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Dec 24, 2004 - pseudosingle-domain (PSD, $100 nm to several microns) particles become significant;. (2) pedogenic particles have a narrow grain size ...
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B12107, doi:10.1029/2004JB003249, 2004

Mechanism of the magnetic susceptibility enhancements of the Chinese loess Qingsong Liu,1,2,3 Michael J. Jackson,1 Subir K. Banerjee,1 Barbara A. Maher,4 Chenglong Deng,2 Yongxin Pan,2 and Rixiang Zhu2 Received 21 June 2004; revised 14 October 2004; accepted 20 October 2004; published 24 December 2004.

[1] Chinese loess/paleosol sequences have been regarded as excellent continental

archives for encoding continuous paleoclimatic variations over the past 2.5 Myr. However, the mechanism for magnetic enhancements (especially the low-field mass-specific magnetic susceptibility, c) of Chinese paleosols is still not completely resolved. This study quantifies contributions of aeolian and pedogenic magnetic particles to the bulk magnetic properties of the Chinese loess/paleosols by using a magnetic extraction technique. Magnetic properties of magnetic separates (extractable) and the corresponding residues (nonextractable) for five characteristic samples covering both loesses and paleosols were comprehensively investigated by hysteresis loops, frequency and low-temperature dependence of magnetic susceptibility, and interparametric ratios. Results show that (1) with moderate degrees of pedogenesis (c < 10  107 m3 kg1), c is enhanced more by increased concentration of stable single domain (SD) magnetic particles than by viscous superparamagnetic (SP) particles. For more mature paleosols with c > (10–12)  107 m3 kg1, contributions of pedogenically related fine-grained pseudosingle-domain (PSD, 100 nm to several microns) particles become significant; (2) pedogenic particles have a narrow grain size distribution concentrated above the SP/SD threshold; and (3) anhysteretic remanent magnetization (ARM) is carried dominantly by SD grains. Moreover, we propose that only the nonextractable fraction of c, saturation magnetization (Ms) and remanent magnetization (Mrs) show a strong relationship with the degree of pedogenesis. This new interpretation of magnetic enhancements helps us to retrieve more accurate and quantitative paleoclimatic signals INDEX TERMS: 1512 Geomagnetism recorded by the Chinese loess/paleosol sequences. and Paleomagnetism: Environmental magnetism; 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; KEYWORDS: Chinese loess, magnetic susceptibility, pedogenesis Citation: Liu, Q., M. J. Jackson, S. K. Banerjee, B. A. Maher, C. Deng, Y. Pan, and R. Zhu (2004), Mechanism of the magnetic susceptibility enhancements of the Chinese loess, J. Geophys. Res., 109, B12107, doi:10.1029/2004JB003249.

1. Introduction [2] The past twenty years have witnessed rapid progress of paleoclimatic studies on the Chinese loess/paleosol sequences using low-field magnetic susceptibility [e.g., Heller and Liu, 1982, 1984, 1986; Kukla et al., 1988]. However, the mechanism of susceptibility enhancement is still a subject of debate. 1 Institute for Rock Magnetism and Department of Geology and Geophysics, University of Minnesota-Twin Cities, Minneapolis, Minnesota, USA. 2 Paleomagnetism Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. 3 Now at Department of Earth Sciences, University of California, Santa Cruz, California, USA. 4 Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, Geography Department, Lancaster University, Lancaster, UK.

Copyright 2004 by the American Geophysical Union. 0148-0227/04/2004JB003249$09.00

[3] Kukla et al. [1988] and Porter et al. [2001] proposed that the susceptibility variations result from the depositional dilution of a constant flux of tropospheric ultrafine magnetic particles during glacial periods. Zhou et al. [1990] showed that the enhanced susceptibility of paleosols is strongly linked to the ultrafine-grained superparamagnetic (SP) particles produced through pedogenesis. Therefore the concentration of these SP and single domain (SD) grains has been proposed to be a sensitive proxy of paleorainfall [Maher et al., 1994]. However, Sun and Liu [2000] and Guo et al. [2001] found that the magnetic susceptibility is not always positively correlated with the degree of pedogenesis. It seems there may be more factors controlling the susceptibility enhancement. [4] To settle these arguments, we need to quantify the contribution of pedogenic fine-grained (SP + SD) particles to the bulk susceptibility. On the basis of hysteresis modeling, Maher and Thompson [1992] concluded that SP grains account for more than 90% of the susceptibility differences between loess and paleosols. Later, Banerjee

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and Hunt [1993] used low-temperature techniques to separate SP contributions from SD and MD contributions. They confirmed that the contribution of saturation magnetization at 15 K, carried by SP grains, to the total magnetization for paleosols (75%) is much higher than that for loess (20%). More recent mathematical ‘‘unmixing’’ techniques involve the separation of coercivity fractions from IRM acquisition curves or alternating field (AF) demagnetization curves [e.g., Robertson and France, 1994; Stockhausen, 1998; Kruiver et al., 2001; Heslop et al., 2002; Egli, 2003]. Chemical separation techniques have also been applied. Verosub et al. [1993] and Fine et al. [1993] found that citrate-bicarbonate-dithionite (CBD) extraction efficiently dissolves small ferrimagnetic grains (magnetite/maghemite). The CBD-soluble magnetic phases carry more than 80% of the bulk susceptibility of paleosols [Verosub et al., 1993; Deng et al., 2004]. More recently, van Oorschot et al. [2002] reported on the acid-ammonium-oxalate/ferrous-iron (AAO-Fe2+) extraction technique as a new approach to distinguish very fine grained magnetite and maghemite from coarse-grained magnetite. The third group of separation methods used is to examine contributions of different grain size fractions of loess/paleosols to bulk susceptibilities. The separation of different grain size fractions generally involves both gravitational settling and centrifuge techniques (e.g., detailed procedures were described by Han and Jiang [1999]). Han and Jiang [1999] suggested that the contribution of fine-grained particles (