Climate, exposed source-rock lithologies, crustal uplift and surface erosion: a theoretical analysis calibrated with data from the Alps/North Alpine Foreland Basin system

Paleofloristic data imply that paleoclimate changed in the Swiss Alps at the Oligocene/Miocene boundary from humid and hot conditions toward a climate with high temperature and low humidity. The aridization is associated with a change in depositional pattern from alluvial fans to lakes and floodplains, suggesting decreasing sediment discharge. A further 25-40% decrease of sediment discharge occurred at ca. 20 Ma when the orogenic core of the Alps became exposed to the surface. We applied a surface processes model to explore potential controls on the pattern of sediment discharge and on the evolution of the Alpine drainage basin. The model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear-stress exerted by the flowing water. The model results imply that the paleoclimate change resulted in an instantaneous decrease of sediment discharge and a vertical topographic growth until steady-state conditions between erosional and crustal mass flux are established. However, exposure of the crystalline core of the Alps at ca. 20 Ma is likely to have resulted in the 25-40% decrease of sediment discharge and the reorganization of the drainage pattern from an orogen-normal to an orogen-parallel orientation of dispersion.     

Petrology of ultramafic lamprophyres from the Beaver Lake area of Eastern Antarctica and their relation to the breakup of Gondwanaland

Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO₂ and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO₂-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of ⁸⁷Sr/⁸⁶Sr, leaving ¹⁴³Nd/¹⁴⁴Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO₂ contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001     

Geschichte und Ursachen des Röhrichtrückgangs am Ammersee (Deutschland)

The development of aquatic reed stands at Lake Ammersee was documented by using vertical airborne photographs which were taken during four partial and five complete flights beginning in 1944. A dramatic decline of reed, especially of those parts which stood in deep water, was demonstrated. Between the years 1963 and 1969 60% and until 1992 another 30% of the Phragmitetum disappeared. Only at the east bank did the lake reed beds slightly expand.The examination of possible factors affecting the development of reed at Lake Ammersee lead to the following scenario: Until 1940 reed stands at Lake Ammersee spread and covered large aquatic areas. Only the east bank of the lake, where the influence of wind and waves was high, and banks near towns and recreation areas remained uncovered. Since the regulation of the River Ammer between 1920 and 1938 flood occurrence increased enormously. The flood disaster of the year 1965 coincided with the period of the highest rate of reed decline. The peak of eutrophication in the lake was reached in 1975. Huge carpets of green algae caused further retreat of reeds.At the present time recovery is not possible because of the negative influence of human recreation and of reed consuming waterfowl. Interestingly enough an expansion of reed has been observed at the east bank of Lake Ammersee at places were there was no Phragmites previously.     

The importance of submerged macrophytes as indicators for the nutrient concentration in a small stream (Rotbach, Bavaria)

The relationship between the macrophyte vegetation and the nutrient concentration of the water and the sediment of a stream was studied. The small stream is fed by calcareous groundwater. The alteration in the macrophyte vegetation of the stream Rotbach from oligotrophic community towards more eutrophic species is associated with an increasing concentration of nutrients in the sediment. Particularly the concentration of SRP-P in the interstitial water changes significantly.

In terms of the macrophyte vegetation the stream can be divided into 4 floristic zones, A-D. The zones are labelled in a sequence according to their sediment's nutrient content. The chemical analyses of the water and the sediment show the highest concentrations in zone D. It is characterized by the presence of Zannichellia palustris. The zones A-C have almost the same very low nutrient concentration in the water but show a different content of nutrients in the sediment, particularly of the SRP-P in the interstitial water. Zone A which is dominated by Chara hispida shows the lowest SRP-P in the interstitial water. Within zone B which is characterized by Mentha aquatica and Nasturtium officinale and even more in zone C where Chara hispida is less abundant and Elodea canadensis occurs, the SRP-P content is elevated.

The nutrient concentration in the sediment is clearly associated with changes in the macrophyte vegetation of the stream Rotbach.