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Geophenomena

Sediments Shed Light on Past Coastal Events
Ilya V. Buynevich

Several forces batter coastal areas everyday: erosion, the onslaught and retreat of floodwaters from tropical cyclones and hurricanes, El Niño events, winter storms and tsunamis. Scientists only have reliable records of the impact of such events on coastal landforms and ecosystems from the past few centuries, particularly in developing countries.

A berm scarp in southern Brazil shows characteristic signatures of erosion. Geophysical imaging (far right) of sediments bearing heavy minerals at the scarp’s base reveal past storm activity and shoreline change over time. All images courtesy of Ilya Buynevich.

But because many coastal barriers around the world developed over the past 3,000 to 5,000 years, most of their dynamic history is archived within their sediments. Using these sediments, researchers can take an integrated approach to understanding how beaches, dunes and coastal inlets respond to a variety of natural phenomena by reconstructing their past behavior over thousands of years. Such reconstructions, which are getting a boost from new below-surface imaging technology, are particularly important to coastal communities coping with rising sea levels, diminishing sand supplies and increasing population pressures.

Using geologic data, scientists are able to determine the locations and extent of past coastal erosion, allowing them to assess the vulnerability of a particular shoreline segment to future storm impacts. For example, in the past 200 years, a number of severe hurricanes and nor’easters (winter storms) have battered the U.S. East Coast, leading to extensive beach and dune erosion, property damage, storm surge inundation and even formation of new inlets. For example, Moriches Inlet along the southern shore of Long Island, N.Y., owes its origin to a 1931 nor’easter, while Shinnecock Inlet to the north was born during a powerful hurricane in 1938.

Massive damage to coastal communities and dramatic landscape changes associated with large storms can be understood better when put in the context of long-term evolution of coastal systems. Studying the layering and composition of beach deposits, especially the historic surface that records the missing sediments removed by erosion, shows how the landscape changed and can thus help model the impact of future storms.

In the past, geologists studied evidence of coastal erosion through the examination of surface features such as dunes, and intrinsic properties such as sediment characteristics. However, natural processes and human-induced changes have modified or obliterated much of the surficial evidence of past erosion. Still, many of these signatures of past coastal behavior remain beneath the surface. Sediment cores and trenches alone may be insufficient to capture subsurface features, and often lack sufficient continuity to fully describe the coastal zone. To remedy these limitations, subsurface imaging technology, such as ground-penetrating radar (GPR), has proven to be successful in providing continuous, high-resolution images of buried beach and dune sediments.

Depending on antenna frequency and properties of geological materials (water retention, texture, porosity), GPR can image decimeter-sized features to depths of more than 20 meters below the surface. The technology enables scientists to visualize the subsurface geology, often in 3-D, by creating a reflection profile based on the travel time of high-frequency electromagnetic waves (see Geotimes, October 2005). The technique does not work well in sediments saturated with saltwater, but many barrier systems contain thick lenses of freshwater, which have a relatively high permeability that enables imaging. These characteristics make GPR a practical imaging tool in a variety of coastal settings, and in recent years, the use of this technology has revolutionized coastal geological research.

A dune scarp in mid-coastal Maine has extensive heavy-mineral deposits at its base that show up in images of the subsurface.

In many parts of the world, GPR surveys have been instrumental in reconstructing past surfaces. In particular, erosional features such as buried storm scarps produce prominent reflections in GPR surveys due to their distinct geometry, often complemented with heavy-mineral concentrations — thus recording major storm events. For example, a series of buried erosional scarps have been imaged at Hunnewell Beach, Maine, and Crane Beach, Mass., where sediments bearing heavy minerals (magnetite, ilmenite, garnet) cover the beach following intense storms. Similar mineralogical signatures of beach erosion, which are sometimes accompanied by gravel or shell-rich horizons, can be found on many beaches around the world. Such signatures also can reveal a several-millennia-long archive of erosional events for expanding coastal systems.

After mapping an area, scientists can then determine the age of a particular coastal event by radiocarbon dating the organic sediments associated with buried erosional features or by dating a sand grain’s last exposure to sunlight at the surface. This information is crucial for determining the timing and recurrence of extreme coastal events in different parts of the world.

During intense storms or tsunamis, incision and breaching of narrow coastal barriers may sometimes lead to formation of new inlets. In areas where sand or vegetation bury and mask historical and relict inlets, their channels may often be recognized in geophysical records as a legacy of the shoreline’s dynamic past. In this case, GPR imaging is used not only for identifying buried channels, but also for providing information on the channel location, dimensions and geometry prior to closure.

Without GPR, such information rarely can be obtained for historical inlets and is unavailable for relict channels. But using GPR at Oyster Pond on Cape Cod, Mass., for example, researchers discovered an old channel beneath a small barrier beach.

This pond was the focus of K.O. Emery’s classic investigation of a coastal pond by oceanographic methods in the 1960s, where he used a suite of geological and geochemical techniques to understand in detail the evolution of a small coastal water body. Similar to many barrier beaches in formerly glaciated regions, the gravel content of the barrier fronting Oyster Pond makes core extraction a challenge. However, this same characteristic produces prominent reflections in geophysical records due to its contrast with the overlying finer-grained sediments.

The recent GPR survey at Oyster Pond not only imaged a small eastern inlet that was depicted on historical charts from the late 1700s to early 1800s, but also revealed a larger channel structure along the central part of the barrier beach. The latter was approximately 60 meters wide and at least 2 meters deep and likely existed prior to or in between the historic mapping efforts. This channel was probably from an inlet that originated during a large storm.

This study shows that a larger proportion of the barrier beach structure consists of channel-fill sediments than previously thought. It also underscores the value of geophysical imaging in providing information that is otherwise unavailable from historical documents and one that would be extremely difficult to obtain using only traditional coring techniques. Such geological information is of immediate use to coastal scientists and managers, who are constantly looking at ways to mitigate the impact of future storms on coastal communities by identifying hotspots of erosion or other types of rapid coastal change. We have already seen, for example, quite a bit of breaching along the barrier islands of the Gulf Coast during Hurricane Katrina.

In many coastal settings, the interpretation of key reflections on geophysical records is greatly facilitated by sediment cores at a few carefully selected locations. Furthermore, with new advances in dating both organic and inorganic materials, it is now possible to establish accurate ages for ancient shorelines and associated erosional features, with the potential of reconstructing coastal evolution over tens of thousands of years. The identification and mapping of erosional signatures in a spectrum of coastal settings will undoubtedly continue to help in evaluating the vulnerability of coastal segments to future storm impacts.


Buynevich is a marine geologist at the Woods Hole Oceanographic Institution who studies coastal landscape change. His research interests include stratigraphy and evolution of barrier islands, geological legacy of tidal inlets, and responses of various components of coastal systems to environmental change.

Links:

"Geophysics and Archaeology Collide," Geotimes, October 2005. Print exclusive

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