Global Change Abstracts The Swiss Contribution - SCNAT

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168 Global Change Abstracts – The Swiss Contribution | Past Global Changes 08.1-341 Excess air as a potential tracer for paleohydrological conditions Klump S, Grundl T, Purtschert R, Kipfer R Switzerland, USA Paleontology , Hydrology , Geochemistry & Geophysics Geochimica et Cosmochimica Acta, 2007, V71, N15, AUG, SS, p A497. 08.1-342 Bedrock landsliding, river incision, and transience of geomorphic hillslope-channel coupling: Evidence from inner gorges in the Swiss Alps Korup O, Schlunegger F Switzerland Paleontology , Geomorphology , Geology The formation of inner gorges cut into bedrock has been explained as relief rejuvenation by fluvial incision in response to rapid base level drop, repeated glaciations, frequent pore pressuredriven landsliding focused at hillslope toes, or catastrophic outburst flows from natural dam failures. Prominent inner gorges occur in soft Mesozoic Bundner schist and lower Tertiary flysch units of the formerly glaciated Alpenrhein catchment, eastern Swiss Alps. Their channel and hillslope morphologies differ from basins characterized by strong glacial or landslide imprints, while formally resembling the theoretically predicted transient response of detachment-limited bedrock rivers to rapid base level fall by headward knickpoint migration. Assuming a postglacial onset of fluvial bedrock incision into a Last Glacial Maximum surface in response to base level drop induced by downwasting of trunk valley glaciers requires downcutting rates E > 20 mm yr(-1) and requires hillslopes to adjust by frequent landsliding toward development of a threshold state. We test this scenario using data on surface uplift, geomorphometry, geomorphic hillslope coupling, and probabilistic slope stability models. We find that adjustment of inner gorge walls through landsliding is mainly strength limited and structurally controlled, and threshold conditions are restricted to the lower 25% of local hillslope relief. Mass movement processes on upper hillslopes remain largely decoupled from channel incision despite inferred postglacial specific sediment yields of 10(4) m(3) km(-2) yr(-1) from the studied basins. Conversely, several constraints imposed by fluvial bedrock detachment, postglacial sediment yields, and bedrock landsliding argue for a pre-Holocene origin for at least some of the inner gorges in the area. This implies partial protection of fluvial gorge topography by subglacial sediment fill during the last (Wurm) extensive glaciation and implies that glaciers were insufficient to fully eradicate fluvially sculpted bedrock topography. This leads us to conclude that lithology and major climate oscillations should be considered as further alternative controls on inner gorge formation. Journal of Geophysical Research Earth Surface, 2007, V112, NF3, SEP 27 ARTN: F03027. 08.1-343 Millenial scale variations of the isotopic composition of atmospheric oxygen over Marine Isotopic Stage 4 Landais A, Masson Delmotte V, Combourieu Nebout N, Jouzel J, Blunier T, Leuenberger M, Dahl Jensen D, Johnsen S Israel, France, Switzerland, Denmark Meteorology & Atmospheric Sciences , Paleontology , Cryology / Glaciology , Geochemistry & Geophysics During rapid events of the last glacial period (DO events), dramatic changes are recorded at high and low latitudes. Without a precise common timescale, links between changes in Greenland temperature and changes in biosphere productivity, hydrology regimes and sea level are difficult to establish. The composition of atmospheric oxygen (delta O-18(atm)) is influenced by global sea level changes, the global hydrologic cycle and the biosphere productivity. Since delta O-18(atm) is measured in ice cores it gives the opportunity to investigate the underlying processes with no timescale uncertainty. Here, we present the first high resolution (50 yrs) record of the isotopic composition of atmospheric oxygen (delta O-19(atm)) measured in the air trapped in a Greenland ice core (NorthGRIP). Our record covers a sequence of DO events (18-19-20) corresponding to the Marine Isotopic Stage 4, similar to 75 to 60 ka ago. Our measurements reveal rapid changes of delta O- 18(atm) associated with the DO events. With a few additional measurements of the third isotope of oxygen (O-17) during the DO event 19, we exclude the hypothesis that sea level changes are responsible for the isotopic variations. They originate more likely from large changes in relative humidity and latitudinal repartition of the continental vegetation over the DO events. Earth and Planetary Science Letters, 2007, V258, N1-2, JUN 15, pp 101-113.
Global Change Abstracts – The Swiss Contribution | Past Global Changes 08.1-344 Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz: Implications for deglaciation chronology Lepper K, Fisher T G, Hajdas I, Lowell T V USA, Switzerland Paleontology , Cryology / Glaciology , Geology Glacial Lake Agassiz has been implicated as the tritmer for numerous episodes of abrupt climate change at the close of the last ice age, yet the beginning age of the lake has never been determined. Here we report the first numerical age data on the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz. Organic remains from lakes, bogs, and channels distal to, and inset to, the Big Stone Moraine require that glacial activity at this moraine ceased prior to 12,000 C-14 yr B.P. (13,950 cal (calendar) yr). A site near New Effington, South Dakota (United States), implies full glacial recession north of the topographic divide prior to 11,810 C-14 yr B.P. (13,670 cal yr), synchronous with the beginning of glacial Lake Agassiz. Lake Agassiz shorelines inset to the moraine yield optically stimulated luminescence (OSL) ages from 14,200-12,600 yr cal. Lower strandlines are younger, but the similarity of ages suggests that initial lake lowering was faster than OSL ages can currently resolve. Nevertheless, the OSL ages represent the first numerical age assignments for the Herman, Norcross, and Upham beach ridges, setting the stage for future numerical age assignments within the Lake Agassiz basin. These two dating methods yield strongly consistent results within stated uncertainties. The age of the Big Stone Moraine implies an interval of rapid retreat for the Des Moines lobe of the Laurentide Ice Sheet during the BiAling-Allerod warm interval. The overlapping ages for the uppermost beach levels and abandonment of the highest Lake Agassiz spillway indicate a rapidly evolving lake until at least 13,500 yr cal. Geology, 2007, V35, N7, JUL, pp 667-670. 08.1-345 Variable Be-10 fluxes in lacustrine sediments from Tristan da Cunha, South Atlantic: a solar record? Ljung K, Bjorck S, Muscheler R, Beer J, Kubik P W Sweden, USA, Switzerland Geology , Geochemistry & Geophysics , Oceanography , Paleontology A 650-yr-long sediment sequence from a crater lake on Tristan da Cunha, South Atlantic, was analysed for its Be-10 content. Based on C-14 dating, and sedimentary, geochemical, magnetic and palynological records, the period between 900 and 1450 AD appears to have been unusually 169 stable in terms of sedimentation and vegetation and therefore this period was chosen for analysis of the Be-10 content. During this period of highly organic sedimentation and closed vegetation, the pattern of Be-10 flux variations follows the Be-10 fluctuations from the GRIP ice core and estimated C-14 production rates well. However, before and after this stable period, variable sedimentation rates have to be accounted for to obtain results that are comparable to the established Be-10/C-14 records. Our data show not only the possibility of obtaining detailed enough Be-10 flux data from sedimentary sequences to reconstruct past solar forcing but also how sensitive this type of record is to sedimentary and environmental changes. If suitable archives can be found, they have the potential to improve reconstructions of solar activity far back in time. Quaternary Science Reviews, 2007, V26, N7-8, APR, pp 829-835 08.1-346 Characterizing carbon isotopic variability in Sphagnum Loader N J, Mccarroll D, van der Knaap W O, Robertson I, Gagen M Wales, Switzerland Paleontology , Plant Sciences , Forestry , Geology To understand more fully the nature of isotopic fractionation in mosses and to explore the potential of stable isotope analyses of selected peat constituents for palaeoenvironmental research, we present results from a study of inter- and intraplant delta C-13 variability in Sphagnum spp. Subdivisions of stem, pendant and horizontal branch elements of modern Sphagnum capillifolium plants revealed consistent and statistically significant differences in their isotopic composition. Sequential (downstem) analysis of a further cohort of four modern Sphagnum capillifolium plants also reveals evidence of common forcing on the isotopic composition of sequentially formed stem and branch increments. This relationship was tested further by analysis of a series of branch and stem samples manually recovered from Sphagnum fuscum preserved within a late Holocene (AD 2003-1970) peat monolith from a European mire. The high degree of isotopic coherence observed between plants supports the analysis of Sphagnum in palaeoecological investigations. However inter- and intra-plant variability between both branch and stem. sections emphasize the need for representative sampling, replication and sample homogeneity when conducting palaeoecological studies. Holocene, 2007, V17, N3, APR, pp 403-410.
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