Magnetic susceptibility in the European Loess Belt: New and existing models of magnetic enhancement in loess

Highlights

• The ways of magnetic enhancement in loess are summarized.

• New ways of detrital enhancement of magnetic susceptibility are introduced.

• A guide for the interpretation of low field and frequency dependent susceptibility is introduced.

Abstract

Magnetic susceptibility measurements play a key role in Quaternary studies. Magnetic proxies, such as low field and frequency-dependent magnetic susceptibility, are widely applied in the reconstruction of terrestrial paleoclimate, e.g., in the study of loess-paleosol successions. In general, the interpretation of loess magnetic susceptibility signals is based on two commonly accepted models: the pedogenic magnetic enhancement and wind-vigour models. However, there are an increasing number of cases where such models cannot be used. These cases show unusual relationships between the two common loess magnetic susceptibility proxies: low field and frequency-dependent magnetic susceptibility. Such relationships have been attributed to various phenomena including the dissolution of fine-grain minerals and the formation of ultrafine magnetic rims on the surface of coarser grains by weathering. Despite the growing number of these exceptional cases of magnetic enhancement, our knowledge about the occurrence and potential causes of the unusual behaviour of magnetic susceptibility parameters is still limited. This, in turn, hinders the wider application of magnetic susceptibility parameters in loess. To fill this knowledge gap, magnetic susceptibility data of various profiles from the European Loess Belt were collected and compared to reveal various enhancement trends in loess. Along with the analysis of magnetic susceptibility parameters, combined scanning electron microscopy (SEM) and rock magnetic analyses were applied to samples from the Paks loess sequence in Hungary to describe some of the irregular cases, notably the cause of increasing frequency-dependent susceptibility in non-altered sediments. Analysis of loess, paleosol and common mineral samples separated from loess (e.g., muscovite) revealed that various features may be responsible for these increasing frequency-dependent susceptibility values: i) surface weathering (maghemitization) of coarser detrital grains, ii) nanofragmentation by physical weathering and iii) the appearance of significant amounts of ultrafine magnetic inclusions in micas. These special modes of magnetic enhancement of loess do not undermine the importance of the basic theories suggested above, but rather provide three mechanisms that account for some of the increasing number of unusual cases. To aid in the wider and more accurate use of magnetic susceptibility parameters in loess, we review the current magnetic enhancement models with special emphasis on the identification of unusual trends in magnetic enhancement and understanding their drivers.

Introduction

The nature of glacial and interglacial paleoenvironments and their role in understanding future climates has become a major focus of Quaternary research over recent decades (Past Interglacials Working Group of PAGES, 2016). As a part of the reconstruction of paleoenvironment, increasingly more information has been revealed about loess records, which are some of the most commonly used terrestrial archives in the reconstruction of glacial and interglacial paleoenvironment over the Quaternary (Pye, 1987). In such reconstructions, mineral magnetic methods play a crucial role since both the type and amount of magnetic contributors in loess are highly sensitive to environmental change during pedogenesis (Maher and Taylor, 1988; Heller et al., 1993; Forster et al., 1994; Dearing et al., 1996; Maher, 2011, Maher, 2016). In fact, basic frequency-dependent magnetic susceptibility analysis has become a routine stratigraphic tool in studies of loess sequences (e.g., Schaetzl et al., 2018 and references therein).

There are two commonly known models for the enhancement of the magnetic susceptibility parameters in loess: pedogenesis and wind vigour (e.g., Forster et al., 1994; Evans and Heller, 1994; Evans, 2001). Both models were built to describe the rhythmic change of environmental magnetic proxies, especially magnetic susceptibility parameters, in loess sequences, most likely related to the cycles of glacial (stadial) and interglacial (interstadial) periods. The widely accepted main magnetic enhancement models seem highly variable geographically. Additionally, there is no consensus about the applicability of the alternative models, and apparently, a number of different hypotheses appear in the literature (see Section 2). Due to the common use of χ_lf and χ_fd, it is essential to resolve these inconsistencies and ambiguities so that these widely used proxies can properly inform debates about past environmental changes. This requires understanding of the fundamental physical meaning of magnetic parameters. The primary goal of this study is to reveal potential influences on magnetic mineral components in sediments that exhibit unusual χ_lf and χ_fd using examples from the Paks loess section in Hungary.