Scientists find slow subsidence of Earth's crust beneath the Mississippi delta

Apr 02, 2012
Geoscientists report new findings on sea level rise and coastal subsidence in Louisiana. Credit: NOAA

The Earth's crust beneath the Mississippi Delta sinks at a much slower rate than what had been assumed.

That's one of the results geoscientists report today in a paper published in the journal .

The researchers arrived at their conclusions by comparing detailed sea-level reconstructions from different portions of .

"The findings demonstrate the value of research on different facets of Earth system dynamics over long time periods," says Thomas Baerwald, and spatial sciences program director at the National Science Foundation (NSF).

NSF's Directorates for and for Social, Behavioral & Economic Sciences funded the research.

"The results provide valuable new insights about the factors that affect shorelines and other locations in the Gulf Coast area now and into the future," says Baerwald.

"Our study shows that the basement underneath key portions of the Mississippi , including the New Orleans area, has subsided less than one inch per century faster over the past 7,000 years than the more stable area of southwest Louisiana," says paper co-author Torbjörn Törnqvist of Tulane University.

This is an aerial view of Delacroix, La., which is mostly abandoned due to sea-level rise and wetland loss. Credit: Tor Törnqvist

The difference is much lower than previously believed.

"Other studies have assumed that a large portion of the Earth's crust underneath the subsided at least 30 times faster due to the weight of rapidly accumulating sediments in the delta," says Törnqvist.

The paper, co-authored by Tulane scientists Shi-Yong Yu and Ping Hu, reveals some good news for residents of the New Orleans area.

Large structures such as coastal defense systems could be relatively stable, provided they are anchored in the basement at a depth of 60-80 feet below the land surface.

Shallower, water-rich deposits subside much more rapidly.

However, the study also provides more sobering news.

Torbjörn Törnqvist determines the elevation of a GPS-antenna in the Mississippi Delta. Credit: Juan Gonzalez

"These subsidence rates are small compared to the rate of present-day sea-level rise from the Florida panhandle to east Texas," says Törnqvist.

"The rate of sea-level rise in the 20th century in this region has been five times higher compared to the pre-industrial millennium as a result of human-induced climate change."

Sea level has risen more than eight inches during the past century.

"Looking forward 100 years, our main concern is the continued acceleration of sea-level rise due to global warming, which may amount to as much as three to five feet," says Törnqvist.

"We can now show that rise has already been a larger factor in the loss of coastal wetlands than was previously believed."

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User comments : 8

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jscroft
1.7 / 5 (6) Apr 02, 2012
Doubtless a consequence of Anthropogenic Climate Change.
tadchem
1.8 / 5 (5) Apr 02, 2012
"These subsidence rates are small compared to the rate of present-day sea-level rise from the Florida panhandle to east Texas," sounds like the sea-level rise is a *local* phenomenon.
The difference between land subsidence and ocean rising is the between a local phenomenon and a global pheonomenon.
Changes in the *land* level are local. Gravity enforces a *global* uniformity of mean sea level.
The ocean has no hills.
Lurker2358
4.3 / 5 (8) Apr 02, 2012
Gravity enforces a *global* uniformity of mean sea level.
The ocean has no hills.


Well, technically you're wrong.

NHC, NOAA and LSU track gulf water levels all the time, and they vary wildly even over a matter of a few tens or hundreds of miles.

Ocean water builds up in ridges and troughs for several reasons, including wind and atmospheric pressure changes.

Also, sub-oceanic sea mounts alter the local gravitation, and can cause water to "pile up" in the region.

So it's not actually as straightfoward as you think, since Earth is not a perfect Sphere.
nkalanaga
not rated yet Apr 02, 2012
Salt content also matters. The Atlantic is denser than the Pacific due to higher salt content.

"Sea level is about 20 cm higher on the Pacific side than the Atlantic due to the water being less dense on the Pacific side, on average, and due to the prevailing weather and ocean conditions. Such sea level differences are common across many short sections of land dividing ocean basins. If the canal was open sea and did not contain locks, i.e. if somehow a deep open cutting had been made rather than the canal system over the mountains, then there would be a current flowing from the Pacific to the Atlantic."
http://www.psmsl....fo/faqs/
Cluebat from Exodar
not rated yet Apr 03, 2012
"Looking forward 100 years, our main concern is the continued acceleration of sea-level rise due to global warming, which may amount to as much as three to five feet," says Törnqvist.

I call shenanigans.

I do not deny that there is cyclic warming- but 3-5 feet in a century?

This is the Kool-Aid talking.
Vendicar_Decarian
not rated yet Apr 03, 2012
Clearly this is where denialists claim the millions of volcanoes that are heating the earth are residing.

GSwift7
not rated yet Apr 03, 2012
I was going to post the same corrections to tadchem's remarks, but it's already covered.

As for the 3-5 feed, the IPCC's latest numbers have been revised down from there. There are draft copies of the 5th AR floating around. Also, the corps of engineers and USGS have been studying coastal land movement for decades. I'm not sure who to believe, but this guy's comments run contrary to every other expert on the subject. I think it might be wise to wait and see if anyone publishes a rebuttal to this guy's work.

Also, when they say "basement", is that some foriegn word for bedrock?
GSwift7
not rated yet Apr 03, 2012
One more thing, sorry: normally when we talk about subsidence, we are talking about surface level. The tectonic subsidence of the crust is an entirely different topic. When the city of New Orleans was built it was several feet above the water level, and it has fallen many feet in the past two centuries so that parts of the city are now way below the water level. It's falling way faster than the rate of sea level change. The same is true for all river deltas, such as the Nile and Amazon. That's why you find cities underwater off the coast of Egypt, for example.

Ground water use and oil drilling can also result in extremely rapid subsidence, such as in Napa Valley California, Boston Massachusets and parts of Texas. There's a picure on wiki on the subsidence page of the former ground level in an area of California where water use has caused extreme subsidence in a short period of time.