Humans' use of land, whether for farming or development, has eroded soil 100 times faster than nature working on its own.
According to new research in the journal Geology, researchers found that European colonisation and agriculture use in North America in the late 1800s and early 1900s caused as much erosion of the landscape over just a few decades' time as would naturally have taken thousands of years.
The team studied the Roanoke, Savannah and Chattahoochee rivers along with seven other large river basins in the US Southeast, and found, for the first time, a precise quantification of this background rate of erosion.
The scientists discovered the rates of hill slope erosion before European settlement were about an inch every 2,500 years; during the period of peak land disturbance in the late 1800s and early 1900s, rates spiked to an inch every 25 years.
''Our study shows exactly how huge an effect European colonization and agriculture had on the landscape of North America,'' said Dylan Rood, who conducted the research while at Lawrence Livermore National Laboratory (he now works as an earth sciences lecturer at Imperial College, London). ''Humans scraped off the soil more than 100 times faster than other natural processes.''
Along the southern Piedmont from Virginia to Alabama - that stretch of rolling terrain between the Appalachian Mountains and the coastal plain of the Atlantic Ocean - clay soils built up for many millennia. Then, in just a few decades of intensive logging and cotton and tobacco production, as much soil eroded as would have happened in a pre-human landscape over thousands of years, scientists note. ''The Earth doesn't create that precious soil for crops fast enough to replenish what the humans took off,'' Rood said. ''It's a pattern that is unsustainable if continued.''
The team collected 24 sediment samples from the river basins and extracted a rare form of the element beryllium, an isotope called beryllium-10 (Be-10), from the quartz in the soil. Formed by cosmic rays, the isotope builds up in the top few feet of the soil. The slower the rate of erosion, the longer the soil is exposed at Earth's surface, and the more Be-10 it accumulates. Using LLNL's Center for Accelerator Mass Spectrometry, Rood measured how much Be-10 was in the samples - giving them a kind of clock to measure erosion over long time spans.
These modern river sediments revealed rates of soil loss over tens of thousands of years. This allowed the team to compare these background rates to post-settlement rates of both upland erosion and downriver sediment yield that have been well documented since the early 1900s across this Piedmont region.
''Whereas our study shows that erosion rates were unsustainable in the past, it also provides a goal for the future,'' Rood said. ''We can use the Be-10 erosion rates as a target for successful resource conservation strategies; they can be used to develop smart environmental policies and regulations that will protect threatened soil and water resources for generations to come.''
The research team includes Lucas Reusser and Paul Bierman of the Rubenstein School of Environment and Natural Resources at the University of Vermont.