Varved lake sediments as indicators of recent cultural eutrophication and hypolimnetic hypoxia in lakes

Varved sediments provide a unique opportunity to track the history of eutrophication and oxygen conditions in lacustrine environments. The well-preserved varves indicate hypoxic/anoxic conditions in hypolimnetic waters. Sediment records from three lakes (Dubie, Salno and Wasoskie) located in northern Poland were investigated to determine the drivers that led to the development of varves in the topmost parts of the sediment profiles. We distinguished lithozones characterized by different sedimentary structures in each sediment core. Age-depth models for each sediment record were established using varve counting and isotope dating. Additionally, an independent marker of the Askja-1875 CE tephra supported the dating of sediments from Lake Wasoskie. Ordination analysis applied to a set of geochemical data enabled identification of three groups of proxies representing the variability in deposition of different material within the lakes: i) terrestrial minerogenic material, ii) organic matter, and iii) carbonates. Our data show that the onset of varve preservation occurred asynchronously in the investigated lakes (from the late 19th century to the mid-20th century) and that the transition from homogeneous to varved sediments was gradual and occurred over several decades. These transformations were consistent with the variability of the proxies, indicating cultural eutrophication and showing a strong relationship with the history of human-induced land-use changes in the catchments. This study shows that wellpreserved varves in the topmost sediments of many lakes are geological evidence of human activity.