Results for “Marine Geology” — Page 93

USGS2006

Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California

Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in?? 800 m of water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in 13C (to -91??? ??13C), the most 13C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used, in part, carbon derived from anaerobically oxidized methane to construct their shells. The ??13C values are highly variable, but most are within the range -12??? to -91???. This variability may be diagnostic for identifying cold-seep-hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to ???1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in 13C (??13C = -46??? to -58???), indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite. Methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane. ?? 2006 Geological Society of America.

Geology↗

USGS1973

Atlas of Mars, Lunae Palus quadrangle, MC-10

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393.4 km.

Open-File Report↗

USGS1973

Atlas of Mars, Memnonia quadrangle, MC-16

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U. S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Atlas of Mars, MC-18, Coprates Quadrangle

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey.

Open-File Report↗

USGS1973

Atlas of Mars, Sinus Sabaeus Quadrangle, MC-20

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Atlas of Mars, Iapygia Quadrangle, MC-21

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Alas of Mars chart, Mare Tyrrhenum Quadrangle, MC-22

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Atlas of Mars chart, Aeolis Quadrangle, MC-23

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Atlas of Mars chart, Thaumasia Quadrangle, MC-25

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1973

Atlas of Mars chart, Argyre Quadrangle, MC-26

This mosaic was made with Mariner 9 pictures provided by the Jet Propulsion Laboratory (JPL), California Institute of Technology. The Image Processing Laboratory (IPL) of JPL processed the digital pictures to remove image artifacts and geometric distortions, to enhance high-frequency topographic detail while subduing low-frequency albedo variation, and to transform the pictures to a Mercator projection. The U.S. Geological Survey performed further artifact removal and modified contrast and tone of each picture analytically on the digital magnetic tapes provided by IPL for uniformity of tone and contrast within the mosaic. Photographic reproductions of the pictures were generated from the magnetic tapes and mosaicked by the U.S. Geological Survey. Primary horizontal control points, where available, controlled the placement of pictures. Where these points were not available, picture placement was controlled by spacecraft tracking data and by matching images in overlap zones between pictures. Discrepancies in matching these images are less than 10 km over 90% of the mosaic. The number designations and positions of the primary control points, shown by broken crosses (-l-), are those given by Davies and Arthur (1973). Aerographic latitudes are shown on this mosaic. The projection is based on a theoretical dynamical ellipticity of 1/192. The scale is based on an equatorial radius of 3393. 4 km.

Open-File Report↗

USGS1995

Data report for onshore-offshore wide-angle seismic recordings in the Bering-Chukchi Sea, Western Alaska and eastern Siberia

This report presents fourteen deep-crustal wide-angle seismic reflection and refraction profiles recorded onland in western Alaska and eastern Siberia from marine air gun sources in the Bering-Chukchi Seas. During a 20-day period in August, 1994, the R/V Ewing acquired two long (a total of 3754 km) deep-crustal seismic-reflection profiles on the continental shelf of the Bering and Chukchi Seas, in a collaborative project between Stanford University and the United States Geological Survey (USGS). The Ewing's 137.7 liter (8355 cu. in.) air gun array was the source for both the multichannel reflection and the wide-angle seismic data. The Ewing, operated by the Lamont-Doherty Earth Observatory, steamed northward from Nunivak Island to Barrow, and returned, firing the air gun array at intervals of either 50 m or 75 m. About 37,700 air gun shots were fired along the northward directed Lines 1 and 2, and more than 40,000 air gun shots were fired along the southward directed Line 3. The USGS and the University of Alaska, Fairbanks (UAF), deployed an array of twelve 3-component REFTEK and PDAS recorders in western Alaska and eastern Siberia which continuously recorded the air gun signals fired during the northward bound Lines 1 and 2. Seven of these recorders also continuously recorded the southward bound Line 3. These wide-angle seismic data were acquired to: (1) image reflectors in the upper to lower crust, (2) determine crustal and upper mantle refraction velocities, and (3) provide important constraints on the geometry of the Moho along the seismic lines. In this report, we describe the land recording of wide-angle data conducted by the USGS and the UAF, describe in detail how the wide-angle REFTEK and PDAS data were reduced to common receiver gather seismic sections, and illustrate the wide-angle seismic data obtained by the REFTEKs and PDAS's. Air gun signals were observed to ranges in excess of 400 km, and crustal and upper /mantle refractions indicate substantial variation in the crustal thickness along the transect.

Alaska↗

USGS2019

Geology and assessment of undiscovered oil and gas resources of the Eurasia Basin Province, 2008

The Eurasia Basin Petroleum Province comprises the younger, eastern half of the Arctic Ocean, including the Eurasia Basin and the outboard part of the continental margin of northern Europe in the Barents and Kara Seas. The province includes the slope and rise sedimentary prism of the Lena Delta, the north-facing outer shelf, slope, and rise of the European passive margin, and sedimentary accumulations in the deep Nansen and Amundsen Basins. The entire province lies north of the Arctic Circle beneath the polar ice cap and includes the North Pole. The province is divided into four assessment units (AUs). The Lena Prodelta AU in the eastern part of the province consists of the deep-marine part of the Lena Delta, which has been deposited across the Gakkel Ridge, an ultraslow spreading ridge. The Nansen Basin Margin AU in the southern part of the province comprises the Cenozoic passive margin sequence of the rift margin of the western (European) part of the Eurasian plate. This AU spans the continent-ocean boundary and includes prerift strata along the outer continental margin. The Nansen Basin and Amundsen Basin AUs encompass the deep abyssal plains of the Eurasia Basin. Lying south of the Gakkel Ridge spreading center, the sedimentary fill of Nansen Basin AU consists of the distal clinoform deposits shed from the western Eurasia passive margin. North of the Gakkel Ridge, Amundsen Basin AU consists of a nascent distal passive-margin sequence derived from the adjacent Lomonosov Ridge and overlying flat-lying deep marine strata shed from distant source areas in Siberia and Greenland after the Lomonosov Ridge subsided below sea level at about 50 Ma. The primary petroleum system thought to be present is sourced in ~50–44 Ma (early to middle Eocene) condensed pelagic deposits that could be widespread in the province. Mean estimates of undiscovered, technically recoverable petroleum resources include <1 billion barrels of oil (BBO) and about 1.4 trillion cubic feet (TCF) of nonassociated gas in Lena Prodelta AU, and < 0.4 BBO and 3.4 TCF nonassociated gas in the Nansen Basin Margin AU. Quantities of natural gas liquids and associated natural gas also are assessed in each of these AUs. The Nansen Basin and Amundsen Basin AUs were not quantitatively assessed because they were judged to have less than 10 percent probability of containing at least one accumulation of 50 MMBOE (million barrels of oil equivalent), the minimum probability required for evaluation in the U.S. Geological Survey Circum-Arctic Resource Appraisal.

Professional Paper↗

USGS2012

Osteosarcoma of the maxilla with concurrent osteoma in a southern sea otter (Enhydra lutris nereis)

Southern sea otters ( Enhydra lutris nereis ) are threatened marine mammals that belong to the family Mustelidae and are native to the coast of Central California. Neoplasia is reported infrequently in sea otters. An adult female free-ranging southern sea otter was found alive at Pebble Beach, Monterey County, California, on January 1st, 1994 and died soon after capture. The carcass was submitted to the US Geological Survey – National Wildlife Health Center for necropsy examination. Grossly, a mass with rubbery texture was firmly attached to the left maxillary region of the skull and the nasopharynx was occluded by soft neoplastic tissue. Post-mortem skull radiographs showed an oval, smoothly marginated mineralized opaque mass centered on the left maxilla, extending from the canine tooth to caudal to the molar and replacing portions of the zygomatic arch and palatine and temporal bones. The majority of the mass protruded laterally from the maxilla and was characterized by central homogeneous mineral opacity. Microscopically, the mass was characterized by fully differentiated lamellar non-osteonal bone that expanded beyond the margins of the adjacent normal osteonal bone. Sections of the nasopharyngeal mass were comprised of moderately pleomorphic cells with bony stroma. Gross, microscopical and radiological findings were compatible with maxillary osteosarcoma with concurrent osteoma.

California↗

USGS2001

Coastal-zone hazard maps and recommendations: Eastern Puerto Rico

A series of coastal zone hazard maps cover the area impacted by Hurricane Hugo (1989) in eastern Puerto Rico. The mapping strategy was to develop a tool for quick visualization of multiple hazards for use by coastal planners, managers, property owners, and potential property owners. The Puerto Rico shoreline is heavily developed in places and also highly compartmentalized in terms of shoreline types, geology, and adjacent shelf conditions. Hazards such as coastal erosion, storm surge, riverine flooding, landsliding, and seismic impact also may be compartmentalized. From a management perspective, resources therefore can be allocated on a compartment-by-compartment basis. Six types of hazards were considered in this investigation: (1) shoreline-setting hazards (long-term coastal problems), (2) marine hazards (short-term impacts of coastal storms), (3) earthquake and slope hazards (ground shaking, landslides, and liquefaction), (4) riverine hazards (historical floods), (5) development hazards (high-density development at risk or lowdensity development in extreme-hazard settings), and (6) engineering hazards (special cases in which shoreline engineering projects such as breakwaters or sand mining have significant detrimental effects on portions of the shoreline). Shoreline segments were ranked as being at extreme, high, moderate, or low risk, depending on the number of hazards present within that segment. These rankings are likely to change, gradually over decades with natural coastal evolution, more rapidly as human development infringes on the coastal zone, or in an instant during a severe storm. The hazard maps provide a basis for hazard mitigation and management recommendations.

Puerto Rico↗

USGS2003

Impact damage to dinocysts from the Late Eocene Chesapeake Bay event

The Chesapeake Bay impact structure, formed by a comet or meteorite that struck the Virginia continental shelf about 35.5 million years ago, is the focus of an extensive coring project by the U.S. Geological Survey and its cooperators. Organic-walled dinocysts recovered from impact-generated deposits in a deep core inside the 85-90 km-wide crater include welded organic clumps and fused, partially melted and bubbled dinocysts unlike any previously observed. Other observed damage to dinocysts consists of breakage, pitting, and folding in various combinations. The entire marine Cretaceous, Paleocene, and Eocene section that was once present at the site has been excavated and redeposited under extreme conditions that include shock, heat, collapse, tsunamis, and airfall. The preserved dinocysts reflect these conditions and, as products of a known impact, may serve as guides for recognizing impact-related deposits elsewhere. Features that are not unique to impacts, such as breakage and folding, may offer new insights into crater-history studies in general, and to the history of the Chesapeake Bay impact structure in particular. Impact-damaged dinocysts also are found sporadically in post-impact deposits and add to the story of continuing erosion and faulting of crater material.

Chesapeake Bay↗

USGS2022

Continuous water-quality and suspended-sediment transport monitoring in San Francisco Bay, California, water years 2020–21

The U.S. Geological Survey (USGS) has continuously monitored real-time water quality and suspended-sediment transport in San Francisco Bay (the Bay) since 1989 as part of a multi-agency effort (see “Acknowledgments” section) to address estuary management, water supply, and ecological concerns. The San Francisco Bay area is home to millions of people and biologically diverse marine and terrestrial flora and fauna. Freshwater mixes with saltwater in the Bay and is subject to riverine influences (floods, droughts, managed reservoir releases, and freshwater diversions) and marine influences (tides, waves, and effects of saltwater). Water temperature, salinity, suspended-sediment concentration (SSC), and turbidity, are used by State and Federal resources managers and are monitored at eight key locations throughout the Bay (fig. 1). Water temperature and salinity affect the density of water, which controls gravity-driven circulation patterns and stratification in the water column. Salinity indicates the relative mixing of fresh and ocean waters in the Bay and is derived from specific conductance measurements. Turbidity is a measure of light scattered from suspended particles in the water that is used to estimate suspended-sediment concentration. Suspended-sediment concentrations also are directly measured through depth-integrated water sampling. Suspended sediment affects Bay water quality in multiple ways. Suspended sediment affects phytoplankton growth by attenuating sunlight in the water column. Suspended sediment deposition on tidal marshes and intertidal mudflats helps to restore and sustain these habitats as sea level rises. Settling of suspended sediment in ports and shipping channels creates the need for more dredging. In addition, suspended sediment often carries adsorbed contaminants as it is transported in the water column, which affects the distributions and concentrations of adsorbed contaminants in the environment. Excessive concentrations of sediment-adsorbed contaminants in deposits on the bottom of the Bay can affect ecosystem health. External factors, such as tidal currents, waves, and wind can also affect water quality in the Bay. Tidal currents in the Bay change direction four times daily, and wind direction and intensity typically fluctuate on a daily cycle. Consequently, salinity, water temperature, and suspended-sediment concentration differ spatially and temporally throughout the Bay. Therefore, high-frequency measurements at multiple locations are needed to monitor these changes. Data collected at eight stations throughout the Bay are transmitted in near real-time using cellular telemetry and posted to the USGS National Water Information System (NWIS; https://waterdata.usgs.gov/usa/nwis ). The purposes of this fact sheet are to (1) provide information about the USGS San Francisco Bay water-quality monitoring network; (2) highlight various applications in which these data can be used; and (3) provide internet links to access the resulting continuous water-quality data collected by the USGS.

California↗

USGS2007

The Distribution of Submersed Aquatic Vegetation in the Fresh and Oligohaline Tidal Potomac River, 2004

Introduction Submersed aquatic vegetation (SAV) is a critical component of the Potomac River ecosystem. Though SAV provides important habitat for fauna and stabilizes bottom sediment, very dense beds may restrict recreational and commercial navigation. Exotic species of SAV are managed by the Metropolitan Washington Council of Governments Potomac Aquatic Plant Management Program (PAPMP). Selected beds of exotic SAV species that limit navigation are harvested mechanically. The program began in 1986 when approximately 40 acres of plants were harvested from 18 sites (Metropolitan Washington Council of Governments 1987). Monitoring efforts are an effective means of quantifying the distribution and abundance of the exotic species, Hydrilla verticillata (hydrilla) and other SAV species. These annual surveys provide a basis for identifying large-scale changes throughout the ecosystem and allow managers to evaluate the effectiveness of resource management policies based on a reliable scientific foundation. The U.S. Geological Survey (USGS) has monitored the distribution and composition of SAV beds in the fresh and oligohaline (salinity 0.5 to 5) tidal Potomac River since 1978 using transect sampling (1978 to 1981, 1985 to 1987, and 2002) and shoreline surveys (1983 to 2004). Shoreline survey data from the tidal Potomac River are incorporated into the Virginia Institute of Marine Science (VIMS) annual report on SAV distribution in Chesapeake Bay. The VIMS report and methods are available at http://www.vims.edu/bio/sav. Additional publications concerning SAV distribution in the Potomac River can be found at http://water.usgs.gov/nrp/proj.bib/sav/wethome.htm.

Open-File Report↗

USGS2008

Assessment of 1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene (DDE) transformation rates on the Palos Verdes Shelf, CA

In 1953, the world's largest producer of DDT, Montrose Chemical Corporation, began to discharge process wastes into sewers of the Los Angeles County Sanitation Districts (LACSD), California. By 1971, when the sewer connection was terminated, approximately 1,500-2,000 metric tons of DDT had been introduced to the LACSD treatment plant in Carson, CA. After treatment, effluent from this plant was released to the ocean through a submarine outfall system on the Palos Verdes Shelf (PVS) near Los Angeles, resulting in the accumulation of highly contaminated marine sediments. Numerous investigations of the PVS have been undertaken since the late 1960s, but few have focused on the biogeochemical fate of DDT and its transformation products. In the early 1990s, it was shown that DDE, the major DDT compound in the sediments, was being reductively dechlorinated by microorganisms resident in sediments on the PVS. The U.S. Geological Survey undertook a study in cooperation with the U.S. Environmental Protection Agency to provide a better understanding of the range of reductive dechlorination rates on the PVS and the environmental factors that control them. Existing data show that rates of reductive dechlorination are variable spatially. A comparison of data from two cores collected approximately 7 kilometers downcurrent from the outfall systems in 1992 and 2003 yielded an average first-order transformation rate of approximately 0.05 yr-1. A multistep reaction model suggests that inventories of DDE in PVS sediments at the study site will continue to decline, whereas the inventory of the metabolite DDNU will reach a maximum around 2014.

California↗