How stable is the Mississippi Delta?

Abstract

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only ϳ0.1 mm yr ؊1. We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata.

Eos, Transactions, American Geophysical Union Volume 87, Number 45, 7 November 2006

“…The trend of RSL rise as reported here is compatible with longer RSL records that cover much of the Holocene at various localities across the Mississippi delta [Törnqvist et al, 2006]. That recent study inferred negligible tectonic subsidence rates in large sections of the Mississippi delta during the past 8000 years.…”

Climate related sea-level variations over the past two millennia

“…3). Salt-marsh proxy records from the Gulf of Mexico (17,18) show stable sea level until AD 1000, followed by rise to a peak at AD 1200. In Connecticut, sea level rose rapidly at AD 1000 (19), although this record may be compromised by sedimentary hiatuses from hurricane erosion (20,21).…”

Peat Accretion Histories During the Past 6,000 Years in Marshes of the Sacramento–San Joaquin Delta, CA, USA

“…Approaches exist that can estimate autocompaction and these need to be explored if a significant relationship is found between depth and accretion rate (Kaye and Barghoorn 1964;Stevenson et al 1986;Pizzuto and Schwendt 1997;Allen 2000). Lastly, for peatforming regions that are known to be tectonically active, an evaluation of the prospects of subsidence or uplift is essential for proper interpretation of the peat record (e.g., Atwater et al 1977;Sawai et al 2002;Tornqvist et al 2004Tornqvist et al , 2006.…”