Grain size distributions of the Bossons glacier (France)

The study presented in this PhD memory aim at better define and quantify the present timeerosion processes in glacial and proglacial domain. The Glacier des Bossons, situated in theMont-Blanc massif (Haute-Savoie, France), is a good example of a natural and nonanthropizedsystem which allows us to study this topic. This glacier lies on two mainlithologies (the Mont-Blanc granite and the metamorphic bedrock) and this peculiarity is usedto determine the origin of the glacial sediments. The sediments were sampled at the glaciersurface and at the glacier sole and also in the subglacial streams in order to understand themechanisms of mechanical erosion and particle transportation in glacial domain. The study ofthe granulometric distribution and the origin of the sediments were performed by a lithologicanalysis at macro-scale (naked-eye) and a geochemical analysis at micro-scale (U-Pb datingof zircons). These analyses allowed specifying the characteristics of glacial erosion andtransport. (1) the supraglacial sediments derived from the erosion of the rocky valley sides aremainly coarse and the glacial transport does not mix these clasts with those derived from thesub-glacial erosion, except in the lower tongue; (2) the sub-glacial erosion rates areinhomogeneous, erosion under the temperate glacier (0.4-0.8 mm/yr) is at least sixteen timesmore efficient than the erosion under the cold glacier (0.025-0.05 mm/yr); (3) the sub-glacialsediments contain a silty and sandy fraction, resulting from processes of abrasion andcrushing, which is evacuated by sub-glacial streams. The high-resolution temporal acquisitionof hydro-sedimentary data during the 2010 melt season, between the May 5th and theSeptember 17th, allowed defining the seasonal behavior of the hydrologic and sedimentaryfluxes. The sediment exportation occurs mainly during the melt season therefore, quantify thesediment fluxes in the Bossons stream and measure regularly the topographic evolution of thefluvio-glacial system allows to perform a sedimentary balance of the erosion of glacial andnon-glacial domains. During the year 2010, about 3000 tons of sediments were eroded with430 tons settled on the fluvio-glacial system. By analyzing the evolution of suspendedparticulate matter concentrations in the Bossons stream upstream and downstream the fluvioglacialsystem, the part of glacial erosion and non-glacial denudation in the sedimentarybalance could be proportioned. The erosion during the stormy events of the uncoveredmoraines, confining the fluvio-glacial system of the Bossons stream, furnishes at least 59% ofthe sediments exported by the Bossons stream and glacial erosion (41% of the flux) istherefore less efficient comparatively. The long-term evolution of the glacial systems inperiod of global warming would show a sustained erosion of proglacial environments(mountain sides and moraines) recently exposed and therefore an increasing of the detritalfluxes. The Glacier des Bossons protects the summit of the Mont-Blanc, the differentialerosion between zones under the ice and non-glacial could lead to an increase of thedifference of altitude between valleys and summits.

(Table 1) Time series of ostracod population density off Bokniseck shore ('Hausgarten'), western Baltic Sea

Seasonal distributions of ostracode species from the Bay of Kiel, western Baltic Sea (Bokniseck, 'Hausgarten') were studied at seven observation stations located between 6 - 23.5 m water depth. During the period from December 1973 to March 1975, 175 samples were taken every two weeks from the sediment surface at each station. Environmental factors were measured simultaneously with sampling. Most of the ostracode species were present in all of the samples throughout the year; four species were found to be seasonal. On the other hand, the population densities of each of the examined ostracode species, as revealed by countings, indicate marked seasonal variations. These variations are attributed to changes in length rate and timing of the reproductive cycles of the different ostracode species, effected mainly by food supply. Maxima in the abundance of the ostracode species with short (1/2 - 1 month) life cycles, occurred simultaneously with peaks of food supply in spring and autumn, whereas the maxima abundance of ostracodes with longer (10 - 12 months) life cycles was delayed.

Water level time series and sediment grain size model

The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 µm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions.