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