Michel Monbaron

400 Years of Debris-Flow Activity and Triggering Weather Conditions: Ritigraben, Valais, Switzerland

Abstract Three major rainfall events have caused considerable damage in the Valais region (Swiss Alps) since 1987. Substantial debris flows originating from periglacial environments were recorded during the August 1987 and September 1993 rainfall events, whereas no debris flows occurred in October 2000. This paper aims at putting these large-area events and the apparent increase in debris-flow frequency into a wider context by reconstructing the past debris-flow activity in the Ritigraben torrent (Mattertal, Valais) with dendrogeomorphological methods. Tree-ring analysis allowed the reconstruction of 53 events, going back to the year 1605. Previously, only 10 debris flows had been known for the torrent, and these were limited to the period between 1922 and 2002. Results further show that the apparently above-average concentration of events since 1987 was mainly caused by insufficient and short archival data. In fact, debris flows occurred even more frequently in the nineteenth century than they do today. The spatial distribution of injured trees in particular years further indicates that significant events, like the one in 1993, always occurred in the torrent. Finally, reconstructed event years were compared with archival data on flooding in neighboring catchments. The comparisons prove that large-area events like those in 1987, 1993, or 2000 have at least been as common in the past as they are today.

AN ABELISAUROID (DINOSAURIA: THEROPODA) FROM THE EARLY JURASSIC OF THE HIGH ATLAS MOUNTAINS, MOROCCO, AND THE RADIATION OF CERATOSAURS

Abstract The fossil record of abelisauroid carnivorous dinosaurs was previously restricted to Cretaceous sediments of Gondwana and probably Europe. The discovery of an incomplete specimen of a new basal abelisauroid, Berberosaurus liassicus, gen. et sp. nov., is reported from the late Early Jurassic of Moroccan High Atlas Mountains. Phylogenetic analysis recovers Ceratosauroidea and Coelophysoidea as sister lineages within Ceratosauria, and Berberosaurus as a basal abelisauroid. Berberosaurus is the oldest known abelisauroid and extends the first appearance datum of this lineage by about 50 million years. The taxon bridges temporal, morphological, and phylogenetic gaps that have hitherto separated Triassic to Early Jurassic coelophysoids from Late Jurassic through Cretaceous ceratosauroids. The discovery of an African abelisauroid in the Early Jurassic confirms at least a Gondwanan distribution of this group long before the Cretaceous.

Atlasaurus imelakei n.g., n.sp., a brachiosaurid-like sauropod from the Middle Jurassic of Morocco alen

The nearly complete skeleton of a large sauropod discovered at Wawmda (High Central Atlas of Morocco) in strata of Bathonian-Callovian age represents a new taxon: Atlasaurus imelakei n.g., n.sp. The sauropod appears to be closer to Brachiosaurus than any other known sauropod, but possesses (relative to the length of the dorsal vertebral column) a larger skull, shorter neck, longer tail and more elongated limbs. The presence of large sauropods of Middle Jurassic age is very important in understanding the history and the evolution of these Mesozoic giants.

Tension wood formed in Fagus sylvatica and Alnus glutinosa after simulated mass movement events

Due to the likelihood of global climate change, the frequency and magnitude of natural hazards such as mass movements may likewise change, thus favouring the refinement of methods to detect and quantify geomorphic events when precise records are not available. Geomorphic events typically have a significant effect on tree growth, e.g., reaction wood marked by changes in ring widths and wood density. To date, several dendroecological techniques have been developed to document the occurrence of these events but it rarely has been possible to retrieve additional information from reaction wood concerning the precise kind and intensity of geomorphic events. Additional qualitative information inferred from reaction wood of trees holds the potential to not only document but also estimate important characteristics of natural hazard events. To refine the methods already used in dendrogeomorpology, experiments simulating various geomorphic events were used to monitor subsequent wood anatomical responses of Fagus sylvatica and Alnus glutinosa. The preliminary results indicate that these two common broadleaf tree species show variations in their reactions to different experimental treatments.