The early impact histories of meteorite parent bodies     

Thomas M. Davison  David P. O'Brien  Fred J. Ciesla  Gareth S. Collins 《Meteoritics & planetary science》2013,48(10):1894-1918

We have developed a statistical framework that uses collisional evolution models, shock physics modeling, and scaling laws to determine the range of plausible collisional histories for individual meteorite parent bodies. It is likely that those parent bodies that were not catastrophically disrupted sustained hundreds of impacts on their surfaces—compacting, heating, and mixing the outer layers; it is highly unlikely that many parent bodies escaped without any impacts processing the outer few kilometers. The first 10–20 Myr were the most important time for impacts, both in terms of the number of impacts and the increase of specific internal energy due to impacts. The model has been applied to evaluate the proposed impact histories of several meteorite parent bodies: up to 10 parent bodies that were not disrupted in the first 100 Myr experienced a vaporizing collision of the type necessary to produce the metal inclusions and chondrules on the CB chondrite parent; around 1–5% of bodies that were catastrophically disrupted after 12 Myr sustained impacts at times that match the heating events recorded on the IAB/winonaite parent body; more than 75% of 100 km radius parent bodies, which survived past 100 Myr without being disrupted, sustained an impact that excavates to the depth required for mixing in the outer layers of the H‐chondrite parent body; and to protect the magnetic field on the CV chondrite parent body, the crust would have had to have been thick (approximately 20 km) to prevent it being punctured by impacts.  相似文献   

Micro-scale biogeochemical heterogeneity in sediments: A review of available technology and observed evidence     

Anthony Stockdale  William Davison  Hao Zhang 《Earth》2009,92(1-2):81-97

The hypothesis that reducing conditions exist in localized zones of high organic matter, termed microniches, was first suggested over a century ago, but only relatively recently have high-resolution techniques been available to investigate them. In any sediment containing benthic fauna, bioturbation affects the distribution of a number of redox-sensitive components. Direct faecal deposition and the death of fauna may be expected to cause particles of labile organic matter (microniches) to be distributed heterogeneously within the sediment. This review discusses the significance and future direction of microniche studies by considering, exclusively, data obtained on a sub-mm scale that provide significant evidence for the existence and properties of microniches. Microelectrodes and planar optodes have shown the significant effect of burrowing organisms on localized O₂ distributions and revealed distinct depletions in O₂ due to microniches. Localized increases in pCO₂ and decreases in pH measured by optodes were attributed to elevated activity at microniches. Diffusive gradients in thin-films have shown isolated supersaturation of metals and sulphide, providing evidence for possible simultaneous oxidation of organic matter by sulphate and iron oxides. The stochastic nature of these data and the lack of information for the same precise location hinders interpretation in terms of sediment diagenesis. If microniches are known to account for a significant proportion of organic matter degradation, re-examination of the current understanding of sedimentary diagenesis may be needed. Further investigation on the distribution and frequency of microniches is required, including a wider range of analytes, in order to estimate their cumulative effect on element diagenesis, immobilisation/remobilisation processes and ultimately pollutant fate.  相似文献   

In-situ determination of dissolved iron production in recent marine sediments   总被引：3，自引：0，他引：3  

Michael D. Krom  Robert J. G. Mortimer  Simon W. Poulton  Peter Hayes  Ian M. Davies  William Davison  Hao Zhang 《Aquatic Sciences - Research Across Boundaries》2002,64(3):282-291

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Seasonal transformations and movements of iron in a productive English lake with deep-water anoxia   总被引：2，自引：1，他引：1  

W. Davison  S. I. Heaney  J. F. Talling  E. Rigg 《Aquatic Sciences - Research Across Boundaries》1980,42(2):196-224

The seasonal and depth distributions of the concentrations of total and soluble iron in a seasonally anoxic lake basin are described for two years. Transport of iron is related to hydrodynamics and fluxes of iron both within and to and from the lake are calculated. The results enabled the construction of a schematic model for iron movement; it is critically dependent upon the many rate processes involved.  相似文献   

Spatial and Temporal Hydrochemical Variations in a Semiconfined Buried Channel Aquifer: Esterhazy, Saskatchewan, Canada     

C. C. Davison  J. A. Vonhof 《Ground water》1978,16(5):341-350

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Formation of pyrite in freshwater sediments: Implications for C/S ratios     

W. Davison  J.P. Lishman  J. Hilton 《Geochimica et cosmochimica acta》1985,49(7):1615-1620

The vertical distribution of pyrite, acid volatile sulphide (AVS), carbon, and total S (St) were determined directly in the sediments of three lakes of different trophic status. The results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters. It appears to form chiefly in reducing sediments which are subject to oxidizing influences, by either a low turnover of organic carbon or periodic incursions of oxygen. Although there are high concentrations of AVS in the near-surface sediments of productive lakes, very little is diagenetically converted to pyrite.The feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed. New values for FeS₂/FeS of 0.5-5 show that this ratio does not provide a reliable test. Values of C/Sp, where Sp represents pyrite sulphur, lie within the range of 160–700 and are much higher than previously measured ratios of C/S_t of 1–50. These new determinations show that, if pyrite sulphur is unequivocally measured, C/S ratios may be a more sensitive indicator of salinity than had been previously thought.  相似文献   

Similarities in the behaviour of Chernobyl derived Ru-103, Ru-106 and Cs-137 in two freshwater lakes     

J. Hilton  W. Davison  J. Hamilton-Taylor  M. Kelly  F. R. Livens  E. Rigg  D. L. Singleton 《Aquatic Sciences - Research Across Boundaries》1994,56(2):133-144

Measurements of the concentrations of Ru-103 and Ru-106 in the water column and sediments of two productive temperate lakes were made during an eight month period following the Chernobyl accident. Ru-103 accumulation in the sediments began simultaneously with that of Cs-134 and a constant ratio (approx. 2.0 on a decay corrected basis) of Ru-103/Cs-137 was observed in surface waters and sediments throughout the measurement period. The ratio is not statistically different from that reported for atmospheric input and for epilimnetic water. The results suggest that the processes of removal of radioactive ruthenium and caesium from the mixed water column to the sediments are the same.Enhancement of Cs, compared with Ru, in the isolated bottom waters of one of the lakes in the summer months provided evidence for Cs release from the sediments during a period of hypolimnetic anoxia.  相似文献   

Precipitation variability and its relationship to hydrologic variability in Alberta   总被引：1，自引：0，他引：1  

Davison Mwale  Thian Yew Gan  Kevin Devito  Carl Mendoza  Uldis Silins  Richard Petrone 《水文研究》2009,23(21):3040-3056

By applying wavelet‐based empirical orthogonal function (WEOF) analysis to gridded precipitation (P) and empirical orthogonal function (EOF) analysis to gridded air temperature (T), potential evapotranspiration (PET), net precipitation (P‐PET) and runoff (Q), this paper examines the spatial, temporal and frequency patterns of Alberta's climate variability. It was found that only WEOF‐based precipitation patterns, possibly modulated by El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation(PDO), delineated Alberta into four major regions which geographically represent northern Alberta Boreal forests, southern Alberta grasslands and Aspen Parklands and the Rocky Mountains and Foothills. The leading mode of wavelet‐based precipitation variability WPC1 showed that between 1900 and 2000, a wet climate dominated northern Alberta with significant 4–8, 11 and 25‐year periodic cycles, while the second mode WPC2 showed that between 1960 and 2000, southern Alberta grasslands were characterized by decreasing precipitation, dominated by 11‐year cycles, and the last two modes WPC3 and WPC4 were characterized by 4–7 and 25‐year cycles and both delineated regions where moisture from the Pacific Ocean penetrated the Rocky Mountains, accounted for much of the sub‐alpine climate. These results show that WEOF is superior to EOF in delineating Alberta precipitation variability to sub‐regions that more closely agree with its eco‐climate regions. Further, it was found that while WPC2 could not explain runoff variations in southern Alberta, WPC1, WPC3 and WPC4 accounted for runoff variability in their respective sub‐regions. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

PSYCHIC – A process-based model of phosphorus and sediment transfers within agricultural catchments. Part 2. A preliminary evaluation     

J. Strmqvist  A.L. Collins  P.S. Davison  E.I. Lord 《Journal of Hydrology》2008,350(3-4):303-316

This paper describes the preliminary evaluation of the PSYCHIC catchment scale (Tier 1) model for predicting the mobilisation and delivery of phosphorus (P) and suspended sediment (SS) in the Hampshire Avon (1715 km²) and Herefordshire Wye (4017 km²) drainage basins, in the UK, using empirical data. Phosphorus and SS transfers to watercourses in the Wye were predicted to be greater than corresponding delivery in the Avon; SS, 249 vs 33 kg ha⁻¹ yr⁻¹; DP, 2.57 vs 1.26 kg ha⁻¹ yr⁻¹; PP, 2.20 vs 0.56 kg ha⁻¹ yr⁻¹. The spatial pattern of the predicted transfers was relatively uniform across the Wye drainage basin, whilst in the Avon, delivery to watercourses was largely confined to the river corridors and small areas of drained land. Statistical performance in relation to predicted exports of P and SS, using criteria for relative error (RE) and root mean square error (RMSE), reflected the potential shortcomings associated with using longer-term climate data for predicting shorter-term (2002–2004) catchment response and the need to refine calculations of point source contributions and to incorporate additional river basin processes such as channel bank erosion and in-stream geochemical processing. PSYCHIC is therefore best suited to characterising longer-term catchment response.  相似文献   

Variations in the depth distribution of phosphorus in soil profiles and implications for model-based catchment-scale predictions of phosphorus delivery to surface waters     

P.N. Owens  L.K. Deeks  G.A. Wood  M.J. Betson  E.I. Lord  P.S. Davison 《Journal of Hydrology》2008,350(3-4):317-328

The PSYCHIC process-based model for predicting sediment and phosphorus (P) transfer within catchments uses spatial data on soil-P derived from the National Soil Inventory (NSI) data set. These soil-P values are based on bulked 0–15 cm depth and do not account for variations in soil-P with depth. We describe the depth distribution of soil-P (total and Olsen) in grassland and arable soils for the dominant soil types in the two PSYCHIC study catchments: the Avon and the Wye, UK. There were clear variations in soil-P (particularly Olsen-P) concentrations with depth in untilled grassland soils while concentrations of total-P were broadly constant within the plough layer of arable soils. Concentrations of Olsen-P in arable soils, however, exhibited maximum values near the soil surface reflecting surface applications of fertilisers and manures between consecutive ploughing events. When the soil-P concentrations for the surface soil (0–5 cm average) were compared to both the profile-averaged (0–15 cm) and the NSI (0–15 cm) values, those for the surface soil were considerably greater than those for the average 0–15 cm depth. Modelled estimates of P loss using the depth-weighted average soil-P concentrations for the 0–5 cm depth layer were up to 14% greater than those based on the NSI data set due to the preferential accumulation of P at the soil surface. These findings have important implications for the use of soil-P data (and other data) in models to predict P losses from land to water and the interpretation of these predictions for river basin management.  相似文献