Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China
Mineral magnetism to probe into the nature of palaeomagnetic signals of subtropical red soil sequences in southern China
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DOI：
10.1111/j.1365-246x.2010.04592.x
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发表时间：
2010-06-01
影响因子：
2.8
通讯作者：
Yuan, Baoyin
中科院分区：
文献类型：
--
作者：
Liu, Caicai;Deng, Chenglong;Yuan, Baoyin
磁极性地层学在世界范围内的海相和陆相沉积层序定年中已被证明是有用的。然而，中国南方亚热带红壤序列磁性地层学结果的可靠性在不同地区存在差异。为探讨红壤记录磁极性地层的能力，选取了3个红壤层序进行了详细的矿物磁性、岩相学和古地磁分析。其中包括位于长江下游的宣城层序和七里亭层序，以及北回归线附近百色盆地的大美层序。古地磁结果表明，宣城序列记录了Brunhes时和Matuyama晚期时，其中包括一个可能代表圣罗莎地磁事件的短时间间隔。宣城序列和七里亭序列忠实地记录了古地磁场的活动特征，而大美序列则不具有古地磁场活动特征。详细的矿物磁性和岩相学测量表明，四种磁性矿物（磁铁矿，磁赤铁矿，赤铁矿和针铁矿）包含在三个研究序列，但特征的Rexonitum磁化（ChRM）载体是不同的。宣城和七里亭样品的ChRM载体为磁铁矿和赤铁矿，均为碎屑成因。然而，大梅样品的ChRM是由成壤赤铁矿携带的，其解除封闭温度（T-B）约为630-640 ℃，这已经覆盖了大梅红壤沉积物的原生剩磁。这种成土赤铁矿主要是由化学风化作用产生的，而化学风化作用通常会因高温和降雨气候而加剧。气候条件对红壤层序古地磁信号的性质有很大影响。此外，红壤中的红色素组分可用来指示重磁化的程度。
P>Magnetic polarity stratigraphy has proved to be useful in dating of both marine and terrestrial sedimentary sequences over the world. However, the reliability of magnetostratigraphic results of red soil sequences in subtropical southern China has been found to be variable in different regions. To probe into the capabilities of recording magnetic polarity stratigraphy in the red soils, three red soil sequences have been selected for detailed mineral magnetic, petrographic and/or palaeomagnetic analyses. These include the Xuancheng and Qiliting sequences, which are located in the lower reaches of the Yangtze River and the Damei sequence in the Bose Basin near the Tropic of Cancer. Palaeomagnetic results indicate that the Xuancheng sequence has recorded the Brunhes Chron and the late Matuyama Chron, including a short interval probably representing the Santa Rosa geomagnetic event. The Xuancheng and Qiliting sequences have faithfully recorded the palaeogeomagnetic field behaviour while the Damei sequence has failed. Detailed mineral magnetic and petrographic measurements suggest that four magnetic minerals (magnetite, maghemite, haematite and goethite) are contained in the three studied sequences, but the characteristic remanent magnetization (ChRM) carriers are different. For the Xuancheng and Qiliting samples, the ChRM carriers are magnetite and haematite, which are all of detrital origin. However, the ChRMs of Damei samples are carried by pedogenic haematite with an unblocking temperature (T-B) of about 630-640 degrees C, which has overprinted the primary remanence of the Damei red soil deposits. This pedogenic haematite is mainly produced by chemical weathering, which is commonly intensified by the climate of high temperature and rainfall. It further suggests that climatic conditions have a great effect on the nature of palaeomagnetic signals of the red soil sequences. In addition, the red pigment components in the red soils may be used to indicate the degree of remagnetization.