Geochemical signature of historical mining: Fowey Estuary, Cornwall, UK     

Duncan Pirrie  Matthew R. Power  Paul D. Wheeler  Andy Cundy  Colin Bridges  Graham Davey 《Journal of Geochemical Exploration》2002,76(1)

Geochemical analyses of intertidal sediments from the northern part of the Fowey Estuary, Cornwall, UK, reveal a clear pulse in Sn concentration in sediments which predate 1880. Sn concentrations at the base of the cores increase rapidly to peak values of 1200 ppm and then decrease to values of 200 ppm at the present-day sediment surface. The mineralogy of the sediments is consistent with an origin from the release of mine waste from tin streaming and smelt products into the estuary. Further to the south, the down-core geochemistry of the estuary sediments is uniform with values of typically 400 ppm. This is interpreted as due to the natural reworking of the recognised pulse in particulate mine waste seen to the north.  相似文献   

Sewage and the community—One view of Sydney's deepwater outfalls, the environmental monitoring programme and the future of sewage disposal     

Duncan Leadbitter 《Marine pollution bulletin》1996,33(7-12):269-272

Sydney's sewage disposal problems have been the catalyst for wider questioning by the community about the management of the water cycle in the Sydney Basin. Public concern over the deepwater ocean outfalls generated an involvement in the Environmental Monitoring Programme (EMP) which provided a number of insights into the marine environment offshore Sydney, interactions between agencies and the role of public interest groups. The EMP has not provided sufficient information to make decisions about the value of higher levels of treatment. However, public interest in these outfalls is being subsumed by demands for the greater recycling of water and mechanisms for reducing water use.  相似文献   

Dynamic behaviour of the K-corona above a type I radio source     

R. A. Duncan 《Solar physics》1983,89(1):63-76

Coronagraph images can be greatly enhanced by subtracting from the brightness of each picture point, the running median brightness over a square surrounding each picture point. The application of this technique to coronal analysis appears to be new. After such enhancement, images recorded by the SMM Coronagraph/Polarimeter reveal a coronal transient with previously undescribed characteristics. The transient began with bright ejecta moving up pre-existing rays and weaker, more diffuse, mass injection in the area between rays. At the sudden onset of this mass ejection the rays began to move apart from one another: during the ensuing 22 hr the separation of one pair of rays increased by 37° of latitude. Towards the end of the event these moving rays looked much like the legs of previously described loop transients. A type I radio source lay close to the centre of symmetry of the transient.  相似文献   

Hardbottom development and significance to the sediment-starved west-central Florida inner continental shelf     

Stephen P. Obrochta  David S. Duncan  Gregg R. Brooks   《Marine Geology》2003,200(1-4):291-306

Hardbottoms are sequence boundaries and condensed sections that offer clues for the interpretation of the incomplete record of Tertiary continental shelf evolution. Seaward of 5 km, 50% of the inner west-central Florida shelf seafloor is flat hardbottom. These lithified surfaces are punctuated by shorefacing, scarped hardbottoms that trend shore-parallel (330°–0°) and vary in relief (up to 4 m). Scarped hardbottoms are the only natural relief on the inner shelf and support a diverse benthic community, the activities of which erode the outcrops, producing undercuts in excess of 1 m. Outcropping hardbottom strata are comprised of distinct, phosphate-rich, mixed carbonate–siliciclastic lithofacies, that range in age from Miocene to Quaternary. Miocene units are dolomite-rich and mark the upper surface of the inner shelf bedrock (Hawthorn Group). Dolomite within these beds (silt-sized, cloudy centered rhombs) fall into two age groups, correlating with highstands at 15 and 5 Ma. This lithofacies is consistent with models that indicate an increased flux of organic matter – resulting from topographically induced upwelling – promoting dolomitization during early burial diagenesis in the sulfate-reduction zone. Quaternary units are calcite-rich and perched atop the shelf bedrock. Samples of these units record a complex diagenetic history and multiple sea-level fluctuations. Based on evidence of primary marine cementation, they are interpreted to be hardground (non-deposition) surfaces, forming as a function of sediment starvation and minimal sediment movement. Decreased highstand magnitude or duration may have resulted in the absence of a significant organic component to Quaternary hardbottoms, which, in turn, may prevent subsequent dolomitization. These outcrops are a potential source for sediments to the inner shelf, not only as habitat for biological sediment production, but also through their destruction. The undercut, shorefacing, scarped hardbottom morphology displayed by west-central Florida hardbottoms is indicative of bio-erosion. Preliminary studies indicate a potential mass of 0.04 kg m⁻² yr⁻¹ of siliciclastic sediment is released to the inner shelf.  相似文献   

Sedimentary records of intense storms in Holocene barrier sequences, Maine, USA     

Ilya V. Buynevich  Duncan M. FitzGerald  Sytze van Heteren   《Marine Geology》2004,210(1-4):135-148

Coastal-morphological, geophysical (ground-penetrating radar [GPR]), and sedimentological data document extreme storm events along the sandy barriers of Maine's south–central (Hunnewell and Flat Point barriers) and southwestern (Saco Bay barriers) coastal compartments. The Hunnewell barrier contains four equally spaced buried storm scarps behind the exposed scarp of the Blizzard of 1978, a 100-year storm that eroded more than 100 m of shoreline, causing extensive property loss. These scarps dip 3–5° steeper than the normal beachface slope and consist of sands with more than 50% heavy minerals. The heavy minerals produce distinct subsurface reflections that facilitate the location of buried supratidal parts of storm scarps and the mapping of ancient poststorm shoreline positions. The imaged scarps likely formed within the past 1.5–2.0 ka BP. The Flat Point barrier consists of a prograded sequence overlain by a laterally extensive, seaward-thinning layer of freshwater peat and capped by aeolian sands. This stratigraphy suggests that the bog varied in size through time, contracting during overwash events and aeolian deposition and expanding across washover sheets during extended periods of barrier stability. The main overwash event accompanied by barrier planation and wetland expansion may be linked to the first historical storm in New England, the “Great Colonial Hurricane” of 1635.

Evidence of near-modern and mid-Holocene storm events along Saco Bay includes washover units and marsh ridges. Washovers interfinger with saltmarsh peat that ranges in age from 4.5 ka BP to modern. The presence of isolated sandy ridges behind existing and former tidal inlets reflects overtopping of marshes and high intertidal mudflats during major storms. Radiocarbon ages indicate that this process took place at different locations along the Saco Bay barrier complex from 3 to 1 ka BP.  相似文献   

The 1919–1920 eruption of Mauna Iki,Kilauea: chronology,geologic mapping,and magma transport mechanisms     

Scott K Rowland  Duncan C Munro 《Bulletin of Volcanology》1993,55(3):190-203

Utilizing historical accounts, field mapping, and photogeology, this paper presents a chronology of, and an analysis of magma transport during, the December 1919 to August 1920 satellitic shield eruption of Mauna Iki on the SW rift zone of Kilauea Volcano, Hawaii. The eruption can be divided into four stages based on the nature of the eruptive activity. Stage 1 consisted of the shallow injection of a dike from the summit region to the eventual eruption site 10 km downrift. During stage 2, a low ridge of pahoehoe formed in the vent area; later a large a'a flow broke out of this ridge and flowed 8.5 km SW at an average flow front velocity of 0.5 km/day. The eruption continued until mid-August producing almost exclusively pahoehoe, first as gas-rich overflows from a lava pond (stage 3), and later as denser tube-fed lava (stage 4) that reached almost 8 km from the vent at an average flow-front velocity of 0.1 km/day. Magma transport during the Mauna Iki eruption is examined using three criteria: (1) eruption characteristics and volumetric flow rates; (2) changes in the surface height of the Halemaumau lava lake; and (3) tilt measurements made at the summit of Kilauea. We find good correlation between Halemaumau lake activity and the eruptive stages. Additionally, the E-W component of summit tilt tended to mimic the lake activity. The N-S component, however, did not. Multiple storage zones in the shallow summit region probably accounted for the decoupling of E-W and N-S tilt components. Analysis of these criteria shows that at different times during the eruption, magma was either emplaced into the volcano without eruption, hydraulically drained from Halemaumau to Mauna Iki, or fed at steady-state conditions from summit storage to Mauna Iki. Volume calculations indicate that the supply rate to Kilauea during the eruption was around 3 m³/s, similar to that calculated during the Mauna Ulu and Kupaianaha shield-building eruptions, and consistent with previously determined values of long-term supply to Kilauea.  相似文献   

Making the 'black box' transparent: publishing and presenting geographic knowledge     

Rob Kitchin  Duncan Fuller 《Area》2003,35(3):313-315

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Comparison of Late Pleistocene and Modern Glacier Extents in Central Nepal Based on Digital Elevation Data and Satellite Imagery   总被引：1，自引：0，他引：1  

Christopher C. Duncan  Andrew J. Klein  Jeffrey G. Masek  Bryan L. Isacks 《Quaternary Research》1998,49(3):241-254

Late Pleistocene and modern ice extents in central Nepal are compared to estimate equilibrium line altitude (ELA) depressions. New techniques are used for determining the former extent of glaciers based on quantitative, objective geomorphic analyses of a 90-m resolution digital elevation model (DEM). For every link of the drainage network, valley form is classified as glacial or fluvial based on cross-valley shape and slope statistics. Down-valley transitions from glacial to fluvial form indicate the former limits of glaciation in each valley. Landsat Multispectral Scanner imagery for the same region is used to map current glacier extents. For both full-glacial and modern cases, ELAs are computed from the glacier limits using the DEM and a toe-to-headwall altitude ratio of 0.5. Computed ELA depressions range from 100–900 m with a modal value of 650 m and a mean of 500 m, values consistent with previously published estimates for the central Himalaya but markedly smaller than estimates for many other regions. We suggest that this reflects reduced precipitation, rather than a small temperature depression, consistent with other evidence for a weaker monsoon under full-glacial conditions.  相似文献   

Nick Rast and the recognition of the Avalonian Arc     

R. Damian Nance  J. Brendan Murphy  J. Duncan Keppie 《Journal of Geodynamics》2004,37(3-5):437

Recognition of the eastern (Avalonian) margin of the northern Appalachian orogen as a Late Precambrian microcontinental arc terrane, rather than the opposing passive margin of the Proto-Atlantic (Iapetus) Ocean to that of eastern Laurentia, constituted a fundamental advance in Appalachian geology that profoundly influenced subsequent models for the orogen's plate tectonic evolution. This advance was first clearly articulated by Nick Rast and his students in 1976, who, by correlating rocks of the Avalon Platform with those of the British Midlands, established the Avalonian volcanic belt as a Japan-like microcontinent. Contrary to contemporary views of the Avalon Platform, which favored an extensional, Basin and Range-like setting for its volcanism, Rast argued on the basis of this correlation that the association of Avalonian volcanism with compressional orogeny, widespread calc-alkaline plutonism and, in Angelsey, with blueschists and ophiolitic rocks, indicated a convergent plate margin setting. Rast further proposed that the Avalonian volcanic belt was ensialic, and was bordered to the northwest and southeast by Precambrian oceanic domains. Contemporary reconstructions of the Avalonian and Cadomian belts as fragments of a Cordilleran-like accretionary orogen that developed along an active margin of Neoproterozoic Gondwana owe their origin to these early ideas and, while far removed from the tectonic model that Rast envisaged, are a direct heritage of his recognition of the Avalonian volcanic belt as a microcontinental arc terrane.  相似文献   

On the relationship of radar backscatter to wind speed and fetch     

Duncan Ross  W. Linwood Jones 《Boundary-Layer Meteorology》1978,13(1-4):151-163

The physics of the interaction of electromagnetic waves with the ocean surface has been an active area of research for a number of years. We present here the results of satellite and aircraft experiments to investigate the ability of active microwave radars to infer surface wind speeds remotely. Data obtained from the recent National Aeronautics and Space Administration (NASA) Skylab experiment are compared with surface wind speeds measured by low-flying aircraft and ships-of-opportunity and found to give useful estimates of the oceanic wind field. We also investigate the influence of varying wave height on radar measurements of wind speed by measuring the backscattering cross-section for constant wind speed but variable wave conditions. We conclude that this effect is of little importance.  相似文献