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Sediment cores were taken at four different sites in the lake. Several chemical, physical and biologic analysis were carried out, including MSCL (density and magnetic susceptibility), XRF & XRD (elemental and mineralogical composition), carbon and oxygen isotopes, carbon content, smear slides, pollen and grain size measurements. The sediment consists of a complex succession of organic-rich, authigenic lake sediments intercalated by light-coloured, detrital layers. These detrital layers consist mainly of quartz, calcite and mica and they are interpreted as Rhone sediments, probably brought into the lake by floods. Together with the age model (C-14 datings), a detailed reconstruction of the climatic and environmental changes in the past is proposed. The longest sediment core Pf07-1_2, taken in the middle of the lake, shows a complete sediment record of the last ≈4000 yBP and reveals the climatic cold phases from Löbben (≈3000 yBP and older) to the Little Ice Age (13th-19th century). The end of the Iron Age, the Roman Period, as well as the mediaeval times could be identified in the pollen and C-14 data, indicating human impacts like forest fires, pasturing, agriculture and forest-clearings. The whole sediment record could additionally be correlated in detail with the glacial advance and retreat curve of the Aletsch glacier (Holzhauser, 1995) and other glacial data (Maisch 2000, Röthlisberger 1986) showing the influence of the Central Alpine climate to the sedimentation regime in Lake Pfafforet. The advance periods of the Aletsch glacier correlate well with the deposition of detrital sediment layers, indicating a connection between detrital sediment input into the lake and cold climate phases. REFERENCES Bendel M.; Tinner W.; Amman B. (2006): Forest dynamics in the Pfyn forest in recent centuries (Valais, Switzerland,Central Alps): interaction of pine (Pinus sylvestris) and oak (Quercus sp.) under changing land use and f ire frequency. The Holocene 16, 81-89 Burri M. (1997): Géologie de Finges et de ses environs (VS). Bulletin de la Murithienne, 115, Sion, 5-27 Holzhauser H. (1995): Gleschterschwankungen innerhalb der letzten 3200 Jahre am Beispiel des grossen Aletsch- und des Gornergletschers. Jubiläums-Symposium der Schweiz, Gletscherkommission 1993 in Verbier (VS). Zürich, 218 S. Maisch M.; Wipf A.;Denneler B. (2000): Die Gletscher der Schweizer Alpen-Gletscherhochstand 1850, Aktuelle Vergletscherungen, Gletscherschwund-Szenarien. 2. Auflage, Geographisches Institut der Universität Zürich Röthlisberger F. (1986): 10000 Jahre Gletschergeschichte der Erde. Verlag Sauerländer, Aarau, Frankfurt am Main. 416 pages .21 Evidence for climatic cooling at Grizzly Lake (Alaska) around 2800 cal. yr. BP van Raden Ulrike *, Beer Ruth**, Tinner Willy **, Gilli Adrian *, Clegg Ben***, Bigler Christian****, Hu Feng Sheng ***, Haug Gerald* *Geological Institute, ETH Zurich, Universitätstrasse 16, CH-8092 Zürich, Switzerland, (vanraden@erdw.ethz.ch) **Oeschger Centre for Climate Change Research & Institute of Plant Sciences, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland ***Program in Ecology and Evolutionary Biology, University of Illinois, 265 Morrill Hall, 505 S. Goodwin Ave, Urbana, IL 61801, USA ****Department of Ecology and Environmental Science, KBC plan 5, Umeå University, S-901 87 Umeå, Sweden Recent studies revealed climatic changes at about 2800 BP in several regions of the northern and southern hemisphere (e.g. Chambers et al. 2007; Swindles et al. 2007). In order to understand environmental and climatic changes as manifestations of this event in northern high latitudes, we investigated a 20 cm sediment core section from Grizzly Lake (Alaska) covering about 900 years of sedimentation. We conducted a high-resolution multiproxy study to reconstruct vegetation and climate changes with a focus on interrelations of different ecosystem aspects and climate. The sediment archive of Grizzly Lake is particularly suitable for paleoenvironmental studies as direct anthropogenic influence is negligible before ≈1950 AD. The high-resolution measurements of numerous proxies enable detailed investigation of leads and lags within the environment. Analyses on pollen, plant macrofossils, charcoal, chironomids and diatoms allude to a potential cooling event similar to the Little Ice Age (Tinner et al. 2008) just before 2800 cal. yr. BP. Preliminary chironomid-assemblage analysis indicates a dramatic change at a core depth of 95.5 cm (about 2900 yrs. cal. BP) where cold stenotherm taxa such as Micropsectra, Corynocera oliveri, Tanytarsus lugens and Heterotrissocladius emerge or increase in abundance. Interestingly, tree pollen (especially Picea glauca) increase at the same time. At 92.75 cm (about 2750 yrs. cal. BP) a marked decrease in tree pollen abundance (Picea mariana, Picea glauca, and Betula) coincides with an increase in pollen from shrub taxa, such as Alnus viridis, a species highly adapted to disturbance (e.g. landslides, avalanches, fire, windthrow). 16 Symposium 5: Quaternary Research
168 Symposium 5: Quaternary Research Optical core descriptions classified the sediment mainly as dark brown gyttja with a light brown silt layer at 94-91.5 cm (about 2800-2680 cal. yr. BP). Complementary sedimentological, mineralogical and geochemical analyses were conducted to verify the macroscopic and paleoecological observations. Ultra-high-resolution XRF scans were used to analyse the elemental composition along the core section. These data, combined with thin section and XRD analyses, reveal constant quiet sedimentation with very little autochthonous material within the dark sediment of the core section. The pale silt interval consists of at least seven event layers, each between 1.5 and 5.5 mm in thickness and some with an erosional base. Every layer shows a gradation with coarse bottom depositions containing high amounts of Si and Ca (evidence for quartz and carbonate) and fine top layers with high amounts of Fe, K, Ti and Al (evidence for feldspars and clays). We interpret these sediments to be washed into the lake from the Alaska Range in the North as a result of the preceding cooling event and associated reduction in forest cover and emerging higher erosion. A potential increase of precipitation that may have altered the hydrological cycle and the lake thermal stratification will be further tested by diatom-assemblage analysis. We suggest that these inferred changes in vegetation and concurrent higher erosion mark the final stage of the cold event. Pollen data and preliminary chironomid analysis reveal a fast recovery of the ecosystem after this series of erosion events and organic rich sediments prevail again after 91.5 cm (about 2680 yrs. cal. BP). All hitherto existing results consolidate the conclusion that the chironomid assemblage denotes the beginning of a cold event. With a lag of about 150 years the vegetation finally responded and forests collapsed, possibly in response to high frostinduced mortality of trees, which were close to their upper distributional limit (see interpretation for Little Ice Age, Tinner et al. 2008). The collapse of forests induced a phase of high erosion events marking the final stage of this period. Taken together our data suggests that the cooling event around 2800 cal. yr. BP had distinct impacts on the environment around Grizzly Lake and presumably large areas of south-central Alaska. REFERENCES Chambers, F.M., Mauquoy, D., Brain, S.A., Blaauw, M. & Daniell, J.R.G. (2007), Globally synchronous climate change 2800 years ago: Proxy data from peat in South America, Earth and Planetary Science Letters, 253 (3-4), 439-444. Swindles, G. T., Plunkett, G., Roe, H. M. (2007), A delayed climatic response to solar forcing at 2800 cal. BP: multiproxy evidence from three Irish peatlands, Holocene, 17 (2), 177-182. Tinner, W., Bigler, C., Gedye, S., Gregory-Eaves, I., Jones, R. T., Kaltenrieder, P., Krahenbuhl, U. & Hu, F. S. (2008), A 700-year paleoecological record of boreal ecosystem responses to climatic variation from Alaska, Ecology, 89 (3), 729-743. .22 From the 'Kanderschnitt' to the 21st century: Human impact and natural hazards in the Lake Thun sediment record Wirth Stefanie*, Girardclos Stéphanie*,** & Anselmetti Flavio S.*** *Geological Institute, ETH Zurich, 8092 Zürich (wirths@student.ethz.ch) **new address: Dept Geology and Paleontology, University of Geneva, Rue des Maraîchers 13, 1205 Geneva ***Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland Lake Thun sediments provide an excellent archive to reconstruct the history of human impact and natural disasters occurring in the lake and the watershed during the last 300 years. 1714 AD is a key year in this history, as it marks the deviation of the sediment load- and carbonate-rich Kander river as new major sediment input to the lake (also called ‘Kanderschnitt’). On the basis of a high-resolution reflection seismic survey and sediment cores (2.5 m maximum length) the consequences of this human-induced change in the lake sedimentary system are investigated. On seismic data, the onset of the Kander input is characterised by proximally deposited sediments imaged on seismic data by chaotic/transparent facies, indicating frequent subaquatic mass-movements certainly released from the overloaded Kander delta slopes. This mass-movement material is deposited distally in the form of turbidite beds imaged seismically with onlapping high amplitude reflections. In the sediment record, the onset of the Kander river input is detected by a relative increase in carbonate elements as Ca, inorganic carbon and Sr, and a decrease in magnetic susceptibility, all reflecting the carbonate-rich input of the Kander river.
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