A preliminary palaeomagnetic study of the Holocene and late Wurmian sediments of Lac Du Bouchet (Haute-Loire, France)

Abstract

Summary. Lac du Bouchet is a small maar type lake located at ∼45°N, 4°E, in central France. the bottom sediments have been sampled down to a depth of 6 m with Mackereth type corers. A chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations. Throughout the period covered, which runs from ∼22 000 years ago through to the present time, declination and inclination variations consist of oscillations of up to about 30° amplitude. No abnormally large amplitude ‘excursions’ in direction are recorded. the main contribution of this study is the extension of the European secular variation (SV) records to the interval from ∼13000 BP (years before present) to ∼22000 BP: the sediments deposited through this interval (Ancient Dryas intc Wurm) originated from the erosion of the internal walls of the volcano. Spectrum analyses of the declination and inclination curves from 13000 to 22000 BP reveal peaks at periods of 3000–4000, ∼1800, ∼1150 and ∼700 yr. Cross-spectrum analysis shows strong coherence of the two signals at the longest of these periods. the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector, possibly suggesting westward drifting sources. Although the palaeomagnetic SV record through the Holocene (0-10300 BP) is compressed into ∼1.7m of highly organic sediments, the principal features exhibited by the ‘type’ curves established for UK can be identified and the resulting ‘magnetic’ ages are consistent with the palynology: the sense of rotation of the magnetic vector is biased equally in both senses. the Late Glacial (∼10300 BP to ∼13000 BP) is characterized by oscillating climatic conditions (Recent Dryas, Allerod and Bolling) which resulted in poorly resolved and probably discontinuous palaeomagnetic SV records which show a strong bias to anti-clockwise rotation. All ages are given in radiocarbon years.