Geomagnetic investigations of some recent British sediments

Abstract

In order to study the geomagnetic secular variation in Britain during the past 10000 years six metre and one metre long cores of post -Glacial sediment have been collected from three British lakes. Magnetic measurements were made on both the whole cores and on sub - samples from them. The natural remanent magnetization is stable, and records well defined declination and inclination swings of 40 -50° and 15 -20 o peak -peak amplitudes respectively. There is also much between swing detail. Both the major swings and many finer details are readily correlated from core to core and lake to lake. Thirty radiocarbon age determinations, pollen analyses and correlations with observatory and archaeomagnetic records have been combined to derive a detailed time scale. This time scale has been transferred to all the ' cores by means of magnetic susceptibility and lithological correl- ations. Fourier analyses of declination and inclination, separately and combined as a complex pair, show that the variations are not simply periodic, nor is their spectrum constant with time. The geomagnetic vector has been looping in a clockwise sense for most of the past 10000 years. The records have been compared with other lacustrine and archaeomagnetic results from other countries, and various models for the non dipole field are discussed in the light of all these records.

Attempts to retrieve palaeointensities from the sediments have shown that ARM may provide an effective normalization parameter in some, but by no means all cases. Determining the suitability of sediments for palaeointensity, studies is complicated.

Field tests were carried out to investigate the magnetic minerals in the soils of the drainage basin of one lake; soil and lake sediment samples were further analysed in the laboratory. In addition to magnetite an impure form of maghaemite appears to play some part in carrying the NRM of this lake sediment.

Continental shelf sediment cores have also been collected from the Firth of Clyde. These have been correlated with each other by their susceptibility logs, and dated by correlation of their secular variation records with the lake sediment record.