Résumé
The drain of the ancient aqueduct of Frejus is in part clogged by a laminated travertine concretion. The hydrology of the springs indicates that the lamination is seasonal and that the dark lamina doublet is annual. 102 doublets have been recorded. With the aim of deciphering possible climatic microcycles, a specific spectral analysis was applied to the chronological series of the thickness of laminae. The various spectrograms obtained revealed a high frequency robust peak at 7.1-7.3 years and a low frequency peak at 39-43 years. Such bipartition of frequencies characterizes the NAO which dominated the climate of Western Europe. As the two modes we present in the studied series, it is possible to infer that the mode of oscillation is not strictly determined. When using a Gaussian filter at 7.3 years, as was to be expected, a low frequency modulation was highlighted at 50+ or -3 years. The considered period of 102 years which extended on the basis of the archaeological data from the mid-Ist century to the mid-IInd century, seems to offer alternative phases of active NAO with phases of less active NAO. It may be described as moderately wet with alternative dryer-colder phases. These results which demonstrate the quality and accuracy of the climatic records in laminated travertines lead us to project other future applications in earlier Holocene series.