HR: 10:20h
AN: GP41D-01    [PDF]
TI: The Sedimentary, Isotopic, and Magnetostratigraphic Signature of a Pair of Pre-Sturtian ($\sim$850 Ma) Rapid True Polar Wander (TPW) Events in the Akademikerbreen Group, NE Svalbard
AU: * Maloof, A C
EM: maloof@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University 20 Oxford Street, Cambridge, MA 02138 United States
AU: Halverson, G P
AF: Department of Geology, University of Namibia, Windhoek, 9000 Namibia
AU: Kirschvink, J L
AF: Division of Geological and Planetary Sciences, California Institute of Technology 1200 E. California Blvd, Pasadena, CA 91125 United States
AB: Since the 1950's, rapid true polar wander (TPW) has been recognized as a physically viable process in which the entire silicate shell of the Earth may shift as much as 90$^{\circ}$ with respect to the spin axis in 10$^{6}$-10$^{7}$ years. Recently, numerous studies have suggested that rapid TPW occurred repeatedly in the latest Proterozoic and Cambrian and may have influenced patterns of climate change and the evolutionary radiation of animals. However, analyses have been plagued by uncertainties in the age, quality, and correlation of paleomagnetic poles collected from various types of rocks on different continents. Using an integrated sedimentary, isotopic, and paleomagnetic approach, we document two possible rapid TPW events of $>$ 50$^{\circ}$ magnitude in the pre-Sturtian carbonate-dominated succession of East Svalbard. The nature of our carbonate magnetostratigraphy, extending through 650 m of coherent section and reproduced at five locations spread out over 160 km on a single craton, ensures that correlation, secular variation, inclination shallowing, tectonic rotation, and uncertainty of paleohorizontal do not contribute significant error. Rigorous AF and thermal demagnetization procedures and at least one semi-quantitative field test constrain each reported paleomagnetic result to date from the time of sedimentation or early diagenesis. Calculated paleomagnetic poles fall on a single great circle swath which is compatible with the occurrence of two rapid TPW events centered around a pole located in Ungava, Canada (after restoration of Eastern Svalbard to the northeastern margin of Laurentia). The older TPW event is marked by a karstic surface and a crash in $\delta^{13}$C from +6 to -1 per mil PDB. Anomalously low $\delta^{13}$C values are maintained for $\sim$300 meters before they rise again sharply, at a second sequence boundary, to the strongly positive values characteristic of the rest of the Akademikerbreen Group. The younger TPW event is constrained to have occurred either within the isotope stage or very shortly thereafter, and likely corresponds to the sequence boundary that delineates the end of the low $\delta^{13}$C interval. We tentatively interpret the stratigraphic and isotopic data in terms of the 'methane-fuse' hypothesis (Kirschvink and Raub, 2003), where TPW-induced sea level changes are manifested as sequence boundaries on continental shelves and destabilize frozen clathrates stored on continental margins, thus contributing isotopically light carbon to the ocean-atmosphere system. Because a rapid TPW event will affect every continent differently but predictably (depending on the continent's changing position relative to the Earth's spin axis), the TPW hypothesis is testable. The well-defined carbon-isotopic stage containing the rapid TPW event in Svalbard is matched in the Shaler Group of Arctic Canada and the Bitter Springs Formation of central Australia. Following correllations made by Walter et al. (2000), radiometric dates from Australia suggest that the isotopic stage ended by 830 Ma. In order to test the TPW hypothesis, we make model predictions about the sedimentary, chemostratigraphic, and paleomagnetic patterns we expect to see in contemporaneous basins of East Greenland, Arctic Canada, and Central Australia.
DE: 1520 Magnetostratigraphy
DE: 1527 Paleomagnetism applied to geologic processes
DE: 4870 Stable isotopes
DE: 9315 Arctic region
DE: 9619 Precambrian
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting