The West has become 500 percent dustier in the past two
centuries due to westward U.S. expansion and accompanying human activity
beginning in the 1800s, according to a new study led by the University of
Colorado at Boulder.
Sediment records from dust blown into alpine lakes in
southwest Colorado's San Juan Mountains over millennia indicates the sharp rise
in dust deposits coincided with railroad, ranching and livestock activity in the
middle of the last century, said geological
sciences Assistant Professor Jason Neff, lead author on the study. The results
have implications ranging from ecosystem
alteration to human health, he said.
"From about 1860 to 1900, the dust deposition rates shot
up so high that we initially thought there was a mistake in our data," said
Neff. "But the evidence clearly shows the western U.S. had it's own Dust Bowl
beginning in the 1800s when the railroads went in and cattle
and sheep were introduced into the rangelands."
A paper on the research funded by the Andrew W. Mellon
Foundation was published in the Feb. 24 issue of Nature Geoscience.
Co-authors included CU-Boulder's Ashley Ballantyne, Lang Farmer and Corey
Lawrence, Cornell University's Natalie Mahowald, the University of Arizona's
Jessica Conroy and Jonathan Overpeck, Christopher Landry of the Center of Snow
and Avalanche Studies in Silverton, Colo., the University of Utah's Tom Painter
and the U.S. Geological Survey's Richard Reynolds.
The study indicates "dust fall" in the West over the past
century was five to seven times heavier than at any time in the previous 5,000
years, said Neff, who is also a faculty member in CU-Boulder's Environmental
Studies Program. While some fine-grained dust from Asia periodically falls on
Colorado's San Juans, the abundance of larger-sized dust particles in the lake
sediments there indicates most of the dust originated regionally in the
Southwest, said the authors.
While droughts can trigger erosion and increased dust
deposition, western U.S. droughts during the past two centuries have been
relatively mild compared to droughts over the past 2,000 years, Neff said.
Instead, the increased dustiness in the West coincides with intensive land use,
primarily grazing, according to radiocarbon dating and lead isotope analysis of
soil cores retrieved from lakebeds, he said.
"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," said Neff. The 1934 Taylor Grazing
Act that imposed restrictions on western grazing lands coincided with a decrease
in accumulation rates of the San Juan lake sediments in the study -- a decrease
that continues to today, he said.
The study also shows more than a five-fold increase in
nutrients and minerals
in the lakebed sediments during the last 150 years, said Neff. Increases in nitrogen,
phosphorus, potassium, calcium and magnesium -- byproducts of ranching, mining
and agricultural activity - have been shown to change water alkalinity,
aquatic productivity and nutrient cycling.
In the Niwot Ridge alpine region west of Boulder, for
example, CU-Boulder researchers have observed increased algal growth in streams
and lakes as a result of rising nitrogen deposition, as well as changes in the
composition and diversity of wildflowers on the tundra. "Because these types of
inputs have the potential to increase plant growth, the ultimate outcome of such
depositions could change the fabric of our ecosystems," said Neff.
Excessive dust also can cause significant human health
problems, including lung tissue damage, allergic reactions and respiratory
problems, Neff said.
The San Juan lakes are located in an area dominated by
rocky talus slopes with little soil and vegetation at about 13,000 feet in
elevation and are located downwind of several major U.S. deserts like the
Colorado Plateau and the Mojave. The site was chosen in part because the San
Juans experience frequent wintertime dust deposition events -- usually between
four to seven episodes annually, Neff said.
A study published in Geophysical
Research Letters in 2007 involving co-authors of the Nature Geoscience paper,
including Neff, showed wind-blown dust from disturbed lands in the Southwest
shortened the duration of San Juan mountain snow cover by roughly a month. "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.
"There seems to be a perception that dusty conditions in
the West are just the nature of the region," said Neff. "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."