摘要
Magnetite and maghaemite are the two main contributors to magnetic susceptibility in theChinese loess/palaeosol sequence. Due to the close similarity in physical properties, earlyinvestigators treated them together as "magnetite". Utilising the property of thermally instability ofmaghaemite, we have been able to separate it from magnetite in loess and palaeosol samples from theupper parts of the Xifeng and Luochuan sections on the central Chinese Loess Plateau and compare itsvariation with depth and horizon. Our results indicate that thermally unstable maghaemite is relativelyenriched in both 1) weakly weathered loess units and 2) towards the west, the direction of dust supply,suggesting that maghaemite is at least partially (coarse grains) of detrital aeolian origin while somepedogenic maghaemite (extra fine grains) occurs in the palaeosols. The physical meaning of magneticsusceptibility, therefore, needs to be reconsidered as a synthetic indicator reflecting thepalaeoenvironment for both source area and deposition area. Thermally unstable goethite is a minormagnetic component of palaeosol samples, but it may play a significant role in determiningcharacteristic magnetic remanence of weakly weathered loess samples, especially when one appliesalternating-field cleaning.
Magnetite and maghaemite are the two main contributors to magnetic susceptibility in the Chinese loess/palaeosol sequence. Due to the close similarity in physical properties, early investigators treated them together as 'magnetite'. Utilising the property of thermally instability of maghaemite, we have been able to separate it from magnetite in loess and palaeosol samples from the upper parts of the Xifeng and Luochuan sections on the central Chinese Loess Plateau and compare its variation with depth and horizon. Our results indicate that thermally unstable maghaemite is relatively enriched in both 1) weakly weathered loess units and 2) towards the west, the direction of dust supply, suggesting that maghaemite is at least partially (coarse grains) of detrital aeolian origin while some pedogenic maghaemite (extra fine grains) occurs in the palaeosols. The physical meaning of magnetic susceptibility, therefore, needs to be reconsidered as a synthetic indicator reflecting the palaeoenvironment for both source area and deposition area. Thermally unstable goethite is a minor magnetic component of palaeosol samples, but it may play a significant role in determining characteristic magnetic remanence of weakly weathered loess samples, especially when one applies alternating-field cleaning.
