Low-temperature oxidation of magnetite in loess-paleosol sequences
Abstract
In many natural samples changes in magnetic parameters occur when samples are heated for 10 minutes at 150 C. These changes are related to a reduction in stress in superficially oxidized magnetite grains. The stress originates from low-temperature oxidation (LTO) at ambient conditions. Measurements on pure magnetite grains are presented to support this interpretation. This phenomenon can be used to supply data for two vital issues concerning the use of magnetic parameters as paleoclimate proxies: 1) which methods can be used to distinguish between the contributions of primary (detrital) magnetic minerals and pedogenic magnetic minerals to the magnetic parameters? and, closely related to that, 2) which part of the magnetic signal is carried by magnetite and/or maghemite grains? Magnetic parameters were measured in a 62 m/ 800 ky core from a loess/paleosol sequence in NE Bulgaria. In the sequence, enhanced susceptibilities occur in paleosols, similar to most sequences from the Chinese loess plateau. With a simple model, it can be shown that both the detrital and the pedogenic fraction are influenced by LTO. Apparently, both fractions partly consist of magnetite (superficially) affected by low-temperature oxidation. Methods to distinguish these fractions can therefore not be based on the supposition that the pedogenic fraction consists of maghemite and the detrital fraction consists of magnetite. The upper half of the section has different properties than the lower half: the modelled detrital susceptibility as well as the measured magnetizations are higher. The ratios of the hysteresis parameters point to slightly larger magnetite (maghemite) grain sizes in the upper part. There is also a difference in response of various parameters to heating to 150 C: viscous decay of remanent magnetization shows an increase in the upper part and decreases in the lowest 15 m. The coercivity record is clearer after heating.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....12996V