Why can thawing permafrost make a stream more acidic instead of simply adding cold meltwater?
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Answer: Sulfide minerals oxidize
Cold water holds acid — Cold water can hold more dissolved gases than warm water, but that is not the explanation for these acid pulses. The diagnostic clue is sulfate rising with metals, which fits sulfide oxidation. Temperature matters because warming thaws the ground and changes flow paths; it is not because cold water magically stores extra acid.
Sulfide minerals oxidize ✓ — Correct. Sulfide minerals such as pyrite can react with oxygen and water to produce acidity and sulfate, a process familiar from acid rock or acid mine drainage. Permafrost thaw changes the plumbing: water reaches rock that used to be locked away. That is why a warming landscape can acidify a stream even when the added water began as snow or ice.
Ice itself is acidic — The ice is not the main acid source. Permafrost matters because it had been acting like a frozen cap over minerals and old flow paths. Once that cap opens, groundwater can reach reactive rock. The acid comes from the chemistry of exposed sulfides, not from meltwater being naturally sour.
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