Tracing the seasonal origin of boreal streamwater over two centuries: Insights from oxygen isotopes of freshwater pearl mussel shells from northern Sweden

Global warming profoundly affects snowmelt-driven runoff regimes in northern Sweden. In particular, shifts in timing and magnitude of spring floods, which often account for more than half of annual streamflow, have the potential to significantly affect summer drought vulnerability. Assessing these changes requires analyzing historical seasonal water budgets, particularly the contributions of winter vs summer precipitation to streamflow. To quantify these contributions, oxygen isotopes (δ¹⁸O) can serve as natural tracers when comparing the seasonal oxygen isotope signatures in precipitation with those in streamwater. The dataset presents shell-based estimates of the seasonal origin index (SOI), developed by Allen et al. (2019a, b), for four northern Swedish streams from 1836 to 2023.