The Ancient Barrier Lake And Geoenvironment, Diexi, Minjiang River

By studying the ancient barrier lake sediments, this paper mainly probes into the ancient climate evolution rule of more scale and less from twenty-two thousand to ten thousand years before present, Diexi, Minjiang River. Moreover, influences to lower reaches of Minjiang river is simply discussed because of die out of the ancient barrier. The using correlative theories in this thesis are MC Chronology, Sedimentology, Isotope Geology, Environmental Geology, Geomorphology. The implying environmental substitute indicators are carbonate carbon and oxygen isotopes and so on. The main conclusions drawn from this paper are as follows:(1) The ancient climate evolution, from twenty-two thousand to ten thousand years before present, Diexi, Minjiang River, is from dryness to wetness and can be divided into six phases and two periods by analyzing the ancient barrier lake sediments through multi-environment indicators, and its evolution cycle is three thousand years or so. Six phases respectively are: ゛bout from twenty-two thousand years to twenty thousands years before present; ゛bout from twenty thousand years to seventeen thousands years before present; ゛bout from seventeen thousand years to fourteen thousand years before present; ゛bout from fourteen thousand years to thirteen thousand and five hundred years before present, ; ゛bout from thirteen thousand and five hundred years to eleven thousand years before present; ゛bout from eleven thousand years to ten thousand years before present. Two periods respectively are:㎜GM, about from twenty-two thousand years to seventeen thousand years before present; ㏑ising Temperature Periods, about from seventeen thousand years to ten thousand years before present.(2) The analysis by characteristic section planes can draw that the ancient climate in deposit period of the characteristic section plane one sediments (about seventeen thousand years before present) is warm-dry and has relative humid and dry evolution rule, from fifteen to thirty years as its evolution period, that the ancient climate in deposit period of the characteristic section plane two sediments (about fourteen thousand years before present) is humid and has relative alternate very humid and humid evolution rule ,from two to fourteen years as its evolution rule.(3) The forming wrapping structure because of interlayer between silt and clay at the transition part from the fourth layer to the fifth layer in the sediments section. According to this, intense change of the geoenvironment in this area at the period of filling up of the ancient barrier lake is presumed.(4) By five terraces surrounding the ancient barrier lake, presuming that destroyed process of the ancient barrier lake is at least divided into five phases at the time span of two thousand years to three thousand years, and it reflects the five times flood in the course of the destroying of the ancient barrier lake.(5) By analyzing the terrace in the lacustrine sediment of the ancient barrier lake, Diexi, Minjiang River, forming of the terrace is not only because of the intermittent arising of the tectonic movement, but also because of descend of the eroding normal covering resulting from destroying of the barrier lake.(6) The destroying of the barrier dam can result in the change of the lower environment. So protecting the upper environment is not only the necessity to the development itself, but the key to the lower environment.