Geomagnetic Polarity Transition Research Articles

The Steens Mountain (Oregon) geomagnetic polarity transition: 2. Field intensity variations and discussion of reversal models

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long‐term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root‐mean‐square of the three field components X, Y, and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr−1, which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

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

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.

Geomagnetic polarity transitions in two oriented sediment cores from the northwest Pacific

The paths described by the geomagnetic poles during Jaramillo and Olduvai polarity transitions, as recorded in two oriented sediment cores from the northwest Pacific Ocean, closely resemble those previously determined in another deep-sea sediment core from the Indian Ocean. The paths determined for a specific transition appear to be the same in all three cores. There seems, however, to be a unique path associated with each of the three successive transitions studied in all three cores.

Large intensity changes of the non-dipole field during a polarity transition

Abstract Directional and paleointensity data for the Steens Mountain geomagnetic polarity transition do not agree with the two simplest models of reversals: rotation of undiminished dipole or gradual diminution followed by change in polarity of the dipole moment. Instead, large and rapid changes in the intensity and direction of the field occur, probably as a result of non-dipole variations.

Non-Dipole Behaviour During an Upper Miocene Geomagnetic Polarity Transition in Oregon

The palaeomagnetism of a section of 71 successive lavas from southeastern Oregon is presented. A geomagnetic polarity change from reversed to normal polarity which took place during the extrusion of the section 15 million years ago has allowed a palaeomagnetic examination of the non-dipole field during a period of diminished dipole intensity. During the polarity transition, there existed an apparent easterlydrifting non-dipole component. Together with an independent determination of the palaeo-intensity on one lava (Coe 1967) and other recently presented data (Goldstein et al. 1968) an application of Irving & Ward's(1964) geomagnetic field model to the data suggests a maximum dipole intensity as low as two per cent of the present value for a period of not less than 100 years during the transition, outside of which the data are consistent with some present-day ratios of non-dipole to dipole field intensity. A very short period of diminished relative dipole intensity occurred during the final stages of the polarity change, when the virtual geomagnetic pole returned to very low latitudes, without any polarity change actually occurring. Because of a great similarity with an independently presented result, it is suggested that this ‘rebound effect’ may reflect a fundamental aspect of the mechanism of at least some polarity changes. Comparison with estimates of the duration of some more recent geomagnetic polarity transitions suggests that the section may have accumulated in less than 50000 years. The lavas of normal polarity, which comprise most of the section, do not show any preferred association with lower or higher oxidation states.

Unstable Components and Paleomagnetic Evidence for a Geomagnetic Polarity Transition

Part of a section of Miocene lavas in southeastern Oregon has been observed to possess natural remanent magnetism (NRM) superficially consistent, in both direction and intensity of magnetization, with the existence of a geomagnetic polarity transition contemporaneous with initial cooling of the lavas. Investigation of unstable magnetic components and other appropriate properties reveals, however, that the critical NRM is a function of petrological and not geomagnetic field properties. The occurrence of maghemite and unoxidized ilmenite lamellae in titanomagnetite are strongly associated with the systematic magnetic instability. The creation of high magnetic stability in the lavas concerned, by natural secondary oxidizing processes, could possibly result in convincing but totally misleading evidence for a Miocene geomagnetic polarity transition.