东海陆架浙闽沿岸泥质区不同属种底栖有孔虫对14C测年的影响及其原因初探

摘要

底栖有孔虫是浅海陆架区钻孔岩芯14C测年的良好材料,但由于其个体细小,为了获得足够的样品量,常常采用不区分属种和大小的混合壳体样品,不利于高分辨率的古环境研究.本研究选择东海陆架浙闽沿岸泥质区的两个钻孔沉积物样品,对其中不同的底栖有孔虫属种壳体化石分别进行14C测年,结果显示同一沉积物样品中不同的底栖有孔虫属种壳体化石14C年龄值存在差异,以Quinqueloculina spp.壳体14C年龄最老,其次是Ammoniacomperessiuscula壳体,最年轻的分别为Elphidium spp.壳体和Bolivina cochei壳体.从底栖有孔虫的生态习性和壳体的结构特征等方面分析推测,同层位不同底栖有孔虫属种壳体年龄差异,不能简单地用它们生活时壳体和所处的微生境水体同位素交换来解释;不同属种有孔虫壳壁构造不同可能是产生壳体化石14C年龄差异的原因.建议为地质钻孔建立高分辨率年代地层挑取底栖有孔虫AMS 14C测年样品时,尽量选择相同的属种,或微生境相同、个体大小相近、壳壁厚度和壁孔大小密度近一致的类别,以降低属种不同对测年结果产生的影响,为海岸带高分辨率的古环境研究提供高精度的测年数据.%Benthic foraminiferas from sediment cores are ideal materials for 14C dating in the shallow continental shelf areas.Usually,mixed benthic foraminiferal shells of different species and sizes from the sediment are picked out to obtain sufficient weight for 14C dating.In this study,two holes D03 (121°47′56″E,27°53′42″N;water depth 42 m,long 41.2 m)and D05(121°36′22″E,27°20′53″N;water depth 57 m,long 40.4 m)were drilled on the muddy coast of Zhejiang and Fujian in the East China Sea shelf.Samples of different benthic foraminiferal species picked from two sedimentary samples at 31.05～31.11 m of Hole D03 and at 12.80～ 12.87 m of Hole D05 were analyzed for 14C dating.The results show that different benthic foraminiferal species from the same sediment units yielded different 14C ages.The 14C age of Quinqueloculina spp.was found to be the oldest,Ammonia comperessiuscula relatively yonger,and Elphidium spp.and Bolivina cochei were the youngest.The observed 14C age differences were interpreted with reference to the benthic foraminiferal microhabitats and the characteristics of shell structure.The water isotope exchange process between the shell and their microhabitat is thought to be less likely the main cause for the age difference.Instead,the characteristic shell structures of foraminiferal taxa used in the study may be responsible for the 14C age difference.In order to reduce the impact of the different foraminiferal species on 14C dating and provide the high precision chronological constraints for coastal zone paleoenvironmental study,it is recommended to use benthic foraminiferal shells from the same genus or species,or taxon that have similar microhabitats,size,wall thickness and pore pattern for AMS 14C analyses.