Northeastern Baffin Island Research Articles

Rock avalanches in northeastern Baffin Island, Canada: understanding low occurrence amid high hazard potential

Rock avalanches in fjord environments can cause direct catastrophic damage and trigger secondary submarine landslides and tsunamis. These are well-documented in Greenland, Norway, and Alaska but have gone largely unreported in the extensive fjord terrain of the eastern Canadian Arctic. We provide the first inventory of rock avalanche deposits in northeastern Baffin Island—a region characterized by moderate to high seismic hazard, steep and high-walled fjords and glacial valleys, active deglaciation, and observed climate warming. Over a broad study area of ~60,000 km2, one sixth of the terrain had sufficient slope height and gradient to potentially generate rock avalanches. Within that hazard zone, we identified eight rock avalanche deposits at six locations. Only three rock avalanche deposits at two locations are dated, using aerial imagery (1958-present), to the last century while five deposits at four locations are inferred as syn- to post-glacial, likely occurring shortly after local debuttressing. These total numbers fall well below documented inventories from Greenland, Norway, and Alaska. We hypothesize that (1) continuous permafrost persists throughout this region and continues to act as a stabilizing factor and (2) rock mass quality is high in areas of most extreme relief contrast within the study region relative to analogous high-latitude fjord systems such as those in southwestern Greenland. We suggest that Baffin Island is currently in a period of quasi-stability that follows the intense instability during initial deglaciation, yet precedes the higher anticipated slope instability that may occur during permafrost degradation.

Retreat and stabilization of a marine-based ice margin along a high arctic fjord-cross-shelf trough system

Multibeam bathymetric and seismostratigraphic data collected in the Clyde fjord-cross-shelf trough system (eastern Baffin Island, Canadian Arctic Archipelago) display glacial landforms and depositional assemblages that enable the identification of the maximal extent of the Laurentide Ice Sheet (LIS) margin and delineating the patterns and controls on its subsequent retreat. Additionally, 10 new sediment cores – from which seven radiocarbon ages were acquired – allow the recognition of depositional processes. Results show that, during the Last Glacial Maximum, the LIS margin extended almost to the edge of the continental shelf. Early deglaciation of the trough was marked by an initial ice-shelf collapse and rapid retreat of the ice stream, as evidenced by the absence of ice marginal landforms and the presence of extensive iceberg ploughmarks across a large portion of the outer trough. It was followed by a slow retreat and successive stabilizations of the ice margin that led to the deposition of recessional moraines and grounding-zone wedges (GZWs). Deglaciation of the fjord in the early Holocene occurred in an episodic style, whereby rapid retreat was punctuated by relatively long standstills that enabled major moraine formation. Long-term stabilizations of the ice margin in the Clyde fjord-cross-shelf trough system are interpreted to coincide with major climatic cooling events, such as the Younger Dryas and early Holocene cold reversals. Ages derived from sediment cores and previous work suggest that higher retreat rates correspond with periods of significant global sea level rise, suggesting that oceanic forcing exerted a minor control on the deglaciation. GZWs and large moraine ridges are observed at pinning points in the trough and fjord, indicating that the location of ice margin stabilizations was influenced by topography. The reconstruction of the deglaciation of the Clyde fjord-cross-shelf trough system allows us to refine deglacial models for similar systems of northeastern Baffin Island, in particular beyond the coast and along the steeper section of the fjord where chronological gaps remained.

Submarine geomorphology of the northeastern Baffin Island fiords and cross-shelf troughs

ABSTRACT This paper presents a series of 50 geomorphological maps of the seabed of northeastern Baffin Island fiords and cross-shelf troughs, in eastern Arctic Canada. The mapping was produced using swath bathymetry imagery from multiple bathymetry datasets. A total of 24 types of landform were systematically mapped (>55,000 landforms) and reflect processes linked to subglacial, ice-marginal or paraglacial to postglacial environments. The landforms record the transition of the seabed from being covered by a marine-terminating ice sheet to the establishment of postglacial conditions. The landform assemblages allow the distinction of ice-flows orientation and ice-stream pathways along fiords and cross-shelf troughs. The multiple moraines and grounding-zone wedges indicate that the ice margin stabilized during retreat and that the overall deglaciation occurred by steps. These maps provide a framework for future investigations in northeastern Baffin Island fiords and shelf and can also provide a template for future seabed geomorphological studies in Arctic Canada.

Ice-stream flow switching by up-ice propagation of instabilities along glacial marginal troughs

Abstract. Ice-stream networks constitute the arteries of ice sheets through which large volumes of glacial ice are rapidly delivered from the continent to the ocean. Modifications in ice-stream networks have a major impact on ice sheet mass balance and global sea level. Reorganizations in the drainage network of ice streams have been reported in both modern and paleo-ice sheets and usually result in ice streams switching their trajectory and/or shutting down. While some hypotheses for the reorganization of ice streams have been proposed, the mechanisms that control the switching of ice streams remain poorly understood and documented. Here, we interpret a flow switch in an ice-stream system that occurred prior to the last glaciation on the northeastern Baffin Island shelf (Arctic Canada) through glacial erosion of a marginal trough, i.e., deep parallel-to-coast bedrock moats located up-ice of a cross-shelf trough. Shelf geomorphology imaged by high-resolution swath bathymetry and seismo-stratigraphic data in the area indicate the extension of ice streams from Scott and Hecla & Griper troughs towards the interior of the Laurentide Ice Sheet. Up-ice propagation of ice streams through a marginal trough is interpreted to have led to the piracy of the neighboring ice catchment that in turn induced an adjacent ice-stream flow switch and shutdown. These results suggest that competition for ice discharge between the two ice streams, which implies piracy of ice drainage basins via marginal troughs, was the driving mechanism behind ice flow switching. In turn, the enlargement of the ice catchment by piracy increased the volume and discharge of Scott Ice Stream, allowing it to erode deeper and flow farther on the continental shelf. Similar trough systems observed on many other glaciated continental shelves may be the product of such competition for ice discharge between catchments.

Glacial to postglacial submarine landform assemblages in fiords of northeastern Baffin Island

Fiord morphology plays a fundamental role in glacier flow dynamics and on ice-margin stability. As most of the present-day margins of the Greenland Ice Sheet lays in fiords, there is a need for understanding short- and long-term glacial dynamics in fiord settings. We investigate ice-sheet retreat patterns in previously glaciated submarine terrain of northeastern Baffin Island fiords to provide analogues for modern and future ice-sheet response to climate change and sea-level rise. Geomorphological maps constructed from the interpretation of swath bathymetry imagery in fiords of northeastern Baffin Island reveal a wide range of glacial to postglacial landforms that allow the reconstruction of past ice-sheet retreat dynamics. Ice-flow landforms such as mega-scale glacial lineations, crag-and-tails, and meltwater channels reveal the direction and behaviour of late-Foxe ice flow through the fiords. The presence of undisturbed elongated landforms within the fiords suggests that ice streams have probably been active until the late stage of deglaciation. Landforms transverse to ice-flow direction include grounding-zone wedges, frontal moraines, grounding-line fans, recessional moraines and De Geer moraines. These landforms are interpreted as the result of former standstills of the ice margin during deglaciation. The occurrence of grounding zones in deep (>800 m) part of the fiords contrasts with studies suggesting instability and rapid retreat of outlet glaciers over deep fiord basins. Sediment-filled basins, often characterised by the presence of turbidity channels, gullies and mass movement scars occur in-between the moraines. Sediment-filled basins with a ponded architecture between sills illustrate that most of the sediment accumulation was ice-proximal during deglaciation and characterised by gravity-driven flows. The proposed landform-assemblage model for northeastern Baffin fiords includes landforms typical of different fiord landsystems.

Holocene alpine glaciation inferred from lacustrine sediments on northeastern Baffin Island, Arctic Canada

AbstractWith accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a−1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a−1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley & Sons, Ltd.

Rapid 20th century environmental change on northeastern Baffin Island, Arctic Canada inferred from a multi-proxy lacustrine record

The Arctic has a disproportionately large response to changes in radiative forcing of climate, and arctic lacustrine ecosystems respond sensitively to these changes. The goal of this research is to generate high-resolution climate records for the past two millennia using multiple proxies in order to place 20th and 21st century climate and environmental change into a long-term context. We use a 14 C- and 210 Pb-dated surface core from Lake CF8 on north- eastern Baffin Island, Arctic Canada to generate a high-resolution multiproxy reconstruction of climate and environmental change. Throughout the late Holocene, primary productivity in Lake CF8 was low, but increased almost 20-fold in the past 200 years. Insect (Chironomidae) assemblages also show dramatic changes since 1950 AD, with cold stenothermous chironomid taxa disappearing from the record altogether. These changes in productivity and chironomid assemblages are unprecedented in the past 5,000 years. The dramatic ecological shifts that occurred at Lake CF8 have also been observed elsewhere in the Arctic, and will likely continue at ever-increasing rates as anthropogenic inputs of green house gases continue to cause climate warming and enhanced lacustrine primary production.

The deglaciation of Clyde Inlet, northeastern Baffin Island, Arctic Canada

AbstractThe behaviour of ice sheets as they retreated from their Last Glacial Maximum (LGM) positions provides insights into Lateglacial and early Holocene ice‐sheet dynamics and climate change. The pattern of deglaciation of the Laurentide Ice Sheet (LIS) in arctic fiord landscapes can now be well dated using cosmogenic exposure dating. We use cosmogenic exposure and radiocarbon ages to constrain the deglaciation history of Clyde Inlet, a 120 km long fiord on northeastern Baffin Island. The LIS reached the continental shelf during the LGM, retreated from the coastal lowlands by 12.5 ± 0.7 ka (n = 3), and from the fiord mouth by 11.7 ± 2.2 ka (n = 4). Rapid retreat from the outer fiord occurred 10.3 ± 1.3 ka (n = 6), with the terminus reaching the inner fiord shortly after 9.4 ka (n = 2), where several moraine systems were deposited between ca. 9.4 and ca. 8.4 ka. These moraines represent fluctuations of the LIS during the warmest summers since the last interglaciation, and this suggests that the ice sheet was responding to increased snowfall. Before retreating from the head of Clyde Inlet, the LIS margin fluctuated at least twice between ca. 7.9 and ca. 8.5 ka, possibly in response to the 8.2 ka cold event. Copyright © 2006 John Wiley & Sons, Ltd.

A multi-proxy lacustrine record of Holocene climate change on northeastern Baffin Island, Arctic Canada

Reconstructions of past environmental changes are critical for understanding the natural variability of Earth's climate system and for providing a context for present and future global change. Radiocarbon-dated lake sediments from Lake CF3, northeastern Baffin Island, Arctic Canada, are used to reconstruct past environmental conditions over the last 11,200 years. Numerous proxies, including chironomid-inferred July air temperatures, diatom-inferred lakewater pH, and sediment organic matter, reveal a pronounced Holocene thermal maximum as much as 5°C warmer than historic summer temperatures from ∼10,000 to 8500 cal yr B.P. Following rapid cooling ∼8500 cal yr B.P., Lake CF3 proxies indicate cooling through the late Holocene. At many sites in northeastern Canada, the Holocene thermal maximum occurred later than at Lake CF3; this late onset of Holocene warmth is generally attributed to the impacts of the decaying Laurentide Ice Sheet on early Holocene temperatures in northeastern Canada. However, the lacustrine proxies in Lake CF3 apparently responded to insolation-driven warmth, despite the proximity of Lake CF3 to the Laurentide Ice Sheet and its meltwater. The magnitude and timing of the Holocene thermal maximum at Lake CF3 indicate that temperatures and environmental conditions at this site are highly sensitive to changes in radiative forcing.

Breeding and Non-Breeding Range of Canada, <em>Branta canadensis</em>, and Cackling Geese, <em>Branta hutchinsii</em>, in the Eastern Canadian Arctic

The accepted breeding distribution of Canada Geese from the Atlantic Population (Branta canadensis interior) in the eastern Canadian Arctic is currently confined to northern Québec and the south coast of Baffin Island. Here we provide evidence based on observations from scientific studies, Inuit hunters, and territorial Wildlife Officers that B. c. interior now breeds in growing numbers 500 km farther north on northeastern Baffin Island than previously reported. Cackling Geese (B. hutchinsii), which breed more widely across eastern Arctic Canada, to about 72°N, may also be increasing there. Moreover, individuals of both species are seen occasionally as far north as Ellesmere Island in small flocks and within migrating or moulting flocks of Snow Geese (Chen caerulescens) or Brant (B. bernicla hrota), though none of these far northern stragglers are known to have bred. Whether these observations reflect a recent range expansion or improved distributional knowledge from more intensive recent survey efforts remains unknown.

Cosmogenic exposure dating in arctic glacial landscapes: implications for the glacial history of northeastern Baffin Island, Arctic Canada

Cosmogenic exposure dating and detailed glacial-terrain mapping from the Clyde Foreland, Baffin Island, Arctic Canada, reveal new information about the extent and dynamics of the northeastern sector of the Laurentide Ice Sheet (LIS) during the last glacial maximum (LGM). The Clyde Foreland is composed of two distinct landscape zones: (1) glacially scoured terrain proximal to the major sources of Laurentide ice that flowed onto the foreland, and (2) ice distal unscoured sectors of the foreland. Both zones are draped with erratics and dissected by meltwater channels, indicating past ice cover. We interpret the two landscape classes in terms of ice sheet erosive ability linked with basal thermal regime: glacially scoured terrain was occupied by erosive warm-based ice, and unscoured terrain was last occupied by non-erosive cold-based ice. Cosmogenic exposure ages from >100 erratics from the two landscape types have different age distributions. Cosmogenic exposure ages from the glacially scoured areas suggest ice cover during the LGM, followed by deglaciation between ~15 and ~12 ka. In the unscoured lowlands, the cosmogenic exposure ages have multiple modes ranging between ~12 and ~50 ka, suggesting multiple periods of cold-based ice cover during the last glacial cycle. In landscapes covered by cold-based ice, large numbers of cosmogenic exposure ages are required for elucidating glacial histories.

Population Genetics, Life History, and Ecology of Arctic Marine Fishes

... My research uses a comparative approach to reveal genetic, ecological, and life-history adaptations of Arctic fish species to the unique challenges of their environment at the northern extreme of their ranges. This paper highlights recent developments in my doctoral research, which can be separated into two general areas. The first is directed towards advancing knowledge of the evolutionary ecology and biodiversity of noncommercial Arctic marine fishes. The second concerns landlocked populations of Atlantic cod (Gadus morhua) persisting as marine relicts in saline coastal lakes on Baffin Island, at the northern extreme of the species' range in Canada. ... Noncommercial fishes were sampled from bycatch taken during Fisheries and Oceans Canada's turbot surveys in Baffin Bay and Davis Strait in September and October 2004. Trawling occurred at randomized stations in depth strata ranging from 400 to 1500 m in depth in Baffin Bay and Davis Strait .... My goal was to obtain approximately 100 specimens of species for which we had already obtained large samples of conspecifics or closely related taxa from temperate Atlantic waters. ... As conspecific samples are obtained, we will collect basic biological data, such as distribution, length, weight, sex, age, growth rate, fecundity, and age and size at maturity, to compare these features between high Arctic and temperate Atlantic populations. The most notable trend in the bycatch obtained during the fall of 2004 surveys ... was the paucity of species occurring to the north of Lancaster Sound. ... Species diversity was slightly higher ... off northeastern Baffin Island .... Our fieldwork in 2003 confirmed that the large cod in these lakes [Qasigialiminiq and Tariujarusiq Lakes, southwestern end of Cumberland Sound] were Gadus morhua, and not the more frequently landlocked Greenland cod, Gadus ogac, and that the lakes were saline meromictic lakes akin to Ogac Lake. ... The objectives of this part of my research include (i) basic physical, chemical, and biotic characterization of Arctic lakes in which Atlantic cod occur, and (ii) the study of aspects of their population biology, life history, and genetics. ... we have collected data on the length, weight, sex, maturity, liver weight, gonad weight, and stomach contents from 100 subadult and adult cod from each lake, and from a further 100 juvenile cod from Ogac Lake. Tissue samples and otoliths were also collected, for genetic and age/growth analysis, respectively. Underwater video was used to qualify benthic macrofauna and to observe cod behaviour in all three lakes. Plankton tows were made ... to compare micro-invertebrate populations among lakes .... Bathymetric measurements were taken using sonar across a number of transects in both Qasigialiminiq and Tariujarusiq to compare them to Ogac Lake.... Salinity, temperature, and dissolved oxygen strata were measured in each basin of each lake, again to compare abiotic limnology across lakes and over time at Ogac Lake. ... Temperature data loggers were deployed near the outflow of each lake to record the timing and frequency of tidal inflows. ... During the series of high spring tides that entered Ogac Lake in early July 2004, we collected samples of marine biota flowing into the lake, .... These samples consisted primarily of marine fish larvae and amphipods, although a high biomass of jelly-plankton ... was also observed entering the lake. ... Although Qasigialiminiq and Tariujarusiq are salt meromictic lakes, similar to Ogac Lake, both are warmer at all depths, and surface salinity is higher at Tariujarusiq (7 per mil) than at the other two lakes <1 per mil). ... Cannibalism appears to be less frequent among adult and subadult cod in the Cumberland Sound lakes (~14%) compared to Ogac Lake (~35%). ... Remarkably, our most conservative estimate of fish and amphipod biomass entering the lake during a 55-minute tidal inflow is 238 kg. .. Our calculations from tide predictions suggest that between 35 and 45 tides enter the lake during the open-water season each year. ... tidal inflows are crucial not only to replenish lake salinity, but also as a vital source of nutritive biomass to support these populations. The results of our genetic studies revealed remarkably low allelic variation at seven polymorphic microsatellite loci, providing evidence that these are small, inbred, and isolated marine relict populations. ... The fact that all three cod lakes occur in areas that were glaciated during the last ice age, and that the contemporary range of Atlantic cod in Canadian waters is constrained to the northern tip of Labrador, suggests that Atlantic cod probably extended their range northward during a postglacial period of ameliorated Arctic Ocean conditions, and colonized these coastal lakes, which would have been in the process of formation by glacioisostatic rebound during that period. ...

Macrobenthos Communities of Cambridge, McBeth and Itirbilung Fiords, Baffin Island, Northwest Territories, Canada

Thirty-eight marine invertebrates, including molluscs, echinoderms, polychaetes and several minor taxa, have been added to the previously described benthic macrofauna inhabiting Cambridge, McBeth and Itirbilung fiords in northeastern Baffin Island, Northwest Territories, Canada. The fiords lie fully within the marine arctic zone and organisms exhibiting panarctic distributions constitute the majority of species collected from them. Macrobenthic associations recorded in the fiords are comparable, both with respect to species composition and habitat, to benthic invertebrate associations occurring on the Baffin Island continental shelf and in east Greenland fiords, reflecting the broad environmental tolerances of the organisms constituting the benthic associations. Deposit-feeding organisms dominate the fiord macrobenthos, notably nuculanid bivalves, ophiuroid echinoderms and elasipod holothurians. The foraging and locomotory activities of these organisms may influence benthic community structure by reducing the abundance of sessile and/or tubiculous benthos. Key words: marine benthos, eastern Baffin Island, fiords, continental shelf, zoogeography, ecology

Four seismogenic environments in eastern Canada

Although the seismogenic environments in eastern Canada have some common features, the respective seismotectonic environments are influenced by local features. The seismotectonic environment in four seismogenic regions are described in order to illustrate how local features are reacting with the ambient plate-tectonic stress (a common feature) to modify the nature of the seismic activity. The Charlevoix earthquakes are occurring along steeply dipping planes formed during the opening of lapetus Ocean in the Proterozoic, perturbed by a meteor impact during the Paleozoic and reactivated by the opening of the present-day Atlantic Ocean in the Mesozoic. Theoretical curves of maximum shear stress versus depth based on a wet upper crust and a wet quartz rheology in the lower crust, for which the heat flow is 1.0 HFU (42 mW/m 2), can account for the depth distribution of earthquakes in this region. Potential trigger mechanisms are gravitationally induced stress, water infiltration and vertical shear strain changes (approximately 3 × 10 −8/ yr) due to post-glacial rebound. The unusually deep focus (29 km) of the M 5.9 Saguenay earthquake of 1988 could be due, in part, to the lower heat flow in this region as compared with that in the Charlevoix region which is about 70 km to the east. The Miramichi earthquake sequence is occurring on conjugate faults within an intrusion at depths shallower than 10 km. Theoretical curves of maximum shear stress versus depth based on a wet quartz rheology in the lower crust for which the heat flow is 1.5 HFU can account for this depth distribution. The steep gradient in the vertical ground movement rate (5 × 10 −8/ yr) in the epicentral area, which is probably due to post-glacial rebound, is a potential trigger mechanism for favorably oriented faults. The normal-fault seismic regime along northeastern Baffin Island can be explained by a combination of topography and post-glacial rebound, which can generate horizontal extensional stresses capable of overcoming the ambient compressional plate tectonic stress. Post-glacial rebound stress can act as a trigger mechanism for the NE-dipping faults in this region. In the Grand Banks region there is seismic evidence indicating that the slump that caused the destructive tsunami also generated the strong seismic ( M 7.2) signals, but double-couple solutions that satisfy certain criteria are not ruled out. The absence of surface faulting, especially in the epicentral regions of large earthquakes, is not properly understood, but several possibilities are near-surface layering that could mask the underlying fault offsets and a spatiotemporal variation in the ambient plate tectonic stress field. As knowledge of the causative factors of eastern Canada earthquakes and also the data base for empirical earthquake scaling relations increase, an overview of the seismotectonics of these mid-plate earthquakes should be enhanced.

Mid and late holocene diatoms from ?Water Supply Lake?, Baffin Island, N.W.T., Canada

Diatom analyses from a 40 cm sediment core from ‘Water Supply Lake’, northeastern Baffin Island, reveal 67 taxa of primarily benthic forms, strongly dominated by small Fragilaria spp. No major stratigraphic changes are noted in the diatom floral record, which encompasses the last 7000 years. In the uppermost 2 cm, there are increases in: a) F. virescens var. subsalina, b) the ratio of symmetric: asymmetric valves of F. construens, and c) species richness. These are interpreted as reflecting anthropogenic modifications of the lake (chlorination and deepening) associated with its use as a source of water for the community of Pond Inlet since 1979.

Holocene sedimentary environment of Cambridge Fiord, Baffin Island, Northwest Territories

The Holocene sedimentary environment of Cambridge Fiord in northeastern Baffin Island is interpreted from six piston cores. Distinct sediment types represent three sedimentary processes. (1) Muds deposited from suspension from fluvial sources at the fiord head and glacial sources near the mouth are the dominant components of the sediments. The rate of accumulation in mid-fiord based on a radiocarbon date on dispersed organic material indicates an average rate of sedimentation of 1.24 mm/a since about 5800 BP. Upper portions of the cores are bioturbated, although bands of well-preserved fine rhythmites indicate the sudden loss of the benthos, perhaps caused by oxygen depletion associated with reduced circulation. Rhythmites near the sediment surface have a mean thickness similar to the calculated annual accumulation, indicating that they probably are varves; those at depth average 8.1 mm thick. Fine sediment at depth in several cores was not bioturbated for long periods, indicating reduced effectiveness of the benthos during periods of higher rates of sedimentation. Graded muds and sandy muds are interpreted as deposits from weak underflows generated in the sediments on the fiord walls. (2) Massive sands deposited near the head of the fiord are the result of cohesionless debris flows from sources of fluvial and littoral sand widely distributed along the fiord floor. (3) Single sand- and pebble-sized particles found everywhere in the muds are interpreted as ice-rafted debris.

Seismogenesis in Eastern Canada

Abstract The pattern of seismicity in eastern Canada depends on the presence of weak zones from previous major tectonic orogenies and how these weak zones are reactivated by local and regional stress fields and geophysical processes. Within the Canadian Shield, away from seismotectonic trends, there is a low level of seismicity and earthquakes tend to be small, less than M5. However, along seismically active trends, earthquakes as large as M7 have occurred. The seismotectonic features fall into four main categories: positive (uplift) continental basement linears; grabens formed by old plate separation; passive rifted margins offshore; and extinct spreading ridges. Two of the positive seismotectonic trends are the Boothia Uplift-Bell Arch that transects the northeastern part of the craton and northeastern Baffin Island, where the effects of postglacial rebound on the upper crustal stress field are the most pronounced. The St. Lawrence Valley (and interconnecting grabens) is a seismically active graben system that contains the most seismically active region (the Charlevoix zone) in eastern Canada. The extinct spreading ridge along the Labrador Sea and the Mesozoic rifted margin along Baffin Bay and Labrador Sea contain clusters of moderate seismicity. There are diffuse zones of moderate seismicity over some geological provinces (e.g. Central Metasedimentary Belt in western Quebec) apart from major tectonic features, a confined seismic zone (within an intrusion) in the Miramichi region and seismicity at the intersection of faults in northern Ontario. In the Nahanni region, which is situated near the boundary between the northeast Cordillera and the Interior Platform, the commencement of a noteworthy earthquake sequence with magnitude up to Ms 6.9 indicates considerable stress-strain build-up over a large area. There is an anticline in the epicentral area that is bounded by thrust faults and mountain ranges. In order to enhance our understanding of causative factors of current seismicity, it is necessary to determine in greater detail the tectonic forces and geophysical processes that are reactivating pre-weakened faults along the seismotectonic trends and over broad, diffuse seismogenic regions. Some of these factors are the rate of stress build-up, stress concentration at the intersection of faults and between mountain ranges, residual stress, the role of pore fluids, individual block movement, whether this movement is due to postglacial rebound or to other underlying viscoelastic phenomena and the rate of sediment deposition along the continental slope. Paleoseismicity is useful not only for the reconstruction of old large earthquakes but also for providing insight as to why surface fault offsets have not been observed in regions where large earthquakes (and associated high rate of microseisrnicity) have occurred within the past several hundred years.

Distribution, Movement and Abundance of Polar Bears in Lancaster Sound, Northwest Territories

Mark-recapture studies show that polar bears are distributed at varying densities throughout Lancaster Sound during winter and spring. Major concentrations occur along the north and south coasts and the transverse floe edge at the mouth of the sound. As the ice melts, some bears move west whereas others move to nearby land areas. There is some evidence that as the ice forms, polar bears return from their summer locations to eastern Lancaster Sound. Polar bears of Lancaster Sound are part of the larger population whose western range extends to Barrow Strait, Prince Regent Inlet, Wellington Channel and Jones Sound. The southern and eastern range limits are unknown although this population may extend at least to Clyde River on northeastern Baffin Island and probably to Greenland. Maternity denning appears to be widespread over the study area probably because of the abundance of suitable habitat. We estimated a population of 1031 &plusmn; 236 polar bears in Lancaster Sound during 1979: however, more estimates are needed to determine if this relatively high number is normal for the area.Key words: polar bear, distribution, movement, abundance, Baffin Bay, Lancaster Sound, summer retreat, denning

Glacier descending toward Ayr Lake, north of Clyde Fiord, in the highlands of northeastern Baffin Island, N.W.T., Canada. (Photo by J. D. Ives.)

Glacier descending toward Ayr Lake, north of Clyde Fiord, in the highlands of northeastern Baffin Island, N.W.T., Canada. (Photo by J. D. Ives.)

The seismicity of northern Canada

The addition of over 1000 earthquakes to the northern Canadian data file during the past 3 years provides sufficient data to delineate distinctive patterns of seismicity, although the short history of low level earthquake monitoring and the temporal and spatial clustering of earthquakes suggests that not all potentially active areas may yet have been identified. The data indicate areas of activity near the larger earthquakes located teleseismically prior to the post-1960 northern expansion of the Canadian Seismograph Network and additional clusters and trends that were not previously apparent. Correlations to seismicity with major deformational trends in the Yukon – Mackenzie Valley, the northern continental margin, the Arctic archipelago and encircling much of the Baffin Island – Foxe Basin area show that structures formed or reactivated by Palaeozoic and later orogenic phases are continuing activity in response to the contemporary stress field.Possible zones of Cenozoic movement show pockets of high seismic activity but important gaps in the trends remain off Banks Island, along Nares Strait, and in Davis Strait. Tectonic forces characteristic of plate margins do not appear to be acting in the Canadian Arctic, and contemporary movement of Greenland with respect to Baffin Island does not need to be invoked to explain the seismicity in the Baffin region.Epicentre clusters in the Beaufort Sea and offshore of Ellef Ringnes Island are distributed mainly over the seaward gradient of elliptically shaped free air anomalies, indicating seismic adjustment in basement structures to uncompensated wedges of Recent–Tertiary sediments. Seismicity around much of the Baffin Island – Foxe Basin block shows a significant correlation with the interval of isostatic equilibrium between broad areas of current postglacial uplift. If northeastern Baffin Island is a hinge zone in the rebound process, and the zone from Hudson Strait to northeastern Keewatin a line of inflection in the rate of uplift contours, the reactivation of structures is occurring along zones of high differential stress.