Drowned shelf-edge deltas, barrier islands and related features along the outer continental shelf north of the head of De Soto Canyon, NE Gulf of Mexico     

J.V. Gardner  B.R. Calder  J.E. Hughes Clarke  L.A. Mayer  G. Elston  Y. Rzhanov   《Geomorphology》2007,89(3-4):370-390

Four drowned shelf-edge delta complexes, two drowned shelf deltas, three drowned barrier islands, large areas of “hardground”, and fields of bedforms on the mid and outer continental shelf and uppermost slope north of the head of De Soto Canyon, NE Gulf of Mexico were mapped with high-resolution multibeam echosounder. Deltas formed not during the last eustatic low stand, but during one or more interstadials when eustatic sea levels were only 60 to 80 m below present sea level. The barrier islands and deltas must have been cemented prior to rapid falls of eustatic sea level that occurred during global glaciations. Cementation is necessary to have preserved the barrier islands from erosion and subsequent destruction by the rapid sea-level rise during the last deglaciation. The preservation of the relict bathymetry is so good that features that superficially resemble trough blowouts are found in association with one of the relict barrier islands. Asymmetric bedforms on the midshelf in water depths of 50 to 60 m indicate transport directions to the SW but asymmetric bedforms in water depths of the upper slope between 100 and 120 m on the S and SE flanks of the drowned shelf-edge deltas indicate a different current direction, a separate flow that is a continuation of a SW-flowing current that was previously found on the upper slope off NW Florida.  相似文献   

Seismotectonic implications of sand blows in the southern Mississippi Embayment     

R.T. Cox  A.A. Hill  D. Larsen  T. Holzer  S.L Forman  T. Noce  C. Gardner  J. Morat 《Engineering Geology》2007,89(3-4):278-299

We explore seismically-induced sand blows from the southern Mississippi Embayment and their implications in resolving the question of near or distal epicentral source region. This was accomplished using aerial photography, field excavations, and cone penetration tests. Our analysis shows that three sand blow fields exhibit a distinct chronology of strong ground motion for the southern embayment: (1) The Ashley County, Arkansas sand blow field, near the Arkansas/Louisiana state border, experienced four Holocene sand venting episodes; (2) to the north, the Desha County field experienced at least three episodes of liquefaction; and (3) the Lincoln–Jefferson Counties field experienced at least one episode. Cone penetration tests (CPT) conducted in and between the sand blow fields suggest that the fields may not be distal liquefaction associated with New Madrid seismic zone earthquakes but rather are likely associated with strong earthquakes on local faults. This conclusion is consistent with the differences in timing of the southern embayment sand venting episodes and those in the New Madrid seismic zone. These results suggest that active tectonism and strong seismicity in intraplate North America may not be localized at isolated weak spots, but rather widespread on fault systems that are favorably oriented for slip in the contemporary stress field.  相似文献   

Mountain hazards and the resilience of social–ecological systems: lessons learned in India and Canada     

James S. Gardner  Julie Dekens 《Natural Hazards》2007,41(2):317-336

Mountain regions are subject to a variety of hazardous processes. Earthquakes, landslides, snow avalanches, floods, debris flows, epidemics and fires, among other processes, have caused injury, death, damage and destruction. They also face challenges from increased populations, and expansion and intensification of␣activities, land uses and infrastructure. The combination of a dynamic bio- geophysical environment and intensified human use has increased the vulnerability of mountain social–ecological systems to risk from hazards. The ability of social–ecological systems to build resilience in the context of hazards is an important factor in their long-term sustainability. The role of resilience building in understanding the impact of hazards in mountain areas is examined and illustrated, in part, through examples from Canada and India. Resilient social–ecological systems have the ability to learn and adjust, use all forms of knowledge, to self-organize and to develop positive institutional linkages with other social–ecological systems in the face of hazards. The analysis suggests that traditional social–ecological systems built resilience through avoidance, which was effective for localized hazards. The more recent development and implementation of cross-scale institutional linkages is shown to be a particularly effective means of resilience building in mountain social–ecological systems in the face of all hazards.  相似文献   

A review of water column processes influencing hypoxia in the northern Gulf of Mexico   总被引：1，自引：0，他引：1  

Michael J. Dagg  James W. Ammerman  Rainer M. W. Amon  Wayne S. Gardner  Rebecca E. Green  Steven E. Lohrenz 《Estuaries and Coasts》2007,30(5):735-752

In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model.  相似文献   

Higher Seismic Activity During Local Night on the Raw Worldwide Earthquake Catalogue   总被引：1，自引：0，他引：1  

L. Knopoff  J. K. Gardner 《Geophysical Journal International》1972,28(3):311-313

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Export of Acidity in Drainage Water from Acid Sulphate Soils     

F.J Cook  W Hicks  E.A Gardner  G.D Carlin  D.W Froggatt 《Marine pollution bulletin》2000,41(7-12):319-326

Disturbed acid sulphate soils are potent sources of acidity in coastal waterways. Monitoring studies of the drainage water for sites at East Trinity, Cairns and Pimpama, south-east Queensland indicate that considerable acidity is found in the drainage water from these sites. Hydrogen (H⁺), ferrous (Fe²⁺) and aluminium (Al) ions are the dominant acid cations involved. When drainage water is mixed with fresh or marine waters the effect of H⁺ on acidity generation is immediate. Aluminium can release acidity on hydrolysis, while the oxidation of Fe²⁺ to Fe³⁺ both acidifies and removes dissolved oxygen from the water. Strongly acidic waters with low levels of dissolved oxygen concentration are undesirable for most forms of aquatic life. Export of acidity from acid sulphate soil is likely to have a major effect on inshore fisheries and breeding grounds especially in periods of flood following drought or periods of low rainfall, where large volumes of acidity can be flushed/leached into sensitive aquatic/marine habitats. Impacts may include low dissolved oxygen, fish kills, epizootic ulceration syndrome and damage to oysters. During the processes of oxidation and hydrolysis, iron and aluminium flocs form, that can smother benthic communities. Heavy metals are found in the drainage water at elevated levels and may also be of concern for aquatic organisms. Chronic effects such as habitat degradation, mortality of marine worms, bivalves, invasion of acid tolerant species (both plant and animal) and avoidance of habitat have been documented elsewhere. These areas require further research.  相似文献   

The caldera-forming eruption of Volcán Ceboruco, Mexico   总被引：1，自引：1，他引：0  

James E. Gardner  Steve Tait 《Bulletin of Volcanology》2000,62(1):20-33

3 of magma erupted, ∼95% of which was deposited as fall layers. During most of the deposition of P1, eruptive intensity (mass flux) was almost constant at 4–8×10⁷ kg s⁻¹, producing a Plinian column 25–30 km in height. Size grading at the top of P1 indicates, however, that mass flux waned dramatically, and possibly that there was a brief pause in the eruption. During the post-P1 phase of the eruption, a much smaller volume of magma erupted, although mass flux varied by more than an order of magnitude. We suggest that caldera collapse began at the end of the P1 phase of the eruption, because along with the large differences in mass flux behavior between P1 and post-P1 layers, there were also dramatic changes in lithic content (P1 contains ∼8% lithics; post-P1 layers contain 30–60%) and magma composition (P1 is 98% rhyodacite; post-P1 layers are 60–90% rhyodacite). However, the total volume of magma erupted during the Jala pumice event is close to that estimated for the caldera. These observations appear to conflict with models which envision that, after an eruption is initiated by overpressure in the magma chamber, caldera collapse begins when the reservoir becomes underpressurized as a result of the removal of magma. The conflict arises because firstly, the P1 layer makes up too large a proportion (∼75%) of the total volume erupted to correspond to an overpressurized phase, and secondly, the caldera volume exceeds the post-P1 volume of magma by at least a factor of three. The mismatches between model and observations could be reconciled if collapse began near the beginning of the eruption, but no record of such early collapse is evident in the tephra sequence. The apparent inability to place the Jala pumice eruptive sequence into existing models of caldera collapse, which were constructed to explain the formation of calderas much greater in volume than that at Ceboruco, may indicate that differences in caldera mechanics exist that depend on size or that a more general model for caldera formation is needed. Received: 18 November 1998 / Accepted: 23 October 1999  相似文献   

The atmospheric lead record preserved in lagoon sediments at a remote equatorial Pacific location: Palmyra Atoll, northern Line Islands     

Collen JD  Baker JA  Dunbar RB  Rieser U  Gardner JP  Garton DW  Christiansen KJ 《Marine pollution bulletin》2011,62(2):251-257

Anthropogenic lead (Pb) inputs to the atmosphere increased greatly over the past century and now dominate Pb supply to the oceans. However, the Pb content of sediments across the equatorial Pacific region is relatively unknown, and data exist only for deep sea sites where Pb deposition lags surface water inputs by up to a century. Here we present ICP-MS analyses of Pb of a core from a lagoon at Palmyra Atoll, northern Line Islands, that spans approximately the past 160 years. The non-bioturbated sediments of the euxinic lagoon, coupled with rapid rates of deposition, provide a unique fine-scale record of atmospheric Pb supply at a remote Pacific location. These first observations of historic Pb sedimentation in an atoll lagoon reveal a 63-fold increase in Pb flux to sediments during the past century and correlate directly with the North American consumption of leaded gasoline that began in 1926.  相似文献   

Commentary on Palmyra atoll     

Gardner J  Collen J  Dunbar R  Garton D 《Marine pollution bulletin》2011,62(12):2876-2877

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Computations and estimates of rate coefficients for hydrocarbon reactions of interest to the atmospheres of the outer solar system     

A.H. Laufer  E.P. Gardner  T.L. Kwok  Y.L. Yung 《Icarus》1983,56(3):560-567

The rate coefficients, including Arrhenius parameters, have been computed for a number of chemical reactions involving hydrocarbon species for which experimental data are not available and which are important in planetary atmospheric models. The techniques used to calculate the kinetic parameters include the Troe and semiempirical bond energy-bond order (BEBO) or bond strength-bond length (BSBL) methods.  相似文献