Superparamagnetic grains, ultrafine (<50 nm) particles that do not retain a magnetic remanence at room temperature, can form through several biogeochemical pathways in marine sediments. Knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change. Herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic (SP) grains. We start with a phenomenological model that describes a dependency of coercivity of remanence (Hcr) with measurement time as a function of superparamagnetic grain content. Calculated time dependencies of Hcr agree with experimental data, suggesting that the model can be used to derive quantitative SP estimates. When applied to pelagic sediments of the western equatorial Pacific Ocean (Ocean Drilling Program Site 805C); the time dependencies of Hcr reveal a 25--30% SP increase starting just above the modern iron redox boundary (IRB). Next, we compare our approach with a previous method that detects SP grains through the thermal demagnetization of saturation remanence acquired at very low temperature (20 K). We find this method fails to record the SP changes detected using the time dependency of Hcr and suggest that the cause is a sensitivity of the low-temperature data to small differences in magnetic mineralogy that accompany diagenesis. Specifically, differential maghemitization causes varying suppression of the remanence changes associated with the Verwey transition at ~120 K. The removal of maghemite shells on magnetic grains below the IRB results in greater remanence changes at the Verwey transition and a corresponding SP underestimate using the low-temperature method. In addition, some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures, resulting in an SP overestimate in oxic sediments above the IRB. The increase of SP content in the zone of magnetic mineral reduction observed with the time dependency of Hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron-reducing bacteria. Superparamagnetism measured by the time dependency of Hcr may be useful for tracking the history of these biogeochemical processes in marine sediments. ¿ 2001 American Geophysical Union