HR: 17:30h
AN: B24A-07    [Abstracts]
TI: On the Origin of Magnetization in Platformal Carbonate Muds
AU: * Maloof, A C
EM: maloofa@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Grotzinger, J P
EM: grotz@MIT.EDU
AF: California Institute of Technology, Department of Geological and Planetary Sciences 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Kopp, R E
EM: rkopp@caltech.edu
AF: California Institute of Technology, Department of Geological and Planetary Sciences 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Weiss, B P
EM: bpweiss@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Vali, H
EM: vali@eps.mcgill.ca
AF: McGill University, Department of Earth and Planetary Sciences 3450 University St., Montreal, QB H3A 2A7 Canada
AU: Kirschvink, J L
EM: kirschvink@caltech.edu
AF: California Institute of Technology, Department of Geological and Planetary Sciences 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: Carbonate muds deposited in peritidal environments have high preservation potential and are abundant throughout the geologic record. Much of what we know about pre-Mesozoic ocean chemistry, carbon cycling, and global change is derived from isotope and trace element geochemistry of platform carbonates. Magnetostratigraphic data from the same sediments would be invaluable, placing records of paleolatitude, paleogeography, and perturbations to the geomagnetic field in context with chemostratigraphy. Many workers, however, have questioned the origin of magnetization in carbonates, suggesting that much of the magnetite found in ancient carbonates may have been modified during burial diagenesis or precipitated by migrating pore-fluids millions of years after deposition. As part of a larger project designed to study sedimentary and hydrological processes on the wind-dominated northwestern edge of Andros Island, Bahamas, we conducted a paleomagnetic and rock magnetic survey of peritidal, often algally microbially bound carbonate muds. All oriented samples of carbonate mud in various stages of cementation preserve a stable natural remanent magnetization (NRM) indistinguishable from the local geomagnetic field observed over the last five years. We found a strong correlation between carbonate facies and NRM intensity, with larger NRMs being more common in carbonate muds bound by Scytonema Algaecyanobacterial filaments. Preliminary results of selected samples suggest that stoichiometric, euhedral single domain magnetite, similar in size and shape to bacterial magnetite, dominates rock magnetic and electron microscope surveys of all but the most bioturbated sediments. So far, no evidence for detrital, multi-domain magnetic minerals from Saharan dust has been found. Vertical cores through the entire interval of Holocene mud (0.5-3.0 m) revealed abrupt changes in rock magnetic properties at mean tide level, where a visible Fe-redox boundary and a major increase in bioturbation also is apparent. More detailed work on the chemistry and mineralogy of the entire sediment column is required to determine whether these carbonate muds are likely to form limestones that preserve an accurate record of the paleomagnetic field and to establish the origin of the magnetic phases in these sediments.
DE: 0419 Biomineralization
DE: 1505 Biogenic magnetic minerals
DE: 1519 Magnetic mineralogy and petrology
DE: 1520 Magnetostratigraphy
DE: 1540 Rock and mineral magnetism
SC: Biogeosciences [B]
MN: Fall Meeting 2005