Canadian Shield Research Articles

National-Scale Geospatial Modelling of Pyrrhotite in Bedrock Across Canada: A Pilot Study

The most abundant iron sulphide minerals in igneous, metamorphic, and some sedimentary rocks are FeS2 (pyrite/marcasite) and Fe1-xS (pyrrhotite). The oxidation of pyrite and pyrrhotite in bedrock aggregates and mine tailings is undesirable to infrastructure and the environment because such chemical reactions are linked with concrete deterioration and acid mine drainage, respectively. The oxidation rate of pyrrhotite is up to one hundred times greater than that of pyrite; thus, pyrrhotite oxidation is of particular concern to society. Pyrrhotite-bearing concrete aggregates may lead to rapid expansion cracking and failure of critical concrete infrastructure including bridges, buildings, and houses, posing a significant safety concern. Additionally, premature disintegration of concrete requires replacement of concrete, leading to excessive aggregate resource extraction and associated increases in CO2 emissions. Considering the widespread concrete infrastructure across Canada, the distribution of pyrrhotite in bedrock used for aggregate is of fundamental importance to the short- and long-term safety of Canadians at the national, regional, and local scales. This pilot study details the initial steps taken to generate national-scale geospatial models of pyrrhotite occurrences in bedrock across Canada and illustrates the associated map products. In total, 12,577 known pyrrhotite occurrences were identified from publicly available provincial and territorial mineral occurrence datasets. The overall modelling strategy involved normalizing the number of pyrrhotite occurrences with respect to the total surface area of major bedrock types and characterizing three different classes of pyrrhotite occurrence density (< 1, 1–4, and 4–10 occurrences/1000 km2). The maps illustrate that pyrrhotite occurrence density is highest in volcanic rocks and undifferentiated sedimentary and volcanic rocks, moderate in intrusive and unknown rocks, and lowest in sedimentary and metamorphic rocks. Sedimentary rocks with no pyrrhotite occurrences span large surface areas across central-western Canada thus resulting in an overall low pyrrhotite occurrence density (< 1 occurrence/1000 km2) for this rock type, despite the fact that numerous pyrrhotite occurrences are identified in sedimentary rocks and may be abundant locally or regionally. Volcanic, undifferentiated sedimentary and volcanic, intrusive, and unknown rocks occur throughout the Canadian Shield of central and northern Canada, the Cordillera of western Canada, and the Appalachians of eastern Canada, but bedrock type and associated occurrence density are highly variable within these geological domains. Comparison of pyrrhotite occurrence density maps for Canada with pyrrhotite permissive geology maps for the United States of America illustrates that rocks with a high pyrrhotite occurrence density in Canada (volcanic rocks and undifferentiated sedimentary and volcanic rocks), are contiguous overall with areas of pyrrhotite potential in the United States. Inconsistencies across the international border reflect the differing methodologies and assumptions consisting of a statistically based approach for Canada and a qualitative approach for the United States. In the Cordillera and Appalachians, such discontinuities across the international border may reflect the underestimation of pyrrhotite occurrences in sedimentary rocks of Canada because of the impact of high surface area on the pyrrhotite occurrence density calculations. The maps presented herein are a first step in illustrating the distribution of pyrrhotite-bearing bedrock across Canada and greater North America. These national-scale map products are useful first-order references for selecting regions for follow-up studies on bedrock pyrrhotite occurrences. Regional and local geospatial analysis combined with field work for ground truthing will be important aspects of future research, especially in the vicinity of population centres where bedrock is utilized for concrete aggregate. Detailed regional and local studies of pyrrhotite occurrences in bedrock will help guide the extraction of safe concrete aggregate and contribute to the long-term sustainability of bedrock resources, safe infrastructure, and a habitable climate.

Constraining seasonal and spatial ambient and urban groundwater contributions to streamflow across a mixed land-use regional-scale Precambrian Shield watershed

Groundwater-surface water (GW-SW) interactions are complex phenomena that vary naturally over space and time and are being influenced by environmental change. The Whitson River watershed is a mixed land-use Precambrian shield watershed located in Northeastern Ontario, Canada in which groundwater is a source of municipal water supply and in which groundwater and surface waters are at risk of potential impacts from urban runoff, ongoing municipal drainage projects, periodic flooding, and climate change. This study used watershed-scale synoptic surveys of stable isotopes (δ18O and δ2H) and geochemistry, and corresponding mixing-model approaches (two-component and three-component) to quantify the seasonal and spatial variation in surface water, ambient, and urban groundwater contributions to streamflow. Multiple mixing models were generated based on isotope and geochemical source water identification, and the performance of these models was compared. Mixing-model analyses showed that groundwater contributes a critical, sustaining source to streamflow (> 50 % or more) during summer baseflow conditions, dropping to ∼ 15 % in fall when connection to shield lakes and wetlands dominate. Three-component mixing models showed similar results to the two-component models, refining groundwater contributions into ambient and urban groundwater sources. Estimated urban groundwater contributed 30 % or more to summer baseflow, dropping to ∼ 4 % in the fall. Baseflow estimates derived from graphical hydrograph separation were similar to mixing model groundwater estimates in the summer but suggest this approach overestimates groundwater contributions during the fall. These results demonstrate the complexity of source water contributions to streamflow and the challenges in their determination in mixed-land-use Precambrian Shield landscapes where communities are largely located. Assessment of source water contributions to streamflow across the Whitson River sub-watershed has generated a region-specific conceptualization (e.g. urban versus ambient groundwater) of a basin that is experiencing increased urban development, providing information that will be of value for long term management.

Is the mantle transition zone uniform beneath Precambrian shields?

In the present study, we examine the mantle transition zone (MTZ) structure below the prominent Precambrian shields on the Earth to understand its thermal and compositional properties and depth distribution of MTZ discontinuities. For this study, we used data from Canadian, Brazilian, Baltic, African and Australian Shields. We migrate the receiver functions to the depth domain using 3D tomographic models to examine the topography of MTZ discontinuities that define the upper and lower boundaries of the MTZ. The depth migration results were obtained using two different 3D tomographic velocity models, LLNL_G3D_JPS and GyPSuM. The analysis shows that both models yield similar results. The findings indicate a thinner than usual MTZ beneath all the Precambrian shields with an average thickness of ∼238 ± 8 km. Notably, the present study reveals the upper boundary of the MTZ (410 km discontinuity) displays distinctive topography; in contrast, the lower boundary of the MTZ (660 km discontinuity) is situated at shallower depths. Therefore, the main factor causing the variation in MTZ thickness is the shallowing of the 660 km discontinuity, indicating that the post-spinel transition occurred at higher temperatures with a negative Clapeyron slope, supporting the theory of whole-mantle convection. This suggests that mantle plumes have some appreciable impact on the MTZ beneath Precambrian shields, which are limited to the base of the MTZ. Alternatively, the global mantle warming explanation could be invoked, as it best explains the similar characteristics of the 660 km discontinuity beneath the Precambrian shields. However, this phenomenon needs to be tested through numerical modelling.

Continental-scale drainage reorganization during Mesoproterozoic orogenesis: Evidence from the Belt Basin of western North America

The Mesoproterozoic Belt Basin of the northwestern United States and southwestern Canada contains a 5–20-km-thick metasedimentary succession deposited during an important transition in the Precambrian development of North America. Key unresolved issues for the Belt Basin include the chronology of deposition, sources of siliciclastic sediment, and regional paleogeography during Laurentian orogenesis. To address these topics, we acquired detrital zircon U-Pb geochronologic data for eastern exposures of the Belt-Purcell Supergroup in the Lewis thrust salient along the USA-Canada border. To define an integrated chronostratigraphic and provenance framework for the Belt Basin, we calculated maximum depositional ages and qualitatively and quantitatively compared our geochronologic data set to a compilation of Laurentian igneous and metamorphic zircon U-Pb ages using multidimensional scaling and an inverse Monte Carlo model. The results suggest a stratigraphic age range of ca. 1495–1380 Ma, constituting a depositional duration of ~115 m.y. with an average sediment accumulation rate of ~40 m/m.y. for the studied locality (extrapolated to ~155 m/m.y. for the basin depocenter). Variations in sediment provenance are expressed by three distinct intervals within the Belt-Purcell Supergroup. The lower Belt Supergroup succession (Waterton to lower Helena Formations; ca. 1495–1440 Ma) is dominated by Paleoproterozoic and Archean grains derived from the northeastern Canadian Shield. The middle Belt Supergroup succession (upper Helena to Sheppard Formations; ca. 1440–1420 Ma) displays mixed early Mesoproterozoic, late Paleoproterozoic, and Archean zircon age groups. The upper Belt Supergroup succession (Gateway to Roosville Formations; ca. 1420–1380 Ma) contains almost entirely late Paleo-proterozoic zircons sourced from the south (Yavapai-Mazatzal and Mojave crustal provinces). We interpret sediment provenance to reflect a continental-scale, fluvial drainage reorganization during middle Belt Supergroup deposition that can be linked to the recently recognized Picuris orogeny.

Fill‐spill‐merge terrain analysis reveals topographical controls on Canadian river runoff

AbstractDigital Elevation Models (DEMs) are a crucial tool for watershed analysis, offering valuable insights into landscape‐scale hydrology. Traditional watershed delineations are derived by filling a DEM to force flow paths through topographic depressions, thus creating a continuous drainage network throughout the domain. However, this approach is challenged in landscapes with abundant real‐world depression storage, intermittently flowing stream channels, and internally drained lake basin (endorheic basin) such as the Canadian Shield (CS). The CS landscape is characterized by “fill‐and‐spill” surface water hydrology, with runoff flow paths controlled by bedrock sills and rocky cascades that overtop when water levels are high but cease flowing when water levels are low. To better represent these intermittent drainage networks, we apply a non‐traditional, less‐aggressive DEM filling model (Fill‐Spill‐Merge or FSM) to a continental‐scale DEM (MERIT) all of Canada. To ensure adequate filling of DEM noise while also preserving real‐world topographic depressions, we propose a climatic method to initialize a key FSM parameter (“runoff depth”) that calibrates observed discharges from 1690 Environment and Climate Change Canada (ECCC) river gauges with climate model P‐ET (precipitation minus evapotranspiration) data. Our application of FSM to all 1690 gauged watersheds identifies 916 significant topographical control points controlling >20% and/or 1000 km2 of their respective areas. The Geikie, Snare, Kazan, Tazin, and Seal rivers may be particularly affected, with impacted watershed areas ranging from 12% to 64%. Extending this approach to ungauged parts of the CS reveals an additional 635 significant topographical control points. Ensemble climate model projections suggest that around 10% of these control points are currently dry but will become active by 2100. This research explicitly determines how CS watersheds are affected by fill‐and‐spill hydrology, and demonstrates the importance of accurate terrain modelling for delineating surface water flow paths in depressional landscapes.

Effect of surficial geology mapping scale on modelled ground ice in Canadian Shield terrain

Abstract. Ground ice maps at small scales offer generalized depictions of abundance across broad circumpolar regions. In this paper, the effect of surficial geology mapping scale on modelled ground ice abundance is examined in the Slave Geological Province of the Canadian Shield, a region where the geological and glacial legacy has produced a landscape with significant variation in surface cover. Existing model routines from the Ground ice map of Canada (GIMC) were used with a 1:125 000-scale regional surficial geology compilation and compared to the national outputs, which are based on surficial geology at a 1:5 000 000 scale. Overall, the regional-scale modelling predicts much more ground ice than the GIMC due to greater representation of unconsolidated sediments in the region. Improved modelling accuracy is indicated by comparison of the outputs to empirical datasets due to improved representation of the inherent regional heterogeneity in surficial geology. The results demonstrate that the GIMC significantly underestimates the abundance and distribution of ground ice over Canadian Shield terrain. In areas with limited information on ground ice, regional-scale modelling may provide useful reconnaissance-level information to help guide the field-based investigations required for planning infrastructure development. The use of current small-scale ground ice mapping in risk or cost assessments related to permafrost thaw may significantly influence the accuracy of outputs in areas like the Canadian Shield, where surficial materials range from bedrock to frost-susceptible deposits over relatively short distances.

Natural metal contents and influence of salinization in deep Canadian Shield groundwater: Base level versus mineral deposit enrichment halos

Groundwater interacts with mineral deposits, making this secondary environment a medium of choice for geochemical exploration. With increasing depth, groundwater in Precambrian shields typically matures and increases in salinity through prolonged water–rock interactions and mixing with geologically old saline fluids. As the focus of mineral exploration is gradually shifting toward deeper targets, the influence of salinity on pathfinder background levels and anomalous thresholds must be considered. We compiled a deep background database (Deep Abitibi Groundwater database; DAGW) containing groundwater samples between depths of 0 m and 690 m from barren exploration boreholes in the Abitibi subprovince, Quebec, to evaluate the natural elemental levels at depth. The DAGW provides valuable knowledge of the base levels of a wide spectrum of elements and of the changes in these base levels along with the increasing salinity in bedrock aquifers of the deep Canadian Shield. This can serve to estimate background values during hydrogeochemical exploration campaigns. The regional Abitibi base level for Zn (median of 7.5 μg/L), a key pathfinder for VMS mineralization, is unaffected by the increasing salinity levels down to depths of 690 m. Hence, down to this depth, the Zn concentrations measured may be used directly for mineral exploration, and no further correction for the effects of salinization is necessary. We conducted a case study near an undisturbed VMS-type deposit (Daniel 25) and a Zn enrichment halo was observed. Within the halo, Zn concentrations increased with depth, indicating that Zn mobility was positively affected by the geochemical conditions of the deeper aquifer. Anomalous Zn thresholds representing the proximal (50–600 m from the mineralization) and direct contact (within 50 m of the mineralization) footprints of the Daniel 25 deposit were estimated at 50 μg/L and 380 μg/L, respectively. An anomalous threshold was also estimated for Co, a secondary pathfinder, at 0.5 μg/L. Unlike Zn, several other elements, which could be used to trace mineral deposits, are affected by salinization. As a final step, we applied a correction to the Daniel 25 data, based on elemental baseline trends observed in the DAGW. The aim was to remove the effects of salinity and enhance the statistical correlations caused by the presence of the mineralized body. This correction led to the emergence of new spatial correlations, notably with Na and Sr, that are coherent with the increasing hydrothermal alteration in proximity to the ore.

Detrital zircon U-Pb + Hf data supports 2.1 Ga extensional and 2.0 Ga syn-orogenic basin in southwest Rae Province during early Nuna assembly

This study examines the Paleoproterozoic history of supracrustal rocks in the western Canadian Shield to better inform the tectonic history of northern Laurentia. Samples for detrital zircon U-Pb geochronology and Hf isotopes were collected from the western Rae Province, Taltson magmatic zone, and the Great Slave Lake shear zone in order to assess depositional ages, provenance and geodynamic significance of the detrital systems. Samples from the western Rae Province were sourced locally and deposited after 2.1 Ga in a broadly extensional regime, based on widespread source terrane ages and few data near the maximum depositional age as well as coeval regional mafic magmatism and absence of coeval metamorphism. By contrast, the Taltson samples were deposited after 2.0 Ga, have a significant 2.2 to 2.0 Ga detrital source of relatively juvenile material, have young zircon that overlap in age with the Taltson and Thelon orogens, and yield a significant fraction of the dates that are close to the maximum depositional age. These Taltson samples were also derived from western Rae Province sources or Buffalo Head and Chinchaga domain sources and are interpreted to have formed in a contractional or collisional regime coeval with Taltson-Thelon orogeny. The 2.2 to 2.0 Ga detritus was likely derived from reworked ca. 2.3 Ga juvenile source. A sample from the Great Slave Lake shear zone was deposited after 2.0 Ga in a contractional/collisional regime and was sourced from Slave Province crust and Taltson-Thelon plutons. Our data support extension along the western Rae Province margin at ca. 2.1 Ga and distinct histories of the Rae and Slave Provinces prior to their collision at ca. 1.95 Ga. Before 2.1 Ga, the Rae Province may have been attached to the Slave, North China, Bastar, Dharwar, or Sinhghbum cratons.

Evaluating the significance of wetland representation in isotope-enabled distributed hydrologic modeling in mesoscale Precambrian shield watersheds

Evaluation of regional-scale models against both streamflow and stable isotopes of water is a relatively new and evolving area of distributed hydrological modeling. While applications of isoWATFLOOD, an isotope-enabled distributed hydrological model are increasing, evaluation of the impacts of model structural features, such as wetland connectivity, on simulation of runoff components is still limited. Wetlands are important water bodies that impact streamflow generation and its spatial variation in Precambrian shield watersheds in parts of Canada and elsewhere. This study investigates the efficiency of isoWATFLOOD in simulating streamflow and instream δ18O and the effects of model percent connected wetlands (%CW) in a mesoscale (∼12,000 km2) Precambrian Shield watershed in Northeastern Ontario, Canada. Model calibration and validation are performed using daily streamflow for five hydrometric gauging stations and biweekly to monthly instream δ18O from 11 river sampling locations from across the watershed. Five model versions with different %CW are generated, calibrated, and validated separately. Results show all models can simulate the timing and pattern of streamflow and δ18O. %CW impacts simulated streamflow during both calibration and validation periods, calibrated parameter sets and simulated internal hydrological processes. Model performance varies spatially with no single value of %CW providing best performance across sub-catchments with 40 % CW providing best mean streamflow simulations in calibration. Increasing %CW (10 % to 40 %) results in higher contribution of lower soil zone water to streamflow that markedly improves baseflow simulation. This study builds on recent isotope-enabled hydrologic simulation capacity of isoWATFLOOD, contributing new examination of model representation of wetland connectivity, a critical component of Precambrian Shield watersheds and evidence of how stable isotopes can contribute to addressing the challenge of equifinality. Results suggest the need to develop model capability for representation of spatial variation in wetland connectivity to recognize this important aspect of heterogeneity in watersheds influencing hydrologic function.

Regional-Scale Paleoproterozoic Heating Event on Archean Acasta Gneisses in Slave Province, Canada: Insights from K–Ar and 40Ar/39Ar Chronology

Slave Province in Canada is an Archean granite–supracrustal terrane at the northwestern corner of the Canadian Shield. It is bordered by the Thelon–Taltson orogen (2.0 to 1.9 Ga) to the southeast and the Wopmay orogen (1.9 to 1.8 Ga) to the west. Acasta gneisses, exposed in the westernmost Slave Province, and the Wopmay rocks, located close to the gneisses, were systematically collected for K–Ar and laser step-heating 40Ar/39Ar single-crystal analyses of the biotite and amphibole. The K–Ar biotite ages of the four Wopmay samples range from 1816 ± 18 Ma to 1854 ± 26 Ma. The 40Ar/39Ar biotite analyses of the three Wopmay samples yield plateau ages of 1826 ± 21 Ma, 1886 ± 13 Ma, and 1870 ± 18 Ma. These ages fall within the reported U–Pb zircon age range of the Wopmay orogen. The K–Ar biotite ages of the fifteen Acasta gneisses range from 1779 ± 25 Ma to 1877 ± 26 Ma, except for one younger sample (1711 ± 25 Ma). The 40Ar/39Ar analyses of the biotite crystals from three samples give the plateau ages of 1877 ± 8 Ma, 1935 ± 14 Ma, and 1951 ± 11 Ma. The K–Ar amphibole ages from twelve samples range from 1949 ± 19 Ma to 1685 ± 25 Ma. Two samples of them give ages older than the zircon U-Pb age of Hepburn plutons. The 40Ar/39Ar analyses of the amphibole crystals show varied age relations. The two samples give plateau ages of 1814 ± 22 Ma and 1964 ± 12 Ma. Some samples exhibit apparent old ages of ~2000 Ma in the middle temperature fractions. These old fractions result from the amphibole crystals, originally formed in the Archean, being affected by the thermal events during the Wopmay orogeny but not fully resetting. These observations suggest that the K–Ar system ages of the biotite and amphibole in the Archean Acasta gneiss were rejuvenated during the Paleoproterozoic ages. The Discussion explores the possibility that the heat source rejuvenating the K–Ar system ages may have arisen due to asthenospheric extrusion into the wedge mantle, a process likely triggered by subduction rollback.

Geodynamic setting of Proterozoic massif-type anorthosites in the Eastern Canadian Shield

Massif-type anorthosites occur worldwide and were predominantly formed during the Paleo- to Mesoproterozoic Era, with an exceptionally large number emplaced in southeastern Laurentia, both within the Grenville Province and in the foreland of the Southeastern Churchill Province and North Atlantic Craton (Nain Province). Their secular nature infers that physical and chemical conditions necessary for their formation were optimized during that time period. A review of geochronological and ambient tectonic regimes that were operating during their emplacement suggests that they formed during four periods, ca. 1.65 Ga, ca. 1.45–1.30 Ga, ca. 1.16–1.14 Ga, and 1.08–1.01 Ga. The first two pulses overlap with continent–ocean convergence and subduction beneath the SE Laurentian margin. The last two pulses correlate with continent–continent collisional tectonics. We argue that higher mantle temperatures during the Proterozoic, as inferred in published models, were not sufficient, by themselves, to produce the necessary volume of basaltic underplate—a key factor in the generation of massif-type anorthosites—and that mantle re-fertilization through metasomatism and/or lateral accretion of oceanic lithosphere might have played a key role. High geothermal gradients acquired during the 2.0–1.2 Ga accretionary and continental arc phase may have helped sustain abnormally high temperatures during the 1.2–1.0 Ga collisional phase of the Grenville Province.

Dynamical response of the southwestern Laurentide Ice Sheet to rapid Bølling–Allerød warming

Abstract. The shift in climate that occurred between the Last Glacial Maximum (LGM) and the Early Holocene (ca. 18–12 kyr BP) displayed rates of temperature increase similar to present-day warming trends. The most rapid recorded changes in temperature occurred during the abrupt climate oscillations known as the Bølling–Allerød interstadial (14.7–12.9 kyr BP) and the Younger Dryas stadial (12.9–11.7 kyr BP). Reconstructing ice sheet dynamics during these climate oscillations provides the opportunity to assess long-term ice sheet evolution in reaction to a rapidly changing climate. Here, we use glacial geomorphological inversion methods (flowsets) to reconstruct the ice flow dynamics and the marginal retreat pattern of the southwestern sector of the Laurentide Ice Sheet (SWLIS). We combine our reconstruction with a recently compiled regional deglaciation chronology to depict ice flow dynamics that encompass the time period from pre-LGM to the Early Holocene. Our reconstruction portrays three macroscale reorganizations in the orientation and dynamics of ice streaming followed by regional deglaciation associated with rapid warming during the Bølling–Allerød interstadial. Initial westward flow is documented, likely associated with an early set of ice streams that formed during the advance to the LGM. During the LGM ice streaming displays a dominant north to south orientation. Ice sheet thinning at ∼15 ka is associated with a macroscale reorganization in ice stream flow, with a complex of ice streams recording south-eastward flow. A second macroscale reorganization in ice flow is then observed at ∼14 ka, in which southwestern ice flow is restricted to the Hay, Peace, Athabasca, and Churchill river lowlands. Rates of ice sheet retreat then slowed considerably during the Younger Dryas stadial; at this time, the ice margin was situated north of the Canadian Shield boundary and ice flow continued to be sourced from the northeast. Resulting from these changes in ice sheet dynamics, we recognize a three-part pattern of deglacial landform zonation within the SWLIS characterized by active ice margin recession, stagnation and downwasting punctuated by local surging (terrestrial ice sheet collapse): the outer deglacial zone contains large recessional moraines aligned with the direction of active ice margin retreat; the intermediate deglacial zone contains large regions of hummocky and stagnation terrain, in some areas crosscut by the signature of local surges, reflecting punctuated stagnation and downwasting; and the inner deglacial zone contains inset recessional moraines demarcating progressive regional ice margin retreat. We attribute these macroscale changes in ice flow geometry and associated deglacial behaviour to external climatic controls during the Bølling–Allerød and Younger Dryas but also recognize the role of internal (glaciological, lithological and topographic) controls in SWLIS dynamics.

Bedrock morphology influences rock barrens turtle nesting habitat energy dynamics.

Energy absorption and flow through a nest is an important aspect of embryonic development in many reptile species including turtles. To date, few studies have explicitly attempted to quantify the energy flow through turtle nests, opting instead for the simplified approach offered by temperature index models. However, the quantification of the energy can provide an explicit abiotic link that can link biological models to biometeorological and ecohydrological processes and models. We investigated the energy flow through turtle nests occupying different bedrock morphologies within a Canadian Shield Rock Barren landscape, in Ontario, Canada. The taxons studied were Spotted Turtle (Clemmys guttata), Midland Painted Turtle (Chrysemys picta marginata), and Blanding's Turtle (Emydoidea blandingii). Nest temperature and soil moisture were measured in 2018 and 2019 using sensors placed in the soil adjacent to 12 turtle nest cavities. Three main rock morphologies were identified for each nest location, Crevice, Ledge, and Flat types, that are in order of decreasing bedrock percentage contact with the nest site. Ground heat flux and change in heat storage were determined using the calorimetric method for each nest, while the direction of energy flux between the atmosphere and the underlying rock was also determined. The Crevice nest morphology experienced the lowest ground heat flux on average (1.56 × 10-1 W m-2) and lowest cumulative heat storage (230 MJ) compared to the Flat (440 MJ) and Ledge (331 MJ) nests. However, over the diurnal cycle, large heat gains by Flat nests were mostly balanced out by nighttime heat losses. While Crevice nests saw the lowest daily heat storage gains, they experienced much lower heat losses over the evening period compared to the other nest types. Furthermore, we found that 59% of the energy is directed from the underlying bedrock into the Crevice nest, highlighting the importance of the bedrock in controlling thermal dynamics in the turtle nesting habitat. The lower variability in energy parameters for Crevice nest types can be attributed to higher amounts of nest-to-bedrock contact, compared to the flat nest types. Our results indicate that Crevice morphology may be ideal for turtles nesting at their northern limits because minimal heat loss during the evening can result in a more stable thermal incubation environment. Future conservation and habitat restoration efforts should consider the importance of bedrock morphology and prioritize the protection of Crevice nest sites. Furthermore, this work highlights important opportunities for potential interdisciplinary work between ecologists, climatologists, biologists, and hydrologists, specifically the integration of ecohydrological and biological models. This work also underscores the potential uncertainty of climate change impacts on turtle egg hatching success and nest sex ratios.

A novel V-cut method for explosive-free breakage of biaxially loaded rock using soundless chemical demolition agents

Interest in soundless chemical demolition agents (SCDAs), also known as expansive cements, as potentially viable alternatives to explosives for rock fragmentation, has been growing in recent years. Consequently, there is an increasing amount of literature on the use of SCDA for the breakage of rock blocks and boulders. Limited research has been conducted so far on the breakage of excavation fronts, such as tunnel or drift faces, using SCDA. This is due to the perception that the planar compressive in-situ stresses in the face would inhibit the creation and propagation of fracturing due to expansive pressure. This study proposes a novel V-cut method for demolishing rock panels under biaxial stress using SCDA. This method was examined through large-scale tests and numerical modelling. The rock panels were subjected to high biaxial confinements of 26 and 40 MPa. Such a level of confinement corresponds to an in-situ stress state 1000 m below the surface in the Canadian shield. The V-cut drillhole pattern employs two sets of three SCDA holes angled at 45° from the face of a Stanstead granite panel. The drillhole arrangement aims to create a V-shaped wedge in the plane of major principal stress. When angled drillholes are subjected to expansive pressure, they tend to cast out of the panel face, causing fragmentation. Two panels of 1 m × 1 m × 0.25 m were successfully demolished using the proposed method. The three-dimensional fast Lagrangian analysis code FLAC3D modelling was used to reconstruct the panel failure mechanism owing to the V-cut. This study demonstrates the feasibility of fragmenting an excavation front, such as a rock excavation face, with SCDA using a V-cut drill hole pattern while subjected to high biaxial confinement.

Improvement of Soil Water Assessment Tool (SWAT) wetland module for modelling of ephemeral pond hydrology

AbstractEphemeral ponds (EPs) are seasonally flooded isolated wetlands that provide a variety of hydroecological benefits, including the provision of breeding habitat for several amphibian and invertebrate species. However, the lack of their explicit representation in hydrological models limits a comprehensive understanding of their interaction with surrounding landscapes and their vulnerability in the context of human interventions and climate change. The purpose of this research was to improve the isolated wetland module of the Soil Water Assessment Tool (SWAT) to better represent EP hydrology. The changes include (1) representation of groundwater and hypodermic flow as the only inflows from the pond drainage surface, due to the intermittent and negligible presence of inflow from surface runoff in forested ponds, (2) revision of how evapotranspiration within EPs is represented and (3) implementation of distinct volume‐area‐depth relationships for ponds based on their geometrical shape. The accuracy of these improvements was assessed against that of a previous isolated wetland formulation in replicating water depth observations of 10 EPs of a portion of the Kenauk forest (68 km2) in the Canadian Shield of the Outaouais region (Québec, Canada). The comparison results show that the revised SWAT model presented here significantly improves the distinct filling and drying water cycle of EPs (average root mean square error of 0.1 m of the revised model vs. 0.23 m for the original model). Besides, the new module allowed to identify that hypodermic flow, evapotranspiration and seepage to the underlying soil are the main EP source and sinks. The new module also allowed to explicitly quantify the differences in filling/drying pattern of the EPs of the Kenauk forest and unlike the original model structure, the new module was able to closely replicate the interannual variation of spring and annual hydroperiod duration.

Phanerozoic Burial and Erosion History of the Southern Canadian Shield from Apatite (U-Th)/He Thermochronology

Patterns of Phanerozoic burial and erosion across the cratonic interior of North America can help constrain the continental hypsometric history and the potential influence of dynamic topography on continental evolution. Large areas of the Canadian Shield currently lack Phanerozoic sedimentary cover, but thermochronology data can help reconstruct the previous extent, thickness, and erosion of Phanerozoic strata that once covered the craton. Here, we report apatite (U-Th)/He (AHe) data for 15 samples of Precambrian basement rocks and 1 sample of Triassic kimberlite from a 1400 km–long east–west transect across the southern Canadian Shield. Single-grain basement AHe dates range from >500 Ma in the west to <250 Ma in the east. AHe dates for the kimberlite in the middle of the transect overlap with the pipe’s Triassic eruption age. These data, combined with previous apatite fission-track data, geologic constraints, and thermal history modeling, are used to constrain the first-order regional thermal history that we interpret in the context of continental burial and erosion. Our burial and erosion model is characterized by Paleozoic burial that was greater to the east, unroofing that migrated eastward through Jurassic time, and little to no post-Triassic burial. This pattern suggests dynamic and tectonic forces related to Appalachian convergence, subduction cessation, and later rifting as drivers. The AHe data contribute to efforts to collect thermochronology data across the Canadian Shield to map out continental-scale burial and erosion patterns. The outcomes can be used to refine mantle dynamic models and test how dynamic topography, far-field tectonics, and other effects influence the surface histories of continental interiors.

Spatial patterns of resource subsidies in Great Lakes tributaries from migratory fishes

Migratory fish can influence the stream ecosystems where they spawn by depositing large amounts of energy rich eggs, carcasses, and excrement which can increase primary and secondary productivity. Past research in the Great Lakes has focused on individual tributaries; accordingly, there is a poor understanding of how wide-spread resource subsidies are amongst tributaries. To determine which Great Lake tributaries received subsidies, we compared carbon stable isotope values (δ13C) of resident stream fishes above and below 54 barriers (e.g., low-head lamprey weirs, natural waterfalls) in 31 tributaries. Subsidies, as indicated by the difference in δ13C values above and below barriers, were common. The magnitude of the subsidy varied among Great Lakes with Lake Superior having significantly larger subsidies than the other three lakes. Barrier type (complete, partial) was not related to subsidy magnitude; however, the classification of barrier type was imperfect. Correspondence analysis of regional and local environmental factors showed that large subsidies were associated with oligotrophic streams with steep channels on the Canadian Shield with little agricultural land. The association between subsidies and canopy cover and substrate size were poor. Migratory fish supply resource subsidies to rivers across the Great Lakes basin, though their magnitude is contextually dependent. The presence of barriers are likely limiting the production of migratory fishes that depend on streams for juvenile production. The importance of resource subsidies should be considered when decisions are made about the fate of existing, and the construction of new, barriers that may reduce stream productive capacity.

Contrasting rare earth element concentrations and mixing behaviors in the St. Lawrence Estuary and Saguenay Fjord

Rare earth elements (REEs) including Yttrium (Y) are commonly used as tracers of estuarine and oceanic mixing. The lanthanide series and yttrium are usually referred to as REYs. The geochemical behavior of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. During mixing, partitioning of these elements occurs according to their source function and the stability of natural complexes, with heavy REEs typically forming more stable complexes than light REEs in solution. In this study, we compare the concentrations and partitioning of the 0.7 μm-filtered and 0.05 μm-filtered fractions of the dissolved REYs collected during the summers of 2017 and 2021 in the surface waters (< 3 m) of the St. Lawrence estuarine system (river, estuary and gulf) with those of the Saguenay Fjord, a tributary of the latter that drains the Mesoproterozoic rocks of the Canadian Shield. Whereas REYs do not mix conservatively in the St. Lawrence Estuary (SLE) in the summer, they nearly do so in the Saguenay Fjord (SF). REY concentrations are 2.5 to 6 times greater in the surface waters of the SF than those of the SLE at the same salinity and, in contrast to most estuaries including the SLE, the fjord waters are enriched in LREEs. The 0.05 μm-filtered REY concentrations are positively correlated with dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) concentrations in the SF but independent of both DOC and CDOM concentrations in the SLE. The CDOM in the fjord differs from that of the estuary as it is more aromatic and has a higher molecular weight. The formation of strong REE-humate complexes stabilizes REY ions in the SF surface waters and impedes their adsorption to and scavenging by solid surfaces during estuarine mixing. The LREE enrichment in the SF surface waters most likely reflects the geology of the fjord's drainage basin, more specifically the exposed Mesoproterozoic granites and gneisses of the Canadian Shield that are enriched in LREE relative to the younger Paleozoic sedimentary rocks exposed along the St. Lawrence Lowlands.

Proterozoic - basal Cambrian stratigraphy across Nares Strait: correlation between Inglefield Land and Bache Peninsula

In the Kane Basin region of Nares Strait (78° to 79°N) unmetamorphosed Proterozoic and basal Cambrian platform strata overlie the Precambrian Shield both in Inglefield Land in Greenland, and on Bache Peninsula in Ellesmere Island. The platform succession is composed of two shallow-water elastic sequences separated by an erosional disconformity. The lower formation, of multicoloured sandstone and siltstone with minor shale and stromatolitic dolomite, is assigned to the redefined Rensselaer Bay Formation; the upper, composed of red and overlying white sandstone with dolomitic sandstone and siltstone at the top, is assigned to the Dallas Bugt Formation. Basic sills from the Rensselaer Bay Formation have yielded isotopic ages as old as 1200 m.y., while the trace fossils Rusophycus and Skolithos suggest a basal Cambrian age for the Dallas Bugt Formation.&#x0D; The platform sequences of Inglefield Land and Bache Peninsula show striking similarity in lithology and thickness and the detail correlation is supported by the evidence from isotopic ages of basic sills and from trace fossils. In both areas the bipartite succession is overlain with apparent conformity by dolomites that pass upwards into limestones containing Early Cambrian macrofauna.&#x0D; The Rensselaer Bay Formation thickens southwards and passes into the much thicker sedimentary sequence of the Thule Basin, while the Dallas Bugt Formation and the overlying dolomites thicken northwards, indicating that the two formations were laid down in separate sedimentary basins. The close correlation of the platform successions and the corresponding configuration of the depositional slopes on both sides of the Kane Basin clearly indicate that the Proterozoic - Early Cambrian sedimentary pattern has not been radically disturbed by tectonism or by any appreciable transcurrent movement along Nares Strait.

Spatial Characterization of Shallow Structures in the Revell Batholith Integrating Seismic Imaging Techniques

The Revell Site, located in Northwestern Ontario within the Canadian Shield, is being assessed as a potential Deep Geological Repository (DGR) for Canada’s used nuclear fuel. Effective DGR establishment requires comprehensive subsurface assessment, particularly in evaluating structural stability, hydrogeological attributes, geological composition, and geochemical properties. Key among these considerations is understanding the three-dimensional characteristics of structural features to ensure the site’s suitability for long-term containment of radioactive materials. This case study focuses on imaging and characterizing structures within the predominantly intact biotite granodiorite-tonalite host rock at the Revell Site. Borehole data reveals these structures as mostly sub-horizontal mafic intrusions, with thicknesses of up to 3.5 m. They appear either as separate, discrete entities or in stacked configurations. Despite their limited thickness, most of these intrusions exhibit discernible attributes in surface seismic images due to their distinct physical properties. Some, however, exhibit velocity variations that decrease their overall reflectivity properties. An integrative approach employing surface seismic, Vertical Seismic Profiles (VSP), and borehole data facilitates the spatial identification of over 30 of these mafic structures. This comprehensive characterization not only lays the foundational framework for future discrete fracture network models but also provides important support for simulations related to fluid flow dynamics, groundwater behavior, contaminant dispersion, and heat transport mechanisms within the Revell Site. This study underscores the critical role of in-depth subsurface characterization in ensuring the secure, long-term management of radioactive materials in DGRs.