FE-OXIDE MINERALS IN SPELEOTHEMS AND CAVE SEDIMENTS AS RECORDERS OF GEOMAGNETIC AND ENVIRONMENTAL HISTORY

Fe-oxide minerals within karstic environments accumulate in sufficient quantities within speleothems and cave sediments to offer unique opportunities to uncover high-resolution records of geomagnetic field behavior and environmental change through time. Records of geomagnetic field behavior are most frequently held by assemblages of magnetite and maghemite, whose grain size distributions match those commonly found in modern soils. Hematite and goethite may also contribute to magnetic recordings in karstic materials, but typically hold only a small fraction of the remanence held by magnetite. Depending on the growth history of a speleothem or depositional history of a clastic cave sediment, such materials are capable of accurately recording short- and long-term geomagnetic phenomena, including geomagnetic excursions, reversals, and secular variation.

Fe-oxides in speleothems and cave sediments can also be used to delineate environmental change. Magnetic methods are exquisitely sensitive to changes in both the relative concentration of specific oxide minerals, as well as their grain size distributions. The ultimate delivery of Fe-oxides into karst environments is dependent on factors such as mean annual precipitation and temperature, seasonality of precipitation, localized flooding, dustiness, and the extent of soil cover and vegetation immediately above a karst system. Several studies have been able to link populations of magnetic minerals to specific surface processes, such as pedogenesis and extreme rainfall events. In several instances, we have observed a lag between the onset of an environmental perturbation at the surface and the expression of the perturbation in a speleothem or cave sediment, which has important implications for comparative paleoclimate studies.

This presentation will review many of the recent geophysical and paleoclimate advances that have been made by measuring the magnetic properties of cave materials.