Preliminary Study On Lacustrine Sediments At Diexi In The Upper Reach Of The Minjiang River During The Last Deglaciation

An 11-m-long lacustrine record was observed and investigated nearby the Diexi town, Maoxian, in the upper reach of the Minjiang River. Based on multiproxies analyses,the tectonic and climatic history were discussed between 20~12 ka. The preliminary conclusions have been achieved as following:(1) OSL dating indicates that the Xinmocun section of lacustrine sediments spans from 20.18 to 12.6 ka. Characteristics of grain size, rare-earth element (REE) geochemistry and morphology of quartz grains from the Xinmocun section suggest that the fine sediments are of eolian origin and transported by the Plateau monsoon and the westerlies.(2) Deposition rate increased quickly and curves of indexs fluctuated dramatically upward especially after the 16 ka. Most of the particles are fine, dominated by silt and clay. The sand-bearing samples (average) are only 3.3%. Magnetic susceptibility and median grain size changed roughly synchronously, showing sudden increase and then gradually decrease. Total organic carbon (TOC) and total inorganic carbon (TIC) concentrations are low, an average of 0.35% and 1.91% respectively, being consistent with ecological conditions of the semiarid region. TOC show a fluctuating decreasing trend and TIC was also decreased but with limited scope. Pollen samples were dominated by herbs which were content of 46.3~94.3%,an average of 55.4%; Arboreal pollen were secondary important, which were 4.5~46.3% in content, an average of 31.2%.(3) The pollen and carbon content of the Xinmocun section show the course of paleoenvironment evolution during 20~12 ka. The study area was mainly covered by steppe vegetation, and the pollen abundances changed steadily, the paleoclimate at Xinmocun could be divided into three periods about Cold and dry-cool and wet-cold and dry climates: the climate was cold and dry with sparse vegetation during 20.18~19 ka; during 19~16 ka, the cover type was steppe with some broad leaved forests, the climate became cool and humid, which was a little warm and dry before 17.4 ka, and transited cool and humid late; The environment changed frequently during 16~12.6 ka, the vegetation was also temperate steppe but the vegetation types became rich, the climate became cold with intense alternation of drying and wetting, which implied that the climate changed dramatically over time.(4) Grain size and magnetic susceptibility clearly record the tectonic activities in the study area. It was speculated that eight relatively strong seismic events occurred in the study area during the last deglaciation, especially during 16~12.6 ka.We can identify eight seismic events according to grain size and magnetic susceptibility curve fluctuations. There were only two earthquakes before 16 ka with intervals of about 1500 years and after 16 ka six earthquakes occurred with an average interval of 512 years.