Magnetic susceptibility as a proxy parameter: assets and pitfalls

Abstract

The merit of the low-field magnetic susceptibility as proxy parameter is concisely reviewed. In many geological samples it is a composite consisting of a diamagnetic, paramagnetic, and ferromagnetic (sensu lato) contribution that varies with rock type. For weakly magnetic samples, the diamagnetic and paramagnetic contributions are an appreciable portion of the measured susceptibility. In such materials equating the measured susceptibility with the ferromagnetic susceptibility is not warranted. Before a susceptibility record is geologically interpreted, the underlying drivers of its variation must be understood. This is achieved by integrating susceptibility data with an extended magnetic property analysis, geochemical information, and a paleomagnetic assessment of the timing of the acquisition of the natural remanent magnetization as the magnetic minerals may post-date the sedimentation age. Measuring the susceptibility at several frequencies and in varying low applied fields provides important information on the grain-size distribution in the ultrafine grain end and, to a lesser extent, on the magnetic mineralogy. Examples discussed include research into loess/paleosol records and high-resolution time scale development. Variable surficial oxidation of magnetite particles and potential remagnetization complicate the interpretation of a susceptibility record in terms of paleoclimate.