A comparison of two sequential geomagnetic polarity transitions (upper Olduvai and lower Jaramillo) from the Southern Hemisphere

Abstract

Two normal to reverse (N to R) geomagnetic polarity transitions were obtained from a Southern Hemisphere deep-sea sediment core (35.91°S, 59.97°E). The upper Jaramillo reversal was continuously sampled by taking 0.5 cm thick samples across 55 cm of section, and the upper Olduvai reversal was sampled in a similar manner across 60 cm. Both records are characterized by full normal and reverse polarity directions which are in very good agreement with those predicted by axial dipole fields for the core site latitude. Each record also exhibits a zone yielding intermediate directions. Because of a large increase in both the within-level and the between-level scatter in the upper Jaramillo transition zone, it is not considered to represent an accurate record of the geomagnetic field. The upper Olduvai transition, however, appears to be a more coherent record. It is characterized by directions that shallow early in the reversal but then rapidly steepen to nearly vertical, upward directions. The declinations do not change until the inclinations have passed through the vertical. The total directional change occurs within a broad NRM intensity minimum. The virtual geomagnetic pole (VGP) path calculated for this record is not longitudinally constrained but instead exhibits a westward progression through the reversal. When considered together with the lower Jaramillo transition previously reported from this core, these records constitute a set of sequential Southern Hemisphere transitions. A number of striking similarities exist in the two records such as steep, upward directions and a shallowing which occurs between the near vertical and the full polarity directions. The results of zonal harmonic modeling of these records are consistent with the idea of a standing field which persisted across both the upper Olduvai and lower Jaramillo reversals.