许家窑泥河湾沉积物的环境磁学特征

ENVIRONMENTAL MAGNETIC RECORDS OF NIHEWAN LACUSTRINE SEDIMENTS AT XUJIAYAO AND THEIR PALEOCLIMATIC SIGNIFICANCE

许家窑泥河湾河 -湖相沉积物磁化率 (k)、天然剩磁 (NRM)、饱和等温剩磁(SIRM4T)、非磁滞剩磁 (ARM)及S比率 (IRM- 0 .3T/IRM4T)等磁学参数与深海氧同位素的对比分析结果显示 :1 )许家窑泥河湾层从布容 /松山界线到氧同位素阶段 5 (0 .1 3MaB .P .)基本上是连续沉积 ,沉积速率保持在约 2 .45cm/1 0 3a左右 ;2 )与同期形成的黄土高原黄土 -古土壤序列类似 ,许家窑河 -湖相层也很好地记录了当时中国北方的第四纪气候和环境特征 ,其典型特征为冰期时高矫顽力赤铁矿含量的显著增加 ,而间冰期则以低矫顽力的磁铁矿及磁赤铁矿为主要载磁矿物 ;3 )由尝试建立的S比率时间标尺所确定的许家窑古人类遗址处的地层年代约 0 .5MaB .P .,与先前报道的化石的U Th绝对年龄 (0 .1MaB .P .)不一致 ;4)与k相比 ,S比率与SIRM更适合作为河 -湖相沉积物的古气候替代性指标 ;5 )以 2 .45cm/1 0 3a的沉积速率来推算 ,沉积物的主要物质来源为正常的河 -湖相沉积 。

A comparative analysis was performed between magnetic parameters of Pleistocene sediments of the Nihewan Group at Xujiayao and deep sea oxygen isotopic data. There seem to be some parallel or corresponding relations between the two kinds of data, which help to deepen the understanding of Nihewan sediments. These sediments are largely continuous from the Brunhes-Matuyama boundary to the oxygen isotope-based stage 5 (0.13MaB.P.), with a sedimentation rate of about 2.45 cm/10 3a. Like the synchronous loess-paleosol sequences on Chinese Loess Plateau to record climatic and environmental characteristics of Quaternary of North China, Nihewan sediments are characterized by the fact that hematite with a high coercive force significantly increased during the glacial ages, whereas magnetite and maghemite with a low coercive force were dominant magnetic minerals during the interglacial ages. According to the S-ratio (IRM -0.3T/IRM 4T)-based time scale, sediments at the Xujiayao paleoanthropological site give an age of 0.5 MaB.P., which is different from the U-Th age given by fossils reported previously (0.1 MaB.P.). Magnetic parameters in Nihewan sediments show that compared with the magnetic susceptibility k , S-ratio and saturation isothermal remanent magnetization (SIRM) are more suitable to use an alternative paleoclimatic indicator of river-lake sediments. When calculating in the sedimentation rate of 2.45cm/10 3a, the source of Nihewan sediments should be mainly of river-lake facies, but the loess-like dust should not be dominant.