Geochemical processes in the Onyx River, Wright Valley, Antarctica: Major ions, nutrients, trace metals   总被引：2，自引：0，他引：2  

William J. Green  Brian R. Stage  Shannon Wagers  Silvia Newell 《Geochimica et cosmochimica acta》2005,69(4):839-850

We present data on major ions, nutrients and trace metals in an Antarctic stream. The Onyx River is located in Wright Valley (77-32 S; 161-34 E), one of a group of ancient river and glacier-carved landforms that comprise the McMurdo Dry Valleys of Antarctica. The river is more than 30 km long and is the largest of the glacial meltwater streams that characterize this relatively ice-free region near the Ross Sea. The complete absence of rainfall in the region and the usually small contributions of glacially derived tributaries to the main channel make this a comparatively simple system for geochemical investigation. Moreover, the lack of human impacts, past or present, provides an increasingly rare window onto a pristine aquatic system.For all major ions and silica, we observe increasing concentrations with distance from Lake Brownworth down to the recording weir near Lake Vanda. Chemical weathering rates are unexpectedly high and may be related to the rapid dissolution of ancient carbonate deposits and to the severe physical weathering associated with the harsh Antarctic winter. Of the nutrients, nitrate and dissolved reactive phosphate appear to have quite different sources. Nitrate is enriched in waters near the Lower Wright Glacier and may ultimately be derived from stratospheric sources; while phosphate is likely to be the product of chemical weathering of valley rocks and soils. We confirm the work of earlier investigations regarding the importance of the Boulder Pavement as a nutrient sink.Dissolved Mn, Fe, Ni, Cu, and Cd are present at nanomolar levels and, in all cases, the concentrations of these metals are lower than in average world river water. We hypothesize that metal uptake and exchange with particulate phases along the course of the river may serve as a buffer for the dissolved load. Concurrent study of these three solute classes points out significant differences in the mechanisms and sites of their removal from the Onyx River.  相似文献   

DISEASE MAPPING AND EARLY THEORIES OF YELLOW FEVER     

Gary W. Shannon 《The Professional geographer》1981,33(2):221-227

For more than two hundred years yellow fever epidemics periodically raged in some of the largest cities of the United States. Physicians of the late 18th and early 19th centuries were ignorant of the source, mode of tranmission, and method of cure. Debate centered upon the issues of foreign vs. local origin, and the contagious vs. noncontagious nature of the disease. Several disease maps were produced in attempts to support these theories. The maps purported to demonstrate the relationship between miasma and yellow fever. Three early maps that were used for deriving and supporting theories of local origin and noncontagion are presented here.  相似文献   

Predicting pipeline corrosion in heterogeneous soils using numerical modelling and artificial neural networks     

Azoor  Rukshan  Deo  Ravin  Shannon  Benjamin  Fu  Guoyang  Ji  Jian  Kodikara  Jayantha 《Acta Geotechnica》2022,17(4):1463-1476

Acta Geotechnica - The influence of soil heterogeneity on the corrosion of underground metallic pipelines and the resulting evolution of localised corrosion patches were examined. A field-validated...  相似文献   

Responses of fish populations in the Benguela ecosystem to environmental change     

Shannon  L. V.; Crawford  R. J. M.; Brundrit  G. B.; Underhill  L. G. 《ICES Journal of Marine Science》1988,45(1):5-12

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Regime shifts in upwelling ecosystems: observed changes and possible mechanisms in the northern and southern Benguela     

Philippe Cury  Lynne Shannon   《Progress in Oceanography》2004,60(2-4):223

A regime shift is considered to be a sudden shift in structure and functioning of a marine ecosystem, affecting several living components and resulting in an alternate state. According to this definition, regime shifts differ from species replacement or alternation of species at similar trophic levels, whereby the ecosystem is not necessarily significantly altered in terms of its structure and function; only its species composition changes. This paper provides an overview of regime shifts, species replacements and alternations that have been observed in the northern and southern Benguela ecosystems over the past few decades. Bottom-up control, initiating and sustaining regime shifts or species replacements via environmental forcing, is documented for both the southern and the northern Benguela ecosystems. Fishing (a case of top-down control) appears to have played an important role in regime shift processes in the Namibian ecosystem. Very low biomass levels of exploited fish stocks associated with less efficient energy transfer in the northern Benguela are indicative of a regime shift. Very high biomass levels have been reached in the southern Benguela in the 2000s. However the alternation between sardine and anchovy that has been observed in the southern Benguela over the last two decades appears not to have had major effects on the overall functioning of the ecosystem. The consequences of regime shifts for exploitation are highlighted, suggesting that fisheries managers should move towards a more effective ecosystem approach to fisheries.  相似文献   

Trophodynamic indicators for an ecosystem approach to fisheries   总被引：8，自引：3，他引：8  

Cury  P.M.; Shannon  L.J.; Roux  J-P.; Daskalov  G.M.; Jarre  A.; Moloney  C.L.; Pauly  D. 《ICES Journal of Marine Science》2005,62(3):430-442

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Anomalous Cenozoic subsidence along the ‘passive’ continental margin from Ireland to mid-Norway     

S. Ceramicola  M. Stoker  D. Praeg  P.M. Shannon  L. De Santis  R. Hoult  B.O. Hjelstuen  S. Laberg  A. Mathiesen 《Marine and Petroleum Geology》2005,22(9-10):1045

Two-dimensional flexural backstripping and thermal modelling (assuming uniform stretching and cooling) is applied to four interpreted, depth-converted seismic profiles across the Rockall, Faroe–Shetland and Vøring basins, along 1600 km of the Atlantic continental margin of NW Europe. The results reveal a significant discrepancy between the modelled palaeo-depths for the base of the Cenozoic succession and those proven by geological evidence at control points (subaerial conditions or depositional depth ranges in wells). The discrepancy is of Rm-scale, much larger than the possible range of parameter error determined by sensitivity tests (up to 0.5 km). Assuming a Cretaceous rift episode (100 Ma), the discrepancy is at least 1.7 km in the Rockall Basin, up to 2.1 km in the Faroe–Shetland Basin and at least 1 km in the Vøring Basin (which also contains evidence of kilometre-scale uplift of the inner margin). Assuming (unproven) a second rift in the early Cenozoic (60 Ma), the discrepancy remains of kilometre-scale in the Rockall and Faroe–Shetland basins. The restorations also provide evidence of uplift, both above compressive structures and across the modelled profiles as seaward rotations of palaeo-bathymetric records. The palaeo-bathymetric discrepancy corresponds to an anomaly in subsidence that is the cumulative product of all the tectonic episodes that have affected the NW European margin, and may incorporate both permanent effects of the last episode of lithospheric extension and transient responses to the interaction of the margin with mantle convective flow. Any explanation must accommodate both the large magnitude of anomalous subsidence along the margin and evidence of its episodic character.  相似文献   

High-pressure synthesis,long-term stability of single crystals of diboron trioxide,B<Subscript>2</Subscript>O<Subscript>3</Subscript>, and an empirical electronic polarizability of <Superscript>[3]</Superscript>B<Superscript>3+</Superscript>     

Manfred Burianek  Johannes Birkenstock  Philipp Mair  Volker Kahlenberg  Olaf Medenbach  Robert D. Shannon  Reinhard X. Fischer 《Physics and Chemistry of Minerals》2016,43(7):527-534

Single crystals of B₂O₃ are needed for the precise determination of the refractive indices used to calculate the electronic polarizability α of 3-coordinated boron. The α(B) values in turn are used to predict mean refractive indices of borate minerals. Since the contribution of boron to the total polarizability of a mineral is very low, the synthetic compound B₂O₃ represents an ideal model system because of its high molar content of boron. Millimeter-sized crystals were synthesized at 1 GPa in a piston-cylinder apparatus. The samples were heated above the liquidus (800 °C), subsequently cooled at 15 °C/h to 500 °C and finally quenched. The refractive indices were determined by the immersion method using a microrefractometer spindle stage. The refractive indices n _o = 1.653 (3) and n _e = 1.632 (3) correspond to a total polarizability for B₂O₃ of α = 4.877 Å³. These values were used to determine the electronic polarizability of boron of α(B) = 0.16 Å³. Although the surface of the B₂O₃ crystals was coated with a hydrous film immediately after being exposed to air, its bulk crystallinity is retained for a period of at least 2 months.  相似文献   

Sediment storage and transport in Pancho Rico Valley during and after the Pleistocene‐Holocene transition,Coast Ranges of central California (Monterey County)     

Antonio F. García  Shannon A. Mahan 《地球表面变化过程与地形》2009,34(8):1136-1150

Factors influencing sediment transport and storage within the 156·6 km² drainage basin of Pancho Rico Creek (PRC), and sediment transport from the PRC drainage basin to its c. 11 000 km² mainstem drainage (Salinas River) are investigated. Numeric age estimates are determined by optically stimulated luminescence (OSL) dating on quartz grains from three sediment samples collected from a ‘quaternary terrace a (Qta)’ PRC terrace/PRC‐tributary fan sequence, which consists dominantly of debris flow deposits overlying fluvial sediments. OSL dating results, morphometric analyses of topography, and field results indicate that the stormy climate of the Pleistocene‐Holocene transition caused intense debris‐flow erosion of PRC‐tributary valleys. However, during that time, the PRC channel was backfilled by Qta sediment, which indicates that there was insufficient discharge in PRC to transport the sediment load produced by tributary‐valley denudation. Locally, Salinas Valley alluvial stratigraphy lacks any record of hillslope erosion occurring during the Pleistocene‐Holocene transition, in that the alluvial fan formed where PRC enters the Salinas Valley lacks lobes correlative to Qta. This indicates that sediment stripped from PRC tributaries was mostly trapped in Pancho Rico Valley despite the relatively moist climate of the Pleistocene‐Holocene transition. Incision into Qta did not occur until PRC enlarged its drainage basin by c. 50% through capture of the upper part of San Lorenzo Creek, which occurred some time after the Pleistocene‐Holocene transition. During the relatively dry Holocene, PRC incision through Qta and into bedrock, as well as delivery of sediment to the San Ardo Fan, were facilitated by the discharge increase associated with stream‐capture. The influence of multiple mechanisms on sediment storage and transport in the Pancho Rico Valley‐Salinas Valley system exemplifies the complexity that (in some instances) must be recognized in order to correctly interpret terrestrial sedimentary sequences in tectonically active areas. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Examining barriers and opportunities for sustainable adaptation to climate change in Interior Alaska     

Shannon M. McNeeley 《Climatic change》2012,111(3-4):835-857

Human adaptation to climate change is comprised of “adjustments” in response to (or anticipation of) climatic impacts. Adaptation does not necessarily imply favorable or equitable change, nor does it automatically imply sustainable use of ecosystems. “Sustainable adaptation” in this case implies strategic, collective action to respond to or anticipate harmful climate change to reduce disruption to key resource flows and adverse effects on general well-being. This research examined social-ecological system responses to recent warming trends in the remote northwest region of Interior Alaska using a unique vulnerability and adaptive capacity assessment (VA) approach that integrated indigenous observations and understanding of climate (IC) with western social and natural sciences. The study found that Alaska Native communities that were historically highly mobile and flexible across the landscape for subsistence hunting are increasingly restricted by the institutional rigidity of the regulatory system for wildlife and subsistence management. This has resulted in negative impacts to game harvest access and success threatening food security and community well-being. This suggests that policies limiting the ability of natural resource-dependent societies to be flexible, diversify, or innovate can threaten livelihoods and exacerbate vulnerability. Nevertheless, opportunities for sustainable adaptation exist where wildlife management is adaptive and includes an understanding of and response to climate variability and slow-onset climate change with the human dimensions of subsistence hunting for more effective “in-season” management.  相似文献