Analysis of geomagnetic secular variations 10 000 to 30 000 years bp, Lac du Bouchet, France

Abstract

Cores of the bottom sediments from Lac du Bouchet, in the Haute Loire, France (44.9° N, 3.8° E) have been taken using Mackereth type pneumatic piston corers. The sediments have been dated by palynological control back to 15 000 years bp (years before present), and by radiocarbon age determinations using both conventional and accelerator methods. The depth/time transform provisionally used here dates the sequence from 10 000 to 30 000 years bp. The stacked records of demagnetised remanent magnetisation from eight cores have been examined. Fourier and maximum entropy method spectral analyses show good agreement. Inclination and declination spectra are different in that the dominant inclination periods tend to fall in the negative part of the complex spectrum, while the dominant declination periods tend to fall in the positive part. The VGP path traced out by the remanent magnetisation vector shows predominantly clockwise looping, consistent with essentially westward drifting geomagnetic sources. The detailed form of the VGP path is affected by the degree of smoothing, but clockwise looping always predominates averaging ∼ 67% over the whole time interval investigated. A band of frequencies between ∼ 0.9 and ∼ 0.3 cycles per thousand years is associated with the strongest bias towards clockwise rotation (∼ 80% to 90%) while a band between ∼ 0.3 and 0.1 cycles per thousand years is associated with only ∼ 50% clockwise rotation. Individual core records show some negative inclinations at horizons where inclination minima occur suggesting that ‘excursions’ should be regarded as extreme values of secular variation rather than aborted polarity reversals of the main field.