In situ magnetic identification of giant, needle-shaped magnetofossils in Paleocene–Eocene Thermal Maximum sediments

Abstract

Near-shore marine sediments deposited during the Paleocene–Eocene Thermal Maximum at Wilson Lake, NJ, contain abundant conventional and giant magnetofossils. We find that giant, needle-shaped magnetofossils from Wilson Lake produce distinct magnetic signatures in low-noise, high-resolution first-order reversal curve (FORC) measurements. These magnetic measurements on bulk sediment samples identify the presence of giant, needle-shaped magnetofossils. Our results are supported by micromagnetic simulations of giant needle morphologies measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments. These simulations underscore the single-domain characteristics and the large magnetic coercivity associated with the extreme crystal elongation of giant needles. Giant magnetofossils have so far only been identified in sediments deposited during global hyperthermal events and therefore may serve as magnetic biomarkers of environmental disturbances. Our results show that FORC measurements are a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their ecology and significance with respect to environmental change. Significance Giant magnetofossils are the preserved remains of iron-biomineralizing organisms that have so far been identified only in sediments deposited during ancient greenhouse climates. Giant magnetofossils have no modern analog, but their association with abrupt global warming events links them to environmental disturbances. Thus, giant magnetofossils may encode information about nutrient availability and water stratification in ancient aquatic environments. Identification of giant magnetofossils has previously required destructive extraction techniques. We show that giant, needle-shaped magnetofossils have distinct magnetic signatures. Our results provide a nondestructive method for identifying giant magnetofossil assemblages in bulk sediments, which will help test their significance with respect to environmental change.