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the instrument can make measurements unattended overnight as well as during regular working hours.
The continuous laser 40 Ar-39 Ar dating system is being used to address a variety of interesting and important problems that could not be as accurately addressed with previous equipment and methods. Among these are the timing of mineralization associated with ore deposits, the age of volcanic units associated with key faunal assemblages, and coal deposits-all of which can be used in mineral assessments and exploration for economic deposits. The ages of seamounts and other volcanic features in the Pacific can also be determined, which provides essential keys to the history of this ocean basin.
Figure 3. Laser microprobe system used in 4oAr-39Ar method of K-Ar dating of rocks and minerals. The laser is a 5-watt continuous argon-ion laser. The laser beam is focused by lenses and directed by mirrors through the glass window of a small, ultra-high vacuum sample chamber where it is used to heat and melt single mineral grains. The argon released by the heating is exposed to the metal-alloy gutters of the cleanup system, which remove the reactive gases. The purified argon is then admitted to the ultra-sensitive, ultra-clean mass spectrometer, and the isotopic ratios-from which the age is calculatedare measured. The laser-fusion process is monitored by a closed circuit television camera mounted on a microscope. A microscopic infrared radiometer, not shown in the photograph, is used to measure the temperature of the sample during laser heating and thus permits incremental heating experiments to be done on the single crystals. (Photograph by G. Brent Dalrymple.)
Loss of Coastal
By S. Jeffress Williams and Asbury H. Sallenger, Jr.
contributes less atmospheric argon (a contaminant in the experiment), and the low level of this contaminant results in very precise age determinations. Some typical single-crystal results from the USGS continuous laser system are shown in table 1.
A final advantage of the continuous laser system is that it is easily automated. In the USGS system, grain selection, laser operation, temperature measurement, valve operation, and video recording can all be controlled by the computer. This automation greatly increases the productivity of the system because
Table 1. 40 Ar/39Ar age results on single crystals of potassium feldspar handpicked from a volcanic unit in the Medicine Lodge Basin, western Montana
Once considered mostly worthless real estate in their natural state, wetlands around the country have long been subjected to draining and filling for additional farmland and for urban expansion. Wetlands also have been used as convenient repositories for waste and trash disposal. In the past two decades, however, people have come to realize that, in fact, wetlands of all varieties are immensely important to both the environmental and the economic health of the Nation.
Wetlands are important as habitats for large and varied populations of aquatic and terrestrial wildlife. They reduce the effects of flooding on developed areas, recharge ground water
The Louisiana Delta Plain
resources, and serve as natural filters for reducing pollutants carried in ground water as well as rivers. In addition, wetlands support a wide array of recreational activities and function as spawning areas and nurseries for the majority of commercial and recreational fisheries, as well as supporting other important industries.
The natural processes of wetlands degradation as well as wetlands destruction and alteration by public agencies and individuals have resulted in the loss of more than 50 percent of the wetlands that existed in the contiguous United States at the start of European settlement. These wetlands losses are continuing, and nowhere is the problem greater than in the Mississippi River delta plain of Louisiana.
Louisiana accounts for an estimated 25 percent of the vegetated wetlands and 40 percent of the tidal wetlands in the 48 conterminous States. Currently, it is undergoing the greatest amount of wetlands loss and deterioration of any State in the Nation. An estimated 80 percent of the Nation's wetlands loss has occurred in Louisiana, and by current estimates, 40 to 60 square miles are lost each year. These losses are the result of a combination of physical erosion by waves and currents as well as conversion from marsh to open water by disintegration of the marshlands and submergence. The U.S. Army Corps of Engineers has predicted that if these rates of loss continue, nearly one million additional acres of valuable wetlands in Louisiana will be lost in the next 50 years.
The physical processes that cause coastal erosion and wetlands deterioration are complex, highly varied, and are still not particularly well defined or understood. The rates and magnitudes of future land loss, therefore, are not predictable with any high degree of confidence. Also, much debate still exists in the technical and scientific community about which of the natural and humaninduced causes are most destructive. The natural causes range from hurricane and winter-storm effects to worldwide changes in sea level. Human-induced causes include dredging of navigation channels and subsidence from groundwater withdrawal.
The geologic record of the coastline and continental shelf of Louisiana shows clearly that over the past 6,000 to 8,000 years, large shifts in the course of the Mississippi River have occurred at about 1,000-year intervals. Such changes in the river's channel have been responsible for repeated cycles. These cycles are followed by rapid sediment compaction, subsidence, and massive erosion and wetlands deterioration as the river abandoned old channels and created new deltas.
Because of these cyclic changes in the delta, sandy barrier islands form at the seaward ends of the delta plain. These barriers provide a buffer from ocean waves and currents and direct storm effects for the wetlands estuaries behind the barriers. With continued subsidence and a lack of coastal sediment, however, the barriers undergo rapid erosion, at rates up to 60 feet per year, and are broken into smaller, less protective segments when tidal inlets open during storms. Eventually, the coastal barriers are unable to maintain their geometry and become submerged sand bodies. Many examples of these former barriers can be seen as buried features on the Louisiana continental shelf.
The widespread loss and deterioration of wetlands
in coastal Louisiana is due to a combination of natural long-term geologic
processes and manmade effects on the Mississippi River and delta-plain
The Effects of Human Activities
In addition to the natural geologic processes that cause coastal erosion and wetlands loss, human activities during the past century and, especially, in the past 50 years have had dramatic effects. Since
Canal and waterway construction
Oblique photograph of the Louisiana coast and wetlands showing the effects of natural processes and the impacts of human activities. (Photograph by S. Jeffress Williams.)
The origin and evolutionary history of the Louisiana delta plain region have been tied in the recent geologic past to shifts of the Mississippi River channel at 1,000-year intervals. Once the active delta is cut off from the source of river sediment, a cycle of coastal erosion and wetlands deterioration proceeds through the three-stage process shown. (Modified from Penland and others, 1988, Relative sea-level rise and delta-plain development in the Terrebonne Parish Region: Louisiana Geological Survey, Coastal Geology Technic cal Reports, no. 4, fig. 7, p. 10.)
of silt and clay from river-borne sediments is necessary in the wetlands in order to counterbalance natural compaction and subsidence that occurs if the wetlands are not nourished and replenished by these sediments.
Additional activities that cause wetlands loss are an extensive system of canals and waterways that serve as pipeline paths, access for hydrocarbon exploration and production, and waterways for boat traffic. Not only do dredging and maintaining these canals impact the wetlands, but many of them that open to the Gulf of Mexico enable saltwater to intrude brackish and freshwater wetlands, which accelerates their deterioration. Other causes that are suspected to be important, but not well documented as yet, involve subsidence that is associated with the extraction of hard minerals and fluids in the shallow subsurface. For example, sulfur mining over salt domes has resulted in localized subsidence of tens of feet in just several decades. Forced drainage, where marsh areas are diked and large pumps are used to draw down the ground water, is a widespread practice for agriculture and development that seems to contribute to soil compaction and subsidence.
ies in Louisiana that focus on coastal erosion and wetlands loss. A cooperative effort with the Louisiana Geological Survey over the past four years has demonstrated the important role that the coastal barrier islands play in providing natural protection to bays, estuaries, and wetlands from ocean waves and surge flooding and from saltwater intrusion accompanying storms. A second study, which was started in late 1988 in cooperation with the U.S. Fish and Wildlife Service and Louisiana State agencies, is the Louisiana Wetlands Loss Study. This 5-year investigation is designed to assess the regional extent of wetlands loss and to provide the information base needed to better define the critical physical processes affecting wetlands environments.
These wetlands studies are being carried out through field research by USGS scientists and through contracts and cooperative agreements with scientists at Louisiana State University, the Louisiana Geological Survey, and private consultants. To fill the information gaps, the current USGS study is focused on: • Mapping and interpreting the physical changes that have taken place along the Louisiana barrier coast and in the wetlands over the past several thousand years and particularly during the past 100 years. • Developing a comprehensive coastal data base and using these data in a network of computer-based geographic information systems available in Federal, State, and local agencies and private companies. • Comparing the sediment-deficient Terrebonne basin in the Louisiana delta plain with the sediment-rich Atchafalaya basin. These comparative investigations will focus on sediment compaction, sealevel rise, and land subsidence; effects of meteorological events such as hurricanes; dispersal of fine-grained sediments; movement of fresh- and saline water; processes of physical erosion; and conditions required for soils to develop in wetlands. • Assessing the potential effectiveness of small-scale freshwater diversions from the Mississippi River as a mitigation measure for wetlands deterioration.
All of these studies will provide significant information on coastal wetlands and will enhance information that is
Role of USGS
Because significant gaps still exist in the information available on the processes of wetlands formation and loss, continued research is needed. Various measures and recommendations have been proposed to mitigate the natural and man-made causes. Considerable controversy exists, however, over some of the measures such as marsh management, river diversions, and barrier island restoration for mitigation and wetlands restoration. Much of the debate has to do with uncertainties in predicting the long-term success of these measures, all of which require large expenditures of time and money to design, construct, and then maintain.
The USGS is conducting research to provide the basic information needed to gain an improved understanding of the geologic processes causing coastal erosion and deterioration of wetlands environments. The USGS has two ongoing stud
currently incomplete, unavailable, or uncertain. Filling these information gaps is a significant step towards the President's stated goal of no net loss of wetlands. In turn, this information, which is provided to the technical community as well as Federal, State, and local coastal zone managers, can then be used to better manage our coastal resources and protect and preserve valuable wetlands.
By Michael H. Bothner and Bradford Butman
cerning sediment distributions and pollutant transport processes in this coastal environment to aid in management and engineering decisions. This need exists in many coastal areas adjacent to major population centers where wastes are often discharged into the ocean, which is also used for recreation, fishing, and transportation.
The USGS study to address contaminant transport in Boston Harbor and Massachusetts Bay, called P-TRACE (Pollutant Transport and Accumulation in Coastal Embayments), uses the tools and disciplines of marine geology to map the distribution of sediment types, determine the present levels of chemical contaminants in the sediments, monitor water currents and sediment transport events, measure physical characteristics of bottom sediments, and estimate rates of sediment accumulation and mixing. The focus on sediments capitalizes on the fact that sediments in the aquatic environment have chemically active surfaces that adsorb a wide variety of dissolved pollutants. A quantitative understanding of sediment distributions and transport mechanisms contributes to the ability to predict the fate and effects of these pollutants. Knowing the location and areal extent of the different sediment types, bedrock outcrops, and bedforms generated by the action of ocean currents is also vital in designing an effective monitoring program. Such a monitoring program is necessary to assess the environmental impacts of ocean outfalls and is critical for selecting the locations for the instruments from which long-term current and sediment-transport measurements can be made.
A first stage in the pilot study was to conduct geophysical surveys of Boston Harbor and selected regions in Massachusetts Bay. Information from these surveys typically include sidescan-sonar, high-resolution seismic-reflection, and bathymetric data. The preliminary maps that have been generated from these data show the morphology and texture of the sea floor as well as the thickness of the sediments. Inferred areas of erosion and of sediment-pollutant accumulation have been identified that will aid significantly in the design of subsequent topical studies.
The major objective of a multidisciplinary study of Boston Harbor and Massachusetts Bay is to provide a regional basinwide perspective of sediment and contaminant transport. The Boston area was selected for a pilot study because the U.S. Environmental Protection Agency (EPA) has designated the harbor as one of the most contaminated in the United States, because a comprehensive plan to cleanup the harbor is presently being implemented, and because the geometry, topography and sediment distribution provide a variety of sedimentary environments for study. The $6 billion cleanup effort, to be completed by the year 2000, includes the elimination of ocean discharge of sewage sludge, upgrading sewage treatment from primary to secondary (allowing for partial detoxification of the sludge), and the construction of a new ocean outfall approximately 8 miles seaward of the harbor mouth.
To assist in this cleanup effort, a clear need exists for basic scientific information that the USGS can provide con