Washington DC sinking fast, adding to threat of sea-level rise

Scientists have found that land under the Chesapeake Bay - the largest estuary in the US - is sinking rapidly and the country's capital, Washington, DC, could drop by six or more inches by 2100 - adding to the problems of sea-level rise.
The falling land will exacerbate the flooding that Washington, DC, faces from rising ocean waters due to a warming climate and melting ice sheets, researchers said.
For sixty years, tide gauges have shown that sea level in the Chesapeake is rising at twice the global average rate and faster than elsewhere on the East Coast.
Geologists have hypothesised for several decades that land in this area, pushed up by the weight of a pre-historic ice sheet to the north, has been settling back down since the ice melted.

The new study - based on extensive drilling in the coastal plain of Maryland - has confirmed this hypothesis, and provides a firm estimate of how quickly this drop is happening.
Additionally, detailed field data make clear that the land sinking around Washington is not primarily driven by human influence, such as groundwater withdrawals, but instead is a long-term geological process that will continue unabated for tens of thousands of years, independent from human land use or climate change, researchers said.

The research was conducted by a team of geologists from the University of Vermont, the US Geological Survey, and other institutions.

Washington's woes come from what geologists call "forebulge collapse." During the last ice age, a mile-high North American ice sheet, that stretched as far south as Long Island, piled so much weight on the Earth that underlying mantle rock flowed slowly outward, away from the ice.
In response, the land surface to the south, under the Chesapeake Bay region, bulged up. Then, about 20,000 years ago, the ice sheet began melting away, allowing the forebulge to sink again.
Ben DeJong, lead author on the new research, who conducted the study as a doctoral student at UVM's Rubenstein School of Environment and Natural Resources, and others drilled seventy boreholes, many up to a hundred feet deep, in and around the Blackwater National Wildlife Refuge, near Washington, on the Chesapeake's eastern shore.

Then they examined layers of sediment in these deep cores, using a suite of techniques to calculate the age of the sand, other rocks, and organic matter in each layer.
Combining this data with high-resolution LiDAR and GPS map data allowed the team to create a detailed 3D portrait of both the current and previous post-glacial geological periods in the Chesapeake, stretching back several million years.
This longer view gives the geologists confidence that they have a "bullet-proof" model, DeJong said, showing that the region today is early in a period of land subsidence that will last for millennia.
The results were published in the journal GSA Today.