A new approach for palaeoaltitude estimates based on pollen records: example of the Mercantour Massif (southeastern France) at the earliest Pliocene

The altitude of the Mercantour Massif (Southern Alps, France) during the earliest Pliocene has been estimated using a new quantification method. Instead of classical methods using geomorphologic criteria, our method uses biogeographic and climatologic criteria: composition of the modern vegetation belts in the European and peri-Mediterranean mountains, and Pliocene annual temperature estimates obtained from fossil pollen data. The climatic transfer function indicates for the coastal earliest Pliocene of the studied area a mean annual temperature of 16.5°C. Such a temperature today occurs at 38.5°N latitude in the Mediterranean region. The relation established by Ozenda between the present-day vertical and latitudinal vegetation assemblages evidences a shift of, on average in Western Europe, 110 m in altitude per degree in latitude. As a consequence, it is possible, taking into account that the Pliocene lapse rate was almost similar to the modern one, to estimate the minimum altitude of the massif at the earliest Pliocene. The so-obtained palaeoaltitude estimate is close to that obtained with the geomorphological method: the Mercantour Massif was almost 2000 meters high at the earliest Pliocene.