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Field surveys and sediment cores were used to estimate marsh erosion and land subsidence at Madison Bay, a well-known wetland loss hot spot in coastal Louisiana. Former marshes of Madison Bay are under about 1 m of water. Nearly two-thirds of the permanent flooding was caused by rapid subsidence in the late 1960s, whereas the other third was caused by subsequent erosion. Subsidence rates near Madison Bay since the 1960s (∼20 mm/yr) are an order of magnitude greater than deltaic subsidence rates averaged for the past 400–4000 yr (∼2 mm/yr).
The rapid acceleration and unexpected decline in wetland losses in the Mississippi delta plain are difficult to explain on the basis of most physical and biogeochemical processes. There are, however, close temporal and spatial correlations among regional wetland loss, high subsidence rates, and large-volume fluid production from nearby hydrocarbon fields. The decreased rates of wetland loss since the 1970s may be related to decreased rates of subsidence caused by significantly decreased rates of subsurface fluid withdrawal.
Annual fluid production from the Lapeyrouse, Lirette, and Bay Baptiste fields that encompass Madison Bay accelerated in the 1960s, peaked about 1970, and then declined abruptly. Large decreases in pore pressure in the Lapeyrouse field have likely altered subsurface stresses and reactivated a major fault that coincides with the wetland loss hot spot. Therefore, wetland losses at Madison Bay can be closely linked to rapid subsidence and possible fault reactivation induced by long-term, large-volume hydrocarbon production.
Bob Morton is a U.S. Geological Survey research geologist. He was employed previously as a senior research scientist at the University of Texas at Austin Bureau of Economic Geology and as a petroleum geologist at Chevron Oil in New Orleans, Louisiana. Throughout his career, Bob has conducted subsurface studies of the Gulf Coast Basin and surficial changes in coastal Texas and Louisiana.Ginger Tiling received a B.S. degree in environmental science in 1999 and is an M.S. candidate in geology at the University of South Florida. She is employed by ETI Professionals and is working, under contract, at the U.S. Geological Survey Center for Coastal and Watershed Studies in St. Petersburg, Florida.
Nicholas F. Ferina received his B.A. degree in environmental geography in 1997 and his M.S. degree in geology from the University of New Orleans in 2002. He is employed by Environmental Careers Organization and is working, under contract, at the U.S. Geological Survey Center for Coastal and Watershed Studies in St. Petersburg, Florida.