甘肃环县剖面记录的末次间冰期地磁场

A DETAILED PALAEOMAGNETIC RECORD FOR THE LAST INTERGLACIAL PERIOD FROM HUANXIAN LOESS SECTION

环县剖面的古地磁研究结果表明，在第一层古土壤Ｓ１的上部和下部分别存在两个地磁场异常。通过剖面磁化率曲线与深海沉积物氧同位素曲线的对比，建立了地磁场变化的时间序列。Ｓ１上部记录的地磁场异常，年龄与挪威－格陵兰海及大西洋中发现的地磁异常年龄相当。Ｓ１下部的地磁异常，以磁偏角方向强烈摆动，偶极磁场强度明显下降为特征，其年龄与报道的布莱克事件的年龄相当，也与索马里海和地中海所记录的大幅度偶极磁场减弱相吻合，但与西宁剖面的记录不同的是，环县剖面记录的地磁场从没有达到完全的倒转，ＶＧＰ曲线表现为围绕采样点旋转的经过南半球低纬地区的大环。这些特征表明，在偶极磁场减弱期，具有强烈区域性的非偶极磁场的变化可能对地磁场起着重要的作用。

Palaeomagnetic studies of marine and lake sediments have established the existence of geomagnetic cpisodes which can be recogmosed over large geographic regions. One of the best documented geomagnetic episodes in the Brunhes normal polarity epoch is the so-called Blake Event at about 0.1 Ma B.P. Anomalous geomagnetic behaviour at this time has been recognised from different locations and lithologies although individual records have indicated durations ranging from 4000 yrs to 50000 yrs.The loess deposits of central China contains a send-continuous terrestrial record of global climatic and geomagnetic changes during the Quaternary. The typical loess-palaeosol stratigraphy is well established and by correlating the loess-palaeosol alternations with the oxygen isotopic stratigraphy of the deep sea sediments,it is possible to use the time scale to assign the boundary age of each loess and soil unit. In this paper we present a continuous palaeomagnctic record from a loess section near Huanxian, Gansu Province.The Huanxian section lies in a region of arid to semi-arid climate with relative ly low mean annual temperature and precipitation. Pedogensis is weak but still pro- duces visible structure in the field such that the loess-soil stratigraphy can be correlated with sections all over the Loess Plateau. The section consists of an interglacial soil complex S1 at the bottom, Malan bass and Holocene soil on the top. In the field a total of 125 oriented overlapping samples with an average size of 10cm×10 cm× 10cm,were collected from the 25 m-thick profile. In the laboratory each block was cut into two parallel sets of cubic specimens, one for palaeomagnetic study and the other for relative palaeointensity estimate. At present mineral magnetic and palaeomagnetic analysis is restricted to the lower 4.6 m of the section which covers the Last Interglacial time period, here represented by soil complex S1 Studies of the magnetic mineralogy of the Chinese loess deposits revealed that magnetite and maghaemite dominate the remanence of loess and palaeosols with a lesser contribution from heamitite. In the current study a number of rock magnetic techniques have been used to investigate the magnetic mineralogy.In this study, the stratigraphic variation of low-frequency magnetic susceptibility X; the frequency dependence of magnetic susceptibility Xfd(%), anhysteretic remanent magnetisation ARM and the ratio of anhysteretic susceptibility to magnetic susceptibility have been measured. Rock magnetic study has shown that the profile exhibits fairly consistency in terms of magnetic mineralogy.All specimens were subjected to stepwise thermal demagnetisation up to 550 ℃ at an interval of 50 ℃ NRM' s and remanent magnetisation were measured using a cryogeuic magnetometer. All samples exhibited secondary magnetisations which were re-moved at demagnetisation temperatures between 150℃ to 300℃.For the relative intensity estimate the second sample of each stratigraphic pair was given an ARM in a peak alternating field of 170mT. This sample was then thermally demagnetised at 300℃. The ratio of NRM at 300℃ to ARM is though to represent the relative intensity.To analyse the palaeomagnetic record of the Huanxian section we have followed convention and converted palaeomagnetic directions into equivalent VGP paths. The Huanxian palaeomagnetic record shows considerable instability in terms of directions and relative intensities.A comparison of this new record with existing records of the Blake Event from the USA and the Mediterranean reveals a number of discrepancies in field behaviour,although all records are consistent in that at no time during the event is full reversed polarity obtained. In contrast to a published record of the Blake Event from the loess at Xining, which shows three short episedes of almost full reversed polarity, in the current record the Blake Event is represented by a loop in the VGP path which passes through shallow southern latitudes. We believe that the Blake Event is a geomagnetic excursion resulting from the increased