U–Pb dating of calcite–aragonite layers in speleothems from hominin sites in South Africa by MC-ICP-MS     

Robyn Pickering  Jan D. Kramers  Tim Partridge  Janos Kodolanyi  Thomas Pettke 《Quaternary Geochronology》2010,5(5):544-558

Speleothems are found in association with hominin fossil-bearing cave deposits in South Africa and can be used to provide valuable chronological constraints. Such material is generally too old for U–Th dating and, although U–Pb geochronology presents a suitable alternative, bulk U concentrations are typically too low (<0.05 μg/g) to provide useful ages. For this reason, we used a simple non-invasive beta-scanner imaging screening step to identify U-rich (≥1 μg/g) domains that could be analyzed with MC-ICP-MS techniques to provide U–Pb ages. We demonstrate the technique using samples from Sterkfontein cave that exhibit infrequent <1 cm-thick layers with U concentrations ≥1 μg/g. Relict aragonite needles are found exclusively in these U-rich layers. We analyzed material from the same flowstone suite as Walker et al. (2006) and obtained a U–Pb age of ～2.3 Ma that agrees well with their estimate of 2.24 ± 0.09 Ma. We also obtained similar U–Pb (0.164 ± 0.026 to 0.200 ± 0.052 Ma) and U–Th (0.148 ± 0.003 Ma) ages for another sample exhibiting U-rich layers. We recognize that open-system behaviour during the partial transformation of aragonite to calcite is a potential problem and argue, on the basis of geochemistry and age consistencies, that recrystallization took place rapidly after speleothem formation and did not significantly affect the U–Pb ages.  相似文献   

Rates of CO2 efflux and changes in DOC concentration resulting from the addition of POC to the fluvial system in peatlands     

Claire S. Goulsbra  Martin G. Evans  Tim E. H. Allott 《Aquatic Sciences - Research Across Boundaries》2016,78(3):477-489

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The fluvial flux of particulate organic matter from the UK: the emission factor of soil erosion   总被引：1，自引：0，他引：1       下载免费PDF全文

Fred Worrall  Tim P. Burt  Nicholas J.K. Howden 《地球表面变化过程与地形》2016,41(1):61-71

Soil erosion has been identified as a potential global carbon sink since eroded organic matter is replaced at source and eroded material is readily buried. However, this argument has relied on poor estimates of the total fate of in‐transit particulates and could erroneously imply soil erosion could be encouraged to generate carbon stores. These previous estimates have not considered that organic matter can also be released to the atmosphere as a range of greenhouse gases, not only carbon dioxide (CO₂), but also the more powerful greenhouse gases methane (CH₄) and nitrous oxide (N₂O). As soil carbon lost by erosion is only replaced by uptake of CO₂, this could represent a considerable imbalance in greenhouse gas warming potential, even if it is not significant in terms of overall carbon flux. This work therefore considers the flux of particulate organic matter through UK rivers with respect to both carbon fluxes and greenhouse gas emissions. The results show that, although emissions to the atmosphere are dominated by CO₂, there are also considerable fluxes of CH₄ and N₂O. The results suggest that soil erosion is a net source of greenhouse gases with median emission factors of 5.5, 4.4 and 0.3 tonnes CO_2eq/yr for one tonne of fluvial carbon, gross carbon erosion and gross soil erosion, respectively. This study concludes that gross soil erosion would therefore only be a net sink of both carbon and greenhouse gases if all the following criteria are met: the gross soil erosion rate were very low (<91 tonnes/km²/yr); the eroded carbon were completely replaced by new soil organic matter; and if less than half of the gross erosion made it into the stream network. By establishing the emission factor for soil erosion, it becomes possible to properly account for the benefits of good soil management in minimizing losses of greenhouse gases to the atmosphere as a by‐product of soil erosion. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Closed Basin Brine Evolution and the Influence of Ca–Cl Inflow Waters: Death Valley and Bristol Dry Lake California, Qaidam Basin, China, and Salar de Atacama, Chile     

Tim K. Lowenstein  François Risacher 《Aquatic Geochemistry》2009,15(1-2):71-94

Diagenetic-hydrothermal brines, here called “hydrothermal Ca–Cl brines,” have compositions that reflect interactions between groundwaters and rocks or sediments at elevated temperatures. Hydrothermal Ca–Cl brines reach the surface by convection-driven or topographically driven circulation, and discharge as springs or seeps along fault zones to become important inflow waters in many tectonically active closed basins. Case studies from (1) Qaidam Basin, China, (2) Death Valley, California, (3) Salar de Atacama, Chile, and (4) Bristol Dry Lake, California illustrate that hydrothermal Ca–Cl inflow waters have influenced brine evolution in terms of major ion chemistries and mineral precipitation sequences. All four basins are tectonically active; three (Death Valley, Salar de Atacama, and Qaidam Basin) have well-documented Ca–Cl spring inflow and Holocene faulting. Bristol Dry Lake has young volcanic deposits and Salar de Atacama has an active stratovolcano on its eastern margin, indicating subsurface magma bodies. A midcrustal magma chamber has been identified in southern Death Valley. Volcanism and faulting in these closed basins provides the heat source for hydrothermal-diagenetic processes and the energy and pathways to deliver these waters to the surface.  相似文献   

A Review of “Agglomeration,Technology and Business Groups”     

Tim Reiffenstein 《The Professional geographer》2013,65(3):418-420

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“Doing” Critical Geographies with Numbers   总被引：2，自引：1，他引：1  

Tim Schwanen  Mei-Po Kwan 《The Professional geographer》2013,65(4):459-464

Criticisms of quantitative geography have a long history in our discipline. Since the emergence of early Marxist geography, the advent of new theoretical traditions has been accompanied by sustained and sometimes vigorous critiques of spatial analysis. In this introductory article to the second focus section on critical quantitative geographies, we discuss the accuracy of some of the criticisms about the use(s) to which spatial analysis is put and about the philosophical presumptions underpinning quantitative geographies. We argue that spatial analysis and critical geographies should not be considered mutually exclusive of one another—although this does not mean that numbers and critical geographies are entirely comfortable with one another. We suggest that critical geographies cannot dispense with numbers and visualizations based on numerical information. We hope that the articles collected here will inspire geographers to engage with numbers and statistics to challenge inequality and imagine alternative futures.  相似文献   

Trading in the dark – The medicinal plants production network in Uttarakhand     

Tim Pauls  Martin Franz 《Singapore journal of tropical geography》2013,34(2):229-243

Medicinal plant cultivation and gathering can play a vital role in the course of livelihood diversification for the marginalized population living in remote areas. However, this requires an integration of the respective production networks that allows the producers a fair and reliable income and does not endanger rare plant species. This paper analyses the situation within the Indian state of Uttarakhand, and identifies structures and entities hindering medical plant cultivation and its potentials. Based on the broad network understanding of the Global Production Network approach, the case study focuses on the middlemen and their hidden embeddedness. It shows how governmental organizations and in particular non‐governmental organizations and farmers' institutions can help to overcome clandestine structures of illegal trade and contribute towards a redesign of the medicinal plant network in a more equitable and transparent way.  相似文献   

Editorial: Jim Blaut and the trajectories of tropical geography     

Tim Bunnell  Chin Ee Ong  James D. Sidaway 《Singapore journal of tropical geography》2013,34(3):285-291

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Scale impacts on spatial variability in reference evapotranspiration     

Tim Hess  Andre Daccache  Ali Daneshkhah  Jerry Knox 《水文科学杂志》2013,58(3):601-609

ABSTRACT

Evapotranspiration (ET) is one of the most important components in the hydrological cycle, and a key variable in hydrological modelling and water resources management. However, understanding the impacts of spatial variability in ET and the appropriate scale at which ET data should be incorporated into hydrological models, particularly at the regional scale, is often overlooked. This is in contrast to dealing with the spatial variability in rainfall data where existing guidance is widely available. This paper assesses the impacts of scale on the estimation of reference ET (ETo) by comparing data from individual weather stations against values derived from three national datasets, at varying resolutions. These include the UK Climate Impacts Programme 50 km climatology (UKCP₅₀), the UK Met Office 5 km climatology (UKMO₅) and the regional values published in the Agricultural Climate of England and Wales (ACEW). The national datasets were compared against the individual weather station data and the UKMO₅ was shown to provide the best estimate of ETo at a given site. The potential impacts on catchment modelling were then considered by mapping variance in ETo to show how geographical location and catchment size can have a major impact, with small lowland catchments having much higher variance than those with much larger areas or in the uplands. Some important implications for catchment hydrological modelling are highlighted.