Earth Planets Space, Vol. 59 (No. 7), pp. 825-840, 2007
Tianshui Yang1,2, Masayuki Hyodo2, Zhenyu Yang3, Lin Ding1, Jianli Fu4, and Toshiaki Mishima5
1Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China
2Research Center for Inland Seas, Kobe University, Kobe 657-8501, Japan
3Department of Earth Sciences, Nanjing University, Nanjing 210093, China
4Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
5Center for Advanced Marine Core Research, Kochi University, Kochi 783-8502 Japan
(Received September 27, 2006; Revised January 11, 2007; Accepted January 15, 2007; Online published July 20, 2007)
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 short-lived 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.
Key words: Geomagnetic excursion, paleomagnetism, magnetostratigraphy, loess, Matuyama Chron, Olduvai, Punaruu, Stage 54, Réunion, cryptochron C2r.2r-1.