HR: 1340h
AN: GP43B-1223    [Abstracts]
TI: Extending the Sensitivity of Paleomagnetic Techniques: Magnetostratigraphy of Weakly- Magnetized, Organic-Rich Black Limestone from the Permian of Japan
AU: * Kirschvink, J L
EM: kirschvink@caltech.edu
AF: Geological & Planetary Sciences, California Institute of Technology 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Isozaki, Y
EM: isozaki@ea.c.u-tokyo.ac.jp
AF: Dept. Earth Science & Astronomy, The University of Tokyo 3-8-1 Komaba, Meguro, Tokyo, 153-8902, Japan
AB: Despite their importance for biostratigraphy, bituminous, organic-rich limestone has rarely yielded reliable paleomagnetic results, principally due to the weak magnetization levels encountered; NRMs for such samples often start with moments below 10^-10Am² (10^-7 emu), and they become unmeasurable or unstable rapidly upon demagnetization. Recently, we have developed a fairly simple and inexpensive automation system for standard paleomagnetic and rock magnetic measurements that has the additional benefit of using an extremely low-noise sample holder. A vacuum pick-and-put system holds samples at the end a thin-walled quartz-glass tube, moving them between a measurement queue and the sense region of a vertically-aligned superconducting rock magnetometer. The system minimizes the amount of extraneous material introduced into the magnetometer's sense region. With frequent cleaning in strong acid, the glass holder system is capable of reliably measuring samples with moments below 1 pAm² (10^-9 emu), on a DC-SQuID magnetometer system with stable instrument noise of a few hundred fAm². Signal averaging and clean-lab handling techniques are often needed to achieve this sensitivity, however. To test this system, we collected a series of densely-spaced samples for magnetostratigraphy from two sections in the Middle-Late Permian limestone at Kamura in Kyushu, Japan, which is the remnant of lagoonal sediments formed on an oceanic island atoll (Isozaki et al., 2007). According to fusulinids, the Saraito section spans Wordian time, which should be near the end of the Kiaman Reversed superchron. Similarly, strata at the Kamura section cover late Capitanian to early Wuchiapingian time, which should display a shift from predominantly Normal to Reversed magnetization. Raman spectroscopy suggests no appreciable heating. NRM intensities of the samples range generally between 10^-10 and 10^-11 Am², with little change in the vectors either upon low-temperature cycling or alternating-field demagnetization up to 7 mT. However, thermal demagnetization to temperatures as low as 75 ° C in an N₂ atmosphere often removes a present-field component, revealing a two-polarity NW/SE shallow magnetization; further thermal cleaning at 10 to 15 ° C intervals up to ~ 250 to 300 ° C allows proper isolation of this characteristic component in about 2/3^rds of the specimens analyzed. When viewed stratigraphically, these data agree remarkably well with the magnetic reversal time scales, and imply formation at about 12° South latitude. MAD values from the principal component analysis, plotted against the intensity of the recovered component, indicate that the system is able to recover reliable data for characteristic components that are initially as weak as 10 pAm² (10^-8 emu). Continued advances in the sensitivity of rock magnetometers could open many additional types of weakly- magnetized sediments to paleomagnetic and magnetostratigraphic analysis. Isozaki, Y., Kawahata, H., and Minoshima, K. (2007). Palaeoworld 16, pp16-30.
DE: 0410 Biodiversity
DE: 1520 Magnetostratigraphy
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting