THE ROLE OF BIOGEOCHEMICAL CYCLING FOR THE FORMATION AND PRESERVATION OF VARVED SEDIMENTS IN SOPPENSEE

Reference
Gruber, N., B. Wehrli, A. Wüest, The role of biogeochemical cycling for the formation and preservation of varved sediments in Soppensee, Journal of Paleolimnology, 24(3) , 277-29, 2000.

Abstract
We analyzed seasonally aggregated observations of temperature, conductivity, dissolved oxygen and dissolved inorganic carbon from Soppensee (District of Lucerne, Switzerland) from the years 1980 to 1993. Holomictic Soppensee is characterized by a strong summer stratification with a thin epilimnion separated from an anoxic hypolimnion by a strong pycnocline formed by thermal and chemical gradients. A dynamic vertical one-dimensional model was developed to simulate the observed seasonal cycles of carbon and oxygen. The processes of net community production, mineralization of organic matter, precipitation and dissolution of calcite, gas exchange, in- and outflow, sedimentation and vertical eddy diffusion are included. According to the model, the annual net community production is estimated to about 110 g C m-2 yr-1 and the annual net primary production to about 330 to 440 g C m-2 yr-1, which is a typical value for eutrophic lakes.
A mass balance of the carbon cycle indicates that most of the inflow comes from groundwater which is super-saturated with respect to atmospheric CO2. Therefore the surface waters exhibit a large capacity for calcite precipitation. The results of the dynamic model are used to constrain the conditions which favor the formation of varved sediments in Soppensee during thousands of years. Model calculations show that the deep waters would still turn anoxic even if the sedimentation rate of organic matter decreases to 25%. Several physical factors such as biogenic stabilization of the deep waters due to calcite dissolution and low imput of wind energy are responsible for the long term anoxia in Soppensee.