Sunspot-like and ENSO/NAO-like periodicities in lacustrine laminated sediments of the Pliocene Villarroya Basin (La Rioja, Spain)

This is a study of an extremely good outcrop of 1745 varves in the Villarroya Pliocene Basin (Spain). The thicknesses of the light and dark layers, which were obtained from enlarged photograph negatives, are dealt withseparately, and three time series, of light, dark, and the sum of both layers, are performed. Periodicities of about 12, 6–7 and 2–3 years have been obtained. The origin of the recurrent behaviour of the sediments with these periods could be related to natural phenomena like sunspot cycles, ElNiño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO) and Quasi-biennial Oscillation (QBO), as these phenomena are capable of modulating the climate in this frequency band. From our point of view, there could be a close relationship between sedimentation in the Villarroya Basin and these natural phenomena, which is moved through climate. The light layers record the run-off of gastropods, ostracods and charophytes from a seasonal productivity cycle, and are generated during late summer and autumn. It seems clear that the climatic information provided by these cycles corresponds mainly to summertime, which would be translated into a variation of the light layer thickness due to a more or less intense development of the flora and fauna, being thicker in years of higher temperatures and thinner in years of lower temperatures. Considering the results, we would suggest that the sedimentation of these materials in the Pliocene was driven by solar activity or/and ENSO, NAO, and QBO phenomena. Nowadays these phenomena produce rainfall and temperature changes with the cyclicities shown in this study and we think they could have operated in the same way in Pliocene times.