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Article Excerpt
ABSTRACT. This study documents the Holocene evolution of lakes located in the Bluefish Basin, northern Yukon, on the basis of lake lithology, distribution of plant macrofossils, and radiocarbon dating of the basal organic material in sediment cores obtained from former lake basins. Basal organic matter from former lake basins is radiocarbon-dated to the late Holocene (< 3770 yr. BP), whereas the (14) C ages from the polygonal peat plateaus ([approximately] 2 m thick) that surround most of the former lake basins cluster in the early Holocene (between 11 435 and 8200 yr. BP). Plant macrofossil distribution in four out of five cores obtained in former lake basins indicates a transition from emergent aquatic vegetation to wetland and terrestrial-type vegetation, suggesting a gradual decline in water levels. The fifth core analyzed for macrofossils showed evidence of sudden lake drainage. The absence of [sup.14]C ages from the middle Holocene (7000 to 4000 yr. BP) suggests that the lakes had a greater spatial coverage and water levels during that period, a conclusion supported by the greater surface area occupied by the former lake basins relative to modern lakes and by the fact that the middle Holocene was a wet period in northern Yukon. The gradual decrease in water levels during the late Holocene could be attributed to partial drainage of lakes, increased evaporation under a drier climate, or a combination of both. A comparison with other regional climate records indicates a change toward drier climate conditions around 4500 yr. BP as a result of a reconfiguration in large-scale atmospheric circulation patterns, suggesting a climate-driven change in hydrological conditions.
Key words: thaw lakes, lake levels, Holocene, lake sediments, plant macrofossils, radiocarbon dating, Bluefish Basin, northern Yukon
RESUME. La presente etude retrace revolution des lacs de l'Holocene situes dans le bassin Bluefish, dans le nord du Yukon. Elle s'appuie sur la lithologie des lacs, la repartition des macrofossiles de plantes et la datation par le radiocarbone des matieres organiques de base se trouvant dans les carottes de sediments provenant d'anciens bassins lacustres. La datation par le radiocarbone de la matiere organique de base d'anciens bassins lacustres fait remonter cette matiere a l'Holocene superieur (< 3770 ans avant le present), tandis que la datation par le radiocarbone des plateaux de tourbe polygonaux ([approximately]2 m d'epaisseur) qui entourent la plus grande partie du groupement d'anciens bassins lacustres remonte a l'Holocene inferieur (entre 11435 et 8200 ans avant le present). La repartition des macrofossiles de plantes dans quatre des cinq carottes provenant des anciens bassins lacustres laisse voir une transition allant d'une vegetation aquatique emergente a une vegetation de zone humide et de type terrestre, ce qui attesterait du declin graduel des niveaux d'eau. Dans la cinquieme carotte, les macrofossiles presentaient des preuves d'un assechement lacustre soudain. L'absence de datation par le radiocarbone de l'Holocene moyen (de 7000 a 4 000 ans avant le present) laisse entrevoir que les lacs s'etendaient sur de plus grandes aires et que les niveaux d'eau etaient plus eleves pendant cette periode, conclusion qui est appuyee par la plus grande surface occupee par les anciens bassins lacustres relativement aux lacs contemporains et par le fait que l'Holocene moyen etait une periode humide dans le nord du Yukon. La diminution graduelle des niveaux d'eau pendant l'Holocene superieur pourrait etre attribute a l'assechement partiel des lacs, a I'evaporation accrue lors d'un climat plus sec ou a une combinaison des deux. La comparaison avec d'autres releves climatologiques regionaux indique un changement s'orientant vers des conditions climatiques plus seches environ 4500 ans avant le present, changement decoulant de la reconfiguration de la circulation atmospherique a grande echelle, ce qui laisse entendre que les conditions hydrologiques auraient evolue en raison du climat.
Mots cles: lacs thermokarstiques, niveaux des lacs, Holocene, sediments lacustres, macrofossiles de plantes, datation par le radiocarbone, bassin Bluefish, nord du Yukon
Traduit pour la revue Arctic par Nicole Giguere.
INTRODUCTION
In the western Canadian Arctic lowlands, lakes and drained lake basins occupy a great portion of the landscape. Climate change in this region has already resulted in changes in hydrological factors, including precipitation, evaporation, runoff, the duration and frequency of ice cover, and the extent of permafrost (Woo, 1996; Rouse et al., 1997). In recent years, many studies have examined the impact of these factors on the distribution and surface area of lakes in the western Arctic (e.g., Smith et al., 2005; Frohn et al., 2005; Hinkel et al., 2005, 2007; Labrecque et al., 2009), and the direction of change seems to be affected mainly by the response of permafrost. For example, in the discontinuous permafrost zone, lakes have shown a substantial decrease in their surface area due to an increase in subsurface drainage as permafrost thaws (Osterkamp et al., 2000; Yoshikawa and Hinzman, 2003; Christensen et al., 2004; Riordan et al., 2006), whereas in areas underlain by continuous permafrost, lakes show signs of expansion (Smith et al., 2005), shrinkage (Smol and Douglas, 2007), or little variation (Plug et al., 2008).
To assess the magnitude and significance of lake responses to present-day climate change, we need to understand the effects of past climate change on hydrological conditions, including lake distribution and lake-level fluctuations, from the late Quaternary to today. Such information can be retrieved by analyzing the distribution of the former lake basins in conjunction with paleohydrological analyses of lake sediments (such as the stratigraphic sequence of diatoms, pollens, and macrofossils). These analyses provide the basis for understanding the natural range of variability in lake-level fluctuations in relation to long-term climate change (Lamoureux and Bradley, 1996; Wolfe et al., 1996; Vardy et al., 1997; Pienitz et al., 2000; Moser et al., 2000; MacDonald et al., 2000; Edwards et al., 2000; Eisner et al., 2005; Ellis and Rochefort, 2006). We can reconstruct long-term regional lake-level variations by analyzing sediment cores from multiple lakes within the same region. Synchronous changes in the different lakes would suggest that the lake levels are influenced mainly by climate, whereas non-synchronous changes would tend to reflect non-climatic factors (e.g., geomorphic effects) (Digerfeldt, 1988; Magny, 2004). Compilations of lake-level data at a regional scale have suggested that in some western Arctic lakes, the water level responded to modifications in regional atmospheric circulation patterns, such as the Arctic Oscillation or Pacific Decadal Oscillation (e.g., Street-Perrott and Roberts, 1983; Harrison and Metcalfe, 1985; Harrison and Digerfeldt, 1993; Yu and Harrison, 1995; Anderson et al., 2005). In other lakes, however, water levels showed no relation with climate change, as their variation was dependant on local events (e.g., Henselmann, 1970). These divergent observations arise from the thermokarstic nature of most lakes in the western Arctic; the water levels of such lakes react to both climatic and geomorphic events (Billings and Peterson, 1980).
[FIGURE 1 OMITTED]
In the continuous permafrost zone of the western Canadian Arctic, several lacustrine plains host thousands of lakes that provide breeding habitat for aquatic animals, waterfowl, and caribou herds. Three of the largest are the Old Crow, Blueflsh, and Bell-Driftwood basins located in northern Yukon (Fig. 1). These basins are situated in the transition zone between the boreal forest and tundra ecoregion, a region that is sensitive to the changing climate. The objective of this study was to determine whether the observed modern trends toward decreasing lake water level in this region (Labrecque et al., 2009) are unique to the ongoing present-day climate change, or part of the natural variability in lake levels over time. The reconstruction of the Holocene evolution of lakes is based on lake lithology, distribution of plant macrofossils, and radiocarbon dating of the basal organic material in sediment cores. These cores were obtained from former lake basins and then compared with other regional paleoclimate records. This study contributes to the knowledge of paleohydrologic conditions of lakes in the western Arctic (Wolfe et al., 1996; Vardy et al., 1997; Edwards et al., 2000; Hinkel et al., 2003; Eisner et al., 2005) and to our understanding of how climate influences the water balance of lakes in a region strongly affected by the recent climate warming.
STUDY AREA
Quaternary and Physiographic Setting
Because of logistical constraints, this study focuses only on the eastern portion of the Bluefish Basin (Fig. 1), located approximately 200 km west of the Mackenzie Delta in the northern Yukon, rather than on the much larger Old Crow Basin located a few kilometers farther north. The Bluefish Basin was formed by the Laramide orogeny during the early Tertiary, with subsequent infilling...
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