Hydrogen isotope ratios of individual n-alkanes in plants from Gannan Gahai Lake (China) and surrounding area

Gannan Gahai Lake, in the northeast of the Tibetan Plateau, is the largest freshwater lake in the Gannan plateau, and is rarely influenced by human activity. Aquatic plants in the lake and terrestrial plants in the surrounding area were systematically collected, and the n-alkane distributions and hydrogen isotopic composition investigated for the first time. The distributions exhibited a unimodal/bimodal distribution with maxima at C17 or C21 and at C23 to C31, and were dominated by odd numbered components with carbon preference index (CPI) values of 2.3–14.7. Average chain length (ACL) values, ranging from 25.7 to 29.6, correlated with the dominant n-alkane carbon number. The average δD values of the n-alkanes ranged from –246‰ to –130‰. In general, the δD values were higher for woody plants, lower for herbaceous plants and in between for aquatic plants. The data show that the hydrogen isotopic composition of the plant n-alkanes was influenced by species, leaf surface size, plant water source and environment. The δD values tended to be lower with increasing ACL values, which results from the reduction in leaf water evaporation. The isotopic fractionation during n-alkane synthesis was larger for herbaceous plants, smaller for woody plants and in between for aquatic plants.