Impact of historical land use changes on lacustrine sedimentation recorded in varved sediments of Lake Jaczno, northeastern Poland

Varved lake sediments provide an invaluable archive of natural and human-induced environmental changes. We present results of sedimentological, geochemical and pollen analyses of recent sediments from Lake Jaczno (northeastern Poland). The sediment record was dated using varve chronology validated with 137Cs fallout peaks. Principal Component Analysis (PCA) and stratigraphically constrained cluster analysis were used to identify four major phases of lake development during the last 200 years. These results were compared with maps and archival data. At the beginning of the studied period (1839-1853), the lake catchment was already partly deforested and used for agriculture, which resulted in intensive soil erosion and high input of minerogenic matter. Next, a short period (1854-1864) characterized by fire events is recorded by microcharcoal in the sediments. The following phase (1865-1971) shows major changes in the catchment area including further deforestation and shortening of the major inflow feeding the lake. This caused a major change from allochthonous (minerogenic) to autochthonous (biogenic) sedimentation, expressed by the transition from clastic-organic to biogenic varves. The most recent phase (1972-2013) is characterized by natural afforestation, reduction of allochthonous inputs and increase in autochthonous deposition. Our study shows how intensification of agriculture and related land use changes in small catchments influences sedimentation processes and the formation of varves.