On the relationship between excursions and paleomagnetic secular variation – Records from MIS 5, North Atlantic Ocean

Abstract

We have recovered four new records of the paleomagnetic field variability within Marine Isotope Stage 5 (MIS 5: 71–130 ka) from deep-sea sediment cores of the Bahama Outer Ridge, western North Atlantic Ocean. These cores contain reproducible evidence of the Blake Event and are located within 300 km of cores used by Smith and Foster (1969) to define the Blake Event. Our new paleomagnetic records have both reproducible directions and relative paleointensities, but they should be considered only medium-resolution records because they were recovered from sediments with bulk accumulation rates of only ~6 cm/kyr. Paleomagnetic interpretations of the records are complicated by the fact that the Blake Event occurs partially within two narrow zones of bacterial magnetite. Our rock magnetic and paleomagnetic studies suggest that the bacterial magnetite has not contributed significantly to the directional or relative paleointensity results. Our records indicate that the Blake event occurred ~119–126,000 years BP, within MIS5e (114–130,000 years BP). The Blake Event is a local magnetic field reversal (Class II excursion) with directions flipping quickly to reversed-polarity directions, hovering statically for at least 6000 years, and then flipping quickly back to normal polarity directions. The core dynamo source for the Blake Event must have a strong axisymmetric and global-scale character to produce such a true local polarity reversal. We have carried out detailed short-interval (3 ka and 9 ka) statistical analysis of the PSV (after removal of true excursional directions) from three of our records to assess the pattern of PSV through MIS 5 and associated with the Blake Event. Reproducible statistical results among the three records suggest significant deviations away from long-term average on time scales greater than 3 ka and 9 ka. The overall angular dispersions are generally low (~10°) except for two narrow intervals (~25°). The high dispersion intervals are associated with the lowest paleointensity intervals and excursions.