DRIVERS OF ISOTOPIC AND GEOCHEMICAL VARIABILITY IN SUBARCTIC PERMAFROST WATERSHEDS

Abstract

Northern environments are undergoing rapid hydrological and geochemical transformations as warming temperatures and changes in precipitation regimes accelerate permafrost thaw. In permafrost-influenced catchments, seasonal thaw regulates vertical and lateral hydrological connectivity of the catchment to the stream, yet there is considerable variability across watersheds reflecting differences in permafrost status, topography, lithology and vegetation. Despite widespread reports of permafrost degradation, uncertainties in the future trajectories of cold regions remain due to limited field-based studies that characterize runoff generation processes across diverse spatiotemporal scales. This study integrates six years of isotopic, hydrochemical and high frequency datasets collected from seven catchments in the Tombstone Waters Observatory, Yukon, to evaluate how seasonality, catchment structure, and lithology influence water storage and runoff mechanisms. Results revealed strong seasonal evolution in source water contributions and flowpath activation. Continuous permafrost catchments showed high isotopic variability in δ²H and δ¹⁸O, strong event signals, and low solute concentrations, consistent with shallow flowpath dominance and minimal subsurface storage. In contrast, lower permafrost extent catchments exhibited reduced isotopic variability, muted event signals, and higher solute concentrations, reflecting enhanced groundwater contributions and greater storage capacities. Observed seasonal and spatial variation in solute concentrations (up to 2 orders of magnitude), Mg2+/Ca2+ and (Ca²⁺ +Mg²⁺) / SO₄²⁻ molar ratios, and concentration-discharge relationships highlight the combined role of permafrost extent in constraining water-rock interaction, and lithology in governing mineral weathering processes and geochemical signatures. High frequency data revealed threshold shifts in subsurface connectivity following late-season storm events, and increased complexity in paired concentration-discharge relationships that reflect subsurface heterogeneity. The findings of this study demonstrate that hydrologic and geochemical variability in subarctic catchments emerge from interacting influences of storage capacity, catchment structure, and lithology. By capturing both seasonal and event scale dynamics over variable catchment characteristics, this work provides new insight into the processes shaping runoff generation and chemical regimes in northern headwaters, and their potential response to climate change.