Isotope Geochemistry Southern Vegetation Reconstruction HUGUANGYAN Code Joel Lake Sediments Record

As a special type of new Cenozoic volcanic lake in the Leizhou Peninsula, maar lake has its unique depositional environment with closure, continuity and high-resolution, which is a better place to reconstruct palaeoenvironment than others. In the thesis, we analyze the compositions of n-alkanes and their stable carbon isotopes in the core sediments taken from the Huguangyan Maar Lake. The paleovegetation and paleoenvironment from21.1ka B.P to5.5ka B.P were reconstructed depending on the molecular compositions.The main research conclusions are as follows:(1) The compositions of n-alkanes mainly consist of long-chain n-alkanes. The n-alkanes exhibited strong odd-over-even predominance between C25and C33, suggesting that the n-alkanes in the lake sediments were mainly derived from terrestrial higher plants.(2) The stable carbon isotopic of n-alkanes range from-19.6‰to-34.2‰, suggesting that land vegetation were formed by C4and C3plants. The compound-spec-ific stable carbon isotopic compositions of C27, C29, C31and C33in the core sediments were determined. The relative contribution of C4and C3plants was estimated by using a binary mixing model. The results showed that vegetation was composed of both C4and C3plants from21.0to18.5ka B.P, however, C4plants were dominant. This vegetation pattern suggested that the climate changed into aridity and promoted the spread of C4plants in South China.There was an increasing trend in C3plants and a decreasing trend in C4plants after19.5ka B.P. C4plants were over C3plants during16.5-9.0ka B.P, and C3plants predominated in South China until6.5ka B.P.(3) The paleovegetation and paleoenvironment from21.1ka B.P to5.5ka B.P in this region are reconstruted by the indicies of CPI, ALC,(C27+C29)/2C3i and compounds-specific stable carbon isotope:Phase I (about21.1-18.5ka B.P):The vegetation was dominated in the Huguangyan Maar Lake area during that time, while the woody plants had expansion trend. The climate was cold-dry;Phase II (about18.5-16.5ka B.P):The woody plants were further expansion trend. And the climate was cold-wet;PhaseIII(about16.5-9.0ka B.P):The vegetation was relatively, but rapid fluctuates, the climate evolution model was cold-wet to cold-dry to warm-dry; PhaseⅣ(about9.0-5.5ka B.P):The vegetation and woody plants were rapidly growing, but the grass vegetation faster than woody plants, the climate was warm and humid.