In environmental magnetism, rock and mineral magnetic techniques are used to investigate the formation, transportation, deposition, and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes. All materials respond in some way to an applied magnetic field, and iron‐bearing minerals are sensitive to a range of environmental processes, which makes magnetic measurements extremely useful for detecting signals associated with environmental processes. Environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics, chemistry, and biology and environmental science, including research on climate change, pollution, iron biomineralization, and depositional and diagenetic processes in sediments to name a few applications. Magnetic parameters are used to routinely scan sediments, but interpretation is often difficult and requires understanding of the underlying physics and chemistry. Thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities, complexities, and limitations to interpretations. In this review, we evaluate environmental magnetic parameters based on theory and empirical results. We describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations. We also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology, chemistry, and physics. Finally, we discuss directions in which environmental magnetism is likely to develop in the future.

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A theoretical model of single‐domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two‐domain configuration takes place at the SD threshold d0. This two‐domain configuration is shown to be more applicable to fine‐grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superparamagnetic threshold size ds by Néel's relaxation equation and calculating the SD threshold d0 at which SD to two‐domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds ≃ 0.05 μm, and d0 ≃ 0.076 μm. For cubic magnetite particles just above d0 a two‐domain configuration is predicted in which a 180° domain wall occupies ∼60% of the particle volume. No SD range exists for cubic magnetites at T > 450°K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo‐single‐domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length : width ratio of 5 : 1, SD limits of ds ≃ 0.05 μm and d0 ≃ 1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3−x Tix04) increase with Ti content. For cubic titanomagnetites of x = 0.6, ds ≃ 0.08 μm, and d0 ≃ 0.3 μm. Comparison of the calculated SD range with the available high‐resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks.

The hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization (DRM/ARM) for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches. One approach is a detailed rock‐magnetic examination of the implicit assumptions of the DRM/ARM method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions. Sediments are ‘uniform’ with respect to DRM/ARM ratio if they contain magnetite in the 1–15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20–30 times the minimum concentration. The DRM/ARM ratios of these sediments should provide estimates of relative geomagnetic paleointensity. Relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility (XARM) versus low‐field susceptibility (X) and the size range ≃ 1–15 μm is approximately identified by high‐field hysteresis parameters. A rock‐magnetic evaluation of LeBoeuf Lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study. The second approach to evaluating the DRM/ARM ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data. A comparison between the LeBoeuf Lake estimates and Thellier‐Thellier results from the western United States supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations.