浙江长兴更新世红土中的分子化石及其古环境意义

MOLECULAR FOSSILS AS INDICATORS FOR PALEOENVIRONMENT FROM QUATERNARY RED EARTH IN CHANGXING,ZHEJIANG

利用 GC/MS从浙江长兴更新世红土中检测出种类繁多但含量甚微的分子化石 ,主要包括正构烷烃、长链三环萜、五环三萜、一元正脂肪酸、正脂肪醇、正烷基 - 2 -酮等系列。红土中的这些分子化石 ,特别是正构烷烃的特征表明本区的红土是在多水的环境下形成的 ,并且其上发育了以草本植物为主的高等植被。三环萜烷与三萜烷比值 (C2 3 /C3 0 )随地层的变化与红土网纹化程度密切相关 。

The south China climate is characterized by the dominance of the East Asian summer monsoon, which leaves the formation of the rubified paleosol during the Pleistocene instead of the loess deposits seen widely in central-north China. The widely developed paleosol in south China (with an area of 2 2×10 6 km 2) known as 'red earth' spans late early Pleistocene to early late Pleistocene and is clay enriched and distinctly reddish in color, and most importantly, develops a kind of worm-form or vermicular structure. These paleosols should provide valuable sources of the reconstruction of the varied strength of the East Asian summer monsoon as has been revealed in the loess stratigraphy. However, owing to the intensive pedogenesis in the tropical-subtropical area, the vermicular rubified paleosol is proposed to be incapable of containing the information about the past climate. Previous studies on bio-climatic approaches including pollen and spore fossils and phytoliths are also believed not suitable for such kind of stratigraphy. Lipid biomarkers (molecular fossils), based on the powerful analysis of gas chromatography (GC) and GC-mass spectrometry (MS), enable a potential molecular record to trace the past changing climate. Here as an initial, we present some significant molecular fossil data from the subtropical rubified paleosols in south China to show the effectiveness in recording the paleo-vegetation change and related climate variation. Series of molecular fossils, including n -alkanes, extended tricyclic terpanes, n -alkanoic acids, n -alkanols, n -alkan-2-ones and pentacyclic triterpanes dominated by hopane series, have been identified through GC/MS analysis from the Quaternary red earth in Changxing, Zhejiang. The n -alkanes have the carbon number distribution of C 14-C 33, with 2 main peaks at C 16 (or C 17, C 18) and C 31 (or C 29) and a distinct odd-even carbon predominance above C 22. The distribution of n -alkanoic acids ranges from C 8 to C 24 with a maximum at C 16 and a strong even-over-odd carbon predominance. n -alkanols extracted from the red earth are mainly distributed from C 10 to C 30 with 2 main peaks at C 16 (or C 18) and C 28 and an obvious even-over-odd predominance. The series of n -alkan-2-ones ranging from C 21 to C 31 with a maximum at C 29 and a strong odd-over-even predominance have also been detected from the Quaternary red earth. The molecular fossil characters, especially the n -alkane distribution dominated by homologues less than the C 22 with the peak at C 16(or C 17,C 18) diagnostic of aquatic micro-organism input indicates the occurrence of an aquatic environment during the formation of the red paleosols. The dominance of C 31 n -alkane over the C 22 n -alkanes implicates the occurrence of a grassy vegetation over this area. Significantly, the index of tricyclic terpanes/hopanes (such as C 23/C 30) has a close correlation with the rubified intensity of vermicular red earth. This index is generally higher in the well developed vermicular red earth than in the weakly developed vermicular red earth, which may indicate the potential parameter for use as an indicator of climatic change. The high value of the index suggests that the parent rocks had suffered strong weathering effect as a result of temperate and moist climate.