Mixing-induced groundwater denitrification beneath a manured field in southern Alberta, Canada     

J.E. McCallum  M.C. Ryan  B. Mayer  S.J. Rodvang   《Applied Geochemistry》2008,23(8):2146-2155

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Postglacial variability in near‐bottom current speed on the continental shelf off south‐west Spitsbergen          下载免费PDF全文

BEATA STERNAL  WITOLD SZCZUCISKI  MATTHIAS FORWICK  MAREK ZAJĄCZKOWSKI  STANISŁAW LORENC  JOANNA PRZYTARSKA 《第四纪科学杂志》2014,29(8):767-777

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Rip currents as a geological tool   总被引：2，自引：0，他引：2  

MICHA&#; GRUSZCZY&#;SKI§  STANIS&#;AW RUDOWSKI†  JULIA SEMIL‡  JAN S&#;OMI&#;SKI†  JERZY ZROBEK‡ 《Sedimentology》1993,40(2):217-236

This paper considers the nature and sedimentary significance of rip currents. Rip currents are the main factor responsible for the transport of coarse sediments from the littoral zone to greater depths. Such sediments, deposited outside the zone of wave deformation, may be identified as storm rip current increments within sediments deposited during fair weather. Composite beds deposited during a given wave cycle by storm rip currents are closely similar to ‘turbidites’ and many so called ‘fluxo-turbidites’ described from flysch deposits. Using data gathered in studies conducted on the modern Baltic coast, supplemented by experimental work and theoretical considerations, a sedimentary model is proposed. The model may be used to interpret possible rip current deposits among shallow water ‘turbidites’, and both modern storm sediments and ancient ‘tempestites’.  相似文献   

The partitioning of Zr and Nb between diopside and melts in the system diopside-albite-anorthite     

Todd Dunn  I.S. McCallum 《Geochimica et cosmochimica acta》1982,46(4):623-629

The crystal liquid partitioning of Zr and Nb has been measured experimentally between diopsidic clinopyroxene and melts in the system Di-Ab-An. Nb was found to be excluded from diopside (D(Nb) is always less than 0.02). D(Zr) is quite variable, ranging from 0.05 to 0.45. D(Zr) is positively correlated with the Al content of both the melt and the pyroxene and is negatively correlated with temperature. Both D(Zr) and D(Nb) were found to be independent of oxygen fugacity. This implies that neither Zr or Nb suffer valence changes over a range of oxygen fugacities spanning both lunar and terrestrial conditions.  相似文献   

Mineralogy and petrology of sample 67075 and the origin of lunar anorthosites     

I.S. McCallum  F.P. Okamura  S. Ghose 《Earth and Planetary Science Letters》1975,26(1):36-53

Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O₂ conditions and/or early crystallization of olivine and plagioclase.The⁸⁷Sr/⁸⁶Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and³⁹Ar/⁴⁰Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.  相似文献   

The effects of frequency-dependent quasar variability on the celestial reference frame     

Stanislav S. Shabala  Jonathan G. Rogers  Jamie N. McCallum  Oleg A. Titov  Jay Blanchard  James E. J. Lovell  Christopher S. Watson 《Journal of Geodesy》2014,88(6):575-586

We examine the relationship between source position stability and astrophysical properties of radio-loud quasars making up the International Celestial Reference Frame (ICRF2). Understanding this relationship is important for improving quasar selection and analysis strategies, and therefore reference frame stability. We construct flux density time series, known as light curves, for 95 of the most frequently observed ICRF2 quasars at both the 2.3 and 8.4 GHz geodetic very long baseline interferometry (VLBI) observing bands. Because the appearance of new quasar components corresponds to an increase in quasar flux density, these light curves alert us about potential changes in source structure before they appear in VLBI images. We test how source position stability depends on three astrophysical parameters: (1) flux density variability at X band; (2) time lag between flares in S and X bands; (3) spectral index root-mean-square (rms), defined as the variability in the ratio between S and X band flux densities. We find that the time lag between S and X band light curves provides a good indicator of position stability: sources with time lags $<$ 0.06 years are significantly more stable ( $>$ 20 % improvement in weighted rms) than sources with larger time lags. A similar improvement is obtained by observing sources with low $(<$ 0.12) spectral index variability. On the other hand, there is no strong dependence of source position stability on flux density variability in a single frequency band. These findings can be understood by interpreting the time lag between S and X band light curves as a measure of the size of the source structure. Monitoring of source flux density at multiple frequencies therefore appears to provide a useful probe of quasar structure on scales important to geodesy. The observed astrometric position of the brightest quasar component (the core) is known to depend on observing frequency. We show how multi-frequency flux density monitoring may allow the dependence on frequency of the relative core positions along the jet to be elucidated. Knowledge of the position–frequency relation has important implications for current and future geodetic VLBI programs, as well as the alignment between the radio and optical celestial reference frames.  相似文献   

Understanding connected surface-water/groundwater systems using Fourier analysis of daily and sub-daily head fluctuations     

R. I. Acworth  Gabriel C. Rau  Andrew M. McCallum  Martin S. Andersen  Mark O. Cuthbert 《Hydrogeology Journal》2015,23(1):143-159

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Can the uncertainty of full carbon accounting of forest ecosystems be made acceptable to policymakers?     

Anatoly Shvidenko  Dmitry Schepaschenko  Ian McCallum  Sten Nilsson 《Climatic change》2010,103(1-2):137-157

In accordance with the concept that only full accounting of major greenhouse gases corresponds to the goals of the United Nations Framework Convention on Climate Change and its Kyoto Protocol, this paper considers uncertainties of regional (national) terrestrial biota Full Carbon Accounting (FCA), both those already achieved and those expected. We analyze uncertainties of major components of the FCA of forest ecosystems of a large boreal region in Siberia (~300 × 10⁶ ha). Some estimates for forests of other regions and Russia as a whole are used for comparison. The systems integration of available information sources and different types of models within the landscape-ecosystem approach are shown to have enabled an estimation of the major carbon fluxes (Net Primary Production, NPP, and heterotrophic respiration, HR) for the region for a single year at the level of 7–12% (confidential interval, CI, 0.9), Net Ecosystem Production (NEP) of 35–40%, and Net Biome Production (NBP) of 60–80%. The most uncertain aspect is the assessment of change in the soil carbon pool, which limits practical application of a pool-based approach. Regionalization of global process-based models, introduction of climatic data in empirical models, use of an appropriate time period for accounting and reporting, harmonization and multiple constraints of estimates obtained by different independent methods decrease the above uncertainties of NEP and NBP by about half. The results of this study support the idea that FCA of forest ecosystems is relevant in the post-Kyoto international negotiation process.  相似文献   

Lead isotopes in sulfides from the Stillwater Complex, Montana: evidence for subsolidus remobilization     

I. S. McCallum  M. W. Thurber  H. E. O'Brien  B. K. Nelson 《Contributions to Mineralogy and Petrology》1999,137(3):206-219

Isotopic ratios of Pb in sulfide minerals (primarily pyrrhotite, chalcopyrite, and pentlandite) from a suite of samples from the platiniferous J-M Reef of the Stillwater Complex were measured to elucidate the temporal and genetic relationship between sulfides and host silicate minerals. Results indicate that sulfides and coexisting plagioclases are generally not in isotopic equilibrium, that both sulfides and feldspars record highly radiogenic initial ratios at 2.7 Ga, and that a component of “post-emplacement” radiogenic Pb has mixed with common Pb in the sulfides. A model involving introduction of radiogenic Pb carried by fluids derived from sources external to the complex is favored. Analyses of the lead isotopic composition of sulfides in veins which cut the complex indicate that a significant fraction of the radiogenic lead which was added to the sulfides was externally derived during an extensive hydrothermal episode, associated with Proterozoic regional metamorphism around 1.7 Ga. The possibility that some fractions of the radiogenic Pb may have been derived from primary minerals altered during the low-grade metamorphism cannot be discounted. The amount of radiogenic lead added is variable and in some cases negligible. There is a good correlation between the lead isotope composition and the nature of the secondary mineral assemblage. Sulfides and plagioclases in samples that show little or no alteration of the primary minerals are generally in isotopic equilibrium and preserve isotope ratios consistent with magmatic crystallization at 2.7 Ga. Samples with the most radiogenic sulfides contain abundant secondary minerals (serpentine, talc, actinolite, chlorite and zoisite) associated with greenschist facies metamorphism. Some of the radiogenic Pb in the sulfides can be removed by progressive stepwise leaching. However, in most samples recrystallization of sulfides during metamorphism has mixed common Pb and radiogenic Pb throughout the crystal structure such that, in these samples, stepwise leaching does not recover initial Pb isotopic ratios. Plagioclases are much more resistant to low temperature recrystallization and in almost all cases, stepwise leaching reveals the initial lead isotopic composition. The reactivity of sulfides over a wide temperature range enhances their utility in understanding not only the processes involved in their formation at the time of magmatic emplacement but also postmagmatic processes which were important in the redistribution and enrichment of platinum group elements (PGE) within the ore zone. Received: 30 December 1998 / Accepted: 16 June 1999  相似文献   

Groundwater age,mixing and flow rates in the vicinity of large open pit mines,Pilbara region,northwestern Australia     

Peter Cook  Shawan Dogramaci  James McCallum  Joanne Hedley 《Hydrogeology Journal》2017,25(1):39-53

Determining groundwater ages from environmental tracer concentrations measured on samples obtained from open bores or long-screened intervals is fraught with difficulty because the sampled water represents a variety of ages. A multi-tracer technique (Cl, ¹⁴C, ³H, CFC-11, CFC-12, CFC-113 and SF₆) was used to decipher the groundwater ages sampled from long-screened production bores in a regional aquifer around an open pit mine in the Pilbara region of northwest Australia. The changes in tracer concentrations due to continuous dewatering over 7 years (2008–2014) were examined, and the tracer methods were compared. Tracer concentrations suggest that groundwater samples are a mixture of young and old water; the former is inferred to represent localised recharge from an adjacent creek, and the latter to be diffuse recharge. An increase in ¹⁴C activity with time in wells closest to the creek suggests that dewatering of the open pit to achieve dry mining conditions has resulted in change in flow direction, so that localised recharge from the creek now forms a larger proportion of the pumped groundwater. The recharge rate prior to development, calculated from a steady-state Cl mass balance, is 6 mm/y, and is consistent with calculations based on the ¹⁴C activity. Changes in CFC-12 concentrations with time may be related to the change in water-table position relative to the depth of the well screen.  相似文献