On the northeast coast of New Zealand's North Island, the Waipaoa River drains into the dazzling sea. Upriver, things are not so pretty. More than a century of land clearing for farming has created some of the most dramatic erosion in the world.
For land managers attempting to restore the Waipaoa Basin — or eroding landscapes anywhere — finding the source of sediment moving downstream is critical. Now, University of Vermont geology professor Paul Bierman and his graduate student Luke Reusser have developed a new rapid method for pinpointing sediment sources and tracking it downstream. Their results were published in the January edition of the journal Geology.
Key to this process is the isotope beryllium-10 and its presence in the rain cycle. If the land it falls on is eroding slowly, beryllium-10 builds up over years. But if it is eroding quickly, little or no beryllium has time to accumulate. Bierman and Reusser have been able to measure the amount of this "cosmogenic" beryllium stuck to the outside of grains of sand.
Reusser began by collecting sediment samples in the Waipaoa's headwaters and worked his way down the river to the ocean. The samples measured a rising number of beryllium-10 atoms as they went downstream.
"People in New Zealand have been trying very hard to find a way to re-stabilize the hill slopes and try to mitigate some of these problems with sediment," Reusser says. "Now we can come in and, in a very accurate manner, characterize what portions of the drainage basins are problematic and which ones aren't so problematic for land managers to target and try to restore."