THE ORIGIN AND PRESERVATION OF MAGNETIZATION IN PLATFORMAL CARBONATE MUDS, ANDROS ISLAND, BAHAMAS

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. Paleomagnetic data from the same sediments would be invaluable, placing records of paleolatitude, paleogeography, and perturbations to the geomagnetic field in the context and relative chronology of 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 magnetic survey of peritidal, often microbially bound carbonate muds. All oriented samples of carbonate mud exposed at the surface preserve a stable natural remanent magnetization (NRM) indistinguishable from the local geomagnetic field observed over the last five years. Vertical cores through the entire interval of Holocene mud (0.5-3.0 m) revealed abrupt changes in rock magnetic properties and NRM at mean tide level, where a visible Fe-redox boundary and a major increase in bioturbation is apparent. We evaluate the preservation potential of paleomagnetic records in ancient carbonates by examining the affect of bioturbation and redox changes on the size, composition, organization and magnetism of magnetic minerals in surface and core environments.