Revisiting the initial sites of geomagnetic field impulses during the Steens Mountain polarity reversal

Abstract

We present a new palaeomagnetic investigation of the two sites from the Steens Mountain (Oregon) volcanic record of a Miocene polarity reversal which were supposed to record very fast changes of the geomagnetic field or impulses (Mankinen et al. 1985; Prtvot et al. 1985a,b). Approximately 130 cores were first drilled from the two initial sites, belonging to sections A and B, in order to obtain at least one detailed and complete vertical sampling of each lava flow. Thermal analyses of natural remanent magnetization, complemented by some alternating field treatments, low-field thermomagnetic curves, microscopic observations and electron probe analyses of key magnetic phases, lead us to somewhat different conclusions for the first and the second impulses. At site B (first impulse), we find that the dependence of the remanence direction on the sample vertical position in flow B51 does not imply a directional field change during flow cooling, but is better explained by a thermochemical overprinting due to the overlying B50 flow. However, this conclusion does not challenge the existence of the first impulse because this field change seems to be recorded some 25 m away in flow B51 (Coe & Prevot 1989), at a place where it is thick enough for this record not to have been erased by the baking due to B50. Regarding the second impulse, restudied at site A, our new findings are more comprehensively explained by a change in the field direction during cooling of flow A41-2 than by some overprinting. Using a simple model of flow cooling, the angular rate of change of the field is estimated to have been of the order of 2 "-3 O or 250-350 nT per day during the impulse. This figure is similar to that previously obtained from site D, some 250m away. However, the directional paths describing the field change are somewhat different at the two sites. New investigations are planned to try to understand the origin of this discrepancy.

Evidence from lava flows for complex polarity transitions: the new composite Steens Mountain reversal record

“…Note the unusual Curie temperature (400°C) of the secondary component. This behaviour has been seen in other Steens Basalt samples (Camps et al . 1995) and was attributed to the mineral magnesioferrite.…”

“…With regard to the hypothesis of extremely rapid field change in the gaps of the benchmark Steens record (Mankinen et al . 1985; Coe & Prévot 1989; Camps et al . 1995; Coe et al .…”

“…The first gap of 60 • is divided into two roughly equal halves, the second gap of 70 • is now replaced by the entirely new T-N-T-R-T part of the transition record that has five new gaps of 90 • , 40 • , 80 • , 150 • and 100 • , and the third gap is little changed. With regard to the hypothesis of extremely rapid field change in the gaps of the benchmark Steens record Coe & Prévot 1989;Camps et al 1995;Coe et al 1995), which was based on anomalously streaked intraflow scattering of specimen ChRM directions associated with gaps one and two, it is still a possibility for the first gap, it is clearly not the case for the second gap, and no additional evidence has been obtained with regard to the third gap.…”

Demise of the rapid-field-change hypothesis at Steens Mountain: The crucial role of continuous thermal demagnetization

“…For this study samples were taken 250 m from where suspicious within-flow directional variation was originally detected because the geometry of the cooling units was simpler and overall exposure better there. Indeed, directional results published later from the original locality were complex (Camps et al, 1995a), whereas at this section the high-temperature direction within the critical 1.3 m flow again moved systematically with distance from the base about 60 • away from the direction of pre-gap flows toward the direction of post-gap flows and then back again, as shown in Figs. 2 and 3 in Coe et al (1995).…”

Orientation of hatchling loggerhead sea turtles to regional magnetic fields along a transoceanic migratory pathway

“…These considerations aside, responses to regional magnetic fields might be rendered useless during limited periods of exceedingly rapid field change (Camps et al, 1995;Coe and Prevot, 1989) that sometimes accompany polarity reversals of the Earth's field. Such transient periods of rapid change, however, do not preclude the evolution of magnetic responses during the intervening and usually longer intervals (Skiles, 1985) when the Earth's field changes slowly and is relatively stable.…”