6. INFLUENCE OF DRILLING ON TWO RECORDS OF THE MATUYAMA/BRUNHES POLARITY TRANSITION IN MARINE SEDIMENT CORES NEAR GRAN CANARIA 1

B. Herr,2 M. Fuller,3 M. Haag,4 and F. Heider 2

ABSTRACT

Two records of the Matuyama/Brunhes transition were obtained from the Ocean Drilling Program (ODP) at Sites 953 and 954, which were drilled in the volcanic apron of Gran Canaria in the Canary Islands. Rock magnetic analyses revealed magnetic carriers close to magnetite in composition and in the pseudo–single-domain grain-size range. The transitions were sampled with u-channels and with discrete samples that were taken on both sides of the u-channels covering the transitional region. Each core exhibits a stable record of the geomagnetic transition. Assuming a dipolar field, the virtual geomagnetic pole (VGP) starts at high southern latitudes and ends with well-established VGPs in high northern latitudes. However, there is a large difference of nearly 60° in the longitude of the VGP paths from the two sites. This difference is not the result of different field directions at the two localities, because the sites are only 48 km apart. The remanent magnetization of the discrete samples taken alongside the u-channels is demonstrably contaminated with a drilling-produced magnetic remanence with a horizontal component that is directed radially inward and hard to demagnetize. In addition, a much softer drill moment was observed that is directed steeply downward. The radial character of the magnetic overprint was found because the samples were taken on both sides of the u-channels. Even in the u-channels taken from the center of the core section, the horizontal radial moment appears to be present. The demagnetization characteristics of the radial moment are comparable to the natural remanent magnetization. The presence of the pervasive drill moments and their ability to introduce false transitional points in the reversal records suggest caution in the interpretation of ODP reversal records, which have not been adequately tested for the presence of such drilling-produced overprints.

1 Weaver, P.P.E., Schmincke, H.-U., Firth, J.V., and Duffield, W. (Eds.), 1998. Proc. ODP, Sci. Results, 157: College Station, TX (Ocean Drilling Program).
2 Institut für Allgemeine und Angewandte Geophysik, Ludwig-Maximilians-Universität, Theresienstrasse 41, 80333 München, Federal Republic of Germany. beru@rockmag.geophysol.und.mueuchen
3 Department of Geological Sciences, University of California, Santa Barbara, CA 93106, U.S.A. (Present address: SOEST-Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, U.S.A.).
4 Centre des Faibles Radioactivites, Laboratoire mixte CNRS-CEA, 91198 Gif-Sur-Yvette Cedex, France (Present address: Institutfür Geophysik, ETH-Hönggerberg, 8093 Zurich, Switzerland).