Modern climate and vegetation variability recorded in organic compounds and carbon isotopic compositions in the Dianchi watershed
2015
Xu, Meina | Wang, Yanhua | Yang, Hao | Xie, Biao | Niu, Xiaoyin
The aliphatic hydrocarbons distribution and compound-specific characteristics of carbon isotopic compositions in the sediments from the small catchment (197 km²) of the Dianchi watershed were investigated for identification of modern climate and vegetation variations in the study area. Results show that a regular bimodal n-alkanes distribution ranged from nC₁₆ to nC₃₃, with strong dominance at nC₁₇ for short-chain n-alkanes and nC₃₁ for long-chain n-alkanes. Mass chromatogram of total fatty acids also indicates corresponding mixed contribution of algae, hydrophilous non-emergent (C₄ plants) and terrestrial plants (C₃ plants) to sedimentary organic matter (OM). At the depth of −24 to −25 cm (early 1970s), nC₃₁/nC₁₇ and terrestrial to aquatic ratio of hydrocarbons (TAR) values decrease, suggesting a shift of OM origins from C₃ terrestrial plants to algae-derived C₄ plants. The highest water stage in 1971 was found to be recorded in the particle size (<4 μm). For long-chain alkanes, the values of δ ¹³Cₒᵣg and δ ¹³C ₙ₋ₐₗₖₐₙₑₛ varied from −26.9 to −22.4 and −33.4 to −27.9 ‰, respectively. Population growth and economic development led to a demand for abundant habitable and cultivable land. Due to unreasonable land expansion, the primordial forest sporadically distributed. A mixture of C₃ and C₄ plants probably replaced C₃ plants as the sources of OM in the past 10 years. The changes of land-use types and severe drought resulted in the excessive OM inputs to the watershed.
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