PALEOCLIMATIC SIGNIFICANCE OF THE 300-KA MINERAL MAGNETIC RECORD FROMTHE SEDIMENTS OF LAC-DU-BOUCHET, FRANCE

A set of three 50-metre sediment cores from a small crater lake, Lac d u Bouchet, show down-core changes in the magnetic mineralogy which ref lect the changing past environmental and climatic conditions around th e lake over the last 300 ka. This work builds on studies of 20 metre a nd shorter cores from the same lake, extending both the time covered b y the sequence, and the details of the nature of the magnetic minerals and their stratigraphic variation. The primary magnetic mineral is a titanomagnetite derived from the basaltic crater walls, and was identi fied by magnetic experiments and microprobe analyses of the sediment. Down-core susceptibility records, at 2.5 cm (150 to 300 year) resoluti on, primarily show the changing titanomagnetite concentration. Concent rations are high in the silty clays deposited under glacial conditions , and are low in the organic-carbon-rich compacted gyttjas deposited u nder temperate conditions. The low concentration layers also have high percentages of arboreal pollen, which have been correlated to the war m stages of the marine delta(18)O stratigraphy back to the beginning o f Stage 8. This correlation is supported by Ar/Ar dates for a tephra l ayer near the base of the cores of 286+/-17 ka BP. The mineral-magneti c Variation is accounted for by: (a) dilution (input of detrital mater ial decreases and input of organic matter increases under more tempera te climates); and (b) dissolution of the titanomagnetite when the orga nic matter content of the sediment is high. In this way, the susceptib ility is related to the palaeoenvironment and palaeoclimate. In additi on to susceptibility, the laboratory magnetizations SIRM and ARM have been implanted in the samples, and they all show the same stratigraphi c sequence of peaks and troughs. Differences between susceptibility, S IRM and ARM are due to variations in the size of the magnetite intergr owths within the titanomagnetite, and the non-linear concentration dep endence of ARM. In the extended older part of the sequence, the minera l magnetic record indicates that, like the Eemian interglacial (delta( 18)O Stage 5e), the warm sub-stages of delta(18)O Stage 7 also have in ternal variability, and that some short duration warm events occur in delta(18)O Stage 8. The mineral magnetic record from Lac du Bouchet ex tends beyond the range of the Greenland ice record, and is more detail ed than typical marine sediment records. (C) 1996 Elsevier Science Ltd