洛川黄土-红粘土序列铁氧化物组成及其古气候指示

对洛川黄土、古土壤和红粘土中磁性矿物组成、成因和相关系进行了研究。结果表明:黄土磁性矿物以风尘磁铁矿为主,少量的成土赤铁矿和成土磁赤铁矿;古土壤磁性矿物以成土磁赤铁矿为主,成土赤铁矿次之,少量的风尘磁铁矿和赤铁矿;红粘土磁性矿物以成土赤铁矿为主,风尘磁铁矿和成土磁赤铁矿次之,少量风尘赤铁矿。黄土、古土壤和红粘土磁性矿物组成差异,反映了其形成期不同的古气候特性以及不同气候条件下生物地球化学作用强度的差异。干冷的冰期,黄土弱成土作用形成了以粗颗粒的风尘磁铁矿核 赤铁矿边的磁化率载体。间冰期的温暖湿润的古气候最有利于生物活动,强烈生物活动导致古土壤中大量纳米超细磁赤铁矿/磁铁矿产生,形成以磁赤铁矿为主,风尘磁铁矿核 赤铁矿边为辅的磁化率载体。红粘土成壤期,强降雨强蒸发的长干短湿的高温炎热的古气候使得红粘土化学风化强烈,生物地球化学活动较弱,形成以磁铁矿核 赤铁矿边和磁赤铁矿核 赤铁矿边的磁化率载体。黄土、古土壤和红粘土磁性矿物组成、磁性矿物相关系是其形成期独特的古气候指示。

COMPOSITION OF FERRIC OXIDES IN THE LUOCHUAN LOESS-RED CLAY SEQUENCES ON CHINA' S LOESS PLATEAU AND ITS PALEOCLIMATIC IMPLICATIONS

Magnetic extraction,CBD method,XRD,SEM and microscopic results showed that in the loess magnetic minerals are mainly detrital magnetite,and minor pedogenic hametite and maghemite.While in the paleosol magnetic minerals are dominated by pedogenic maghemite,next to pedogenic hematite rather than aeolian magnetite and hematite.The pedogenic hametite dominates red clay magnetic minerals,coming next is pedogenic maghemite with minor aeolian magnetite and hametite.Among the loess,paleosol and red clay,there are differences in magnetic mineral composition,which strongly indicates the paleoclimate characteristics from the loess,paleosol to the red clay stage.During the dry/cold ice age,the detrital magnetite-core/hametite-rim boundary arouse as a result of weak-degree weathering in the loess layer.At the time of paleosol deposition,the warm/wet paleoclimate favors nanometer-scale magnetite formation resulting from biochemical process and low temperature oxidation of magnetite to maghemite and hametite,and is directly related to the amount of magnetic susceptibility carriers consisting mainly pedogenic maghemite and minor aeolian magnetite-core/ hematite-rim.Through the red clay pedogensis,the dry and warm climate of long dry and short wet characterized by the combination of strong rainfall and evaporation caused stronger pedogenesis charactered by higher degree low temperature oxidation of autogenic maghemite to hematite and detrital magnetite to hematite.