Geochemical signals from biogenic varves reflect hydroclimate and lake oxygen conditions in central Finland

Annually laminated (varved) lacustrine sediments are sensitive recorders of climatic-induced variability in the catchment. In a Boreal setting, climatic information is usually extracted from clastic-biogenic varves, although the potential of biogenic varves remains almost unexplored. The organic-rich sublayers of Boreal biogenic varves usually include thicker growing season lamina enriched with amorphous organic matter and thinner winter lamina reinforced with fine-grained organic matter settled under ice cover during the winter season. This study explores the properties and controls of varve formation in Lake Kallio-Kourujärvi and their implications in understanding past hydroclimate and lake oxygen conditions using micro-XRF combined with stable carbon and nitrogen isotope analysis. Lake Kallio-Kourujärvi is located in central Finland and has organic-rich varves. The thickness of these varves is controlled by the accumulation of biogenic matter that originates from terrestrial sources and autochthonous production. The varve counting provides an age estimate for the 1,8 m long varved sediment sequence of approximately 3700 years before the present (BP). The results from major elemental data reveal that changes in iron and sulfur are consistent with the varve thickness data previously shown to be sensitive for precipitation, as well as decadal changes in North Atlantic Oscillations (NAO) forced winter precipitation. Precipitation likely increases the transport of soluble Iron(II) from the catchment, which settles into sediment as particulate Iron(III) after being oxidised in the water column. Strong changes of redox conditions by the elements iron, manganese, and sulphur are indicated between around 1600 BP and 3700 BP. The variation of redox-sensitive elements suggests that changes in hydroclimatic conditions and past water mixing conditions can be reconstructed from the biogenic varve records.