亚热带落叶乔木叶蜡正构烷烃单体碳同位素季节变化特征及影响因素

Seasonal variations in the carbon isotope compositions of leaf wax n-alkanes in two subtropical deciduous tree species

叶蜡正构烷烃单体碳同位素组成已经成为陆相沉积古生态重建的重要研究方法。现代过程研究是提升叶蜡正构烷烃指标解译的重要环节,而目前关于落叶乔木叶蜡烷烃单体碳同位素组成(δ^(13)C_(alk))的季节性变化研究还比较有限。本文对两种典型亚热带落叶乔木(小叶栎、枫香)开展了为期2年的叶蜡烷烃δ^(13)C_(alk)研究。两个树种叶蜡烷烃的δ^(13)C_(alk)值在春季偏正,随后迅速偏负,秋季变幅较小,趋于稳定。这种季节性变化特征和初期利用储存的老碳而后期利用新合成的糖类有关。在生长季末期,两种落叶乔木叶蜡烷烃相对于叶片的碳同位素分馏系数处于−6‰~−8‰,相对于文献报道的平均值偏负,这可能是因为生长季后期仍有少量的长链正构烷烃合成。研究结果揭示,基于落叶乔木叶蜡烷烃单体碳同位素进行古环境重建时需要考虑环境信号的季节性。

Carbon isotope compositions of leaf wax n-alkanes (δ^(13)C_(alk)) have been essential tools for paleoclimate and paleoecological reconstructions.Research of modern processes has the potential to improve the interpretation of leaf wax n-alkanes in geological archives.However,the understanding of seasonal variations of δ^(13)C_(alk)in subtropical deciduous species remains limited.Therefore,we investigated the seasonal variations in the carbon isotope compositions of two typical subtropical deciduous trees (Quercus chenii,Liquidambar formosana) over two years.During the spring,the δ^(13)C_(alk)values in the two tree species were relatively positive,then turned negative,and finally showed near-constant in the fall.Such a seasonal pattern possibly relates to the utilization of stored carbohydrates at the initial stage of leaf development and the use of newly synthesized sugars in the later stages.At the end of the growing season,the apparent fractionation of carbon isotopes between the bulk leaves and n-alkanes ranged from-6‰to-8‰,which may be due to the secondary synthesis of long chain n-alkanes in small amounts at the end of the growth period.These results suggest that the seasonality of environmental factors may affect the δ^(13)C_(alk)signals of deciduous trees preserved in geological archives.