USGS geologists Robert Morton and Julie Bernier constructed new subsidence histories by integrating data sets from the National Oceanic and Atmospheric Administration (NOAA), including National Ocean Service tide-gauge records, National Geodetic Survey releveling data (elevation surveys repeated periodically at established bench marks), and Global Positioning System (GPS) elevations at Continuously Operating Reference Stations. (These "CORS" continuously record extremely accurate positional data; the first station in the southern Mississippi River delta plain was set up in 2002.) Bernier announced their results at the Geological Society of America Annual Meeting held October 5-9, 2008, in Houston, Texas.
Wetland loss is a natural part of coastal-delta cycles and has occurred for thousands of years in Louisiana, but until recently land losses have been counterbalanced by various wetland-building processes. Beginning in the 20th century, however, coastal land has been disappearing much faster than it is being replaced. Among the contributing factors are dams and levees on the Mississippi River, which prevent coastal wetlands from receiving the river water, nutrients, and sediment needed to nourish wetland vegetation and counteract long-term natural subsidence. Wetland loss degrades animal habitats, depletes fisheries, decreases such ecological services as water filtration and nutrient cycling, and increases the vulnerability of coastal areas to rising sea level and storms. Accelerated wetland disappearance in the Mississippi River delta plain allowed storm surge and flooding from Hurricane Katrina (August 2005) to move farther inland and persist longer than if healthy wetlands had been in place.
Led by Morton, scientists with the USGS Florida Integrated Science Center (FISC), St. Petersburg, began investigating wetland loss in Louisiana's Mississippi River delta plain in 1999. They initially focused on five study sites, with the aim of quantifying the difference between land loss caused by subsidence and land loss caused by erosion. Using water-level measurements, core samples, isotopic analyses, aerial photographs, and satellite imagery, they documented areas and rates of wetland loss and identified subsidencethe gradual settling or sudden sinking of the Earth's surface relative to sea levelas the main cause of wetland loss in the studied areas. What's more, they showed that the highest rates of wetland loss correlate significantly with periods of peak extraction of subsurface oil, gas, and water from below ground: their results document a rapid increase in land loss in the late 1960s and early 1970s, followed by a decrease in land-loss rates from the 1980s to the 1990s. This pattern corresponds to the region's history of underground-fluid extraction, which also peaked in the late 1960s and early 1970s and subsequently declined. (See "Rapid Subsidence and Historical Wetland Loss in the Mississippi Delta Plain: Likely Causes and Future Implications," USGS Open-File Report 2005-1216.)
"It's well known that what occurs below the surface can lead to surface subsidence," said Morton. "The extraction of oil, gas, and associated formation water in the Mississippi Delta region caused reduced subsurface pressures in the hydrocarbon reserves, leading to reservoir compaction and fault reactivation that contributed to land-surface subsidence and wetland loss."
Land subsidence from natural processes, such as sediment compaction, and from human activities, such as extraction of water and hydrocarbons, occurs throughout the United States. More than 38,000 km2 of land from California to the Florida Everglades has undergone some form of subsidence. Over the past 70 years, the Mississippi River delta ecosystem has lost more than 4,000 km2 of wetland as land areas have been submerged. The calculated natural rate of subsidence for the delta plain over the past 5,000 years is 1 to 5 mm per year. The average subsidence rate in recent decades has been 8 to 12 mm per year, nearly double the natural background rate. Peak wetland loss occurred in the 1970s, when onshore activities such as oil drilling and hydrocarbon production also peaked. Subsidence rates and wetland losses have slowed in recent years, as extraction activities have declined or moved offshore. "It is pretty striking how closely parallel wetland loss and extraction are," said Morton. "With less subsurface extraction taking place, we should see the subsidence rate and wetland losses slowing down, which is happening."
The connection between wetland loss and subsurface extraction has significant implications for future coastal-restoration projects. "If high subsidence rates are caused entirely by natural processes, then high rates of wetland loss should continue into the future, and restoration would be extremely difficult to support if the end result is going to be submerged wetlands. But if much of the subsidence and wetland loss was induced, then, as extraction in the area declines and subsidence largely slows down, you can begin to restore what was lost," said Morton. "Coastal-restoration strategies vary depending on whether the cause of wetland loss is subsidence or erosion; and in the past, most coastal-restoration projects were designed to counter the effects of erosion," said Bernier. The correlation between wetland loss, subsidence, and subsurface-resource extraction will help restoration projects target areas where conservation techniques will be most effective. The researchers' discovery that subsidence rates have slowed since the 1990s supports their theory that accelerated subsidence in the 1960s and 1970s was caused by subsurface-resource extraction. And if the slower rates persist, coastal-restoration projects will have a better chance for long-term success.
In June 2008, the USGS group expanded their studies to a new area, the Louisiana chenier plain, which backs the western third of the State's coastline. Here, mud and marsh sediments lie between shore-parallel ridges"cheniers"of sand and shell fragments, named for the oak trees ("chênes" in French) that commonly inhabit them. Morton, Bernier, Gary Hill (biological science technician), and contractor Kyle Kelso (geologist) of the USGS FISC office in St. Petersburg and Gregg Snedden (ecologist) of the USGS National Wetlands Research Center's Coastal Restoration Field Station in Baton Rouge, Louisiana, spent a week in Sabine National Wildlife Refuge on the western chenier plain, where they collected sediment cores and water-level readings along transects across areas that have undergone historical wetland loss. The team will analyze the core samples to determine whether the uppermost peat deposits have been removed by erosion or are still present but have subsided relative to the emergent marsh.
In August 2008, Kelso and geologist Nancy DeWitt (USGS, FISC, St. Petersburg) returned to the Mississippi River delta plain to take a bathymetric survey of study sites that once were land, as shown in historical aerial photos, but have since become submerged. Some study sites that once were emergent are now a meter below water. Bathymetric data obtained from the survey, geologic cores, and satellite images of the region will help the team map out the next areas most likely to sink below sea level.
To learn more about wetland loss in Louisiana, visit the USGS National Wetlands Research Center's "Louisiana Coastal Land Loss" page. Information about coastal-restoration projects in Louisiana, plus additional information about the State's coastal land loss, is posted at URL http://www.lacoast.gov/, a Web site dedicated to the implementation of the Coastal Wetlands Planning, Protection and Restoration Act in Louisiana. To explore the varied wetlands of coastal Louisiana, Mississippi, and the Pearl and Atchafalaya River basins, visit the USGS Northern Gulf of Mexico Project's Web page, where several sets of photographs from aerial and boat surveys provide a virtual tour. To learn more about Louisiana's Christmas tree program, visit the Parish Coastal Wetlands Restoration Program Web site.
About the author: Lead author Matthew Cimitile holds a B.A. in history from the University of Tampa and is obtaining an M.A. in environmental journalism from Michigan State University. He spent part of summer 2008 gaining experience in science communications by working with Ann Tihansky at the USGS Florida Integrated Science Center office in St. Petersburg.
in this issue:
Slowing of Coastal Subsidence in Louisiana Wetlands