The world''s large lake basins as denudation-accumulation systems and implications for their lifetimes

The mechanical denudation rates of 81 large lake basins (lake area > 500 km²) were determined from long-term river loads and erosion maps. Using the drainage area/lake area ratios the mean sedimentation rates of the lakes were calculated for a porosity of 0.3. The mean sedimentation rates of different lake types vary between 0.1 mm/a (glacial lakes, lowland) and 5.4 mm/a (mostly sag basin lakes). The calculated lifetimes of the lakes are based on the lake volumes and mean sedimentation rates, assuming steady-state conditions and solely clastic material. On average, glacial lakes in highlands and fault-related lakes show the shortest lifetimes (c. 70 ka), glacial lakes in lowlands and rift lakes have the longest lifetimes (c. 1 Ma). Some lakes remain unfilled for very long time spans due to rapid subsidence of their basin floors. The calculated lifetimes are compared with those derived from sediment core studies. Most core studies indicate lower mechanical sedimentation rates than the calculated ones because a major part of the incoming sediment is trapped in deltas. However, a number of lakes (e.g., the Great Lakes of North America) show the opposite tendency which is largely caused by extensive shoreline erosion and resuspension. The lifetimes of large glacial lakes often exceed the duration of interglacials. Hence, their lifetimes are restricted by glaciation and not by sediment infill. Rift lakes persist for long time periods which exceed the calculated lifetimes in some cases. Time-dependent subsidence, basin extension, as well as the impact of climate change are briefly described.     

Location and height of intertidal bars on macrotidal ridge and runnel beaches

Previous studies devoted to the morphology and hydrodynamics of ridge and runnel beaches highlight characteristics that deviate from those initially postulated by King and Williams (Geographical Journal, 1949, vol. 113, 70–85) and King (Beaches and Coasts, 1972, Edward Arnold). Disagreements on the morphodynamics of these macrotidal beaches include the position of the ridges relative to the mean neap and spring tide levels, the variation in the height of the ridges across the intertidal profile and, most importantly, whether the ridges are formed by swash or surf zone processes. The morphological characteristics of ridge and runnel beaches from three locations with varying wave, tidal and geomorphic settings were investigated to address these disagreements. Beach profiles from each site were analysed together with water‐level data collected from neighbouring ports. It was found that the ridges occur over the entire intertidal zone. On one site (north Lincolnshire, east England), the ridges are uniformly distributed over the intertidal beach, whereas on the two other sites (Blackpool beach, northwest England, and Leffrinckoucke beach, north France) there is some indication that the ridges appear to occur at preferential locations. Most significantly, the locations of the ridge crests were found to be unrelated to the positions on the intertidal profile where the water level is stationary for the longest time. It was further found that the highest ridges generally occur just above mid‐tide level where tidal non‐stationarity is greatest. These findings argue against the hypothesis that the ridges are formed by swash processes acting at stationary tide levels. It is tentatively suggested that the ridges are the result of a combination of swash and surf zone processes acting across the intertidal zone. Elucidation of the morphodynamic roles of these two types of processes, and other processes such as strong current flows in the runnels, requires further comprehensive field measurements complemented by numerical modelling. Copyright © 2001 John Wiley & Sons, Ltd.     

A novel ozone sensor for direct eddy flux measurements

A small, lightweight (1.5 kg) and fast-response ozone sensor for direct eddy flux measurements has been built. The basis for detection is the chemiluminescence of an organic dye adsorbed on dry silica gel in the reaction with ozone. The chemiluminescence is monitored with a cheap and small blue-sensitive photomultiplier. At a flow rate of 100 l min^-1 the ozone sensor has a 90% response time of significantly better than 0.1 s with a detection limit lower than 50 ppt at S/N=3. There are no interferences from other atmospheric trace gases like NO_x, H₂O₂ and PAN. Water vapour and SO₂ enhance the chemiluminescence efficiency of the ozone sensor. Since their response times are 22 seconds and 30 minutes, respectively, no correlation between rapid ozone fluctuations and those of these two trace gases is noticed by the ozone sensor when operating at a frequency of 10 Hz.The ozone sensor was tested for several weeks in continuous measurements of ozone fluxes and deposition velocities over different croplands using the eddy correlation technique. Good agreement was found between ozone dry deposition velocities derived from profile measurements and by eddy correlation.     

Kontinentale und marine Senkungsbecken als Ausdruck epirogener Bewegungen

Zusammenfassung Wir unterscheiden innerhalb der Geosynklinalen langgestreckte, schmale kontinentale Stenosynklinalen (besonders der Permzeit) im Gegensatz zu den aus ihnen herauswachsenden breiten, flächenhaften Eurysynklinalen. Während bei der ersten Form keine eigentlichen Geantiklinalen zu beobachten sind, können die Flanken der Eurysynklinalen als Kippungsflächen um eine oder mehrere Achsen (bzw. Achsenbündel) mit aufsteigendem Geantiklinal-Hebelarm und absteigendem Geosynklinalarm aufgefa\t werden.Der Betrag der Senkung wird ausgedrückt durch den stets nur zu errechnenden, niemals zu messenden absoluten oder relativen Senkungswinkel. Der Höchstbetrag der Senkung wird bezeichnet durch den Trogwinkel, d. h. die Abweichung der ganzen Beckenfläche von der ursprünglichen Horizontalebene. Der Durchschnitts-Senkungswinkel ist bei den Eurysynklinalen nur durch Bogenminuten oder -Sekunden auszudrücken. Senkungs- und Trogwinkel sind in der Mitte der Senke am kleinsten, wachsen dann nach den Rändern hin etwas an, um nach den Rändern hin schlie\lich wieder abzunehmen; nur bei den Stenosynklinalen kann der Trogwinkel auf wenige Grade anwachsen.Das Wachsen der Sedimentationsräume ist bedingt durch Zunahme des Senkungswinkels. Verschiebt sich die Kippungsachse gegen das Senkentiefste, so steigt bei gleichbleibender Kippungstendenz das zwischen der alten und neuen Achse gelegene Stück wieder auf, es beginnt das Schwinden des Sedimentationsraumes, das bei den Stenosynklinalen zu einem völligem Verschwinden durch Zuschüttung wird.Im Bereich der Kippungsachse findet, sofern diese nicht gerade untermeerisch liegt, weder Ablagerung noch Abtragung statt, während mit wachsender Entfernung von ihr im sinkenden Hebelarm Vermehrung der Ablagerungen, im aufsteigenden Hebelarm Verstärkung der Abtragung eintritt.     

Röntgenographische Gefügeanalyse mit dem Texturgoniometer am Beispiel von Quarziten aus kaledonischen Überschiebungszonen

Zusammenfassung Quantitative Gefügediagramme können röntgenographisch mit einem Zählrohr-Textur-Goniometer gemessen werden. Diese Methode kommt besonders in Frage bei sehr feinkörnigen Gesteinen oder aber dann, wenn etwa bei einachsigen Mineralen auch die Regelung anderer Gitterrichtungen außer der c-Achsen-Richtung interessiert. Das beim Quarz anzuwendende Verfahren wird behandelt und gezeigt, wie man hier im Einzelfall Prismenregelungen nachweisen kann. Als praktische Beispiele werden Quarzitgefüge mit Prismenregelung aus kaledonischen Überschiebungszonen Schottlands und Norwegens abgebildet und ihre Entstehung diskutiert. Die Messungen scheinen darauf hinzuweisen, daß beim Quarz bevorzugt Translation mit (11¯20) als Gleitfläche und c als Gleitgerade maßgebend war.