HR: 11:20h
AN: GP42A-05    [Abstracts]
TI: Evidence for Two New Paleomagnetic Field Excursions ~2,500 and ~12,500 Years Ago from the South Pacific Ocean Region (Tahiti)
AU: * Lund, S P
EM: slund@usc.edu
AF: University of Southern California, University Park, Los Angeles, CA 90089-0740, United States
AU: Platzman, E
EM: platzman@usc.edu
AF: University of Southern California, University Park, Los Angeles, CA 90089-0740, United States
AU: Thouveny, N
EM: thouveny@cerege.fr
AF: CEREGE CNRS, Europole Mediterraneen de l Arbois, Aix-en Provence, Cedex 4, France
AU: Camoin, G
EM: camoin@cerege.fr
AF: CEREGE CNRS, Europole Mediterraneen de l Arbois, Aix-en Provence, Cedex 4, France
AB: Paleomagnetic measurements have been made on 250 oriented samples from IODP Expedition 310 Sites 5, 7, 15-18 in the MARAA region of the Tahiti coral reef tract. The paleomagnetic inclinations (unoriented cores preclude declination measurements) average -31.1 degrees with an approximate 95 percent confidence interval of 2.8 degrees. This value is not significantly different from the axial-dipole expected inclination for this region of - 32.6 degrees. We believe that the paleomagnetic inclinations reflect the actual geomagnetic field values at this region over time. The directions are due to detrital magnetic grains (less than 1 μm to ~10 μm) from the Tahiti Island volcanics being washed out into the coral reef, oriented with the ambient field in quiet water conditions within the reef, and locked into place by primarily microbial precipitation of carbonate (95 percent of the samples come from the microbialite component of the reef). Relative paleointensity measurements have been made as well by normalizing the demagnetized natural remanence to magnetic susceptibility and demagnetized isothermal remanence. Both the inclination and relative paleointensity variability in each hole/site can be correlated with variability at the other independent holes/sites. Moreover, we can correlate this paleomagnetic variability with other sites from the Pacific Ocean Basin that surround Tahiti. This permits us to estimate a chronostratigraphy for the postglacial MARAA coral reef tract that extends from about 18,000 years ago to the present. This also lets us estimate the spatial pattern and timing of MARAA coral reef development over the last 18,000 years. We note, as well, the existence of 10 inclinations that average +30 degrees, almost antipodal to the rest of the inclinations. These anomalous inclinations are consistently located at two horizons in independent holes. Four anomalous inclinations from four different holes all occur at about 2,500 years ago and six anomalous inclinations from six different holes occur about 12,500 years ago. Our working hypothesis is that these are magnetic field excursions, which have never been recorded elsewhere. Yet, these two excursions occur at the two times of most anomalous paleomagnetic directions noted previously from sites that surround Tahiti: Indonesia, New Zealand, Hawaii, and Chile.
DE: 1507 Core processes (1213, 8115)
DE: 1513 Geomagnetic excursions
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting