青藏高原北部中新统五道梁群湖相沉积碳氧同位素变化及古气候旋回

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

通过对中新统五道梁群湖相沉积进行全岩心钻探和碳氧同位素测试,获得青藏高原北部中新世早期古环境变化和古气候旋回的重要地质记录。五道梁群约150m湖相沉积以灰岩、白云质灰岩与泥灰岩为主,仅在深度50.0～51.0m出现湖相沉积间断,上下均为连续湖相沉积。深度140～145m湖相沉积碳氧同位素剧烈变化,碳同位素(δ13C/‰)出现2次最低峰值,氧同位素(δ18O/‰)出现2次最高峰值;深度140.7m湖相沉积碳同位素(δ13C/‰)和氧同位素(δ18O/‰)同时出现极低值,对应于渐新世/中新世界线深海沉积记录的Mi-1全球古气候事件。深度140.7～14.2m湖相沉积碳氧同位素记录了Mi-1期后7次1.2Ma天文周期的古气候旋回,深度62.6～69.86m湖相沉积碳氧同位素记录了9次周期约17.4ka的古气候旋回。根据湖相沉积碳氧同位素记录的古气候旋回,推断青藏高原北部五道梁盆地中新世早期古大湖发育时期为(24.1±0.6)Ma～(14.5±0.5)Ma,年均气温变化范围为19～21℃,平均约20.0℃。

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 δ 13C occur two times. δ18O and δ13C reach to the minimum at the depth of 140.7m, 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℃ to 21℃（-20℃ on average） in Wudaoliang basin in Early Miocene.