Records of magnetic properties in Quaternary loess and its paleoclimatic significance: a brief review

Abstract

Due to the successive deposition, Quaternary loess–paleosol sequences potentially provide one of the best terrestrial records of paleoclimatic changes. Magnetostratigraphy and paleoclimate are two major aspects of magnetic investigation in northern China and other regions of the world. The classic loess sections across the Chinese Loess Plateau indicate that wind-blown loess deposition began close to the base of Matuyama (2.6 Ma). However, progress has also been made in extending the record below the loess into the Red Clay, and the aeolian record can now be carried back beyond 7 Ma. The surprising similarity between the initial susceptibility (IS) curve obtained from numerous loess–paleosol sequences and the oxygen isotope fluctuation from the deep-sea sediments provides impressive evidence of the global significance of the magnetic record. As a useful proxy in paleoclimatic studies, magnetic susceptibility (MS) has been extensively used to reconstruct the paleoclimate. However, the actual mechanism of MS enhancement of paleosols remains controversial. The important questions—why does IS fluctuate in loess sections, and what are the causes of the IS enhancement in paleosols?—have been matters of debate in the last 10 years. Currently, the viewpoint that pedogenesis plays an important role in the MS enhancement of paleosols has been generally accepted. Pedogenic magnetite is an important contributor to the MS enhancement. IS is not only simply controlled by the abundance of strong magnetic minerals, but also by their grain-size distributions. The increment of superparamagnetic (SP) grains (<0.1 μm), which is induced by pedogenesis, could enhance the IS of paleosols. The formation of SP minerals has been debated as either biotic or abiotic. It is now possible to propose a quantitative model for the reconstruction of paleoclimate according to the paleoprecipitation deduced from proxy indicators, such as IS fluctuation, grain size, stable isotope composition, CaCO 3 content, Rb/Sr, and ratio of citrate-bicarbonate-dithionite-extractable Fe 2O 3 to total Fe 2O 3 (FeD/Fet).