次人类对湖泊水系的改造是引起湖泊环境两次突变的主要因素。;The sediment from a columnar core with a depth of 6. 2 min Gucheng Lake was analyzed with regard to radioisotope, grain size, organic matter content, δ¹³C of organic matter and pollen. ¹⁴C dating of 3630+230a was obtained from the organic mud sample 6. 1 m below the core top. The average sedimentary rate of recent sediments is 0. 71mm/a resulting from ²¹⁰Pb dating technique. According co the lithology and various environmental m ar kers of the sediment five lithoiogical beds and five correspondins stages of environmental evolution may be divided from the core profile:(1)5.1-6.2m , green-greymud and siltymud.Clay content(CL), organic matter conent(LOI)and δ¹³C values are all higher(77.5 , 5.6 and - 25.36‰ PDB respectively ) in th interva1. High-lake level was reflected during the period .(2) 3 .5- 5· 1n1, grey silty m ud. CL, LOI and δ¹³C va]ues are reduced slightly (62.2% , 5.0% , and-25.98%0 respectively). Lake-level was also higher in the period .(3) 1.2- 3.5m , grey muddy fine grained sand and sir. CL .LOI an d 6 C are all at their low est values in the profile (13.3 .3.7 and 26.31‰ respectively).Thelake—level was verylow in thetime. (4) 0.3- 1.2m. grey sandY an d silty mud. CL , LOI, and 6 C values are risen again to some extent(41.2% , 7.8 and -25.`` respectively ).The lake was expan ded during the t[nle. (5) 0 0.3m , dark—grey silty mud. CL , LOI and δ¹³C values are all quite high (70.3-10.6 and - 22.1 4‰ respectively).The Inke-level was probably higher during the period-According to the pollen forms and contents in the core profile, eight palynological assemblage zones are divided as follows;(I)Quercus-Casta~ea zone, (Ⅱ)Pinus-Quercus- Polypodiaceae zone, (Ⅲ )Quercus-Graminca- Typha zone, (IV)Graminea-Arteinsria-Quercus zone, (V ) Quercus-Graminca- Typha zone, (VI) Gram inea Pigza-Quercu zone, (Ⅶ) Qercus-Pinus-Castanea zone an d (Ⅷ )P inus-Graminea-Ariternaisira-Qzone. Com par ison of climatic change sequence resulting from pollen an alysk to the lake—level evolution m ay lead to the discovery that the clim ate change was coincident w ith lake—level change before 2.5 ka B.P.(6.2— 3.5m in the core)and the climate waS the main controlling factor for the lake expansion. But the regularitlos of lake—level fluctuation ar e not conformable to the clim atic change s.nce 2.5 ka B.P.(0- 3.5m in the core). Obviously the lake environm ent change was not controlled dom inantly by clim ate factor during the time. On the contrar y, human activities were the leading factor that affected lake environment evolution. In the historical records, the Xux[River channaled at caused Gucheng Lake water our eastwards an d also caused the lake to shrink intensely.As a result, the lake was converted into Taihu Lake drainage bas in. U P to 893 A .D. as five bars were built along the Xuxi River.the lake water flowing eas twards was CUt off resultin g in an expansion of lake ar ea and the lake beca me the Changjan g River drainage bas in again. Therefore, it is inferred that the twice abrupt changes of lake environment at 3.5m and 1.2m of the core are resulted from hum an’g tras form atinn on the lake water system in historical time.