等时线Al-26/Be-10埋藏测年法研究泥河湾下沙沟和东窑子头动物群年代

The hundreds of meters-thick fluvio-lacustrine sediments in Nihewan Basin, Northern China are rich in mammalian fossil and Paleolithic remains, for which the basin has been a key region for studying Quaternary land mammalian biochronology and early human settlements in East Asia. Due to the lack of suitable radio-isotopic dating method, the chronological study of sites in Nihewan Basin depends mainly on the relative dating method of magnetostratigraphy. However, the lower part of Nihewan beds is intercalated by multiple layers of sandy-gravels, which is usually a sign of depositional discontinuity and violates the reliability of magnetostratigrapy dating results of the faunas from this segment of strata. Isochron Al-26/Be-l0 burial dating is a newly established dating method. By analyzing several samples from the same horizon level, and plotting their Al-10 and Be-10 data on the Cartesian coordinates, a straight line, the slope of which is dependent on the burial age, can be plotted out through fitting. This provides a means to reveal the true burial age of shallowly buried samples, and also offers a check for internal consistency. The present proposal aims at constraining the age of Xiashagou and Dongyaozitou fauna, two of the earliest and most important faunas in Nihewan Basin, by using isochron burial dating. It is expected to obtain robust age controls for the two faunas, which is the key to establish a more reliable chronological framework for the Nihewan mammalian faunas. The results of this project is also important for contextualizing early human occupation in Nihewan Basin and constraining the age of the lower boundary of Nihewan Beds.

泥河湾盆地巨厚河湖相沉积中埋藏了大量哺乳动物化石和旧石器遗存，是研究晚新生代陆相生物年代学和早期人类活动的宝库。因缺乏可靠的放射性同位素测年手段，相关地点年代学研究长期以来只能依靠磁性地层学等相对定年法。但泥河湾下部地层普遍分布砂砾层，指示地层存在不连续的可能，给泥河湾早期动物群的磁性地层研究带来较大困难。等时线埋藏测年法是近年发展起来的新方法，通过将同一层位多个砾石样品的Al-26和Be-10浓度拟合成等时线，可规避后期生成宇生核素的影响，获得真实的埋藏年龄，亦可剔除经历复杂埋藏史的样品，大幅提高年代结果的可信度。下沙沟和东窑子头动物群是泥河湾最早期动物群的代表，也是泥河湾动物演化序列中的关键节点。本项目拟运用等时线埋藏测年法精确限定这两个动物群的年代，以此重新厘定泥河湾动物群的年代框架。相关结果对探讨早期人类在泥河湾盆地活动的环境背景、确定泥河湾层的底界年代亦有重要意义。

泥河湾盆地巨厚河湖相沉积中埋藏了大量哺乳动物化石和旧石器遗存，是研究晚新生代陆相生物年代学和早期人类活动的理想区域。但因缺乏可靠的放射性同位素测年手段，相关地点年代学研究长期以来只能依靠磁性地层学等相对定年法。本项目采用等时线埋藏测年法测定了泥河湾盆地下沙沟和大南沟东陡壁两个经典的动物群地点。.下沙沟六个石英质砾石样品的等时线埋藏测年结果为2.03±0.11Ma，与Olduvai亚时的绝对年代在误差范围内一致，支持前人依据磁性地层学数据得出的年代范围（2.2~1.7Ma）。这一结果在误差范围之内也与地层古生物学的年代判断（~1.8Ma）基本吻合。本研究是下沙沟动物群发现近一个世纪以来首次获得第一个可靠、独立的放射性同位素年龄。与前人的研究相结合，基本夯实了狭义的泥河湾动物群的年代位置。.大南沟东陡壁第3层被认为是稻地组和泥河湾组的岩石地层界线，亦是生物地层学意义上稻地动物群和狭义泥河湾动物群的分界。采自这一层位的六个燧石样品等时线埋藏测年结果为2.69±0.15Ma，恰是古地磁中Gauss和Matuyama的界限（2.6Ma）即第四纪的底界年龄。由此确定了产自其上第3-6层产出的大哺乳动物群不仅组合面貌上与下沙沟动物群相似，其年代也基本相当。而产自第1-2层的更为原始的稻地动物群年代应早于~2.69Ma，确属于上新世。由此可见产自大南沟东陡壁的东窑子头动物群确为一个上新世动物群（稻地）和早更新世泥河湾动物群的混合。本研究补足了泥河湾动物群年代序列中的重要一环。.本项目是等时线埋藏测年法在泥河湾盆地的首次应用，不仅从放射性同位素测年的角度精确限定了下沙沟动物群，东窑子头动物群（以及稻地动物群）的年代，为泥河湾盆地陆生动物群年代框架的建立提供了重要的证据。此外，本项目成果同时也验证了磁性地层学中Olduvai和G/M界限等重要古地磁事件的划分，为整个盆地大量的磁性地层学研究工作提供了绝对年代支撑。