N-alkanes distributions and compound-specific carbon isotope records and their paleoenviromental significance of sediments from Lake Sifangshan in the Great Khingan Moun-tain, Northeastern China
对位于大兴安岭中北段四方山天池全新世以来湖泊沉积物中正构烷烃的分布特征及其单体碳同位素组成进行了讨论分析.结果表明,正构烷烃碳数分布范围为nC17 ~nC33,绝大多数样品具有单峰型的分布特征,少数样品呈现双峰型分布特征,单峰型以nC_(27)为主峰,双峰型分布中前峰群以nC_(21)为主峰、后峰群以nC_(27)为主峰;短链正构烷烃(nC_(27))具有明显的奇碳优势;正构烷烃的分布特征揭示四方山天池湖泊沉积物中的有机质来源于陆生植物和水生植物的共同输入,并以陆生植物贡献为主.全新世以来,四方山天池湖泊沉积物中长链正构烷烃单体碳同位素值(delta~(13)C_(27~31))逐渐偏负,与太阳辐射的变化表现出明显的一致性,表明在轨道尺度上该区域的有效湿度的逐渐增加主要受太阳辐射的控制,湖泊水位的变化则既受到西太平洋副热带高压和鄂霍次克海高压相对位置的影响,又受到东南季风和东北季风势力强弱的制约.根据多种气候代用指标的变化,全新世以来四方山天池湖泊环境及其区域气候演化可以划分为5个阶段:(1)11.2 8.0 ka BP:区域有效湿度较低,陆生C3植物中木本植物比例略有增加,湖泊水位频繁波动,湖泊初级生产力下降,湖泊营养状态发生贫化;(2)8.0 6.4 ka BP:区域有效湿度增加,陆生C3植物中草本植物比例略有增大,湖泊水面收缩、水位下降,湖泊初级生产力变化不大,湖泊营养状态较为稳定;(3)6.4 3.4 ka BP:区域有效湿度比上一阶段更高,陆生C3植物中木本植物扩张,草本植物比例相对收缩,湖泊水位上升、水面扩大,湖泊初级生产力增加,湖泊营养状态发生好转;(4)3.4 2.4 ka BP:区域有效降水量继续增加,陆生C3植物中草本植物比例升高,湖泊水位下降、水面收缩,湖泊初级生产力下降,湖泊营养状态发生贫化;(5)2.4 0.9 ka BP:区域有效湿度较高,陆生C3植物中木本植物逐渐增加,湖泊水位上升、水面扩张,湖泊初级生产力增加,湖泊营养状态有所好转.四方山天池湖泊沉积物所记录的全新世以来区域气候环境演化历史,既与中国东北地区其他高分辨率气候记录有较好的一致性,也存在着差异性,表现出独有的区域气候特征.
英文摘要:
Lake Sifangshan (49°22'32.97N, 123°27'49.90E, altitude: 933 m asl) is a nearly dried up volcanic lake, located on the central-northern part of the Great Khingan Mountain, northeast China. This study obtained n-alkane samples from Holocene sediment cores in the lake, and analysed their distribution and compound specific carbon isotope compositions. The distribution of n-alkanes showed the four following characteristics: (1) a homologous series of n-alkanes was detected with carbon numbers ranging from nC17 to nC33; 2) Most samples show a single peak, while others have double peeks; 3) For single peak samples, the peak is nC_(27), and for double peak samples, the first peak is nC_(21) and the second peak is nC_(27); 4) For short chain (nC_(27)) chain n-alkanes have an odd-over-even carbon number predominance. These characteristics show that the organic matter in the lake came from both terrestrial and aquatic plants, the former of which is the main input. The isotope ratios (delta~(13)C_(27~31)) of n-alkanes in these samples are negative, and gradually decreases along the time, in good accordance with changes of summer solar radiation in the Northern Hemi- sphere during the Holocene. This implies that effective precipitation changes on the orbital scale were directly controlled by changes in summer solar radiation. Based on above proxy indicators, the environmental evolution of Lake Sifangshan during Holocene is di- vided into five stages: (1) In the 11.2-8.0 ka BP,effective precipitation was low and proportion of woody plants in terrestrial C3 plants increased slightly.The lake water level fluctuated frequently, but the primary productivity and nutritional status of the lake deteriorated. (2) In the 8.0-6.4 ka BP,effective precipitation increased and proportion of woody plants in terrestrial C3 plants in- creased slightly. The lake surface area shrunk and water level dropped.The primary productivity and nutritional status of the lake were stable. (3) In the 6.4-3.4 ka BP, effective precipitation was higher than that in the previous stage, and the proportion of woody plants increased but that of herbs decreased.The lake surface area expanded and water level rised.The primary productivity increased, and the tropical level of the lake started to be lower; (4) In the 3.4-2.4 ka BP,effective precipitation continued to rise, and the proportion of herbs increased. The lake surface area shrunk and water level dropped. The primary productivity de- creased, and depletion of nutrition occurred again. (5) In the 2.4-0.9 ka BP,effective precipitation was in a high level and the proportion of woody plants increased gradually. The lake surface area expanded and water level rised.The primary productivity in- creased and the tropical level of the lake started to get lower. The climatic evolution produced by this study is in good agreement with other high-resolution climate records of Northeast China, and the differences show unique regional climate characteristics of the Lake Sifangshan.