Radiogenic Sr isotopes reveal extended seasonal windows of soil-river dissolved organic carbon transfer in an Arctic permafrost catchment

Permafrost thaw mobilizes substantial quantities of dissolved organic carbon (DOC) to Arctic rivers. Yet the seasonal timing and sources of DOC transfer remain poorly constrained, particularly during shoulder seasons. In this study, we combine radiogenic strontium isotopes (87Sr/86Sr), water isotopes (δ18O & δD), radiocarbon (Δ14C), and DOC, Sr and iron concentrations to investigate soil-river connectivity across contrasting seasons in Panguingue Creek, a headwater stream in Interior Alaska, USA.Our dataset reveals that stream chemistry is primarily controlled by binary mixing between groundwater and a soil-derived source, with occasional inputs from snow/rainfall. Despite progressive seasonal soil thaw, the consistent stream 87Sr/86Sr signature indicates a singular source from soil organic layers. We quantify soil-derived contributions of 76% during Autumn, with detectable inputs persisting over 30 days beyond traditional summer monitoring periods. During Spring, soil-derived inputs are detected prior to ice break-up, contributing up to 35% as snowmelt mobilizes organic material across the watershed. Winter baseflow exhibits anomalous geochemical signatures (enriched δ18O, elevated 87Sr/86Sr, ancient DOC with apparent ages of 5.0-7.2 ka), hypothesized to result from groundwater rerouting or year-round talik connectivity accessing isolated permafrost carbon.These findings demonstrate that soil-river connectivity extends beyond conventional monitoring periods, implying that current Arctic carbon flux models may underestimate annual DOC export. As climate warming lengthens shoulder seasons, these previously unaccounted export windows will expand, increasing permafrost carbon release. Despite winter disconnection between rivers and active layers, talik could remain hydrologically connected, enabling continuous DOC mobilization throughout the year. Arctic headwaters require year-round monitoring as their direct connection to permafrost soils enables early detection of changes in soil-river transfer dynamics under rapid climate change.