Changes in biotic conditions and metal deposition in the last millennium as reflected in ombrotrophic peat in Draved Mose, Denmark

Surface samples of Sphagnum magellanicum and samples from peat columns in a profile from the raised bog Draved Mose, Denmark, have been studied to illuminate palaeobotanical changes and past and present deposition rates of metals. A Sphagnum imbricatum sociation dominated extended areas of the bog in the Middle Ages, whereas it is not found at present. The extinction of the sociation is closely related to the formation of hummock and hollow structures. In hollow areas the eradication was related to increased wetness while increased dryness caused the extinction on hummocks. Surface samples were separated into annual layers, and increment and dry matter production were calculated. The exposure of the Sphagnum patches to the prevailing wind direction has a considerable influence on increment rates and dry matter productivity. The possibility of using the metal-uptake in the annual peat layers as a recorder of the present deposition rate has been evaluated. Some trace metals and Mg seem to be sorbed and retained in quantities reflecting the present deposition level. Cs-137 and tritium-analyses have been used to elucidate the leaching process in the bog. Based on C-14 dates and the chemical composition of the peat columns, the mean annual storage of elements (A.D. 1300-1973) has been calculated. The relations between storage and past deposition rates are discussed.