科研之友
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摘要
A set of lake surface sediment samples and aquatic macrophytes from the eastern and central Tibetan Plateau and a sediment core from Koucha Lake (northeastern Tibetan Plateau) were analysed for hydrogen isotope signals of aliphatic lipid biomarkers. The delta D values of n-alkanes were correlated with those of meteoric water in the summer months and showed increasing Pearson correlation coefficients with increasing n-alkane chain length (r, 0.80 for n-C-31). While in samples from the most humid regions (southeastern Tibetan Plateau), the apparent enrichment factors between meteoric water and lipids are close to those reported for other humid sites (epsilon approximate to -135%o for mid-chain and approximate to -125 parts per thousand for long chain n-alkanes), they become smaller (epsilon approximate to -118 parts per thousand to -41 parts per thousand; mean value: -86 parts per thousand) at sites with a negative moisture balance (northeastern and central Tibetan Plateau and Qaidam Basin). This might be caused by enrichment in deuterium of the source water used for lipid synthesis as a result of lake water evaporation and soil and leaf water evapotranspiration. In the core from Lake Koucha, decreasing delta D values of all n-alkanes between 15 and 10 cal ka BP give evidence for an intensification of the Asian summer monsoon. At ca. 9 cal ka BP, a ca. 15 parts per thousand shift towards higher delta D values of mid-chain n-alkanes suggests enhanced evaporation of the lake water, which outweighs higher precipitation amounts because of a temperature increase during the early Holocene monsoonal maximum. Only a few surface sediment samples show a significant offset between delta D values of mid-chain n-alkanes and n-C-31 (A delta Dmid-n-C31) and no systematic dependence between this offset and environmental parameters, such as mean annual precipitation, was found. In the core, available data for most samples also do not show a significant offset, only a slight increase (approximate to + 15 parts per thousand) of Delta delta Dmid-n-C31 at ca 10.0 cal ka BP and a decrease (approximate to -17 parts per thousand) at ca 6.8 cal ka BP might indicate wetter and drier conditions, respectively. However, interpretation of Delta delta Dmin-n-C31 throughout the record is difficult because of a lack of reliable data for n-C31 in key sections of the core. So, more records are necessary to further test the potential applicability of Delta delta Dmin-n-C31 as a proxy for palaeo-hydrological conditions.
- 出版日期2010-8
