Lake sediment as environmental archive : natural and anthropogenic influence on the chronology of trace elements

Sammanfattning: The focus of this thesis is the historical pollution of some seldom-monitored trace elements (SMTEs; Ag, Be, Ga, In, Sb and Tl) that have been involuntarily released for several thousands of years but whose usage have increased during the industrial era. Sediment cores from four rural lakes in a south to north transect in central Sweden, and two urban lakes have been used as environmental archives for chronological studies. The historical development of the SMTEs is put in perspective of frequently monitored elements (Cd, Cu, Pb and Zn) that serve as a well-known anthropogenic signal. Dating of the sediments is made with 210Pb, 137Cs and 239,240Pu as well as acid leachable lead and the 206Pb/207Pb ratio. The impact of diagenetic redistribution is included, when necessary.The results show that site-specific reference concentrations are required in order to estimate the present pollution as well as its historical development. Atmospheric deposition is the principal pathway of transport in rural lakes and the concentrations are lowered towards the north. It is not possible from this material, however, to separate the direct deposition on the lake surfaces from the contribution from their catchments. From 21Pb, acid-leachable Pb and the 206Pb/207Pb ratio a minimum of four periods of pollution can be discerned. For the SMTEs the quantitatively most important period of pollution took place from the Second World War until present. The growth of the domestic industrialisation as well as the early industrialisation of central Europe and the British Isles are also distinguishable. The pollution history of Ag, In and Sb goes back for several centuries as a result of metal processing whereas the levels of thallium mainly increase as a result of industrialisation. For Ag and Tl in-sediment diagenetic redistribution limits the precision of the historical estimate. The preliminary interpretation of 137Cs and 239,240Pu indicates that they are less suitable as chronological markers in the system studied. The urban impact on the trace metal sediment content in the urban lakes was lower than expected, except for Au. There is a large impact from hydrological conditions on the studied system why further investigations are recommended.