Environmental magnetism is the study of

magnetism Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...

as it relates to the effects of climate,

sediment transport Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural system ...

, pollution and other environmental influences on magnetic minerals. It makes use of techniques from rock magnetism and

magnetic mineralogy Magnetic mineralogy is the study of the magnetic properties of minerals. The contribution of a mineral to the total magnetism of a rock depends strongly on the type of magnetic order or disorder. Magnetically disordered minerals (diamagnets and para ...

. The magnetic properties of minerals are used as proxies for environmental change in applications such as

paleoclimate Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...

, paleoceanography, studies of the

provenance Provenance (from the French ''provenir'', 'to come from/forth') is the chronology of the ownership, custody or location of a historical object. The term was originally mostly used in relation to works of art but is now used in similar senses i ...

of sediments, pollution and

archeology Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...

. The main advantages of using magnetic measurements are that magnetic minerals are almost ubiquitous and magnetic measurements are quick and non-invasive.

History

Environmental magnetism was first identified as a distinct field in 1978 and was introduced to a wider audience by the book ''Environmental Magnetism'' in 1986. Since then it has grown rapidly, finding application in and making major contributions to a range of diverse fields, especially paleoclimate, sedimentology, paleoceanography, and studies of particulate pollution.

Fundamentals

Environmental magnetism is built on two parts of rock magnetism:

, which looks at how basic magnetic properties depend on composition; and magnetic hysteresis, which can provide details on particle size and other physical properties that also affect the hysteresis. Several parameters such as

magnetic susceptibility In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...

and various kinds of

remanence Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...

have been developed to represent certain features of the hysteresis. These parameters are then used to estimate mineral size and composition. The main contributors to the magnetic properties of rocks are the iron oxides, including magnetite, maghemite,

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

; and iron sulfides (particularly greigite and pyrrhotite). These minerals are strongly magnetic because, at

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

, they are magnetically ordered (magnetite, maghemite and greigite are ferrimagnets while hematite is a canted antiferromagnet). To relate magnetic measurements to the environment, environmental magnetists have identified a variety of processes that give rise to each magnetic mineral. These include erosion, transport, fossil fuel combustion, and bacterial formation. The latter includes extracellular precipitation and formation of

magnetosomes Magnetosomes are membranous structures present in magnetotactic bacteria (MTB). They contain iron-rich magnetic particles that are enclosed within a lipid bilayer membrane. Each magnetosome can often contain 15 to 20 magnetite crystals that form a ...

by magnetotactic bacteria.

Applications

Paleoclimate

Magnetic measurements have been used to investigate past climate. A classic example is the study of

loess Loess (, ; from german: Löss ) is a clastic, predominantly silt-sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits. Loess is a periglacial or aeolian ...

, which is windblown dust from the edges of glaciers and semiarid desert margins. In north-central China, blankets of loess that were deposited during glacial periods alternate with paleosols (fossil soils) that formed during warmer and wetter interglacials. The

profiles of these sediments have been dated using magnetostratigraphy, which identifies geomagnetic reversals, and correlated with climate indicators such as oxygen isotope stages. Ultimately, this work allowed environmental magnetists to map out the variations in the monsoon cycle during the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

. Magnetic measurements of lacustrine sediments can also be used to reconstruct the upland surface processes that were associated with past climate.

Notes

References