A 10 000 yr geomagnetic secular variation record from three Australian maars

Abstract

Detailed within-lake magnetic stratigraphies enable composite declination and inclination versus depth curves to be derived for sets of 6 m cores from each of three volcanic crater lakes in western Victoria. Comprehensive radiocarbon chronologies for each sequence enable reliable timescales to be assigned to the variations in direction of the remanence. The resultant time-scaled curves are in satisfactory agreement and are therefore thought to provide an estimate of the geomagnetic secular variation in SE Australia during the last 10 000 yr. Amplitude attenuation occurs as an increasing function of age, and probably results from long time-scale remagnetization processes. Mean directions coincide with the geocentric axial dipole field directions at the three sites. Inclination oscillations are stronger and better resolved than those in declination, whereas the opposite tends to be true for the European records. Reduction of records to a common 'virtual' site suggests that the difference is largely of geometrical origin. Comparisons of VGP paths do not reveal a common dipole wobble component. The sense of looping of the magnetic vector favours westward rather than eastward drift of features of the non-dipole field. Relative field strength estimates based on normalization of NRM with respect t o measures of the magnetic mineral content do not agree with Australian and worldwide' archaeointensity results. IntroductionDetailed variations in the geomagnetic field beyond the range of magnetic observatory records can be reconstructed from archaeomagnetic data, palaeomagnetic measurements on recent lava flows, and from the remanent magnetism of lacustrine or rapidly deposited marine sediments. The advantage of sediments in providing a continuous, or piece-wise continuous, time sequence is, unfortunately, offset by their questionable fidelity as palaeomagnetic recorders compared with materials which have acquired a thermoremanent magnetization.

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

“…Around 7000 BC all data series consistently show a clear inclination minimum, which is predicted by pfm9k.1a and CALS10k.OL2c, but not by SHA.DIF.14k. This is because at that time the few available data (only 355 values for the whole interval of 12000 BC to 6000 BC) come from a small area at the western coast (Stockhausen 1998); WAI -Lake Waiau, Hawaii (Peng & King 1992); GNO -Lake Gnotuk, Australia (Barton & McElhinny 1981) and ERH -Erhai Lake, China (Hyodo et al 1999). (A colour version of this figure is available in the online version.)…”

Geomagnetic Secular Variation Reconstructed from Magnetizations of Wide-Diameter Cores of Holocene Sediments in Japan.

“…The SV records from Britain (TURNER and THOMPSON, 1981) and Australia (BARTON and MCELHINNY, 1981) exhibit complex VGP paths with both clockwise and anti-clockwise loopings, and irregular linear trends. Dominant clockwise looping has been observed during the Holocene time only in North America (CREER and TUCHOLKA, 1982b;LUND and BANERJEE, 1985).…”

“…Unfortunately, such studies can only be carried out at archeological sites and *Dept . ing Holocene times have been obtained in the U.K. (TURNER and THOMPSON, 1981), Australia (BARTON and MCELHINNY, 1981), and North America (LUND and BANERJEE, 1985;VEROSUB et al, 1986). These records are all constructed by stacking data from many cores.…”

Propriétés magnétiques et sédimentologiques de séquences holocènes de l'estuaire du St-Laurent et du fjord du Saguenay, Canada /

“…L'intensité et l'orientation de ce champ magnétique varient dans le temps selon des échelles allant de quelques secondes à des millions d'années {e.g., Merril et al, 1996 ;McElhinny et McFadden, 2000 ;Dormy et al, 2000). Aujourd'hui, les variations d'intensité et d'orientation peuvent être mesurées grâce aux observatoires géomagnétiques, à certains satellites et à quelques archives historiques, mais ces enregistrements ne remontent jamais au-delà des derniers -400 ans (McElhinny et McFadden, 2000 Stoner et al, 1995;Guyodo et Valet, 1996, 1999Roberts, 1997 ;Lund, 1996 ;Peck et al, 1996 ;Saarinen, 1999 ;Ali et al 1999 ;, même si une partie des processus responsables de l'acquisition de la magnétisation dans les sédiments demeure incomprise (Tauxe, 1993 ;Tauxe, 1998 ;Katari et al, 2000 ;Katari et Bloxham, 2001 Barton et McElhinny, 1981 ;Créer and Tucholka, 1982 ;Créer et al, 1983 ;Constable et McElhinny, 1985 ;Verosub et al, 1986;Lund, 1996 ;Peck et al, 1996 ;Saarinen, 1999 ;Ali et al 1999 ;. Des décalages temporels dans les différents enregistrements lacustres ont été observés et parfois interprétés comme la dérive de la composante non-dipolaire du champ magnétique terrestre {e.g., Créer, 1981 ;Verosub et al, 1986).…”