Tracing lake mixing and oxygenation regime using the Fe/Mn ratio in varved sediments: 2000 year-long record of human-induced changes from Lake Żabińskie (NE Poland).

This study explores the long-term drivers of changes in lake mixing regime and the causes of lacustrine anoxia in the 2000 year-long, varved sediment record from Lake Żabińskie in northeastern Poland. Annually resolved geochemical data (μXRF and CNS) and pollen data were used to find links between changes in catchment land use and lake mixing regime. Furthermore, we tested the applicability of the Fe/Mn ratio for the reconstruction of past water ventilation. Multivariate statistical analyses show that Fe and Mn were mostly independent of terrestrial inputs and lake trophy, meaning that the Fe/Mn ratio mainly responded to changes in redox potential in hypolimnetic waters. Characteristic changes of the Fe/Mn ratio corresponded to transformations of woodland cover in the catchment as registered by changes of the arboreal/non-arboreal pollen ratio. Six main phases of different lake mixing intensity were distinguished. The local landscape was more open during phases of intensified human impact and catchment deforestation, leading to the intensification of lake mixing. At times of negligible human impact, the catchment was mostly wooded, and thus the lake was well sheltered from intense wind-driven mixing. This led to periods of prolonged anoxia. The Fe/Mn record accurately traced past changes in lake mixing intensity and related shifts in water column oxygenation. During the last two millennia, the mixing regime of Lake Żabińskie mostly depended on human activity in the catchment and landscape openness, while long periods of anoxia were not exclusive to only the most recent sediments.