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Sites, shapes and texture of oyster shells reveal changes in water level and salinity
If oysters could talk, Darrin Lowery would probably understand their language.
A fireside chat is unlikely, so Lowery studies their shells instead - mounds of them. As a "geo-archaeologist" for the Smithsonian Institution, Lowery is part of a team investigating the history of coastal change along the Chesapeake's Eastern Shore.
Oysters, it turns out, have secrets to tell.
Many are revealed through the large, historic collections of oyster shells accumulated at American Indian fishing camps, called "middens."
Lowery is coaxing information from the placement, shape and texture of midden shells that help to detail the complicated coastal dynamics of the Chesapeake's past.
"Middens tell us more than what people were eating. Within the shells of these creatures is information about the temperature and salinity of the world they were living in," Lowery said.
Lowery and his colleagues have documented 25-30 middens along the waterways of Maryland's Dorchester County.
The oldest, at Snake Island in Fishing Bay, is roughly 1,600 years old. Lowery said that most middens in the area are approximately 600-800 years old - in use several centuries before Capt. John Smith and his crew explored the Bay in the early 1600s.
Because American Indians ate oysters close to where they caught them, middens are especially helpful in characterizing the geological history of a local area.
On a bright morning near Fishing Bay, Lowery aimed his boat toward a barely noticeable break in the marsh, where dense stands of grass and brush give way to a tiny patch of exposed land on Guinea Island.
The bow of his boat nosed onto shore, and Lowery clambered onto the rough layer of oyster shells that coated the short stretch of beach.
The shells had arrived by land, not water. They were displaced from the island's midden, dispersed from the original mound over centuries.
"The midden itself is massive. Anywhere from 12-20 acres," Lowery said.
His hand dipped to the beach and returned with a number of shells. He ran his thumb over the surface and flipped them deftly as if sorting them for sale.
"What we have here is an ecological fingerprint of Fishing Bay before John Smith's time," Lowery said.
The edges of the shell, he explained, reflect water quality. A scalloping effect indicates plenty of ultraviolet light, which means the oysters grew in clear shallow water.
Their shape reveals more. Shells from water with a high current are long and skinny, while those in gentle flow are rounded.
On the underside of a shell, Lowery rubbed dirt from the hinge site that once connected the top and bottom pieces. Variations in the pearly white color resemble the texture on the outer shell, but Lowery sees it differently.
"These lines are calcium deposits, like rings on a tree," Lowery said. "The hinge is the signature of its life."
An oyster lays down regular calcium deposits in its shell. But in times of stress-anything from breeding or storm events with a high, freshwater flow - the deposits slow down.
Some of the shells Lowery handled had neat round holes through the surface. The holes paint a dramatic contrast between the past and present ecosystem.
The gastropods that bore those holes were a species that live only in salty water. The salinity levels needed to support them would have been much higher than levels found in the Bay's water today.
"One thousand years ago, the water was near marine salinity. All of the plants you see here would not have existed," Lowery said.
The salinity and plants of today's marshes are quite similar to those in more recent centuries. Lowery believes that colonization and the deforestation that came with it played a role in the change.
"When you remove the forest - the sponge - you get much more freshwater into the system," he said.
The greater mixture of freshwater and saltwater gave rise to extraordinary diversity in the Chesapeake ecosystem. Lowery's findings suggest that this is a more recent development. Ironically, the shift may have been nurtured by the same types of human activity that over time have also led to the Bay's decline.
"What should we conserve or try to reconstruct to? What's here may be largely a byproduct of us. This landscape is in flux and has been for 20,000 years," Lowery said.
The location of oyster middens is also helping to document changes in sea level. Their presence serves as a "data print" for when an area was last dry.
Lowery demonstrated the point by stepping from his boat to showcase another midden near Fishing Bay. There was no beach. The water reached his calves.
"This was a big Native American encampment that's been drowned," Lowery said. "It's about 800â€"1,000 years old and extends about 200 yards into the marsh. Where I'm standing now was once dry land, where folks were eating oysters."
But just as water has swallowed some middens, erosion has shed daylight on others.
Erosion can be driven by chafing water from rising sea level, or by pressure from stormwater runoff, high wave energy and wind.
Along the broad expanse of Fishing Bay, water gnaws the shoreline in an area known as Elliott Island. The process has toppled large trees and left remnants of their root systems protruding from the water.
Land rises sharply above the sandy beach, exposing layers of soil in a convenient geological display.
A crusty layer of oyster shells lies within it, more than a foot thick and clearly exposed to the naked eye. The midden is close to the topsoil, but the shells are intact because the land was never farmed.
"An unploughed midden like this is extremely rare," Lowery said.
The exposure of such sites offers great research opportunities, but the pace of erosion can also threaten their existence.
"Erosion is exposing the site but there is nothing here to protect it," Lowery said.
Lowery and his colleagues hope to make the most of the research window at Elliott Island, because the oyster midden is only one of its valuable features.
Its geology demonstrates the variations that can take place in a relatively small area. Upstream, a midden has been drowned, but here the land appears to have risen.
Lowery believes that wind from the northwest built a dune over the wetlands long ago. Ancient glacier pressures from as far as New Jersey could also have lifted the land like the opposite end of a seesaw.
A few feet off shore, a black band of peat is compressed into the sand. Roots and fibers extracted from the peat are the physical remains of the Chesapeake ecosystem more than 20,000 years ago.
"In that peat is all the plant material that grew here during the last ice age," Lowery said.
Lab analysis shows evidence of trees like red spruce, balsam fir and yellow birch that today are common in much colder settings. These studies, combined with a close reading of the middens, will continue to unravel the continually changing geology of the area.
"Coastal plain geography is complicated and rarely do you have a full book. It's like having "War and Peace" with some of the chapters ripped out," Lowery said. "I've developed a great respect for complexity."
This article credits Bay Journal News Service
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