Rock-magnetic properties of Eemian maar lake sediments from Massif Central, France: a climatic signature?
Résumé
The hypothesis of a climate variability within the last interglacial (Eemian), suggested by oxygen isotope data of a Greenland ice core (GRIP) was later supported by magnetic susceptibility, organic matter and pollen signatures in the Eemian lake sediments of the Lac du Bouchet (France). In the present paper, we analyse in detail the rock-magnetic properties of these sediments in order to determine the origin of the magnetic susceptibility variations. Furthermore, in order to verify the empirical relation between magnetic susceptibility and climate, we studied two Eemian sequences from neighbouring lakes (Lac St. Front and Ribains maar lake). Rock-magnetic investigations mainly consisted of analyses of anhysteretic and isothermal remanent magnetizations along with hysteresis measurements carried out on pilot samples. Lac du Bouchet susceptibility variations are at least partly affected by postdepositional magnetite dissolution. Lac St. Front sediments show a complex behaviour resulting in partly contradicting interpretations of rock-magnetic parameters. Possible explanations may be the presence of a very fine-grained magnetite or the presence of maghemite. Ribains maar lake sediments show a homogeneous magnetic mineralogy without major variations in most of the rock-magnetic parameters. Susceptibility records of the three lakes were compared under the simplified assumption that magnetic susceptibility mainly reflects variations of the detrital (titano-)magnetite input. Only a poor correlation between the three Eemian susceptibility sequences is observed, suggesting that local effects (including dissolution and dilution effects) may have altered the original detrital (climatic) signal. The purely detrital origin of the Lac du Bouchet Eemian susceptibility variations must thus be questioned but it nevertheless appears to be the only lake whose susceptibility variations can tentatively be correlated with the GRIP Eemian oxygen isotope record.