Emmanuel Tric

Paleointensity of the geomagnetic field during the last 80,000 years

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

Abstract A detailed record of the Upper Olduvai polarity transition, composed of > 100 transitional directions, has been obtained from the Crostolo section in northern Italy. A careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques, thermomagnetic, scanning electron microscopy, microprobe, X-ray diffraction and Mossbauer analysis, shows that greigite is the main magnetic carrier of the remanence. The correlation of a shift in the magnetic record to a small-scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition, or very shortly after. The virtual geomagnetic pole (VGP) path is largely confined along a great circle over North and South America ∼ 90° west of the site and consists of three stages: first, the VGP moves to southern mid-latitudes, then, after a period of standstill, it comes back to almost true north, and, finally, the south pole is reached in a third step. Although different from a record from the southern Indian Ocean, this path is virtually identical to those obtained for the same transition from North Pacific deep-sea cores, and partly coincides with a North Atlantic record, which suggests that a dipolar component may be present in the transitional field during the Upper Olduvai reversal. A review of the recently obtained records of various transitions shows that in more than two-thirds of the cases the VGP paths are similarly confined along a meridian over the Americas or antipodal to them, irrespective of the sampling site and of the sense of the transition. Although not deterministic, this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages.

The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance.

Abstract Two records of the Blake geomagnetic event have been obtained from marine cores in the Mediterranean. The upper bound for duration of the event, calculated from accurate oxygen isotope stratigraphy and/or tephrochronology, is ∼ 4000 years. One record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved. The directional changes do not occur at constant speed, rather a stop-and-go behaviour is observed as in detailed records of reversals. The virtual geomagnetic pole (VGP) path, confined to two bands of longitude over the Americas and antipodal to them, is virtually identical to the only other path of this event obtained from lacustrine sediments in the western USA, suggesting that a dipolar component may be present in the transitional field during the Blake event. The two bands of longitude coincide with those of many recent reversals, suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time.

Absolute paleointensity and magnetomineralogical changes

Determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings. Such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization (PTRM) are reflected by negative pTRM checks, a deviation between the original pTRM and a repeated measurement after heating at a higher temperature. Because the deviations indicated by the PTRM checks can provide a direct estimate of the effects to the pTRMs, we suggest that they can be used to correct pTRMs. The correction method we present requires additional measurements, in particular pTRM checks performed after each pTRM and specific diagrams to detect acquisition of chemical remanence. Performing multiple successive pTRM checks can also provide information on the blocking temperatures of the alteration product. These experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step. The corrections have been tested on recent and historic lava flows from several localities. We have also compared results on samples from the same lava flows that were heated in air and in vacuum. The mean paleointensity values were found to lie within less than 10% from the expected field intensity at each site. The corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65% additional data for specimens heated in air. Successive heatings at the same temperature in air are often accompanied by a time-dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field, a problem that is not related to the corrections. We conclude that corrections can be used with fairly high confidence to the pTRMs performed in vacuum and in air.

ABSOLUTE PALEOINTENSITY BETWEEN 60 AND 400 KA FROM THE KOHALA MOUNTAIN ( HAWAII)

Abstract Magnetic experiments including thermal demagnetization of the natural remanent magnetization (NRM), mineralogical studies and paleointensity measurements have been conducted on ten lava flows, with ages between 60 ka and 400 ka, from the Kohala Mountain, Hawaii. Of the samples, 62 were subjected to double heating paleointensity experiments under vacuum. In total, 35% of the specimens did not exhibit significant magnetomineralogical changes during heating and met all the criteria for successful determinations of absolute paleointensity. A technique of corrections [1] was attempted for samples that exhibited changes in their ability to acquire partial thermoremanent magnetization (pTRM) during heating but did not show acquisition of chemical remanence (CRM). This procedure doubled the rate of success, with consistent results between the uncorrected and the corrected data from within the same flows. The successful paleointensity estimates obtained for 8 lava flows are found to be in good agreement with previous absolute paleointensities obtained from other areas. The results are also consistent with the synthetic curve (Sint-200) of relative paleointensity obtained for the past 200,000 years from deep-sea sediment cores [2]. There is thus no reason to infer the presence of large non-dipole fields in the vicinity of Hawaii. Overall, the geomagnetic intensity appears to have the same variability for at least the past 200 ka.