GP004-01
In-situ identification of giant, needle-shaped magnetofossils in Paleocene-Eocene Thermal Maximum Sediments using high-resolution, low-noise FORC measurements

Monday, 14 December 2020: 08:31
Virtual
Peter C Lippert1,2, Courtney Wagner1, Ramon Egli3, Ioan Lascu4, Ken Livi5 and Helen Sears6, (1)University of Utah, Department of Geology and Geophysics, Salt Lake City, UT, United States, (2)University of Utah, Global Change and Sustainability Center, Salt Lake City, UT, United States, (3)Zentralanstalt für Meteorologie und Geodynamik, Wien, Austria, (4)Smithsonian Institution, Washington DC, United States, (5)The Johns Hopkins University, Baltimore, United States, (6)Colby College, Department of Geology, Waterville, ME, United States
Recorded Presentation
Abstract:
Near shore marine sediments deposited during the Paleocene-Eocene Thermal Maximum at Wilson Lake, New Jersey contain abundant conventional and giant magnetofossils. Giant magnetofossils are the preserved remains of iron-biomineralizing organisms that so far have been identified only in sediments that were deposited during Eocene hyperthermal events. Before now, the identification of giant magnetofossils required a combination of expensive and destructive techniques. 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. FORC diagrams of marine sediments containing abundant conventional magnetofossils are typically associated with a distinct central ridge, a narrow region of strong FORC amplitudes between Bc = ~10 and 120 mT. Our results show a high-coercivity magnetofossil contribution to the central ridge beyond the coercivity range of conventional magnetofossils. In the lower half of the FORC diagram this component extends above the limit for reversible magnetic moment rotation in conventional magnetofossils, reaching Bc = 210 mT. This component is associated with an unusually large vertical offset of the central ridge, which reaches the peak value of ~0.7 mT at Bc = 170 mT. These are the first reported magnetic measurements on bulk sediment samples that uniquely identify the presence of giant, needle-shaped magnetofossils. Micromagnetic simulations of giant needle morphologies, measured from transmission electron micrographs of magnetic extracts from Wilson Lake sediments, independently suggest that giant needles have single domain characteristics and large magnetic coercivities associated with extreme crystal elongation. Our results provide a non-destructive method for identifying giant magnetofossil assemblages in bulk sediments and a way to test their ecological significance with respect to environmental change.
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