Abstract
Bulk magnetic properties of soils and sediments are often sensitive proxies for environmental change but commonly require interpretation in terms of the different sources of magnetic minerals (or components) that combine to generate them. Discrimination of different components in the bulk magnetic record is often attempted through endmember unmixing and/or high resolution measurements that can require intensive measurement plans, assume linear additivity, and sometimes have difficulty in discriminating a large number of sources. As an alternative, magnetic measurements can be made on isolated sediment fractions that constitute the bulk sample. When these types of measurements are taken, heterogeneity is frequently observed between the magnetic properties of different fractions, suggesting different magnetic components often associate with different physical grain sizes. Using a particle size-specific methodology, individual components can be isolated and studied and bulk magnetic properties can be linked to, and isolated from, sedimentological variations. Deconvolving sedimentary and magnetic variability in this way has strong potential for increased understanding of how magnetic fragments are carried in natural systems, how they vary with different source(s), and allows for a better assessment of the effect environmental variability has in driving bulk magnetic properties. However, despite these benefits, very few studies exploit the information they can provide. Here, I present an overview of the different sources of magnetic minerals, why they might associate with different sediment fractions, how bulk magnetic measurements have been used to understand the contribution of different components to the bulk magnetic record, and outline how particle size-specific magnetic measurements can assist in their better understanding. Advantages and disadvantages of this methodology, their role alongside bulk magnetic measurements, and potential future directions of research are also discussed.
Highlights
Magnetic properties of soils and sediments are among the most commonly made physical property measurements to understand records of past environmental change [1,2,3,4,5,6,7,8,9,10,11,12]
A simple suite of magnetic measurements can rapidly and non-destructively characterize the concentration, mineralogy, and magnetic grain size of magnetic minerals present in samples. These properties are strongly sensitive to the abundance and type of Fe-bearing minerals, which are both ubiquitous in environmental systems and sensitive to environmental change, such that magnetic measurements offer the opportunity to study temporal and spatial environmental variability in a range of settings
Instead of just a sum of different components, these results suggest that bulk magnetic properties should be considered as a convolved signal of the magnetic heterogeneity of different sediment size fractions (that is influenced by source(s)) weighted by their abundance within the bulk sediment
Summary
Magnetic properties of soils and sediments are among the most commonly made physical property measurements to understand records of past environmental change [1,2,3,4,5,6,7,8,9,10,11,12]. Inter-property correlations often sidestep addressing the more fundamental questions about what drives magnetic property variations in any particular record Lack of this understanding may be a major reason why magnetic measurements are often poorly understood and overlooked as potential investigative tools by the wider geoscience community. The second approach aims to characterize heterogeneity in the magnetic record and isolate different components through measurement of the magnetic properties of different sediment particle size constituents of the bulk sediment [10,11,12,17,30,31,32] Both approaches have provided greater insights into what drives bulk magnetic properties in a variety of different environments, they are rarely employed in routine magnetic studies. As relatively underused, yet simple techniques, I shift to focus on the particle size dependence of magnetic properties and outline some important considerations when designing a particle size specific magnetic study, discuss their role alongside bulk magnetic property measurements, before providing an outlook and direction for future research
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