Chemostratigraphy of the Holocene sediments of Lake Työtjärvi in southern Finland and its limnological significance

Abstract

ALHONEN, P., 1987: Chemostratigraphy of the Holocene sediments of Lake Työt-järvi in southern Finland and its limnological significance. Bull. Geol. Soc. Finland 59, Pari 2, 97-107.The significance of some chemical elements during the limnological history of Työtjärvi, a small oligohumic lake in southern Finland, is discussed. Sedimentary chlorophyll, organic matter, phosphorus, iron, manganese, magnesium, aluminium, zinc, copper, sodium, potassium and calcium were determined from the sediment core investigated, and their stratigraphical distribution was interpreted in terms of limnology. Sodium, potassium, aluminium, magnesium and, possibly also, calcium can be used as indicators of the intensity of erosion during the developmental history of Lake Työtjärvi, especially since the first signs of land use in the catchment of the lake. In contrast, the sedimentary chlorophyll, phosphorus and Fe/Mn ratio throw light on the typological development of Lake Työtjärvi, adding more details to earlier biostratigraphical results.By applying the trophic terminology it can be concluded that Lake Työtjärvi was eutrophic in its early stages during the Preboreal and Boreal periods, but that it rapidly became increasingly dys-oligotrophic at the beginning of the Atlantic period as a result of the ombrotrophic development in the adjacent bog, Varrassuo. The eutrophication associated with prehistoric land use in the catchment was the second conspicuous change in the limnological history of Lake Työtjärvi. It was not, however, continuous, and the lake remained dys-oligotrophic. The most recent change in the limnology of the lake has been the fall of the water colour, probably due to acid precipitation of anthropogenic origin.

Deposition of metals in glacial lake sediment

“…Reducing conditions are result of O 2 consuption during organic matter reminaralisation, which leads to the release of Fe and Mn. After oxygenation at the chemocline due to partial or total mixing of the water column, Fe and Mn precipitates are deposited in the lake sediment (Alhonen 1986, Naeher et al 2013). Because of the fact that iron is oxidised more easily than Mn, parts of sediment with higher Mn/Fe ratio have higher amounts of dissolved oxygen.…”