Tuesday, 11 July 2006 - 8:30 AM

Magnetism and Moessbauer Spectroscopy of Loessic Soils/Paleosols as a Key to Pedogenic Transformation of Fe Minerals and Climate Change.

Tatyana S. Gendler, United Institute of Physics of the Earth RAS, Bol'shaya Gruzinskaya 10, Moscow, 123810, Russia, Friedrich Heller, Institut fur Geophysik ETH, Zurich, CH-8093, Switzerland, Alexander Tsatskin, Zinman Institute of Archaeology University of Haifa, Mount Carmel, Haifa, 31905, Israel, and Alla A. Novakova, Physics Faculty Moscow State University, Vorob'evu Gory, Moscow, 119899, Russia.

Rock magnetic methods, Mössbauer Spectroscopy (MS), micromorphological observations and analysis of Organic Matter content (OM) have been combined to study two Quaternary loess/soil sequences in the SW of the East European. Being situated above the established Matuyama/Brunhes boundary, the sequences are well correlated and contain gray and brown variants of mollisols (chernosems). Magnetic key parameters such as low field susceptibility (κlf), high field induced (Ji) and remanent (Jr) magnetization, coercivity and remanent coercivity (Hc, Hcr), as well as thermomagnetic curves of clay extracts exhibit clear differences 1) between loess and paleosols, and 2) between different paleosol varieties. All of the paleosols, including even poorly expressed ones, show magnetic enhancement vs. loesses. The increase in κlf, Ji, Jr values is in accord with soil morphology peaking in the A horizons of stronger developed paleosols, and is significantly higher in clay fractions because of the neoformed partially oxidized magnetite. Likewise, the MS spectra total areas, as well as OM, increase in the A horizons. At room temperature MS reveals a broadened poorly resolved doublet originating from different paramagnetic minerals and superparamagnetic (SP) Fe oxides and hydroxides. Computer fitting of the spectra shows that the paleosols are basically enriched in Fe(III)-smectite, amorphous or poorly crystallized ferrihydrite, goethite and hematite (<20 nm size) which are intimately associated with clay minerals. In contrast, Fe-compounds in loess consist mainly of Fe(II)-alumosilicates without any traces of nano-sized SP Fe oxides. Figure 1 shows that the Jr values, reflecting concentration and size of ferrimagnets alone, and the MS areas, which depend on the amount of Fe in clays, amorphous and SP hydroxides and hematite, are in good agreement. The same trend of proportional accumulation holds true between OM and Fe in paramagnetic and SP phases as identified by MS (Fig. 2). The data suggest the existence of genetic relationships among different Fe minerals and indirectly between Fe with OM (the latter poorly preserved in paleosols). Since enhanced SP magnetite accumulation in mollisols is accounted for by microbiological and pedogenic processes, the related occurrence of SP nano-sized Fe oxihydroxides and Fe(III) in clay minerals of mollic paleosols is explained in the same way. We emphasize that the various nano-sized Fe minerals in paleosols indicate former pedobiogenesis with no conspicuous hydromorphism. That association of pedogenic ferri- and paramagnetic and SP particles is clearly distinguished from those inherited or attached through the ensuing dust influx. Although magnetic and Mössbauer methods are not strictly quantitative, their integration provides comprehensive information on the Fe-containing minerals and emphasizes the crucial role of nano-sized Fe minerals in the genesis/diagenesis of soils in a wide variety of paleoclimatic situations. Keywords: soil magnetism, Mössbauer, paleosols, iron minerals, neoformation Fig 1. Correlation between Jr and normalized total Mössbauer spectra area S. Fig 2. Correlation between S hydroxides and OM (Corg) for different paleomollisols.

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