Beschreibung: Diatom oxygen isotopes (δ18Odiatom) from lacustrine sediments have proven to be reliable proxies to trace the hydrological and climate dynamics in the catchment of a respective lake, and are generally linked to the temperature and the water isotope composition (δ18Olake). These background conditions may vary over time constrained by the individual hydrological changes in a given climate field. Diatom oxygen isotopes are excellent recorders of these changes, especially in high-latitudes where carbonates are widely absent, and have been linked to changes in the individual hydrological characteristics in the catchment, such as: (1) air temperature, (2) air-mass contributions, (3) inflow changes, (4) evaporation rates and (5) glacial meltwater influx. In general, the corresponding diatom isotope-based hydroclimate records show a ~5‰ variability over the Holocene and are rather smooth depending on the residence time and turn-over rate of each lake. Lake Bolshoye Shchuchye (67°53’N; 66°19’ E; 187 m a.s.l) is the largest and deepest freshwater lake located in the Polar Urals, Arctic Russia. Its δ18Odiatom record generally follows a decrease in summer insolation, in line with the northern hemisphere (NH) temperature history. However, Lake Bolshoye Shchuchye is exceptional, as short-term, centennial-scale changes of 5-7‰ are notable in the δ18Odiatom values. As most of these minima and maxima are confirmed by more than one data point, and were measured twice, these are no methodological artefacts. The question arises why these extremes occur and what could be the responsible mechanism(s) behind this short-term variability. The recent isotope geochemical background helps setting Lake Bolshoye Shchuchye in its recent hydrological context as a well-mixed monomictic lake, covered more than half of the year by ice, implying negligible evaporative effects. As a deep and voluminous lake, ca. 30-50% of the water of Lake Bolshoye Shchuchye need to be exchanged with isotopically light water within short time to account for these 5‰-isotopic shifts in δ18Odiatom. These changes occur in the Holocene, contemporaneous with and similar to NH glacier advances (Nesje, 2009). However, potential Holocene glacier advances in the Lake Bolshoye Shchuchye catchment are not known and have left no significant imprint on the lakes’ sediment biogeochemistry. Accordingly, the only other source of light isotope composition is snow, known to be transported in significant quantities and with large variability to the leeward side of the Polar Urals (Mangerud et al., 2008). Hence, we hypothesize snow being the dominant agent responsible for the observed short-term changes in the δ18Odiatom record. To our understanding, this is the first time such drastic hydrological changes have been documented for a lacustrine diatom oxygen isotope record.