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At least 541 records · Page 30Linked to original sources
USGS2013

Marsh collapse thresholds for coastal Louisiana estimated using elevation and vegetation index data

Forecasting marsh collapse in coastal Louisiana as a result of changes in sea-level rise, subsidence, and accretion deficits necessitates an understanding of thresholds beyond which inundation stress impedes marsh survival. The variability in thresholds at which different marsh types cease to occur (i.e., marsh collapse) is not well understood. We utilized remotely sensed imagery, field data, and elevation data to help gain insight into the relationships between vegetation health and inundation. A Normalized Difference Vegetation Index (NDVI) dataset was calculated using remotely sensed data at peak biomass (August) and used as a proxy for vegetation health and productivity. Statistics were calculated for NDVI values by marsh type for intermediate, brackish, and saline marsh in coastal Louisiana. Marsh-type specific NDVI values of 1.5 and 2 standard deviations below the mean were used as upper and lower limits to identify conditions indicative of collapse. As marshes seldom occur beyond these values, they are believed to represent a range within which marsh collapse is likely to occur. Inundation depth was selected as the primary candidate for evaluation of marsh collapse thresholds. Elevation relative to mean water level (MWL) was calculated by subtracting MWL from an elevation dataset compiled from multiple data types including light detection and ranging (lidar) and bathymetry. A polynomial cubic regression was used to examine a random subset of pixels to determine the relationship between elevation (relative to MWL) and NDVI. The marsh collapse uncertainty range values were found by locating the intercept of the regression line with the 1.5 and 2 standard deviations below the mean NDVI value for each marsh type. Results indicate marsh collapse uncertainty ranges of 30.7–35.8 cm below MWL for intermediate marsh, 20–25.6 cm below MWL for brackish marsh, and 16.9–23.5 cm below MWL for saline marsh. These values are thought to represent the ranges of inundation depths within which marsh collapse is probable.

Louisiana↗
USGS1999

Diagenesis of lower Cretaceous pelagic carbonates, North Atlantic: Paleoceanographic signals obscured

The stable isotope and minor element geochemistry of Neocomian (Lower Cretaceous) pelagic carbonates of the North Atlantic Basin (Deep Sea Drilling Project Sites 105, 367, 387, 391, and 603) was examined to develop a diagenetic model for pelagic limestones. In particular, we hoped to test the fidelity of whole-rock geochemical records as paleoceanographic indicators for pelagic deposits of pre-Aptian age, in which individual microfossils are not available for analysis. Data indicate that in addition to depth of burial, rhythmic variations in primary carbonate content have strongly controlled diagenetic patterns and associated geochemical signatures in these Neocomian sequences. Samples become increasingly depleted in Sr and 18O with increasing CaCO3 content. Within individual sedimentary sections, substantial decreases in Sr/Ca ratios and ??18O values are evident over a range of 4 to 98% CaCO3. However, even over a relatively narrow range of 50 to 98% CaCO3 a 2.5%c variation in ??18O values and a change of a factor of 1.7 in Sr/Ca ratios are observed. Carbon isotope compositions do not vary as extensively with CaCO3 content, but carbonate-rich intervals tend to be relatively depleted in 13C. Petrographic analysis reveals that these geochemical patterns are related to the transfer of CaCO3 from carbonate-poor intervals (calcareous shales and marlstones) to adjacent carbonate-rich intervals (limestones) during burial compaction and pressure solution. This process results in the addition of diagenetic cement to carbonate-rich intervals to produce a bulk composition that is relatively depleted in Sr and 18O and, at the same time, enables the retention of more-or-less primary carbonate that is relatively enriched in Sr and 18O in adjacent carbonate-poor intervals. Thus, although cyclic variations in CaCO3 content are primary in the Neocomian sequences examined, measured variations in Sr/Ca ratios and ??18O values are not and, as such, do not provide reliable proxies for past variations in climate, oceanographic conditions, or global ice volume.

Journal of Foraminiferal Research↗
USGS2003

Reduction in sea lamprey hatching success due to release of sterilized males

Male sea lampreys ( Petromyzon marinus ), sterilized by injection with bisazir, were released in Lake Superior tributaries from 1991 to 1996 and exclusively in the St. Marys River (the outflow from Lake Superior to Lake Huron) since 1997 as an alternative to chemical control. To determine effectiveness in reducing reproductive potential through the time of hatch, males were observed on nests and egg viability was determined in nests in selected Lake Superior tributaries and the St. Marys River. The proportions of sterilized males observed on nests were not significantly different than their estimated proportion in the population for all streams and years combined or for the St. Marys River alone. It was concluded that sterilized males survive, appear on the spawning grounds, and nest at near their estimated proportion in the population. There was a significant reduction in egg viability corresponding with release of sterilized males for all streams and years combined or for the St. Marys River alone. In the St. Marys River from 1993 to 2000, the percent reduction in egg viability was significantly correlated with the observed proportion of sterile males on nests. It was further concluded that sterilized males remain sterile through nesting and attract and mate with females. Reduction in reproductive potential in the St. Marys River due to both removal of females by traps and sterile-male-release ranged from 34 to 92% from 1993 to 2001 and averaged 64%. From 1999 to 2001, when the program stabilized, reductions ranged from 71 to 92% and averaged 81%. The current release of sterile males in the St. Marys River effectively reduced reproductive potential through the time of hatch and did so near theoretical levels based on numbers released, estimates of population size, and the assumptions of full sterility and competitiveness.

Journal of Great Lakes Research↗
USGS2007

Deep-water chaunacid and lophiid anglerfishes (Pisces: Lophiiformes) off the Southeastern United States

Recent research cruises to deep (80–910 m) reef habitats off the south-eastern U.S. and in the northern Gulf of Mexico have provided new information on the diagnostic characteristics, behaviours, colour patterns in life, bottom associations, distributions and maximum sizes of species of the anglerfish genera Chaunax , Lophiodes and Sladenia . Chaunax stigmaeus occurred much further south than previously known (Blake Plateau off South Carolina), and all C. stigmaeus observed were found associated with dense beds of dead coral ( Lophelia pertusa ) rubble or on broken hard bottom. In contrast, Chaunax suttkusi was found on soft bottoms. Chaunax stigmaeus and C. suttkusi appear to be sympatric over a major portion of their ranges. Because knowledge of pigmentation in live or freshly caught Chaunax is critical to distinguish some members of the genus, changes in the colouration of C. suttkusi were noted and documented photographically immediately after death and after fixation. The yellow spots found on some, but not all specimens, temporarily disappeared completely after death, but they reappeared after fixation, slowly disappearing thereafter along with other carotenoid pigments. Lophiodes beroe and Lophiodes monodi were collected for the first time off the Atlantic coast of the U.S., being previously known only from the Gulf of Mexico, Caribbean Sea and the northern coast of South America. For both species ( L. beroe and L. monodi ), the collections included the two largest known representatives of the species (400 and 325 mm standard length, respectively). Lophiodes beroe commonly occurred on L. pertusa rubble, and seemed to prefer this habitat. Occupying such a habitat that is deep and difficult to sample probably explains how this common species escaped detection. Only a single L. monodi was collected or observed, so this species appears to be uncommon in this geographic area or at least so on coral rubble habitat. Detailed aspects of the colour patterns of both species were noted. In particular, L. beroe displayed a characteristic pattern of white patches in life that were not apparent after death. The first photographic documentation of the colour pattern in life and of the pharyngeal pigmentation of Lophiodes reticulatus is provided. The third known specimen of Sladenia shaefersi , and the first to be taken in U.S. waters was collected from coral rubble near the base of a steep 200 m scarp on the Blake Plateau.

Gulf of Mexico↗
USGS2025

Multiscale framework for assessing land cover change on barrier islands from extreme storms and restoration

Often found along the estuarine-marine interface, barrier islands and mainland coastal zones are shaped by tides, currents, extreme storms, and relative sea-level rise. These systems provide ecosystem services such as storm surge and wave attenuation, erosion protection to inland areas, habitat for fish and wildlife, recreation, and tourism. Given the importance of these ecosystems coupled with their dynamic nature, information on how these coastal systems are changing can help to inform natural resource management. Remote sensing advancements have led to an abundance of data for monitoring change in coastal settings. This study developed a multiscale framework that can provide trajectory information from screening-level analyses by using existing or custom moderate spatial resolution land cover maps. Using the north-central Gulf Coast as a case study, the trajectory of land cover area for barrier islands and mainland coastal zones was assessed using several geospatial data sets, including: (1) long-term moderate-resolution remote sensing products with an annual (or more frequent) temporal frequency; (2) a restoration database ( e.g. , beach/dune restoration, sediment placement, and dune enhancement); and (3) a tropical storm database. Due to the coarser spatial resolution of data sets used for screening-level analyses, detailed or application-specific analyses are often needed to reduce uncertainty in smaller changes that may not be captured. These may include land cover change analyses ( i.e. this study), periodic land cover maps with higher spatial resolution and more detailed land cover classes, or elevation-related analyses ( e.g. , dune change or inundation change). Using this framework, abrupt changes in land cover on Dauphin Island, Alabama, resulting from extreme storms were detected using moderate spatial resolution screening-level data, while restoration impact analyses may require higher resolution data. Further, land cover change analyses that incorporate change allocation provide robust information for understanding land cover change in dynamic coastal settings.

Alabama, Florida↗
USGS2012

High paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the Mid- Atlantic Ridge

Nineteen pillow basalts dredged within the rift valley of the Mid-Atlantic Ridge at36.8°N were studied by the Thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor. Previously reported fission track ages are 2,000 to 6,000 years for the youngest rocks (mainly olivine basalts) and 10,000 to 100,000 years for the others (mainly plagioclase basalts and pyroxene basalts). All but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates: stability of the direction of NRM during its thermal demagnetization, constant ratio of NRM/TRM (natural remanent magnetization to thermoremanent magnetization) over 50% or more of the original NRM intensity (80 to 94% for 11 specimens), and reproducibility of low-temperature partial TRM (PTRM). However, strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in Curie temperature (15 to 80°C) during the paleointensity experiments below 250°C, notwithstanding the linearity of the NRM-TRM plot in this temperature interval. This alteration, probably due to low-temperature oxidation of the specimens, seems typical of young pillow basalts and may result in paleointensity estimates which are too high. This result shows that excellence of remanence tests (NRM-TRM linearity and PTRM stability) does not ensure the absence of chemical changes during the Thellier experiments and therefore the validity of the paleointensity obtained. The assumption that reliable paleointensities are obtained when the Curie point increase is 10°C or less led to the selection of five of our specimens, all from the youngest group. Their mean paleointensity is 64.2±20.5 μT (standard deviation) and the corresponding virtual axial dipole moment (VADM) is 11.5±3.7× 10 22 A m 2 . Given the variations of the VADM of the Earth's magnetic field over the last 6,000 years, as established from archeomagnetic studies, our paleointensity results suggest that the latest eruptions on the inner floor of the Rift Valley at 36.8°N occurred 1,500±1,000 years ago.

Journal of Geophysical Research Solid Earth↗
USGS1972

Factors of ecologic succession in oligotrophic fish communities of the Laurentian Great Lakes

Oligotrophic fish communities of the Great Lakes have undergone successive disruptions since the mid-1800s. Major contributing factors have been intensive selective fisheries, extreme modification of the drainage, invasion of marine species, and progressive physical–chemical changes of the lake environments. Lake Ontario was the first to be affected as its basin was settled and industrialized earliest, and it was the first to be connected by canals to the mid-Atlantic where the alewife ( Alosa pseudoharengus ) and sea lamprey ( Petromyzon marinus ) which ultimately became established in the Great Lakes were abundant. Oligotrophic fish communities were successively disrupted in Lakes Erie, Huron, Michigan, and Superior as the affects of population growth, industrialization, and marine invaders spread upward in the Laurentian drainage.The degree and sequence of response of families offish and species within families differed for each factor, but the sequence of change among families and species has been the same in response to each factor as it affected various lakes at different times. The ultimate result of the disruption of fish communities has been a reduction of productivity of oligotrophic species that ranges from extreme in Lake Ontario to moderate in Lake Superior, and which has reached a state of instability and rapid change in the upper three Great Lakes by the rnid-1900s similar to the situation in Lake Ontario in the mid-1800s. Since oligotrophic species (primarily salmonines, coregonines, and deepwater cottids) are the only kinds of fish that fully occupied the entire volume of the deepwater Great Lakes (Ontario, Huron, Michigan, and Superior), the fish biomass of these lakes has been reduced as various species declined or disappeared. In Lake Erie, which is shallow, and in the shallow bays of the deep lakes, oligotrophic species were replaced by mesotrophic species, primarily percids, which have successively increased and declined. All oligotrophic species are greatly reduced or extinct in lakes Ontario and Erie, and are in various stages of decline in lakes Huron, Michigan, and Superior, from greatest to least, respectively. The percids appear to be near the end of their sequence of succession in lakes Erie, Ontario, and Huron (primarily Saginaw Bay) where only the yellow perch ( Perca flavescens ) remains abundant. The yellow perch appears to be on the brink of decline in Lake Erie, which has been more severely influenced by water quality change than the other lakes.

Journal of the Fisheries Research Board of Canada↗
USGS2000

Bottom currents and sediment transport in Long Island Sound: A modeling study

A high resolution (300-400 m grid spacing), process oriented modeling study was undertaken to elucidate the physical processes affecting the characteristics and distribution of sea-floor sedimentary environments in Long Island Sound. Simulations using idealized forcing and high-resolution bathymetry were performed using a three-dimensional circulation model ECOM (Blumberg and Mellor, 1987) and a stationary shallow water wave model HISWA (Holthuijsen et al., 1989). The relative contributions of tide-, density-, wind- and wave-driven bottom currents are assessed and related to observed characteristics of the sea-floor environments, and simple bedload sediment transport simulations are performed. The fine grid spacing allows features with scales of several kilometers to be resolved. The simulations clearly show physical processes that affect the observed sea-floor characteristics at both regional and local scales. Simulations of near-bottom tidal currents reveal a strong gradient in the funnel-shaped eastern part of the Sound, which parallels an observed gradient in sedimentary environments from erosion or nondeposition, through bedload transport and sediment sorting, to fine-grained deposition. A simulation of estuarine flow driven by the along-axis gradient in salinity shows generally westward bottom currents of 2-4 cm/s that are locally enhanced to 6-8 cm/s along the axial depression of the Sound. Bottom wind-driven currents flow downwind along the shallow margins of the basin, but flow against the wind in the deeper regions. These bottom flows (in opposition to the wind) are strongest in the axial depression and add to the estuarine flow when winds are from the west. The combination of enhanced bottom currents due to both estuarine circulation and the prevailing westerly winds provide an explanation for the relatively coarse sediments found along parts of the axial depression. Climatological simulations of wave-driven bottom currents show that frequent high-energy events occur along the shallow margins of the Sound, explaining the occurrence of relatively coarse sediments in these regions. Bedload sediment transport calculations show that the estuarine circulation coupled with the oscillatory tidal currents result in a net westward transport of sand in much of the eastern Sound. Local departures from this regional westward trend occur around topographic and shoreline irregularities, and there is strong predicted convergence of bedload transport over most of the large, linear sand ridges in the eastern Sound, providing a mechanism which prevents their decay. The strong correlation between the near-bottom current intensity based on the model results and the sediment response, as indicated by the distribution of sedimentary environments, provides a framework for predicting the long-term effects of anthropogenic activities.

New York↗
USGS2013

Diablotin Pterodroma hasitata : a biography of the endangered Black-capped Petrel

The Black-capped Petrel Pterodroma hasitata was believed extinct throughout much of the 20th century. It is the only gadfly petrel currently known to breed in the Caribbean Basin. Now seriously endangered, the species is presumed extirpated from Martinique, Dominica, and Guadeloupe, and breeding populations currently occur only on Hispaniola and perhaps Cuba. A related form (now considered a full species) once bred, but is now apparently extinct, on Jamaica. The Black-capped Petrel breeding population may number as few as 500 breeding pairs. Remaining populations suffer from multiple threats to terrestrial and pelagic habitats, including harvest by humans and predation by introduced mammals. The exact sizes, locations, and detailed chronologies of all Black-capped Petrel breeding sites remain poorly studied, although major colonies are today apparently restricted to steep sea and inland cliffs along the La Selle Ridge in Hispaniola. The largest known breeding population occurs in Haiti, although there is continued discussion about a possible breeding site in Cuba in the Sierra Maestra mountain range. Accounts from Cuba are based on the unverified assumption that birds observed at sea just offshore of that island are breeding locally. All evidence at present indicates that waters in or adjacent to the Florida Current and the Gulf Stream between north Florida and southern Virginia provide the primary foraging range of Black-capped Petrels. A small foraging area just off of southeast Cuba has also been reported, but the extent and seasonal use of this area are unknown. Concentrations of birds can be found along the Gulf Stream in southeastern US waters throughout the year, but are particularly common in May, August, and late December through early January. Concentrations of adult birds during winter, when peak breeding activity is underway, suggest that breeding birds forage along the Gulf Stream while commuting to and from breeding colonies. Such long-distance foraging is certainly possible for Pterodroma species. Potential threats to Black-capped Petrels include introduced predators, human encroachment on breeding and foraging habitats, and offshore oil, gas, and wind energy development. Increased mercury levels associated with petroleum production also pose a potential threat, as the Black-capped Petrel seems to be highly susceptible to mercury bioaccumulation compared with other pelagic species. In addition, fires and other bright light sources are known to attract Black-capped Petrels, making collisions with wires and other structures on lighted ships and platforms a potential concern. Haitian social-economic instability and increasing habitat loss suggest the likelihood of further population declines and increasing vulnerability of the species to extinction. Our findings are in accord with the recent decision by the US Fish and Wildlife Service (USFWS) to evaluate the need for additional protection of the species and the primary foraging habitat off the southeastern United States under the Endangered Species Act (USFWS 2012). Additional conservation measures and research strategies that warrant further consideration include (1) protection, monitoring, and management of known breeding populations and nesting habitat in the Dominican Republic and Haiti through controlling predators, installing artificial nest burrows in appropriate sites and hiring local wardens at breeding sites during the nesting season; (2) local and regional training, education and public awareness (e.g. Blanchard & Nettleship 1992); (3) restoration of the original common name Diablotin to common usage to promote the historical and cultural importance of this species; (4) studies to determine the distribution and genetic variability in the remaining populations; and (5) studies of satellite-tagged birds to assess their seasonal and geographic use of pelagic habitats.

Marine Ornithology: Journal of Seabird Research an↗
USGS1995

Progress toward lake trout restoration in Lake Michigan

Progress toward lake trout restoration in Lake Michigan is described through 1993. Extinction of the native lake trout fishery by sea lamprey predation, augmented by exploitation and habitat destruction, resulted in an extensive stocking program of hatchery-reared lake trout that began in 1965. Sea lamprey abundance was effectively controlled using selective chemical toxicants. The initial stocking produced a measurable wild year class of lake trout by 1976 in Grand Traverse Bay, but failed to continue probably due to excessive exploitation. The overall lack of successful reproduction lakewide by the late 1970s led to the development and implementation in 1985 of a focused inter-agency lakewide restoration plan by a technical committee created through the Lake Committee structure of the Great Lakes Fishery Commission. Strategies implemented in 1985 by the plan included setting a 40% total mortality goal lakewide, creating two large refuges designed to encompass historically the most productive spawning habitat and protect trout stocked over their home range, evaluating several lake trout strains, and setting stocking priorities throughout the lake. Target levels for stocking in the 1985 Plan have never been reached, and are much less than the estimated lakewide recruitment of yearlings by the native lake trout stocks. Since 1985, over 90% of the available lake trout have been stocked over the best spawning habitat, and colonization of the historically productive offshore reefs has occurred. Concentrations of spawning lake trout large enough for successful reproduction, based on observations of successful hatchery and wild stocks, have developed at specific reefs. Continued lack of recruitment at these specific sites suggests that something other than stotk abundance has limited success. Poor survival of lake trout eggs, assumed to be related to contaminant burden, occurred in the late 1970s and early 1980s, but survival has since increased to equal survival in the hatchery. A recent increase in lamprey wounding rates in northern Lake Michigan appears to be related to the uncontrolled build-up of lampreys in the St. Marys River a tributary of Lake Huron. If left uncontrolled, further progress toward restoration in the Northern Refuge may be limited.

Journal of Great Lakes Research↗
USGS1999

Analysis of stress-induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific

Deep Sea Drilling Project (DSDP) Hole 504B is located in the eastern equatorial Pacific Ocean and extends to a total depth of 2111 m beneath the seafloor (mbsf). Several acoustic televiewer logs have been obtained in this well during successive stages of drilling, and the resulting digital images have revealed numerous oval-shaped fractures seemingly etched into the borehole wall. A theoretical examination of these stress-induced features identifies a unique and ephemeral set of stress distributions and magnitudes that are necessary for their production. Consequently, the ovals provide a basis for quantifying the magnitudes and orientations of the maximum and minimum horizontal principal stresses, S H and S h at this site. Vertical, truncated breakouts and horizontal tensile fractures define the spatial boundaries of the ovals. Explicit criteria for their occurrence are combined with estimates for various physical properties of the rock to yield a range of possible values for the horizontal principal stresses. The conspicuous oval geometry is completed by a curved fracture that joins the vertical and horizontal components. Its degree of curvature is delineated by the modified Griffith failure criterion and is directly related to the principal stress difference ( S H − S h ). Matching a series of type curves corresponding to specific values for ( S H − S h ) with the actual undistorted well bore images allows the magnitude of the stress difference to be further constrained. With a value for ( S H − S h ) of 45 ± 5 MPa the individual magnitudes of S H and S h are determined more precisely. Final estimates for the horizontal principal stresses in DSDP Hole 504B at a depth of 1200 mbsf are 141 MPa ≤ S H ≤ 149 MPa and 91 MPa ≤ S h ≤ 109 MPa. Stress magnitudes derived from this approach rely heavily upon the values of a variety of physical properties, and complementary laboratory measurements performed on relevant rock samples provide critical information. Uncertainties in estimating these properties translate into less precise predictions of principal stresses.

Journal of Geophysical Research B: Solid Earth↗
USGS2000

Influence of inner-continental shelf geologic framework on the evolution and behavior of the barrier-island system between Fire Island Inlet and Shinnecock Inlet, Long Island, New York

High-resolution, sea-floor mapping techniques, including sidescan-sonar and subbottom profiling, were used to investigate how the geologic framework of the inner-continental shelf influenced the Holocene evolution and modern behavior of the Fire Island barrier-island system, Long Island, New York. The inner-continental shelf off Long Island is divided into two physiographic provinces by a broad outcrop of Cretaceous coastal-plain strata offshore of Watch Hill; this outcrop was part of a subaerial headland during the Holocene marine transgression. Erosion of the headland during transgression furnished sediment to the inner-continental shelf downdrift to the west. The sediment was, in turn, reworked by oceanographic processes into a series of shoreface-attached sand ridges. The oldest (~1200 yr BP) and most stable part of the barrier-island system is immediately landward of the outcropping coastal-plain strata and thickest sand ridges. East of Watch Hill, Pleistocene sediment either is exposed on the inner-continental shelf or is buried by a veneer of modern reworked sediment. Here the barrier-island system has migrated landward at a faster rate than the segment west of Watch Hill and has been breached by numerous historic inlets. Because the Pleistocene sedimentary deposit is generally of uniform thickness throughout the study area and unconformably overlies the Cretaceous coastal-plain strata, both the Holocene and historical evolution of the Fire Island barrier-island system are controlled by the physiography of this regional unconformity. In particular, the shoreface-connected sand ridges appear to be a significant source of sediment to the western portion of Fire Island. Previous attempts to develop a sediment budget for this coastal system have failed to explain volumetric discrepancies, primarily because poor assumptions were made about the nature of sediment transport in the system. A more realistic sediment budget must include a significantly larger spatial scale, including sediment input from the inner-continental shelf.

New York↗
USGS2001

Sources of global warming in upper ocean temperature during El Niño

Global average sea surface temperature (SST) from 40°S to 60°N fluctuates ±0.3°C on interannual period scales, with global warming (cooling) during El Niño (La Niña). About 90% of the global warming during El Niño occurs in the tropical global ocean from 20°S to 20°N, half because of large SST anomalies in the tropical Pacific associated with El Niño and the other half because of warm SST anomalies occurring over ∼80% of the tropical global ocean. From examination of National Centers for Environmental Prediction [ Kalnay et al. , 1996] and Comprehensive Ocean-Atmosphere Data Set [ Woodruff et al. , 1993] reanalyses, tropical global warming during El Niño is associated with higher troposphere moisture content and cloud cover, with reduced trade wind intensity occurring during the onset phase of El Niño. During this onset phase the tropical global average diabatic heat storage tendency in the layer above the main pycnocline is 1–3 W m −2 above normal. Its principal source is a reduction in the poleward Ekman heat flux out of the tropical ocean of 2–5 W m −2 . Subsequently, peak tropical global warming during El Niño is dissipated by an increase in the flux of latent heat to the troposphere of 2–5 W m −2 , with reduced shortwave and longwave radiative fluxes in response to increased cloud cover tending to cancel each other. In the extratropical global ocean the reduction in poleward Ekman heat flux out of the tropics during the onset of El Niño tends to be balanced by reduction in the flux of latent heat to the troposphere. Thus global warming and cooling during Earth's internal mode of interannual climate variability arise from fluctuations in the global hydrological balance, not the global radiation balance. Since it occurs in the absence of extraterrestrial and anthropogenic forcing, global warming on decadal, interdecadal, and centennial period scales may also occur in association with Earth's internal modes of climate variability on those scales.

Journal of Geophysical Research C: Oceans↗
USGS1996

Crustal structure of the Colorado Plateau, Arizona: Application of new long-offset seismic data analysis techniques

The Colorado Plateau is a large crustal block in the southwestern United States that has been raised intact nearly 2 km above sea level since Cretaceous marine sediments were deposited on its surface. Controversy exists concerning the thickness of the plateau crust and the source of its buoyancy. Interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick. A thick crust could support the observed topography of the Colorado Plateau isostatically, while a thinner crust would indicate the presence of an underlying low-density mantle. This paper reports results on long-offset seismic data collected during the 1989 segment of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that extended from the Transition Zone into the Colorado Plateau in northwest Arizona. We apply two new methods to analyze long-offset data that employ finite difference travel time calculations: (1) a first-arrival time inverter to find upper crustal velocity structure and (2) a forward-modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals. We find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern Basin and Range province to about 42 km beneath the northern Transition Zone and southern Colorado Plateau margin. We observe some crustal thinning (to ∼37 km thick) and slightly higher lower crustal velocities farther inboard; beneath the Kaibab uplift on the north rim of the Grand Canyon the crust thickens to a maximum of 48 km. We observe a nonuniform crustal thickness beneath the Colorado Plateau that varies by ∼15% and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations. Crustal compositions (as inferred from seismic velocities) appear to be the same beneath the Colorado Plateau as those in the Basin and Range province to the southwest, implying that the plateau crust represents an unextended version of the Basin and Range. Some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the Proterozoic Era.

Journal of Geophysical Research B: Solid Earth↗
USGS1978

Origin of major element chemical trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge

In this paper we summarize the major element chemical variation for basalts from the Deep Sea Drilling Project Leg 37 and relate it to stratigraphic position in each of five drilling sites. Least-squares techniques are successfully used to quantify the nature and extent of alteration in these basalts, and to correct the major element analysis back to a magmatic, or alteration-free, composition on the assumption that alteration takes place in two ways: (1) secondary minerals are introduced into veins and vesicles, and (2) CO2 and H2O react with components in the rock to form a simple alteration assemblage. A chemical stratigraphy is defined for these basalts by grouping lavas whose chemistries are related by low-pressure phenocryst-liquid differentiation as identified by least-squares calculation. Major chemical-stratigraphic units are as much as 200 m thick; correlations of these units can be made between the holes at site 332 (about 100 m apart), but not between the other sites. Compositions of parental magmas are calculated by extrapolating low-pressure variations to a constant value of 9% MgO. The differences in these extrapolated compositions reflect high-pressure processes, and suggest that clinopyroxene may be an important phase in either intermediate-level fractionation of basaltic liquids, or as a residual phase during the partial melting which produces these basaltic liquids. Several of the basaltic liquids calculated as parental to the Leg 37 basalts have CaO contents greater than 14% and indicate that the oceanic mantle is richer in CaO and Al2O3 than values used in pyrolite models for the upper mantle. A model for magma generation and eruption beneath the Mid-Atlantic Ridge embodies the following characteristics: 1. (1) Separate magma batches are generated in the mantle. 2. (2) Each of these may be erupted directly or stored at shallow depth where significant fractionation takes place. Common fractionation processes are inferred to be gravitative settling of olivine, flotation (?) of plagioclase, and flow differentiation of an olivine-plagioclase-augite assemblage. 3. (3) Eruption of fractionated lava derived from earlier magma batches may alternate with eruption of younger less-fractionated or unfractionated magma. ?? 1978.

Journal of Volcanology and Geothermal Research↗
USGS2013

Injection, transport, and deposition of tephra during event 5 at Redoubt Volcano, 23 March, 2009

Among the events of the 2009 eruption at Redoubt Volcano, Alaska, event 5 was the best documented by radar, satellite imagery, and deposit mapping. We use the new Eulerian tephra transport model Ash3d to simulate transport and deposition of event 5 tephra at distances up to 350 km. The eruption, which started at about 1230 UTC on 23 March, 2009, sent a plume from the vent elevation (estimated at 2.3 ± 0.1 km above sea level or a.s.l.) to about 16 ± 2 km above sea level in 5 min. The plume was a few kilometers higher than would be expected for the estimated average mass eruption rate and atmospheric conditions, possibly due to release of most of the eruptive mass in the first half of the 20-minute event. The eruption injected tephra into a wind field of high shear, with weak easterly winds below ~ 3 km elevation, strong southerly winds at 6–10 km and weak westerlies above ~ 16 km. Model simulations in this wind field predicted development of a northward-migrating inverted “v”-shaped cloud with a southwest-trending arm at a few kilometers elevation, which was not visible in IR satellite images due to cloud cover, and a southeast-trending arm at > 10 km elevation that was clearly visible. Simulations also predicted a deposit distribution that strongly depended on plume height: a plume height below 15 km predicted ash deposits that were located west of those mapped, whereas good agreement was reached with a modeled plume height of 15–18 km. Field sampling of the deposit found it to contain abundant tephra aggregates, which accelerated the removal of tephra from the atmosphere. We were able to reasonably approximate the effect of aggregation on the deposit mass distribution by two methods: (1) adjusting the grain-size distribution, taking the erupted mass < = 0.063 mm in diameter and distributing it evenly into bins of coarser size; and (2) moving 80–90% of the mass < = 0.063 mm into a single particle bin ranging in size from 0.25 to 1 mm. These methods produced an area inside the 100 g m − 2 isomass lines that was within a few tens of percent of mapped area; however they under-predicted deposit mass at very proximal (< 50 km) and very distal (> 250 km) locations. Modeled grain-size distributions at sample locations are also generally coarser than observed. The mismatch may result from a combination of limitations in field sampling, approximations inherent in the model, errors in the numerical wind field, and aggregation of particles larger than 0.063 mm.

Alaska↗
USGS2012

An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala

Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake-generating stresses. The data permit construction of a magnitude-duration scale, calibrated with m b magnitudes for events located by the World-Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 m b , and the threshold magnitude of locatable events was about 2.8 m b . Two distinct composite focal mechanisms were determined. One appears to indicate high-angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.

Journal of Geophysical Research Solid Earth↗
USGS2012

Analysis of geophysical well logs obtained in the State 2-14 borehole, Salton Sea geothermal area, California

A complete suite of conventional geophysical well logs was obtained in the upper part of a 3220-m- deep borehole drilled into geothermally altered alluvial sediments on the southeastern edge of the Salton Sea. In situ temperatures greater than 300°C and an inability to cool parts of the borehole by circulation limited the suite of logs run below 2000 m in depth to deep induction, spontaneous potential, un-calibrated neutron, natural gamma, and temperature. Bottom-hole temperature trends given by repeat temperature logs were extrapolated to undisturbed temperatures approaching 355°C at a depth of 3220 m. Geophysical logs obtained in the State 2–14 borehole indicate that neutron porosity, gamma-gamma, and deep-induction logs provide useful information on lithologic trends with depth. The natural gamma log contains almost continuous, high-frequency fluctuations that obscure lithologic trends and that may be related to recent radioisotope redistribution and departure from radiometric equilibrium. Acoustic transit time logs give unrealistically low in situ compressional velocities ranging from 1.8 to 3.0 km/s, whereas acoustic waveform logs indicate that sediment compressional velocities range from less than 3.0 km/s shallower than 1000 m in depth to almost 5.0 km/s at depths greater than 2000 m. Analyses indicate that most log values lie between two lithologic end points: an electrically conductive claystone with moderate neutron porosity, but no effective porosity, and an electrically nonconductive, fully cemented siltstone that has small but finite porosity. A limited number of clean sandstones depart from this trend; geophysical logs from these sandstones indicate an effective porosity ranging from 5 to 14%, and saturation with brines having equivalent NaCl concentrations greater than 100,000 mg/L. Depth- averaged trends in neutron porosity and deep-induction logs, along with trends in acoustic velocity determined from acoustic waveform logs, demonstrate that major changes in the properties of alluvial sediments occur within the depth range from 1200 to 1800 m. Although caliper logs were not obtained deeper than 2000 m, resistivity values less than 10 ohm m at those depths probably correspond to borehole enlargements in production zones rather than local increases in effective porosity. The transition in sediment properties indicated by the geophysical logs in the depth interval from 1200 to 2000 m apparently represents a detailed vertical profile of the transition from relatively unaltered clay minerals in alluvial sediments to electrically nonconductive alteration products such as epidote and feldspar.

Journal of Geophysical Research Solid Earth↗