Geotimes

Geophenomena

Evidence for Dust Bowl dust in Greenland
New sinking rates for Louisiana

Evidence for Dust Bowl dust in Greenland

The 1930s Dust Bowl years marked an unusual period in the climate history of the United States. Now scientists examining ice cores from Greenland have reason to believe that the Dust Bowl years were an even greater meteorological fluke than previously documented.

Most of the dust that makes its way around the globe to settle on the snow and ice of the Greenland ice cap is known to come primarily from China and Mongolia, based on evidence of the mineral particles found. But some researchers, seeing discrete spikes in the ice record, have suggested that the dust that lofted into the air during the dry and windy episodes that struck the Great Plains region of the United States between 1930 and 1940, specifically during 1935 and 1936, may have left a fingerprint in Greenland's ice. But until now none have shown enough evidence to convince the scientific community that the dust indeed marked its territory that far north.

This photo, taken April 18, 1935, shows a dust storm approaching Stratford, Texas. Photo courtesy from NOAA George E. Marsh Album.

Much of the skepticism comes in part because no known period in Greenland's ice record to date has shown a dust trail from North America. So it was with caution that geologist John Donarummo Jr. and physicist Michael Ram, both of the University of Buffalo in New York, along with biologist Eugene Stoermer of the University of Michigan in Ann Arbor, presented the evidence supporting their find of Dust Bowl dust in Greenland. "We didn't find a smoking gun," Donarummo says. But if presented in a court of law, the evidence, he says, would be enough to convince a jury. "It shows that when conditions are right, the continental U.S. can contribute a significant amount of dust," Donarummo says. "The end goal is to understand how dust behaves and how it contributes to climate."

In their report published in the March 18 Geophysical Research Letters, Donarummo and his co-authors presented three lines of evidence for Dust Bowl dust in Greenland's ice sheet summit: they dated the dust and found that it arrived in Greenland between 1933 and early 1934; the clay minerals of the dust correlate with either an American or African origin; and the diatoms they found — the silica shells of aquatic algae that can travel with the wind — were all common North American species, although not diagnostically North American.

Pierre Biscaye of the Lamont-Doherty Earth Observatory at Columbia University, who with colleagues was first to report the East Asian source of Greenland dust, and who has continued to study that source, criticized their analytical methods but agreed with their conclusion. "I believe they have shown for the first time dust other than from China/Mongolia in Greenland."

Donarummo and his colleagues based their standard for what distinguishes one clay mineral source from another on Biscaye's analysis of dust in Greenland ice and snow. However, the samples Donarummo and Ram collected were too small to have been analyzed using either bulk X-Ray diffraction or strontium and neodymium isotopes, as Biscaye and colleagues had done. Instead, they used a Scanning Electron Microscope (SEM) to analyze the particles seen in the ice. "They had to use SEM because of the small size of their samples, but the standard for where they say the dust comes from is our literature and strictly speaking, they should have used a standard from the source area that's based on SEM," Biscaye says. He adds that his point is a technical one and the quality of their evidence, nonetheless, indicates the weather did indeed throw the planet a small curve-ball in terms of wind patterns in 1933.

"It certainly has not been a very frequent transport pathway," Biscaye says. "It took an extraordinarily unusual circumstance to allow that 1933 dust to be transported to Greenland, if, as it seems, it is from the southwestern U.S. It must, however, have been an enormous quantity of Dust Bowl dust in order for it to have swamped the usual Chinese/Mongolian dust signal that was surely transported to Greenland that year."

Christina Reed

Back to top


New sinking rates for Louisiana

Last year, a field crew from the National Geodetic Survey (NGS) and Louisiana State University (LSU) drove a 70-mile stretch of road along coastal Louisiana. A GPS receiver mounted to the back of their van continually measured the height of the road. Similar crews had measured the road heights in 1993 and in 1982. The conclusion: the road, which spans much of the wetland areas west of the Mississippi River, has been sinking, and sinking quickly, up to a foot per decade.

A tugboat pushes a barge load of rock up Locust Bayou in Point au Fer Island, Louisiana. A NOAA restoration project, completed in 1997, used the rock as riprap to protect the island's coast from erosion. Every year, land subsidence and sea-level rise allow the Gulf of Mexico to creep farther inland, drowning marshes and increasing the risk of damaging storm surges. Photo from NOAA Restoration Center, Erik Zobrist.

Resource managers have long known that coastal Louisiana is sinking; each year, hundreds of acres of wetland sink below sea level, inundated by the Gulf of Mexico. At the same time, the new NGS data indicate that subsidence rates are nearly double past estimates. On April 17, Charlie Challstrom, director of NGS, a branch of the National Oceanic and Atmospheric Administration (NOAA), presented the subsidence rates at the National Hurricane Conference. With hurricane season right around the corner, he warned that subsidence would increase the likelihood of coastal flooding.

Scientists at NGS and LSU estimate that at the current rate of subsidence, 15,000 square miles of land along south Louisiana will be at or below sea level within the next 70 years, according to a NOAA press release.

The new elevation data will be used to fine-tune storm surge projections, says Paul Trotter, a meteorologist at the National Weather Service in New Orleans. Most damage during coastal storms comes when sea level rises quickly, and a wall of water that can be greater than 20 feet high advances inland.

Forecasters at the National Hurricane Center will incorporate the new road heights to predict how far those storm surges will advance, Trotter says. "A difference of a foot in elevation means that the storm surge could go hundreds of meters farther inland."

Shea Penland, a geologist at the University of New Orleans, warns that the high rates of subsidence measured along the road, which have not yet been published in a peer-reviewed journal, may not give an accurate picture of what is happening in coastal Louisiana as a whole. "You shouldn't extrapolate from manmade objects into a natural landscape; that would be mixing apples and oranges," he says.

The roads, and the cars pounding along the roads, add weight, creating local compaction rates that may be higher than those in the surrounding wetlands, Penland says. Compaction is particularly prevalent in coastal Louisiana, which sits on relatively unconsolidated sediments that have accumulated over the past 5,000 years. In addition, Penland says, wetland areas accumulate sediment and organic matter over time, adding height that offsets subsidence. The road calculations do not include accumulation. That omission may exaggerate the severity of the problem, he says.

Geologist Sherwood Gagliano, president of Coastal Environments Inc. in Baton Rouge, says that accumulation rates can be as high as a third of a foot per decade. That rate would offset the recent NOAA-LSU subsidence measurements by roughly a third.

Robert Morton, a geologist at the U.S. Geological Survey Center for Coastal & Watershed Studies in St. Petersburg, Fla., also cautions against extrapolating current rates of subsidence into the future. His research indicates that oil and gas drilling in coastal Louisiana has been causing a substantial amount of the subsidence in the region. Removing the petroleum and associated water reduces belowground pressure, which can trigger blocks of wetland to drop. Most of the wetlands in coastal Louisiana sit on irregularly shaped blocks defined by faults that run 25,000 to 40,000 feet deep. Because oil and gas production peaked in the 1970s, and has continued to decline since then, Morton expects that subsidence rates will lessen over time.

Resolving the true rate of current and future subsidence is at the heart of long-term efforts to restore Louisiana's coast, Morton says. Responding to rapid land loss, a host of federal, state and local resource management agencies have constructed Coast 2050, a plan that outlines steps to regenerate wetlands. Hallmarks of the $14 billion plan to be submitted to Congress include reinforcing existing barrier islands and allowing more of the Mississippi River to flood coastal regions. Dams and levees along the river have reduced, by more than half, the amount of sediment that the river carries and deposits to wetlands, Gagliano says.

"The rates of subsidence are obviously critical," Morton says. "If the land is going down a foot every 10 years, why bother? Why spend that much money?" he asks rhetorically, noting that subsidence of that rate would outpace restoration efforts. If the rates are lower, as he believes, restoration becomes more plausible.
The NOAA-LSU data are still brand new, says Bill Good, an ecologist at the Louisiana Department of Natural Resources, and a lead developer of Coast 2050. "People have not had a chance to digest it yet, to evaluate it and compare it with other data." But, he adds, properly incorporating the data will be extremely important: "We don't want to start a project and have it submerged before we are done."

Greg Peterson

Back to top


Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers

© 2014 American Geological Institute. All rights reserved. Any copying, redistribution or retransmission of any of the contents of this service without the express written consent of the American Geological Institute is expressly prohibited. For all electronic copyright requests, visit: http://www.copyright.com/ccc/do/showConfigurator?WT.mc_id=PubLink