A summary of paleomagnetic secular variation and excursions for the last 380 ky of the Brunhes normal polarity Chron

Abstract

This study develops composite high-resolution, full-vector PSV records for the last 380 ka from three regions associated with ODP Leg 172 in the western North Atlantic Ocean. These records identify 207 inclination features and 196 declination features that are common to all three regions. The average sediment accumulation rates are ~25 cm/ky, which produce sampling intervals of ~100–200 years. Each region has been dated by a combination of oxygen isotope stratigraphy and carbonate stratigraphy. We also have developed relative paleointensity records from all three regions that have 34 features in common. The paleointensity features can also be correlated to the PISO-1500 global paleointensity record of Channell et al. (2009) and provide an independent chronostratigraphy. The directions have near Gaussian distributions with inclinations averaging about 6° lower than axial dipole expectation due to the ΔI anomaly. The intensities also have a near Gaussian distribution, but have some more low intensities than expected. There is reproducible evidence for 7 excursions in these records with good waveform information. The excursions last from ~900 to ~8400 years. Five of the excursions are Class I excursions with out-of-phase inclination and declination variability that produces clockwise or counter-clockwise circularity. Two of the excursions (Blake Event, 8α Excursion) are Class II excursions with in-phase inclination and declination variability. Both of these excursions should be considered local reversals due to their very fast (<200 year) in-phase directional movement to reversed polarity directions that hover there before returning quickly to normal polarity directions. We have complete full-vector PSV records that surround each of these excursions. Some excursions occur quickly with no distinctive pre- or post-excursion anomalous directional variability. Other excursions are preceded by 10–20 ky of anomalous directional variability (high angular dispersion). Some also show continuing anomalous directions for 10–20 ky after the excursions. All of these features suggest a close relationship between excursions themselves and the surrounding PSV. Statistical studies of the PSV also show distinctive patterns to the PSV variability. The angular dispersion in 3-ky intervals shows a distinctive bimodal distribution with high (low) values associated with low (high) intensities. The high angular dispersion (and low intensity) are associated with the seven recorded excursions in these regions. There is reproducible evidence for another 5 excursions elsewhere in the world, but not in our records, that are also associated with high angular dispersion intervals. We think this is corroborative evidence for the reality of these excursions. That means that 12 excursions have occurred in the last 380 ka. Overall assessment of the PSV records suggests that important non-linear behavior occurs in the geodynamo as a function of intensity. This behavior is noted in the sharp changes between low/high angular dispersion intervals. It suggests that the 9-ky averaged field is more multipolar and far from axial dipole expectation during low paleointensity. Non-linearity is also seen in the fast polarity flips associated with the two Class II excursions.