HR: 10:35h
AN: U42A-02 INVITED     [Abstracts]
TI: Calibration of a Pleistocene Geomagnetic Instability Time Scale (GITS) using ⁴⁰Ar/³⁹Ar-dated lavas
AU: * Singer, B S
EM: bsinger@geoogy.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison 1215 W. Dayton St., Madison, WI 53706 United States
AU: Hoffman, K A
EM: khoffman@calpoly.edu
AF: Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407 United States
AB: Advances in measuring paleomagnetic intensity recorded by marine sediments, and ⁴⁰Ar/³⁹Ar dating of paleomagnetic directional recordings in lava flows, offer a means of calibrating a global magnetostratigraphy for the last 2 m.y. This involves moving beyond the classic geomagnetic polarity time scale (GPTS) and resolving not only the undisputed polarity reversals, but also the many short-lived geomagnetic "events" or cryptochrons thought to signal brief periods of geodynamo instability. Many short events are distinguished as intensity minima in global sediment records (SINT-800; GLOPIS-75) that are dated by astrochronology. Thus, when the degree of stability of the geodynamo is considered, rather than lengths of polarity intervals, an alternative approach to the GPTS is appropriate. We are developing a Geomagnetic Instability Time Scale (GITS) via ⁴⁰Ar/³⁹Ar dating of transitionally-magnetized lava flows younger than 2 Ma. As an example, the Laschamp event--expressed as a sharp intensity minimum in the GLOPIS-75 sediment stack--was dated by matching O-isotope variations in North Atlantic sediments to those recorded in annually counted layers of the GISP2 ice core. Matching ¹⁴C ages from the sediments to specific varves in the ice core shows the paleointensity minimum to span 1500 yr between 41 and 39 ka. ⁴⁰Ar/³⁹Ar and unspiked K-Ar dating of two basaltic lava flows that record the event at Laschamps, France yield an age of 40.4±1.1 ka (± 2 sigma, analytical uncertainty). Thus, despite systematic uncertainty in the ⁴⁰K decay constant, both the accuracy and precision of the K-Ar clock can be remarkably good, i.e., better than 2% for the Pleistocene. Intercomparison of several ⁴⁰Ar/³⁹Ar-dated geomagnetic events, including the Matuyama-Brunhes polarity reversal (776 ± 2 ka), Big Lost event (579 ± 6 ka) and Pringle Falls/Albuquerque event (211 ± 11 ka) implies either that: 1) the astrochronology-based age models used for the SINT-800 paleointensity stack are inaccurate between 150 and 600 ka, or 2) the excursional directions recorded in the dated lava flows do not correlate with global lows in paleointensity.
DE: 1115 Radioisotope geochronology
DE: 1135 Correlative geochronology
DE: 1520 Magnetostratigraphy
DE: 1535 Reversals: process, timescale, magnetostratigraphy
DE: 1560 Time variations: secular and longer
SC: Union [U]
MN: Fall Meeting 2005