Across large swathes of southern Louisiana, average annual subsidence of five-to-10 millimetres (0.2 to 0.4 inches) have contributed to sea-level rise, shoreline erosion and wetland loss, they said.

The findings have implications for delta regions around the world -- home to tens of millions of people -- already threatened by rising sea levels caused by global warming, the researchers told AFP.

But the study, published in Nature, also shows for the first time that the sinking of the Mississippi Delta is, geologically speaking, only skin deep, limited to a layer of peaty sediment less than 100 meters (300 feet) thick.

This is good news, say the researchers, because it means that flood-control structures -- if anchored in the rock-hard stratum below this softer, organic-rich layer -- stand a better chance of remaining stable and not subsiding.

The new findings could also help determine the effect of coastal restoration projects, such as plans to divert water and sediment from the Mississippi River to depleted wetlands.

The prevailing theory up to now has been that subsidence occurs deeper beneath the surface in Earth's crust due to the crushing weight of accumulating sediment.

But the study "shows that the high rates of subsidence at the land surface can be quite easily explained by what happens in the shallow subsurface, say 50 to 100 meters (150 to 300 feet)," Torbjorn Tornqvist, director of the Coastal Centre of the US Department of Energy's National Institute for Climate Change Research, told AFP.

Some parts of New Orleans, he said, had sunk more than two meters in five or six decades, caused in large measure by artificial drainage.

Tornqvist, a professor at Tulane University in New Orleans, and colleagues punched more than 100 holes up to 15 meters (50 feet) deep in the Bayou Lafourche region and used radiocarbon-dating to examine the sediment cores.

They found that rates of subsidence over the last 1,000 years were at least five millimeters a year, and up to twice that in shallower sediment.

Tens of millions of people around the world live in low-lying deltas caught between the scissors of land subsidence -- caused by depleted water tables and dense human habitation -- and rising sea levels driven by climate change.

"Compaction is a problem in many of these cities, often enhanced by human activities such as groundwater extraction," said Tornqvist, citing Manila and Bangkok as good examples.

Oil and gas withdrawal can also be factors in subsidence, he added.

Compaction is the process that squeezes water out of sediment, leading to a decrease in volume and accelerated sinking of land surfaces.

Other major delta areas already feeling the impact of rising sea levels include Mumbai and Kolkata in India, as well as Shanghai and Dhaka. "Tokyo also has a major subsidence problem," he added.

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