THE ROCK MAGNETIC SIGNAL OF CLIMATE-CHANGE IN THE MAAR LAKE SEQUENCE OF LAC ST FRONT (FRANCE)

Rock magnetic properties of the maer lake sediments of Lac St Front (M assif Central, France) reflect environmental changes during the last c limatic cycle. High magnetic concentrations are measured in the sedime nts deposited under glacial climatic periods. Low- and high-temperatur e measurements indicate that the remanence is carried by (titanium-poo r) magnetic. However, some maghemite and haematite is present in sedim ents deposited under temperate conditions. Normalized intensities and coercivities of the anhysteretic remanent magnetization (ARM) are clea rly higher for the sediments deposited during the temperate climatic p eriods of the Eemian, St Germain I, II and Mid-glacial than for glacia l sediments, but other magnetic parameters hardly differ between these groups. Due to slight differences in magnetic composition and possibl e effects of grain interactions, it is not straightforward to relate t his different ARM behaviour to magnetic grain-size variations. For the Holocene sediments, rock magnetic parameters indicate a larger grain size. This trend is also suggested by granulometric experiments with a n optical laser granulometer. Dissolution of smaller grains is the mos t likely explanation for this larger grain size. Changes in magnetic c omposition and grain size are extremely limited for the glacial sedime nts, but magnetic concentration varies considerably. Magnetic concentr ation maxima in the glacial sediments of Lac St Front correlate with t hose of the nearby Lac du Bouchet (Thouveny et al. 1994). Correlating the susceptibility records of these sequences with the delta(18)O reco rd of the GRIP ice cores (thouveny et al. 1994) suggests that magnetic concentration maxima may corresppond with short cold climatic episode s, associated with Heinrich events.