Those dust-caked frontier towns depicted in Western movies are no exaggeration: The West has become a much dustier place over the past few hundred years.
Westward U.S expansion and changes in humans' land use have made the West five times dustier in the past two centuries than it had been in the past 5,000 years, a new study of lakebed sediments finds.
"There seems to be a perception that dusty conditions in the West are just the nature of the region," said lead researcher Jason Neff of the University of Colorado at Boulder. "We have shown here that the increase in dust since the 1800s is a direct result of human activity and not part of the natural system."
Researchers analyzed sediment records from dust blown into alpine lakes in southwest Colorado's San Juan Mountains over several thousand years. They found a sharp rise in dust deposits coinciding with the boom in railroad, ranching and livestock activity that began in the middle of the 19th century.
"From about 1860 to 1900, the dust deposition rates shot up so high that we initially thought there was a mistake in our data," Neff said. "But the evidence clearly shows the western U.S. had its own Dust Bowl beginning in the 1800s when the railroads went in and cattle and sheep were introduced into the rangelands."
While droughts can trigger erosion of soils and increased dust deposition in lakes, western U.S. droughts in the past two centuries have been mild compared to those over the past 2,000 years, Neff said, likely ruling droughts out as the cause of the rise in dust. Radiocarbon dating of soil cores taken from the lake beds indicates that the dust pile-up began around the time of more intensive land use, primarily grazing.
"There were an estimated 40 million head of livestock on the western rangeland during the turn of the century, causing a massive and systematic degradation of the ecosystems," Neff said. Dust levels started to fall again around the passage of the 1934 Taylor Grazing Act that imposed restrictions on western grazing lands.
The results of the study, funded by the Andrew W. Mellon Foundation, are detailed in the Feb. 24 issue of the journal Nature Geoscience.
The dust includes nutrients such as nitrogen, phosphorus and calcium — by-products of ranching, mining and agricultural activity — that can change the balance of plant growth in an ecosystem. High dust levels can also cause respiratory problems in humans, Neff noted.
A 2007 study published in the journal Geophysical Researcher Letters, which Neff co-authored, linked wind-blown dust from disturbed lands to earlier snowmelt in the San Juans.
"The dust we see in these lakes is the same dust that causes earlier spring snowmelt here, so we can now definitively say that humans are in large part responsible for this melt," said Neff.