AND 3
AND 3Records 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 carbonate variations (Lund et al., in press; Keigwin, Rio, Acton, et al., 1998; see also Lund et al., Chap. 10, this volume).
It is clear for all three sites that selected inclination and declination features (numbered in Figs. F2, F3, and F4) can be traced among the records from independent holes separated by distances of <1 km. The most diagnostic features are narrow intervals of distinctive low/high inclination or easterly/westerly declination. In between these most easily correlated features, it is common to see more subtle patterns of variability that are also consistent between holes. The directional secular variation in selected time intervals can also be correlated between sites up to 1600 km apart. For example, compare the inclination and declination variability from ~15,000 to 45,000 k.y. as recorded at Site 1063 (Bermuda Rise) (Fig. F4) vs. Site 1061 (Blake Outer Ridge) (Fig. F2).
The secular variation
records in Figures F2,
F3, and F4
consistently contain two intervals (highlighted in Figs. F2,
F3, and F4)
wherein inclination and declination variability is significantly higher in
amplitude and inclinations are negative. These directions (after correction for
overall core orientation) yield virtual geomagnetic poles (VGPs) that are more
than 45° away from the North Geographic Pole; such VGPs are termed excursional
(Verosub and Banerjee, 1977) and may reflect times of unusual geomagnetic field
behavior. We identified these two intervals as "plausible" excursions
on the basis of three criteria: (1) presence of true excursional directions, (2)
occurrence in at least four different holes at two or more sites, and (3)
location of excursional directions within a reproducible and correlatable
pattern of more typical secular variation. The younger excursion was labeled
"3"
and the older was labeled "3."
More careful assessment of
the paleomagnetic results in Figures F2,
F3, and F4
raises some concern about the reality of excursion 3.
Excursion 3
is present and associated with inclination low "17" in three of five
holes at Site 1061 and in both holes at Site 1063. Excursion 3
is missing from the other holes either because of some type of magnetic
"smearing" or stratigraphic gaps. At Site 1062, what we originally
interpreted as excursion 3
is associated with inclination low "12´"
in two of five holes and is associated with inclination low "17´"
in only one of eight holes. Thus, evidence that excursion 3
occurs within a reproducible pattern of secular variation is less certain than
noted previously (Keigwin, Rio, Acton, et al., 1998; Lund et al., 1998).
