The frequency dependence of low field susceptibility in loess sediments

Abstract

S U M M A R YThe low field susceptibility variations in the Chinese and Tajik loess sequences reflect the palaeoclimatic fluctuations of the Quaternary on the continents in a very direct and complete way. It can be demonstrated that the fine-grained ferromagnetic mineral fraction is largely responsible for the climatically controlled susceptibility enhancement in the loess sediments. In order to gain more information about the origin and quality of the susceptibility enhancement we are scrutinizing the F-factor (F,) which is conventionally used as a measure of the frequency dependence of the susceptibility. For qualitative estimates of the grain sizes present, F, seems to be a rather misleading parameter because it is flawed when variable amounts of minerals with both frequency-independent and frequency-dependent susceptibility are present. We introduce a new concentration-independent F-factor (K) which takes the influence of the mineral fractions with frequency-independent susceptibility on the total susceptibility into consideration. F, is fairly constant in all the investigated sections. This result is explained theoretically following NCel's (1949) fine-particle theory and Stephenson's (1971a) modelling of various single-domain grain distributions. It is found that the grain-size distribution is essentially the same throughout all loess sections investigated and the upper and Iower grain-size limits are essentially invariable. In addition, susceptibility and its frequency dependence were measured at low temperatures on selected loess samples in order to determine the maximum grain size present in the fine-grained magnetic fraction.

Magnetic enhancement paths in Loess sediments from Tajikistan, China and Hungary

“…Heller and Liu (1982) were the first to point out that the climatic information recorded in the loesspaleosol sequence at Luochuan (Central Chinese loess plateau) can be reconstructed using magnetic low field susceptibility. Strong evidence has accumulated during recent years which supports an in situ production of fine-grained ferrimagnetic minerals to enhance the susceptibility in the paleosols (Zhou et al, 1990, Maher and Thompson, 1992, Verosub et al, 1993, Eyre and Shaw, 1994, Forster et al, 1994. Detrital magnetic minerals also contribute to the susceptibility signal to some extent (Heller and Liu, 1984).…”

“…(Fig. 2a) although the low field susceptibility in these enhanced samples (paleosols) is controlled by the contribution from magnetite grains at the SP/stable single domain (SSD) boundary (Forster et al, 1994). Thus the influence of magnetite grains at the SP/SSD threshold on Zh can be ruled out.…”

Spectroscopic and ancient geomagnetic field intensity studies on archaeological pottery samples, India

“…The parameters, mass-specific frequency-dependent susceptibility χ FD defined as χ FD = χ LF −χ HF and percentage frequency-dependent susceptibility χ FD % defined as χ FD % = (χ LF −χ HF )/χ LF ·100, are used to detect ultrafine (<0.03 µm) ferrimagnetic minerals lying in the superparamagnetic grain size [12,13]. Temperature dependence of magnetic susceptibility χ-T (in air atmosphere) was measured using Bartington MS2WFB with high temperature furnace by heating samples up to 700 • C and cooling down to 100 • C in steps of 2 • C. Saturation isothermal remanent magnetization (SIRM) studies were conducted using pulse magnetizer (IM-10-30 Impulse Magnetizer, ASC Scientific, USA).…”

Pleistocene climate and environment reconstruction by the paleomagnetic study of a loess-paleosol sequence (Cérna Valley, Vértesacsa, Hungary)

“…The paleosol layers are characterized by the highest magnetic susceptibility, due to the higher concentration of multi-domain ferromagnetic grains and the superparamagnetic component (Thompson and Oldfield 1986;Forster et al 1994;Eyre 1997;Worm 1998). The superparamagnetic minerals were probably generated during pedogenesis (e.g.…”