The Environmental Magnetism Study Of Pliocene Red Clay

There is a vast of aeolian deposits in the north of China. The thickness of deposits in the middle of the Yellow River is about several hundreds of meters formed by splendid Chinese Loess Plateau (CLP). The Quaternary deposits consist of alternating loess and paleosol units. Underlying the Quaternary loess/paleosol sequences is the Hipparion Red-Earth Formation, named after the Hipparion fossils from there. It was also called the Red Clay afterwards. The Red Clay consists of many paleosol complexes. Recent evidence suggests that the Pliocene Red Clay is of aeolian origin. The CLP adjoin central Asia and the Tibet Plateau, at the same time has been controlled by Global important atmospheric circulation systems. During the summer season, the CLP is controlled by the East Asian summer monsoon, which brings a lot of precipitation. While during the winter season, it is controlled by the East Asian winter monsoon (Siberian High), which takes the aeolian from the central Asia to the CLP. Generally, the CLP is also influenced by the westerlies in the whole year. Moreover, the uplifting of Tibet Plateau exerted a great impact on CLP. The loess-Red Clay sequence combining with the continent, Ocean and atmosphere circulation systems, they systematically contain abundant information of paleoclimatic evolution.Variations of magnetic susceptibility can be used to trace the summer paleomonsoon intensity, and the variation of medium diameter can be used to trace the winter paleomonsoon in the Quaternary loess/paleosol sequence in the CLP. These two proxies are comparable to marine oxygen isotope, so Chinese loess provides one of the best geologic records for paleoclimatic research, along with ice cores and marine oxygen isotopes. Magnetic properties and paleoclimatic significance of the Quaternary loess have been greatly studied in the past decades, the Pliocene Red Clay has not been studied intensively until now, so it needs to be studied immediately. In this study, we focus on the Pliocene Red Clay, in order to understand the development of Asian aridification and cooling, the uplift of Tibet Plateau, the development of Arctic icecap and the evolution of Asian paleomonsoon and so on.We have conducted systematic magnetic analysis on Jiaxian Red Clay, combined with un-magnetic methods (geochemical analysis, magnetic extraction and XRD analysis). We tried to discuss the magnetic minerals, concentration and grain size distribution of Jiaxian Red Clay, the susceptibility enhancement mechanism of Red Clay, and tried to make sure whether the variation of magnetic susceptibility can be used as a proxy for summer paleomonsoon intensity, why the strongly developed Red Clay gets low susceptibility and also analyzed the paleoclimatic evolution lastly.Through the analysis of magnetic property of Jiaxian Red Clay, we can get the following conclusions:(1) Ferrimagnetic minerals (magnetite and maghemite) dominate the Jiaxian Red Clay, antiferromagnetic minerals (hematite and goethite, mainly hematite) are also present. It is similar with Quaternary loess/paleosol sequence. There are a lot of SP and SD ferrimagnetic grains in the Red Clay sequence. King plot analysis shows that magnetic grain size is nearly finer than 0.2?m. The strongly developed paleosols contain more ferrimagnetic minerals, with thinner magnetic grain sizes. The weakly developed paleosol and loess layers contain more antiferromagnetic minerals, with coarser magnetic grain sizes.(2) The generated ultrafine ferrimagnetic grains (SP and SD) during pedogenesis are responsible for an increase in magnetic susceptibility, and therefore the susceptibility enhancement mechanism of Red Clay is similar to that of the overlying loess/paleosol sequences.(3) The magnetic minerals of strongly developed paleosol are the same as the weakly developed paleosol in the Red Clay sequence, and the magnetic minerals of loess are the same as the paleosol in the Quaternary loess/paleosol sequence. The magnetic minerals of loess are different from the paleosol in the high latitude loess/paleosol sequence (Alaska and Siberia loess), the loess contains more ferrimagnetic minerals and the paleosol has more iron hydroxide. At the same time, the susceptibility enhancement mechanism of Red Clay is similar to that of the overlying loess/paleosol sequences. These phenomenon illustrate that the pedogenic environment of Red Clay is similar with Quaternary loess/paleosol sequence in the CLP, but it obvious different with the one of high latitude area.(4) The magnetic susceptibility variations of six south-north transect sections in the CLP have similar trend, the value increases from northwest to southeast. They indicate that magnetic susceptibility of Red Clay can be used to trace the variation of paleomonsoon intensity at a spatial scale. It was found that the strongly developed Red Clay unit did not necessarily lead to high susceptibility, based on the comparison whether of Tertiary and Quaternary sequences at Lingtai section or of Tertiary Red Clay units at Jiaxian section. These illustrate that the magnetic susceptibility of Red Clay cannot be used to trace the variation at temporal scale. The reason is that there are some differences between them, so the susceptibility cannot be used to reflect paleomonsoon intensity in the same quantitative way between Quaternary loess/paleosol sequence and late Tertiary Red Clay sequence. We should consider all different factors to produce an adjusted calibration function of Red Clay sequence, thus magnetic susceptibility can be used to properly trace paleomonsoon intensity not only from spatial but also from the temporal scale.(5) The value of magnetic susceptibility is controlled by the variation of ferrimagnetic minerals. We found that the content of ferrimagnetic mineral decrease from the top to bottom of Jiaxian Red Clay sequence through systematic magnetic analysis, which is the main reason for the strongly developed Red Clay with lower magnetic susceptibility. The pedogenic intensity of Red Clay is stronger than that of Quaternary paleosols, and also the Red Clay undergo a long-term iron oxidation stage, so unstable maghemite can be transformed into stable hematite. The magnetic susceptibility decreased with diagenesis. All these factors probably can reduce the content of ferrimagnetic minerals in Red Clay sequence.(6) The magnetic "unmixing" technique of stepwise AF demagnetization of SIRM (SIRMxmT) is applied for the first time to isolate the magnetic components of Red Clay sequence. We get that the ratios of SIRM100mT/SIRM, SIRM100mT/SIRM60mT and SIRM100mT/SIRM30mT have a long-term up-section decease during 5-2.6 Ma B.P., which has similar trend to the long-term up-section increase in ?18O values of the marine oxygen isotope records. The ratios suggest that a long-term increasing aridification and cooling trend and decreasing paleomonsoon intensity mainly in the CLP during the Pliocene.