SLIDE 1

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

SLIDE 2

Introduction - Historic Wetland Loss
Delta plain: ~ 4000 km2 land loss since 1930s
image: map of Louisiana showing 1932-2006 land loss

notes: Louisiana's coastal wetlands, which extend up to 100 km inland along the length of its coastline, have been severely degraded by high land-loss rates (up to 70-100 km2/yr) over the past 50+ years with significant environmental, ecological, and economic impacts. Accelerated historic wetland loss has resulted in submergence of ~ 4000 km2 formerly emergent delta-plain wetlands since 1930s. Understanding historical and current trends in subsidence rates and their causes is critical for designing and successfully implementing coastal-restoration activities and for modeling and predicting expected impacts of future RSLR on the delta plain. 

Recent USGS studies (south-central d.p.) have identified rapid subsidence leading to marsh-surface submergence as a primary cause of historic delta-plain wetland loss, especially at interior hotspots that opened up in the late 1960s and 1970s. However, although it is generally agreed that the delta plain has historically been an area of high subsidence rates, there is a poorer understanding of the magnitude of the most recent rates. 


SLIDE 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

SLIDE 4

Integrated Datasets
image: map of Louisiana showing CORs sites, tide gauges, and benchmarks
notes: RSLR trend - proxy for vertical land motion if long enough record
Grand Isle tide gage: near-continuous record since 1947
Repeat leveling surveys along B. Petit Caillou, B. Lafourche, and Mississippi R: decadal-scale elevation changes between leveling epochs (early 1950s, early to mid-1960s, early 1980s, 1993)
Identify general trends by comparing magnitudes and average rates of subsidence for the same area for consecutive periods focus on BLF line (Raceland to GI)
continuous GPS measurements at CORS stations (Houma, Cocodrie, and Boothville): high-accuracy elevation changes since established (2002-2003)

SLIDE 5

Grand Isle Tide Gauge
Mid-1960s to early 1990s: accelerated relative sea level rise
image: graph showing relative sea level rise at Pensacola and Grande Isle tide gauges
Grand Isle RSLR for 1947-2006: 9.3 mm/yr
Grand Isle RSLR for 1947-1964: 2.2 mm/yr
Grand Isle RSLR for 1964-1991: 11.5 mm/yr
Grand Isle RSLR for 1991-2006: 3.4 mm/yr
Pensacola RSLR for 1924-2006: 2.1 mm/yr

notes: average rate of RSLR at GI (9.3 mm/yr) is not constant through time
2.2 mm/yr (1947-1964) to 11.5 mm/yr (1964-1991) to 3.4 mm/yr (since 1991)

interannual and decadal variations in regional water levels - ÒnoiseÓ in tide-gage records, but:
comparison w/ Pensacola (nearest gage in a relatively stable geologic setting) - relatively uniform rate of RSLR (2.1 mm/yr)
no pronounced period of RSLR acceleration and deceleration as at GI

SLIDE 6

Bayou Lafourche Leveling Line
1982-1993: accelerated subsidence
image: graph showing subsidence rates
1952-1965: average -7.8 mm/yr
1965-1982: average -7.9 mm/yr
1982-1993: average -11.1 mm/yr

notes: temporal trends from elevation changes at BMs along B. Lafourche:~ 8 mm/yr (1952-1965-1982) to ~ 11 mm/yr (1982-1993)
spatial trends: highest subsidence rates occur over same approximate locations through time

SLIDE 7 

Integrated Datasets: Temporal Trends
image: map of Louisiana showing CORs sites, tide gauges, benchmarks, and subsidence rates for several locations
notes: general decadal pattern of slow - rapid - slow subsidence:
- earliest subsidence rates derived from RSLR trends at GI (~ 3 mm/yr)
- mid-1960s to early 1990s average rates derived from leveling surveys (Bs. Petit Caillou - Lafourche) and GI tide gage are similar (~ 10 mm/yr)
- most recent GPS rates (3.5-6.3 mm/yr) are similar to RSLR at GI since early 1990s 
- higher historic subsidence rates along MSR, but similar trend: most recent rates << historic rates

SLIDE 8

Delta-Plain Subsidence Rates
Source: Radiocarbon ages; Period: Holocene; Rate (mm/yr): 1-5; reference: Penland et al, 1988; Roberts et al, 1994; Morton et al, 2006;
Source: Numerical model; Period: Holocene; Rate (mm/yr): less than 5; reference: Meckel et al., 2006;
Source: NGS leveling - Bayou Lafourche; Period: 1965-1982; Rate (mm/yr): 7.9; reference: Shinkle & Dokka, 2004;
Source: NGS leveling - Bayou Lafourche; Period: 1982-1993; Rate (mm/yr): 11.1; reference:Shinkle & Dokka, 2004;
Source: NGS leveling - Bayou Petit Caillou; Period: 1966-1993; Rate (mm/yr): 11; reference: Shinkle & Dokka, 2004;
Source: NGS leveling - Mississippi River; Period: 1961-1984; Rate (mm/yr): 13.6-18.7; reference: Shinkle & Dokka, 2004;
Source: NOS tide gauge - Grand Isle; Period: 1947-1964; Rate (mm/yr): 2.2; reference: Morton and Bernier, this study;
Source: NOS tide gauge - Grand Isle; Period: 1964-1991; Rate (mm/yr): 11.5; reference: Morton and Bernier, this study;
Source: NOS tide gauge - Grand Isle; Period: 1991-2006; Rate (mm/yr): 3.4; reference: Morton and Bernier, this study;
Source: NGS CORS stations; Period: 2002-2007; Rate (mm/yr): 3.5-6.3; reference: Dokka et al, 2006;

notes: table - summary of geologic and historic subsidence rates from various methods
   highest historic rates >> geologic rates
   historic data sources: urban areas and along natural levees

historic subsidence rates from stratigraphic analysis of shallow cores at interior d.p. sites >> rates measured along natural levees:
   50 to > 100 cm subsidence at wetland loss hotspots over ~ 50 yr _ min 10-20 mm/yr (Morton et al., 2005)
   historic imagery - rapid submergence at these sites during 1960s-1970s, most subsidence likely occurred over << time

most recent rates (GI since 1993 and CORS) are if similar magnitude to geologic subsidence rates

SLIDE 9

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

notes: competing hypotheses to explain high rates of historical subsidence:

compaction of unconsolidated Holocene sediments within the incised MR valley:
   rates from 14C analyses and numerical modeling < 5 mm/yr - component of total subsidence but cannot explain high historical rates

neotectonics - ongoing salt evacuation, sediment loading, and growth faulting since middle to late Miocene:
   operate on geological time scale - hard to explain decadal-scale acceleration and deceleration of subsidence rates
   some subsidence trends are opposite what would be expected: historic subsidence rather than uplift over subsurface salt domes (Leeville, Valentine)

close temporal and spatial correlation between rates of wetland loss, subsidence, and hydrocarbon production - anthropogenic induced subsidence:
   numerical modeling: rapid subsurface fluid withdrawal (1960s-1970s) and associated large reductions in reservoir pore pressures - 
       induce land subsidence and cause slip on growth faults after threshold stresses are exceeded
   predicts that subsidence rates should decrease after overburden load and subsurface pressures re-equilibrate following decline in production


SLIDE 10

Delta-Plain Oil-and-Gas Fields

image: map of Louisiana showing benchmarks, oil-and-gas fields, and 1932-2006 land loss
notes: location of o+g fields relative to historic d.p. wetland losses
look BLF line in more detail

SLIDE 11

Bayou Lafourche Leveling Line
- Highest rates occur over nearby producing fields
image: graph showing subsidence rates for Raceland, Valentine, Cutoff - Bully Camp, Leeville
notes: spatial trends: highest subsidence rates occur over same approximate locations through time (over nearby o+g fields)
   similar spatial trends along BPC and MSR lines
   
SLIDE 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
notes: In conclusion, integration of datasets reveals a decadal-scale acceleration and subsequent deceleration of historic subsidence rates that was likely induced by deep subsurface hydrocarbon production. The most recent subsidence rates are comparable to rates averaged over geological time scales and the recent reductions in subsidence rates likely reflect balancing of subsurface stresses and a return to near-equilibrium condition. A better understanding of the most recent trends and process causing subsidence needs to be incorporated into coastal restoration efforts and efforts to model expected impacts of increased RSLR in a time of global climate change.