Carbonate-Free Hard Isothermal Remanent Magnetization Record Of The Pliocene Red Clay Sequence On The Chinese Loess Plateau

Chinese Loess Plateau?CLP?is composed of hundreds-meters-thick windblown sedimentary sequences,which is famous for its continuous sedimentation,long chronological sequences and high resolution.Such a whole deposition sequences can completely document the evolution history of Asian paleomonsoon,the uplift history of the Tibet Plateau,and the process of aridification and desertification in Asian interior.The research on paleoclimate of Quaternary loess-paleosol has been quite mature,and there is no dispute about the paleoclimatic evolution information during this period by environmental magnetic parameters,but the research on tertiary red clay overlying quaternary sedimentary sequences is relatively weak.The hard isothermal remanence magnetization?HIRM?indicates the content of weakly magnetic minerals?hematite/goethite?,which is different from magnetic susceptibility(?_lf)indicating the content of strongly magnetic minerals?magnetite/maghemite?.What's more,HIRM records orbital period?eccentricity?signal and contains abundant paleoclimatic evolution information of tertiary red clay,which deserves to be further explored and studied.The zhaojiachuan?ZJC?red clay sequence of CLP was selected as the research profile in this paper,and a series of environmental magnetic parameters(?_lf,?_fd,HIRM and S-ratio)and the non-magnetism method?carbonate content measuring,size sorting,diffuse reflection spectrum?DRS?,X-ray diffraction?XRD?,and soil micromorphology?were measured systematically.A comprehensive analysis of the magnetic properties of red clay were conducted to mainly make further clear the indication of HIRM in red clay,quantity the proportion of hematite content reflected by HIRM and identify the source of hematite in red clay.The systematic investigation and study of the ZJC red clay sequence obtain the following three understandings:1.The carbonate content of red clay sequence is 3.15%-45.97%??=22%,?=6.74%?,showing the high discrete degree.Therefore,it's not allowed to ignore the effect of carbonate leaching on the fluctuation of environmental parameters.It's significant to use the high carbonate content with high amplitude change to correct environmental parameters to reveal their accurate indication,making HIRM truly recover the hematite variation in red clay.2.The comparison between the hematite content obtained by HIRM and the DRS shows that the hematite content indicated by HIRM accounts for 62-95%of the total hematite contained in the red clay.HIRM predominately reflects the hematite content of red clay sequences.3.The HIRM values of red clay sequence on different grain size fractions?<2?m?2-8?m?8-31?m and 31-63?m?are very close,the maximum is on the coarsest fraction?31-63?m?,and the fluctuation range is very limited.While the HIRM values of Quaternary loess increases as the particle size becomes finer,and the maximum value appeared in pedogenic grain size fraction?<2?m?.The HIRM of red clay shows significant differences from other magnetic parameters indicating pedogenesis,which differs from the in-phase change patterns in Quaternary loess.To sum up,we suggest that the hematite reflected by HIRM in Pliocene Red Clay sequences was wind-blown and derived from the source area.