GDGTs揭示的西宁盆地中中新世环境变化

摘要

西宁盆地关家山剖面微生物细胞膜脂的生物标志化合物甘油二烷基甘油四醚类脂物(Glycerol DialkylGlycerol Tetraethers,简称GDGTs)古温度指标MATmr记录了从16.8 Ma到13.4 Ma的持续降温过程(下降约5.1℃),与全球深海氧同位素记录的中中新世气候适宜期(MMCO)及其后的中中新世气候转型(MMCT)事件一致,揭示了西宁盆地古温度在此期间受全球温度变化控制.其环化参数CBT'重建的古pH值(CBT'-pH)平均为6.56±0.50,且80％样点的干旱指标Ri/b(古菌类异戊二烯型isoprenoid GDGTs(简称iGDGTs)与细菌支链型branched GDGTs(简称bGDGTs)相对含量比值)小于0.5,表明西宁盆地在此期间整体上处于较为湿润的环境,但是在14.0 Ma和14.8 Ma,Rvb值记录了两次干旱事件.总之,虽然在古温度绝对值计算等问题上仍存在一定困难,GDGTs展现了在西宁盆地乃至青藏高原东北部在新近系甚至更老地层中应用的潜力.%The MATmr,records of Glycerol Dialkyl Glycerol Tetraether lipids (GDGTs) from Guanjiashan section (36°40.55′N,101°50.24′E),Xining Basin,NE Tibetan Plateau,suggest the continuous cooling between 16.8 Ma and 13.4 Ma (droping in temperature about 5.1 ℃).Our result is consistent with the climatic events responding to Middle Miocene Climate Optimum(MMCO) and Middle Miocene Climate Transition(MMCT) recorded by global deep-sea oxygen isotope records.Thus.,it reveals that MATmr were controlled by global temperature change during this period.The mean value of CBT'-derived paleo-pH (CBT'-pH)values is 6.56±0.50,and the abundances of b GDGTs(branched GDGTs)relative to that of archaeal isoprenoidal(iso) GDGTs (Ri/b,as a drought proxy in Chinese soils)for 80％ samples are less than 0.5,together indicating relatively humid in that period as a whole in the Xining Basin.But two drought events were recorded by Ri/b values in 14.0 Ma and 14.8 Ma.Although it's difficult to estimate the absolute paleo-temperature at present,GDGTs have important potentiality to reconstruct paleo-temperature of Xining Basin,and so far as to NE Tibetan Plateau since Neogene and even older sediments.