Handaxe and non‐handaxe assemblages during Marine Isotope Stage 11 in northern Europe: recent investigations at Barnham,Suffolk, UK          下载免费PDF全文

Nick Ashton  Simon G. Lewis  Simon A. Parfitt  Rob J. Davis  Chris Stringer 《第四纪科学杂志》2016,31(8):837-843

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Long‐term streamflow trends in the middle reaches of the Yellow River Basin: detecting drivers of change          下载免费PDF全文

Zhaoliang Gao  Lu Zhang  Xiaoping Zhang  Lei Cheng  Nick Potter  Tim Cowan  Wenju Cai 《水文研究》2016,30(9):1315-1329

The catchments in the Loess Plateau, in China's middle reaches of the Yellow River Basin, experienced unprecedented land use changes in the last 50 years as a result of large‐scale soil conservation measure to control soil erosion. The climate of the region also exhibited some levels of change with decreased precipitation and increased temperature. This study combined the time‐trend analysis method with a sensitivity‐based approach and found that annual streamflow in the Loess Plateau decreased significantly since the 1950s and surface runoff trends appear to dominate the streamflow trends in most of the catchments. Annual baseflow exhibited mostly downward trends, but significant upward trends were also observed in 3 out of 38 gauging stations. Mean annual streamflow during 1979?2010 decreased by up to 65% across the catchments compared with the period of 1957?1978, indicating significant changes in the hydrological regime of the Loess Plateau. It is estimated that 70% of the streamflow reduction can be attributed to land use change, while the remaining 30% is associated with climate variability. Land use change because of the soil conservation measures and reduction in precipitation are the key drivers for the observed streamflow trends. These findings are consistent with results of previous studies for the region and appear to be reasonable given the accelerated level of the soil conservation measures implemented since the late 1970s. Changes in sea surface temperature in the Pacific Ocean, as indicated by variations in El Niño–Southern Oscillation and phase shifts of the Pacific Decadal Oscillation, appear to have also affected the annual streamflow trends. The framework described in this study shows promising results for quantifying the effects of land use change and climate variability on mean annual streamflow of catchments within the Loess Plateau. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Anisotropy is Everywhere,to See,to Measure,and to Model     

Nick Barton  Eda Quadros 《Rock Mechanics and Rock Engineering》2015,48(4):1323-1339

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Conceptualising the PhD as preparing for academic practice in geography     

Nick Hopwood  Lynn McAlpine 《GeoJournal》2015,80(2):203-207

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http://www.sciencedirect.com/science/article/pii/S1674987114001583   总被引：4，自引：3，他引：1  

Christopher J.Spencer  Peter A.Cawood  Chris J.Hawkesworth  Anthony R.Prave  Nick M.W.Roberts  Matthew S.A.Horstwood  Martin J.Whitehouse  EIMF 《地学前缘(英文版)》2015,6(3):357-372

Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of 'peaks' and 'troughs'.The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events,or preferential preservation of continental crust during global collisional orogenesis.The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks.Here these two end-member models(episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis) are assessed using U-Pb,Hf,and O analysis of detrital zircons from sedimentary successions deposited during the ~ 1.3-1.1Ca accretionary.~1.1-0.9 Ga collisional,and 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland.The pre-collisional,accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology.Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from~ 1500 to 1100 Ma.However,post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma,similar to that observed in modern North American river sediments.Zircons within the post-orogenic sedimentary successions have progressively lower εHf and higherδ~(18)O values from ~ 1800 to ~ 1200 Ma whereupon they have higher εHf and δ~(18) within the dominant1085-985 Ma age peak.Furthermore,the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material.The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Crenville Orogeny,which is a typical collisional orogenic belt.The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision.This temporal change in provenance highlights that the source(s),from which detrital zircons within syn-orogenic strata were derived,was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle.This may reflect some combination of tectonic burial,erosion,or possibly recycling into the mantle by tectonic erosion of the source(s).During continental collision,the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins.This tectonic isolation combined with sedimentaiy recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments.These results imply that zircon age peaks,which developed in conjunction with supercontinents,are the product of selective crustal preservation resulting from collisional orogenesis.  相似文献   

Geochemistry and chronology of the Bunburra Rockhole ungrouped achondrite          下载免费PDF全文

Lev J. Spivak‐Birndorf  Audrey Bouvier  Gretchen K. Benedix  Samantha Hammond  Gregory A. Brennecka  Kieren Howard  Nick Rogers  Meenakshi Wadhwa  Philip A. Bland  Pavel Spurný  Martin C. Towner 《Meteoritics & planetary science》2015,50(5):958-975

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca‐phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The ²⁶Al‐²⁶Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of ²⁶Al, but a slight excess in the radiogenic ²⁶Mg in a bulk sample allows the determination of a model ²⁶Al‐²⁶Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The ²⁰⁷Pb‐²⁰⁶Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole‐rock isochron provides a re‐equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.  相似文献   

Load or concentration,logged or unlogged? Addressing ten years of uncertainty in neural network suspended sediment prediction   总被引：1，自引：0，他引：1  

Nick J. Mount  Robert J. Abrahart 《水文研究》2011,25(20):3144-3157

This paper addresses current inconsistencies in methodological approaches for neural network modelling of suspended sediment. An expansion in the number of case studies being published over the last decade has yet to result in agreed guidelines on whether suspended sediment load or concentration should be modelled, and whether log‐transformation of data is either necessary or potentially beneficial. This contrasts with the well‐recognized guidelines that direct traditional sediment rating curve studies. The paper reports a comprehensive set of single‐input single‐output neural network suspended sediment modelling experiments performed on two catchments in Puerto Rico. It examines the impact of internal complexity, input variable choice and data transformation on the form, consistency and physical rationality of model outputs, the existence of localized overfitting and the usefulness of global performance metrics. Sound guidance on whether to model sediment load or concentration, and whether to model log‐transformed data is provided. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Radiocarbon and 230Th data reveal rapid redistribution and temporal changes in sediment focussing at a North Atlantic drift     

Gesine Mollenhauer  Jerry F. McManus  Thomas Wagner  I. Nick McCave  Timothy I. Eglinton 《Earth and Planetary Science Letters》2011,301(1-2):373-381

In locations of rapid sediment accumulation receiving substantial amounts of laterally transported material the timescales of transport and accurate quantification of the transported material are at the focus of intense research. Here we present radiocarbon data obtained on co-occurring planktic foraminifera, marine haptophyte biomarkers (alkenones) and total organic carbon (TOC) coupled with excess Thorium-230 (²³⁰Th_xs) measurements on four sediment cores retrieved in 1649–2879 m water depth from two such high accumulation drift deposits in the Northeast Atlantic, Björn and Gardar Drifts. While ²³⁰Th_xs inventories imply strong sediment focussing, no age offsets are observed between planktic foraminifera and alkenones, suggesting that redistribution of sediments is rapid and occurs soon after formation of marine organic matter, or that transported material contains negligible amounts of alkenones. An isotopic mass balance calculation based on radiocarbon concentrations of co-occurring sediment components leads us to estimate that transported sediment components contain up to 12% of fossil organic matter that is free of or very poor in alkenones, but nevertheless appears to consist of a mixture of fresh and eroded fossil material. Considering all available constraints to characterize transported material, our results show that although focussing factors calculated from bulk sediment ²³⁰Th_xs inventories may allow useful approximations of bulk redeposition, they do not provide a unique estimate of the amount of each laterally transported sediment component. Furthermore, our findings provide evidence that the occurrence of lateral sediment redistribution alone does not always hinder the use of multiple proxies but that individual sediment fractions are affected to variable extents by sediment focussing.  相似文献   

Trial by Fire: natural hazards,mixed-methods and cultural research     

Christine Eriksen  Nick Gill  Ross Bradstock 《The Australian geographer》2011,42(1):19-40

This paper considers the issues of research ‘relevance’ and ‘use’ to reflect upon a cultural geography research project on bushfire that did not begin with any specific aim of being useful to policy makers but which has garnered considerable and ongoing interest from a broad audience. It provides an example of how the integration of quantitative and qualitative research methods and data can enhance research into cultural aspects of natural hazards whilst simultaneously playing a key role in ensuring that the research results are of interest to a wide range of groups. Using a mixed-methods research approach was found to provide insight into complex factors that influence attitudes and actions towards bushfire amongst diverse landholders in rural–urban interface areas in south-east Australia. We argue that mixed-methods research is a powerful tool in building and enhancing a cultural geography that has policy relevance, retains analytical depth, and is acceptable to risk managers. The ability of cultural geography through mixed-methods research to illuminate how socio-cultural processes are central to environmental attitudes and preparedness behaviour has direct relevance to recent international discussions of how to manage the vulnerability of the growing number of people living in bushfire-prone rural–urban interface areas.  相似文献   

The effect of hedgerow wild-margins on topsoil hydraulic properties,and overland-flow incidence,magnitude and water-quality     

Ethan E. Wallace  Gareth McShane  Wlodek Tych  Ann Kretzschmar  Thomas McCann  Nick A. Chappell 《水文研究》2021,35(3):e14098

Overland and shallow-subsurface flows from agricultural catchments are believed to contribute towards flood-risk and water-quality degradation across the globe. Hedgerows are commonplace agricultural features that may disrupt these rapid hydrological pathways. Research into the hydrological functioning of hedgerows is very limited however, with no field-based quantitative comparison of overland-flows within hedgerows versus other land-uses. This research is the first globally to observe changes in overland-flow incidence, volume and water-quality, alongside topsoil hydraulic and physico-chemical properties, induced by a hedgerow and adjoining wild-margin within a grassland landscape. Observations were conducted within two replicated paired-plots between a hedgerow wild-margin and a bordering pasture, within Cumbria, UK. Compared to adjacent pasture, hedge-margins significantly reduced topsoil dry bulk-density and increased porosity, and significantly increased the topsoil median permeability by a factor of 22–27. Overland-flow models, based on direct observations, highlight that hedge-margins are slower to produce overland-flows than pastures, requiring an equal or greater amount of saturation before the onset of overland-flow generation. Hedge-margins resultantly produced less overland-flow volume, likely due to increased infiltration, percolation and/or evapotranspiration. Soil saturation models, also based on direct observations, confirm pastures saturate faster than hedge-margins, with hedge-margins having extremely variable dynamics in relation to precipitation, whereas pastures have more moderate and consistent dynamics. Overland-flow water-quality from ‘wash-off’ experiments highlight that hedge-margins may store substantially more nitrate (70–260%), nitrate-nitrite (640–650%), and loose sediment (540–3970%) on the ground surface compared to pastures; although further experimentation is needed to determine contaminant mobilization potential.  相似文献