Changes in lake-sediment structure and composition caused by human impact:: repeated studies of Lake Martiska, Estonia

This research uses a comparison of the sediment record of Lake Martiska (NE Estonia) with well-documented historical changes in human impact to identify the factors dominantly affecting the sediment lithological composition, and the accumulation of heavy metals and other microelements into the sediments. To this end, comprehensive lithological-geochemical studies of the upper sediment were undertaken in 1986 and repeated in 2003 and 2005. Oil shale mining and processing heavily impacted the area via atmospheric pollution and groundwater extraction. As a result of the fly-ash deposition clear marker horizons of chemical compounds were formed. Historical water-level fluctuations are clearly reflected in the lithological composition and grain-size variations of the studied sediment cores. During regression and transgression phases displacement of the erosion-transport-accumulation limits caused redistribution of previously accumulated sediments and their return into the biogeochernical matter cycling of the lake. The Pb-210 chronology of the sediment records is in contradiction with the historical records of fly-ash emissions, suggesting that changes in Pb-210 flux and focusing of sediments caused by lake-level change have invalidated the dating models.