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Savard M.M.,Geological Survey of Canada
Environmental Pollution | Year: 2010

Hydrogen (δ2H), carbon (δ13C), oxygen (δ18O) and nitrogen (δ15N) isotopes of tree rings growing in field conditions can be indicative of past pollution effects. The characteristic δ13C trend is a positive shift generally explained by invoking closure of stomata, but experimental studies suggest that increased rates of carboxylation could also generate such trends. In many cases the δ18O and δ2H values decrease in trees exposed to pollution and exhibit inverse coinciding long-term trends with δ13C values. However, some trees exposed to diffuse pollution and experimental conditions can show an increase or no δ18O change even if δ13C values increase. These diverse responses depend on how stress conditions modify physiological functions such as stomatal conductance, carboxylation, respiration, and perhaps water assimilation by the root system. Recent studies suggest that δ15N changes in trees can be caused by soil acidification and accumulation of anthropogenic emissions with isotopic signals deviating from natural N. Crown Copyright © 2009.


Buchan K.L.,Geological Survey of Canada
Precambrian Research | Year: 2013

Key paleomagnetic poles are poles that are well defined and precisely dated. The rock unit from which the pole is derived must have a precise (usually U-Pb) age and the pole itself must be demonstrated primary with a rigorous field test. The use of key poles is essential in defining reliable apparent polar wander paths (APWPs) and establishing continental reconstructions. Many hundreds of Proterozoic paleopoles have been published from around the globe, but only ~45 are from large craton interiors and pass the key pole criteria. Most key poles are from mafic dykes and sills in the Superior craton (pre-1.83. Ga) or Laurentia (post-1.80. Ga) or Baltica. As a result, with occasional exceptions, it is difficult to define or compare reliable APWP segments in order to test Proterozoic continental reconstructions. However, there are now sufficient age matches or approximate age matches for pairs of key poles from a number of cratons to help constrain their relative locations. In this analysis, Proterozoic key poles are identified and their use in constructing APWPs and testing continent and supercontinent reconstructions is discussed. This key pole database establishes a well constrained Superior craton-Laurentia APWP for much of the Proterozoic that can be used as a reference track against which a growing number of individual key poles from other cratons can be compared. There is now a robust Baltica-Laurentia reconstruction for ~330. m.y. between 1.59 and 1.26. Ga using this approach and potentially for ~570. m.y. between 1.83 and 1.26. Ga if additional key and non-key poles from well-dated units are considered. Key pole comparisons for several other cratons yield preliminary constraints on the relative movement of cratons (e.g., Slave and Superior cratons in the Paleoproterozoic) or on specific elements of continental reconstructions (e.g., Amazonia and Baltica in the Mesoproterozoic, South China craton and Australia in the Neoproterozoic, or Baltica and Laurentia also in the Neoproterozoic). © 2013.


Hyndman R.D.,Geological Survey of Canada
Bulletin of the Seismological Society of America | Year: 2015

This article provides a summary of the structure and tectonic history of the Queen Charlotte transform fault zone off western Canada, as background to understanding the 2012 Mw 7.8 thrust earthquake off Haida Gwaii. There was margin subduction prior to the Eocene. The fault zone then became the mainly transcurrent Pacific–North America boundary. There was mid-Tertiary oblique extension, then 15°–20° oblique convergence from ∼6 Ma to the present that resulted in underthrusting and subduction initiation. The total underthrusting has been too small for Benioff–Wadati seismicity or arc volcanics but is indicated by (1) a trench, the Queen Charlotte Trough, into which the oceanic plate bows downward and an offshore flexural bulge, the Oshawa rise; (2) the Queen Charlotte terrace, an accretionary sedimentary prism; (3) seismic receiver function delineation of the underthrusting Pacific plate; (4) heat flow decreasing landward as predicted for underthrusting; (5) low gravity offshore and high onshore, consistent with underthrusting; and (6) late Tertiary uplift and erosion of the west coast of the islands. Oblique convergence is partitioned into nearly marginnormal underthrusting (i.e., Mw 7.8 event) relative to the terrace, which is moving along the margin, and margin parallel on the Queen Charlotte strike-slip fault just off the coast that produced the 1949 Ms 8.1 earthquake. Landward on the mainland, Global Positioning System data suggest slow coast-parallel shear with no historical seismicity. The convergence rate decreases to the north of Haida Gwaii off Dixon Entrance, but large thrust earthquakes are possible. To the south, underthrusting of the Winona basin margin also could generate large earthquakes. © 2015 Seismological Society of America. All rights reserved.


Key paleomagnetic poles are poles that are well defined and precisely dated. The rock unit from which the pole is derived must have a precise (usually U-Pb) age and the pole itself must be demonstrated primary with a rigorous field test. The use of key poles is essential in defining reliable apparent polar wander paths (APWPs) and establishing continental reconstructions. Many hundreds of Proterozoic paleopoles have been published from around the globe, but only ~45 are from large craton interiors and pass the key pole criteria. Most key poles are from mafic dykes and sills in the Superior craton (pre-1.83. Ga) or Laurentia (post-1.80. Ga) or Baltica. As a result, with occasional exceptions, it is difficult to define or compare reliable APWP segments in order to test Proterozoic continental reconstructions. However, there are now sufficient age matches or approximate age matches for pairs of key poles from a number of cratons to help constrain their relative locations. In this analysis, Proterozoic key poles are identified and their use in constructing APWPs and testing continent and supercontinent reconstructions is discussed. This key pole database establishes a well constrained Superior craton-Laurentia APWP for much of the Proterozoic that can be used as a reference track against which a growing number of individual key poles from other cratons can be compared. There is now a robust Baltica-Laurentia reconstruction for ~330. m.y. between 1.59 and 1.26. Ga using this approach and potentially for ~570. m.y. between 1.83 and 1.26. Ga if additional key and non-key poles from well-dated units are considered. Key pole comparisons for several other cratons yield preliminary constraints on the relative movement of cratons (e.g., Slave and Superior cratons in the Paleoproterozoic) or on specific elements of continental reconstructions (e.g., Amazonia and Baltica in the Mesoproterozoic, South China craton and Australia in the Neoproterozoic, or Baltica and Laurentia also in the Neoproterozoic). © 2014.


Some researchers have suggested that Phanerozoic land-based ice sheets extended occasionally into the tropical realm. If true, the tropical ocean at those times must have been distinctly colder than at the Last Glacial Maximum (LGM) when northern hemisphere ice sheets did not extend below 38° latitude. Low-latitude ocean temperatures derived from oxygen isotopes of brachiopod shells test this hypothesis by comparing the temperature regime for Paleozoic and early Mesozoic low-latitude settings to the tropical temperature regime of the modern interstadial ocean, and to mean temperatures estimated for the tropical ocean at LGM. A running mean of pH-adjusted brachiopod habitat temperatures (BHTs) shows that Paleozoic low-latitude oceans were, on average, cool to cold relative to the modern interstadial tropical ocean. At times during Pennsylvanian, Serpukhovian, Tournaisian and Ordovician-Silurian glaciations, these tropical seas were indeed significantly colder on average than at the LGM. Ice-sheets within tropical latitudes can be reasonably predicted at those times. Abundant and diverse Paleozoic brachiopod communities reflect these cool tropical oceans, consistent with modern brachiopod ecological preference for colder waters. Amplified Paleozoic temperature oscillations suggest recurring global warming events which episodically drove these cold tropical oceans to temperatures significantly higher than the warmest modern tropical ocean. © 2012.


Agterberg F.P.,Geological Survey of Canada
Journal of Geochemical Exploration | Year: 2012

In several recent studies, 2-dimensional applications of local singularity analysis including regional studies based on stream sediment data show local minima that are spatially correlated with known mineral deposits. These minimal singularities, which may provide targets for further mineral exploration, generally are smoothed out when traditional geostatistical contouring methods are used. Multifractal analysis based on the assumption of self-similarity predicts strong local continuity of element concentration values that cannot be readily determined by variogram or correlogram analysis. This paper is concerned with multifractal and geostatistical modeling of the largest and smallest geochemical element concentration values in rocks and orebodies. These extreme values correspond to local singularities with near-zero fractal dimensions that occur close to the minimum and maximum singularity in the multifractal spectrum. The latter cannot be determined by means of the method of moments because of small-sample size problems arising when the largest and smallest concentration values are raised to very large powers q. It is shown by means of a computer simulation experiment and application to copper determinations from along the 7-km deep KTB borehole in southeastern Germany, that local singularity analysis can be used to determine all singularities including the extreme values. The singularities estimated by this method are linearly related to logarithmically transformed element concentration values. This simple relation also can used to measure the small-scale nugget effect, which may be related to measurement error and microscopic randomness associated with ore grain boundaries. © 2012 Elsevier B.V.


Bédard (2006) proposed that Archaean cratons formed above large, long-lived mantle plumes, where the thick basaltic crust cannibalized itself to generate TTGs (tonalite-trondhjemite-granodiorite) and complementary eclogitic restites. In this model the dense eclogitic restites foundered into the depleted upper mantle and refertilized it, triggering generation of new basaltic melt pulses, and so eclogite represents a catalyst for coupled crust-mantle differentiation. Since most of the eclogite is destroyed in the upper mantle, voluminous hidden eclogitic reservoirs are not predicted. The model was not intended to explain the generation of overprinting fabrics and terrane assembly, but to account for chemical evolution of the coupled crust-mantle system in the initial stages of craton development. Wyman (2013) argues that the models and hypotheses presented in Bédard (2006) are unrealistic and irrelevant, and reaffirms his opinion that the Archaean world was dominated by plate tectonics. The criticisms and arguments of Wyman (2013) are invalid. © 2012 Published by Elsevier B.V.


Wang K.,Geological Survey of Canada | Bilek S.L.,New Mexico Institute of Mining and Technology
Geology | Year: 2011

Seamount subduction is a common process in subduction zone tectonics. Contradicting a widely held expectation that subducting seamounts generate large earthquakes, seamounts subduct largely aseismically, producing numerous small earthquakes. On rare occasions when they do produce relatively large events, the ruptures tend to be complex, suggesting multiple rupture patches or faults. We explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault. The complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures. © 2011 The Geological Society of America.


Time-lapse seismic interval travel time differences are determined during 7years of CO2 injection within the Weyburn oil field, Saskatchewan, Canada. Travel time difference maps are used in conjunction with geophysical and geological logs, and depth dependant fluid properties to provide upper bounds on the amount of CO2 that may reside within various geological intervals. Calculated travel time sensitivity values increase from 0.01ms/m to ∼0.25ms/m moving upward from the reservoir depth (1450m) to the shallowest aquifer considered (600m). The seismic-based apportionment of CO2 shows the fraction of CO2 within or below the reservoir increasing from at least 62-70% after 1year of injection to at least 92-94% after 7years of injection, whereas the proportion of the total injected CO2 in the zone immediately overlying the reservoir decreases from a maximum of 30-36% to ∼5-6% over the same time period. The estimates of CO2 quantities residing within this zone are significantly overestimated due to clearly identified pressure effects in this zone. The maximum estimated proportion of CO2 residing in either interval above the regional sealing formation is ≤1%. The maximum amount of CO2 potentially residing above the regional seal by 2007 is <56, 000tonnes, with a maximum of 0.31Mtonnes just above the reservoir, and 4.6-4.7Mtonnes in the reservoir-containing interval. © 2013 .


White D.,Geological Survey of Canada
International Journal of Greenhouse Gas Control | Year: 2013

3D time-lapse seismic monitoring has been conducted over a seven-year period of CO2 injection within the Weyburn field, Saskatchewan, as part of an enhanced oil recovery operation. 3D monitor seismic volumes, acquired in 2001, 2002, 2004 and 2007, were processed in parallel resulting in good data repeatability with global NRMS values of 0.30-0.34. Time-lapse amplitude and travel time difference maps that include the reservoir interval depict changes that exceed background noise levels. Comparison with reservoir flow simulations show good correlation with injection-related reservoir changes, and demonstrate that the CO2 plume outline can generally be tracked. Pressure changes within the reservoir are inferred to have a limited contribution to the time-lapse signal based on qualitative comparison with flow simulations. Lateral heterogeneity within the reservoir and injection procedures affect the observed seismic response due to CO2 in the subsurface, emphasizing the need to combine seismic observations with reservoir simulations, calibration and an appropriate rock physics model in order to achieve robust semi-quantitative CO2 quantity estimates. © 2013 .

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