Partly pedogenic origin of magnetic variations in Chinese loess

QUATERNARY sequences of interbedded aeolian silts (loess) and buried soils (palaeosols) potentially provide one of the best terrestrial records of past climates1–5. Magnetic susceptibility variations in loess and palaeosol sequences from China are strongly correlated with climate-induced fluctuations of oxygen isotope ratios in deep-sea sediments2–4,6–8. As an explanation of this correlation, it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non-magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6–8. It has also been suggested that such a model, involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles, provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7. Here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size. Our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units. This suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non-magnetic loess is inadequate. Our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols.

[1]  G. Kukla,et al.  Pleistocene Climates in Central Europe: At least 17 Interglacials after the Olduvai Event , 1977, Quaternary Research.

[2]  Joseph L. Kirschvink,et al.  Magnetite biomineralization and magnetoreception in organisms , 1985 .

[3]  D. Dunlop.  The rock magnetism of fine particles , 1981 .

[4]  Subir K. Banerjee,et al.  Ferrimagnetic Properties of Magnetite , 1985 .

[5]  B. Maher.  Characterisation of soils by mineral magnetic measurements , 1986 .

[6]  K. Verosub,et al.  Role of pedogenesis in distribution of magnetic susceptibility in two California chronosequences , 1989 .

[7]  K. Pye.  Aeolian dust and dust deposits , 1987 .

[8]  Peter Self,et al.  Magnetite in soils: I. The synthesis of single-domain and superparamagnetic magnetite , 1986, Clay minerals.

[9]  Zhang Shuguang,et al.  Soils of the loess region in China , 1983 .

[10]  U. Schwertmann.  The effect of pedogenic environments on iron oxide minerals , 1958 .

[11]  B. Maher,et al.  Formation of ultrafine-grained magnetite in soils , 1988, Nature.

[12]  K. Pye,et al.  Late Pleistocene and Holocene aeolian dust deposition in North China and the Northwest Pacific Ocean , 1989 .

[13]  Subir K. Banerjee,et al.  A preliminary magnetic study of soil samples from west-central Minnesota , 1982 .

[14]  G. Kukla.  Loess stratigraphy in central China , 1987 .

[15]  F. Oldfield,et al.  A Multivariate Mixing Model for Identifying Sediment Source from Magnetic Measurements , 1989, Quaternary Research.

[16]  J. Prospero,et al.  Magnetic differentiation of atmospheric dusts , 1985, Nature.

[17]  J. Beget,et al.  Influence of orbital parameters on Pleistocene loess deposition in central Alaska , 1989, Nature.

[18]  F. Heller,et al.  Palaeoclimatic and sedimentary history from magnetic susceptibility of loess in China , 1986 .

[19]  Hojatollah Vali,et al.  Occurrence of magnetic bacteria in soil , 1990, Nature.

[20]  J. King,et al.  A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments , 1982 .

[21]  M. S. Tite,et al.  ENHANCEMENT OF THE MAGNETIC SUSCEPTIBILITY OF SOILS ON ARCHAEOLOGICAL SITES , 1971 .

[22]  M. S. Tite,et al.  Magnetic viscosity, quadrature susceptibility, and frequency dependence of susceptibility in single‐domain assemblies of magnetite and maghemite , 1973 .

[23]  G. Kukla,et al.  Pleistocene climates in China dated by magnetic susceptibility , 1988 .

[24]  Dongsheng Liu,et al.  Loess and the environment , 1985 .

[25]  G. Kukla.  Loess stratigraphy in central China , 1987 .

[26]  L. Parry.  Magnetic properties of dispersed magnetite powders , 1965 .

[27]  David B. Stone,et al.  Paleoclimatic forcing of magnetic susceptibility variations in Alaskan loess during the late Quaternary , 1990 .

[28]  B. Maher.  Magnetic properties of some synthetic sub-micron magnetites , 1988 .

[29]  F. Heller,et al.  Magnetism of Chinese loess deposits , 1984 .

[30]  G. Turner,et al.  Environmental Applications of Magnetic Measurements , 1980, Science.

[31]  F. Oldfield,et al.  Particle-size related, mineral magnetic source sediment linkages in the Rhode River catchment, Maryland, USA , 1985, Journal of the Geological Society.