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No speed limit for soil in New Zealand's mountains

Isaac Larsen collects some sediment from a tributary to the Whataroa River. [Image courtesy of Andre Eger]

Scientists working in the mountains of New Zealand report very fast rates of soil weathering, a new study in the 17 January issue of the journal Science reports, contradicting previous studies that suggest mountainous soil weathering has a speed limit a rate at which it cannot go any faster.

Since the weathering process can pull carbon dioxide from the atmosphere, the discovery of a high rate of weathering in these mountains has important implications for scientists' understanding of mountains as carbon sinks.

Carbon sinks are natural sources that trap atmospheric carbon dioxide, locking it away in compounds on the Earth's surface for a long time. This means less of it is in the air to hold solar radiation, and as a result, the Earth's temperature cools.

Soil acts as a carbon sink. It is involved in a process called chemical weathering in which carbon dioxide in the air reacts with it and "weathers" soil, getting locked inside its particles in the process.

Though weathering happens in the soil on mountains, scientists have debated whether mountains act as carbon sinks. This is in large part because scientists have not understood how the weathering process changes in mountainous regions, particularly those that are regularly uplifted by large tectonic plates.

The uplift process exposes new soil for chemical weathering to act on (such that weathering could continue, locking away carbon dioxide as it goes), but some scientists have suggested the erosion caused when mountains uplift removes so much soil that the weathering process slows. They've said weathering in mountains has a "speed limit," limiting their ability to serve as carbon sinks.

To shed some light on the matter, which is debated, Isaac Larsen and colleagues took soil samples from the western Southern Alps of New Zealand, some of the fastest-uplifting mountains in the world.

To gauge soil weathering rates over time, the researchers evaluated the removal of certain elements, like Beryllium, from the soil. Contrary to previous studies, which suggest that soil weathering declines as erosion from uplift activity increases, here, soil weathering increased as erosion did, the researchers say.

This is likely because the jostled mountain surfaces were not entirely stripped of the soil, as might be expected; rather, the wetter climate kept it in place for weathering to act on.

The researchers compared their soil weathering rates against a compilation of worldwide data and found theirs to be the highest anywhere to date.

The work of Larsen et al. helps show the importance of tectonic activity and erosion as drivers of global weathering changes, and thus as controls on atmospheric carbon dioxide. It also has important consequences for the current debate about whether mountains "matter" for global weathering, carbon dioxide cycling, and climate.