Rock magnetic properties of the late Pleistocene Loess-Paleosol deposits in Haro River area, Attock basin, Pakistan: Is magnetic susceptibility a proxy measure of paleoclimate?

摘要

Variations of rock-magnetic properties in a late Pleistocene (approx. 18-130 ka) Loess-Paleosol sequence of Haro River area, Attock basin, Pakistan were studied. Isothermal remanent magnetization (IRM) acquisition and thermomagnetic analyses of the samples indicate that magnetite and maghemite are the major magnetic minerals in these samples. Saturation isothermal remanent magnetization (SIRM), anhysteretic remanent magnetization (ARM), and the ratio ARM/SIRM disclose relative enrichment of ultra-fine magnetite/maghemite grains in the paleosols as compared to loess deposits, which suggests that the magnetite/maghemite is of pedogenic origin. Despite this enrichment, the low-field magnetic susceptibility is not always enhanced in paleosols in this sequence. In the Chinese Loess it has been believed that the observed lows and highs of magnetic susceptibility values respectively correspond to loess and paleosol sequences which indicates paleoclimatic change during the formation of the sequences. However, our observations in Pakistan's Loess-Paleosol sequence demonstrate that, although the ultrafine grained magnetite/maghemite may be produced by the process of pedogenesis, the low-field magnetic susceptibility record does not directly correlate with the alternation of loess and paleosol, and therefore the magnetic susceptibility can not simply be interpreted as a proxy for paleoclimate in the area.