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A palaeobotanical study shows that La Cerdanya and the eastern Pyrenees had already reached their current height at least 10 million years ago.

Image: Fossil maple leaf (Acer tricuspidatum) from the Miocene of La Cerdanya. Author: Aixa Tosal.

La Cerdanya is a Pyrenean tectonic rift formed about 10 million years ago, in the Upper Miocene.  At present, the Cerdanya valley floor is at an average altitude of 1100 m, in the middle of the axial zone of the Pyrenees. However, in the last decade there has been some scientific controversy as to whether this height is the result of a relatively modern geological phenomenon (hundreds of thousands of years ago) or whether it is the result of the original uplift of the Pyrenees (tens of millions of years ago). The work of researcher AixaTosal, a member of the Biodiversity Research Institute of the University of Barcelona (IRBio), on plant fossils preserved in the sediments of an ancient lake in Cerdanya may help to resolve this conflict. The fossil record of the Miocene of Cerdanya is quite complete and has been studied continuously from the beginning of the 19th century to the present day. Particularly noteworthy are the plant and insect fossils, which have been preserved buried in the clays that settled to the bottom of a lake located around Bellver some 10 million years ago.
A team from the Department of Earth and Ocean Dynamics of the UB led by Dr. Carles Martín-Closas, member of IRBio, has carried out a study of the leaves of the fossil dicotyledons from La Cerdanya based on the CLAMP method (Climate Leaf Analysis Multivariate Program), which has provided information on the main climatic parameters of that time (Tosal et al., 2021). This method is based on the comparison of the adaptive characteristics of the leaves (size, shape, types of margin, apex and base) with those of the leaves of present-day plants. The climatic database of these present-day plants has been obtained from meteorological stations distributed all over the world and which have records of more than thirty consecutive years. The results showed that the mean annual temperature was 11.4 ± 2.1 ºC, the mean temperature of the warmest month was 21.6 ± 2.5 ºC, the mean temperature of the coldest month was 1.8 ± 3.4 ºC, the rainfall of the 3 rainiest months was 661 ± 38 mm and that of the 3 driest months was 194 ± 229 mm. Consequently, Cerdanya had a much warmer and more humid climate than today, corresponding to the Cfb climate of the Köppen classification, i.e. a warm temperate and very humid climate, similar to that which currently exists in certain mountains in Japan. It is significant that these climatic characteristics were cooler and wetter than those obtained in other European localities of the same age, almost all of which were situated at low altitudes above sea level in the Upper Miocene.
It is well known that temperature decreases in altitude according to a constant that varies with latitude. Therefore, if we know the temperature of a weather station at a known altitude and compare it with another one located at a higher altitude, we can know the altitude of the second station. In the case of the Miocene of Cerdanya, its temperature has been compared with that obtained on the ancient coastline, which was located 10 million years ago in the Vallès, specifically near Terrassa, where numerous fossil plant remains have also been preserved. The result was clear: Cerdanya was already in the Upper Miocene between 1100 and 1500 m above sea level. The differences with previous studies, which obtained heights of a few hundred metres for Cerdanya, have been attributed to biases, either in the insufficient preservation of the materials used in geochemical analyses (the phenomenon known as diagenesis) or in the precision of the method used (indicators that are not very specific, as in the case of fossil pollen). Similar studies should be repeated in other parts of the Pyrenees, where the results may be different, in order to achieve a more general knowledge of the height of the mountain range in the geological past.
Tosal, A.; Verduzco, O.; Martín-Closas, C. 2021. CLAMP-based palaeoclimatic analysis of the late Miocene (Tortonian) flora from La Cerdanya Basin of Catalonia, Spain, and an estimation of the palaeoaltitude of the eastern Pyrenees. Palaeogeography Palaeoclimatology Palaeoecology, 564: 110186