川西峨眉地区古近纪早期气候特征:来自名山组下段地球化学及磁化率的证据*

古近纪早期是显生宙典型的“温室气候”期,也是气候向“冰室气候”转型前的最后温暖期。川西峨眉地区发育有完整的古近系名山组湖相地层,对该地层的古气候研究可以为了解和探索新生代重大地质事件和气候的演化过程提供参考依据。在前人研究的基础上,以常量元素与微量元素比值和磁化率为依据,结合宏观沉积特征和岩石薄片镜下特征,对峨眉地区古近纪早期气候演化与磁化率形成机理进行讨论。研究结果表明: 川西峨眉地区古近纪早期总体为热带—亚热带炎热干旱气候,该气候为晚白垩世炎热干旱气候的延续,但较晚白垩世干热程度明显降低,期间出现多次炎热干旱—温暖湿润变化,并存在气候波动较为剧烈的时段。气候的干湿变化是造成研究区磁化率变化的主要原因,磁化率高值反映气候趋于湿润,低值则反映干旱程度增加,剖面中磁化率出现明显波动的位置可能对应东亚季风开始对峨眉地区产生影响的时期。

Early Paleogene climatic features in Emei area,western Sichuan: evidence from geochemistry and magnetic susceptibility in the lower part of the Mingshan Formation

The Early Paleogene is a typical “greenhouse climate” period of the Phanerozoic,and also the last warm period prior to the Cenozoic “icehouse climate”. There is a complete set of lacustrine strata of the Paleogene Mingshan Formation in the Emei area,western Sichuan,which can provide useful information about the Cenozoic major geological events and evolution of palaeoclimate. On the basis of previous studies,element ratio,magnetic susceptibility and macroscopic and microscopic characteristics of rocks are used to study the Early Paleogene climate evolution and the formation mechanism of magnetic susceptibility in the Emei area. The results show that the study area had a tropical-subtropical hot and arid climate during the early Paleogene,which was a continuation of the late Cretaceous hot and arid climate,but the dry and heat degree significantly reduced compared with that in the Late Cretaceous. Moreover,there were some cycles of hot and arid-warm and humid climate,and severe climatic fluctuation stage during the Early Paleogene. The dry and wet change of the climate is the main reason for the variability in the magnetic susceptibility of the strata in the study area. Its high values reflect the humid climate,and low values reflect increase of drought. The position where the magnetic susceptibility fluctuates obviously in the strata may correspond to the period when the East Asian monsoon began to affect the Emei area.