Pollution history from 256 BC to AD 2005 inferred from the accumulation of elements in a varve record of Lake Korttajärvi in Finland

In order to assess the importance of different pollution sources for the increase in element concentration and accumulation, historical changes in selected elements were studied in the annually laminated sediment of Lake Korttajärvi in Central Finland (62°20′N; 25°41′E). The sediment chronology based on varve counting (256 BC to AD 2005) provided a unique opportunity to explore and date signals of metal emissions, including the ancient metallurgical activities of the Roman Empire at the beginning of the Current Era. Records of this kind are mostly lacking in Finland and northernmost Europe. The stratigraphic sequence of element concentrations did not reflect any major changes in the lake, but changes in element accumulation rates provided distinct pollution signals caused by airborne fallout, catchment erosion, and to some extent municipal wastewater loading. The maximum bulk sedimentation recorded in the twentieth century was 11-fold and organic sedimentation 4-fold higher than the mean background sedimentation rate (256 BC to AD 1019). The increase in the accumulation rates of the majority of the elements, such as Cd, Sn, Pb, Si, Ni, B, Cu, Zn, Sr, Na, K, Sb, Ca, Cr, U and Mg, in descending order, was at least equal to that of bulk sedimentation or much greater, especially for Cd, Sn, and Pb. Changes in the accumulation of Co, Fe, Mn, Mo and As were small and mainly followed those of organic sedimentation. The earliest pollution signals were those of Pb recorded in AD 1055–1141. A weak signal of Pb pollution from the Roman Era was detected in metal concentrations, but this could not be confirmed by the accumulation rate data for Pb.