泸沽湖沉积物正构烷烃分布特征及其古植被意义

DISTRIBUTION OF N-ALKANES IN THE LUGUHU LAKE SEDIMENTS AND THEIR PALEOVEGETATION SIGNIFICANCE

利用气相色谱-质谱联用仪从泸沽湖沉积物中检测出丰富的正构烷烃系列分子化石,泸沽湖沉积物正构烷烃分布特征可归纳为双峰前峰型和双峰后峰型,说明泸沽湖沉积有机质母质来源为混源。较高丰度的中等长度正构烷烃及Paq和（C15＋C17＋C19）/（C27＋C29＋C31）比值等指标指示挺水植物及沉水/漂浮植物等水生植物贡献量较大,同时,也说明较大的内源输入可能是泸沽湖TOC含量较高的原因之一。主峰碳及正构烷烃代用指标（C27＋C29）/（C31＋C33）比值和ACL指示泸沽湖沉积有机质木本植物输入量占优势,这与现今泸沽湖流域植被类型特征相符。纵向上,（C27＋C29）/（C31＋C33）比值和ACL没有明显的阶段性和趋势性变化,反映出正构烷烃记录的泸沽湖流域草本植被与木本植被相对变化不明显。不同沉积时期正构烷烃分布特征有所不同。3万年以来泸沽湖流域植被演化特征可以分为3个阶段：30～20cal.kaBP气候偏干冷,沉积有机质母质主要为低等水生生物和藻类,沉水和漂浮植物也有较大贡献;20～5cal.kaBP为相对暖湿的气候环境,陆源高等植被对泸沽湖沉积有机质的贡献增加;5～0cal.kaBP（对应剖面205～0cm）气候有变冷干的趋势,沉积有机质物源以低等水生生物和藻类为主,同时,具有较多高等植物和沉水/漂浮植物输入。泸沽湖流域12kaBP为干冷气候时期,这在时间与气候特征上与YD事件相符,可能是西南季风对YD事件的响应。

Using GC/MS analysis,n-alkanes have been identified in the sediments of the Luguhu Lake,southeastern Tibetan Plateau.The n-alkanes for all the samples are distributed in a bimodal pattern,indicating a mixed source.They are classified into two types.The TypeⅠ found in seven samples is characterized by significant abundance of C27,C29 and C31,indicating a substantial input of greater and higher plant.The other 9samples,which fall in TypeⅡ,are characterized by lower microorganisms input.The significant abundance of the medium-chain（C23and C25）n-alkanes and the value of Paqand the ratio of（C15＋C17＋C19）/（C27＋C29＋C31）indicate an input of greater aquatic plant.The grater endogenous input maybe one of the reasons for higher TOC in the Luguhu Lake.The parameters of（C27＋C29）/（C31＋C33）and ACL,which represents relatively high woody plants compared to grassy plants,is 1.73～4.28and26.19～28.1,respectively,indicating that wood plants have dominated the Luguhu Lake area since 30 kaBP.It is consistent with the present vegetation distribution pattern in the Luguhu Lake area.The vertical distribution of（C27＋C29）/（C31＋C33）and ACL does not suggest a obvious change trend.It means the change in relative contribution of woody plants and grassy plants recoded by n-alkanes was rather small.It is worthy to note that the paleoclimate of the Luguhu Lake area was cold and dry about 12 kaBP,which could be the response to the Younger Dryas Event.