Variations in the Earth's magnetic field arising from local concentrations of shallow subsurface magnetite were mapped in the Arctic National Wildlife Refuge and elsewhere in northern Alaska. The anomalies were delineated with a magnetic horizontal gradiometer mounted on a low-flying (300 ft or approximately 90 m above ground) fixed-wing airplane. Limited data from stable carbon-isotope and remanent magnetism measurements of rock cores from the Cape Simpson region strongly suggest that the magnetic anomalies result from the chemical reduction of iron oxides in the presence of seeping hydrocarbons. The magnetic contrast between sedimentary rocks of normally low magnetic susceptibility and those locally enriched with this epigenetic magnetite results in distinctive high-wavenumber and low-amplitude total-field anomalies. Magnetometers extended from each wing tip and in a tail stinger permit calculation of the resultant horizontal gradient vector relative to the flight path. This calculation allows more meaningful interpolation of data for the unsurveyed area between adjacent flight lines spaced at 1.0 mile (1.6 km), thereby allowing generation of accurate computer-enhanced images or maps. Problems related to diurnal variations and solar storms at high magnetic latitudes are largely overcome because changes in total magnetic field do not significantly affect the magnetic gradient. Analysis of an experimental survey, covering 2745 line-miles (4418 line-km), reveals numerous anomalies, the most prominent of which parallels the Marsh Creek anticline. The data provide further evidence that the Marsh Creek anticline is prospective for oil and/or gas. Although the effect of permafrost on epigenetic processes has not been investigated, the data suggest that special-purpose aeromagnetic surveying may be a useful and inexpensive way to explore for oil and gas in this harsh environment.