Abstract. The stable isotope composition of lacustrine sediments is routinely used to infer Late Holocene changes in precipitation over Scandinavia and, ultimately, atmospheric circulation dynamics in the North Atlantic realm. However, such archives only provide a low temporal resolution (ca. 15 years), precluding the ability to identify changes on inter-annual and quasi-decadal timescales. Here, we present a new, high-resolution reconstruction using shells of freshwater pearl mussels, Margaritifera margaritifera, from three streams in northern Sweden. We present seasonally to annually resolved, calendar-aligned stable oxygen and carbon isotope data from 10 specimens, covering the time interval from 1819 to 1998. The bivalves studied formed their shells near equilibrium with the oxygen isotope signature of ambient water and, thus, reflect hydrological processes in the catchment as well as changes, albeit damped, in the isotope signature of local atmospheric precipitation. The shell oxygen isotopes were significantly correlated with the North Atlantic Oscillation index (up to 56 % explained variability), suggesting that the moisture that winter precipitation formed from originated predominantly in the North Atlantic during NAO+ years but in the Arctic during NAO− years. The isotope signature of winter precipitation was attenuated in the stream water, and this damping effect was eventually recorded by the shells. Shell stable carbon isotope values did not show consistent ontogenetic trends, but rather oscillated around an average that ranged from ca. −12.00 to −13.00 ‰ among the streams studied. Results of this study contribute to an improved understanding of climate dynamics in Scandinavia and the North Atlantic sector and can help to constrain eco-hydrological changes in riverine ecosystems. Moreover, long isotope records of precipitation and streamflow are pivotal to improve our understanding and modeling of hydrological, ecological, biogeochemical and atmospheric processes. Our new approach offers a much higher temporal resolution and superior dating control than data from existing archives.

The stable isotope composition of lacustrine sediments is routinely used to infer Late Holocene changes in precipitation over Scandinavia and ultimately, atmospheric circulation dynamics in the North Atlantic realm. However, such 15 archives provide only low temporal resolution (ca. 15 years) precluding the ability to identify changes on inter-annual and quasi-decadal time-scales. Here we present a new, high-resolution reconstruction using shells of freshwater pearl mussels, Margaritifera margaritifera, from three rivers in north Sweden. We present seasonally to annually resolved, calendar-aligned stable oxygen and carbon isotope data from ten specimens covering the time interval of 1819 to 1998. The studied bivalves formed their shells near equilibrium with the oxygen isotope signature of ambient water and thus reflected hydrological 20 processes in the catchment as well as changes, albeit damped, of the isotope value of local atmospheric precipitation. Shell oxygen isotopes were correlated significantly with the North Atlantic Oscillation index (up to 56 % explained variability) suggesting that moisture from which winter precipitation formed originated predominantly in the North Atlantic during NAO + years, but in the Arctic during NAOyears. The specific isotope signature of winter precipitation was damped in stream water and this effect was recorded by the shells. Shell stable carbon isotope values did not show consistent ontogenetic trends, but 25 rather oscillated around an average that differed slightly among the studied rivers (ca. -12.00 to -13.00 ‰). Results of this study contribute to an improved understanding of climate dynamics in Scandinavia and the North Atlantic sector and can help to constrain ecological changes in riverine ecosystems. Moreover, long isotope records of precipitation and streamflow are pivotal for improving our understanding and modeling of hydrological, ecological, biogeochemical and atmospheric processes.Our new approach offers a much higher temporal resolution and superior dating control than existing archives. 30