新生代地外物体撞击事件诱发的古气候环境灾变

本文在论述新生代以来６５、３４、１５、２．４、１．０、０．７ＭａＢ．Ｐ．六次重大撞击作用的地质地球化学证据基础上，详细地讨论了撞击作用诱发古气候灾变的过程和机制，认为撞击作用产生的尘埃、炭黑和气溶胶可产生短期的快速降温效应，温度最低可降至２２５Ｋ；撞击后大气中ＣＯ_２含量的下降则可导致最大时间尺度为１０５ａ的地球气候的寒冷期。目前撞击界线层中的氧同位素记录所反映的古气温变化支持本模式的结论。

PALAEOENVIRONMENTAL AND PALAEOCLIMATE CATASTROPHE INDUCED BY CENOZOIC BOLIDEIMPACT EVENTS

Based on geologic and geochemical evidence, we demonstrated that there existed six huge bolide-impact events happened at 65, 34, 15, 2.4, 1.0 and 0.7 Ma B.P., which affected the palaeoenvironment and palaeoclimate on the Earth. Among them, 34, 15, 1.0 and 0.7Ma B. P. impact events were represented by four known famous tektite-strewn events.In this paper we in detail discussed the general physical scenior of the extraterrestrial body impact process, common methods for researching impact records in sediment columns and the process and mechanics of palaeoclimatic catastrophe induced by impact. By using the zero-dimensional energy balance model the simulating calculation showed that impact-generated dusts floated in stratosphere could result in a short-term (<20 years) rapid drop in temperature immediately after impact.Especially for the largest energetic 65 Ma B. P. impact event the effect was the most obvious, giving the lowest temperature to 225K. Considering the effect of soots and gas colloid particles, drop in temperature on the surface of the Earth would be more strong. When impact-generated dusts, soots and gas colloid particles in stratosphere fell in the short time, the temperature on the surface of the Earth would not raised up rapidly. Inversely, the effect of drop in temperature would persist for a longer time because of the positive feedback of the high reflect ratios of snow sheets and ice covers produced in the process of initial temperature-drop immediately after bolide-impact. The modelling results of the water-rock interaction between dusts generated by impact and seawater revealed that the temperature lowering and the increase of PH value of seawater after impact would decrease the amount of CO² in the atmosphere, resulting in a long-term cold period with the longest time scale to 105 year. We suggest that the decrease of the amount of CO² in atmosphere was the main factor to cause long-term colting. The conclusion deduced from this model is supported by palaeotemperature changes recorded by oxygen isotope in sediments across impact boundary.The Cretaceous/Terdary bolide-impact event yielded an important geologic boundary in history of Earth evolution. Although it is nuclear about its magnitude and dimension until now, the research results achieved in Chinese loess sections imply that the 2.4 Ma B. P. multi-impact event could be a possible cause to produce another important geologic boundary: Tertiary/Quaternary.