Biologically formed nanoparticles of the strongly magnetic mineral, magnetite, were first detected in the human brain over 20 y ago [Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ (1992) Proc Natl Acad Sci USA 89(16):7683-7687]. Magnetite can have potentially large impacts on the brain due to its unique combination of redox activity, surface charge, and strongly magnetic behavior. We used magnetic analyses and electron microscopy to identify the abundant presence in the brain of magnetite nanoparticles that are consistent with high-temperature formation, suggesting, therefore, an external, not internal, source. Comprising a separate nanoparticle population from the euhedral particles ascribed to endogenous sources, these brain magnetites are often found with other transition metal nanoparticles, and they display rounded crystal morphologies and fused surface textures, reflecting crystallization upon cooling from an initially heated, iron-bearing source material. Such high-temperature magnetite nanospheres are ubiquitous and abundant in airborne particulate matter pollution. They arise as combustion-derived, iron-rich particles, often associated with other transition metal particles, which condense and/ or oxidize upon airborne release. Those magnetite pollutant particles which are <∼200 nm in diameter can enter the brain directly via the olfactory bulb. Their presence proves that externally sourced iron-bearing nanoparticles, rather than their soluble compounds, can be transported directly into the brain, where they may pose hazard to human health.brain magnetite | magnetite pollution particles | Alzheimer's disease | combustion-derived nanoparticles | airborne particulate matter

The grain size dependence of various mineral (rock) magnetic parameters has been determined, using a series of essentially pure, fine-grained (single-domain, SD) and ultrafine-grained (superparamagnetic, SP) magnetites. The parameters measured include low-field susceptibility (x), frequency-dependent x(xFD), saturation remanence (SIRM), anhysteretic susceptibility (xARM), and coercivity of remanenceThe magnetites were produced in experiments designed to simulate possible pedogenic and biogenic pathways of magnetite formation. Their mean grain sizes range from 0.012 p m to 0.06 pm, and hence span the SP/SD boundary. Isothermal magnetic measurements were performed on two separate subsets of differing packing densities. The response of the magnetic parameters is modified by interaction effects, but they display continuous variation across the entire grain size range, confirming their value for rapid magnetic granulometry. Within the fine and ultrafine end of the magnetite grain size spectrum, x, xFD and xARM are notably responsive to grain size change. In terms of magnetic response (and also possibly of grain size, shape, and absence of cation substitution), these synthetic magnetites represent close analogues of those found in some soils and sediments.

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S U M M A R YThe theory, measurement and interpretation of frequency-dependent susceptibility ( xFD) are examined. A new model is proposed which explains xFD in terms of the behaviour of all superparamagnetic grains (SP) with diameters between 0 and ~0 . 0 3 pm. The model predicts maximum xFD percentage values of 14-17 per cent for spherical SP ferrimagnetic grains in the grain-size range 0.01-0.025 pm, and a maximum value of 10-12 per cent for grain assemblages spanning a wider range of grain sizes (0-0.03 pm). Synthetic and experimental data support the model predictions in terms of both maximum xFD percentage values and the relationship between xFD percentage and mass specific xFD, which exhibits an envelope of data points partly related to grain-size distributions within the SP range. When the xFD percentage is at a maximum, the mass specific xFD term can be used to estimate the concentration of SP grains in a sample.Lower values of xFD percentage in soils are caused by the presence of narrow distributions of ultrafine SP grains, frequency-independent stable single and multidomain ferrimagnetic grains. Some soils with low susceptibilities may have low xFD percentages because of an appreciable content of paramagnetic and canted antiferromagnetic minerals. A simple mixing model predicts proportions of SP grains in mixed grain assemblages, but model validation requiring further characterization of grain interaction and grain-size distributions is needed before it can be applied to environmental data.

Palaeo-dust records in sediments and ice cores show that wind-borne mineral aerosol ('dust') is strongly linked with climate state. During glacial climate stages, for example, the world was much dustier, with dust fluxes two to five times greater than in interglacial stages. However, the influence of dust on climate remains a poorly quantified and actively changing element of the Earth's climate system. Dust can influence climate directly, by the scattering and absorption of solar and terrestrial radiation, and indirectly, by modifying cloud properties. Dust transported to the oceans can also affect climate via ocean fertilization in those regions of the world's oceans where macronutrients like nitrate are abundant but primary production and nitrogen fixation are limited by iron scarcity. Dust containing iron, as fine-grained iron oxides/oxyhydroxides and/or within clay minerals, and other essential micronutrients (e.g. silica) may modulate the uptake of carbon in marine ecosystems and, in turn, the atmospheric concentration of CO2. Here, in order to critically examine past fluxes and possible climate impacts of dust in general and iron-bearing dust in particular, we consider present day sources and properties of dust, synthesise available records of dust deposition at the last glacial maximum (LGM); evaluate the evidence for changes in ocean palaeo-productivity associated with, and possibly caused by, changes in aeolian flux to the oceans at the LGM; and consider the radiative forcing effects of increased LGM dust loadings.