U.S. Department of the InteriorU.S. Geological Survey
Recent Reductions of Subsidence Rates in the Mississippi River Delta Plain
Julie C. Bernier1 and Robert A. Morton2
1U.S. Geological Survey, Florida Integrated Science Center, St. Petersburg, FL 2U.S. Geological Survey, Florida Integrated Science Center, Austin, TX
Introduction – Historic Wetland Loss
Delta plain: ~ 4000 km2 land loss since 1930s
Objective
Identify historic trends and most recent subsidence rates through integration of:
tide-gauge records (NOS) → relative sea-level rise (RSLR)
repeat leveling surveys (NGS) → decadal elevation change
continuous GPS (CORS) → short-term elevation change
Integrated Datasets
Grand Isle Tide Gauge
Mid-1960s to early 1990s: accelerated RSLR
Bayou Lafourche Leveling Line
1982-1993: accelerated subsidence
Integrated Datasets: Temporal Trends
1947-1965: -3.3 mm/yr1965-1993: -10.7 mm/yr1993-2006: -4.1 mm/yr
1965-1993:-9.6 mm/yr
1966-1993:-11 mm/yr
2003-2007:-4.4 mm/yr
2003-2007: -6.3 mm/yr
2002-2007:-3.5 mm/yr
1961-1984:-13.6 to -18.7 mm/yr
Delta-Plain Subsidence Rates
Source PeriodRate
(mm/yr)Reference
Radiocarbon ages Holocene 1-5Penland et al, 1988; Roberts et al, 1994;
Morton et al, 2006
Numerical model Holocene < 5 Meckel et al., 2006
NGS leveling –
Bayou Lafourche
1965-1982 7.9
Shinkle & Dokka, 2004
1982-1993 11.1
NGS leveling –
Bayou Petit Caillou1966-1993 11
NGS leveling – Mississippi River
1961-1984 13.6 – 18.7
NOS tide gauge –
Grand Isle
1947-1964 2.2Morton and Bernier,
this study1964-1991 11.5
1991-2006 3.4
NGS CORS stations 2002-2007 3.5 - 6.3 Dokka et al, 2006
Subsidence Mechanisms
Mechanism Scale Reference
Compaction – Holocene sediments
< 5 mm/yrPenland & Ramsey, 1990; Roberts et al., 1994;
Meckel et al., 2006
Neotectonics – salt tectonics, sediment loading, growth faulting
geological time scaleDokka et al., 2006;
Gagliano et al., 2003
Fluid withdrawal – hydrocarbon production
decadal time scaleMorton et al., 2006;
this study
Delta-Plain Oil-and-Gas Fields
Bayou Lafourche Leveling Line
Highest rates occur over nearby producing fields
Conclusions and Implications
Decadal-scale acceleration and subsequent deceleration of historic subsidence rates was likely induced by deep subsurface hydrocarbon production
Most recent subsidence rates are comparable to rates averaged over geological time scales
A better understanding of most recent trends and processes causing subsidence needs to be incorporated into coastal restoration efforts and efforts to model expected impacts of increased RSLR
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