Rioual, Patrick; (2000) Diatom assemblages and water chemistry of lakes in the French Massif Central: A methodology for reconstruction of past limnological and climate fluctuations during the Eemian period. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is a contribution to the ongoing research on palaeoclimate carried out in French crater lakes, and especially the site of Ribains (French Massif Central). The first objective was to reconstruct palaeolimnological and palaeoclimatic changes recorded in the sediments of Ribains crater during the last interglacial period (Eemian). The chosen approach was to examine the sedimentary record of diatoms. A robust knowledge of the modem ecological distributions of individual taxa and species assemblages was required. A data-set was produced from the analysis of 207 water samples collected between May 1996 and May 1998 in 25 lakes of the Massif Central. The relationships between diatom communities and their physical-chemical environment were statistically analysed using multivariate techniques. The ecological niches of the most common planktonic species were defined. The second objective was to quantify the fossil records in terms of the dominant environmental variable through the development of a transfer function. The transfer function was thus built upon a training-set of 29 subfossil diatom assemblages collected from surface sediments. Canonical correspondence analyses (CCA) and weighted averaging partial least squares (WA-PLS) regression and calibration were used to determine that water-alkalinity was the variable explaining most of the diatom distribution. The two-component WA-PLS model was found to have the lowest error of prediction. To assess its reliability, the alkalinity transfer function was first applied on down-core diatom assemblages found in contemporary crater lakes of the Massif Central. The high-resolution diatom analysis of the Ribains mastercore focused on two transition periods. The first was the transition between the penultimate glacial and the Last interglacial stages. The second period studied was the end of the Eemian and the transition to the last glacial period. The diatom assemblages present in the Ribains sediment core were largely dominated by planktonic species. Interpretation of the Ribains diatom sequence was made using the ecological preferences of the dominant taxa, as defined earlier, supported by the application of the alkalinity transfer function and by analogue matching. Diatom and pollen records were in broad agreement but the diatom record appears to be more sensitive to changes of short duration (e.g. a climatic oscillation at the transition between the Saalian glacial and the Eemian). In conclusion, the variations observed in the Ribains diatom record were discussed in terms of climate, and a comparison with the pollen record was made.

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