Researchers & Affiliations: Shi-Yong Yu, Torbjörn Törnqvist and Ping Hu, Department of Earth and Environmental Sciences, Tulane University
Written by Linda Rowan
6 February 2013
The Mississippi Delta along the Gulf Coast of the United States is a major site of sediment deposition from the Mississippi River and conversely a major site of wetland loss from rising seas and subsidence. There is debate about how much and when the delta has risen or fallen due to deposition, subsidence, sea level change, and erosion. Using geology and geodesy, the researchers found a steady rise of relative sea level along the southwest Louisiana Chenier Plain that is similar to the rise over the Mississippi Delta. Subsidence associated with glacial isostatic adjustment is the main mechanism affecting the area over the past 7,000 years.
The Mississippi Delta is one of the largest deposition centers in the world and is experiencing among the highest rates of wetland loss as well. The area is a major population and economic center with trade, transport, petroleum production, fisheries, and tourism all flowing through the delta. Determining the sinking of the land related to subsidence as well as the rise of the sea related to global sea level change (eustasy) over time is essential for understanding the evolving environment of the coast. A better grasp of the environment will help to understand shorter term changes related to abrupt geologic events or human-induced changes.
Peat beds in the subsurface suggest continuous submergence at a decreasing rate, while beach ridges have suggested sea-level highstands (with sea levels as much as 2. 5 meters higher than present) in the middle and late Holocene. To try to untangle these differences, the authors studied the Louisiana Chenier Plain that has both peat beds and beach ridges. High precision differential GPS measurements at multiple geodetic stations along the Louisiana Coast were made to determine the elevation of the sampling sites. Sediment samples were obtained from cores and dated using the radiocarbon method.
The Chenier Plain shows a relatively steady rise in relative sea level with a tapering off of the rate of rise in more recent times. There is no evidence for any sea-level highstands. The steady rise along the Chenier Plain is similar to the rise along the Mississippi Delta, suggesting that glacial isostatic adjustment across the continent driving land subsidence by forebulge collapse is the main mechanism that explains relative sea level rise in the region. While the pattern of sea level rise is similar for both areas, the Chenier Plain sea-level curve is consistently higher than that for the delta, which the authors attribute to lithospheric flexural subsidence beneath the delta due to sediment loading.
Significantly, the results of this study are consistent with other measurements of the accelerated rate of sea level rise in the twentieth century outside of the Mississippi Delta. The findings revitalize the hypothesis that the beach ridges might be related to a variable wave climate in the past and thus the ridges may serve as proxy for past storm activity. Thus studies of the past, such as this one, may help to decipher current environmental conditions and how much current changes are related to human-induced factors like climate change and resource extraction.
Quantifying Holocene Lithospheric Subsidence Rates Underneath the Mississippi Delta, Shi-Yong Yu, Torbjörn Törnqvist and Ping Hu, (2012) Earth Planet. Sci. Let. 331-332, 21-30, doi:10.1016/j.epsl.2012.02.021.
glacial isostatic adjustment, eustasy, subsidence and sea level rise
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