晚新生代以来天山北缘沉积记录及其稳定同位素变化的环境意义

Sedimentary records and paleoenvtronmental significance of stable isotopic evidences from the late Cenozoic,northern Tianshan.

晚新生代以来,天山北缘的沉积记录与天山的隆升密切相关。研究表明:始-渐新世安集海河组主要沉积发育水平层理的绿色、灰绿色湖泊相泥岩;晚渐新世沙湾组中上部岩石组合特征为紫红色曲流河相砂岩、粉砂岩和砾岩夹层;早中新世塔西河组下部主要为灰绿色、褐红色砂、泥岩互层,上部主要为土黄色砂岩和粉砂岩夹层;晚中新世-早上新世独山子组为辫状河相灰色砾岩和砂砾岩沉积;晚上新世-早更新世西域组主要为厚层砾岩堆积。土壤碳酸盐中稳定同位素数据表明:安集海河组δ^(13)C平均值为-3.9‰,δ^(18)O平均值为-5.0‰;沙湾组中上部δ^(13)C平均值为-8.1‰,δ^(18)O平均值为-11.0‰;塔西河组δ^(13)C平均值为-5.9‰;δ^(18)O平均值为-7.7‰;独山子组δ^(13)C平均值为-8.5‰,δ^(18)O平均值为-10.3‰;西域组δ^(13)C平均值为-6.0‰,δ^(18)O平均值为-10.4‰。同位素结果和沉积记录共同表明,始-渐新世为湿润温暖气候;晚渐新世为干旱寒冷气候;早中新世早期为半湿润、半温暖气候,晚期为干旱、寒冷气候;晚中新世-早上新世为干旱寒冷气候;晚上新世-下更新世为干旱、半温暖气候。结合古地磁数据和全球气候变化,天山北缘晚新生代以来发生五次主要气候变化事件,即晚渐新世(沙湾组时期中上部)气候转换,约24Ma(沙湾组时期顶部)气候突变,早中新世内部(约24～19Ma)气候转换事件,晚中新世(约7Ma)C_4植被迅速扩张和约3.17Ma以来气候的剧烈震荡。

Sedimentary records in northern Tianshan are in response to the uplift of the Tianshan Mountain from the late Cenozoic. Eocene-Oligocene Anjihaihe formation mainly deposite gray or dark gray lacustrine mudstone with flat bedding; Late Oligocene Shawan formation are overlain purple meander sandstone, sihstone and interbedded conglomerate are dominant; the lower of Early Miocene Taxihe formation consist of gray or brownish sihstone and mudstone frequently coexist; the upper are brownish or gray-yellowish sandstone with interbedded sihstone; Late Miocene-Early Pliocene Dushanzi formation are composed of braided-fluvial accumulation of dark gray conglomerate or associated with brownish or gray-yellowish sandstone; Late Pliocene-Early Pleistocene Xiyu formation consist of a large amount of alluvial fan pebble to boulder conglomerate. Stable carbon and oxygen isotopic data show： average δ^13C values from Eocene-Oligocene, through Late Oligocene, lower of Early Miocene, upper of Early Miocene, Late Miocene-Early Pliocene to Late Pliocene-Early Pleistocene are ～ - 3.9‰, ～ - 8.1‰, ～ - 5.9‰, ～ - 8.5‰, ～ - 6.0‰ respectively, while average δ^18O values are ～ -5.0‰,～ -11.0‰, ～ -7.7‰, ～ -10.3‰, ～ -10.4‰ respectively. Stable isotopic changes and sedimentary records indicate paleoenvironmental changes： warm-wet during Eocene-Oligocene, arid and cold during Late Oligocene, semi-humid and semiwarm during lower of Early Miocene, arid and cold during upper of Early Miocene, arid and cold during Late Miocene-Early Pliocene, arid and semi-warm during Late Pliocene to Pleistocene. Combined with paleomagnetic data in this area, stable isotopic results, sedimentary records and global climate change, five paleoenvironmental changes have developed in northern Tianshan since the late Cenozoic： climate conversion during late of Qligocene, climate break at Early Miocene（ ～ 24Ma）, climate conversion during early of Miocene （24 ～19Ma）, C4 rapidly expand at Late Miocene （ ～ 7Ma） and climate intensely vibrate at Late Pliocene to Early Pleistocene （-3.17Ma）.