The Southern Alps of New Zealand are some of the fastest growing mountains in the world—but they’re also eroding quickly. Wind, rain, and a variety of natural chemical processes are breaking down rock into 2.5 millimeters of soil each year. That’s about four times the highest rate previously measured anywhere else in the world, according to a new study. Researchers tallied the figure by measuring the concentrations of beryllium-10, an isotope produced naturally when cosmic rays strike rocks at Earth’s surface, in sediments gathered from slopes and riverbeds (image). They also measured the concentration of zirconium in the samples, which helps estimate the rates of various chemical changes in the soil. Together, physical and chemical weathering conspire to bring down mountains—and typically soak up CO 2 in the process, but the overall magnitude of this climate-cooling effect has been long debated. Contrary to previous studies, the new data suggest that there isn’t a “speed limit” on the rates of chemical weathering in mountain soils, the researchers report online today in Science. The disparity with previous analyses, the researchers contend, stems in large part from the environment found in the Southern Alps. With an average annual precipitation of more than 10 meters in some locales, slopes sport temperate rainforests and shrubby ecosystems that trap soil before it can wash away to the seas, where its ability to scrub CO 2 from the air would cease. In many of the areas previously studied elsewhere in the world, some of them relatively arid, erosion sweeps away soil quickly—there, as a general rule, the steeper the slopes, the less time soil sticks around. The longer the soil stays in place, the more time there is for the soil to chemically interact with the atmosphere. The new results may help scientists better assess how episodes of mountain-building deep in Earth’s past have affected climate over the long term.

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