Modern climate and vegetation variability recorded in organic compounds and carbon isotopic compositions in the Dianchi watershed.

The aliphatic hydrocarbons distribution and compound-specific characteristics of carbon isotopic compositions in the sediments from the small catchment (197 km(2)) 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 (13)Corg and δ (13)Cn-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.