Hydrogen isotope ratios of leaf wax n-alkanes in loess and floodplain deposits in northern China since the Last Glacial Maximum and their paleoclimatic significance

摘要

The hydrogen isotopic composition of leaf-wax n-alkanes (delta Dn-alkane) is increasingly used as a proxy for estimating delta D of past precipitation (delta D-p). However, aridity can also affect sedimentary delta Dn-alkane, complicating the interpretation of paleo-delta Dn-alkane records. In order to evaluate the effects of the complex interactions and balance between precipitation hydrogen isotope composition and aridity on sedimentary delta Dn-alkane records, we present two delta Dn-alkane records from the North China Plain and the western Chinese Loess Plateau across a steep east-west climatic gradient in the East Asian Summer Monsoon area since the Last Glacial Maximum. The estimated delta D-p changes, based on a present-day relationship between delta D-p and rainfall amounts, can explain much of the delta Dn-alkane variations in the North China Plain, but do not explain the large variations of delta Dn-alkane in the western Chinese Loess Plateau. Comparison of delta Dn-alkane records with observed On-alkane values in modern plants and published pollen data indicates that changes in vegetation play a minor role in the variations of delta Dn-alkane. Instead, delta Dn-alkane, values are coincident with aridity both temporally and spatially, i.e. the higher delta Dn-alkane values the drier climate and vice versa. Aridity effects are more evident in the western Chinese Loess Plateau than in the North China Plain. Therefore, on glacial-interglacial scale, sedimentary delta Dn-alkane records from the North China Plain mostly reflect changes in delta D-p values, whereas those from the western Chinese Loess Plateau largely reflect the effects of aridity, mainly due to a greater evapotranspirational deuterium-enrichment caused by a drier climate in the western Chinese Loess Plateau than in the North China Plain. Further calibrations from modem sediments are required to better define the quantitative relationship between the delta Dn-alkane and climate in northern China