Scientists working in some of the fastest uplifting mountains in the world report very high rates of soil weathering there, a process that can contribute to removal of carbon dioxide from the atmosphere. This find, reported in the 17 January issue of Science, contradicts previous studies suggesting that soil weathering rates in fast-uplifting mountains have a "speed limit." It also has important ramifications for the currently contentious debate about whether mountains matter for global weathering, carbon dioxide cycling, and climate change.
Mountains get steeper when the tectonic plates below collide, pushing their misty tops higher. Research has suggested that this uplift process influences carbon dioxide cycling because it exposes new rock and soil that chemical weathering acts on. The more new rock and soil, the more opportunity for chemical weathering to occur, pulling atmospheric carbon dioxide into the Earth and locking it away in a process that involves formation of limestone.
But whether mountains that uplift very quickly can influence climate through chemical weathering has been debated, in large part because scientists have lacked a clear grasp of how the chemical weathering process changes in such locations. If rigorous uplift exposes new rock and soil too fast, erosion may proceed too quickly for chemical weathering reactions to occur, reducing the potential for mountains to lock away much greenhouse gas.
"If weathering rates level off or even decline as erosion rates increase to very high values, then mountains may not be very efficient sites for weathering, and the link between mountain uplift and climate begins to break down," explained University of Washington researcher Isaac J. Larsen, first author of the Science report.
"I have done field work in a lot of different mountains, and the work in New Zealand was by far the most challenging."
The researchers use a mountain radio to call the helicopter pilot to arrange a pick-up, after being stranded on top of the mountain due to cloudy weather. | Courtesy Andre Eger
To better understand if chemical weathering of soil could increase in the face of erosion, Larsen and colleagues took soil samples from some of the fastest uplifting mountains in the world, the western Southern Alps of New Zealand.
"The technology for making soil weathering rate measurements is relatively young, only about 20 years old," Larsen explained, "and prior to our work no measurements had been attempted in Earth's most rapidly uplifting and eroding mountain ranges."
Larsen described the hill slopes of the Southern Alps as steep "yet curiously not stripped to bare bedrock." In other words, despite the rapid erosion there, they were continuously covered in soil. The wet climate helps, Larsen explained. "The annual rainfall supports extremely dense vegetation with roots that pry into fractured bedrock. Prying roots contribute to soil production by breaking up the rock and mixing it into the soil."
This setting was perfect for the team's investigation because it provided high erosion and ample soil, the fuel for the weathering process.
To gauge weathering rates over time here, the researchers evaluated the concentration of certain elements, like beryllium, from soil samples. They also made measurements in sand collected from local rivers and streams. "To get to one of the streams," Larsen said, "we had to cross the Hokitika River in a metal basket attached to a cableway. I have done field work in a lot of different mountains and the work in New Zealand was by far the most challenging."
The researchers found that, contrary to predictions, soil weathering increased as erosion did. Indeed, despite the erosion, which limited the duration of weathering reactions, many such reactions could still take place in this soil-rich setting. "A whole lot of minerals are being weathered incompletely," Larsen said, "which leads to a high weathering rate when you sum it all up."
The researchers compared the soil weathering rates from the western Southern Alps against a compilation of worldwide data and found their rates to be the highest anywhere to date.
"Our results show that soil chemical weathering rates increase even as soil physical erosion rates increase to the highest values ever measured," Larsen said. "It suggests that mountains are important sites for chemical weathering, which has potentially important implications for linking mountain uplift with the carbon dioxide cycle and global climate."