Normal Alkane Distributions in Soil Samples along a Lhasa-Bharatpur Transect

We present n-alkane distributions in the soil samples along a transect from Lhasa on the Tibetan Plateau to Bharatpur in Nepal, which covers a large geographical area and a wide range of climatic conditions. These data allow us to assess the significance of n-alkane distributions in different vegetation types and their relationships to temperature and precipitation. In the tropical rainforest and broadleaved forest zones, n-alkanes exhibit a bimodal distribution pattern with dominant homologues around n-C_（31） and n-C_（23）. The bimodal distribution of tropical rainforest n-alkanes may be a general pattern because of the presence of many lianas, epiphytic plants, algae and the strong microbial activity and degradation involved in the postdepositional process. In the warm-temperate mixed forest and needle-leaved forest zones, the long-chain alkanes have a pattern of n-C_（31） n-C_（33） n-C_（27）. In the alpine shrub and grassland zone, although the most abundant homologue is n-C_（31） , relatively high n-C_（23） concentrations have also been observed in some samples. The statistical results show a good correlation between n-alkane proxies and climatic factors. The average chain length（ACL） values are positively correlated with precipitation and temperature across the transect. The carbon preference index（CPI） values show a negative correlation with temperature and precipitation along a Lhasa–Bharatpur transect. Although there is a positive relationship between ACL and temperature, it is difficult to separate the two climatic variables（temperature and precipitation） because they are well coupled in the monsoon region.