分子地层学的原理、方法及应用实例

当前分子地层学研究已涉及蛋白质、核酸、碳水化合物、类脂物和木质素等多种生物化学组分。在地层中,分子化石具有分布的广泛性、定量的准确性、应用的指纹性和信息的多样性等特点,其地层学应用的主要原理是依据分子化石的生物源信息和其离开生物体后发生的一系列转化途径来实现的,其表述方法可以是含量、相对丰度、碳数分布和单体同位素特征等。在各类年代学框架下,由这些分子化石参数所揭示的各类生物事件和环境事件可以成为区域性乃至全球性地层对比的主要依据。分子地层学与分子古生物学、生物地球化学、有机地球化学密切相关,它与传统三大地层学分支学科明显不同,目前还没有明确的分子地层单位,也没有进行广泛的分子地层划分与对比工作。对各类生物事件与环境事件有重要指示作用的分子地层学,与生态地层学、事件地层学等地层学分支学科类似,其主要任务是在传统生物地层学或其他年代学框架下,提高地层划分和对比的精度。以浙江长兴煤山二叠纪-三叠纪界线地层和第四纪泥炭为例,以高分辨率的生物事件与环境事件为切入点,分别探讨在生物地层学或其他年代学框架下的分子地层工作,由此提出了分子地层学的分类单位——分子化石带。

PRINCIPLES, METHODOLOGY AND APPLICATION OF MOLECULAR STRATIGRAPHY

As a newly developed subdiscipline of stratigraphy,molecular stratigraphy focuses on a variety of molecular fossils derived from biochemicals including protein,nuclear acids,carbohydrates,lipids and lignin.These organic molecules widely distributed in geological bodies are fingerprints in geological application and can be precisely quantified by using modern techniques such as gas chromatography(GC) and GC-mass spectrometry.They preserve both biological and geological information,which can be revealed by the content,the relative abundance(mainly biomarker ratios),the carbon number distribution and the isotope composition.The application principles are based on known information inherited from biological precursors as well as transformation pathways during diagenesis.Molecular stratigraphy shows a close relationship with biogeochemistry and organic geochemistry,but differs from the conventional stratigraphy such as biostratigraphy,lithostratigraphy and chronostratigraphy in that a molecular stratigraphic unit has not yet been proposed.The unavailable stratigraphic unit prevents the subdivision and correlation of strata using molecular data.We proposed that the molecular stratigraphy unit can be termed as the molecular fossil zone,analogous to a biozone in biostratigraphy.The regional and global correlation could thus be conducted on the basis of the identification of various biological and environmental events using molecular stratigraphy under the control of chronological data.Identical to ecostratigraphy and eventostratigraphy,molecular stratigraphy aims to provide a precise subdivision and correlation of strata under the constraint of chronological data.In this paper,the application of molecular fossils in the subdivision and correlation of the strata is presented,exemplified by the Permian/Triassic boundary strata at Meishan,Zhejiang Province and the Quaternary peat deposits in northern England.