8 research outputs found
Varve formation since 1885 and high-resolution varve analyses in hypertrophic Baldeggersee (Switzerland)
According to their microstratigraphical composition the laminated sediments of Baldeggersee
are true varves. Two varve time-series from freeze cores taken in the deepest part (66 m) of
Baldeggersee have been analysed for annual and seasonal layers. The varve time-series covers the
period of 1885 to 1993.
The two freeze-cores from Baldeggersee can be accurately correlated by means of distinct layers
(e.g. marker varves, turbidites), lithological units, and varve measurements. The Baldeggersee
varve chronology has been checked by independent high-resolution 137Cs dating. Mobility of
cesium has not been detected in Baldeggersee. A highly positive correlation between phosphorus
concentration in the lake water and median grain size of calcite crystals has been observed
and allows, together with the results of geochemical analyses, to hindcast the trophic state in
Baldeggersee for periods where no limnological data are available.
The main features of the Baldeggersee varve time-series can be summarized as follows: before
1885 there are packets of 5 – 10 varves interrupted by massive, homogeneous marl beds, indicating
oscillating hypolimnetic oxygen levels until the lake ultimately became anoxic in 1885. Between
1885 and 1905 varves are thin, with a high carbonate content. Between 1905 and 1910 the thickness
of varves increased steadily, and an increase in calcite grain size suggests a major step in eutrophication.
Median grain size values increase again at the end of the 1940s, whereas the thickness of
the seasonal layers decreased between the early 1940s and the early 1960s. At the onset of the
1960s, during the time of highest epilimnetic phosphorus concentrations in Baldeggersee, the total
phosphorus concentrations in the sediments as well as the varve thickness and the total accumulation
rates all increased. At the beginning of the 1980s the median grain size decreased substantially
as a result of lower phosphorus concentrations in Baldeggersee