Numerical modelling provides evidence of a Baltic Ice Stream during the Younger Dryas

In an effort to analyse the complex Younger Dryas event in central Scandinavia a finite-element method solution of the continuity equation has been used to describe the glaciological processes involved. In order to make the model compatible with the geologic evidence, it is suggested that the ice sheet was drained by a 'Baltic Ice Stream'. The Ice Stream was steered by differences in basal conditions. We also conclude that the climatic event responsible for the Younger Dryas stillstand was probably short (< 500 years), and that different regions of the ice sheet responded in different ways. During a simulated termination it was shown that there was broad agreement about the marginal positions in Sweden and Finland if it was assumed that there was a general sliding zone for elevations below 100 m. with an enhanced sliding zone through the centre of the Baltic and the Gulf of Bothnia. A stillstand near the position of the Younger Dryas moraines is attained with a climatic equilibrium line altitude (ELA) depression of 600 m for a period of 500 years. Agreement of simulated behaviour with observed behaviour is less consistent for the more maritime areas of western Sweden and western Norway.     

Fishing Arctic Foxes Alopex lagopus on a rocky island in West Greenland

On a small barren island in Disko Bay, West Greenland, 195 hours were spent observing Arctic Foxes at their den and the nearby coast. Their main food was found to be live, caught fish, and the activity of the foxes was correlated with the tidal height. S. M. Nielsen, Arctic Station, University of Copenhagen. DK-3953 Godhavn, Greenland.     

The influence of palaeosalinity, grain size distribution and clay minerals on the content of B, Li and Rb in Quaternary sediments from Eastern Jutland, Denmark

Bulk sample analysis of Late- and Postglacial sediments from the Randers fjord area, Eastern Jutland, seem to indicate a relationship between palaeosalinity and B, Li and Rb contents determined on weak acid extracts of the sediment. Investigations of different size fractions, grain size distribution, and of clay minerals show, however, that variations in palaeosalinity as determined by palaeontological methods in this case have no direct influence on the chemistry. The variation of Li and Rb is related to variation in grain size, while the B variation seems to be related to the content of montmorillonite in the clay fraction.     

Conditions of meteoric calcite formation along a Variscan fault and their possible relation to climatic evolution during the Jurassic–Cretaceous

Two calcite cements, filling karst cavities and replacing Lower Carboniferous limestones at the Variscan Front Thrust, were precipitated after mid-Jurassic Cimmerian uplift and subsequent erosion but before late Cretaceous strike-slip movement. The first calcite (stage A) is nonferroan and crystals are coated by hematite and/or goethite. These minerals also occur as inclusions along growth zones. The calcite lattice contains < 0·07 mol.% Fe, but Mn concentrations can be as high as 0·72 mol.% in bright yellow luminescent zones. Primary, originally one-phase, all-liquid, aqueous inclusions have a final melting temperature between ?0·2° and +0·2 °C, indicating a meteoric origin of the ambient water. The δ¹³C and δ¹⁸O values of the calcites are between ?7·3‰ and ?6·3‰, ?7·8‰ and ?5·5‰ on the Vienna PeeDee Belemnite (VPDB) scale, respectively. The second calcite (stage B) consists of ferroan (0·13–0·84 mol.% Fe) blocky crystals with Mn concentrations between 0·34 and 0·87 mol.%. Primary, single-phase aqueous fluid inclusions indicate precipitation from a meteoric fluid below 50 °C . The δ¹³C values of stage B calcites vary between ?7·3‰ and ?2·1‰ VPDB and the δ¹⁸O values between ?7·9‰ and ?7·2‰ VPDB. A precipitation temperature below 50 °C for the stage A calcites and the presence of iron oxide/hydroxide inclusions in the crystals indicate near-surface precipitation conditions. Within this setting, the geochemistry of the nonferroan stage A calcites reflects precipitation under oxic to suboxic conditions. The ferroan stage B calcites precipitated in a reducing environment. The evolution from the stage A to stage B calcites and the associated geochemical changes are interpreted to be related to the change from semiarid to humid conditions in western Europe during late Jurassic–Cretaceous times. A change in humidity can explain the evolution of groundwater from oxic/suboxic to reducing conditions during calcite precipitation. The typically higher δ¹³C values of the stage B compared to the stage A calcites can be explained by a smaller contribution of carbon derived from soil-zone processes than from carbonate dissolution in the groundwater under humid conditions. The small shift to lower δ¹⁸O between stage A and B calcites may be caused by a higher precipitation temperature or a decrease in the δ¹⁸O value of the meteoric water. This decrease could have been caused by a change in the source of the air masses or by an increase in the amount of rainfall during the early mid-Cretaceous. Although the latter interpretation is preferred, it cannot be proven.     

Multiple-step recrystallization within massive ancient dolomite units: an example from the Dinantian of Belgium

South of the Caledonian Brabant-Wales Massif a more than 200 m thick Tournaisian to Lower Visean replacive dolomite unit can be followed for several hundred kilometres from the Boulonnais (France) to Aachen (Germany). Field observations, of features such as karst cavities occurring at the top of the Lower Visean dolomite which are filled by Lower Visean crinoidal limestone, indicate that dolomitization and karstification took place during the Early Visean. This early development of the dolomite is in agreement with the presence of stylolites cutting the dolomite fabric. The minor element composition of the majority of the dolomites remains almost uniform throughout the entire studied area. Values for Fe, Mn, Na and Sr are normally in the range 700–4700 ppm, 15–400 ppm, 80–300 ppm and 50–200 ppm, respectively. The δ¹³C values (range-0.72 to +5.31%_o) mainly reflect the carbon isotopic composition of the precursor limestones. The δ¹⁸O values, in contrast, are highly variable: ranging from-19.15 to +0.85%_o. This rather large range of δ¹⁸O values is explained by multiple-step re-equilibration/recrystallization during progressive burial and subsequent uplift of the dolomites. These processes are also responsible for the high ⁸⁷Sr/⁸⁶Sr values of the dolomites which range from about 0.7088 to 0.7098. They are distinctly more radiogenic than Lower Visean marine carbonates (0.7076–0.7078). Correlation, however, of δ¹⁸O values or ⁸⁷Sr/⁸⁶Sr ratios with dolomite and/or cathodoluminescenec (CL) textures has not been very successful. This suggests that recrystallization may remain unrecognized if only petrographic techniques are used. Nevertheless, certain CL textures can be related to specific interactions with the ambient recrystallizing fluids.     

Coastal morpho-stratigraphy and Holocene relative sea level changes at Tuapaat, southeastern Disko Island, central West Greenland

The geomorphology of a coarse clastic coastal landscape at Tuapaat (69°24'N 52°36'W), southeastern Disko Island, central West Greenland, is described, and a coastal morpho-stratigraphy of the landscape is constructed. ¹⁴C ages on marine shells, whale bones, peat and gyttja are used to construct relative sea level changes throughout the Holocene. The emergence of SE Disko Island occurred in the early part of the Holocene. The Holocene marine limit is situated ca. 80m a.s.l. Between 4.7 and 1.0ka BP, the relative sea level approached the present sea level and it has probably been below present sea level between 4.7 ka BP and the present. The morpho-stratigraphy in the lowest part of the coastal landscape at Tuapaat suggests a complex late Holocene relative sea level history which includes at least 3-4 transgressions during the last ca. 2.5 ka.     

Recurrent interaction between the Norwegian Channel Ice Stream and terrestrial‐based ice across southwest Norway

The occurrence of till beds alternating with glaciomarine sediment spanning oxygen isotope stages 6 to 2, combined with morphological evidence, shows that the southwestern fringe of Norway was inundated by an ice stream flowing through the Norwegian Channel on at least four occasions, the last time being during the Late Weichselian maximum. All marine units are deglacial successions composed of muds with dropstones and diamictic intrabeds and a foraminiferal fauna characteristic of extreme glaciomarine environments. Land‐based ice, flowing at right angles to the flow direction of the ice stream, fed into the ice stream along an escarpment formed by erosion of the ice stream. Each time the ice stream wasted back, land‐based ice advanced into the area formerly occupied by the ice stream. During the last deglaciation of the ice stream (c. 15 ka BP), the advance of the land‐based ice occurred immediately upon ice stream retreat. As a result, the sea was prevented from inundating the upland areas, allowing most of the glacioisostatic readjustment to occur before the land‐based ice melted back at about 13 ka BP. This explains the low Late Weichselian sea levels in the area (10–20 m) compared with those of the Middle Weichselian and older sea‐level high stands (～200 m). Regional tectonic movements cannot explain the location of the observed marine successions. The highest sea level recorded (>200 m) is represented by glaciomarine sediments from the Sandnes interstadial (30–34 ka BP). Older interstadial marine sediments are found at somewhat lower levels, possibly as a result of subsequent glacial erosion in these deposits. Ice streams developed in the Norwegian Channel during three Weichselian time intervals. This seems to correspond to glacial episodes both to the south in Denmark and to the north on the coast of Norway, although correlations are somewhat hampered by insufficient dating control.     

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.