1. U.S. Department of the Interior U.S. Geological Survey Recent Reductions of Subsidence Rates in the Mississippi River Delta Plain Julie C. Bernier 1 and Robert A. Morton 2 1 U.S. Geological Survey, Florida Integrated Science Center, St. Petersburg, FL 2 U.S. Geological Survey, Florida Integrated Science Center, Austin, TX

2. Introduction – Historic Wetland Loss  Delta plain: ~ 4000 km 2 land loss since 1930s

3. 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

4. Integrated Datasets

5. Grand Isle Tide Gauge  Mid-1960s to early 1990s: accelerated RSLR

6. Bayou Lafourche Leveling Line  1982-1993: accelerated subsidence

7. Integrated Datasets: Temporal Trends 1947-1965: -3.3 mm/yr 1965-1993: -10.7 mm/yr 1993-2006: -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

8. Delta-Plain Subsidence Rates SourcePeriod Rate (mm/yr) Reference Radiocarbon agesHolocene1-5 Penland et al, 1988; Roberts et al, 1994; Morton et al, 2006 Numerical modelHolocene< 5Meckel et al., 2006 NGS leveling – Bayou Lafourche 1965-19827.9 Shinkle & Dokka, 2004 1982-199311.1 NGS leveling – Bayou Petit Caillou 1966-199311 NGS leveling – Mississippi River 1961-198413.6 – 18.7 NOS tide gauge – Grand Isle 1947-19642.2 Morton and Bernier, this study 1964-199111.5 1991-20063.4 NGS CORS stations2002-20073.5 - 6.3Dokka et al, 2006

9. Subsidence Mechanisms MechanismScaleReference Compaction – Holocene sediments < 5 mm/yr Penland & Ramsey, 1990; Roberts et al., 1994; Meckel et al., 2006 Neotectonics – salt tectonics, sediment loading, growth faulting geological time scale Dokka et al., 2006; Gagliano et al., 2003 Fluid withdrawal – hydrocarbon production decadal time scale Morton et al., 2006; this study

10. Delta-Plain Oil-and-Gas Fields

11. Bayou Lafourche Leveling Line  Highest rates occur over nearby producing fields

12. 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