New Wine in an old Bottle? A Model for Magnetism-Climate (tele)Connection

Inversion of modern topsoil or paleosol magnetism to environmental change parameters require that we understand correctly the physical, chemical and biogeochemical controls on soil formation per se, and loessic soil formation in particular. Accumulating field evidence from loess deposits worldwide point decisively to the formation of both magnetically enhanced and depleted soils compared to parent loess. Only in some cases magnetic enhancement in parent loess may be explained by detrital windblown magnetite. In general however, neoformed ultrafine (less than a few micrometers in size) 'iron oxide' ("sensu lato"), particles include strongly magnetic ferrimagnets and weakly magnetic antiferromagnets. A knowledge of the balance between the two may lead to a more correct and comprehensive understanding of multiple environmental parameters of the past, rather than simply the past rainfall, as has been done in the past. An approach based on the fundamental magnetochemistry of alteration is not only desirable but feasible. Thus, the formation and stability of ferrous ions on nanoparticles of ferric oxides and hydroxides may be the most fundamental step for neoformation of magnetite and maghemite. The ferrous ion may, however, be unstable in the Earth's surface environment and invert back to ferric, generating weakly magnetic hematite and goethite. In both cases, information about past environmental parameters (temperature, precipitation minus evaporation, soil acidity, microbial presence) may be embedded in soil magnetic (and other) parameters. To test the validity of the above heuristic model, we review recent experimental evidence (low temperature magnetism, Mössbauer spectra, surface chemical reactivity, and conventional and synchrotron X-ray diffraction and absorption, etc). We then advance model environments which would lead to the magnetic observables. In addition, we inspect computational models of a reactive 'iron oxide' surface to investigate their compatibility with magnetic and non-magnetic evidence of alteration from the laboratory and the field situations.