by Adityarup "Rup" Chakravorty Thursday, January 17, 2019
High in Earth’s atmosphere, cosmic rays collide with oxygen atoms, shattering the oxygen into smaller atoms, including radioactive beryllium-10. Atmospheric beryllium-10 that falls to Earth’s surface — in precipitation or aboard dust particles — is known as meteoric beryllium-10. Researchers often use the ratio of meteoric beryllium-10 to nonradioactive beryllium-9 in soil as a tracer of soil age and processes. As beryllium-10 has a long half-life — about 1.4 million years — scientists have used it for studies of both short- or long-term soil dynamics.
But doubts have remained about how much meteoric beryllium-10 reaches Earth’s surface and how well it’s retained in soils. In a new study in Geology, researchers suggest that meteoric beryllium-10 may be lost from soils faster than anticipated in environments with high rainfall and weathering rates, indicating the proxy may be of limited use in such areas.
The researchers measured meteoric beryllium-10 and beryllium-9 levels in soil samples from 10 sites along a 12-kilometer transect in the Kohala region on Hawaii’s Big Island. They chose Kohala as their study site for several reasons: The volcanic soils there have been well studied, and their ages and compositions are known. Also, prior research led by Oliver Chadwick, a geologist at the University of California, Santa Barbara, and co-author of the new study, had characterized weathering intensity and soil geochemistry in the area.
The team found that at sample sites that receive less than 1,400 millimeters of rain per year, soil meteoric beryllium-10 levels correlated with rainfall amounts, consistent with the hypothesis that precipitation is a major depositor of meteoric beryllium-10 in these soils. At these sample sites, soil ages were calculated to be about 145,000 years based on beryllium dating, very close to the 150,000-year age previously determined through potassium-argon dating. But meteoric beryllium-10 levels dropped by nearly an order of magnitude in soils from sites that receive more than 1,400 millimeters of annual rainfall; and although potassium-argon dating indicated these soils were also about 150,000 years old, the beryllium-based soil age was only about 17,000 years old.
“This study documents a significant change in the behavior of meteoric beryllium-10 at high rainfall rates in soils derived from basaltic rocks,” says Paul Bierman, a geologist at the University of Vermont who was not involved in the study. “While the threshold of precipitation may vary for different soils and different rock types, this is an important finding and one that will change the way people use this isotopic system.”
The 1,400-millimeter threshold in the Kohala region is the level of rainfall at which the amount of incoming water from precipitation equals water exiting by evapotranspiration. Above the threshold, “the annual water availability shifts from a deficit to a surplus,” says Jean Louise Dixon, a geologist at Montana State University and lead author of the study. This inflection point will vary in different parts of the world and in different ecosystems, however, and “it will also evolve with climate change,” she says. So researchers “must be really careful about aging soils with meteoric beryllium-10” and must account for modern and paleoclimatic conditions.
In Kohala, higher rainfall drives physical and chemical changes in the soil that could all contribute to the loss of meteoric beryllium-10. For instance, more rainfall means more water coursing through soil profiles, which can increase leaching and removal of meteoric beryllium-10.
Higher rainfall also shifts soil pH, which affects beryllium retention. Previous research led by Chadwick showed that the pH of Kohalan soils swings from about 7.5 in areas that receive less than 1,400 millimeters of rainfall to below 5 in areas with more than 1,400 millimeters. And laboratory experiments by other researchers have shown that beryllium binds tightly to clay soils at near-neutral pH, but that a shift into acidic conditions can release up to 97 percent of the element from soils. Finally, the increased weathering that accompanies higher rainfall can release other cations bound to soil, like aluminum, that can outcompete meteoric beryllium-10 for retention in acidic soils.
“We think it’s this combination of three factors — increased water flow that causes more weathering and erosion, pH changes, and competition from other cations — that causes the increased loss of meteoric beryllium-10 from soils in areas with high rainfall,” Dixon says.
That’s important because, for instance, “if meteoric beryllium-10 is used to determine soil residence time, results are likely to be biased at sites with high rainfall,” Bierman says, “or if meteoric beryllium-10 is used as an erosion rate monitor — as has been done extensively in the Amazon, where precipitation can be quite high — there could be biases as well.”
This work has established physical and chemical constraints for beryllium-10 mobility in soils, Dixon says. But the isotope is still “a really powerful tracer,” she says, noting that her team’s findings should help improve its application.
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