A stable isotope record of Holocene precipitation dynamics in the Baltic region from Lake Nuudsaku, Estonia

A long-term perspective of past hydroclimate variability for the Baltic region provides important context for evaluating the potential impacts of Northern Hemisphere temperature changes and shifting ocean-atmospheric interactions on northern European climate. Lake Nuudsaku is an open-basin lake in south-central Estonia with seasonal overflow fed by active springs and groundwater. Stable isotope analyses of surface water samples from regional lakes, rivers, wetlands, and precipitation indicate that Lake Nuudsaku water δ18O values are consistent with the isotopic composition of inflowing meteoric water. The isotopic composition of fine-grained (<63 μm) endogenic CaCO3 (δ18Ocalcite) samples from Lake Nuudsaku vary between ∼14 to -9‰ (VPBD) over the last ∼9400 years. Based on sensitivity tests, we interpret the δ18Ocalcite variations from Lake Nuudsaku as a record of meteoric inputs; principally reflecting variations in the seasonality of precipitation, with temperature changes as a secondary influence. The δ18Ocalcite record indicates the early Holocene at ∼9400 calendar years before present (cal yr BP) was characterized by a high proportion of cold season (November through April) precipitation and lower temperatures, followed by waning cold season precipitation and higher temperatures until ∼8200 cal yr BP. The middle Holocene (8200-4200 cal yr BP) was likely a period of overall reduced cold season precipitation and higher temperatures, and/or increased warm season (May through October) precipitation. The late Holocene transition at ∼4200 cal yr BP generally marks a shift to lower δ18Ocalcite values that we interpret largely the result of increased cold season precipitation amounts. This was followed by the onset of more variable precipitation amounts and temperatures after ∼3600 cal yr BP, which gave way to less variable conditions during the last ∼2600 years. The Medieval Climate Anomaly (MCA) is marked by δ18Ocalcite values that are near the Holocene and present-day averages. The period spanning the Little Ice Age (LIA) is marked by low δ18Ocalcite values, suggesting temperatures were lower and cold season conditions were wetter than present-day. Proxy evidence from Lake Nuudsaku further indicates that centennial to millennial-scale hydroclimate shifts in the Baltic region were likely associated with changes in North Atlantic ocean-atmospheric circulation dynamics.