Discovery of Giant Magnetofossils Within and Outside of the Palaeocene-Eocene Thermal Maximum in the North Atlantic

Giant magnetofossils have exceptionally large grain sizes and peculiar morphologies compared to conventional biogenic magnetite nanoparticles. The origin of these unusual magnetic crystals is a mystery because there are no known modern analogues. Giant magnetofossils have so far been identified in marine sediments deposited during past warming periods, leading to the assumption that these fossils were uniquely tied to ancient hyperthermal events. Here we describe the occurrence of abundant giant magnetofossils within North Atlantic pelagic sediments from International Ocean Discovery Program (IODP) Sites U1403 and U1409 at distinct palaeodepths not only during the Palaeocene-Eocene Thermal Maximum (PETM; ∼56 Ma) intervals but also far before (>700 ky earlier than the PEB, Palaeocene-Eocene boundary) and after (>300 ky later than the PEB) the PETM, and in a sample of ∼70 Ma age. Our results indicate that giant biogenic magnetite crystals were not uniquely produced during ancient hyperthermal events. Magnetic domain states of giant magnetite particles are investigated using dimensional analysis and micromagnetic simulations. Morphological, compositional, and crystallographic data point towards a potential biogenic origin of those unusual crystals.