Permafrost thaw controls iron flux from wetlands and sulfide-bearing rocks to Arctic rivers and streams

Recent warming has caused widespread iron mobilization into Arctic waterbodies that degrades ecosystems and threatens natural resources. Yet, understanding where and when iron flux occurs remains limited. Here we investigate iron loading across regional to local scales in Arctic Alaska using climate, water chemistry, and borehole data together with mapped geology and permafrost presence, showing both anoxic microbial iron reduction and acid rock drainage from iron-sulfide oxidation mobilize iron. At a subregional scale, iron influx is associated with sulfide-rich bedrock and presence of near-surface permafrost. Locally, acid rock drainage chemistry correlates strongly with the depth of seasonal thaw above permafrost from the previous year, indicating a temporal lag. These findings clarify the spatial and temporal dynamics of Arctic river rusting, provide a mechanistic understanding of the phenomenon, and may allow anticipation of its occurrence and assessment of its implications for aquatic ecosystem health and subsistence resources under ongoing climate change.