Utilisation des isotopes stables de l’oxygène des cernes d’arbres pour déterminer l’origine des inondations passées : premiers résultats pour la péninsule ibérique

Abstract

Tree-ring studies have been used for over fifty years to date and quantify past flood events. Stable C and O isotopes in tree-rings have also been extensively applied to the reconstruction of past environmental conditions and their changes over time. However, the two approaches have not previously been combined. In this study we explore whether the meteorological origin of precipitation causing past flood events might be assessed by investigating oxygen stable isotopes in tree rings. It is well known that floods may have different origins, e.g. heavy convective rainstorms, frontal precipitation, snow melting, etc.; each of these floodwater sources bears a particular isotopic fingerprint. This paper presents the first results of this methodology applied to a recent flash flood event occurring in Central Spain. For this purpose, a well-known heavy-rain convective event was chosen from the recent flood record. In the forested area affected by this event, six cores from each of four selected species (Pinus sylvestris L., Pinus pinaster Ait., Quercus pyrenaica L. and Alnus glutinosa (L) Gaertn), were sampled using an incremental borer. After α-cellulose extraction, the oxygen-isotope composition (δ18O) in tree-rings was analyzed and compared with climate data and δ18O values in precipitation. The isotope signal in tree-ring cellulose was dominated by spring climatic conditions but, after removing this “spring signal”, a clear and site-specific signal in latewood δ18O emerged, which was associated with the heavy-rain event. These preliminary results encourage more in-depth analyses aimed at recognizing specific precipitation sources and separating different populations of past floods according to their cause.