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An evaluation of biological and climatic effects on plant n-alkane distributions and δ2Halk in a field experiment conducted in central Tibet

Bai, Yan, Azamdzhon, Murodov, Wang, Shiping, Fang, Xiaomin, Guo, Haichao, Zhou, Pengchao, Chen, Chihao, Liu, Xiaomin, Jia, Shugang, Wang, Qi
Organic geochemistry 2019 v.135 pp. 53-63
Angiospermae, alkanes, carbon, field experimentation, herbaceous plants, hydrogen, isotopes, paleoclimatology, temperature, China
The distributions of long chain n-alkanes and their hydrogen isotope values (δ2Halk) can be used to investigate paleoclimatic changes, but, as yet, the degree to which plant types, temperature and/or hydrology may drive such variations, cannot be discriminated. Seven co-occurring herbaceous plants (angiosperms) were studied in climate-controlled growing conditions in an open-top chamber located in Naqu, Tibet, in 2015 and 2017, using generalized n-alkane parameters, viz. n-alkane concentrations (Concalk), the carbon preference index (CPI) and the average chain length (ACL), and δ2Halk values. These plants were subjected to warming (W, spanning a < 4 °C mean annual temperature (MAT) gradient) and the amount of water that plants received (100% precipitation differences) conditions in 2015 and 2017, respectively. Growth experiments showed that: (1) No significant relationships were found between n-alkane parameters (Concalk, ACL and CPI) and temperature (a 4 °C difference) or precipitation (100% precipitation differences); (2) The δ2Halk increased ca. 10–30‰ during W 4 °C conditions, with differences in the sensitivity of responses to W between different species. During P treatments, the variability in δ2Halk values was close to that caused by a W 4 °C response; and 3) the interspecies variability in n-alkane distributions and δ2Halk (ca. 30–50‰) was higher than that caused by these P and W responses. These results highlight the importance of the potential impact of any changes in the floral assemblage when interpreting sedimentary n-alkane distributions and δ2Halk records.