What actually turns some newly thawed Arctic streams orange when permafrost exposes sulfide-rich rock to water and air?

Answer: Rusty iron particles

Dust washed from bare soil — Bare soil can make water cloudy or brown, so it is a tempting guess, but the reported Arctic streams show a chemical fingerprint: lower pH plus higher sulfate, iron, and trace metals. That points to mineral weathering, not simple erosion. The surprising part is that a remote creek can start behaving like mine drainage without a mine.

Rusty iron particles ✓ — Yes. When thaw lets oxygenated water reach sulfide minerals such as pyrite, the reaction makes acid and releases dissolved iron. As that iron later meets more oxygen and less-acidic water, it forms ferric iron particles - the same basic rust color people recognize on metal. The orange stain is the visible end of a hidden groundwater chemistry change.

Extra colored algae — Algae can color ponds green, red, or brown in other settings, but the Brooks Range cases are tied to metals and sulfate rather than a biological bloom. Some affected waters also changed over weeks to months and showed sharp chemistry shifts. The color is more like rust precipitating out of water than plants multiplying in it.