August 2009

Magnetic anomalies are deviations from an internal planetary magnetic field produced by crustal materials. Crustal anomalies, measured over a wide range of vertical distances, from near-surface to satellites, are caused by magnetic minerals that respond to the changing planetary field. Previously, magnetism of continental crust was described in terms of the bulk ferrimagnetism of crustal minerals, which is mostly due to induced magnetization. The recent discovery of lamellar magnetism, a new interface-based remanence type, has changed our thinking about the contribution of remanent magnetization. Lamellar magnetism may also be an important contributor to deep-seated anomalies in the crust of the Earth and in other planets with highly magnetic crusts, like Mars.

Ferrimagnetic nanocrystals are present in virtually every organism. They are used by bacteria, algae, mollusks, insects, and vertebrates either for navigating in the geomagnetic field or for hardening their tissues. Advanced transmission electron microscopy techniques, including electron holography, reveal the complex interplay between the physical and magnetic properties and biological functions of ferrimagnetic nanocrystals in bacteria. Although some information is now available about magnetic sensory systems in more complex organisms, much further research is required to understand fully the origin and function of biomagnetism.

Two contrasting examples of the application of mineral magnetism to environmental problems are discussed. Magnetic susceptibility measurements of sediments from the Chinese Loess Plateau – the biggest accumulation of windblown sediments on Earth – reveal one of the best records of continental climate change available. These records provide a detailed picture of glacial and interglacial cycles and variations in the East Asian summer monsoon stretching back more than 2 million years. In the case of anthropogenic airborne particles, the spread of particulate pollutants can be robustly traced throughout a city environment by measuring the magnetic properties of leaves, which trap magnetic particles released from vehicle exhausts and/or industry emissions.

Extraterrestrial materials contain a diversity of ferromagnetic phases, ranging from common terrestrial oxides to exotic metal alloys and silicides. Because of their great age and remote provenance, meteorites provide a unique window on early solar system magnetic fields and the differentiation of other bodies. Interpreting the records of meteorites is complicated by their poorly understood rock magnetic properties and unfamiliar secondary processing by shock and low-temperature phase transformations. Here we review our current understanding of the mineral magnetism of meteorites and the implications for magnetic fields on their parent bodies.

The long-term history of the geodynamo provides insight into how Earth’s innermost and outermost parts formed. The magnetic field is generated in the liquid-iron core as a result of convection driven by heat carried across the core–mantle boundary and freezing of the solid inner core. Earth’s magnetic field acts as a shield against energetic solar radiation, and therefore the geodynamo played an important role in the development and retention of our atmosphere, ultimately setting the stage for the evolution of life. A new analytical approach, using single silicate crystals that host minute magnetic particles, can reveal heretofore hidden aspects of Earth’s magnetic history. This method is being used to address some of the outstanding questions regarding the long-term behavior of the geodynamo.

Magnetic minerals are ubiquitous in the natural environment, and they are also present in a wide range of biological organisms, from bacteria to human beings. The last ten years have seen a striking improvement in our ability to detect and image the magnetization of minerals in geological and biological samples. These minerals carry a wealth of information encoded in their magnetic properties. Mineral magnetism (together with the related disciplines of rock magnetism, paleomagnetism, environmental magnetism, and biomagnetism) decodes this information and applies it to an ever increasing range of geoscience problems, from the origin of magnetic anomalies on Mars to quantifying variations in Earth’s paleoclimate.