The Chemical Signal of Clastic-biogenic Varves during the Last 2000 Years from Lake Nautajärvi (Southern Finland)

Annually laminated (varved) lake sediments provide high-resolution information on past environmental and limnological conditions. The study site, Lake Nautajärvi (24°41’E/61°48’N), is a small dimictic lake located in southern central Finland. Lake Nautajärvi forms and preserves clastic-biogenic varve sediments that are comprised of light (minerogenic) and dark (organic) couplets (approximately 0.6 mm thick). The minerogenic layer is deposited during spring when snow melt waters transport fine allogenic mineral matter from the catchment area into the lake. The organic layer is composed of authigenic organic matter and is deposited during summer, autumn and winter. Hence, the climatic factors (seasons, precipitation, snow and ice cover, lake productivity) have an affect on the varve structure, formation and chemical composition. The Lake Nautajärvi varve chronology extends up to 10 000 years from present day. The enhancement of X-ray fluorescence (XRF) instruments has opened up new applications for lacustrine and marine sediments. The ITRAX Core Scanner is a fast and non-destructive way to produce high-resolution (100 µm) elemental profiles. Elemental profiles can provide accurate information on annual and seasonal sedimentary fluctuation. The ITRAX chemical analysis was performed on epoxy impregnated sediment blocks that cover the last 2000 years. The µ-XRF chemical analysis (µ-XRF Mo tube with a 20 s acquisition time and step size of 100 µm) detected 24 elements of which Si, K, Ti, Ca, Cl, Mn and Fe showed the most relevant elemental profiles. Selected elemental signals (K, Ti, Ca, Mn, Cl) reveal two opposite trends that show similarities with X-ray densitometry based organic and minerogenic layer thicknesses. In this study, we combine XRF element profiles with the existing X-ray densitometry data and focus on interpreting the high-resolution elemental signals from Lake Nautajärvi over the last 2000 years.