On the relationship between paleomagnetic secular variation and excursions – Records from MIS 6 and 7 - ODP Leg 172

Abstract

This paper summarizes the pattern of paleomagnetic secular variation (PSV) and excursions within MIS 6–7 (130–243 ka) from the western North Atlantic Ocean – ODP Sites 1060–1063. Composite high-resolution PSV records (both directions and relative paleointensity) have been developed for each site and inter-compared. There are 65 correlatable inclination features, 67 correlatable declination features, and 15 correlatable paleointensity features in the four PSV records. We have used the results of Grutzner et al. (2002), who compared the calcium carbonate records of ODP Leg 172 sediments and tuned them with Milankovich cyclicity, to date our records. We also dated our PSV records by comparing our paleointensity variability with the study the PISO-1500 global paleointensity record. The two methods agree well. We use the Grutzner phase-shifted chronology to date and analyse our cores because it has lower age uncertainties (~±2 ka). We noted two excursions in our PSV records – Excursions 7α and 7β. These are equivalent to the Iceland Basin and Pringle Falls Excursions. Our revised age estimate for the Iceland Basin Excursion is 196 ± 3 ka. Our revised age estimate for the Pringle Falls Excursion is 215 ± 2 ka. Both are Class I excursions noted by 2–3 cycles of high-amplitude oscillatory inclinations and declinations. We note a strong similarity in the oscillatory kinematic pattern of the two excursions. We also note evidence for similar but lower amplitude directional PSV between and after the excursions. This complex pattern suggests that the dynamics of the regional dynamo process responsible for these excursions is a continuing process for more than 35 ky. We have carried out a statistical study of the PSV records over the interval 120–250 ka after removing all true excursional directions. We have averaged directions and paleointensity in 3 ka and 9 ka overlapping intervals to evaluate the space/time character of PSV and its geodynamo cause(s). We see significant PSV directional variability over 104 yr time scales that is regionally correlatable among the four sites. There is a notable pattern of angular dispersion variability with most time spent with low (~10°) dispersion, with four shorter intervals of high (~25°) dispersion. There are also four notable intervals of low paleointensity in all four records. There is a one-to-one correspondence between the four low paleointensity intervals and the four intervals of high angular dispersion. The Blake Event (123 ± 3 ka) and Excursion 8a (238 ± 2 ka) occur in two of the low intensity/high dispersion intervals that bound MIS 6/7. The two magnetic field excursions within MIS 7 (Iceland Basin and Pringle Falls Excursions) occur in one distinctive long (~35 ka) interval of low paleointensity/high dispersion interval.