Continental-scale magnetic properties of surficial Australian soils

Soil magnetism reflects the physical properties of mainly iron oxide and oxyhydroxide minerals, which provides important information for deciphering soil environments. Establishing national scale soil magnetic databases can provide important reference information that can assist mineral surveying and agricultural planning. Our aims are to provide visualizations and to describe multiple magnetic properties across Australia, to evaluate the relationship between soil magnetism and soil forming factors, and to interpret the mechanisms responsible for surface soil magnetism in Australia. We present the first surficial Australian soil magnetic database, which contains 471 topsoil samples of natural and unpolluted materials. The samples were characterized with detailed magnetic measurements, which show that the magnetic properties of Australian soils vary considerably, but most surficial soils have small concentrations of coarse-grained magnetic minerals. The vast central Australian interior is characterized by weak magnetism, with more hematite and goethite contribution. Strong magnetic hotspots occur in the northwestern plateau, Nullarbor Plain, and eastern highlands. Parent material acts as the dominant control on soil magnetic properties, influencing magnetic mineral concentration and grain size, and controlling the contribution and relative importance of hematite to goethite. Temperature and rainfall both have a weak negative influence on superfine ferrimagnetic particles, due to progressive transformation to hematite and particle migration driven by intensive rainfall in sandy soils. Biota and land use changes tend to have a more complex and integrated local influence on hematite and goethite formation and preservation.