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Abstract

A high-resolution paleomagnetic record of a magnetic polarity episode has been recovered from an 8-m section of diato-maceous lacustrine sediments from Pringle Falls near La Pine, Oregon (43.7°N, 238.6°E). A total of 79 samples (172 specimens) was collected at about 2- to 5-cm intervals from the 20-m section. Biostratigraphic dating from diatoms gives an approximate age around 150 ka for this record. The characteristic magnetization of the samples was determined by AF demagnetization to 15 mT, after progressive demagnetization experiments from 2.5 to 60 mT. The average normal polarity stable declination and inclination of the non-reversing polarity episode sampled section (61°; α95 = 2.6°) is statistically equivalent to that expected for this site for a geocentric axial dipole (62°). Rock magnetic experiments such as isothermal remanent magnetization (IRM) indicate that titanomagnetite is the dominant ferrimagnetic mineral. Interpretation of saturation IRM, susceptibility (X), and cleaned intensity (J) over anhysteretic remanent magnetization (J/ARM) experiments indicate that the behavior of the intensity of magnetization of these sediments is a reflection of the decrease of the intensity of the Earth’s magnetic field during the Blake polarity episode. The record provides clear documentation of three conspicuous inclination features, labelled A, B, and C. A well-defined virtual geomagnetic pole (VGP) path also has been obtained and shows two preferred bands of longitude over the Americas and antipodal to them. Correlation between a previously published record obtained from a locality about 1.5 km from the site of the record reported here indicates that there is a repeated sequence of inclination features (labelled A, B, and C) between the two records. These inclination features from the Pringle Falls sites can be correlated with a record from the Mediterranean (oceanic record) and from Gioia Tauro, Italy. Also, the same inclination features are repeated and correlated with the only igneous record for the Blake episode, obtained from the island of Reunion in the South Indian Ocean. The Pringle Falls sections, as well as the oceanic cores from the Mediterranean, produced a series of intermediate directions that coincide with the more continuous sedimentary record of the Blake polarity episode. The correlation within (local) and between (global) sites of lake sediments (Pringle Falls), oceanic sediments (Mediterranean cores), sedimentary rocks (Gioia Tauro, Italy), and ultimately lava flows (Réunion record) is important because the VGP paths correlate with other geophysical phenomena which indicate that episodes and reversals yield key data about the dynamo process and the core-mantle boundary. It appears that the paths show channels of north-south flow in the outer core, which are related to the nature of the base of the mantle, and that the geometry of the transitional field and the fluid motion in the Earth’s core are to a large extent determined by the thermal structure of the lower mantle.

Thus, paleomagnetic records of the geomagnetic field on the surface of the Earth during polarity transitions may lead to a better understanding of the dynamics of the Earth’s deep interior. In summary, it is valid to assume that if the record of the Blake episode presented here and its correlation to other records of the same episode are valid, the Blake geomagnetic polarity episode can and should be used as a stratigraphic marker because of its world-wide presence as a geomagnetic feature.

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