Carbon and oxygen isotope changes and palaeoclimate cycles recorded by lacustrine deposits of Miocene Wudaoliang Group in northern Tibetan Plateau

Carbon and oxygen isotopes in drill core of Wudaoliang Group have well recorded Early Miocene environment changes and palaeoclimate cycles in northern Tibetan Plateau.The Wudaoliang Group as thick as ～ 150 m consists mainly of limestone,dolomite limestone and marl,with a discontinuity occurring at the depth of 50.0-51.0 m in the drill core.Carbon and oxygen isotopes change considerably at the depth of 140-145 m,where both maximal δ18O and minimal δ13C occur two times.δ18O and δ13C reach to the minimum at the depth of 140.7 m,corresponding to global cooling Mi-1 recorded by the deep-sea drilling of oceanic deposits at the Miocene/Oligocene boundary.Seven eccentricity cycles of 1.2 Ma are recorded between 140.7 m and 14.2 m in depth,and 9 minor cycles of 17.4 ka are recorded from 69.86 m to 62.60 m in depth.It is further inferred that Early Miocene lake in northern Tibetan Plateau initiated at(24.1±0.6) Ma and ended at(14.5±0.5) Ma according to Palaeo-climate cycles recorded by carbon and oxygen isotope changes of lacustrine deposits,and that annual temperatures changed from 19°C to 21°C(～20°C on average) in Wudaoliang basin in Early Miocene.