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At least 973 records · Page 54Linked to original sources
USGS2016

Viral lysis of photosynthesizing microbes as a mechanism for calcium carbonate nucleation in seawater

Removal of carbon through the precipitation and burial of calcium carbonate in marine sediments constitutes over 70% of the total carbon on Earth and is partitioned between coastal and pelagic zones. The precipitation of authigenic calcium carbonate in seawater, however, has been hotly debated because despite being in a supersaturated state, there is an absence of persistent precipitation. One of the explanations for this paradox is the geochemical conditions in seawater cannot overcome the activation energy barrier for the first step in any precipitation reaction; nucleation. Here we show that virally induced rupturing of photosynthetic cyanobacterial cells releases cytoplasmic-associated bicarbonate at concentrations ~23-fold greater than in the surrounding seawater, thereby shifting the carbonate chemistry toward the homogenous nucleation of one or more of the calcium carbonate polymorphs. Using geochemical reaction energetics, we show the saturation states (Ω) in typical seawater for calcite (Ω = 4.3), aragonite (Ω = 3.1), and vaterite (Ω = 1.2) are significantly elevated following the release and diffusion of the cytoplasmic bicarbonate (Ω calcite = 95.7; Ω aragonite = 68.5; Ω vaterite = 25.9). These increases in Ω significantly reduce the activation energy for nuclei formation thresholds for all three polymorphs, but only vaterite nucleation is energetically favored. In the post-lysis seawater, vaterite's nuclei formation activation energy is significantly reduced from 1.85 × 10 −17 J to 3.85 × 10 −20 J, which increases the nuclei formation rate from highly improbable (<<1.0 nuclei cm −3 s −1 ) to instantaneous (8.60 × 10 25 nuclei cm −3 s −1 ). The proposed model for homogenous nucleation of calcium carbonate in seawater describes a mechanism through which the initial step in the production of carbonate sediments may proceed. It also presents an additional role of photosynthesizing microbes and their viruses in marine carbon cycles and reveals these microorganisms are a collective repository for concentrated and reactive dissolved inorganic carbon (DIC) that is currently not accounted for in global carbon budgets and carbonate sediment diagenesis models.

Frontiers in Microbiology↗
USGS1978

Impact of coastal processes on resource development with an example from Icy Bay, Alaska

The coastline of Alaska is dynamic and continually readjusting to changes in the many processes that operate in the coastal zone. Because of this dynamic nature, special consideration must be made in planning for development, and. caution must be exercised in site selection for facilities to be emplaced in the coastal zone. All types of coastal processes from continuously active normal processes to the low frequency-high intensity rare event must be considered. Site-specific evaluation-s considering the broad range of possible processes must precede initiation of development. An example of the relation between coastal processes and a proposed resource treatment facility is presented for Icy Bay, Alaska. Icy Bay is the only sheltered bay near many of the offshore tracts leased for petroleum exploration in the 1976 northern Gulf of Alaska OCS (Outer Continental Shelf) lease sale. Consequently, it has been selected as a primary onshore staging site for the support of offshore exploration and development. The environment of Icy Bay has many potentially hazardous features, including a submarine moraine at the bay mouth and actively calving glaciers at the bay's head which produce many icebergs. But most significant from the point of view of locating onshore facilities and pipeline corridors are the high rates of shoreline erosion and sediment deposition. If pipelines or any onshore staging facilities are to be placed in the coastal areas of Icy Bay, then the dynamic changes in shoreline position must be considered so that man-made structures will not be eroded away or be silted in before the completion of development.

Open-File Report↗
USGS1988

Eocene to Miocene biostratigraphy of New Jersey core ACGS #4; implications for regional stratigraphy

Lower Eocene through probable upper Miocene sediments were recovered in a 945-foot core (ACGS #4) taken near Mays Landing, New Jersey. Calcareous nannofossils and planktic foraminifers are common to abundant in most of the lower part of the core. All standard calcareous nannofossil zones from lower Eocene Zone NP 12 through lower Oligocene Zone NP 21 are identified, and the midlatitude Eocene planktic foraminifer zones based on the Turborotalia cerroazulensis lineage are recognized. The Paleogene section of this core is unique for the New Jersey coastal plain because it includes a nearly continuous lower Eocene to lower Oligocene sequence of marine sediments. In contrast, previous analyses of this area indicate that a major unconformity is present between the middle Eocene and the Oligocene. A time versus depth plot controlled primarily by nannofossil zone boundaries shows that sediment accumulation rates during the early and middle Eocene were in the range of 6 to 15 feet per million years. During the late Eocene, accumulation rates were much higher, perhaps exceeding 70 feet per million years. The only clear hiatus detected in the Paleogene part of ACGS #4 on the basis of microfossils is between the early and (?)late Oligocene. However, hiatuses are suspected at the early-middle Eocene boundary and within the late Eocene. Occurrences of calcareous nannofossils and planktic foraminifers are documented, and a number of key taxa are illustrated.

New Jersey↗
USGS2018

Suspended-sediment concentrations and loads in the lower Mississippi and Atchafalaya rivers decreased by half between 1980 and 2015

The Weighted Regressions on Time, Discharge, and Season (WRTDS) model was used to derive estimates of suspended-sediment concentration (SSC) and suspended-sediment load (SSL), their dependence on discharge, and their trends with confidence intervals, for one site each on the lowermost Mississippi and Atchafalaya Rivers. The WRTDS model reduces uncertainty in SSCs related to variable streamflow conditions. Flow-normalized SSCs in each river were similar, and decreased from about 260 mg/L to 130 mg/L from 1980 through 2015; combined annual SSL in the two rivers decreased from about 200 Megatons per year (MT/y) to about 100 MT/y. Declines in SSC and SSL were more gradual from 2005 through 2015 and show signs of stabilizing. Our estimates of SSL in 2015 differ markedly from several recently published estimates of current and projected future Mississippi River SSLs, which were generally around 200 MT/y. However, these values came mostly from a different site upstream on the Mississippi River. The relationship between SSC and streamflow differed in an important way between the two rivers. SSC increased as streamflow increased for the entire range of observed streamflow in the Atchafalaya River. In the Mississippi River, SSC followed the same pattern during low and moderate streamflow but decreased at the highest streamflow that tended to occur between January and July. Since much of the water flowing in the Atchafalaya originates from the Mississippi River, the difference suggests a within-basin source of suspended sediment for the Atchafalaya River that is absent in the lower Mississippi River. These findings have important implications for the restoration of deltaic wetlands in coastal Louisiana. Accurate estimates of the SSL available in each river are crucial for understanding how effective diversions of river water into adjacent estuaries will be in sustaining these wetlands. Our study demonstrates that there might be far less sediment available than previously reported. Further, the difference in the relationship between SSC and streamflow in the two rivers is highly relevant to the ongoing discussion of coastal restoration strategies because the delta building that is occurring at the mouth of the Atchafalaya River is frequently used as a model of what could be expected with controlled diversions in the lower Mississippi River delta. The differences in the SSC behavior with changes in streamflow between the two rivers needs to be considered when results from the Atchafalaya River system are projected to those of the Mississippi River.

Louisiana↗
USGS1975

Stratigraphy of Atlantic coastal margin of United States north of Cape Hatteras: Brief survey

A synthesis of studies of sea-floor outcrops of the sedimentary wedge beneath the northeastern United States continental shelf and slope and a reassessment of coastal plain Mesozoic stratigraphy, particularly of the coastal margin, provide insight for estimating the oil and gas potential and provide geologic control for marine seismic investigations of the Atlantic continental margin. The oldest strata known to crop out on the continental slope are late Campanian in age. The Cretaceous-Tertiary contact along the slope ranges from a water depth of 0.6 to 1.5 km south of Georges Bank to 1.8 km in Hudson Canyon. Few samples are available from Tertiary and Late Cretaceous outcrops along the slope. Sediments of the Potomac Group, chiefly of Early Cretaceous age, constitute a major deltaic sequence in the emerged coastal plain. This thick sequence lies under coastal Virginia, Maryland, Delaware, southeastern New Jersey, and the adjacent continental shelf. Marine sands associated with this deltaic sequence may be present seaward under the outer continental shelf. South of the Norfolk arch, under coastal North Carolina, carbonate rocks interfinger with Lower Cretaceous clastic strata. From all available data, Mesozoic correlations in coastal wells between coastal Virginia and Long Island have been revised. The Upper-Lower Cretaceous boundary is placed at the transition between Albian and Cenomanian floras. Potential hydrocarbon source beds are present along the coast in the subsurface sediments of Cretaceous age. Potential reservoir sandstones are abundant in this sequence.

Atlantic coastal plain↗
USGS1997

Correlation of Miocene sequences and hydrogeologic units, New Jersey Coastal Plain

We have developed a Miocene sequence stratigraphic framework using data from recently drilled boreholes in the New Jersey Coastal Plain. Sequences are shallowing upward, unconformity-bounded units; fine-grained shelf and prodelta sediments grade upward to delta front and shallow-marine sands, corresponding to confining bed-aquifer couplets. By dating Miocene sequences using Sr-isotope stratigraphy, and mapping with borehole data and geophysical logs, we can predict the continuity and effectiveness of the confining beds and aquifers. The following are illustrated on a 90-km basinward dip section: (1) the composite confining bed is comprised of the KwO and lower Kw1a (ca. 23.8-20.5 Ma) sequences downdip at Atlantic City, and the Kw1b, Kw1a and older sequences updip (ca. 69.3-20.6 Ma), and is continuous throughout most of the coastal plain; (2) the major confined aquifer, the Atlantic City 800-foot sand, is comprised of the upper Kw1a and Kw1b sequences (ca. 20.5-20.2 Ma) and is an areally continuous sand that is interconnected with the Kirkwood-Cohansey aquifer system updip of Mays Landing; (3) the confining bed above the Atlantic City 800-foot sand is comprised of the Kw2a, Kw2b, and Kw3 sequences (18.1-13.3 Ma) and is an extensive confining bed that pinches out updip. These sequences and aquifer-confining bed couplets are linked to global sea-level changes evinced by the ??18O record. We conclude that sequence stratigraphy is a powerful tool when applied to regional hydrogeologic problems, although basinal tectonic differences and localized variations in sediment supply can affect aquifer thickness and permeability.

New Jersey↗
USGS2003

Rivers, runoff, and reefs

The role of terrigenous sediment in controlling the occurrence of coral reef ecosystems is qualitatively understood and has been studied at local scales, but has not been systematically evaluated on a global-to-regional scale. Current concerns about degradation of reef environments and alteration of the hydrologic and sediment cycles place the issue at a focal point of multiple environmental concerns. We use a geospatial clustering of a coastal zone database of river and local runoff identified with 0.5?? grid cells to identify areas of high potential runoff effects, and combine this with a database of reported coral reef locations. Coastal cells with high runoff values are much less likely to contain reefs than low runoff cells and GIS buffer analysis demonstrates that this inhibition extends to offshore ocean cells as well. This analysis does not uniquely define the effects of sediment, since salinity, nutrients, and contaminants are potentially confounding variables also associated with runoff. However, sediment effects are likely to be a major factor and a basis is provided for extending the study to higher resolution with more specific variables. ?? 2003 Elsevier B.V. All rights reserved.

Global and Planetary Change↗
USGS1989

Nitrogen cycling between sediment and the shallow-water column in the transition zone of the Potomac River and Estuary. II. The role of wind-driven resuspension and adsorbed ammonium

During periods of sediment resuspension, desorption of ammonium from sediment solids can be the major pathway for enriching the water column with the ammonium that is produced by bacterial degradation of organic matter in the bottom material. This hyopthesis is based on a three-year study of diffusive flux in the transition zone of the Potomac River at a site 35 m from the Virginia shore where the average water-column depth is approximately 1 m over sandy sediment. A diffusion-controlled sampler was used to collect water samples at the interface between the water column and sediment and at several tens of centimeters into the sediment. Interstitial water concentration gradients showed that diffusive flux of ammonium from the sandy shallow-water sediments was approximately 1% of the diffusive flux of ammonium from the silty channel sediments in the same zone of the Potomac River. Organic nitrogen and bound or adsorbed ammonium were the predominant nitrogen forms in the sediment. Adsorbed ammonium concentrations ranged from nondetectable to 3·7 μmol g −1 of sediment. Concentrations of adsorbed ammonium per gram of sediment were one to three orders of magnitude more than interstitial water ammonium concentrations. Desorption of ammonium from sediment solids appeared to be the controlling factor in the degree of water-column ammonium enrichment. In laboratory experiments that simulated sediment resuspension, 40–80% of the adsorbed ammonium predicted to desorb did so after approximately 30 min of mixing. Based on calculations for 1 m 2 to a depth of 4 cm, one resuspenion event lasting minutes could mix more ammonium into the water column from desorption of ammonium from sediment solids than could be delivered to the water column by diffusive flux from shallow-water sediments in 10–1000 days and would be comparable to enrichment by ammonium diffusive flux for 5–50 days from channel sediments in the same river zone.

Maryland, Virginia↗
USGS2000

Sedimentary petrology of a declining reef ecosystem, Florida reef tract (U.S.A.)

Petrologic evaluation of biogenic sediments collected shelf-wide along the Florida reef tract in 1989 revealed three principal components: coral, the calcareous green alga Halimeda, and mollusc. The dominant grain was dependent in part upon local morphology that controlled composition and vitality of the biota. Either Halimeda or mollusc grains prevailed in sands off the upper Keys. In the middle and lower Keys, Halimeda grains prevailed nearshore and coral grains offshore. Comparison with similar analyses of samples collected in 1952 and 1963 indicates that, over 37 years, the relative abundance of molluscan grains more than doubled in the upper Keys and that of particulate coral tripled in the middle Keys. These changes can be interpreted in the context of physical and biological events that affected Florida Keys reefs over that period of time. In the 1970s, outbreaks of extremely cold water killed even representatives of the hardiest coral species. In the 1980s, black-band and white-band diseases decimated the major reef-building acroporid corals, and the pivotal herbivore, Diadema antillarum, disappeared. Although Diadema is a major coral bioeroder, the sea urchin is also essential to healthy reef growth. The increase in coral debris in the middle Keys may be related to Hurricane Donna in 1960, but it is also consistent with the prediction of accelerated bioerosion by boring organisms in response to increased plankton productivity. Plankton productivity is stimulated by nutrients from Florida Bay and by well-documented eutrophication of nearshore environments. In the upper Keys, where reefs are somewhat removed from bay and nearshore influence, a relative decrease in coral debris over the 37 years may reflect proliferation of algae and algae-grazing molluscs as well as suppressed rates of bioerosion in the absence of Diadema. Human activities have substantially increased the natural flux of fixed nitrogen to coastal systems worldwide. Waters in the Florida Keys are no exception. Spatial and temporal trends in sediment constituents are compared to a previously published model that predicts the response of benthic biota to changes in nutrient supply. As adapted to interpret changes observed in reef-tract sediments, the model provides insights into the natural nutrient gradient along the Florida reef tract.

Journal of Coastal Research↗
USGSDate not supplied

Sand and gravel resources of Puerto Rico

Many of Puerto Rico's beaches are eroding, and though rates of erosion vary, it is a major concern for the tourism and residential development industries. More than 85 percent of the population lives within 7 kilometers of the coast and they are heavily dependent on tourists that are attracted by the island's beaches and coral reefs. High-quality scientific data are needed to help formulate public policy regarding residential and commercial construction along the coast, beach replenishment, and future use of marine resources. Scientists have long recognized that the causes of coastal land loss are not limited to a relative rise in sea level, but can be manmade as well. For example, sediment supply to beaches especially along the north shore of Puerto Rico has been strongly affected by upstream river channeling, dam construction, various agricultural practices, paving and urbanization, as well as shallow-water oceanographic processes. The response to coastal erosion in Puerto Rico has been mostly crisis based leading to engineered solutions that have a negative effect on the coastal environment.

Puerto Rico↗
USGS2006

usSEABED: Gulf of Mexico and Caribbean (Puerto Rico and U.S. Virgin Islands) offshore surficial sediment data release

Over the past 50 years there has been an explosion in scientific interest, research effort and information gathered on the geologic sedimentary character of the United States continental margin. Data and information from thousands of publications have greatly increased our scientific understanding of the geologic origins of the shelf surface but rarely have those data been combined and integrated. This publication is the first release of the Gulf of Mexico and Caribbean (Puerto Rico and U.S. Virgin Islands) coastal and offshore data from the usSEABED database. The report contains a compilation of published and previously unpublished sediment texture and other geologic data about the sea floor from diverse sources. usSEABED is an innovative database system developed to bring assorted data together in a unified database. The dbSEABED system is used to process the data. Examples of maps displaying attributes such as grain size and sediment color are included. This database contains information that is a scientific foundation for the USGS Marine Aggregate Resources and Processes Assessment and Benthic Habitats projects, and will be useful to the marine science community for other studies of the Gulf of Mexico and Caribbean continental margins. This publication is divided into ten sections: Home, Introduction, Content, usSEABED (data), dbSEABED (processing), Data Catalog, References, Contacts, Acknowledgments and Frequently Asked Questions. Use the navigation bar on the left to navigate to specific sections of this report. Underlined topics throughout the publication are links to more information. Links to specific and detailed information on processing and those to pages outside this report will open in a new browser window.

Caribbean Sea, Gulf of Mexico↗
USGS2005

The distribution of phosphorus in Popes Creek, VA, and in the Pocomoke River, MD: Two watersheds with different land management practices in the Chesapeake Bay Basin

This paper compares phosphorus (P) concentrations in sediments from two watersheds, one with, and one without, intensive animal agriculture. The watersheds are in the coastal plain of the Chesapeake Bay and have similar physiographic characteristics. Agriculture in the Pocomoke River, MD, watershed supplied 2.7 percent of all broiler chickens produced in the USA in 1997. Poultry litter is an abundant, local source of manure for crops. Broiler chickens are not produced in the Popes Creek, VA, watershed and poultry manure is, therefore, not a major source of fertilizer. The largest concentrations of P in sediment samples are found in floodplain and main-stem bottom sediment in both watersheds. Concentrations of total P and P extracted with 1N HCl are significantly larger in main-stem bottom sediments from the Pocomoke River than in main-stem bottom sediments from Popes Creek. Larger concentrations of P are associated with what are potentially redox sensitive iron oxyhydroxides in sediment samples from the Pocomoke River watershed than are associated with what are potentially redox sensitive iron oxyhydroxides in sediment samples from the Popes Creek watershed. Data for P and iron (Fe) concentrations in sediments from the Popes Creek watershed provide a numerical framework (baseline) with which to compare P and Fe concentrations in sediment from the Pocomoke River watershed. ?? Springer 2005.

Water, Air, & Soil Pollution↗
USGS2021

Quantifying thresholds of barrier geomorphic change in a cross-shore sediment-partitioning model

Barrier coasts, including barrier islands, beach-ridge plains, and associated landforms, can assume a broad spectrum of morphologies over multi-decadal scales that reflect conditions of sediment availability, accommodation, and relative sea-level rise. However, the quantitative thresholds of these controls on barrier-system behavior remain largely unexplored, even as modern sea-level rise and anthropogenic modification of sediment availability increasingly reshape the world's sandy coastlines. In this study, we conceptualize barrier coasts as sediment-partitioning frameworks, distributing sand delivered from the shoreface to the subaqueous and subaerial components of the coastal system. Using an idealized morphodynamic model, we explore thresholds of behavioral and morphologic change over decadal to centennial timescales, simulating barrier evolution within quasi-stratigraphic morphological cross sections. Our results indicate a wide diversity of barrier behaviors can be explained by the balance of fluxes delivered to the beach vs. the dune or backbarrier, including previously understudied forms of transgression that allow the subaerial system to continue accumulating sediment during landward migration. Most importantly, our results show that barrier state transitions between progradation, cross-shore amalgamation, aggradation, and transgression are controlled largely through balances within a narrow range of relative sea-level rise and sediment flux. This suggests that, in the face of rising sea levels, subtle changes in sediment fluxes could result in significant changes in barrier morphology. We also demonstrate that modeled barriers with reduced vertical sediment accommodation are highly sensitive to the magnitude and direction of shoreface fluxes. Therefore, natural barriers with limited sediment accommodation could allow for exploration of the future effects of sea-level rise and changing flux magnitudes over a period of years as opposed to the decades required for similar responses in sediment-rich barrier systems. Finally, because our model creates stratigraphy generated under different input parameters, we propose that it could be used in combination with stratigraphic data to hindcast the sensitivity of existing barriers and infer changes in prehistoric morphology, which we anticipate will provide a baseline to assess the reliability of forward modeling predictions.

Earth Surface Dynamics↗
USGS1988

Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California

The south-central Monterey Bay shelf is a high-energy, wave-dominated, tectonically active coastal region on the central California continental margin. A prominent feature of this shelf is a sediment lobe off the mouth of the Salinas River that has surface expression. High-resolution seismic-reflection profiles reveal that an angular unconformity (Quaternary?) underlies the entire shelf and separates undeformed strata above it from deformed strata below it. The Salinas River lobe is a convex bulge on the shelf covering an area of approximately 72 km 2 in water depths from 10 to 90 m. It reaches a maximum thickness of 35 m about 2.5 km seaward of the river mouth and thins in all directions away from this point. Adjacent shelf areas are characterized by only a thin (2 to 5 m thick) and uniform veneer of sediment. Acoustic stratigraphy of the lobe is complex and is characterized by at least three unconformity-bounded depositional sequences. Acoustically, these sequences are relatively well bedded. Acoustic foresets occur within the intermediate sequence and dip seaward at 0.7° to 2.0°. Comparison with sedimentary sequences in uplifted onshore Pleistocene marine-terrace deposits of the Monterey Bay area, which were presumably formed in a similar setting under similar processes, suggests that a general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequences are interpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposits formed during interglacial highstands and/or during early stages of falling sea level. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities that separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions.

Marine Geology↗
USGS2000

A method for measuring vertical accretion, elevation, and compaction of soft, shallow-water sediments

High-resolution measures of vertical accretion, elevation, and compaction of shallow-water sediments are fundamental to understanding the processes that control elevation change and the mechanisms of progradation (e.g., development of mudflats and intertidal wetlands) in coastal systems. Yet, measurements of elevation by traditional survey methods often are of low accuracy because of the compressible nature of the substrates. Nor do they provide measures of vertical accretion or sediment compaction. This paper evaluates the use in shallow-water systems of an approach designed to measure these variables in vegetated wetlands. The approach employs simultaneous measures of elevation from temporary benchmarks using a sedimentation-erosion table (SET) and vertical accretion from marker horizons with sediment cores collected with a cryogenic coring apparatus. The measures are made with a level of resolution sufficient to distinguish between the influence of surface and subsurface processes on elevation, thus providing quantitative estimates of shallow subsidence. The SET-marker horizon approach was evaluated on a developing splay created by an artificial crevasse of a distributary in the Mississippi River delta. The approach provided high-resolution measures of vertical accretion (48.3 ' 2.0 cm.) and elevation (36.7 ' 1.6 cm) over a 4-year period, with the difference between the two indicating the amount of shallow subsidence. In addition, by laying new marker horizons in later years, the approach provided rates not only of shallow subsidence (3.9 ' 0.5 cm y-1) but also compaction of newly deposited seiments (2.1 ' 0.6 cm y-1) and compaction of underlying sediments (1.8 ' 2.0 cm y-1 ) over a two-year period. Hence, the SET-marker horizon approach has widespread applicability in both emergent wetland and shallow water environments for providing high resolution measures of the processes controlling elevation change.

Journal of Sedimentary Research↗
USGS1980

Structural framework, stratigraphy, and petroleum geology of the area of oil and gas lease Sale No. 49 on the U.S. Atlantic continental shelf and slope

On September 23, 1977, the U.S. Department of the Interior announced the tentative selection of 136 tracts for Sale No. 49 of oil and gas leases in the Baltimore Canyon Trough on the U.S. Atlantic Continental Shelf and Slope. This report summarizes the geology and petroleum potential of the area. The Baltimore Canyon Trough is an elongate, seaward-opening sedimentary basin filled by as much as 14 km of Mesozoic and Cenozoic sedimentary rocks. The basin first formed under the New Jersey shelf and gradually spread west and south as the area subsided after the rifting that formed the Atlantic basin. Rocks of the Triassic and Jurassic Systems together are more than 8 km thick in a depocenter areally restricted to the northern part of the trough. Basal Jurassic rocks are apparently nonmarine sedimentary rocks bedded with evaporite deposits. Direct evidence that some salt is in the basal Jurassic section comes from the Houston Oil and Minerals 676-1 well, which penetrated salt at a depth of about 3.8 km. During the Middle and Late Jurassic, more open marine conditions prevailed than in the Early Jurassic, and carbonate banks and reefs formed discontinuously along the seaward side of the shelf. Sand flats likely occupied the central part of the shelf, and these probably graded shoreward into nonmarine red beds that accumulated in a bordering coastal plain. Thick nonmarine sands and silty shales of Late Jurassic age were deposited in what is now the nearshore and midshelf area. These sedimentary rocks probably grade into thick marine carbonate rocks near the present shelf edge. During the Cretaceous, less sediment accumulated (about 4 km) than during the Jurassic, and most was deposited during Early Cretaceous time. The Cretaceous units show two main trends through time-a diminishing rate of sediment accumulation and an increase in marine character of sediments. During the Middle and Late Cretaceous, calcareous sand and mud filled the basin, buried the shelf-edge reefs and later spilled across the reefs into the oceanic basin as worldwide sea level reached a maximum. Cenozoic deposits are spread over the present shelf and adjacent Coastal Plain in overlapping sheets of marine and nonmarine sediment. The maximum thickness (1.5 km) is along the outer part of the present shelf. Major tectonic deformation in the Baltimore Canyon Trough area appears to have terminated near the end of the Early Cretaceous, when at least one large mafic intrusion (Great Stone dome) was emplaced. Upper Cretaceous sedimentary rocks are arched above older uplifted fault blocks near the shelf edge; this arching may be the result of draping due to differential compaction or, perhaps, minor movement of the fault blocks during Late Cretaceous time. The dominance of terrestrial over marine-derived organic matter in sediment samples from the COST No. B-2 well indicates that economic amounts of liquid petroleum hydrocarbons were probably not generated in the area but suggests a high potential for generation of wet or dry gas. Supporting evidence for the presence of natural-gas deposits on the slope comes from AMCOR 6021, the upper 305 m of which penetrated sediments that contained methane, ethane, and propane. Texaco, Inc., has announced that its 598-1 well yielded nearly 479,000 m s of natural gas per day from two zones during early testing. Further indication of possible gas deposits comes from analyzing the amplitude (bright spots) of seismic data. Geochemical studies of the COST No. B-2 well have shown that the shelf area of the Baltimore Canyon Trough has a relatively low geothermal gradient today and that it apparently has had a gradient as low or even lower throughout the Cretaceous to Holocene. A controversy exists concerning the maturity of the basal sediments penetrated by the COST No. B-2 well. Although significant amounts of gaseous hydrocarbons may have been generated, large amounts of liquid petroleum hydrocarbons probably hav

Circular↗
USGS2010

Complexities in barrier island response to sea level rise: Insights from numerical model experiments, North Carolina Outer Banks

Using a morphological-behavior model to conduct sensitivity experiments, we investigate the sea level rise response of a complex coastal environment to changes in a variety of factors. Experiments reveal that substrate composition, followed in rank order by substrate slope, sea level rise rate, and sediment supply rate, are the most important factors in determining barrier island response to sea level rise. We find that geomorphic threshold crossing, defined as a change in state (e.g., from landward migrating to drowning) that is irreversible over decadal to millennial time scales, is most likely to occur in muddy coastal systems where the combination of substrate composition, depth-dependent limitations on shoreface response rates, and substrate erodibility may prevent sand from being liberated rapidly enough, or in sufficient quantity, to maintain a subaerial barrier. Analyses indicate that factors affecting sediment availability such as low substrate sand proportions and high sediment loss rates cause a barrier to migrate landward along a trajectory having a lower slope than average barrier island slope, thereby defining an “effective” barrier island slope. Other factors being equal, such barriers will tend to be smaller and associated with a more deeply incised shoreface, thereby requiring less migration per sea level rise increment to liberate sufficient sand to maintain subaerial exposure than larger, less incised barriers. As a result, the evolution of larger/less incised barriers is more likely to be limited by shoreface erosion rates or substrate erodibility making them more prone to disintegration related to increasing sea level rise rates than smaller/more incised barriers. Thus, the small/deeply incised North Carolina barriers are likely to persist in the near term (although their long-term fate is less certain because of the low substrate slopes that will soon be encountered). In aggregate, results point to the importance of system history (e.g., previous slopes, sediment budgets, etc.) in determining migration trajectories and therefore how a barrier island will respond to sea level rise. Although simple analytical calculations may predict barrier response in simplified coastal environments (e.g., constant slope, constant sea level rise rate, etc.), our model experiments demonstrate that morphological-behavior modeling is necessary to provide critical insights regarding changes that may occur in environments having complex geometries, especially when multiple parameters change simultaneously.

North Carolina↗
USGS2004

Belowground dynamics in mangrove ecosystems

Mangrove ecosystems are tropical/subtropical communities of primarily tree species that grow in the intertidal zone. These tidal communities are important coastal ecosystems that are valued for a variety of ecological and societal goods and services (fig. 1). Mangrove wetlands are important filters of materials moving between the land and sea, trapping sediment, nutrients, and pollutants in runoff from uplands and preventing their direct introduction into sensitive marine ecosystems such as seagrass beds and coral reefs. Mangroves serve as nursery grounds and refuge for a variety of organisms and are consequently vital to the biological productivity of coastal waters. Furthermore, because mangroves are highly resilient to disturbances such as hurricanes, they represent a self-sustaining, protective barrier for human populations living in the coastal zone. Mangrove ecosystems also contribute to shoreline stabilization through consolidation of unstable mineral sediments and peat formation. In order to help conserve mangrove ecoystems, scientists with the United States Geological Survey (USGS) at the National Wetlands Research Center are working to more fully understand the dynamics that impact these vital ecosystems.

Fact Sheet↗