The last Welsh Ice Cap: Part 1 – Modelling its evolution,sensitivity and associated climate     

Henry Patton  Alun Hubbard  Neil F. Glasser  Tom Bradwell  Nicholas R. Golledge 《Boreas: An International Journal of Quaternary Research》2013,42(3):471-490

A high‐resolution, three‐dimensional, thermomechanical ice‐flow model is used to investigate the glaciodynamics of the Last Glacial Maximum Welsh Ice Cap – a large, independent ice centre of the British–Irish Ice Sheet. The model uses higher‐order physics to solve longitudinal stresses, and is coupled to climate via a distributed, positive degree‐day mass‐balance scheme. A suite of model experiments driven by the GISP2 δ¹⁸O curve was initiated from a climatic optimum at 38.3 ka BP through to the Devensian/Holocene boundary to identify an icecap configuration compatible with available empirical evidence. An enhanced cooling from present of 11.85°C and strong precipitation suppression are required between 27.4 and 23.5 ka BP for the modelled icecap to attain well‐established empirical limits, a scenario probably associated with Heinrich Event‐2 and the potential collapse of thermohaline circulation in the North Atlantic. The experiments indicate ice‐dispersal centres located in North and Mid Wales, the latter being essential for forcing ice southwards of the Brecon Beacons during the Last Glacial Maximum. Deglaciation of the Welsh Ice Cap was relatively rapid, occurring within one millennium. Dynamic stability is governed largely by the dominance and vigour with which fast‐flowing outlet glaciers drain the icecap interior, which in turn are linked to variations in the climatic forcing. The distribution of permanently cold‐based ice across the uplands and summits indicates the probable preservation of relict landscapes in these areas throughout the full glacial cycle.  相似文献   

Changes in volcanic hazard exposure in the Northwest USA from 1940 to 2100     

Stephen M. Strader  Walker Ashley  James Walker 《Natural Hazards》2015,77(2):1365-1392

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Experimental partitioning of uranium between liquid iron sulfide and liquid silicate: Implications for radioactivity in the Earth’s core     

Kevin T. Wheeler  David Walker  William G. Minarik 《Geochimica et cosmochimica acta》2006,70(6):1537-1547

Measurable uranium (U) is found in metal sulfide liquids in equilibrium with molten silicate at conditions appropriate for a planetary magma ocean: 1-10 GPa, 1750-2100 °C, 0-28 wt% S, and fO₂ 2 log units below IW. However, the transfer of U from metal sulfide to silicate under our experimental conditions is so complete that insufficient U would remain so as to be of any importance to the core’s heat budget. U content in the sulfide phase increases strongly with S content but shows no significant variability with either pressure or temperature. Maximum is 0.001 while most values are considerably lower.  相似文献   

Unraveling the sequence and structure of the protein osteocalcin from a 42 ka fossil horse     

Peggy H. Ostrom  Hasand Gandhi  Angela K. Walker  Joseph Leykam  Robert L. Kelly  Mike Buckley 《Geochimica et cosmochimica acta》2006,70(8):2034-2044

We report the first complete amino acid sequence and evidence of secondary structure for osteocalcin from a temperate fossil. The osteocalcin derives from a 42 ka equid bone excavated from Juniper Cave, Wyoming. Results were determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-MS) and Edman sequencing with independent confirmation of the sequence in two laboratories. The ancient sequence was compared to that of three modern taxa: horse (Equus caballus), zebra (Equus grevyi), and donkey (Equus asinus). Although there was no difference in sequence among modern taxa, MALDI-MS and Edman sequencing show that residues 48 and 49 of our modern horse are Thr, Ala rather than Pro, Val as previously reported (Carstanjen B., Wattiez, R., Armory, H., Lepage, O.M., Remy, B., 2002. Isolation and characterization of equine osteocalcin. Ann. Med. Vet.146(1), 31-38). MALDI-MS and Edman sequencing data indicate that the osteocalcin sequence of the 42 ka fossil is similar to that of modern horse. Previously inaccessible structural attributes for ancient osteocalcin were observed. Glu₃₉ rather than Gln₃₉ is consistent with deamidation, a process known to occur during fossilization and aging. Two post-translational modifications were documented: Hyp₉ and a disulfide bridge. The latter suggests at least partial retention of secondary structure. As has been done for ancient DNA research, we recommend standards for preparation and criteria for authenticating results of ancient protein sequencing.  相似文献   

Sediment pore-water interactions associated with arsenic and uranium transport from the North Cave Hills mining region,South Dakota,USA     

Lance N. Larson  Gregory G. Kipp  Henry V. Mott  James J. Stone 《Applied Geochemistry》2012

The extent of historical U mining impacts is well documented for the North Cave Hills region of Harding County, South Dakota, USA. While previous studies reported watershed sediment and surface water As and U concentrations up to 90× established background concentrations, it was unclear whether or how localized changes in sediment redox behavior may influence contaminant remobilization. Five pore-water equilibration samplers (peepers) were spatially and temporally deployed within the study area to evaluate seasonal solid–liquid As and U distributions as a function of sediment depth. Pore-water and solid phase As and U concentrations, Fe speciation, Eh and pH were measured to ascertain specific geochemical conditions responsible for As and U remobilization and transport behavior. At a mine overburden sedimentation pond adjacent to the mine sites, high total aqueous As and U concentrations (4920 and 674 μg/L, respectively) were found within surface water during summer sampling; however pond dredging prior to autumn sampling resulted in significantly lower aqueous As and U concentrations (579 and 108 μg/L, respectively); however, both As and U still exceeded regional background concentrations (20 and 18 μg/L, respectively). At a wetlands-dominated deposition zone approximately 2 km downstream of the sedimentation pond, pore-water geochemical conditions varied seasonally. Summer conditions promoted reducing conditions in pore water, resulting in active release of As(III) to the water column. Autumn conditions promoted oxidizing conditions, decreasing pore-water As (As_pw) 5× and increasing U_pw 10×. Peak U pore-water concentrations (781 μg/L) were 3.5× greater than determined for the surface water (226 μg/L), and approximately 40× background concentrations. At the Bowman–Haley reservoir backwaters 45 km downstream from the mine sites, As and U pore-water concentrations increased significantly between the summer and autumn deployments, attributed to increased Fe reduction processes. Geochemical modeling suggests solid-phase Fe reduction promotes the liberation of pore-water As and U via suppressing the formation of thioarsenite. Intermittent hydrological processes facilitate As and U transport and deposition throughout the watershed, while biogeochemical-influenced redox changes cycle As and U between pore and surface water within localized environments.  相似文献   

A Dye Tracer Approach for Quantifying Fluid and Solute Flux Across the Sediment–Water Interface     

Ryan N. Cascarano  Donald M. Reeves  Mark A. Henry 《Ground water》2021,59(3):428-437

We propose a dye tracer method to characterize fluid and solute fluxes across the sediment-water interface. Zones of groundwater discharge within the streambed are first identified, and small volume slugs of 0.5 to 1 mL fluorescein dye are released at known subsurface depths. Fluorescein dye allows for visual identification of interface breakthrough locations and times, and dye concentrations at the point of discharge are recorded over time by a fluorometer to generate high resolution breakthrough curves. Groundwater velocities and dispersivities at the demonstration site are estimated by numerically fitting dye breakthroughs to the classical advection-dispersion equation, although the methodology is not limited to a specific transport model. Breakthroughs across the stream-sediment interface at the demonstration site are nonlinear with tracer release depth, and velocity estimates from breakthrough analysis are significantly more reliable than visual dye (time to first dye expression) and Darcy methods which tend to overestimate and underestimate groundwater velocity, respectively. The use of permanent injection points within the streambed and demonstrated reproducibility of dye breakthroughs allow for study of fluid and solute fluxes under seasonally varying hydrologic conditions. The proposed approach also provides a framework for field study of nonconservative, reactive solutes and allows for the determination of characteristic residence times at various depths in the streambed to better understand chemical and nutrient transformations.  相似文献   

Could the Ancestral Puebloans of Chaco Canyon,New Mexico,Have Depended on a Groundwater Ecosystem?     

Henry L. Short 《Ground water》2021,59(6):925-934

A thousand years ago, a population of Ancestral Puebloans occupied a high desert canyon in northwestern New Mexico, USA, where precipitation was limited and surface water scarce. Geological conditions, however, seem favorable for the production of a large Hypocrene springs system near the south canyon walls sufficient to have produced a groundwater ecosystem favorable for agriculture, tree growth, and human occupancy. A human-induced ecological impact is suggested as contributing to the dewatering of the springs, eventually reducing local agricultural production and, presumably, the suitability of Chaco Canyon for human occupancy.  相似文献   

The orbital distribution of Near-Earth Objects inside Earth’s orbit     

Sarah Greenstreet  Henry Ngo  Brett Gladman 《Icarus》2012,217(1):355-366

Canada’s Near-Earth Object Surveillance Satellite (NEOSSat), set to launch in early 2012, will search for and track Near-Earth Objects (NEOs), tuning its search to best detect objects with a < 1.0 AU. In order to construct an optimal pointing strategy for NEOSSat, we needed more detailed information in the a < 1.0 AU region than the best current model (Bottke, W.F., Morbidelli, A., Jedicke, R., Petit, J.M., Levison, H.F., Michel, P., Metcalfe, T.S. [2002]. Icarus 156, 399–433) provides. We present here the NEOSSat-1.0 NEO orbital distribution model with larger statistics that permit finer resolution and less uncertainty, especially in the a < 1.0 AU region. We find that Amors = 30.1 ± 0.8%, Apollos = 63.3 ± 0.4%, Atens = 5.0 ± 0.3%, Atiras (0.718 < Q < 0.983 AU) = 1.38 ± 0.04%, and Vatiras (0.307 < Q < 0.718 AU) = 0.22 ± 0.03% of the steady-state NEO population. Vatiras are a previously undiscussed NEO population clearly defined in our integrations, whose orbits lie completely interior to that of Venus. Our integrations also uncovered the unexpected production of retrograde orbits from main-belt asteroid sources; this retrograde NEA population makes up ?0.1% of the steady-state NEO population. The relative NEO impact rate onto Mercury, Venus, and Earth, as well as the normalized distribution of impact speeds, was calculated from the NEOSSat-1.0 orbital model under the assumption of a steady-state. The new model predicts a slightly higher Mercury impact flux.  相似文献   

The Large Magellanic Cloud and the distance scale     

Alistair R. Walker 《Astrophysics and Space Science》2012,341(1):43-49

The Magellanic Clouds, especially the Large Magellanic Cloud, are places where multiple distance indicators can be compared with each other in a straight-forward manner at considerable precision. We here review the distances derived from Cepheids, Red Variables, RR Lyraes, Red Clump Stars and Eclipsing Binaries, and show that the results from these distance indicators generally agree to within their errors, and the distance modulus to the Large Magellanic Cloud appears to be defined to ±3% with a mean value (m−M)₀=18.48 mag, corresponding to 49.7 kpc. The utility of the Magellanic Clouds in constructing and testing the distance scale will remain as we move into the era of Gaia.  相似文献   

Ranching and the new global range: Amazônia in the 21st century   总被引：1，自引：0，他引：1  

Robert Walker  John Browder  Cynthia Simmons  Marcellus Caldas  Sergio de Zen 《Geoforum》2009,40(5):732-745

This paper seeks to understand how the Brazilian Amazon, which many thought unsuitable for agricultural development, has yielded to a dynamic cattle economy in only a few decades. It does so by embedding the Thunian model of location rents within the regime of capital accumulation that has driven the Brazilian economy since the mid-20th century. The paper addresses policies that have created location rents in Amazônia, the effect of these rents on land managers, and the spatial implications of their behavior on forests. Thus, the paper connects macro-processes and structures to agents on the ground, in providing a political ecological explanation relevant to land change science. The policy discussion focuses on reductions in transportation costs, improvements in animal health, and monetary and trade reforms. To illustrate the impact of policy, the paper presents data on the geography of Amazonian herd expansion, on the growth of Amazonian exports, and on the profitability of the region’s cattle economy. It follows the empirical presentation with more abstract consideration of the spatial relations between cattle ranching and soy farming, and implications for deforestation. The paper concludes on a speculative note by considering the likelihood of forest transition in the region, given the transformation of Amazônia into a global resource frontier.  相似文献