title: Quantified abundance of magnetofossils at the Paleocene-Eocene boundary from synchrotron-based transmission X-ray microscopy.
abstract: The Paleocene-Eocene boundary (∼55.8 million years ago) is marked by an abrupt negative carbon isotope excursion (CIE) that coincides with an oxygen isotope decrease interpreted as the Paleocene-Eocene thermal maximum. Biogenic magnetite (Fe3O4) in the form of giant (micron-sized) spearhead-like and spindle-like magnetofossils, as well as nano-sized magnetotactic bacteria magnetosome chains, have been reported in clay-rich sediments in the New Jersey Atlantic Coastal Plain and were thought to account for the distinctive single-domain magnetic properties of these sediments. Uncalibrated strong field magnet extraction techniques have been typically used to provide material for scanning and transmission electron microscopic imaging of these magnetic particles, whose concentration in the natural sediment is thus difficult to quantify. In this study, we use a recently developed ultrahigh-resolution, synchrotron-based, full-field transmission X-ray microscope to study the iron-rich minerals within the clay sediment in their bulk state. We are able to estimate the total magnetization concentration of the giant biogenic magnetofossils to be only ∼10% of whole sediment. Along with previous rock magnetic studies on the CIE clay, we suggest that most of the magnetite in the clay occurs as isolated, near-equidimensional nanoparticles, a suggestion that points to a nonbiogenic origin, such as comet impact plume condensates in what may be very rapidly deposited CIE clays.
authors.affiliation: Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139;
authors.forename: Huapei
authors.initials: H
authors.lastname: Wang
authors.affiliation: Photon Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Jun
authors.initials: J
authors.lastname: Wang
authors.affiliation: Photon Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973;
authors.forename: Yu-Chen Karen
authors.initials: YC
authors.lastname: Chen-Wiegart
authors.affiliation: Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854; Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 dvk@rutgers.edu.
authors.forename: Dennis V
authors.initials: DV
authors.lastname: Kent
mesh_terms: D001419:Bacteria / Q000737:chemistry* / Q000648:ultrastructure*; D005580:Fossils / Q000648:ultrastructure*; D055944:Magnetosomes / Q000737:chemistry* / Q000648:ultrastructure*; D008853:Microscopy; D010103:Oxygen Isotopes; D017356:Synchrotrons; D014965:X-Rays
chemical_list: D010103:Oxygen Isotopes
keywords: Marlboro Clay; New Jersey Atlantic Coastal Plain; Ocean Drilling Program Leg 174AX; Paleocene–Eocene thermal maximum; impact plume condensate