Columbia University New York, N.Y. 10027 Office of Public Information (212) 854-5573
Scientists at Columbia University's Lamont-Doherty Earth Observatory and the U.S. Geological Survey have found--unexpectedly--that recent violent and destructive storms, including a "storm-of-the century" Nor'easter in 1992, left the bottom of Block Island Sound virtually untouched.
The likely reason, the scientists found, is the existence of a protective, shallow sand bar stretching from Montauk Point at the tip of Long Island to Block Island that keeps storm-whipped waves from entering the deeper parts of the Sound. In other coastal areas without such a fortuitous geological remnant from the ice age, storms would have substantially more impact on the ocean bottom, they said.
The finding has important implications for many coastal activities and structures affected by seafloor changes, including offshore cables, pipelines and wells; boating and submarine channels; fishing beds, and deposit sites for potentially toxic dredge spoils.
In a study designed to explore the effects of major storms on the seafloor, scientists Neal Driscoll of Lamont-Doherty and David Twichell Jr. of the USGS compared sonar images of the ocean bottom obtained last summer with a similar survey supported by the Navy in 1991. In the interim, the region was hit by several damaging storms: Hurricane Bob in 1991, the Halloween Nor'easter of 1991, a "storm-of-the-century" Nor'easter in December 1992, and the blizzard of March 1993.
"We had expected that waves whipped up by these storms would have rearranged seafloor sands and sediments and resculpted the ocean bottom," said
Dr. Driscoll, a geophysicist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y. "Although we had some major storms, including a storm of the century, we found that the passage of these large storms did not cause any significant changes on the seafloor in Block Island Sound--and that was rather surprising."
"If people are going to use coastal places like these responsibly, we really need to begin to learn about the dynamics of these areas and understand how they are affected by storms, currents and other factors," said Dr. Twichell, a marine geophysicist at the USGS in Woods Hole, Mass. "What we learned in this study can help us understand how big storms affect the lee, or protected, side of embayments in other coastal regions."
Last summer, Drs. Driscoll and Twichell co-led an expedition in the Sound aboard the R/V Cape Henlopen, a 120-foot coastal zone research vessel operated by the University of Delaware. The research was funded by the Office of Naval Research.
The scientists surveyed a 5-by-10-nautical-mile area ranging from the waters off Stonington, Conn., and Montauk Point in the west to Weekapaug, R.I., and Block Island to the east. They used two types of sonar to map detailed features of the seafloor and underlying structures. They also towed an underwater camera in the 50- to 120-foot deep waters to photograph seafloor features and took numerous samples of the sand and on the bottom of Block Island Sound.
The survey revealed large mounds and valleys and a rich tapestry of boulders, gravel beds, mud-filled depressions and sand waves. The latter are crafted by currents and tides much the way that winds sculpt undulating sand dunes in deserts. The images were so detailed that they showed spaghetti-like patterns of trenches cut by fishing trawlers dragging nets along the sandy bottom.
Down to small details, seafloor features seen in the 1991 survey remained virtually unchanged in the 1994 survey, the scientists said.
The study showed that the Sound's seafloor is protected by "a natural jetty that was built thousands of years ago by the glaciers," Dr. Driscoll said--a shallow sand bar that stretches 12 nautical miles between Montauk Point and Block Island. The sandy shoal lies 20 to 40 feet below the water's surface, and the seafloor drops to 50 to 120 feet behind the shoal. Though submerged, the sandy shoal acts as a barrier at the entrance to the Sound, blocking waves from disturbing the seafloor.
"It is the front line, the first trench taking the brunt of the assault of the waves," Dr. Driscoll said. "Without that barrier, the deeper portions of the continental shelf in the Sound would have been reshaped by large storms."
The shoal was first created some 18,000 years ago when Long Island Sound was covered by great glaciers. The glaciers surged from Canada, carrying great accretions of boulders and sediments that remained as moraines when the ice retreated. The spine of Long Island, the South Fork and Block Island are vestiges of one of these ramparts formed by periodically surging glaciers. The North Fork, Plum Island and Fishers Island are remnants of another.
The moraines formed earthen dams that trapped water melted from the glaciers. Some 16,000 years ago, one of these glacial dams created a large lake, called Lake Connecticut, where Long Island Sound now lies. At some point, the dam was breached, draining the lake into the ocean. The glaciers melted and sea level rose, submerging the original wall. But remnants of the wall between Montauk Point and Block Island still provide a bulwark against storm waves and protect the seafloor in the Sound's interior, the scientists said.
Since the original breach, the submerged sand bar has actually been reinforced by sands transported eastward along Long Island's South Shore by winds and waves. Once offshore, some sand hit the shoal rising from the seafloor and became incorporated in it.
The study also uncovered two other unexpected findings, which the scientists hope will foster two new research collaborations--the first with University of Connecticut scientists and the second with Block Island Sound fishermen.
The scientists found vibrant green algal mats covering seafloor samples.
"These mats, sampled in box cores, looked as lush as grass on a golf course," Dr. Driscoll said. The mats, composed of photosynthesizing algae, may be a rich source of oxygen in Block Island Sound--a provocative contrast to the more confined western parts of Long Island Sound, which in summer months occasionally experiences hypoxia, a lack of oxygen that threatens sea life. The researchers plan to pursue further research on these mats with biologists and geochemists at the University of Connecticut.
The study also showed that seafloor patterns left by fishing nets closely followed seafloor sediment patterns inferred from the sonar data.
"The correlation is quite remarkable: By looking at where trench marks left by trawlers along the seafloor stopped and started, you could roughly predict where sediment types changed," Dr. Driscoll said. "The fishermen seemed to turn on a dime at precisely the points where sediment types changed."
The finding suggests that certain sediments may provide conditions, food sources or habitats that attract specific fish species and allow them to flourish.
"The fishermen have a vast amount of knowledge, and scientists can learn a lot from them," Dr. Driscoll said. "We'd like to talk to them and share our seafloor maps and geologic information in exchange for their wisdom on fish habitats--to our mutual benefit."