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1.
B. G. Piner S. C. Unwin A. E. Wehrle P. G. Edwards A. L. Fey K. A. Kingham 《New Astronomy Reviews》1999,43(8-10)
We have produced 22 VLBI images of the TeV blazar Markarian 421 at 11 epochs, including a Space VLBI observation with the HALCA satellite. We measure the speeds of the three innermost jet components to be 0.64±0.33, 0.48±0.09, and 0.06±0.09c (H0=65 km s−1 Mpc−1). Interpretation of these subluminal speeds in terms of the high Doppler factor demanded by the TeV observations is discussed. 相似文献
2.
Electrogeochemical sampling with
Reinhard W. Leinz Donald B. Hoover David L. Fey David B. Smith Thaxson Patterson 《Journal of Geochemical Exploration》1998,61(1-3)
Electrogeochemical extraction methods are based on the migration of ions in an electric field. Ions present in soil moisture are transported by an applied current into fluids contained in special electrodes placed on the soil. The fluids are then collected and analyzed. Extractions are governed by Faraday's and Ohm's laws and are modeled by the operation of a simple Hittorf transference apparatus. Calculations show that the volume of soil sampled in an ideal electrogeochemical extraction can be orders of magnitude greater than the volumes used in more popular geochemical extraction methods, although this has not been verified experimentally.
is a method of in-situ electrogeochemical extraction that was developed in the former Soviet Union and has been tested and applied internationally to exploration for buried mineral deposits. Tests carried out at the United States Geological Survey (
) indicated that there were problems inherent in the use of
technology. The cause of the problems was determined to be the diffusion of acid from the conventional electrode into the soil. The
electrode incorporates two compartments and a salt bridge in a design that inhibits diffusion of acid and enables the collection of anions or cations. Tests over a gold-enriched vein in Colorado and over buried, Carlin-type, disseminated gold deposits in northern Nevada show that there are similarities and differences between
results and those by partial extractions of soils which include simple extractions with water, dilute acids and solutions of salts used as collector fluids in the electrodes. Results of both differ from the results obtained by total chemical digestion. The results indicate that
responds to mineralized faults associated with disseminated gold deposits whereas partial and total chemical extraction methods do not. This suggests that faults are favored channels for the upward migration of metals and that
may be more effective in exploration for the deposits. It defines anomalies that are often narrow and intense, an observation previously made by
researchers. The field tests show that
is less affected by surface contamination. A test over the Mike disseminated gold deposit indicates that the method may not be effective for locating deposits with impermeable cover. Faradaic extraction efficiencies of 20–30%, or more, are frequently achieved with
and the method generally shows good reproducibility, especially in extraction of major cations. However, ions of other metals that are useful in exploration, including Au and As, may be collected in low and temporally variable concentrations. The reason for this variability is unclear and requires further investigation. 相似文献
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Susan Ringrose Philippa Huntsman-Mapila William Downey Stephan Coetzee Martin Fey Cornelis Vanderpost Bernard Vink Thebe Kemosidile Dikitso Kolokose 《Geomorphology》2008,101(4):544-557
This work considers the spatial distribution and ages of western MOZ basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands. Previously published and new TL/OSL ages imply that extensive over-washing of dune sands took place at least 100 ka ago while ages on Okavango floodplains imply that the fan was formed ca. 40 ka and has since undergone periods of higher and lower flood regimes. Sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay-enhanced amorphous silica (CEAS) which bonds weakly formed aggregates. The time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using OSL/TL techniques. Hence earlier dune over-washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation > 100 000 years ago. The relatively abundant CEAS matrix in floodplain sands implies more recent semi-continuous flood events again of an acidic-near neutral nature leading to the formation of smectite. In this case the floodplain sediments are dated as having been deposited around 40 and 11 ka, when porewater content may have accelerated clay formation and silica dissolution. The dual nature of the CEAS in the islands reflects a changing environment from smectite-dominated flooding events to sepiolite-dominated desiccation events. Flooding may also correspond to TL/OSL ages over the past 40 000 years which contributed to accelerated CEAS formation. The sepiolite is associated with a Ca-rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation. This work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events. The implications of different stages of sand diagenesis may be significant in enhancing palaeo-environmental interpretations in semi-arid fluvial environments. 相似文献
5.
Torsten?HaberzettlEmail author Michael?Fey Andreas?Lücke Nora?Maidana Christoph?Mayr Christian?Ohlendorf Frank?Sch?bitz Gerhard?H.?Schleser Michael?Wille Bernd?Zolitschka 《Journal of Paleolimnology》2005,33(3):283-302
The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The total inorganic carbon content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the Medieval Climate Anomaly in southeastern Patagonia. At the beginning of the Little Ice Age the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century. 相似文献
6.
Kathryn M. Stack John P. Grotzinger Michael P. Lamb Sanjeev Gupta David M. Rubin Linda C. Kah Lauren A. Edgar Deirdra M. Fey Joel A. Hurowitz Marie McBride Frances Rivera‐Hernndez Dawn Y. Sumner Jason K. Van Beek Rebecca M. E. Williams Robin Aileen Yingst 《Sedimentology》2019,66(5):1768-1802
Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick fine‐grained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be sub‐divided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low‐angle cross‐stratified mudstone; (iii) cross‐stratified sandstone; and (iv) thickly laminated mudstone–sandstone. The very fine grain size of the mudstone facies and abundant millimetre‐scale and sub‐millimetre‐scale laminations exhibiting quasi‐uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low‐angle geometric discordances in the mudstone facies are interpreted as ‘scour and drape’ structures and suggest the action of currents, such as those associated with hyperpycnal river‐generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial‐deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 103 to 107 Earth years. 相似文献
7.
Production of CH4 and CO2 was quantified in anoxically incubated soil samples taken from an Italian rice field. The rates increased with temperature between 10 and 37°C. The δ13C of the accumulated CO2, CH4 and acetate changed with time in a systematic way. The data were used in mass balance equations to constrain isotopic fractionation factors and pathways of CH4 production. The calculations were further constrained by the determination of 14CH4 production from 14CO2 at steady state. At 50°C, CH4 was exclusively produced from CO2, indicating a fractionation factor of αCO2/CH4 = 1.073. Between 10 and 37°C, the results showed a temporal change in the methanogenic pathway. A relatively high (40-60%) CO2-derived fraction of CH4 production in the beginning was followed by a phase in which contribution of CO2-derived CH4 decreased to low (<15%) values, and ultimately by the steady state phase in which values increased to <40% (the theoretically expected value). The rate of change from one phase to the next increased with temperature. Incubation temperature had a strong effect on the overall fractionation of 13C during the formation and consumption of acetate, with stronger fractionation at low than at high temperature. The results further showed that, especially at low temperatures, fractionation occurred during acetate turnover and acetoclastic methanogenesis, despite the fact that steady-state conditions caused (apparent) substrate-limitation. 相似文献
8.
Torsten Haberzettl Flavio S. Anselmetti Sabine W. Bowen Michael Fey Christoph Mayr Bernd Zolitschka Daniel Ariztegui Barbara Mauz Christian Ohlendorf Stephanie Kastner Andreas Lücke Frank Schäbitz Michael Wille 《Quaternary Science Reviews》2009,28(25-26):2927-2939
Paleoenvironmental records extending well into the last glacial period are scarce in the steppe regions of southern South America. Here, we present a continuous record for the past 55 ka from the maar lake Laguna Potrok Aike (51°58′ S, 70°23′ W, southern Patagonia, Argentina). Previous studies on a sedimentary core from a lake level terrace near the northern margin of the lake covered parts of Oxygen Isotope Stage (OIS) 3 (59–29 ka) whereas a second core from the centre of the basin comprised the last 16 ka. Tephrostratigraphical constraints and OSL ages from a third core located below the lake level terrace provide the crucial piece to close the gap between the previous coring sites. High-resolution XRF and magnetic susceptibility as well as grain size data indicate a positive hydrological balance alongside with relatively high aeolian activity during the glacial which is contemporaneous with increased dust fluxes in Antarctica. This is therefore the first evidence for contemporaneity of aeolian deposition in both the target area (Antarctica) and in the major source area of Patagonia. During the Holocene climatic conditions driving sediment deposition seem to have been more variable and less dominated by wind compared to glacial times. The identification of a minor lake level lowering at approximately 4 cal ka BP allows to refine earlier paleoenvironmental reconstructions for the Holocene. Within error margins the OSL ages are consistent with published radiocarbon-dated records offering hence a valuable tool for further studies of the sediments from Laguna Potrok Aike. The new chronology confirms the age of three tephra layers up to now only found in Laguna Potrok Aike sediments and ascribed to OIS 3. 相似文献
9.
We review observational constraints on the structures responsible for extreme scattering events, focussing on a series of
observations of the quasar PKS 1741–038. VLA observations were conducted to search for changes in the rotation measure and
HI absorption during the ESE, while VLBI observations sought ESE-induced changes in the source's image. No RM changes were
found implying B
∥ < 12 mG, and no HI opacity changes were found implying N(HI) < 6.4 × 1017cm-2. No multiple imaging was observed, but the diameter of the source increased by 0.7 mas, contrary to what is predicted by simple refractive lens modeling of ESEs. We summarize what these limits imply
about the structure responsible for this ESE.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
M.L. Francis M.V. Fey H.P. Prinsloo F. Ellis A.J. Mills T.V. Medinski 《Journal of Arid Environments》2007,70(4):588
The soil mantle in Namaqualand is immature in terms of weathering. Despite or even because of this, the diversity and spatial variability of soil physical and chemical properties is exceptionally high and could be fundamental in governing biodiversity and ecosystem function. In arid landscapes, the way the soil sheds, admits, stores, diverts and transmits water is crucially important. Namaqualand's soils possess special features, which modify water infiltration, hydraulic conductivity and subsurface storage and provision of water to plants. These include pedodermal crusts; a natural mulch of surface gravel or desert pavement; water-repellent soils with fingered infiltration of rain water; textural discontinuities at depth; subsurface horizons cemented with silica, calcite, fibrous clays or gypsum; soluble salts that affect plant water uptake osmotically; clay minerals having exceptional water-absorbing capacity and that are unique to arid environments; and a mechanism, in deeper sandy soils, for thermally induced upward transport of water vapour—a kind of nocturnal distillation—that seems to lessen the need for plants to root deeply.Superimposed on this already impressive pedodiversity are the almost ubiquitous heuweltjies—broad, low mounds which add further complexity to fluxes of water, salts, energy and nutrients in the landscape. 相似文献