鹤庆盆地湖相岩心磁化率记录及其古环境意义

MAGNETIC SUSCEPTIBILITY OF HEQING DRILL CORE AND ITS PALAEOENVIRONMENTAL IMPLICATIONS

鹤庆盆地位于受印度季风影响的中国西南地区,沉积连续,厚度大,是古环境研究的理想场所。本文主要通过对鹤庆湖相岩心磁化率记录的分析研究,并结合碳酸盐、烧失量和粒度特征对古环境变化的响应机理,探讨了沉积物中磁化率的古环境指示意义。本研究发现,磁化率与碳酸盐和烧失量具有显著的相反变化;而与粒度的关系较为复杂。磁化率与指示粗颗粒变化的中值粒径在含砂(砾)层具有较好的正相关关系;与指示细颗粒变化的<4μm颗粒的相对含量,在湖相粘土沉积中具有较为明显的正相关关系,表明湖相沉积物中的亚铁磁性矿物主要来自于外源带入的粘土粒级碎屑物中。通过分析磁化率对古环境变化的响应机制,并结合碳酸盐含量、烧失量和粒度变化特征,本研究认为鹤庆盆地湖相岩心的磁化率记录可以用来反映水动力对物源区地表的侵蚀强度和搬运状况,与流域内植被覆盖和降雨量密切相关。在干旱的冰期,植被覆盖度较低,地表侵蚀加剧,带入湖泊的粘土粒级碎屑含量增加,磁化率增高;在湿润的间冰期,植被覆盖度较高,地表侵蚀减弱,粘土粒级碎屑物的带入量减少,磁化率降低。

Located in southwestern China,the Heqing Basin received continuous lacustrine sediments of great thickness,which could be an excellent continental record for Indian monsoon.In this study,we choose the lake sediments from Heqing drill core to analyze the paleoenvironmental significance of magnetic susceptibility.By analyzing the magnetic susceptibility in combination with the response of carbonate content,Loss on Ignition(LOI),and grain size to paleoenvironmental changes,we get some useful information: 1) magnetic susceptibility increase when carbonate and LOI decrease;2) magnetic susceptibility increase with increase of < 4μm grains in lacustrine clayey sediments,and with increase of middle grain size in sections containing sands and pebbles.Based on comparison between these proxies,we conclude that magnetic minerals got into the lake mainly together with clay-sized terrestrial detritus,and magnetic susceptibility is related to the vegetation cover and rainfall in the watershed and therefore could be considered to be a good indicator for monsoon-induced erosion around lake catchments.In the warm-humid interglacial periods,due to well-developed vegetation cover in the catchment area,erosion is relatively weak and clay-sized terrestrial detritus transported into the lake are reduced,so sediments have low concentration of magnetic minerals and low magnetic susceptibility.In the cold-arid glacial periods,however,because of the vegetation development is restricted,erosion is relatively strong and more clay-sized sediments are transported into the lake,the sediments have high concentration of magnetic minerals and high magnetic susceptibility.