Abstract
Superparamagnetic grains, ultra‐fine (d < 0.05µm) 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.
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