Archaeomagnetic and radiocarbon studies of aboriginal fireplaces

Abstract

This thesis presents the results of the first detailed archaeomagnetic and radiocarbon study of ancient Aboriginal fireplaces in Australia. A furnace, designed and built during the present study, is described in detail, and a reappraisal is made of the method of calculating radiocarbon ages. A pilot study of some undated Aboriginal fireplaces demonstrated the suitability of their baked clay ovenstones for Thellier palaeointensity studies, and allowed estimates of their ages to be made by comparing the measured ancient field intensities with the known prehistoric fluctuation of the Earth's magnetic field. Measurements on oriented clay-sand ovenstones from a series of ovens exposed on the open plains revealed that the ovenstones had not been displaced since the time of last cooling, and provided a preliminary archaeo-secular variation curve for southeastern Australia. A comparison of these results with those from other parts of the world suggests that the non-dipole contribution to the field in southeastern Australia was small during the period 1500 yr to 500 yr B.P Measurements were also made on a series of fireplaces exposed by modern erosion of ancient sediments in the Lake Mungo lunette. The fireplaces recorded a geomagnetic excursion occurring between 31,000 yr and 25,000 yr B.P. It appears that there were two excursion loops, one characterized by a high geomagnetic moment (about 50 x 10²⁵ gauss.cm³ ), and a second loop at a time of low dipole moment (about 50 x 10²⁵ gauss.cm³ ). During both these excursion loops, the geomagnetic field rotated more than 90° away from the axial dipole configuration. The evidence suggests that the geomagnetic field was dipolar during both excursion loops. The excursion at a time of low dipole moment may be an aborted reversal of the field, while the observed excursion with a large dipole moment is thought to be a rare and perhaps new type of dynamo behaviour. The similarity of virtual geomagnetic pole positions in both types of excursion to those observed in other excursions or polarity transitions during the last 15 million years suggests that some common mechanism is controlling or restraining the processes occurring in the Earth's core.