The role of biogeochemical cycling for the formation and preservation of varved sediments in Soppensee (Switzerland)

We analyzed seasonally aggregated observations of temperature, conductivity , dissolved oxygen and dissolved inorganic carbon from Soppensee (District of Lucerne, Switzerland) for the yrs 1980 to 1993. Holomictic Soppensee is characterized by a strong summer stratification with a thin epilimnion sepa rated from an anoxic hypolimnion by a strong pycnocline formed by thermal a nd chemical gradients. A vertical one-dimensional model was developed to si mulate the observed seasonal cycles of carbon and oxygen. The processes of net community production, mineralization of organic matter, precipitation a nd dissolution of calcite, gas exchange, in- and outflow, sedimentation and vertical eddy diffusion are included. According to the model, the annual n et 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 i s a typical value for eutrophic lakes. A mass balance of the carbon cycle i ndicates that most of the inflow comes from groundwater which is super-satu rated with respect to atmospheric CO2. Therefore the surface waters exhibit a large capacity for calcite precipitation. The results of the model are u sed to constrain the conditions that favor the formation of varved sediment s in Soppensee during thousands of yrs. Model calculations show that the de ep waters would still turn anoxic even if the sedimentation rate of organic matter were decreased to 25%. Several physical factors such as biogenic st abilization of the deep waters due to calcite dissolution and low input of wind energy are responsible for the long term anoxia in Soppensee.