A rapid method for magnetic granulometry with applications to environmental studies

We have developed a rapid method of characterizing the fine‐scale variation (sub‐micron to hundreds of microns, µm) in the grain size of magnetite in long sediment cores. This method is a new rock magnetic tool and can be readily applied to the following disciplines: (1) environmental studies; (2) hydrology; (3) petrology; and (4) geomagnetism. The method consists of a comparative plot of the depth variation of two magnetic parameters. The first parameter, low field (less than 0.1 millitesla, mT, or 1 Oe) susceptibility in a steady field is plotted on the abscissa. The second parameter, anhysteretic remanent magnetization (ARM) imparted by a smoothly decreasing AF of peak value 100 mT (1,000 Oe) in the presence of a weak steady field (0.01‐0.1 mT, or 0.1‐1 Oe) is plotted on the ordinate. For a sediment column in which the magnetic mineral is magnetite, a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points (χ, ARM).