Chemical alteration of tephra in the depositional environment: theoretical stability modelling     

A. M. Pollard  S. P. E. Blockley  K. R. Ward 《第四纪科学杂志》2003,18(5):385-394

The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Simulating methane emission from a Chinese rice field as influenced by fertilizer and water level     

Z. Yu  X. Shangguan  D. Pollard  E. J. Barron 《水文研究》2003,17(17):3485-3501

Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH₄). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Palaeotemperature reconstructions of the European permafrost zone during marine oxygen isotope Stage 3 compared with climate model results     

Ko van Huissteden  Jef Vandenberghe  David Pollard 《第四纪科学杂志》2003,18(5):453-464

A palaeotemperature reconstruction based on periglacial phenomena in Europe north of approximately 51 °N, is compared with high‐resolution regional climate model simulations of the marine oxygen isotope Stage 3 (Stage 3) palaeoclimate. The experiments represent Stage 3 warm (interstadial), Stage 3 cold (stadial) and Last Glacial Maximum climatic conditions. The palaeotemperature reconstruction deviates considerably for the Stage 3 cold climate experiments, with mismatches up to 11 °C for the mean annual air temperature and up to 15 °C for the winter temperature. However, in this reconstruction various factors linking climate and permafrost have not been taken into account. In particular a relatively thin snow cover and high climatic variability of the glacial climate could have influenced temperature limits for ice‐wedge growth. Based on modelling the 0 °C mean annual ground temperature proves to be an appropriate upper temperature limit. Using this limit, mismatches with the Stage 3 cold climate experiments have been reduced but still remain. We therefore assume that the Stage 3 ice wedges were generated during short (decadal time‐scale) intervals of extreme cold climate, below the mean temperatures indicated by the Stage 3 cold climate model simulations. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

The transition between the Sheba Ridge and Owen Basin: rifting of old oceanic lithosphere     

Carol A. Stein  James R. Cochran 《Geophysical Journal International》1985,81(1):47-74

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Mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at Sheep Mountain, Wyoming     

Pablo F. Sanz  David D. Pollard  Patricia F. Allwardt  Ronaldo I. Borja 《Journal of Structural Geology》2008,30(9):1177-1191

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Reply to the comment on “Some numerical considerations in the geochemical analysis of distal microtephra”, by Nicholas J.G. Pearce,Charles A. Bendall and John A. Westgate     

A.M. Pollard  S.P.E. BlockleyC.S. Lane 《Applied Geochemistry》2008

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Environmental hydrogeology of the southern sector of the Samborombon Bay wetland,Argentina     

Eleonora Carol  Eduardo Kruse  Jorge Pousa 《Environmental Geology》2008,54(1):95-102

The Samborombon Bay wetland is located on the west margin of the Rio de la Plata estuary, in the Province of Buenos Aires, Argentina. This paper analyses the geological, geomorphologic, soil and vegetation characteristics of the southernmost sector of this wetland and their influence on surface water and groundwater. The study area presents three hydrologic units: coastal dunes, sand sheets and coastal plain. Coastal dunes and sand sheets are recharge zones of high permeability with well-drained, non-saline soils, and a few surface water flows. Changes in the water table are related to rainfall. Groundwater in coastal dunes is Ca–Mg–HCO₃ to Na–HCO₃, and of low salinity (590 mg/l). Groundwater in sand sheets is mainly Na–HCO₃ with a salinity of about 1,020 mg/l. The coastal plain exhibits medium to low permeability sediments, with submerged saline soils poorly drained. Groundwater is Na–Cl with a mean salinity of 16,502 mg/l. A surface hydrological network develops in the coastal plain. Surface water levels near the shoreline are affected by tidal fluctuations; far from the shoreline water accumulates because of poor drainage. Both sectors have Na–Cl water, but the former is more saline. Human intervention and sea level rise may affect the wetland severely.  相似文献   

Studies on the binding site of the cadmium-binding protein (CdBP) from the american oyster (Crassostrea virginica)     

Bruce Fowler  Carol Czop  David Elliott  Prakash Mistry  Colin Chignell 《Marine environmental research》1984,14(1-4)

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Manganese and iron distributions off central California influenced by upwelling and shelf width   总被引：4，自引：1，他引：4  

Zanna Chase  Kenneth S. Johnson  Virginia A. Elrod  Joshua N. Plant  Steve E. Fitzwater  Lisa Pickell  Carol M. Sakamoto 《Marine Chemistry》2005,95(3-4):235-254

In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L⁻¹ and dissolved manganese reached 25 nmol L⁻¹. In contrast, during non-upwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L⁻¹ and dissolved manganese concentrations were less than 5 nmol L⁻¹. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L⁻¹, and dissolvable iron concentrations of 12–20 nmol L⁻¹ in the upper 100 m in the north, compared to 3.5–2 nmol L⁻¹ Mn and 0.6 nmol L⁻¹ Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf.These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 μm, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling.Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L⁻¹ dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L⁻¹ dissolved iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out.  相似文献   

Characterization of zebrafish metallothionein gene promoter in a zebrafish caudal fin cell-line,SJD. 1     

Yan CH  Chan KM 《Marine environmental research》2002,54(3-5):335-339

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