泥炭样品的AMS 14 C年龄测定: 全样、植物残体和孢粉浓缩物*

沉积物¹⁴ C的年龄测定一直是第四纪年代学研究的热点。文章对中国干旱、半干旱地区的内蒙古库伦泥炭剖面的泥炭全样、植物残体和孢粉浓缩物进行了AMS ¹⁴ C测年比较研究。从泥炭沉积物中提取用于AMS ¹⁴ C测年的孢粉浓缩物的关键步骤包括:过筛(125μm,63μm和10μm),重液浮选(比重1.9)及在180倍的体视显微镜下进行手工挑选。测年结果表明:孢粉浓缩物的测年值比同一层位的泥炭沉积物全样或植物残体的测年值老225~340年; 植物残体与孢粉浓缩物的测年结果较为接近。但是,距地表24~27cm处孢粉浓缩物给出了610~780A.D.的年龄值,远老于其下样点的年龄,而泥炭全样为现代样品,二者相差1255年,这可能与该样品孢粉纯度较低、掺杂有一些老碳物质燃烧后形成的微粒杂质有关。因此,提高孢粉纯度对孢粉浓缩物¹⁴ C测年至关重要。

AMS 14 C DATING USING BULK SAMPLES, PLANT RESIDUES AND POLLEN CONCENTRATES FROM A PEAT PROFILE AT KULUN, INNER MONGOLIA

AMS ¹⁴ C dating using bulk samples, plant residues and pollen concentrates from 12 samples of six levels of a 140cm peat profile at Kulun, Inner Mongolia, which is situated in a mid-latitude ecotone between a semi-humid zone and a semi-arid zone in Northern China (42°44'N,122°00'E) was conducted. The key steps of efficient extraction of pollen concentrates for AMS dating include (1) sieving (125μm,63μm and 10μm), heavy liquid separation (density 1.9) and hand picking under stereomicroscope (up to x 180). Percentage of main taxa between samples prepared by the conventional method (conventionally extracted fossil pollen samples for identification) and the purified method (purified pollen concentrates for AMS ¹⁴ C dating) is different. Conventionally extracted fossil pollen samples are predominated by Artemisia and Chenopodiaceae; while purified pollen concentrates for AMS ¹⁴ C dating are dominated by increased Cyperaceae and Gramineae. Our dating results reveal that AMS ¹⁴ C ages of the pollen concentrates are consistently 225~340 years older than those obtained with the bulk samples and plant residues. However, the topmost pollen concentrate sample from 24~27cm depth yields an age of 610~780A.D. , which is markedly different from that of the bulk sample with an age discrepancy of 1255 years. This may be caused by the relatively poor pollen purity of the pollen concentrate sample and contamination by microparticles from old carbon.