Topics: All magnetic methods in geophysics rely on less than ten natural iron minerals, which all have complex and characteristic ferromagnetic properties. We command over a range of highly sensitive automated instruments to determine composition, concentration, grain-size and oxidation state of magnetic mineral assemblages in sediments and rocks.
Coupled compass needles are a simplified analogy of atomic exchange interaction
Summary: The magnetic micro- and nanoparticle inventories of oxic sediments from the equatorial Atlantic were investigated by electron microscopy to classify all present detrital and authigenic magnetic mineral species and to investigate their regional distribution, origin, transport, and preservation with the objective to establish source-to-sink relations and to constrain environmental magnetic proxy interpretations. Crystallographic, elemental, morphological, and granulometric data of more than 2000 magnetic particles were collected enabling us to define nine particle classes: (1) detrital magnetite, (2) titanomagnetite (fragmental and euhedral), (3) titanomagnetite-hemoilmentite intergrowths, (4) silicates with magnetic inclusions, (5) microcrystalline hematite, (6) magnetite spherules, (7) bacterial magnetite, (8) goethite needles, and (9) nanoparticle clusters. Each class was associated with fluvial, eolian, subaeric, and submarine volcanic, biogenic, or chemogenic sources. Large-scale sedimentation patterns were also delineated as well: Detrital magnetite is typical of Amazon discharge, fragmental titanomagnetite is a submarine weathering product of mid-ocean ridge basalts, and titanomagnetite-hemoilmenite intergrowths are common magnetic particles in West African dust. This clear regionalization underlines that magnetic petrology is an excellent indicator of source-to-sink relations. Hematite encrustations, magnetic spherules, and nanoparticle clusters were found at all investigated sites, while bacterial magnetite and authigenic hematite were only detected at the more oxic western site. At the eastern site, surface pits and crevices were seen on the crystal faces indicating subtle early diagenetic reductive dissolution. It was observed that paleoclimatic signatures of magnetogranulometric parameters such as the ratio of anhysteretic and isothermal remanent magnetizations can be formed either by mixing of multiple sources with separate, relatively narrow grain size ranges (western site) or by variable sorting of a single source with a broad grain size distribution (eastern site). Hematite, goethite, and possibly ferrihydrite nanoparticles occur in all sediment cores studied and have either high-coercive or superparamagnetic properties depending on their partly ultrafine grain sizes.