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Geochemistry

The mystery of Don Juan Pond

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Naturally occurring nitrous oxide, a potent greenhouse gas, is mainly produced by biological processes. So how can it have formed in high concentrations in one of the harshest environments on Earth, a place where attempts at finding life have failed? Writing in Nature Geoscience, Vladimir Samarkin et al. provide the answer: an unexpected abiotic mechanism for nitrous oxide production (V. Samarkin et al. Nature Geosci. doi:10.1038/NGEO847; 2010).

The authors report their discovery of nitrous oxide emissions from the soil around Don Juan Pond in Antarctica (pictured), a lake that is so salty it almost never freezes, even in the cold Antarctic winter. Surprisingly, Samarkin and colleagues found that the flux of gas from the lifeless soil was comparable to that of certain tropical soils that teem with microorganisms. An abiotic process must be the source, but no such process was known.

Credit: M. MADIGAN

Samarkin et al. realized that the environment of Don Juan Pond contains chemical ingredients that might take part in reactions that produce nitrous oxide: nitrate and nitrite salts in the brine, and iron(II) salts in the rocks around the pond's edges. Back in the laboratory, they found that nitrous oxide was indeed immediately produced when they mixed sterile brine from the pond with minerals found in the pond's vicinity, and at temperatures as low as −20 °C. Hydrogen gas was also produced.

The newly discovered process for nitrous oxide production might well occur elsewhere on Earth. More intriguingly, the authors propose that it may also occur on Mars, which harbours minerals, brine geochemistry and sub-zero temperatures remarkably similar to those at Don Juan Pond.

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Mitchinson, A. The mystery of Don Juan Pond. Nature 464, 1290 (2010). https://doi.org/10.1038/4641290a

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