Brunhes Chron magnetic field excursions recovered from Leg 172 sediments

Abstract

During Ocean Drilling Program Leg 172, we recovered long-core paleomagnetic records of 14 “plausible” magnetic field excursions within the Brunhes Chron at Sites 1060–1063, separated by more than 1200 km. New U-channel paleomagnetic studies of all 14 excursions indicate that 12 have true excursional directions and are almost certainly “real” geomagnetic field excursions, but five excursions still require discrete sample paleomagnetic measurements to further replicate these results. U-channel measurements for two of the original 14 “plausible” excursions (3α and 5α) did not show evidence of true excursional directions and are no longer considered real. U-channel measurements also identified one new excursion not identified during shipboard measurements. We also identified other types of anomalous directional variability that we currently think may be due to systematic biases in the long-core and U-channel measurement process. Further study using discrete samples will be necessary to resolve such uncertainties. All of these observations suggest that excursions are not rare, random perturbations of the stable geomagnetic field, but rather an important systematic and distinct component of the Earth’s magnetic field variability between field reversals. 1Lund, S.P., Williams, T., Acton, G.D., Clement, B., and Okada, M., 2001. Brunhes Chron magnetic field excursions recovered from Leg 172 sediments. In Keigwin, L.D., Rio, D. Acton, G.D., and Arnold, E. (Eds.), Proc. ODP, Sci. Results, 172, 1–18 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Department of Earth Sciences, University of Southern California, University Park, Los Angeles CA 90089, USA. slund@usc.edu 3Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom. 4Ocean Drilling Program, Texas AM Table T1) (Lund et al., 1998; Keigwin, Rio, Acton, et al., 1998). This number of excursions is larger than all previous good-quality Brunhes excursions noted worldwide. This paper summarizes new paleomagnetic and rock magnetic results derived from U-channel measurements of more than 150 m of Brunhesaged sediment collected from Sites 1060–1063. These results confirm the existence of 12 of the 14 originally defined excursions. They also identify one new excursion not previously noted in the long-core measurements. Comparison of the U-channel and long-core measurements indicates that long-core measurements, on the whole, can almost always identify paleomagnetic secular variation (PSV) and excursions, but biases in the measurements are present that slightly alter the true paleomagnetic record (as defined by U-channel and selected discrete sample measurements). In a few unusual cases associated with low intensity natural remanent magnetization (NRM) or fast directional changes, the long-core results are significantly different from U-channel or discrete sample measurements. DIRECTIONAL SECULAR VARIATION DURING THE BRUNHES CHRON Records of magnetic field secular variation within Leg 172 cores were routinely recovered aboard ship by measuring the NRM of all cores (archive halves) at 5-cm spacing after 20-mT AF demagnetization. The 20mT demagnetization removed a ubiquitous low-coercivity drill string magnetic overprint that has been noted previously during many ODP legs (e.g., Nagy and Valet, 1993; Weeks et al., 1993). Further AF demagnetization of selected core segments always showed good characteristic remanences that decayed toward the origin (Keigwin, Rio, Acton, et al., 1998). Stepwise AF demagnetization of discrete samples from selected horizons also displayed the same behavior (Keigwin, Rio, Acton, et al., 1998). The patterns of directional variability observed after demagnetization could commonly be correlated between holes at individual sites for F1. Summary of the Brunhesaged magnetic field excursions, p. 8. C A C D A B C F H A B A B D E C D Site 1060 Site 1061 Site 1062 Site 1063 0 20 40 60 80 100 120 140 160 180 200 220 S ed im en t d ep th ( m bs f) Brunhes/Matuyama Boundary 3b (~40 ka) 5b (~125 ka) 7a (~195 ka) 7b (~235 ka) 8a (~260 ka) 9a (~310 ka) 9b (~330 ka) 11a (~410 ka) 13a (~510 ka) 14a (~535 ka) 15a (~575 ka) 15b (~605 ka) 3a 5a