黄土高原西部红粘土岩石磁学性质及其指示的亚洲内陆中中新世气候变化特征

对黄土高原西部庄浪红粘土进行详细岩石磁学分析表明,沉积物磁性由磁铁矿和赤铁矿主导,并且含有一定量的磁赤铁矿。中中新世大暖期红粘土中磁铁矿、磁赤铁矿含量显著升高; 中中新世大暖期前后红粘土中的磁铁矿含量相对较低,赤铁矿相对含量较高,但并未显示较强的磁赤铁矿信息。磁性矿物的整体粒径表现为以SP,SD和PSD颗粒为主,MD颗粒较少。庄浪红粘土磁化率在16.5~13.8Ma出现峰值,与当时的夏季风强盛、降水丰富、成壤作用较强造成的亚铁磁性矿物相对含量增加密切相关。在全球气候变暖的中中新世大暖期,综合庄浪岩石磁学特征以及陆地和海洋的其他证据可以推测当时东亚冬、夏季风可能同时增强。

MID-MIOCENE CLIMATE IN THE ASIAN INTERIOR ,BASED ON THE MINERAL-MAGNETIC RECORD OF THE RED CLAY SEQUENCE ON THE WESTERN CHINESE LOESS PLATEAU

Many significant warm and cool climate events occurred during the Cenozoic,and one of the most intriguing climate events of the Neogene was the Middle Miocene Climate Optimum(MMCO),which occurred at about 15Ma and represents a geologically recent warming event unrelated to human activity that may mirror future climate change in terms of the average global surface temperature increase.Zhuanglang(ZL)County(35°13'N,106°05'E),on the western CLP,is 50km northeast of Qin'an section and is widely covered with red clay sediments,in order to get a continuous high-resolution record,the ZL drilling core was obtained.From the study of the magnetostratigraphic we can know that the Late Cenozoic eolian red clay sequences of ZL covering an age range from 25.6Ma to 4.8Ma also include the warm climate change period of the Middle Miocene Climate Optimum,which is rich archive of paleoclimatic changes.We choose red clay experimental samples of susceptibility with a spacing of 20cm,then according to the susceptibility variations choose typical samples to do other rock magnetic experiment.The MMCO representative samples were collected from 300~420m,covering an age range of 17.5~13.5Ma.To make a comparison of the characteristic of rock magnetism about the MMCO representative samples,we also collected representative samples from the Preceding-MMCO(samples collected from 420~450m,covering an age range of 18.0~17.5Ma)and Post-MMCO(samples collected from 250~300m,covering an age range of 13.5~12.0Ma).In this study,a multi-parameter rock magnetic investigation such as the magnetic susceptibility(X),temperature-dependent magnetic susceptibility curve(X-T curves),X-T curves of the representative samples heated to different temperatures,IRM acquisition and reverse field demagnetization of IRM2.7T(IRM acquires at an applied field of 2.7T)and magnetic hysteresis loops were carried on samples collected from the Zhuanglang red clay drilling.The result of the characters of X-T curves,X-T curves of different temperatures,IRM acquisition,hysteresis loops indicate the major magnetic material and magnetic carrier of the characteristic remanent magnetization are magnetite,hematite and maghemite.Zhuanglang red clay is characterized by significantly high concentration of magnetite and maghemite during the Middle Miocene Climate Optimum,but considerably lower concentration before and after the MMCO.To the contrary,the content of hematite is lower in the MMCO than the period before and after MMCO.Combined with Day-plot we can know the average grain size of the samples is pseudo single domain(PSD).According to magnetic parameters such as Xfd,SIRM,S-ratio curves varitations along with depth,we can conclude that the enhancement of magnetic susceptibility during 16.5~13.8Ma is probably dependent on the ferriomagnetic mineral concentration,the grain size of which are the superparamagnetic(SP),single domain(SD),pseudo single domain(PSD).This obviously high concentration of magnetite and maghemite during the MMCO may indicate an enhancement of the Asian summer monsoon and pedogenesis in the Chinese Loess Plateau.Through other climate parameters from the continent and ocean we suppose that the summer monsoon and the winter monsoon in East Asia are all strengthened during the Middle Miocene Climate Optimum.