HR: 11:20h
AN: GP22A-04 INVITED     [Abstracts]
TI: Quantifying the diagenetic pathway of iron oxides in soils: a modeler's perspective
AU: * Egli, R
EM: eglix007@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, United States
AU: Banerjee, S K
EM: banerjee@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, Minneapolis, MN 55455, United States
AU: Geiss, C
EM: christoph.geiss@trincoll.edu
AF: Department of Physics, Trinity College, Hartford, CT , United States
AB: The first observation of enhanced magnetic susceptibility in topsoils by LeBorgne in 1955 has driven several studies on the transformation pathway of magnetic iron oxides in soils, whose complexity became immediately evident. The principal factor responsible for the magnetic enhancement (ME) is ultrafine (5-100 nm) pedogenic, cation-deficient magnetite (PCDM), whose low concentration - sometimes well below 1% - makes magnetic measurements the only reliable quantification method. It is now widely accepted that PCDM originates from the partial reduction of ferrihydrite, however, the detailed diagenetic path and the final fate of magnetite or maghemite nanoparticles is not clear. Nevertheless, an empirical correlation is observed between ME and the climate in which modern soils from selected regions of the World were formed. Unfortunately, this correlation is not as good as we wish for using magnetic measurements as a precise climatic proxy. What is the reason for the scatter observed in the ME of soils formed under similar conditions? Is it entirely due to the existence of "hidden variables", such as pH, Eh, vegetation and soil age? To address this question in a bottom-up approach, we first need to test the equation ME = PCDM, which implicitly underlies many studies on the magnetic properties of soils. For example, the definition of ME as the difference between magnetic measurements of the topsoil and of the "unaltered" parent material does not allow us to discriminate the accumulation of diagenetic minerals from other processes, such as weathering of magnetic minerals contained in the parent material, or changes of the dust accumulation rate. Furthermore, hypothetical differences in the grain size distribution of PCDM would be "seen" as concentration changes, because bulk magnetic parameters are grain size dependent. How can we deal with the complex magnetic properties of ferrimagnetic nanoparticles and a plethora of concomitant geochemical processes related to the iron cycle in soils? To address this question, we explored a rigorous modeling approach to (1) isolate the magnetic contribution of PCDM and other magnetic iron oxides, (2) quantify the concentration and grain size distribution of each diagenetic mineral, and (3) test the hypothesis that PCDM is characterized by specific physical and chemical "fingerprints", regardless of the degree of ME, which can be used to develop simple quantification methods. This approach produces a clear definition of ME and sets precise constraints on the diagenetic pathway of pedogenic iron oxides. Preliminary results from a selected group of soils show a dramatic improvement in the correlation between rainfall and our estimates of the PCDM concentration. Other pedogenic minerals, such as hematite and goethite, display a less clear correlation with climate and a far smaller enhancement that is not confined to topsoils.
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly