Superparamagnetic grains, ultra-fine (d<0.05 &mgr;m) particles unable to retain a remanence at room temperature, have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks. In marine sediments, fine-grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction. On the basis of new magnetic hysteresis, low-temperature, and geochemical studies of pelagic sediments from the western equatorial Pacific Ocean, it is proposed that reduction processes increase rather than deplete the ultra-fine magnetic grain population. At the modern Fe-redox boundary enhanced superparamagnetism coincides with a coarsening of remanence-carrying grains. Superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically-driven fluctuations in organic carbon (Corg) supply. Together with hysteresis characteristics, changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady-state magnetic mineral reduction. ¿ American Geophysical Union 1995