Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California     

L. Allan James  Jon Harbor  Derek Fabel  Dennis Dahms  David Elmore 《Quaternary Research》2002,57(3):409

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 ¹⁰Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 ¹⁰Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.  相似文献   

The Need for Public Intellectuals in the Trump Era and Beyond: Strategies for Communication,Engagement, and Advocacy     

Derek H. Alderman  Joshua F. J. Inwood 《The Professional geographer》2019,71(1):145-151

  相似文献   

Low‐Temperature Thermal Remediation: Gaining Traction as a Green Remedial Alternative          下载免费PDF全文

John Horst  Cullen Flanders  Mark Klemmer  Davinder Singh Randhawa  Derek Rosso 《Ground Water Monitoring & Remediation》2018,38(3):18-27

  相似文献   

Spatial variability in beach biogeomorphology in a tropical archipelago     

Shannon Gore  J. Andrew G. Cooper  Derek W.T. Jackson  Lianna Jarecki 《地球表面变化过程与地形》2019,44(9):1860-1875

Beaches of tropical island coasts exhibit high levels of diversity in composition and form in comparison with their continental counterparts. To investigate the nature and origin of this diversity, individual beach morphology and sedimentology was investigated in the British Virgin Islands (BVI), a Caribbean archipelago of > 60 high volcanic and low reef islands. The islands exhibit a diversity of orientations (some facing the Atlantic and some the Caribbean), elevation and gradient, rock type and wave energy. An examination of 100 beaches in the archipelago revealed a first-order division into sand (70 beaches) and coral rubble (30 beaches). These beaches occur in seven planform types (determined by the antecedent geological framework) and are further subdivided according to shoreface type (seagrass, sandy shoreface, or reef). Mainland-attached headland-embayment beaches are the most common form of sand beach while coral rubble beaches usually occur as barriers that enclose salt ponds and wetlands. Among sand beaches, carbonate content is greatest on Atlantic-facing beaches, and coral rubble beaches are more common on Caribbean-facing beaches. Grain size characteristics on sandy beaches are highly variable and range from fine to very coarse sands while coral rubble beaches range up to boulder-sized clasts. The local source material is a primary determinant of sediment composition. The local factors such as the underlying geology, source and availability of sediments are the primary determinants of beach form, composition and texture in the BVI. Oceanographic and climatic conditions such as the prevailing easterly trade winds and waves which seasonally range in direction from east-northeast to southeast as well as beach orientation to Atlantic- or Caribbean-facing waves also contribute to the variability, but in a secondary role. © 2019 John Wiley & Sons, Ltd.  相似文献   

Morphodynamic interactions of continental shelf,beach and dunes: the Cabopino dune system in southern Mediterranean Spain     

Gonzalo Malvárez  Fatima Navas  Emilia Guisado-Pintado  Derek W.T. Jackson 《地球表面变化过程与地形》2019,44(8):1647-1658

The complex morphodynamic interactions between nearshore, shoreface and dune systems are usually simplified by studying these zones and their associated processes in isolation. However, the established relationships between each of them suggests that an integrated approach is required to examine the genesis, evolution and adaptation of the entire morphodynamic system. The Cabopino dune system in the southern Spanish Mediterranean Sea provides a clear example of a linked morphodynamic system in which a relatively large dune system has been generated and grown through the supply of sediments from an adjacent littoral supply environment. Here, we present a conceptual model of how the nearshore has provided suitable conditions for beach and dune development. We purport that synchronization of sediment activation in the marine and aeolian sections of the system have played a major role in this microtidal setting in which temporal aspects are not only tied to storm action, but to large sedimentary features moving alongshore. © 2019 John Wiley & Sons, Ltd.  相似文献   

Geoarchaeological investigations of St. Ann's Bay,Jamaica: The search for the Columbus Caravels and an assessment of 1000 years of human land use     

Michael R. Waters  John R. Giardino  Derek W. Ryter  James M. Parrent 《Geoarchaeology》1993,8(4):259-279

A complex interbedded sequence of shallow bay, alluvial fan, and lagoon/marsh facies was revealed during geoarchaeological investigations of St. Ann's Bay, Jamaica. Eighteen radiocarbon ages and time-diagnostic artifacts were obtained from these sediments. From these data, the landscape evolution of St. Ann's Bay, Jamaica, during the last 4800 years is reconstructed. The landscape reconstructions for St. Ann's Bay are used to identify areas where the two caravels abandoned by Christopher Columbus in A. D. 1504 might be located. In addition, two periods of rapid sedimentation, one from A. D. 1000 to 1300 (lagoon filling) and the other between A. D. 1700 and modern times (bay filling), are attributed to human land use around St. Ann's Bay. The earlier period of increased sedimentation corresponds to the introduction and expansion of native peoples to the north side of the island, who brought with them slash-and-burn agriculture. The later period of increased sedimentation in the bay corresponds to the intensification of agricultural use of the area by the Spanish and British. © 1993 John Wiley & Sons, Inc.  相似文献   

Radial Permeability Measurements for Shale Using Variable Pressure Gradients     

FAN Kunkun  SUN Renyuan  Derek ELSWORTH  DONG Mingzhe  LI Yajun  YIN Congbin  LI Yanchao  CHEN Zhongwei  WANG Chunguang 《《地质学报》英文版》2020,94(2):269-279

Shale gas is becoming an important component of the global energy supply, with permeability a critical controlling factor for long-term gas production. Obvious deviation may exist between helium permeability determined using small pressure gradient(SPG) methods and methane permeability obtained under actual field production with variable pressure gradients(VPG). In order to more accurately evaluate the matrix permeability of shale, a VPG method using real gas(rather than He) is established to render permeability measurements that are more representative of reservoir conditions and hence response. Dynamic methane production experiments were performed to measure permeability using the annular space in the shale cores. For each production stage, boundary pressure is maintained at a constant and the gas production with time is measured on the basis of volume change history in the measuring pump. A mathematical model explicitly accommodating gas desorption uses pseudo-pressure and pseudo-time to accommodate the effects of variations in pressuredependent PVT parameters. Analytical and semi-analytical solutions to the model are obtained and discussed. These provide a convenient approach to estimate radial permeability in the core by nonlinear fitting to match the semi-analytical solution with the recorded gas production data. Results indicate that the radial permeability of the shale determined using methane is in the range of 1×10-6– 1×10-5 mD and decreases with a decrease in average pore pressure. This is contrary to the observed change in permeability estimated using helium. Bedding geometry has a significant influence on shale permeability with permeability in parallel bedding orientation larger than that in perpendicular bedding orientation. The superiority of the VPG method is confirmed by comparing permeability test results obtained from both VPG and SPG methods. Although several assumptions are used, the results obtained from the VPG method with reservoir gas are much closer to reality and may be directly used for actual gas production evaluation and prediction, through accommodating realistic pressure dependent impacts.  相似文献   

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin     

David H. Roberts  Colm Ó Cofaigh  Colin K. Ballantyne  Matthew Burke  Richard C. Chiverrell  David J. A. Evans  Chris D. Clark  Geoff A. T. Duller  Jeremy Ely  Derek Fabel  David Small  Rachel K. Smedley  S. Louise Callard 《Boreas: An International Journal of Quaternary Research》2020,49(3):438-460

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.  相似文献   

Brachiopods from the maastrichtiandanian boundary sequence at Nye Kløv,Jylland, Denmark by Marianne Bagge-Johansen,Fossils & Strata,Oslo, No. 20, 1987. No. of pages: 99. Price: 170NDK (soft covers).     

Derek Ager 《Geological Journal》1989,24(1):82-82

  相似文献   

Land use and the spatial distribution of perfluoroalkyl compounds as measured in the plasma of bottlenose dolphins (Tursiops truncatus)     

Adams J  Houde M  Muir D  Speakman T  Bossart G  Fair P 《Marine environmental research》2008,66(4):430-437

The distribution of perfluoroalkyl compounds (PFCs) in the environment is well documented with higher concentrations observed in wildlife located in industrial and urban areas. This study examined the distribution of PFCs in relation to land use using blood samples collected from bottlenose dolphins during capture-release health assessment surveys conducted in Charleston, SC. The study area was partitioned into three subareas (ACW, CHS, and SRE) based upon habitat and land use characteristics. The ACW and CHS subareas are characterized by high degrees of industrial and urban land uses, while the SRE subarea is more residential and characterized by a lower degree of developed land use. Long-term monitoring data from photo-identification surveys were used to group bottlenose dolphins based on their proportions of sightings in the different subareas. Dolphins affiliated with both the ACW and CHS subareas were observed to have significantly higher mean plasma concentrations of perfluorooctane sulfonate (PFOS), perfluorodecanoic acid (PFDA) and perfluoroundecanoic acid (PFUnA) than those affiliated with the SRE subarea. Dolphins affiliated with the ACW subarea were found to have a significantly higher mean plasma concentration of PFUnA than those affiliated with the CHS subarea. Further examination of the distribution of the PFCs revealed positive correlations with developed land uses and negative correlations with wetland/marsh land cover. A positive correlation was also observed between PFUnA and agricultural land use. The variability and scale of the observed contaminant burdens have important implications for the conservation and management of living marine resources and illustrates the importance of long-term monitoring of free-ranging wildlife species.  相似文献