GLACIER RETREAT IN THE MARITIME ALPS AREA

In the southernmost tract of the Alps (Italian‐French Maritime Alps), extensively covered by glaciers during the Last Glacial Maximum, about 30 small glaciers were present by the end of the Little Ice Age. The aim of this paper is to document the progressive decrease towards exhaustion of these glaciers, located at the latitude of 44° N, highlighting the factors affecting their retreat. All available data sources were investigated for this work including: the annual glaciers fluctuations record, comparative analyses of historical maps and multi‐temporal oblique photographs and direct surveys in the field. The history of the Maritime Alps glaciers fluctuations was thoroughly reconstructed. Stationary conditions were observed from 1896 up to the beginning of the 1930s; since then they underwent phases of withdrawal with variable intensity. In the early 1990s, only six glaciers were still present, the extent of which were all was dramatically reduced. In the past two decades, the Maritime Alps glacier fronts experienced a global retreat of about 100 m, with a sharp acceleration after 2002. Currently ice patches along cirque walls and/or semi‐buried lenses of ice are still present; morphological evidence of permafrost creeping in the glacier forefield accounts for the incipient transition to periglacial landforms (i.e. rock glaciers). The main factors controlling glaciers retreat seem to have been their original extent at the beginning of the current regressive phase and their distance from the main chain divide. From a climatic point of view unfavourable factors for glaciers persistence have been in the last decades a remarkable and sharp temperature increase, a decrease in winter snowfall and a shift of the rainfall peak from autumn to spring.     

Palaeoenvironments associated with caddisfly-dominated microbial-carbonate mounds from the Tipton Shale Member of the Green River Formation: Eocene Lake Gosiute

Caddisfly‐dominated microbial‐carbonate mounds and avian eggshell fragments are common in a nearshore, oolite facies of the Tipton Shale Member of the Eocene Green River Formation. The fossils occur in a 9 m thick carbonate sequence exposed on the south‐west flank of Essex Mountain, Sweetwater County, Wyoming. The eggshell was determined to be of avian origin by examination of the radial eggshell microstructure by scanning electron microscopy and polarized light microscopy. Common allochems in the limestone include: ooids, pisoids, oncoids, ostracods, gastropods, intraclasts, caddisfly larval/pupal cases, fish bones, avian bones and avian eggshell fragments. Carbonate mineralogy varies between 95% calcite and 95% dolomite. Oxygen stable isotopes vary between −10·1 and −1·3‰ (V‐PDB). Carbon and oxygen stable isotopes co‐vary. Co‐variance of oxygen and carbon stable isotopes indicates that Lake Gosiute was hydrologically closed during the formation of this oolite sequence. Positive excursions of oxygen stable isotope values are correlated with dolomite, caddisfly cases and avian eggshell fragments. These factors are probably associated with lake regression and more saline conditions. Negative excursions of oxygen stable isotope values are correlated with calcite, gastropods and intraclasts. These factors are probably associated with lake transgression and lake freshening. In the 9 m oolite sequence studied there are at least two inferred transgressions, three regressions and two storm events. The caddisfly cases and avian eggshell fragments occur in sediments formed only during low‐stands of the lake as determined by other lake level proxies (dolomites and more positive oxygen stable isotope values). As avian eggshell fragments were formed under subaerial conditions, the avian eggshell fragments independently confirm lake regression. In addition, the occurrence of avian eggshell fragments during saline phases of the lake may indicate that the birds which produced the eggshell preferred saline conditions (or that their food preferred saline lake water).     

Farola Monte Hermoso: Fission-track Dating of Darwin's mammals deposit in Argentina

The Montehermosan Land Mammal Age has been etablished by fission-track dating of the mammal deposit of Farola Monte Hermoso. The fossil bones were found in the lowest part of the outcrop, consisting of fluvially derived clay and silt. The erosional surface between the lowest portion and the overlying aeolian sands is charcterised by a compact unit of rounded volcanic glass shards; a second fresh volcanic ash level is about 1 m above. EDS analyses and fission-track age analyses were performed on the two levels. The age determined for the lower volcanic ash level, 1.7 ± 0.4 Ma, is a minimum age for the Chapadmalalan mammal assemblage and a maximum age for the Uquian mammal assemblage in the Bonaerense area.     

Deposits and physical properties of pyroclastic density currents during complex Subplinian eruptions: the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy

Small-scale pyroclastic density currents (PDCs) associated with the AD 472 (Pollena) eruption of Somma-Vesuvius, Italy, were generated by both magmatic and phreatomagmatic explosive fragmentation. The resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction, fluid escape and traction processes. Stratigraphic and lithofacies analysis of these PDCs offers a new perspective on the en masse versus progressive aggradation debate for PDC deposition. In particular, the analyses indicate that PDCs were density stratified with a basal underflow dominated by grain interactions. The underflows comprised trains of self-organized granular pulses of variable thickness and magnitude, depending on the overall particle concentration and fluid turbulence. A change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially (stepwise). In this model each pulse stops en masse and the whole deposit aggrades progressively. Particle concentration, density, mean velocity, and flow height were assessed for the studied PDCs using differaent methods for massive and stratified deposits. The calculated mobility of the flows was 0·2 to 0·3, in the expected range for small-scale PDCs.