Paleomagnetic records of Stage 3 excursions, Leg 172

Shipboard long-core paleomagnetic measurements made during Ocean Drilling Program Leg 172 identified a reproducible pattern of directional secular variation between ~15,000 and 45,000 k.y. that could be correlated between Sites 1061, 1062, and 1063, which span a distance of more than 1200 km. Two intervals of excursional paleomagnetic directions were identified at all three sites and were labeled as excursions 3α and 3β. New analysis of excursions 3α and 3β, based on reassessment of the shipboard measurements, new U-channel paleomagnetic records of the excursions, and independently published discrete sample paleomagnetic records of the same intervals all indicate that excursion 3α is not real; its directional variability is less than originally estimated and not excursional. Excursion 3β is real and easily identified in both U-channel and discrete sample measurements. Excursion 3β is the Laschamp Excursion noted in previous published studies. The best U-channel record of excursion 3β from Hole 1063C is almost identical to the best discrete sample paleomagnetic records. The Uchannel records from Holes 1061B and 1061C are similar to one another but somewhat different from the other records (more smoothed). We attribute that difference to systematic biases in the U-channel measurement process when large-amplitude, fast directional changes occur. INTRODUCTION During Ocean Drilling Program (ODP) Leg 172, almost 6 km of Pliocene/Pleistocene deep-sea sediments was recovered from three sedi1Lund, S.P., Acton, G.D. , Clement, B. , Okada, M. , and Williams, T. , 2001. Paleomagnetic records of Stage 3 excursions, Leg 172. In Keigwin, L.D. , Rio, D. , Acton, G.D. , and Arnold, E. (Eds.), Proc. ODP, Sci. Results, 172, 1–20 [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 3Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station TX 77845-9547, USA. 4Department of Geology, PC344, Florida International University, University Park, Miami FL 33199, USA. 5Department of Environmental Sciences, Ibaraki University, Bunkyo 21-1, Mito 310, Japan. 6Department of Geology, University of Leicester, Leicester LE1 7RH, United Kingdom. Initial receipt: 24 November 1999 Acceptance: 18 January 2001 Web publication: 6 June 2001 Ms 172-217 S.P. LUND ET AL. PALEOMAGNETIC RECORDS OF STAGE 3 EXCURSIONS 2 ment drifts (Keigwin and Jones, 1989) of the western North Atlantic Ocean: the Blake Outer Ridge (Sites 1054–1061), the Bahama Outer Ridge (Site 1062), and the Bermuda Rise (Site 1063) (Keigwin, Rio, Acton, et al., 1998). These sediment drifts (see Fig. F1) are regions of anomalously high sediment accumulation rates (typically 10–40 cm/ k.y.), and they contain perhaps the highest resolution record of geomagnetic field variability ever recovered from deep-sea sediments. Sediments in the uppermost 150–220 m of each site were collected using the advanced piston corer (APC), which is capable of recovering almost pristine cores of soft sediment. Aboard ship, we measured the archive halves of all sediment cores using a new long-core cryogenic magnetometer (Model 760 from 2G Enterprises) with an in-line alternating magnetic–field (AF) demagnetizer, which was installed after ODP Leg 169. From the long-core measurements, we were able to estimate the pattern of geomagnetic field secular variation (both directions and intensity) for the Brunhes Chron and identify 14 “plausible” Brunhesaged magnetic field excursions (Keigwin, Rio, Acton, et al., 1998; Lund et al., 1998). This paper considers in more detail excursions labeled 3α and 3β by Lund et al. (1998; Chap. 10, this volume). We reconsider the shipboard long-core paleomagnetic results that led us to suggest these excursions existed, we present new U-channel paleomagnetic records for three replicate sediment sections containing excursions 3α and 3β (= Laschamp Excursion), and we compare these results with independent discrete sample paleomagnetic results from the same time interval (Schwartz et al., 1997; Lund et al., in press). These results confirm the existence of excursion 3β but not 3α. They also show that long-core and U-channel measurements in excursion intervals, characterized by fast changes in field direction and/or intensity, may not be able to routinely recover the detailed pattern of directional field variability. INITIAL IDENTIFICATION OF EXCURSIONS 3α AND 3β Records of magnetic field secular variation within ODP Leg 172 cores were estimated aboard ship by measuring the sediment natural remanent magnetization (NRM) of all cores (archive halves) at 5-cm spacing after 20-mT AF demagnetization. The 20-mT demagnetization removed a ubiquitous low-coercivity drill-stem magnetic overprint that has been noted previously on many ODP legs (e.g., Nagy and Valet, 1993; Weeks et al., 1993). Further AF demagnetization of selected core segments usually 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 Sites 1060–1063. For example, inclination and declination variability at Site 1061 (Blake Outer Ridge, five holes) from ~15,000 to 45,000 k.y. is shown in Figure F2; similar variability at Site 1062 (Bahama Outer Ridge, eight holes) and Site 1063 (Bermuda Rise, four holes) is shown in Figures F3 and F4. The chronologies for these three records was developed using radiocarbon dates from Keigwin and Jones (1994), which were correlated to these sites using magnetic susceptibility and calcium F1. Map of the western North Atlantic Ocean showing site locations, p. 8. Bermuda Rise ew Egland Semounts Sohm Abyssal Plain M id A tla nt ic R id ge Nares Abyssal Plain Hattaras Abyssal Plain 40 0 m 40 00 m 500 0 m Corner Rise 400 m 4000 m 5000 m 5000 m 40 00 m BlakeBahama Outer Ridge 25° 30° 35° 40° 45° 50° N 80°W 70° 60° 50° 40° Site 1062 Site 1061 Site 1063 F2. Long-core inclination and declination measurements, Site 1061, p. 9. 5 10 15 20 Meters composite depth In cl in at io n (1 0° in cr em en ts ) 1061E 1061D 1061C 1061B 1061A 45 40 35 30 25 20 Radiocarbon years before present (x103) 8' 16 17 18 25 26 21 22 7 8 9 10 11 12 15 16