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This paper presents data on and discusses the degree of postglacial stability of the northern Canadian tree limit. Holocene climatic changes have been both directional and synchronous for this region, though deteriorations/ameliorations either (1) lagged from west to east, (2) were buffered by local geographic factors, or (3) were not registered in the fossil record owing to the inherent persistence of the vegetation at some sites. The present northern tree limit in eastern Canada is in equilibrium with the prevailing climate, with the tree populations successfully regenerating both sexually and vegetatively. These tree stands are still occupying their maximum Hypsithermal positions. In contrast, the tree limit, tree line, and intervening forest-tundra ecotone in central and western Canada are out of equilibrium with today's climate. These populations are not sexually regenerative and apparently became established during warmer climates where the tree line was north of its present position. As such, they are susceptible to destruction by further climatic deterioration or anthropogenic disruption. [ABSTRACT FROM AUTHOR]

This synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the Holocene in Arctic Canada and Greenland. Our synthesis includes 47 records from a recently published database of highly resolved Holocene paleoclimate time series from the Arctic (Sundqvist et al., 2014). We analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records, mostly from datasets that did not fit the selection criteria for the Arctic Holocene database. Combined, we review evidence from a variety of proxy archives including glaciers (ice cores and glacial geomorphology), lake sediments, peat sequences, and coastal and deep-marine sediments. The temperature-sensitive records indicate more consistent and earlier Holocene warmth in the north and east, and a more diffuse and later Holocene thermal maximum in the south and west. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene. The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C on average (n = 11 sites). The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 ka, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time. The spatial pattern of temperature change through the Holocene was likely driven by the decrease in northern latitude summer insolation through the Holocene, the varied influence of waning ice sheets in the early Holocene, and the variable influx of Atlantic Water into the study region. [ABSTRACT FROM AUTHOR]

A major achievement in research supported by the Kluane Lake Research Station was the recovery, in 2001 - 02, of a suite of cores from the icefields of the central St. Elias Mountains, Yukon, by teams of researchers from Canada, the United States, and Japan. This project led to the development of parallel, long (10³ - 104 year) ice-core r ecords of climate and atmospheric change over an altitudinal range of more than 2 km, from the Eclipse Icefield (3017 m) to the ice-covered plateau of Mt. Logan (5340 m). These efforts built on earlier work recovering single ice cores in this region. Comparison of these records has allowed for variations in climate and atmospheric composition to be linked with changes in the vertical structure and dynamics of the North Pacific atmosphere, providing a unique perspective on these changes over the Holocene. Owing to their privileged location, cores from the St. Elias Icefields also contain a remarkably detailed record of aerosols from various sources around or across the North Pacific. In this paper we review major scientific findings from the study of St. Elias Mountain ice cores, focusing on five main themes: (1) The record of stable water isotopes (δ18O, δD), which has unique characteristics that differ from those of Greenland, other Arctic ice cores, and even among sites in the St. Elias; (2) the snow accumulation history; (3) the record of pollen, biomass burning aerosol, and desert dust deposition; (4) the record of long-range air pollutant deposition (sulphate and lead); and (5) the record of paleo-volcanism. Our discussion draws on studies published since 2000, but based on older ice cores from the St. Elias Mountains obtained in 1980 and 1996. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years (calendar years BP) in the northern boreal zone of Québec. Three trenches were excavated across the floodplain''s full width and permitted the interpretation of morphosedimentary units in relation to modern analogs. Chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks. From 4600 to 2900 cal. BP and from 1000 to 120 cal. BP, the channel migrated and constructed its floodplain at very slow rates, mostly because of low flow velocities, vegetated streambanks and the cohesive texture of marine sediments reworked by the channel. Vertical accretion rates were extremely variable on the floodplain, with high rates proximal to the modern channel and low rates over distal (also older) portions of the floodplain. Following a major channel shift (meander cut-off or avulsion) around 2900 cal. BP, channel migration appears to have been constrained to a narrow zone adjacent to the modern channel. Within this constrained zone, the migrating channel has reworked its own sediments leading to a marked unconformity between 2900–1000 cal. BP. It is thought that underlying marine sediments protrusions, and perhaps the forested banks, protected older alluvial sediments from being eroded during the last three millenniums. Our study shows that small boreal floodplains may contain, in a very small area, abundant and diversified archives of their evolution. [Copyright &y& Elsevier]

Paludification is the most common process of peatland formation in boreal regions. In this study, we investigated the autogenic (e.g., topography) and allogenic (fire and climate) factors triggering paludification in different geomorphological contexts (glaciolacustrine silty-clayey and fluvioglacial deposits) within the Québec black spruce (Picea mariana)–moss boreal forest. Paleoecological analyses were conducted along three toposequences varying from a forest on mineral soil to forested and semi-open peatlands. Plant macrofossil and charcoal analyses were performed on basal peat sections (≤50 cm) and thick forest humus (<40 cm) to reconstruct local vegetation dynamics and fire history involved in the paludification process. Results show that primary paludification started in small topographic depressions after land emergence ca. 8000 cal yr BP within rich fens. Lateral peatland expansion and secondary paludification into adjacent forests occurred between ca. 5100 and 2300 cal yr BP and resulted from low-severity fires during a climatic deterioration. Fires that reduced or eliminated entirely the organic layer promoted the establishment of Sphagnum in microdepressions. Paludification resulted in the decline of some coniferous species such as Abies balsamea and Pinus banksiana. The paleoecological approach along toposequences allowed us to understand the spatiotemporal dynamics of paludification and its impacts on the vegetation dynamics over the Holocene. [ABSTRACT FROM AUTHOR]

This study presents the long-term (over the last 8000 years) natural variability of a portion of the Picea mariana-moss bioclimatic domain belonging to Québec's Clay Belt. The landscapes are dominated by mesic-subhydric clay and early successional forests composed of Populus tremuloides, Pinus banksiana and Picea mariana. The natural variability (fires and vegetation) of one of these landscapes was reconstructed by means of pollen and macroscopic charcoal analysis of sedimentary archives from two peatlands in order to assess when and how such landscapes were formed. Following an initial afforestation period dominated by Picea (8000-6800 cal. Years BP), small and low-severity fires favored the development and maintenance of landscapes dominated by Picea and Abies balsamea during a long period (6800-1000 BP). Over the last 1000 years, fires have become more severe and covered a larger area. These fires initiated a recurrence dynamic of early successional stands maintained until today. A decline of Abies balsamea has occurred over the last centuries, while the pollen representation of Pinus banksiana has recently reached its highest abundance. We hypothesize that the fire regime of the last millennium could characterize Québec's Clay Belt belonging to the western Picea mariana-moss and Abies balsamea-Betula papyrifera domains. [ABSTRACT FROM AUTHOR]

Differentiating between forced regressive deposits from deglacial periods in high latitude domains and forced regressive deposits from the onset of glacial periods in low latitude domains is fundamental for the accurate interpretation of glacial cycles within the geological record and then for the reconstruction of palaeogeography and palaeo-climate. A forced regressive deglacial sequence is documented from the Lake Saint-Jean basin (Québec, Canada). In this area, the Late Pleistocene to Holocene sediments have recorded the Laurentide ice sheet retreat accompanied by the invasion of marine waters (Laflamme Gulf) from ca 12·9 cal kyr bp. Subsequently, fluvio-deltaic and coastal prograding wedges were deposited; they followed the base-level fall due to glacio-isostatic rebound. This succession, representing a transition from glacial to post-glacial periods within a previously glaciated area, was investigated through recent mapping, preserved landforms, facies analysis, and new optical stimulated luminescence and radiocarbon dates. Three basin-scale geological sections share a common lower part made of isolated ice-contact fan deposits overlying bedrock. Throughout the entire basin, ice-contact fans are capped by glacimarine muds. Above, fluvial and coastal prograding systems were deposited and evolved through four steps: (i) deltaic systems progressively increased in width; (ii) coastal influence on sedimentation increased; (iii) hydrographic drainage systems became more organised; and (iv) deltas graded from steep (Gilbert delta) to low-angle foresets (mouth-bar delta). Deposited during the base-level fall from glacio-isostatic rebound, the complete succession has been designated as a single falling stage system tract referred to as a deglacial falling stage system tract. It is representative of a deglaciation sequence in areas previously covered by ice during glacial periods (i.e. medium to high latitude domains). Diagnostic criteria are provided to identify such a deglacial falling stage system tract in the geological record, which may aid identification of previously unknown glacial cycles. [ABSTRACT FROM AUTHOR]

Franklin Glacier is an 18-km-long valley glacier that originates in a broad icefield below the west face of Mt. Waddington in the central British Columbia Coast Mountains, Canada. Radiocarbon-dated wood samples from the proximal faces of lateral moraines flanking Franklin Glacier show that the glacier expanded at least nine times since 13,000 cal. yr BP. A probable Younger Dryas advance of Franklin Glacier at 12,910–12,690 cal. yr BP followed the late glacial retreat and down wasting of the Cordilleran Ice Sheet from ca. 16,000 to 12,900 cal. yr BP. During the succeeding early Holocene warm period, Franklin Glacier appears to have retreated significantly, leaving no record of glacial expansion until the mid-Holocene when it repeatedly advanced at 6360–6280, 5470–5280 and 4770–4580 cal. yr BP. Down wasting of the glacier surface after ca. 4770–4580 cal. yr BP was followed by intervals of expansion at 4260–4080, 3210–3020 and 2620–2380 cal. yr BP. Following ice expansion at ca. 2620–2380 cal. yr BP into trees over 224 years in age, there is no record of the glacier activity until 1570–1480 cal. yr BP when Franklin Glacier thickened and advanced into young subalpine fir trees. During the ‘Little Ice Age’, advances at 800–680, 610–560 and 570–510 cal. yr BP preceded a mid-19th to early 20th century advance that saw Franklin Glacier attain its maximum Holocene extent. The dendroglaciological record at Franklin Glacier is among the most comprehensive recovered from the British Columbia Coast Mountains and showcases the complexity of mid- to late Holocene glacier expansion in the region. [ABSTRACT FROM AUTHOR]

ABSTRACT Understanding fire regime dynamics is central to predicting forest structure and the compositional dynamics of boreal forests. Spatial and temporal variations in fire frequency in central Canadian boreal forests over the last 10 000 years were examined to evaluate the influence of bottom-up controls on the regional fire regime. We analysed macroscopic charcoal larger than 160 μm from sediment cores from six lakes to reconstruct fire history and performed GIS analysis of regional landscape features to investigate how fire frequency has changed temporally and how non-climatic factors may have affected long-term fire frequency. Our generalized linear mixed model revealed that temporal changes in fire return intervals (FRIs) were highly dependent on landscape connectivity as inferred through the abundance of natural firebreaks in the form of open water lakes and wetlands. FRIs did not change significantly among highly connected landscapes throughout the Holocene; in contrast, FRIs were significantly longer among poorly connected landscapes in the early Holocene (10-5 cal ka BP), suggesting that the abundant regional firebreaks limited fire spread. All sites had similar FRIs in the late Holocene. The diminishing influence of firebreaks suggests that the regional climate during the late Holocene has overshadowed the influences of the bottom-up controls on fire activities. [ABSTRACT FROM AUTHOR]

Here, we present results of Holocene carbon accumulation from seven new peat cores at four fen sites from Alberta, western Canada. Along with two other published fen records in the same region, we provide a regional synthesis of carbon accumulation patterns, as well as peat property data in this continental climate. On the basis of >3000 peat sample measurements at 1-cm intervals, organic matter content in peat samples is on average at 91.0% (±9.3% in SD) and ash-free bulk density at 0.141 g/cm3 (±0.045 g/cm3 in SD) with a range of 0.018–0.381 g/cm3. On the basis of 109 radiocarbon age determinations from these cores, the overall mean carbon accumulation rate at these fens since 9 kyr (1 kyr = 1000 cal. yr BP) is 32.5 g C/m2/yr, which is much higher than the northern peatland average of 18.6–22.9 g C/m2/yr from large-scale syntheses. The ‘instantaneous’ apparent carbon accumulation rates from all these individual measurements at mostly 1-cm intervals are highly variable throughout the Holocene, ranging from 6 to 504 g C/m2/yr. The carbon accumulation rates appear to show peak values at 8.3, 6.8, 5.3, 3.8, 2.7, and 1.1 kyr, suggesting a ~1500-year periodicity in carbon accumulation, possibly in response to Holocene climate change. Furthermore, carbon accumulation shows a gradual declining trend from 7 to 2 kyr, reaching a minimum at 2 kyr, likely in response to neoglacial climate cooling during the later part of the Holocene in the high-latitude regions. Overall, these fens accumulated and store on average 264 kg C/m2 during the Holocene, among the highest soil carbon storage reported from terrestrial ecosystems. Our results, along with previous global and regional syntheses, suggest that in contrast to conventional wisdom, fens in continental climates accumulate peat at similar or greater rates than do bogs in oceanic climates. [ABSTRACT FROM PUBLISHER]

Zoological remains were examined from the sediments of the Pingualuit Crater Lake, Nunavik, Canada. Our objective was to describe past climate events in the area of delayed deglaciation in northernmost Ungava Peninsula. Our record covers 3 separate sections of deglacial and postglacial invertebrate dynamics interrupted by laminated proglacial sediments and a basin-scale erosive slumping event. The abundance of animal remains in the ultra-oligotrophic and extremely deep arctic lake was low, but distinct faunal assemblages were found among the intervals, results implying that they were environmentally heterogeneous. The lowermost fine-grained interval (before 6850 calibrated years before present [cal BP]), revealed that Cladocera Chydorus sphaericus-type and Bosmina (Eubosmina) longispina-type were common in the lake, whereas Chironomidae were relatively rare. The dominance of B. longispina-type showed that planktonic communities were successful at the time, probably indicating more favorable climatic conditions than today soon after the last deglaciation (~7000 cal BP). In the middle interval (between~6850 and 5750 cal BP), chironomids became more common and were dominated by Heterotrissocladius subpilosus-type and Protanypus, taxa that are characteristic of oligotrophic lakes. The extirpation of B. longispina-type suggests that planktonic invertebrate communities were not successful, probably because of predation by Arctic char. The presence of the chironomid Oliveridia tricornis-type during the late Holocene (between ~4200 and 600 cal BP) suggested general climate cooling. Our paleoclimatic conclusions on the regional environmental history suggest a stationary ice front in the initial stages of the Holocene, favorable climatic conditions in the mid- Holocene and a general late-Holocene cooling. Our records also indicate a subtle increase in nutrient availability throughout the Holocene. The paleoecological record from Pingualuit Crater Lake is valuable in describing the faunal history and biotic resilience in this environmentally extreme lake, which presently contains one of the world's softest and most transparent waters. [ABSTRACT FROM AUTHOR]

Changes in late Neoglaical climate resulted in extensive modification of Arctic terrestrial ice cover. A substantial reduction in terrestrial ice cover in the Queen Elizabeth Islands (QEI) following the 'Little Ice Age' (LIA) (~AD 1250-1900), is indicated by widespread, light-toned patches of poorly vegetated terrain, extending back to the modern ice mass. These patches display abrupt outer margins (trimlines), which record the former position and maximum extent of perennial snow/ice and, in many cases, mark the former equilibrium-line altitude (ELA). Trimlines surrounding terrain formerly covered by LIA perennial snow/ice were mapped using multispectral classification approaches applied to high-resolution satellite imagery. ELAs were reconstructed from trimlines associated with former perennial snow/ice produced by long-term snowline lowering. Between the end of the LIA and 1960, the area of terrestrial ice in the QEI decreased by 37% (62 387 km2). Most of this reduction (94%) occurred in the eastern QEI where the majority of the ice exists today; however, a 100% reduction in ice cover occurred on many of the western islands by 1960, an effect largely controlled by the subtle topography of these islands. The reconstructed LIA ELA trend surface was used with the 1960 mapped ELAs to calculate spatial variations in the change in height (Δh) of the ELA trend surface throughout the QEI during the first half of the twentieth century. ELA Δh between the LIA and 1960 reveal a high degree of local variability in the mountainous regions, ranging from 0 to >600 m; however a strong regional-scale pattern of change is shown over the QEI as a whole. [ABSTRACT FROM AUTHOR]

Long-term vegetation succession and permafrost dynamics in subarctic peat plateaus of west-central Canada have been studied through detailed plant macrofossil analysis and extensive AMS radiocarbon dating of two peat profiles. Peatland inception at these sites occurred around 5800-5100 yr BP (6600-5900 cal. BP) as a result of paludification of upland forests. At the northern peat plateau site, located in the continuous permafrost zone, palaeobotanical evidence suggests that permafrost was already present under the forested upland prior to peatland development. Paludification was initiated by permafrost collapse, but re-aggradation of permafrost occurred soon after peatland inception. At the southern site, located in the discontinuous permafrost zone, the aggradation of permafrost occurred soon after peatland inception. In the peat plateaus, permafrost conditions have remained very stable until present. Sphagnum fuscum-dominated stages have alternated with more xerophytic communities characterized by ericaceous shrubs. Local peat fires have occurred, but most of these did not cause degradation of the permafrost. Starting from 2800-1100 yr BP (2900-1000 cal. BP) consistently dry surface conditions have prevailed, possibly related to continued frost heave or nearby polygon crack formation. [ABSTRACT FROM AUTHOR]

Crawford Lake, Ontario, Canada (43°28.1' N, 79°56.9' W, 278 masl) has varved and AMS dated sediments containing fossil pollen that record native Iroquoian farming ca. AD 1268 to 1520. From before AD 1000 to 1268, bioturbating organisms caused poor varve preservation but since then, well-preserved varves and dung pellets reflect anoxic bottom water due to meromixis. The onset of varve preservation coincides with the occurrences of pollen grains of Zea (maize), Helianthus (sunflower), Phaseolus (bean), Cucurbita (squash) and Portulaca (purslane), and spores of Ustilago cf. maydis (maize smut). These pollen grains and spores are more abundant in pellets between varve laminae than in the surrounding sediment matrix. Analyses of DNA from five pellets demonstrate that they are dung from wild Canada geese (Branta canadensis). In the autumn, as geese foraged in Iroquoian fields, they inadvertently ingested pollen and spores before flying to the lake. There they roosted and cast the pollen-rich dung pellets, which became part of the sediment. This study demonstrates that birds, wild geese and perhaps ducks, can be important vectors of pollen to lake sediments located near agricultural settlements. [ABSTRACT FROM AUTHOR]

The north-flowing, low-gradient section of the Red River in Manitoba has lost ∼60% of its valley gradient since 8 ka cal. BP. An existing hydraulic model of the modern Red River flood zone was used to examine the change in flood extent and depth of a discharge equivalent to the 1997 Red River flood (3970 m3/s) for scenarios of gradients at 8, 6, 4 and 2 ka cal. BP as well as 2 ka in the future. The modelling indicates a broad, shallow flood zone for all of the gradient scenarios, with extent and depth increasing over time. Between the 8 ka cal. BP and present-day scenarios, the flood zone increased from 1186 km2 to 1531 km2 (∼29%) with depth increasing along four east – west cross-sections by 0.69 m (∼61%), 0.91 m (∼82%), 0.56 m (∼64%) and 0.48 m (∼86%). The flood extent and depths increased by a further 18 km2 (∼5%) and 0.04–0.06 m (2–5%), respectively, by 2 ka in the future. Most of these changes to the flood zone occurred between 8 and 2 ka cal. BP, reflecting an exponential loss of gradient. A rise in flood depth equivalent to that which occurred between 8 ka cal. BP and the present-day, is assessed as increasing the long-term flood hazard; in contrast, the slight rise in depth between the present-day and 2 ka in the future does not. [ABSTRACT FROM AUTHOR]

A 485cm sediment core from a lake unofficially called JR01, Boothia Peninsula, Nunavut, Canada, yielded a high-resolution diatom record documenting environmental change in the mid-arctic. Five radiocarbon dates provided the chronology. Changes in diatom composition and sediment characteristics indicated distinct shifts in the Holocene climate. A more diverse and productive diatom flora implies warmer temperatures in the middle Holocene. A subsequent complete shift in diatom composition to a predominantly Fragilaria sensu lato flora and a reduction in diversity and production suggests cooler climates in this region after 4600cal. BP. Smaller-scale climatic fluctuations, such as the 'Little Ice Age'(LIA, 600-150cal. BP) and the 'Mediaeval Warm Period' (MWP, 1150-600cal. BP), caused shifts in the diatom flora and production. Subtle shifts in floristic diversity within the LIA may reflect climatic variability at a century scale. A gradual shift to a more diverse and productive flora in the last 150 years suggests a response to the recent warming trend. [ABSTRACT FROM AUTHOR]

Changes in pollen and stomata assemblages in sediment cores recovered from tundra and forest-tundra lakes in alpine regions of northeastern British Columbia reflect vegetation and inferred climatic change throughout the Holocene. Pollen and stomata records are presented from two lakes, BC2 located in the alpine-tundra zone and Dead Spruce Lake at the present elevation of subalpine treeline. The pollen and stomata records from BC2 indicate that an ephemeral shrub and herb assemblage was rapidly replaced by an aspen (Populus)-spruce (Picea)-birch (Betula) woodland at ~10600 cal. yr BP. The occurrence of stomata suggests that treeline was at least 235 m higher than present from ~10 600 until ~7500 cal. yr BP and temperatures were at least 1.4°C warmer than at present. Analyses from Dead Spruce Lake indicate that the density of trees was also higher between ~9000 and 4800 cal. yr BP. High concentrations of macroscopic charcoal between ~9800 and 8000 cal. yr BP suggest that conditions were also drier at this time. Changes in the position of treeline during the early to mid-Holocene appear to track closely shifts in climate, while forest development at our forest-tundra site reflects a combination of both heightened summer insolation and increased fire activity. The fossil stomata record from BC2 suggests that a rapid cooling event at ~8200 cal. yr BP may have been the trigger that resulted in the downslope movement of treeline to its present elevation. Increased Picea percentages, pollen-accumulation rates (PARs) and concentration of stomata between ~3600 and 2700 cal. yr BP provide evidence for a late-Holocene increase in forest-tundra density in response to warmer temperatures. [ABSTRACT FROM AUTHOR]

Palaeoecological techniques and radiocarbon datings were used to reconstruct the initiation and development of a sloping bog system near Prince Rupert, British Columbia, Canada. Peat initiation began around 12 000 cal. BP, possibly as a result of a cool, wet climate prior to the insolation maximum. A Picea sitchensis-Tsuga heterophylla-Alnus minerotrophic wet fern forest occupied the study site from 9300 to 8300 cal. BP. Between 8300 and 7500 cal. BP, cooler and wetter climatic conditions promoted increased organic matter accumulation and paludification by Sphagnum species. Subsequent lateral expansion has been very slow in the later Holocene. Paludification resulted in a shift to a scrubby, less productive Pinus contorta-T. heterophylla-Cupressaceae-Cyperaceae bog woodland around approximately 7500-3000 cal. BP. The modern Cupressaceae-Pinus contorta-T. heterophylla-Ericaceae-Cyperaceae mire was well established by 2000 cal. BP and a modern oceanic climate was predominant. The initiation and development of the studied bog corresponds to regional climatic and successional trends. However, the average long-term apparent rate of carbon accumulation (LORCA) was low compared to rates from other northern mires, only 6.3 ± 0.6 (SE) g m[sup -2] yr[sup -1]. About 53% of the present carbon store was already accumulated by 6200 cal. BP. This study emphasizes that mire initiation and slow forest paludification in north coastal British Columbia is a multidirectional process resulting from complex interactions between climatic and autogenic factors. [ABSTRACT FROM AUTHOR]

A major aggradation took place from 8000 BP to 4000 BP in the Porcupine and Old Crow rivers, northern Yukon. The aggradation was a consequence of an increase in flow and the result of a perturbation of the permafrost active layer following the early-Holocene climatic warming of northwestern Canada. These findings are useful for improving our understanding of how natural landscapes and river systems evolved in regions that experienced a permafrost history and, more particurlarly, an increase in climatic warming. This article also contributes to an improved understanding of natural landscape evolution along the Porcupine and Old Crow rivers in eastern Beringia, where there has been much interest in questions related to animal and human migration and adaptation. [ABSTRACT FROM AUTHOR]

Sediment stratigraphy and palaeobotanical data from five cores were used to reconstruct the postglacial water-level changes of a small lake in southern Québec. The cores were taken along two transects from the centre of the lake to the margins. Lake level was relatively high before 11 000 cal. BP, when sedimentation was characterized by the deposition of silty gyttja (.13 000 cal. BP) and marl (13 000–11 000 cal. BP). From 11 000 to 7000 cal. BP, a sediment hiatus was indicated by the lack of two regional pollen zones between the marl and the overlying gyttja, suggesting either nondeposition or erosion of the previously deposited sediments due to a low lake level. The lake level was approximately 2 m lower than present on the basis of macrofossil assemblages. A rise in the water level of about 1 m occurred around 7000 cal. BP, which was interrupted by a second lowering between 6100 and 4400 cal. BP, as indicated by a hiatus in the pollen stratigraphy. During this low period, the water level was at least 2 m lower than present. The last 4400 years are characterized by a continuous rise. The lake-level fluctuations are broadly synchronous with other palaeohydrological records in northeastern United States, Ontario, and Subarctic Québec, implying broad-scale climatic control. [ABSTRACT FROM AUTHOR]

Average long-term apparent rates of carbon (C) accumulation (LARCA) were estimated for four peat cores from Arctic and Subarctic Canada. Detailed analyses of dry bulk-density and C content were used to determine variations in C accumulation rates throughout the cores. LARCA range from 12.5 to 16.5 g C m -2 yr -1 over the past 6700–10 000 years. Rates are lower for the surface layers of Arctic high-centred peat polygons, at 5.3 to 7.1 g C m -2 yr -1 for the last 3500–4500 years. By comparison, the rate for the near-surface peat from a Sphagnum fuscum hummock in the high Subarctic was considerably higher, at 24.1 g C m -2 yr -1 . The highest carbon accumulation rates were from core segments older than 4500 BP, which represent fen stages according to palaeoecological analysis. The average LARCA in our study are considerably lower than recent estimates of average carbon accumulation in Boreal peatlands. This difference is attributable partly to lower carbon percentages in our cores compared to the mean or estimated values of 50 to 51.7% used in those studies. Another factor is the presence of ground ice, which exaggerates the apparent peat depth and leads to erroneously high values if cumulative carbon estimates are based on depth. Using cumulative dry bulk-density, as we have done, eliminates the influence of ground ice and thus makes more accurate estimates possible. [ABSTRACT FROM AUTHOR]