Magnetism of Chinese loess deposits

Abstract

The age of Chinese loess deposits has long been disputed. Biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the Pleistocene and ascertained a maximum loess age of about 1.2 Myr. A new collection of nearly 500 samples from a natural outcrop and a borehole section near Lochuan (lat. 35.8 O N , long. 109.2"E; Shaanxi province) has been dated by magnetic stratigraphy. Thermal cleaning of the natural remanent magnetization (NRM) removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite. The characteristic NRM component is due to haematite which is thought to be of chemical origin. Rhythmical intensity variations of NRM and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition.The palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation. The Brunhes-Matuyama boundary and the Jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level. The Olduvai subchron has been found in the borehole section which records the entire loess sequence. Most probably the formation of Chinese loess began shortly after the Matuyama-Gauss polarity transition. Therefore a late Pliocene age of about 2.4 Myr is assigned to the oldest loess sediments measured.

Effect of burning C3 and C4 plants on the magnetic susceptibility signal in soils

“…Magnetic enhancement of paleosols compared with intervening loess has been widely considered as a proxy for palcoclimate changes in the Loess Plateau. Several possible mechanisms for the magnetic enhancement of the palcosols have been suggested [Heller and Liu, 1984 The average MS value of C4 plant ashes after natural burning is about (298+132)x10-am3kg -•, much higher than the corresponding average MS value of (66+35) x 10-8m3kg -• of C3 plant ashes (Fig. 1).…”

Lack of correlation between paleoprecipitation and magnetic susceptibility of Chinese Loess/Paleosol Sequences

“…Kukla et al [1988] proposed that the X variation in loess-paleosol sequences is due to depositional dilution of a constant rainout of ultra-fine magnetic particles from the troposphere during glacial and interglacial periods. Other interpretations include enrichment of magnetic minerals in paleosols due to decalcification and soil compaction [Heller and Liu, 1984], and the heating of loess by frequent natural fires [Kletetschka and Banerjee, 1995]. The prevailing hypothesis for enhancement of X in paleosols is post-depositional weathering and/or formation of magnetic phases by pedogenic processes [Zhou et al, 1990 proposed that X enhancement in paleosols has multiple origins.…”

How does Chinese loess become magnetized?

“…Since Heller and Liu (1984) confirmed the presence of hematite in Chinese loess by thermal demagnetization and microscope observations, the existence of hematite and goethite has been demonstrated by thermomagnetic techniques (Heller et al, 1991;Liu et al, 2004c), X-ray diffraction (Zhu et al, 1994a;Deng et al, 2000;Chen et al, 2005;Torrent et al, 2006), and Mӧssbauer spectroscopy (Vandenberghe et al, 1992). The abundance of hematite in Chinese loess has been estimated and considered to be similar for loess and paleosol samples (Maher and Thompson, 1992;Evans and Heller, 1994).…”