通过泸沽湖沉积物样品的年代学、穆斯堡尔谱学和有机分子地球化学分析,研究了泸沽湖三万年以来的铁元素与正构烷烃记录,及其古气候/环境变化特征。泸沽湖沉积速率稳定,平均沉积速率为0.28mm/a。沉积物中Fe~(2+)相对含量低于Fe~(3+),Pr/Ph为0. 54~ 1. 48,平均为1.04,表明泸沽湖总体属于弱氧化环境。正构烷烃碳数分布在C_(11)~C_(33)之间,呈双峰分布形式,C_21-/C_(22+)为0. 12~2.66、CPI_h为1.20~4.37、ACLh为25. 68~27. 97以及P_(aq)为0.25~0.63,指示泸沽湖沉积有机质母质既有高等植物也有水生生物。泸沽湖沉积物中总铁相对含量(Tot-area), Fe~(2+)/Fe~(3+)比值与生物标志化合物参数(Pr/Ph、C_(21-)/C_(21+)、CPI_h、ACLh和P_(aq))相关性较强,均是指示古气候/环境变化的有用指标:沉积物中总铁相对含量(Tot-area)与湖区降水量成负相关;Fe~(2+)/Fe~(3+)不仅能指示湖泊沉积环境特征,而且与古气温成负相关。利用上述指标重建了三万年以来泸沽湖湖区的古气候/环境,将泸沽湖湖区古气候分为3个阶段:30~20cal.ka B. P.寒冷干燥期、20~5cal. ka B. P.温暖潮湿期和5~0cal. ka B. P.寒冷干燥期。其中在12cal.ka B.P.左右出现气候突变为干冷,应为新仙女木冷期。
英文摘要:
Lugu Lake, located at the southeastern Tibetan Plateau (27°40'~ 27°44'N, 100°46' ~ 100。55'E),is the second largest deep plateau-lake in Yunnan-Guizhou Plateau. A 880cm long core (at 27°40'49"N, 100°48'00"E)taken from water to a depth of 44. 5 meters shows that its sediments are rich in organic matters with leaf remains but without distinguishable physical features. The age-depth model of the Lugu Lake core is established from 16 AMS~(14)C dates. This paper presents iron and n-alkanes evidences for reconstructing paleoclimate of Lugu Lake region during the past 30,000 years. 16 samples from the core spanning the last 30ka have been studied, combining with chronology, ~(57)Fe Mossbauer spectroscopy (MBS) and gas chromatography-mass spectrometry. The rate of sedimentation in Lugu Lake is stable, and its average value is 0. 28 mm/a. Two species of Fe were identified as paramagnetic Fe~(3+) and paramagnetic Fe~(2+) by MBS, and the Fe~(2+)/Fe~(3+) radio<1. At the same time, the ratio of Pr/Ph is 0. 54 ~1.48 with an average of 1. 04. Both above two indexes indicate that the oxidation-deoxidation environment of Lugu Lake is weak oxidation environment. The total percentage composition of iron (Tot-area) shows a strong linear relationship with the ratio of Fe2+/Fe3+ in the samples, which suggest that the change of paleo-climate and sedimentary environments not only affect the chemical forms and oxidation states of iron but also relative content of iron. A series of n-alkanes were identified in Lugu Lake sediments range from C_(11) to C_(33) and show a double-modal distribution pattern. The n-alkanes indexes of C_(21-)/C_(22+), CPI_h, ACL_h and Paq are 0. 12 ~ 2. 66, 1.20~4.37,25. 68 ~27. 97 and 0. 25 ~ 0. 63, respectively, suggesting that the organic matters in the lake sediments were mainly derived from higher terrestrial plants and the lower aquatic organism. Both of total iron (Tot-area) and Fe_(2+)/Fe_(3+) radio in this core has a strong correlation with n-alkanes indexes (C_(21-)/C_(22+), CPIh, ACL_h and P_(aq)), which suggest that all of these indexes are valid proxies for paleo-climate and paleo-environment reconstruction. The abundances of iron in sediments may negatively related to precipitation of Lugu Lake basin. All of above indexes showed that since 30cal. ka B. P. the environmental change in this region can be divided into three stages. The early stage is from 30 cal.ka B.P. to 20cal. ka B. P., which is characterized by cold-dry. 20 ~ 5cal. ka B. P. is the middle stage, during which the climate changed from cold-dry to warm-humid with a cold-dry process should be the Younger Dryas Event about 12cal. ka B.P. The late stage was around 5cal. ka B.P. to present with the characteristics of a general tendency toward cold-dry.