The South Atlantic Geomagnetic Anomaly: a recent anomalous feature of the secular variation during the Holocene. Paleomagnetic evidence from Holocene Chilean volcanic rocks.
Résumé
In order to better describe the secular variation during the Holocene in the southern hemisphere, we sampled 21 dated lava flows or pyroclastic flows from several Chilean volcanoes (Lonquimay, Llaima, Solipulli, Villarrica, Mocho-Choshuenco, Osorno, Calbuco). We also sampled 56 sites in Holocene lava flows with only relative ages with respect of the dated units. The steepest inclination of the geomagnetic field (-71.6°) and the highest intensity (70µT±5) are found at Osorno volcano (calibrated age range of 782-966AD). The steep inclination is also observed in dated lava flows of the same age range at Llaima volcano. The VGP associated with the steep inclination is not much different from the VGP recorded at European sites suggesting a significant dipole wobble at that time.
Mean paleointensities of 63, 60.3 and 57.5µT obtained in three dated units in the time range 0-2000BC confirm the high geomagnetic dipole moment of the Earth’s magnetic field for the two millennia BC. In contrast, paleointensity results in late Pleistocene units show that the magnetic field strength was low just prior to the Holocene (-14000 -15,000 BC).
The available paleomagnetic results from Chile indicate little geomagnetic secular variation in direction during the Holocene. This observation contrasts with the large and rapid secular variation observed during the last two centuries (Roperch et al., 2015). Thus the South Atlantic Geomagnetic Anomaly appears to be a recent and unique anomalous feature of the Earth’s magnetic field during the Holocene.
Roperch et al. (2015), Phys. Earth. Planet. Int. doi:10.1016/j.pepi.2015.03.005