泥河湾盆地基于粒度气候指标的轨道调谐时间标尺

泥河湾盆地东部出露一套厚近百米的以湖相为主的地层.我们测量了洞沟剖面的粒度和磁化率.通过磁化率记录的对比, 我们可以将前人确定的古地磁界线转移到我们的深度坐标上来, 并建立了初步的时间标尺.样品中粒径在20~50μm之间的颗粒含量时间序列可以与ETP曲线对比, 即低含量对应高ETP值.这样的对比意味着在北半球夏季太阳辐射量相对增加的时期, 进入泥河湾盆地沉积物中的风尘物质减少了.这与已知的北方风尘的演化历史是吻合的, 所以我们认为粒度(20~50μm) 含量是泥河湾盆地沉积物中比较可靠的古气候变化的替代性指标.对于沉积速率相对高的时段, 即 > 780ka的地层, 我们尝试用轨道调谐的方法建立了高分辨率的时间标尺.新的粒度(20~50μm) 含量时间序列滤波获得的20ka和41ka的周期成分与岁差及地轴倾斜度的理论值可以重合.这显示洞沟剖面的粒度(20~50μm) 含量记录可以作为轨道调谐的气候曲线. 

Orbitally Tuned Time Scale Based on Climate Proxy Indicator of Grain Size Distribution in Nihewan Basin

The Donggou Section located in the east of Nihewan basin contains a suite of strata consisted mainly of lacustrine sediments approximate to 100 m thick. Both grain size distribution and magnetic susceptibility are measured in successive samples collected from this section. A preliminary time scale can be derived, based on the comparison between records of magnetic susceptibility, from the geomagnetic polarity reversals determined by previous studies. In this study, the grain (20-50 μm) content record of Donggou Section can be correlated to the ETP curve. The low value of the former is compared with the high value of the latter. This negative correlation implies that the dust inputs into sediments in the Nihewan basin decreases while the solar radiation on the north hemisphere increases. This scenario is compatible with the known evolution of the aeolian activity in North China. Therefore, the grain (20-50 μm) content is a reliable proxy indicator of the paleoclimate change. Orbital tuning method is performed and a time scale of high resolution is obtained for the strata older than > 780 ka since the sedimentation rate here is relatively high. The 20 ka and 41 ka periodic components filtered from the obtained grain (20-50 μm) content time series are consistent with the theoretic cycles of the precession and the tilt, respectively, indicating that the grain (20-50 μm) content record of Donggou Section can be used as the climatic curve for orbital tuning.