Some patches of Arctic permafrost are bleeding acid as they melt. The dribble of acid is destroying rocks and releasing more carbon dioxide into the air 鈥� but it鈥檚 not clear how much.
Permafrost is soil and sand that is permanently frozen. Climatologists have warned for years that Arctic permafrost is thawing due to climate change. This will transform the landscape, and release carbon that is locked away in the permafrost in the form of carbon dioxide and methane 鈥� adding to the greenhouse effect. However, most climatologists think the extra warming will be minor compared to that directly caused by our emissions.
Now it seems that some regions of the Arctic might release more carbon dioxide than expected.
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Scott Zolkos at the University of Alberta in Canada and his colleagues studied permafrost in the western Canadian Arctic, which is different to that in other areas. 鈥淭he permafrost is more ice-rich and more sediment-rich,鈥� says Zolkos. 鈥淭here鈥檚 more minerals. So when that permafrost thaws, the material it exposes is different.鈥�
The team analysed samples of water from sites upstream and downstream of thawing permafrost patches that have subsided. Zolkos describes them as 鈥済iant mud pits鈥� that can be hundreds of metres across and up to 30 metres deep.
A dribble of acid
They found that the runoff water contained significant amounts of sulphuric acid, which formed when sulphide minerals were exposed by permafrost melt. The sulphuric acid then began reacting with limestone rocks, releasing carbon dioxide.
This doesn鈥檛 happen in other regions, Zolkos says, because the permafrost doesn鈥檛 contain sulphide minerals. Instead carbonic acid forms when carbon dioxide dissolves in the melt-water. This acid also destroys limestone, but the chemical reaction consumes carbon dioxide rather than releasing it.
What will happen next depends on which parts of the Arctic melt first. 鈥淚f there is carbonate weathering where sulphides are present, that can create CO2,鈥� says Zolkos. 鈥淏ut if it鈥檚 carbonate weathering where sulphides are absent, that will likely consume CO2.鈥�
Unfortunately we don鈥檛 know how much of the Arctic permafrost contains sulphides. 鈥淭here really hasn鈥檛 been much work characterising the mineral composition of permafrost,鈥� says Zolkos. That means we don鈥檛 know how much extra carbon dioxide might emerge due to permafrost acid.
鈥淲e can control many sources of CO2 to the atmosphere, caused by human activities, like fossil fuels and land-use change,鈥� says Zolkos. 鈥淏ut once permafrost starts to thaw and release CO2 and methane, that鈥檚 beyond our control. It鈥檚 not like we can put a giant thermal blanket on the Arctic.鈥�
Geophysical Research Letters
