True impact of settlers on erosion rates in North America revealed

European colonisation of North America caused soil erosion to occur a hundred times faster than it would have done naturally.

The research focussed on the eastern seaboard of America, along a 600 km long stretch of land between the coast and the Appalachian Mountain range. Consisting of a series of river basins at the foothills of the Appalachians, the area was cleared of native forest and used intensively for agriculture by settlers after they arrived in the 1700s and peaking in the late 1800s and early 1900s.

Intensive agriculture and deforestation are some of the most significant human activities that can lead to erosion, which is the loss of the top layer of soil. Problems associated with erosion include decreases in agricultural productivity, ecological collapse and in some cases the eventual desertification of areas. Other effects include sedimentation of waterways, which can harm or destroy habitats, as well as sediment-related damage to roads and houses through mudslides.

Scientists have previously known that human-induced soil erosion has a dramatic impact on the Earth. However, the new research is the first to provide a method to compare this to natural rates of erosion, to determine the full impact human land practices on the environment.

The team was able to determine natural rates of erosion by detecting the rare radioactive isotope Beryllium-10 in soil samples deposited in rivers. The longer soil stays on the surface, the more Beryllium-10 is produced in it, enabling researchers to analyse the movement of the soil.

The researchers compared their results to historical data for the activities after colonisation. The team found that peaking in the decades between the late 1800s and early 1900s the settlers caused widespread erosion that would have taken thousands of years to occur under natural rates.

The team also found that so much soil was washed off the mountain slopes that the rivers could only move less than ten percent of this eroded soil, with the rest being left behind in the valleys. This means that it will take the rivers a long time to remove this “legacy sediment,” and the waters will likely be laden with more sediment than they would naturally for decades to come.

The study, published today in the journal Geology, was carried out by a team from Imperial College London and the University of Vermont. It throws into sharp contrast the dramatic effect that European colonisation has had on the landscape of the eastern seaboard of North America.

The researchers say their technique for comparing human induced erosion to natural erosional rates could be used by conservationists around the world to determine the impact of historical and current land use practices. This could lead to smarter environmental policies and better strategies and regulations to protect threatened soil and water resources.

Dr Dylan Rood, from the Department of Earth Science and Engineering at Imperial, said: “Soil is the lifeblood of our planet, providing a nutrient rich environment in which to grow the world’s staple crops. The loss of soil through erosion can have a devastating effect on our environment and our way of life. Our study shows exactly how huge an effect European colonisation and agriculture had on the landscape of North America. In only a handful of decades, humans had scraped off the soil more than 100 times faster than would have happened naturally. We can now apply our technique to other areas of the world to determine the historical impact of humans on the erosion rates. Looking to the future, we believe our method could be used to help conservationists and governments develop more informed policies to ensure that erosion rates are sustainable.”

The team collected river samples from the Roannoke, Savannah and Chattahooche Rivers. They analysed the samples using an accelerator mass spectrometer located at Lawrence Livermore National Laboratory, which enabled them to sift through billions of atoms in sand grains to find Beryllium-10.

Beryllium-10 is created when cosmic rays from space hit minerals, such as quartz, in the soil grains, which splits the oxygen inside the grain, leaving Beryllium-10. The scientists liken this process to the grains getting sunburn, because the longer the grain remains on the surface of Earth, the more exposed it is to cosmic rays, and the more Beryllium-10 is created inside the grain. Counting the amount of Beryllium-10 in each grain enables the researchers to determine the rate of natural erosion.  The team then compared this data to well-documented historical records since the early 1900s to determine the impact of human induced erosion.

The team are now applying their methods to help the Chinese authorities to understand in more detail the impact of land practices on their environment. Intensive agriculture practices have been carried out in China for hundreds of years and the country has for decades meticulously kept daily records of erosion, which the team believe will enable them to carry out a comprehensive analysis on the impact of people on the environment.

This research follows on from a study carried out by Dr Rood and his colleagues on the Andes Mountain range in South America, which showed that earthquakes, not an ancient wetter climate as previously thought, are eroding the mountains in Peru.