Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties
Identifieur interne : 000F59 ( Main/Exploration ); précédent : 000F58; suivant : 000F60Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties
Auteurs : Diana Jordanova [Bulgarie] ; Nikolai Petersen [Allemagne]Source :
- Geophysical Journal International [ 0956-540X ] ; 1999-08.
Descripteurs français
- Wicri :
- geographic : Bulgarie.
English descriptors
Abstract
A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess (Hus & Han 1992; Maher & Thompson 1992; Heller & Evans 1995; Hunt 1995). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S0 and first and second palaeosols S1 and S2 developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force Hc showing even higher values than those of the parent loess material. The older palaeosols (S4 to S6 ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200°C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.
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DOI: 10.1046/j.1365-246X.1999.00874.x
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A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess (Hus & Han 1992; Maher & Thompson 1992; Heller & Evans 1995; Hunt 1995). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S0 and first and second palaeosols S1 and S2 developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force Hc showing even higher values than those of the parent loess material. The older palaeosols (S4 to S6 ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200°C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases.</div></front></TEI><affiliations><list><country><li>Allemagne</li><li>Bulgarie</li></country><region><li>Bavière</li><li>District de Haute-Bavière</li><li>Sofia-ville (oblast)</li></region><settlement><li>Munich</li><li>Sofia</li></settlement></list><tree><country name="Bulgarie"><region name="Sofia-ville (oblast)"><name sortKey="Jordanova, Diana" sort="Jordanova, Diana" uniqKey="Jordanova D" first="Diana" last="Jordanova">Diana Jordanova</name></region><name sortKey="Jordanova, Diana" sort="Jordanova, Diana" uniqKey="Jordanova D" first="Diana" last="Jordanova">Diana Jordanova</name></country><country name="Allemagne"><region name="Bavière"><name sortKey="Petersen, Nikolai" sort="Petersen, Nikolai" uniqKey="Petersen N" first="Nikolai" last="Petersen">Nikolai Petersen</name></region></country></tree></affiliations></record>Pour manipuler ce document sous Unix (Dilib)
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