Whole-core magnetic susceptibility can sometimes be used as a rapid and sensitive indicator of variations in the concentration of terrigenous material. We apply this approach to study the evolution of Plio-Pleistocene climatic cycles of terrigenous sedimentation at Ocean Drilling Program Site 721, on the Owen Ridge in the Arabian Sea. Aerosol and sediment studies have shown that terrigenous sedimentation on the Owen Ridge is dominated by variations in the supply of eolian dust borne by summer southwest monsoon winds. Terrigenous extraction analyses of the Site 721 sediments show that magnetic susceptibility is a conservative and sensitive tracer of the terrigenous (eolian) fraction variations at Site 721. Spectral analysis of the susceptibility time series spanning the last 3.2 Myr show the record varies strongly at earth orbital periodicities. Prior to about 2.4 Myr, the record varies predominantly at the 23 and 19 kyr periodicities, corresponding to orbital precession; after 2.4 Myr it shows a significant increase in 41-kyr power corresponding to orbital obliquity. This shift coincides with the rapid expansion of Northern Hemisphere ice sheets at about 2.37 Myr. General circulation model experiments and paleoclimatological evidence from northeast Africa suggest the increase in 41-kyr power after 2.4 Myr may be reflecting periodic increases in monsoon dust source area aridity associated with the coeval expansion of Northern Hemisphere ice sheets, which varied predominantly at this periodicity.