The magnetic properties of loess and paleosol samples from the Chinese Loess Plateau spanning the last 22 million years

Abstract

Magnetic measurements (susceptibility, isothermal and anhysteretic remanence, and quotients derived from these) are presented for a suite of loess and paleosol samples from sites on the Chinese Loess Plateau. The three sites, Xifeng, Dongwan and Qinan (QA-I), together span most of the last 22 million years. The results provide the first multi-parameter magnetic measurements from pre-Pleistocene loess/paleosol sequences that began accumulation in the early Miocene. Mass specific measurements have been corrected for carbonate concentrations which peak in the loess layers and range up to 57%. All the magnetic properties recorded indicate variability on both orbital and supra-orbital timescales. The changing magnetic properties allow the record to be divided into a sequence of eight sample groups that broadly parallel the main supra-orbital changes in magnetic susceptibility. The pre-Pleistocene parts of the record include intervals (mainly late Miocene and Pliocene) during which loess as well as paleosol layers are characterized by high values for properties such as frequency dependent susceptibility ( χ fd), usually associated with strong weathering. Most of the Miocene record prior to 8.5 Ma, with the exception of a brief interval from ca. 14.4 to 15.9 Ma, shows reduced values for indicators of pedogenic ferrimagnetic concentrations in both the loess and paleosol layers, as well as a low amplitude of variability between loess and paleosol values. The palaeoenvironmental implications of this part of the sequence are unclear since they hinge on the type of model of pedogenic magnetic mineral formation adopted. High-field remanence measurements indicate that the coercivity of the antiferromagnetic minerals represented declined progressively throughout the whole period in both the loess and the paleosol samples, though there is no unambiguous evidence for long-term diagenetic alteration of the magnetic properties on timescales beyond the orbital ones that drive the loess/paleosol alternations. The present results suggest caution in applying existing climofunctions based on magnetic susceptibility to deposits spanning the whole of the last 22 Ma.