黄土高原S_5古土壤形成期的气候环境——陆生蜗牛化石的证据

PALEOENVIRONMENTAL STUDY OF S_5 PALEOSOL FORMATION:BASED ON MOLLUSK FOSSIL EVIDENCE

通过对黄土高原西峰黄土剖面S5古土壤层的蜗牛化石组合研究发现，西峰地区在S5形成时（对应海洋氧同位素MIS 15-13期）气候环境经历了几次显著的波动。MIS15期地层中绝大多数蜗牛壳体被溶蚀，气候非常暖湿，夏季风强盛；MIS14是个弱冰期，蜗牛组合显示气候以凉湿为主，仍然有较强的夏季风影响；MIS13期以暖湿的气候为主，夏季风增强。对温湿蜗牛种类冠状砂螺（Gastrocoptacoreana）含量变化的功率谱分析显示出明显的23ka周期，表明岁差轨道驱动的低纬太阳辐射是控制黄土高原夏季风变化的主导因素；冷干种类杂色虹蛹螺（Pupilla aeoli）含量变化具有明显的100ka和弱的43ka周期，说明即使在气候特征总体暖湿的背景下，冬季风仍具有以全球冰量（100ka）为主导周期的变化特征。在海洋和冰芯记录中MIS13和MIS15时期并不是最温暖的间冰期，但在黄土高原受到强夏季风的持续影响，这可能与此时北半球夏季太阳辐射量的增强和北大西洋深层流强度的加强有关。S5中SS1与S5SS3两层强烈发育的古土壤可能有着不同的发育机制，S5SS1因其较发育的母质、超长的成壤期，形成了比S5SS3更强的成壤特征。分析认为全球冰量与轨道驱动的北半球太阳辐射对MIS15～13时期黄土高原气候环境的演替和冬、夏季风的消长起了主导作用。

The S5 is the most prominent paleosol unit in the Chinese loess stratigraphy, and is distinguished by its great thickness,dark color,and well-developed pedogenic features. Previous studies showed that the S5 developed under much warmer and wetter conditions than at present. However, the S5 is a paleosol complex, corresponding to a period of marine oxygen-isotope stages（MIS） 15 through 13, characterized by weak glacial-interglacial oscillations. Here we present a high-resolution terrestrial mollusk record from the Xifeng （35°46′N, 107°41′E） loess-paleosol sequence to illustrate changes in climate and environment in the Loess Plateau during MIS 15 ~ 13. A total of 149 mollusk assemblage samples from the Xifeng section have been taken and analyzed with a sampling interval of 10cm in the L4 -L6（425 -649kaB. P. ）,and each sample analyzed weighs about 15kg. The result shows that terrestrial mollusk fossils are abundant,and 16species have been identified. These species can be divided into three ecotype groups,namely cold-aridiphilous, thermo-humidiphilous, and oriental group. Our study reveals that the climate in the Xifeng region experienced warm-humid,mild,temperate-humid,mild,warm-humid conditions during MIS 15 to MIS 13, which is attributed to the effect of winter and summer monsoon interactions on the loess region. Our results suggest that global ice volume and insolation are the major factors of the change of climate and monsoon in the loess region. Our study also shows that strong summer monsoon, having prominent procession-induced 23ka cycle, continuously influenced the loess region during the ＇global cool＇ interglacials MIS 15 and MIS 13. The possible explanation to the seeming paradox is the intensification of both summer insolation of the Northern Hemisphere and the North Atlantic deep-water circulation during these two periods. Anyway, as the S5SS1 was formed on weakly developed loess layer and experienced more prolonged pedogenesis, it showed more prominent pedogenetic characteristics and yielded more mollusk individual shells than S5SS3. For this,more study is needed in the future.