The deglaciation history of the Simplon region (southern Swiss Alps) constrained by 10Be exposure dating of ice-molded bedrock surfaces

The deglaciation history of the Swiss Alps after the Last Glacial Maximum involved the decay of several ice domes and the subsequent disintegration of valley glaciers at high altitude. Here we use bedrock exposure dating to reconstruct the temporal and spatial pattern of ice retreat at the Simplon Pass (altitude: ∼2000 m) located 40 km southwest of the ‘Rhône ice dome’. Eleven 10Be exposure ages from glacially polished quartz veins and ice-molded bedrock surfaces cluster tightly between 13.5 ± 0.6 ka and 15.4 ± 0.6 ka (internal errors) indicating that the Simplon Pass depression became ice-free at 14.1 ± 0.4 ka (external error of mean age). This age constraint is interpreted to record the melting of the high valley glaciers in the Simplon Pass region during the warm Bølling–Allerød interstadial shortly after the Oldest Dryas stadial. Two bedrock samples collected a few hundred meters above the pass depression yield older 10Be ages of 17.8 ± 0.6 ka and 18.0 ± 0.6 ka. These ages likely reflect the initial downwasting of the Rhône ice dome and the termination of the ice transfluence from the ice dome across the Simplon Pass toward the southern foreland. There, the retreat of the piedmont glacier in Val d’Ossola was roughly synchronous with the decay of the Rhône ice dome in the interior of the mountain belt, as shown by 10Be ages of 17.7 ± 0.9 ka and 16.1 ± 0.6 ka for a whaleback at ∼500 m elevation near Montecrestese in northern Italy. In combination with well-dated paleoclimate records derived from lake sediments, our new age data suggest that during the deglaciation of the European Alps the decay of ice domes was approximately synchronous with the retreat of piedmont glaciers in the foreland and was followed by the melting of high-altitude valley glaciers after the transition from the Oldest Dryas to the Bølling–Allerød, when mean annual temperatures rose rapidly by ∼3 °C.

Late Glacial and Holocene environmental history of the Pirin Mountains (SW Bulgaria): a paleolimnological study of Lake Dalgoto (2310 m)

Diatoms, Cladocera, and chironomids preserved in the sediments of Lake Dalgoto were studied to reconstruct the history of the lake ecosystem in the context of the vegetation history as represented by the pollen stratigraphy. Younger Dryas silty sediments at the base of the core are characterized by low diversity of aquatic organisms. The transition to the Holocene is indicated by a sharp change from silt to clay-gyttja. The migration and expansion of trees at lower elevations between 10200 and 8500 14C-yr BP, along with higher diversities and concentrations of aquatic organisms and the decreased proportion of north-alpine diatoms, point to rapidly rising summer temperatures. After 6500 14C-yr BP the expansion of Pinus mugo in the catchment coincides with signs of natural eutrophication as recorded by an increase of planktonic diatoms. In the late Holocene (4000–0 14C-yr BP) Pinus peuce and Abies are reduced and Picea expands. Cereal grains and disturbance indicators suggest late-Holocene human modification of the vegetation.

Middle to Late Holocene vegetation history of the Upper Engadine (Swiss Alps): the role of man and fire

o reconstruct the vegetation and fire history of the Upper Engadine, two continuous sediment cores from Lej da Champfèr and Lej da San Murezzan (Upper Engadine Valley, southeastern Switzerland) were analysed for pollen, plant macrofossils, charcoal and kerogen. The chronologies of the cores are based on 38 radiocarbon dates. Pollen and macrofossil data suggest a rapid afforestation with Betula, Pinus sylvestris, Pinus cembra, and Larix decidua after the retreat of the glaciers from the lake catchments 11,000 cal years ago. This vegetation type persisted until ca. 7300 cal b.p. (5350 b.c.) when Picea replaced Pinus cembra. Pollen indicative of human impact suggests that in this high-mountain region of the central Alps strong anthropogenic activities began during the Early Bronze Age (3900 cal b.p., 1950 b.c.). Local human settlements led to vegetational changes, promoting the expansion of Larix decidua and Alnus viridis. In the case of Larix, continuing land use and especially grazing after fire led to the formation of Larix meadows. The expansion of Alnus viridis was directly induced by fire, as evidenced by time-series analysis. Subsequently, the process of forest conversion into open landscapes continued for millennia and reached its maximum at the end of the Middle Ages at around 500 cal b.p. (a.d. 1450).

The palaeoecological history of the Praz-Rodet bog (Swiss Jura) based on pollen, plant macrofossils and testate amoebae (Protozoa)

Stratigraphy, radiocarbon dating and analyses of pollen, plant macrofossils and testate amoebae were used to reconstruct the development and ecology of a small raised bog in a karst-dominated landscape in the Swiss Jura Mountains. Special focus was on past vegetation and on the history of Pinus rotundata in relation to anthropogenic and climatic influences. Testate amoebae were used to reconstruc-t past local soil pH and water-table depth. The inferred development of the Praz-Rodet bog typifies a classic hydroseral tefrestrialization of a small basin. Two features are specific for this site. First, the bog was much wetter than today for a long period; according to our hypothesis, this only changed as a consequence of human activities. Second, two hiatuses are present at the coring location (Younger Dryas--early Preboreal, and 4700-2800 cal. yr BP), the former probably caused by low lake productivity due to cold temperatures and the latter by the erosional activity of the adjacent small river. The date of 2800 cal. yr BP for renewed peat accumulation may be related to climatic change (Subboreal-Subatlantic transition). Pollen indicators failed to show one hiatus: an apparently complete pollen sequence is therefore no guarantee of an uninterrupted sediment accumulation. Evidence of early minor human impact on the vegetation in the Joux Valley dates back to c. 6850 calendar years, congruous with the early Neolithic in the Jura Mountains. The history of Pinuis rotindata appears to be more complex than previously believed. Human activity is clearly responsible for the present abundance of this species, but the tree was naturally present on the bog long before the first evidence of important human disturbance of the site (1500 cal. yr BP). It is suggested that, in karst-dominated landscapes, dense forests growing on mineral soils around raised bogs may significantly reduce summer evapotranspiration by acting as windbreaks. Forest clearance results in increased evapotranspiration, causing a lowering of the water table on the bog and a modification of the vegetation cover. This hypothesis has implications for the management of similar small raised bogs in karst-dominated landscape.

Influence of human impact and bedrock differences on the vegetational history of the Insubrian Southern Alps

Vegetation history for the study region is reconstructed on the basis of pollen, charcoal and AMS14C investigations of lake sediments from Lago del Segrino (calcareous bedrock) and Lago di Muzzano (siliceous bedrock). Late-glacial forests were characterised byBetula andPinus sylvestris. At the beginning of the Holocene they were replaced by temperate continental forest and shrub communities. A special type of temperate lowland forest, withAbies alba as the most important tree, was present in the period 8300 to 4500 B.P. Subsequently,Fagus, Quercus andAlnus glutinosa were the main forest components andA. alba ceased to be of importance.Castanea sativa andJuglans regia were probably introduced after forest clearance by fire during the first century A.D. On soils derived from siliceous bedrock,C. sativa was already dominant at ca. A.D. 200 (A.D. dates are in calendar years). In limestone areas, however,C. sativa failed to achieve a dominant role. After the introduction ofC. sativa, the main trees were initially oak (Quercus spp.) and later the walnut (Juglans regia). Ostrya carpinifolia became the dominant tree around Lago del Segrino only in the last 100–200 years though it had spread into the area at ca. 5000 cal. B.C. This recent expansion ofOstrya is confirmed at other sites and appears to be controlled by human disturbances involving especially clearance. It is argued that these forests should not be regarded as climax communities. It is suggested that under undisturbed succession they would develop into mixed deciduous forests consisting ofFraxinus excelsior, Tilia, Ulmus, Quercus and Acer.

Holocene vegetation and fire history of the mountains of Northern Sicily (Italy)

Knowledge about vegetation and fire history of the mountains of Northern Sicily is scanty. We analysed five sites to fill this gap and used terrestrial plant macrofossils to establish robust radiocarbon chronologies. Palynological records from Gorgo Tondo, Gorgo Lungo, Marcato Cixé, Urgo Pietra Giordano and Gorgo Pollicino show that under natural or near natural conditions, deciduous forests (Quercus pubescens, Q. cerris, Fraxinus ornus, Ulmus), that included a substantial portion of evergreen broadleaved species (Q. suber, Q. ilex, Hedera helix), prevailed in the upper meso-mediterranean belt. Mesophilous deciduous and evergreen broadleaved trees (Fagus sylvatica, Ilex aquifolium) dominated in the natural or quasi-natural forests of the oro-mediterranean belt. Forests were repeatedly opened for agricultural purposes. Fire activity was closely associated with farming, providing evidence that burning was a primary land use tool since Neolithic times. Land use and fire activity intensified during the Early Neolithic at 5000 bc, at the onset of the Bronze Age at 2500 bc and at the onset of the Iron Age at 800 bc. Our data and previous studies suggest that the large majority of open land communities in Sicily, from the coastal lowlands to the mountain areas below the thorny-cushion Astragalus belt (ca. 1,800 m a.s.l.), would rapidly develop into forests if land use ceased. Mesophilous Fagus-Ilex forests developed under warm mid Holocene conditions and were resilient to the combined impacts of humans and climate. The past ecology suggests a resilience of these summer-drought adapted communities to climate warming of about 2 °C. Hence, they may be particularly suited to provide heat and drought-adapted Fagus sylvatica ecotypes for maintaining drought-sensitive Central European beech forests under global warming conditions.