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HR: 1340h
AN: GP13A-0020
TI: How small is pedogenic magnetite? Size estimates for loessic soils based on remanence and hysteresis measurements.
AU: * Geiss, C E
EM: christoph.geiss@trincoll.edu
AF: Trinity College, 300 Summit St, Hartford, CT 06106 United States
AU: Machac, T
EM: tamara.machac@trincoll.edu
AF: Trinity College, 300 Summit St, Hartford, CT 06106 United States
AB: Pedogenic magnetite (or maghemite) is generally believed to be fine-grained, consisting mainly of superparamagnetic (SP) and single-domain (SD) particles. These grain size estimates are based on increased values of ARM/IRM or frequency dependent susceptibility (χ_FD) in the magnetically enhanced horizons. However, these changes in grain size dependent parameters are generally quite small, compared to the often large increase in concentration dependent parameters, such as low field susceptibility, saturation magnetization (J_S), or various remanence parameters (RM). In addition, a recent study by Dunlop (2002b) suggested that hysteresis properties of several Chinese soils (modern and buried) do not suggest a fining of the magnetic component in the magnetically enhanced soil horizons. We used the model of Dunlop (2002a)to analyze hysteresis data from twenty modern soil profiles from the Midwestern United States. In contrast to the data analyzed by Dunlop (2002b), our data can be modeled equally well with binary mixtures of SD-MD and SD-SP particles. To complement the ambiguous hysteresis data we generated a simple mixing model to constrain the size distribution of pedogenic magnetite. Our model assumes remanence acquisition efficiencies f = RM / J_S for coarse (MD - PSD) and fine (SD-SP) magnetite for ARM and IRM. It then uses ARM/IRM ratios to estimate the relative abundances of coarse and fine magnetite. The validity of our grain size distribution estimates can be checked by comparing measured J_S values to J_S calculated from our model output and measured values of IRM (or ARM). A comparison of our ARM/IRM modeling and hysteresis data shows that the magnetic properties of magnetically enhanced soil horizons can be explained with the addition of a fine grained magnetite component. This component, however, has a wide grain size distribution which includes SP, SD and likely PSD particles. Dunlop, D. J. (2002a). Theory and application of the Day plot (M_rs/M_s versus H_cr/H_c) 1. Theoretical curves and tests using titanomagnetite data. Journal of Geophysical Research 107, 10.1029/2001JB000486. Dunlop, D. J. (2002b). Theory and application of the Day plot (M_rs/M_s versus H_cr/H_c) 2. Application to data for rocks, sediments and soils. Journal of Geophysical Research 107, 10.1029/2001JB000487.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005