St. Petersburg Coastal and Marine Science Center
Subsidence and wetland loss in coastal Louisiana continue to create concerns for government officials because their inferred causes influence plans for coastal restoration and hazard mitigation. Recent studies by the USGS show that in the Mississippi delta plain, subsidence and wetland loss correlate both temporally and spatially with long-term, large-volume production of oil, gas, and associated formation water. All three parameters generally followed the same historical trends with low rates before the 1960s, accelerated rates during the 1960s and 1970s, and decreasing rates since the 1970s. Subsidence-rate and wetland-loss declines generally have lagged behind the declined rates of fluid extraction. Large decreases in reservoir pore pressures associated with fluid withdrawal likely caused reservoir compaction, altered subsurface stresses, and reactivated some faults that were near the threshold of failure.
Many processes other than subsidence (e.g., direct removal, storms) can also convert wetlands to open water, but subsidence accounts for much of the extensive interior wetland losses in the Mississippi delta plain that occurred during the 1960s and 1970s. During this period, subsidence rates were an order of magnitude greater than rates averaged over geological time scales (millennia). Natural internal basin processes such as deep-seated salt mobilization and fault movement cannot be ruled out entirely, but there is no compelling evidence that these processes were responsible for the observed changes. In fact, repeat leveling surveys show historical subsidence over salt domes rather than uplift, as expected. Furthermore, it would be difficult to explain the recent reductions in subsidence rates if rapid subsidence were related to tectonic processes.
Former delta-plain marshes are now submerged beneath water that averages 0.5 to 1.0 m deep. Although magnitudes of subsidence are not large relative to other examples of production-induced subsidence, minor reductions in elevation result in extensive permanent submergence when coastal wetlands are near sea level.
1U.S. Geological Survey, FISC, Austin, TX
Additional information and supporting documents can be found at http://coastal.er.usgs.gov/gc-subsidence/.
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