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New Evidence for the Origin of Ferrimagnetic Minerals in Loess from China

Inst. of Soils and Water, ARO, The Volcani Center, Bet Dagan 50250, Israel

Michael J. Singer and Jeff TenPas

Dep. of Land, Air and Water Resources

Kenneth L. Verosub

Dep. of Geology, Univ. of California, Davis, CA 95616

*Corresponding author.

ABSTRACT

The sequence of loess and interbedded paleosols of north-central China has accumulated for the last 2.4 million yr. This offers a unique location to study pedogenesis. The paleosol units are easily differentiated from zones of unaltered parent material by their higher low-field mass magnetic susceptibility (MS). We have studied the MS of 69 loess and paleosol samples to determine if MS could be used to understand more about pedogenesis in the loess column. The MS ranged from 11 x 10^–8 m³ kg^–1 in an unaltered loess to 324 x 10^–8 m³ kg^–1 in a paleosol. We present magnetic and chemical data that support the in situ formation of ferrimagnetic material during periods of pedogenesis. An inherited ferrimagnetic component, which is predominantly silt-sized magnetite, is evenly distributed throughout the loess column with an average MS of 17 ± 4 x 10^–8 m³ kg^–1. This component is resistant to removal by citrate-bicarbonate-dithionite (CBD) extraction. A second ferrimagnetic component, with is CBD soluble, occurs predominantly in the clay (<2-µm) fraction. We assume this material is a newly formed maghemite that has accumulated in the paleosols as a result of pedogenesis. This CBD-soluble ferrimagnetic phase is enriched in the paleosols, as shown by the linear (P < 0.001) increase of the ratio of MS to Fe oxides and oxyhydroxides. The ferrimagnetic content, ferrimagnetic minerals, and particle-size distribution of the ferrimagnetic phase in the paleosols are all related to the extent of pedogenesis in the loess column.

Work supported by National Science Foundation Grant EAR 9117790. Contribution from the ARO, The Volcani Center, Bet Dagan, Israel, no. 1065-E, 1993 series.

Received for publication July 1, 1992.