Canadian Shield Research Articles

Wind variability over a large lake with complex topography: Lake of the Woods

Lake of the Woods spans two geological regions: the Precambrian Shield and the Glacial Lake Agassiz Lakebed. The lake watershed’s complex topography influences wind speeds and directions. We study wind variability over Lake of the Woods using reanalysis data and wind observations at seven sites. A wind variability index is proposed as a measure of lake-wide spread in wind direction. Our results show that the directional variability depends on wind speeds, with higher variability at lower speeds. The wind variability also changes throughout the year; it is more pronounced in spring (Apr-Jun) and fall (Sep-Oct) and less pronounced during winter (Dec-Jan). Observed wind directions are 60% more variable than reanalysis wind, indicating a need for higher-resolution reanalysis to resolve local wind effects due to topographic changes. Based on the statistical hierarchical clustering method, our analysis shows that geographically variable wind can be grouped into three regions to represent the predominant wind directions accurately. Clustering results also suggest spatial coverage gaps in the existing meteorological observation network. We reassess the existing network and propose a new meteorological observation site in the southeast basin near Morson, Ontario. Our study is of significance for water quality management because the wind is the key driver of water circulation, transport, and mixing in the lake, and accurate wind measurements are necessary to better constrain the underpinning physical, biological, and biochemical processes in mechanistic ecological models.

Modern style nappe stacking in the Paleoproterozoic lower crust: An example from the snowbird tectonic zone, Canadian Shield

The Intra Tantato shear zone (ITSZ), which occurs within the East Athabasca mylonite triangle in the southern portion of the ∼1.9 Ga Snowbird tectonic zone in northern Saskatchewan, Canada, is a low to high angle, curvi-planar, kilometer-thick mylonite zone hosted within granulite-facies gneiss that records a complex history with both thrust- and normal-sense motion. New field mapping, microstructural analysis, and quartz crystallographic preferred orientations (CPO) detail a composite history for the ITSZ that includes top-to-the-ENE thrusting of the hanging wall over the footwall that is overprinted by normal-sense top-to-the-WSW reactivation. Quartz CPOs further indicate constrictional deformation related to both events and quartz c-axis opening angles define deformation temperatures of ∼750 ± 50 °C for the thrusting event and ∼550 ± 50 °C for the later, normal-sense reactivation.Monazite and garnet geochronology on rocks within and around the ITSZ indicate it may have nucleated along a pre-existing 2.6–2.5 Ga sub-horizontal fabric. This appears to have occurred immediately after the ∼1.920 Ga metamorphic peak of the hanging wall and during the ∼1.905 Ga metamorphic peak of the footwall. Monazite ages from small matrix grains in interstitial positions in specimens that record reactivation kinematics indicate overprinting occurred at ∼1.860–1.830 Ga, contemporaneous with the earliest recorded activity along the nearby Grease River Shear Zone.Overall, this tectonic evolution can be attributed to a crustal thickening process that occurred in the Athabasca mylonite triangle during the Snowbird orogeny at ∼1.9 Ga and during the post-collisional exhumation processes. Moreover, these results show that the lower continental crust can actively accommodate shortening during large-scale collisional events with tectonic units piling up via nappe-stacking processes similarly to what is typically observed in the middle crust.

Potential energy sources for the deep continental biosphere in isolated anoxic brines

In isolated fracture networks in the Precambrian Shield, long-term water and rock interactions produce saline anoxic fluids that host extant microbial communities deep within the continental subsurface. Light and oxygen (O2) are absent in these environments. Thus, chemotrophic organisms inhabiting these systems rely on anaerobic reactions for energy. Viable electron donors include short-chain alkanes, such as methane (CH4) and C2+ alkanes, while alternative electron acceptors include sulfate (SO2−4), nitrate (NO−3), and ferric iron (Fe3+). Here, we constrain the potential sources of energy for microorganisms in Neoarchean bedrock on the 27th level west drift of the Soudan Underground Mine State Park, MN, USA (713.5 meters below the surface). The Gibbs Free Energy (ΔG) of 11 reactions are modeled and expressed as available chemical potential energy per mass fluid (J/kgfluid). Metabolic reactions involving CH4 oxidation by SO2−4 would yield the highest potential energy of reactions modeled in this study (−111 J/kgfluid). The free energy for methanogenesis via the breakdown of dimethylamine (DMA; ∑(CH3)2NH(aq)) is exergonic but with near-zero available energy per mass fluid, suggesting that DMA may be cycled quickly to produce biological CH4 at Soudan. We examine all the possible pathways by which CH4 and other short-chain alkanes may be formed. Conventional δ13CCH4 values and C1/C2+ abundance ratios support a mixed biological and non-biological origin of CH4. Doubly substituted ‘clumped’ CH4 isotope 13CH3D values are consistent with formation temperatures of 84-89°C that exceed current environmental conditions of 11.5-12.1°C. These estimated formation temperatures are too low for CH4 to be formed solely through thermogenic degradation of organic matter. Further, low or undetectable H2 rules out active abiogenesis of CH4 from CO2 reduction. It is more likely that the bulk CH4 pool reflects a mixture of microbial CH4 with Δ13CH3D values equilibrated at 11.5-12.1°C and thermogenic CH4 formed at temperatures >100°C. Understanding the origin and cycling of these electron donors contributes to a fundamental understanding of how microbial activity may promote, maintain, or suppress the habitability of these isolated systems over long timescales.

Mineral and isotopic (Nd, Sr) signature of fine-grained deglacial and Holocene sediments from the Mackenzie Trough, Arctic Canada

ABSTRACT Dolomites in Arctic Ocean sediments are widely attributed to erosion and transport of sediments from northern Canada and Greenland. Coarse-grained dolomite-rich ice-rafted debris is often linked to iceberg transport, but the origin of fine-grained dolomite is less well constrained. A presumed source is the Mackenzie River. In this article, we fingerprint the minerogenic and isotopic (Nd and Sr) composition of deglacial and Holocene fine-grained sediments (<38 μm) from the shallow Mackenzie Trough. Sediments from an 81.5-m borehole (MTW01) were analyzed. The borehole is composed of a progradational (deltaic), transitional (transgressive), and marine unit (<9.4 cal. ka BP). The average dolomite content (~7 percent) and ɛNd signals (−13.3) are surprisingly constant in the progradational and marine units. The isotopic signature is inherited from the Interior Platform, the major underlying bedrock region of the Mackenzie River mainstream. The transitional unit contains fluctuations in ɛNd (−11.0 and −14.6), reflecting enhanced input from the North American Cordillera and Canadian Shield that are not associated with elevated amounts of dolomite. Additional studies combining minerogenic and detrital ɛNd analyses from sites proximal to the paleo-icestreams draining the Canadian Arctic are required to ascertain the origin of dolomite enrichment in central Arctic Ocean sediments.

Hydrological mass balance of boreal watersheds in the Canadian Shield

This study aims at evaluating the hydrological balance of large watersheds of the Canadian Shield in the James Bay area in Northwestern Quebec, Canada. The focus is set on six rivers of the Canadian Shield altogether draining more than 185,000 km2 of the Boreal Shield, Taiga Shield and Hudson Plains ecozones of Canada. River discharge measurements, geochemical data (δ2H, δ18O and electrical conductivity [EC] of water), remote sensing, and GIS models are used jointly to calculate water balances. The approach allows for partitioning the influence of rainwater, snowmelt, surface runoff, evaporation, transpiration, and groundwater discharge to the hydrological balances of watersheds. On an annual basis, the results suggest that runoff from rainwater (30–61 % of total precipitation) and snowmelt (18–40 % of total precipitation) are the main contributions to river discharge, while the contribution of groundwater discharge to rivers represents < 12 % of the total precipitation. Over the study area, this contribution represents 2–5 km3 of water. The stable isotope mass balances allow for estimating watershed-scale evaporation over inflow ratios ranging between 2 and 10 % and suggest that transpiration has an isotopic composition close to summer rainwater. The hydrological balances further suggest that the total pool of water stored in the active portion of watersheds represents 10–20 % of the total annual precipitation, while the exports of groundwater beyond the limits of surface watersheds are negligible. The seasonal trends in the hydrological balances of monitored watersheds were further documented to provide insights into the sensitivity of watersheds as they face climate change. The observations are used to propose recommendations for monitoring of rivers in the Canadian Shield and to identify future research needs.

Geology, geochemistry, and apatite/titanite U–Pb geochronology of ca. 1.88 Ga alkaline ultrabasic dykes in the Southern Province near Sudbury, Ontario

The area northeast of Sudbury, Ontario, is well known for hosting one of the largest unexplained geophysical anomalies in the Canadian Shield, the Temagami Anomaly. In search of a geological explanation for this anomaly, low-grade metamorphic ultrabasic dykes have been discovered in the overlying Huronian Supergroup sedimentary rocks, both in outcrop and in a deep drill core. Here, we report on the first geochemical and geochronological data obtained on these dykes and compare these data with known magmatic units in and around the 1850 Ma impact-generated Sudbury Igneous Complex (SIC). The NW-striking dykes, which cut across sedimentary rocks of the ca. 2.3 Ga Cobalt Group, and which are, in turn, crosscut by pseudotachylitic breccia, are characterized by distinctively high concentrations of Ti, P, Nb, and Zr, highly fractionated rare earth element patterns (La/YbN 7.6–15.5), and a lack of crustal contamination (Nb/Th &gt; 10). Such features are typical of modern ocean island basalts but very different from Palaeoproterozoic rocks previously documented in the wider region. Multigrain U–Pb laser ablation inductively coupled plasma mass spectrometry analyses performed on magmatic titanite and apatite with high Th/U ratios yielded 1876.0 ± 8.7 and 1880.9 ± 8.3 Ma, respectively, which we interpret as the intrusion age of the dykes. This interpretation is supported by similar whole-rock Sm-Nd model ages of 1890–2000 Ma (initial εNd +2.5). This magmatic event in the footwall of the SIC shortly before the impact was coeval with, and likely genetically related to, the 1.88–1.87 Ga Circum-Superior Large Igneous Province.

Evaluation of the Effect of Geomechanical Parameters and In Situ Stress on Tunnel Response Using Equivalent Mohr-Coulomb and Generalized Hoek-Brown Criteria

The generalized Hoek-Brown (GHB) failure criterion can estimate the rock mass parameters required for rock mechanics–related analyses such as numerical modeling in geomechanics. The determination of GHB parameters has been developed in the field of rock mechanics. Due to the wide use of the Mohr-Coulomb criterion and the lack of an existing relationship for determining its parameters for a rock mass, equivalent Mohr-Coulomb parameters (EMC) can be derived from the GHB. To determine the differences in the use of these two criteria, we analyzed the behavior of a deep circular tunnel in nine stress states for three metamorphic rocks recovered from the Canadian Shield from rock masses that present a very blocky structure. We carried out 241 simulations using the finite element code RS2 to assess the effect of the geological strength index (GSI), in situ stress, and rock type on the deviation of wall displacement, the number of yielded elements, and the differential stress obtained by the GHB and EMC parameters. A combination of low in situ stress and high GSI yielded similar results when using both failure criteria.

Numerical analysis of midwinter infiltration along the soil-rock interface: A pathway for enhanced bedrock recharge

Shallow bedrock aquifers are prevalent across the Northern Hemisphere and can be the primary source of freshwater for many communities. Cryospheric impacts from climate warming are particularly concerning for bedrock aquifers where recharge is dominated by winter precipitation. For the management and protection of fractured rock groundwater supplies, the processes influencing infiltration and recharge under warmer winter conditions require a greater understanding. As such, a numerical investigation was undertaken with the objective of testing a conceptual model of midwinter infiltration and recharge near a granitic Canadian Shield bedrock outcrop using data collected over an unseasonably warm and rainy winter in eastern Ontario, Canada. Additional objectives were to determine if infiltration and recharge would be limited through frozen soils without an outcrop and to explore the hydrologic implications of an additional midwinter snowmelt event. Results supported the concept that outcrops provide preferential infiltration along the soil-rock interface at outcrop margins, which enables localized bedrock recharge under frozen conditions. To simulate soil moisture content near the outcrop, bedrock runoff was needed as additional lateral input in the lower soil profile (75 cm). The amount of runoff required as input was controlled by the hydraulic conductivity of the soil-rock interface. In the absence of an outcrop, limited midwinter infiltration occurs due to reduced permeabilities of frozen soils from repeated early winter rainfall and snowmelt events. Additional midwinter melt events can likely increase bedrock recharge, but the associated decline in snow depth deepens soil frost which delays further infiltration and increases surface runoff. Simulations suggest winter is becoming a more active time for bedrock recharge where outcrops are present, but wetter conditions have implications for frozen soil permeability. Findings extend to similar seasonally frozen bedrock environments which are relied on as a source of freshwater and are rapidly changing under a warming climate.

The Importance of Lake Emergent Aquatic Vegetation for Estimating Arctic‐Boreal Methane Emissions

AbstractAreas of lakes that support emergent aquatic vegetation emit disproportionately more methane than open water but are under‐represented in upscaled estimates of lake greenhouse gas emissions. These shallow areas are typically less than ∼1.5 m deep and can be detected with synthetic aperture radar (SAR). To assess the importance of lake emergent vegetation (LEV) zones to landscape‐scale methane emissions, we combine airborne SAR mapping with field measurements of vegetated and open‐water methane flux. First, we use Uninhabited Aerial Vehicle SAR data from the NASA Arctic‐Boreal Vulnerability Experiment to map LEV in 4,572 lakes across four Arctic‐boreal study areas and find it comprises ∼16% of lake area, exceeding previous estimates, and exhibiting strong regional differences (averaging 59 [50–68]%, 22 [20–25]%, 1.0 [0.8–1.2]%, and 7.0 [5.0–12]% of lake areas in the Peace‐Athabasca Delta, Yukon Flats, and northern and southern Canadian Shield, respectively). Next, we account for these vegetated areas through a simple upscaling exercise using paired methane fluxes from regions of open water and LEV. After excluding vegetated areas that could be accounted for as wetlands, we find that inclusion of LEV increases overall lake emissions by 21 [18–25]% relative to estimates that do not differentiate lake zones. While LEV zones are proportionately greater in small lakes, this relationship is weak and varies regionally, underscoring the need for methane‐relevant remote sensing measurements of lake zones and a consistent criterion for distinguishing wetlands. Finally, Arctic‐boreal lake methane upscaling estimates can be improved with more measurements from all lake zones.

Garnet Geochemistry and Lu-Hf Geochronology of a Gold-Bearing Sillimanite-Garnet-Biotite Gneiss at the Borden Lake Belt

The Kapuskasing Structural Zone transects the Wawa and Abitibi greenstone belts in the Superior province of the Canadian Shield. At the southern margin of the Kapuskasing Structural Zone (KSZ), the Borden Gold deposit hosts low-grade-gold mineralization within upper amphibolite- to granulite-facies sillimanite-garnet-biotite gneisses. Here, we present the geochemistry and Lu-Hf geochronological studies of a gold-hosting garnet-bearing gneiss to constrain the formation history of the deposit and regional geological history. The garnet is almandine-rich, and contains inclusions of quartz, biotite, pyrite, and zircon. The absence of chemical zoning in garnet is likely due to intracrystalline diffusion at peak metamorphic temperatures. The 176Lu-176Hf internal isochron of two garnet fractions and three corresponding whole-rock splits from a gneissic unit yield a precise internal isochron age at 2629.0 ± 4.3 Ma (MSWD = 0.66), with an initial 176Hf/177Hf = 0.281210 ± 0.000010 corresponding to an εHfi = +4.1. This signature suggests a mafic source, which was derived from a long-term depleted reservoir, for this metasediment. The Lu-Hf age is consistent with the youngest age population of zircons from the paragneisses, which were previously dated in the area. Our results thus provide upper constraints on the timing of retrograde upper-amphibolite metamorphism and gold mineralization at Borden.

Late Neoarchean terrane and Paleoproterozoic HT–UHT metamorphism on southern Devon Island, Canadian Arctic

The northernmost exposures of the Laurentian shield in Canada outcrop on Devon and Ellesmere islands, near the tectonic domains of the ≥2.6 Ga Rae Province and ca. 2.0–1.9 Ga Thelon–Inglefield orogenic arc. We report (Th)–U–Pb zircon and monazite ages along with phase equilibrium modelling pressure–temperature estimates for southern Devon Island, the only area on Devon or Ellesmere islands with previously known Archean rocks. Our results demonstrate that southern Devon Island is a late Neoarchean (ca. 2.55–2.47 Ga) terrane composed of lower to middle crustal, granulite facies rocks with evidence of anatexis and pervasive solid-state ductile deformation. Protoliths of clinopyroxene–garnet quartzofeldspathic gneiss, orthopyroxene quartzofeldspathic gneiss, two-pyroxene metabasite and biotite granite gneiss were emplaced between ca. 2.55–2.51 Ga, and a clastic sedimentary protolith for rutile–garnet metasedimentary rocks was deposited at ca. ≥2.47 Ga. Earlier, poorly constrained metamorphic events are recorded at ca. 2.54, 2.47 and 2.30 Ga, possibly associated with pulses of the Arrowsmith orogen (ca. 2.5–2.3 Ga). Most recently, this terrane was reworked by high temperature (HT) to ultrahigh temperature (UHT) metamorphism at ca. 1.91 Ga during with the Thelon–Inglefield orogen; obtaining peak P–T conditions of ca. 9 kbar and >850 °C. The discovery of a sapphirine–quartz metapelite assemblage is a first for the Canadian Arctic and is diagnostic evidence of UHT conditions within the Thelon-Inglefield orogenic zone.

A lithospheric-scale Arrowsmith (2.4 Ga) detachment system with major Trans-Hudson (1.8 Ga) reactivation documented in the Howard Lake shear zone, Rae craton, Canada

This study characterizes the polyphase tectonometamorphic history of a major shear system in the Rae craton, the Howard Lake shear zone (HLsz), which exhumed high-grade rocks in the distant hinterland of the Paleoproterozoic Trans-Hudson Orogen. Prior work established a Mesoarchean mantle model-age discrepancy in basement rocks across the HLsz, which we interpret to have contributed a fundamental lithospheric rheology contrast that allowed for HLsz localization. Our work shows that the HLsz has a long-lived history starting in the early Paleoproterozoic as it abruptly separates 2.36 Ga granulite-facies metamorphic units to the east from 2.4 Ga greenschist-facies metamorphic basement rocks to the west. Subsequent reactivation during Trans-Hudson Orogen time between 1.86 and 1.82 Ga accommodated re-burial and re-exhumation of high-grade rocks in its footwall coeval with Sask craton collision. Strong penetrative overprinting deformation in metaigneous rocks and a newly discovered < 2.04 Ga schist preserve evidence for right-lateral transpression within the HLsz at ca. 1.82 Ga, which is coincident with the terminal collision of the Superior craton. Our work demonstrates that the HLsz is one of several major crustal-scale anisotropies in the south Rae craton and it is broadly analogous to major detachment fault systems documented in numerous Phanerozoic orogenic systems. Furthermore, the polycyclic deformation history of the HLsz, along which regional burial and exhumation was strongly localized, demonstrates the considerable influence crustal scale anisotropies can have on the evolution of craton architecture such as that presently found in the south Rae craton of the Canadian Shield.

The Great Preglacial “Bell River” of North America: Detrital Zircon Evidence for Oligocene–Miocene Fluvial Connections Between the Colorado Plateau and Labrador Sea

The idea of a great pre-glacial river that drained much of North America into the Arctic waters of modern Canada was first suggested in 1895 by Robert A. Bell. In the 1970s, petroleum exploration in Hudson Strait and the Labrador Sea located the massive, submerged delta of what is now known as the Bell River. Reconstructions suggest that three main branches of the Bell River joined up near modern Hudson Bay. The eastern branch largely drained the Canadian Shield, but the central and western branches had headwaters in the Cordilleran orogenic belt and its foreland in the present-day U.S. and northwestern Canada, respectively. We present new detrital zircon U–Pb data from Lower Oligocene and Lower Miocene sand from an exploration well in the Saglek delta of the northern Labrador Sea. In conjunction with other detrital zircon results from the Labrador Sea (and elsewhere) these data record the configuration and history of this continental-scale drainage basin in more detail. Mesozoic and younger detrital zircon grains (&lt; 250 Ma) are subordinate to Precambrian age groupings, but Cenozoic populations become more abundant during the Oligocene, suggesting that the basin had expanded into areas now occupied by the Colorado Plateau and the Basin-and-Range Province. Proterozoic and Phanerozoic detrital zircon grain populations in Saglek delta sediments are similar to those of the Pliocene Colorado River. The results support an earlier idea that initial incision of the Grand Canyon and denudation of the Colorado Plateau were associated with a north-flowing paleo-river that fed into the Bell River basin. This contribution continued until the Pliocene capture of this ancestral river by the Gulf of California basin, after which the excavation of the modern Grand Canyon was completed. The Bell River drainage basin was later blocked by the expansion of Pleistocene ice sheets.

Latest Silurian syntectonic sedimentation and magmatism and Early Devonian orogenic gold mineralization, central Newfoundland Appalachians, Canada: Setting, structure, lithogeochemistry, and high-precision U-Pb geochronology

Abstract The eastern Dunnage Zone of the central Newfoundland Appalachians hosts Paleozoic orogenic gold mineralization along a northeast-trending, crustal-scale fault corridor that extends for more than 200 km. This orogenic gold system is characterized by polyphase, structurally controlled, quartz vein systems that cut Neoproterozoic granitoid rocks and unconformably overlying syntectonic, polymict conglomerate and associated transitional to calc-alkaline bimodal igneous rocks. High-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology confirms a widespread, latest Silurian magmatic pulse (422–420 Ma) that is attributed to a transient phase of lithospheric extension resulting from asthenospheric and crustal melting related to slab break-off. Syntectonic conglomerate was deposited as a basal unit during extension-related uplift and erosion that lasted until ca. 418 Ma in north-central Newfoundland. Orogenic gold mineralization associated with syntectonic sedimentation and magmatism is hosted within third-order shear vein systems that form offshoots within a triangle zone–like structural corridor between southeast- and northwest-dipping, second-order fault splays and shear zones. The southeast-dipping fault system formed during northwest migration of the Acadian thrust front, whereas the northwest-dipping faults and shear zones are Salinic structures that were reactivated in the Early Devonian. Primary hydrothermal rutile in the orogenic gold-mineralized quartz veins produced ages of ca. 410 Ma. These ages are consistent with quartz vein emplacement and orogenic gold mineralization as a result of hydrothermal fluid-pressure cycling related to far-field compression and thermal perturbations during the Early Devonian Acadian orogenic cycle. The setting and process evolution of the central Newfoundland gold district are remarkably similar to that of world-class orogenic gold systems of the Canadian Shield.

Water and Rock Chemistry Inform Our Understanding of the Deep Biosphere: Case Study in an Archaean Banded Iron Formation

Research into the deep biosphere requires an understanding of both the microbial community at a given site and the geochemical and hydrological factors that support that microbial community. To highlight the interplay between geochemistry and microbiology in these deep environments, we characterized the hydrogeologic and geochemical systems of a 2.7 Ga banded iron formation within the Canadian Shield in the Soudan Underground Mine State Park in Minnesota, United States, a site known to host a lithotrophic microbial community. Calcium-sodium-chloride brines, characteristic of deep groundwaters throughout the Canadian Shield, were found in the site with total dissolved constituents (&amp;lt;0.2 micron) as high as 116,000 mg/L (ppm) in one borehole. Comparison of the Soudan waters to those found at other sites in the Canadian Shield or other sites of deep biosphere research indicate that they are notable for their high magnesium concentrations relative to total salinity. Additionally, the most saline Soudan waters have distinct 2H and 18O water isotope values suggesting long periods of isolation from the surface, which would allow for the evolution of a distinctive subsurface community. The presence of the banded iron formation along with the long-term isolation of the shield waters make Soudan a site of great potential for future research into deep crustal life. Furthermore, our work at Soudan highlights how geochemical data can inform future research into the deep biosphere and highlights a path for future research at the mine.

Investigating hydrologic controls on 26 Precambrian shield catchments using landscape, isotope tracer and flow metrics

AbstractAlthough many studies focus on catchment classification, hydrologists are challenged to find the most important variables for a meaningful catchment classification, especially in the heterogeneous environment of the Precambrian Shield. This study investigates landscape controls on hydrologic response by conducting a catchment classification of 26 catchments located within two Precambrian Shield watersheds (Sturgeon–French–Nipissing [SNF] and Muskoka) in central and Northeastern Ontario, Canada, using combinations of landscape characteristics (e.g. topography, geology, landcover), hydrometric variables, and stable isotopes of δ18O and δ2H in river flow. Weekly to monthly surveys of δ18O and δ2H in river flow were collected between 2013 and 2019. Flow metrics (e.g. Pardé coefficient and coefficient of variation of streamflow) were generated for 14 of the 26 rivers between 2008 and 2018. Principal component analyses (PCA) and Hierarchical Clustering on Principal Components (HCPC) analyses were used to identify variables controlling catchment clustering according to their similarities for four different scenarios. Despite their similar location along the southern edge of the Precambrian Shield, the 26 catchments generally clustered by watershed (SNF and Muskoka) with some exceptions. Differences in wetland and lake area (%), mean slope, and % area covered by glacialacustrine and glaciofluvial outwash deposits were the most influential variables in catchment classification. A positive correlation between % wetland area and streamflow stable isotope damping ratios suggests greater % wetland area (with shallow and potentially seasonally variable surface areas and/or hydrologic connection) observed in the SNF catchments increases variability in the influence of evaporative enrichment in SNF catchments. The catchment classification analyses in combination with stable isotopes of δ18O and δ2H were functional tools to investigate the combined influences of diverse types of catchment characteristics that lead to differences in hydrometric response. These results could support future studies focusing on generating hydrologic models and representing wetlands connectivity in the region.

Physical Circulation in the Coastal Zone of a Large Lake Controls the Benthic Biological Distribution

AbstractGradients of conductivity and major ions in the coastal zone of the eastern Georgian Bay of Lake Huron appear to limit the spatial distribution of invasive dreissenid mussels. Rivers flowing into Georgian Bay from the Canadian Shield have relatively low conductivity compared to the main body of Lake Huron, which creates a gradient of solutes near the river mouths. Field observations show a strong positive correlation between conductivity and calcium concentration. Thus, we use conductivity to infer the calcium concentrations required for the successful growth of dreissenid mussels. Most dreissenid mussels were observed in regions where specific conductivities were greater than 140 μS/cm. Field observations were used to examine how the calcium poor river water mixes within the coastal zone, resulting in solute gradients that determine mussel distribution. When river flows are low in late summer, there is only a weak solute gradient across the coastal zone, implying an intrusion of open bay waters into the shallow embayments, that favor the growth of dreissenid mussels. In contrast, during spring when river flows are as much as 10 times higher, there is a strong solute gradient that extends further into the lake, and the low calcium appears to limit the growth of dreissenid mussels. Thus, the seasonal character of solute gradients helps describe the spatial distribution of dreissenid mussels and explains the localized absence of a species that is otherwise prevalent in much of the Laurentian Great Lakes.

A cluster-based multiparametric similarity test for the compartmentalization of crystalline rocks into structural domains

Usually, fracture sampling studies comprise the collection of several fracture samples that involve many fracture clusters. Grouping fracture samples into structural domains is generally useful for geologists, hydrogeologists and geomechanicians as a region of fractured rock is subdivided into subregions with similar behaviour in terms of their hydromechanical properties. One of the common methods used for grouping fracture samples into structural domains considers the fracture orientation of clusters and ignores several fracture parameters, such as fracture spacing, aperture and persistence, which are important for fluid circulation in the rock mass. In this study, we proposed a new cluster-based similarity method that considers the orientation of clusters as well as the clusters’ aperture, persistence and fracture spacing. Field investigations were conducted in the Grenville geological province of the Canadian Shield in the Lanaudière region, Quebec, Canada, where fractures were sampled from 30 outcrops and four boreholes. The proposed method is more suitable than other methods, and has applications in hydrogeology, rock mechanics and particularly in studies of fluid circulation in the rock mass. In addition, a method for the compartmentalization of a given study area into structural domains by means of Voronoi diagrams is also proposed.

The DESC catchments: Long‐term monitoring of inland Precambrian shield catchment streamflow and water chemistry in Central Ontario, Canada

AbstractSince the early 1970s, the Dorset Environmental Science Centre (DESC) research catchments have been home to long‐term monitoring and study of terrestrial headwater catchment processes, their linkages to inland aquatic ecosystems and the influence of both natural variation and human activities. Located on the Precambrian Shield in central Ontario, Canada, covered by mixed Great Lakes‐St. Lawrence forest, the 29 catchments, defined by inflows and outflows to eight lakes, have been monitored for streamflow, meteorology and water chemistry, with long‐term datasets spanning from 1976 to the present. These datasets have provided insights into cold region hydrologic processes such as runoff generation, wetland and groundwater–surface water interactions, snow and ice processes, and catchment linkages to lake nutrient budgets and ecology. The datasets have supported catchment transit time estimates, hydrological modelling and cold region intercomparison studies. Starting with early research efforts driven by concerns over impacts from cottage development and acid deposition on soils, rivers and lakes, the DESC catchment datasets have supported study of impacts of stressors of forest harvesting, calcium depletion, road salt application and climate change. Ongoing monitoring of streamflow, meteorology and water chemistry in the DESC catchments continues to offer unique opportunities for investigation of critical zone processes in Precambrian shield catchments, their model representation and anthropogenic impacts.

Ideas and perspectives: Emerging contours of a dynamic exogenous kerogen cycle

Abstract. Growing evidence points to the dynamic role that kerogen is playing on Earth's surface in controlling atmospheric chemistry over geologic time. Although quantitative constraints on the weathering of kerogen remain loose, its changing weathering behavior modulated by the activity of glaciers suggests that this largest pool of reduced carbon on Earth may have played a key part in atmospheric CO2 variability across recent glacial–interglacial cycles and beyond. This work enunciates the possibility of kerogen oxidation as a major driver of atmospheric CO2 increase in the wake of glacial episodes. This hypothesis of centennial- and millennial-timescale relevance for this chemical weathering pathway is substantiated by several lines of independent evidence synthesized in this contribution, including the timing of atmospheric CO2 increase, atmospheric CO2 isotope composition (13C and 14C), kerogen oxidation kinetics, observations of kerogen reburial, and modeling results. The author hypothesizes that the deglaciation of kerogen-rich lithologies in western Canada contributed to the characteristic deglacial increase in atmospheric CO2, which reached an inflection point ≤ 300 years after the Laurentide Ice Sheet retreated into the kerogen-poor Canadian Shield. To reconcile the release of isotopically light carbon via kerogen oxidation with Earth surface carbon pool constraints, major oceanic degassing and biospheric regrowth must have acted in concert across glacial–interglacial transitions. Additionally, a process such as a strong shift in the ratio of C3 to C4-derived organic matter must be invoked to maintain isotope mass balance, a point key for reconciling the hypothesis with the carbon isotope record of marine dissolved inorganic carbon. In order to test this hypothesis, quantitative constraints on the contribution of kerogen oxidation to CO2 rise at glacial terminations are needed through systematic studies on (1) CO2 fluxes emanating from the weathering of different lithologies, (2) oxidation kinetics of kerogen along glacial chronosequences, and (3) high-resolution temporal changes in the aerial extent of glacially exposed lithological units and glacial flour.