Abstract
Abstract A detailed paleomagnetic and rock-magnetic investigation on the early and middle Matuyama loess-paleosol sediments has been carried out at the Baoji section, Shaanxi province, southern Chinese Loess Plateau. Our new magnetostatigraphy revises the position of the lower Olduvai boundary from L27 to S26. Seven shortlived geomagnetic excursions, tentatively named as E1, E2, E3, E4, E5, E6, and E7, have been recognized in the L13, S22, L26, L27, S29, and upper and middle parts of L32, respectively. Results of the anisotropy of low-field magnetic susceptibility (AMS) show that the studied loess-paleosol sediments retain the primary sedimentary fabric. Rock magnetic experiments reveal that the sediments from the excursional and polarity transitional intervals have the same magnetic characteristics as those from the surrounding normal and reversed polarity intervals. Assuming a constant accumulation rate between polarity boundaries, these seven excursions are estimated to be at about 1.11 Ma (E1), 1.58 Ma (E2), 1.92 Ma (E3), 2.11 Ma (E4), 2.25 Ma (E5), 2.35 Ma (E6), and 2.42 Ma (E7) Ma. The E1 and E2 in the middle Matuyama Chron can be correlated with the Punaruu and Stage 54 (Gilsa) excursions, respectively. The E4, E5, and E7 in the early Matuyama Chron can be correlated with the Réunion II, Réunion I, and cryptochron C2r.2r-1 (X-subchron), respectively. The E3 in the lower Olduvai subchron and E6 in the early Matuyama Chron have no comparable events. At present they can only be correlated with the anomalous directions observed in the Osaka Bay core (Biswas et al., 1999). Therefore, further investigations are necessary to support their global occurrences. The present result together with the two late Matuyama excursions dated at about 0.89 Ma and 0.92 Ma (Yang et al., 2004) show that the Baoji section yields at least nine Matuyama excursions which, along with the results of the study, suggests that eight excursions occur at 0.9–2.2 Ma (Channell et al., 2002), thereby providing evidence that the short-lived geomagnetic excursions may also be a basic characteristic of the geomagnetic field during the Matuyama Chron.
Highlights
After removing a Low-temperature components (LTC), a high-temperature magnetization component (HTC) that decayed toward the origin was isolated between 250◦C and 620–650◦C (Fig. 3)
It is noted that even such specimens with anomalous paleomagnetic directions and demagnetization behaviors have a welldefined LTC that is quite consistent with the present geomagnetic field direction (Fig. 5). 4.3 Rock-magnetic analyses
A high-resolution early and middle Matuyama paleomagnetic record with detailed rock-magnetic investigation was obtained from an aeolian loess-paleosol sequence near Baoji city, the southern Chinese Loess Plateau (CLP)
Summary
The geomagnetic excursion is an important systematic and distinct component of the Earth’s magnetic field variability between field reversals, showing a larger swing of the geomagnetic field beyond the range of secular variation, i.e., the virtual geomagnetic pole (VGP) latitude deviates more than 45◦ from the normal position (e.g., Merrill and McFadden, 1994; McElhinny and McFadden, 1997).Recent paleomagnetic studies have recognized numerous geomagnetic excursions that occurred in the Brunhes Chron (e.g., Langereis et al, 1997; Lund et al, 2001, 2006; Singer et al, 2002; Oda, 2005); in contrast, the number of papers reporting on the Matuyama Chron is much smaller, and more research is clearly necessary.Loess-paleosol sequences in the Chinese Loess Plateau (CLP) consist of aeolian dust originating from the dry Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.deserts of the north and northwest (Liu, 1985). Loesses are mainly deposited under the cold and dry conditions of the East Asian winter monsoon system (glacial), and paleosols are formed under the warm and moist conditions of the summer monsoon (interglacial). Several researchers have continued investigations on the underlying red clay (Sun et al, 1998; Ding et al, 1998, 2001; Qiang et al, 2001; Guo et al, 2002), and the initiation of aeolian dust deposition has been extended back to more than 22 Ma, marking the onset of the present-day East Asian monsoon system (Guo et al, 2002)
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