Natural Subsidence and Sea-Level Rise - Subsidence and Wetland Loss Related to Fluid Energy Production, Gulf Coast Basin

Science Center Objects

Geologic proxies may shed light on long-term environmental trends and stability of the Louisiana coastal marsh. The combined processes of accretion, sea-level rise and subsidence influence wetland elevation and determine marsh stability. 

photos of shallow-water benthic foraminifera

Shallow-water benthic foraminifera produce an elaborate carbonate test, or shell, that is used for identification. The presence or absence of certain species can be used as a proxy for environmental characteristics.

(Public domain.)

Microfossils, such as foraminifera (fig. 1) and diatoms (fig. 2), are sensitive to environmental conditions, and present meaningful long-term proxies for investigating wetland stability and persistence. Once protocols are established, these geologic proxies are valuable for rapid assessment of environmental trends.


The USGS is investigating landscape change in coastal wetlands using microfossil assemblages and physical characteristics in marsh cores as proxies for storm impact, subsidence, and sea-level rise. Surface and down-core microfossil distributions will be correlated with sedimentologic and stratigraphic parameters to identify changes in environmental conditions. The initial study area will focus on southwest Louisiana, where rapid subsidence and erosion of wetlands is resulting in unprecedented rates of sea level rise.

The project goals are to:

  • Develop and update protocols for extracting microfossil samples from marsh sediment samples

  • Identify the taxonomy and distribution of key diatom and foraminifera species that record accurate indications of wetland environmental characteristics

  • Provide a record of diatom species distribution for coastal Louisiana, since current knowledge is limited

  • To refine the marsh foraminifera biofacies of the Mississippi delta region

  • Evaluate the ability of microfossils to indicate extreme events, such as storms


photographs of diatoms

The silica cell-wall, or frustule, of a diatom preserves well in most sedimentary environments and can be used in species identification. Since certain species exist only in specific habitats, the composition and quantity of diatoms can be used to figure out past environmental conditions. 

(Public domain.)

Paleoecological and sedimentological analyses are conducted on several coastal marsh sediment cores. Sediment core sites are selected to represent a gradient of saline to brackish marsh. Cores are X-rayed and sampled for microfossil assemblages, grain-size, bulk density, organic carbon, and radioisotopes (210Pb and 137Cs). Statistical analyses of downcore abundance curves for microfossils and sedimentological data are examined with chronology data to isolate proxies most clearly indicative of marsh accretion, storm-impact, subsidence, and sea-level rise.


James Flocks
Chandra Dreher
Kathryn Smith


  • Smith, K.E.L., and Flocks, J.G., 2010, Environmental investigations using diatom microfossils: U.S. Geological Survey Fact Sheet 2010–3115, 2 p. [URL:]

  • Dreher, C.A., 2006, Modern Foraminiferal Bio-facies within a Transgressive Saline Influenced Deltaic Headland, South-Central Louisiana: Masters Thesis, University of New Orleans, 136 p. [URL:,443]