Using lakes as isotopic precipitation gauges to constrain inflow seasonality and evaporative enrichment

Lake-water-derived proxies have the potential to store information about both large-scale circulation changes and catchment-integrated processes. Lacustrine carbonates, cellulose, and aquatic organisms incorporating the stable isotopic composition of lake water hydrogen (d2H) and oxygen (d18O) can indirectly archive the isotopic composition of precipitation in the geological past. However, despite similar precipitation input, the lake water isotope values may vary greatly within a region due to variable influence of inflow (precipitation) seasonality and evaporative enrichment. Moreover, the relative sensitivity of each lake to these controls may vary through time. One way to quantify spatial and temporal variation in lake water isotopic composition is modern lake water monitoring. Furthermore, studying present-day conditions allows us to directly link isotopic changes to observations of atmospheric circulation change. From 2018 to 2022, we have measured the stable water isotopic composition of 135 lakes in northern Norway, Finland, and Sweden through a sampling campaign in the first week of July each year. The sample collection is focused along a coastal-to-inland transect, extending from the Norwegian Sea in northwest to Bothnian Bay in southeast. We find that the lakes display variable sensitivity to changes in inflow isotopic composition and to evaporative enrichment, and that these controls vary in both time and space. The water isotopic composition of coastal lakes is mostly sensitive to distillation during moisture transport, whereas the isotopic composition of inland lakes also reflects evaporation. To estimate the inflow isotopic composition of evapo-concentrated lakes (d2HI), we used a Bayesian method. For instance, more depleted d2HI in 2020 than in 2019 indicated more 2H-depleted and/or more winter precipitation inflow to the lakes in 2020, explained by extremely positive Arctic Oscillation (AO+) conditions. We find evidence that lake water isotopic variability in this region reflects a combination of seasonal precipitation changes associated with atmospheric circulation changes, and catchment-integrated evaporation. Careful consideration of the variable sensitivity to these processes is essential when inferring past climate from lake water isotope proxies.