Environmental impact of metalliferous black shales at Talvivaara in Finland, with indication of lake acidification 9000 years ago

Abstract

Environmental impact of metalliferous black shales was studied in the Talvivaara area in eastern Finland with 194 lake sediment and 48 organic stream sediment samples and 57 surface water and 9 ground water samples. The samples were analysed using a combination of ICP–MS and ICP–AES methods. Continuous undisturbed lake sediment cores were obtained from seven lakes. Cores from three of the lakes penetrated through the sediment column into the underlying clay, which was deposited in a late glacial ice-dammed lake between 12,000 and 9000 years ago. The concentrations of Ni, Zn, and Cd in surface water and organic stream and lake sediments were significantly greater if the bedrock of the catchment area consisted of black shales than if it consisted of gneiss granite or quartzite. It appeared that this was also the case in the past. The influence of bedrock chemistry is particularly pronounced in the Lake Härkälampi profile at a depth of 358 cm, which records the abrupt isolation of the lake from the Sotkamo ice-dammed lake 9000 years ago. At this time, the metalliferous black shales of the catchment area were exposed to prolonged wave action and erosion, and pH dropped to as low as 3.8. Following isolation of the lake the organic content of the sediment steadily increased, attaining a maximum value of 13% at a depth of 310 cm. Maximum Ni (0.1%), Cu (0.03%), Zn (0.5%), and Al (3.4%) concentrations are recorded shortly after isolation, when the proportion of organic material was still as low as 2.7%. Evidently vegetation effectively reduces bedrock weathering even in cool climate, since Ni, Cu, Zn, and Al concentrations decreased and pH increased in Lake Härkälampi due to colonization of tundra vegetation.