滇池晚更新世末期以来湖积环境分析

ENVIRONMENT ANALYSIS OF DIANCHI LAKE EVER SINCE LAST STAGE PLEISTOCENE EPOCH

通过对滇池湖心区晚更新世末期以来的沉积剖面、沉积物及其化学组合、古地磁特征及孢粉组合的研究,初步确定了滇池上更新统与全新统的分界,认为自晚更新世末期以来,滇池一直维持着稳定的湖相沉积。晚更新世末期,滇池处于温凉偏于的相对浅水环境。全新世早期气候逐渐转暖,湿度渐增,湖水面积扩大且渐深,至全新世中期,气候变得温暖湿润,湖水面积进一步扩大变深。全新世晚期滇池气候又变得温凉偏干、处于湖水面积缩小的浅水环境。

Depositional section of corehole ZK6(0-6.5m)from the central place of Dianchi Lake was studied in this paper. The interface between Holocene and Pleistocene series and environmental changes ever since last stage Pleistocene Epoch in the lake were discussed on the basis of sporo-pollen assemblage and paleomagnetic features in the sediment of the corehole from the lake.Clay is a principal composition and grain size is finer in the sediment of the corehole. It was deduced base on above facts, that depositional environment of Lake facies was stable ever since last stage Pleistocene Epoch in the lake. Four sporo-pollen and magnetic susceptibility zones are respectively distinguished from the top to bottom in the section and they are approximately comparable in the depth of the section.There are four features of sporo-pollen assemblage in the fourth zone(5.10-6.50m)below:(1) Some species of arbor plant pollen is superior. (2) Pinus pollen belonged among coniferous vegetation is a primary type among them and there is a little of Abies and Picea Pollen etc. (3) Broadleaf plant pollen is mainly composed of Alnus.Quercus and Carpinus,but their contents are much less than that of Pinus. (4) Herb pollen is mainly composed of Gramineae and Artemisia, and Pteridophyta spore is mainly composed of Polypodiaceae and Polypodium. Both percentage contents are lower than arbor plant in the zone, but are higher than themselves in above three zones. It was deduced, based on above features, that paleoclimate might be a arid type of cool-temperate zone and the lake was shallow at that time and it was similar with the paleoclimate of Late Pleistocene Epoch. Reversed geomagnetic polarity was measured at the top sediment(5.10-5.20m)in the zone and the polarity may be corresponded to that of Gothenburg discovered in North Europe. So it may be considered as the interface between Pleistoncene and Holocene Series. Magnetic susceptibility value in the Zone is moderate and undulates frequently, it should be the reflection of that the climate fluctuated in last stage Late Pleistoncene Epoch.In the third(5.10~3.37)and second(3.37~0.90m)sporo-pollen zones the content of arbor plant pollen increased further and attained the highest value in the second zone. But the content of Pinus pollen reduced and broadleaf plant pollen such as Alnus,Quercus,Castanopsis,Carya and Lliquidambar etc. increased. Specially Castanopsis.Carya and Liquidambar lived in tropic and subtropical zone began to appear in the third zone and got to the highest value in the second zone. The composition of Herb pollen and pteridophyta spore in the third, second zone is basically similar with that in the fourth zone, but their contents were less. It was indicated that the paleoclimate in Early Holocene Epoch began to warm and wetness increased gradually and it had become warm and wet, the lake area was become larger and the water was deeper in Middle Holocene Epoch. Amplitude of susceptibility in the third zone was larger. It might be the result of that the paleoclimate became wet. The fact that susceptibility value was comparatively lower in the second zone than in other zones should be related with that the sediment size was coarser.In the first sporo-pollen zone(0.9-0m)arbor plant pollen was less than that in the second zone, but Pinus was in majority again and broadleaf plant pollen decreased obviously, Carya and Liquidambar were extinct in the zone. The content of Pteridophyto spore was the lowest, but that of Herb pollen attained the highest. Thus it was indicated that the climate became of cool-temperate and lightly dry, and the lake area drew back and became shallow again in Late Holocene Epoch. The susceptibility value was the highest in the first zone, which may be related with that clay is more and the sediment size is finer in the zone than in other zones.