Dieter M. Imboden

AMS radiocarbon dating and varve chronology of Lake Soppensee: 6000 to 12000 14C years BP

For the extension of the radiocarbon calibration curve beyond 10000 14C y BP, laminated sediment from Lake Soppensee (central Switzerland) was dated. The radiocarbon time scale was obtained using accelerator mass spectrometry (AMS) dating of terrestrial macrofossils selected from the Soppensee sediment. Because of an unlaminated sediment section during the Younger Dryas (10000–11000 14C y BP), the absolute time scale, based on counting annual layers (varves), had to be corrected for missing varves. The Soppensee radiocarbon-verve chronology covers the time period from 6000 to 12000 14C y BP on the radiocarbon time scale and 7000 to 13000 calendar y BP on the absolute time scale. The good agreement with the tree ring curve in the interval from 7000 to 11450 cal y BP (cal y indicates calendar year) proves the annual character of the laminations. The ash layer of the Vasset/Killian Tephra (Massif Central, France) is dated at 8230±140 14C y BP and 9407±44 cal y BP. The boundaries of the Younger Dryas biozone are placed at 10986±69 cal y BP (Younger Dryas/Preboreal) and 1212±86 cal y BP (Alleröd/Younger Dryas) on the absolute time scale. The absolute age of the Laacher See Tephra layer, dated with the radiocarbon method at 10 800 to 11200 14C y BP, is estimated at 12350 ± 135 cal y BP. The oldest radiocarbon age of 14190±120 14C y BP was obtained on macrofossils of pioneer vegetation which were found in the lowermost part of the sediment profile. For the late Glacial, the offset between the radiocarbon (10000–12000 14C y BP) and the absolute time scale (11400–13000 cal y BP) in the Soppensee chronology is not greater than 1000 years, which differs from the trend of the U/Th-radiocarbon curve derived from corals.

Phosphorus model of lake eutrophication

A two-box lake model is presented with the subsystems epilimnion (E) and hypolimnion (H) and phosphorus as the limiting nutrient factor. The mean O2 consumption in H as a function of P loading is calculated and critical P-loading figures above which the lake turns toward eutrophy are given for varying lake mean depth z and hydraulic loading factor qs. The results agree with an empirical relation between z and P loading given by Vollenweider. In addition, P retention factors are calculated using the model and compared to measured values.

Mixing processes in lakes: mechanisms and ecological relevance

In Lake Baldegg, Switzerland (surface area 5.3 km2, maximum depth 66 m) the analysis of data from moored instrument systems (atmospheric boundary layer, lake temperature distribution, bottom currents) was correlated to the long-term development of vertical mixing as seen from profiles of natural isotopes (radon-222, tritium and helium-3) and chemical species. The investigation shows: 1. Vertical mixing coefficients below 25 m are small. Consequently the vertical concentration distribution of sediment emanating species in the deep hypolimnion is controlled by the bottom topography. 2. Renewal of deep hypolimnic water is significant even during stratification. 3. Weakly damped internal waves characterize the internal dynamics during stratification. 4. Horizontal bottom currents play an important role in the hypolimnion mixing and can be correlated to internal waves during stratification.ZusammenfassungMischungsprozesse in Seen finden auf einer breiten Skala von Längen- und Zeitmaßstäben statt. Für die Entwicklung aquatischer Ökosysteme ist dabei die vertikale Mischung von besonderer Bedeutung. In einer Langzeitstudie im Baldeggersee, einem hocheutrophen Schweizer See (Oberfläche 5,3 km2, grösste Tiefe 66 m), wurde die Dynamik physikalischer Parameter (Meteorologie, Wassertemperatur, Bodenströmung) mit verankerten Messgeräten erfasst. Gleichzeitig wurden Profile natürlicher Isotope (Radon-222, Tritium und Helium-3) sowie chemischer Parameter (Phosphor, Methan, Sulfid, Ammonium und andere) genommen. Die Analyse der Daten zeigt; 1. Im Hypolimnium ist die vertikale Mischung klein. Dadurch gewinnt im tiefen Hypolimnium die Bodentopographie entscheidenden Einfluss auf die vertikale Konzentrationsverteilung sedimentbürtiger Stoffe. 2. Die Erneuerung von Tiefenwasser findet auch während der Stratifizierung statt. 3. Interne Wellen bestimmen während der Stratifizierung die interne Dynamik des Sees. 4. Horizontale Strömungen am Seegrund, die sich mit den internen Wellen korrelieren lassen, beeinflussen entscheidend die Mischung im Hypolimnion.RésuméUne étude à long terme a été faite dans le lac de Baldegg, un lac suisse très eutrophe, dune superficie série 5,3 km et dune profondeur maximale de 66 m. Des paramètres physiques ont été pris par une série dinstruments ancrés (météorologie, température de leau, courants de fond). En même temps, le profil des isotopes naturels (radon-222, tritium et hélium-3) ainsi que des paramètres chimiques (phosphore, méthane, sulfure, ammonium et autres) ont été mesurés. Lanalyse des données montre: 1. Le mélange vertical dans lhypolimnion est faible. Par ce fait, dans lhypolimnion profond, la distribution verticale de la concentration des matières chimiques provenant du sédiment est largement influencée par la topographie du fond. 2. Le renouvellement de leau profonde a aussi lieu pendant la stratification. 3. Des vagues internes déterminent la dynamique interne du lac pendant la stratification. 4. Des courants de fond horizontaux jouent un rôle important dans le mélange de lhypolimnion et peuvent être corrélés aux vagues internes pendant la stratification.

Interstitial transport of solutes in non-steady state accumulating and compacting sediments

Abstract A general one-dimensional equation for interstitial transport in accumulating and compacting sediments under non-steady state conditions is derived. As a consequence of compaction the metric along the path of a given horizon, i.e. the spatial distance between neighbouring particles, changes continually. Special emphasis is put on the treatment of advection caused by compaction. The resulting partial differential equation for the interstitial concentration of a given solute contains terms which can be evaluated based on data from a single sediment core. In addition, an integral over the time-derivative of porosity appears which would make it necessary to compare cores from the same site but at different times. Under quite general assumptions this last term may, however, be transformed into a form for which evaluation from a single core becomes possible. Several special solutions are treated such as total steady state, steady state at the surface, non-constant sedimentation rate with steady state compaction, and non-steady state with steady state compaction. The last case applies, e.g., to diffusion under the influence of changing boundary conditions at the water/sediment interface while the accumulation process remains in steady state.

Annual heat balance and equilibrium temperature of Lake Aegeri, Switzerland

The mean heat budget of Lake Aegeri, Switzerland, is 950 MJ·m−2, comparable to that of neighbouring lakes. The annual variation in the net heat flux can be adequately described using a six-term heat balance equation based on 12 years of monthly mean meteorological and surface temperature data. Although the magnitude of the net heat flux is dominated by the radiative terms of the equation, the one-month backward shift of the net flux and total heat content extrema from the solstices and equinoxes respectively is due to the phase shift of the non-radiative with respect to the radiative terms. A linear approximation was used to express the net heat flux in terms of a heat exchange coefficient and an equilibrium temperature. The former varies from 17 to 28 W·m−2·K−1 in the course of a year; fluctuations in the latter are found to depend mainly on fluctuations in cloud cover and relative humidity, whilst the effect of fluctuations in air temperature and wind speed is slight.

Thermische Energieflüsse an der Wasseroberfläche: Beispiel Sempachersee

Data collected every 20 minutes for 18 months by a meteorological buoy moored on Lake Sempach in Switzerland (maximum depth 86 m, surface area 14.1 km2) are used to calculate different processes contributing to the net heat flux between water and atmosphere. The processes considered are shortwave and longwave radiation, evaporation/condensation and sensible heat transfer. The temporal resolution of the measurements allows the evaluation of the processes occurring on three different time scales: diurnal variations, weather events of a few days and yearly cycles. The heat content of the lake is calculated from quasi-continuous measurements of water temperature at different depths. The yearly amplitude of the heat content is 1100·106 J/m2. Short-time variations of the heat flux determined from water temperature analysis agree well with the flux variations modeled using meteorological data. However, the latter generally underestimates the measured net heat flux in the long term. Wind measurements, together with the net heat flux, are used to calculate the Flux Richardson Number and the Monin-Obukov Length. Examples are given to show the predominant influence of the wind on the stratification of the upper water column and thus on the surface water temperature.

Chemical and geochemical studies of Lake Biel

The mixing and transport processes in Lake Biel have been studied using the heat and excess Rn-222 as natural tracers. The mixing parameters were established in terms of coefficients of eddy diffusion in a two-dimensional box model. To account for observed transient Rn-222 distribution, transport by advection had to be included in the model calculations.

Internal seiche pumping between sill-separated basins

Abstract Despite their close proximity and similar dimensions (∼ 200m deep × 10km long × 2km wide) the two eastern basins of Lake Lucerne, Gersauersee and Urnersee, exhibit considerable differences in their internal behaviour, particularly during late winter and spring. The two lakes are separated by a small intermediate basin (∼ 120m deep × 4km long × 1km wide) with sills of approximately 90m depth at each end. We report results of a field program conducted over the period February—May, 1988, when observations were obtained from weekly CTD transects and from three thermistor string/current meter moorings deployed for two months, one near each sill and the third at the southern end of Urnersee near Fluelen. During the observation period the stratification, relative surface to bottom density difference, Δρ/ρ, was 12 × 10−6 in Gersauersee and 4 × 10−6 in Urnersee. Following wind events a large amplitude internal seiche in Gersauersee (vertical excursions of ∼50m and period ∼60 hours) effectively pumpe...

MELIMEX, an experimental heavy metal pollution study: Vertical mixing in a large limno-corral

In a limno-corral (diameter 12 m, depth to sediments 10 m), located in Baldeggersee (Switzerland), vertical mixing has been measured during more than one year and compared to the conditions in the open lake (maximum depth 65 m, surface area 5.3 km2). The temperature method by McEwen and Hutchinson yields Kz values between 5×10−2 cm2s−1 at the upper boundary of the thermocline and 2×10−3 cm2s−1 at the bottom, a value near the molecular diffusion of heat at 4°C (1.36×10−3 cm2s−1). Kz calculated from profiles of excess radon-222 generally agree with those from the temperature data. Compared to the open lake, the corral has a more shallow epilimnion. However, during calm meteorological conditions, vertical mixing in the upper 10 m is similar outside and inside the corral.

Von der Industriellen Revolution zu den Grenzen des Wachstums

Heute ist der 11.11, es ist 1108 h, in 3 Minuten beginnt die Fasnacht. Die Fasnacht war einst ein Ventil gegen die strikten Regeln der Kirche und der Gesellschaft, das wir heute nicht mehr notig zu haben scheinen, leben wir doch in einer Gesellschaft ohne Tabus und Autoritat. Und doch erfreut sich die Fasnacht zunehmender Beliebtheit (es soll sogar eine Fasnacht in Zurich geben!). Mag da etwa mitspielen, dass wir die Verkleidung vermehrt dazu brauchen, vor uns selbst, vor unserem im Wohlstand ertrinkenden und daher beangstigenden Leben Distanz zu gewinnen?