HR: 0800h
AN: GP21B-03    [Abstracts]
TI: Deposition, Diagenetic, or Bacterial Origin of Magnetite in Sediment Drifts From the Western Antarctic Peninsula
AU: * Dulgar, S E
EM: stephaniedulgar@yahoo.com
AF: Department of Geology, Southern Illinois University Carbondale, 1259 Lincoln Drive, Carbondale, IL 62901, United States
AU: Brachfeld, S
EM: brachfelds@mail.montclair.edu
AF: Department of Earth and Environmental Studies, Montclair State University, 1 Normal Avenue, Montclair, NJ 07043, United States
AU: Ishman, S
EM: sishman@siu.edu
AF: Department of Geology, Southern Illinois University Carbondale, 1259 Lincoln Drive, Carbondale, IL 62901, United States
AU: Ferré, E C
EM: eferre@geo.siu.edu
AF: Department of Geology, Southern Illinois University Carbondale, 1259 Lincoln Drive, Carbondale, IL 62901, United States
AB: A 607 meter sediment core from the western Antarctic Peninsula was collected during Ocean Drilling Program (ODP) Leg 178 at Site 1096. An abrupt shift, first recognized during the construction of the geomagnetic reversal stratigraphy, is observed in the magnetic mineral assemblage in core 1096A at 18 meters below seafloor (mbsf). This magnetic shift led us to question the fidelity of the geomagnetic reversal record. We evaluated the magnetic mineral assemblage using rock magnetic and electron microscopy techniques. Magnetic susceptibility showed variable concentration of magnetic material with depth, but not an abrupt change in concentration at 18 mbsf. Hysteresis data reveal a grain size shift at 18 mbsf. Magnetic particle size measurements show that with increasing depth the magnetic grain size increases. One possible explanation for the grain size shift is the presence of bacterial magnetite above 18 mbsf. However, TEM analysis did not conclusively prove the presence of magnetosomes. TEM images show grains in the 20-60 nm size range, but these grains were not in chains and they do not have the typical morphologies associated with bacterial magnetite. It is possible that these ultra fine grains are authigenic. Dissolution of these ultra fine magnetic particles could cause the grain size shift, with the smaller grains dissolving below18 mbsf. Temperature dependent FORC analyses are in progress to further characterize the particle size distribution above and below 18 mbsf. Curie temperature analyses indicate that magnetite is present above and below 18 mbsf. A magnetic iron sulfide with a Curie temperature of 350° C is present above 18 mbsf. X-ray microanalysis results agree with Curie temperature analyses, and show the presence of both Ti-poor iron oxides and iron sulfides in the magnetic extract. The presence of magnetic iron sulfides above 18 mbsf suggests iron-sulfur diagenesis is occurring with pyrrhotite preserved above 18 mbsf, and non-magnetic pyrite present below 18 mbsf. This could explain why the bottom of the core has a weak-amplitude natural remnant magnetization and a noisy geomagnetic reversal record.
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
DE: 9310 Antarctica (4207)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly