Paleoenvironnement and paleoclimate reconstruction for the early to middle Miocene from stable isotopes in pedogenic carbonates (Digne-Valensole basin, southeastern France)

Stable isotope (C and O) analyses were carried out on pedogenic nodules from carbonate-rich paleosols from early to middle Miocene strata in the Digne-Valensole foreland basin (southeast France). Paleosols from three sections corresponding to several paleoenvironments were sampled in order to investigate the local influence on the isotope record of the depositional sites: a deltaic floodplain (Châteauredon section, BCR), an inland floodplain (Saule Mort section, SM) and a shallow lacustrine system (Pont d’Aiguines section, PA). According to their biostratigraphy, they correspond to the same stratigraphic interval. Paleosols are regularly distributed along the three sections. They have developed either on floodplain alluvium or on palustrine carbonate mud. They are characterized by fersiallitic red soils with a calcic horizon, typical of a xeric soil moisture conditions with a marked seasonality. Carbonate nodules sampled at depths of 0.3 to 1 m below the preserved top soil are considered suitable for paleoecological interpretation. The cross-plot of the δ18O and δ13C values shows well-distributed data indicating that the pedogenic nodules were not affected by any major recrystallizations during burial. Their distribution indicates climate conditions similar to those of modern Mediterranean soils, characterized by carbonates precipitating from enriched meteoric water. A refined stratigraphic attribution of all the sections is based on the carbon isotope chemostratigraphy developed on the Châteauredon section by correlation with marine carbon isotope data [Bialkowski et al., 2006]. All sections extend from very early Aquitanian to middle Serravallian. No major hiatus was identified except for the one associated with a highly karstified surface in the Pont d’Aiguines section. The hiatus duration, estimated to 1 Ma, is interpreted as the record of some deformation at the front of the Digne thrust sheet during the Langhian. Most carbon isotope values are heavier than −8‰, suggesting a water-stressed C3 vegetation growing in a semi-arid environment. These semi-arid conditions are found in all sections during early Aquitanian and late Burdigalian to early Serravallian (Middle Miocene climatic optimum, MMCO), indicating similar environmental conditions between the coastal sites (BCR and PA sections) and the inland SM section. From middle Aquitanian to late Burdigalian, data from the coastal sites indicate typical C3 vegetation whereas xeric conditions characterized the inland site, suggesting distinct local ecosystems. The interpretation of the oxygen isotope record is more complex due to the different interactions during carbonate precipitation. In the early Aquitanian, the concomitant decrease in the pedogenic δ13C and δ18O isotope values is interpreted as a decrease of the water-stressed environmental conditions due either to a change in precipitation patterns resulting in low evaporation or to a fall in temperature. For the Langhian, the high δ13C values indicate xeric conditions throughout the area, and the high δ18O values at the coastal site suggest an increase in the combined effect of temperature and evaporation. Evidence of deformation related to the foreland basin evolution during this period is supported by the great differences in the oxygen isotope record of the different sections. The observed changes in the environmental conditions of the Digne-Valensole basin agree with the climate fluctuations identified on the basis of paleobotanical assemblages and mammal teeth isotope records from western European continental basins and of marine data. Our results show that a study of paleosols and the isotope composition of the pedogenic carbonate of the early to middle Miocene in the Digne-Valensole basin provide valuable information concerning the regional paleotopographic evolution of the foreland basin, as well as the distinct paleoecological systems that developed between the inland and the coastal sites and the regional paleoclimatic trends that correspond to those identified in other European continental basins. These reconstructions based on paleosols, the dry-end member of the climatic record, are complementary to those based on the paleofloras, mostly obtained from the wet-end member.