Redefinition of the Blake Event

We have recovered four new records of the paleomagnetic field variability within Oxygen Isotope Stage 5 from deep-sea sediment cores of the Blake/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. The paleomagnetic records are complicated by the fact that the Blake Event occurs within a narrow zone 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. We propose that our four new paleomagnetic records be used to redefine the Blake Event because these records significantly improve our understanding of the age and paleomagnetic field variability associated with the Blake Event in its type area. Our records indicate that the Blake event occurred ~119126,000 years BP, within Oxygen Isotope Stage 5e (114-131,000 years BP). The Blake Event is a local magnetic field reversal (Class II excursion) with directions flipping quickly to reversedpolarity 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. 2 Introduction Geomagnetic field excursions are defined as short intervals of locally anomalous field directions that occur within a broader interval of 'stable' (normal or reversed) magnetic polarity. By anomalous, we mean that the directions have equivalent virtual geomagnetic poles (VGPs) more than 45° away from the geographic North (South) Pole during normal (reversed) polarity (Watkins, 1976; Verosub and Banerjee, 1977). (VGPs are calculated from local magnetic field directions that locate the magnetic-field North Pole by assuming that the directions are caused by a simple dipole or bar magnet situated at the center of the Earth.) There have been enough independent paleomagnetic records of excursions documented (e.g.; Champion et al., 1988; Nowaczyk and Baumann, 1992; Langereis et al., 1997; Lund et al., 1998, 2001) to be confident that excursions really do exist and are a fundamental aspect of geomagnetic field variability. We need to understand the detailed pattern of spatial and temporal variability associated with excursions if we are going to estimate what excursions indicate about the outer-core dynamo process that generates them. Three of the best-known and most recent excursions are the Mono Lake Excursion (Denham and Cox, 1971; Liddicoat and Coe, 1979) (ca. 33,000 cal. yrs. BP), Laschamp Excursion (Bonhommet and Babkine, 1967) (ca. 41,000 cal. yrs. BP), and Blake Event (Smith and Foster, 1969) (ca., 119,000 cal. yrs. BP). For all of these excursions, there are continuing arguments about their space/time pattern of field variability and their relationships to both normal secular variation and the geomagnetic field reversal process. Of these three excursions, the Blake Event is the most poorly defined in its type locality (Blake/Bahama Outer Ridge, western north Atlantic Ocean, Figure 1) with lingering questions about the Blake event’s exact age, duration, and pattern of directional variability. In this paper, we present new paleomagnetic and chronostratigraphic results from four deep-sea sediment piston cores from the Blake/Bahama Outer Ridge that redefine its age and directional variability in the type locality and significantly improve our overall understanding of the Blake Event. Previous Paleomagnetic Records of the Blake Event