Search

The Arctic has warmed significantly over the past decades. However, the evolution of Arctic climate during the Holocene remains to be clarified in more detail, and regional factors controlling aqua...

Although widely distributed throughout Arctic and subarctic regions, thermokarst ponds and lakes remain relatively unexplored regarding geomorphological changes in their catchments and their internal properties in relation to climate change over the past decades. This study synthesizes recent landscape evolution and modern sedimentology of limnologically diverse thermokarst ponds near southeastern Hudson Bay, Canada. Spatio-temporal analysis of permafrost mounds, thermokarst ponds, and vegetation surface areas over the past five decades revealed that the recent climate-induced decrease of permafrost-affected areas was not primarily compensated by thermokarst pond development, but rather by a remarkable increase in vegetation cover. These changes appeared to be modulated by topographical and hydrological gradients at the study site, which are associated with east-ward increasing thickness of postglacial marine deposits. At a more contemporary timescale, physico-chemical measurements made on sedimenting materials (sediment traps) and freshly deposited lacustrine sediments of selected thermokarst ponds revealed striking differences both among ponds and between the oxic epilimnion and the oxygen-depleted hypolimnion. These findings underscore the major influence of local landscape properties and oxycline development on pond sedimentology and geochemistry, such as the transport of detritic particles and the concentration of redox-sensitive elements.

Arctic and sub-arctic lake sediment sequences from the Pleistocene are uncommon due to multiple glacial–interglacial cycles and the associated advances and retreats of Pleistocene ice sheets. Pleistocene strata are preserved in a 9-m-long sediment core recovered from Pingualuit Crater Lake, Nunavik, northern Quebec (Canada). In addition to tychoplanktonic Aulacoseira species, the Pleistocene planktonic flora comprises representatives from the thalassiosiroid genera Cyclotella, Discostella and Puncticulata, of which most species are extant in perennially ice-free sub-arctic and arctic lakes. One Cyclotella species, C. pingualuitii is described as new and is characterized by a small central area, multiple central and scattered marginal fultoportulae with triangular satellite pore covers, a single large submarginal rimoportula and alveolate striae of unequal length. Cyclotella pingualuitii is the most prominent species during the oldest diatom-rich interval (DR3) recovered, which was deposited during the Lat...

The Great Whale River region on the eastern shore of Hudson Bay, Canada, encompasses the villages of Whapmagoostui (Cree First Nation) and Kuujjuarapik (Inuit) and surrounding areas. The principal field station of Centre d'etudes nordiques (CEN: Centre for Northern Studies) has operated at Whapmagoostui-Kuujjuarapik (W-K; 5 5° 1 5' n, 77° 45' w) since the 1970s, with diverse research projects on past and present environments. The climate at W-K is strongly influenced by the proximity of Hudson Bay, and the recent pronounced loss of sea ice in this sector of northern Canada has been accompanied by large increases in air temperature. Discontinuous or scattered permafrost occurs throughout the region and is degrading rapidly. The W-K region continues to experience particularly rapid isostatic uplift in response to the retreat of the Laurentide Ice Sheet. Parabolic dunes occur along the coast and are strongly influenced by the plant cover. Paleoecological studies have documented the Holocene evolution of landscapes, including lakes, wetlands, and forests. The vegetation type is coastal forest tundra, with some 400 recorded species. Studies on certain insect groups provide a baseline for assessing future ecological change. The first signs of human occupation in the W-K region have been dated at 3800 BP . The arrival of the Hudson's Bay Company in the 18 th century marked the onset of continuous occupation. Rapid social, economic, and environmental change initiated in the mid-20 th century continues to this day.

The formation of many arctic wetlands is associated with the occurrence of polygonpatterned permafrost. Existing scenarios to describe and explain surface landforms in arctic wetlands (low-center and high-center polygons and polygon ponds) invoke competing hypotheses: a cyclic succession (the thaw-lake hypothesis) or a linear succession (terrestrialization). Both hypotheses infer the predictable development of polygon-patterned wetlands over millennia. However, very few studies have applied paleoecological techniques to reconstruct long-term succession in tundra wetlands and thereby test the validity of existing hypotheses. This paper uses the paleoecological record of diatoms to investigate long-term development of individual polygons in a High Arctic wetland. Two landform processes were examined: (1) the millennial-scale development of a polygon-pond, and (2) the transition from lowcenter to erosive high-center polygons. Diatom assemblages were quantified from habitats associated with contrasting landforms in the present-day landscape, and used as an analog to reconstruct past transitions between polygon types. On the basis of this evidence, the paleoecological record does not support either of the existing models describing the predictable succession of polygon landforms in an arctic wetland. Our results indicate a need for greater paleoecological understanding, in combination with in situ observations in present-day geomorphology, in order to identify patterns of polygon wetland development and elucidate the long-term drivers of these landform transitions.

The influence of natural terrestrial disturbances on the limnology of northern lakes is little known, yet important for understanding the ecology of these remote ecosystems. The Riviere George Caribou Herd (RGCH) in northern Quebec has undergone a large population increase since the 1950s, which has been accompanied by pronounced impacts (i.e., degradation of vegetation cover, soil erosion) on terrestrial environments of the Riviere George region. The goal of our study was to evaluate if the increased caribou activity and resulting terrestrial impacts have been accompanied by impacts on adjacent aquatic ecosystems. We studied the recent diatom assemblages (i.e., about the last two centuries) preserved in six sediment cores taken from lakes located in the most heavily impacted region, and in one core from a lake showing little evidence of recent caribou activity in the catchment basin. Core chronologies based on 210Pb dating techniques indicated that average mass sedimentation rates in this subarctic region were very low (0.0039 to 0.012 g cm-2 yr-1). Perhaps surprisingly, diatom assemblages from all seven cores showed very little change, indicating remarkably stable limnological conditions throughout the past -200 yr. The lack of a signal from caribou activity is most likely due to (1) short-term impacts being too transitory to be registered in the sediment record, and (2) long-term impacts not representing a significant perturbation outside the natural variability of these aquatic ecosystems. In contrast to other arctic regions, the limnological stability of our study sites suggests that recent climatic change impacts have been negligible in this region of northern Quebec, which seems to confirm climate model predictions that northern Quebec and Labrador will remain climatically stable under global warming scenarios.

Knowledge of the composition and ecological preferences of diatom assemblages in northern regions is important for paleoenvironmental reconstructions of variables related to climate and/or anthropogenic disturbances. Relatively little is presently known about diatoms from circumpolar treeline areas, especially for lakes located in northern Russia. Our study set included lakes in the Siberian tundra, forest-tundra, and boreal forest regions close to Norilsk, of which nine sites were subject to anthropogenic disturbances, such as mining and housing developments. We enumerated surficial sediment diatom assemblages and used multivariate statistical techniques to investigate which environmental variables were important in explaining the variation in diatom assemblages within our lake set. Deeper lakes were associated with higher abundances of planktonic centric taxa, such as Cyclotella Kutzing species. Warmer, higher conductivity forested lakes were associated with higher abundances of planktonic pennate taxa,...

Paleolimnological analyses of two lakes located near the northern treeline in the central part of the Canadian Northwest Territories document a history of abrupt postglacial climatic and limnological changes. A diatom-based transfer function, based on weighted-averaging partial least squares regression (WA-PLS) techniques, was used to give quantitative estimates of past trends in lakewater dissolved organic carbon (DOC), a limnological variable strongly aligned with catchment vegetation and soils. The diatom record from the sediment cores provides evidence for profound limnologic change during the mid-Holocene, corresponding to maximum forest-tundra development between 5000 and 3000 '4C yr BP in both Queen's and Toronto lakes, with a diatom-inferred relative change in lakewater DOC of up to 5.8 mg L-1 between the mid-Holocene and the present-day. Comparison of the diatom-inferred environmental changes with other proxy data (pollen, stable isotopes) from the same lakes provides strong evidence for an episode of climatic amelioration and lake responses to the associated vegetational changes (from tundra to forest-tundra) at sites near the central Canadian treeline. This study illustrates the usefulness of diatoms as quantitative indicators of past climate-related environmental change in northern treeline regions, and implies that aquatic ecosystems at high latitudes might respond with extreme sensitivity to climate warming.

Les diatomees des eaux douces sont de plus en plus utilisees pour reconstituer les environnements du passe. Cette recherche a ete realisee dans le but d’evaluer ce potentiel dans une region ou peu d’informations sur les diatomees etaient disponibles. L’etude a permis d’identifier un ensemble de 516 especes de diatomees preservees dans les sediments de surface d’un transect latitudinal de 59 lacs du Nord-Ouest quebecois. A l’aide d’analyses statistiques multivariees, il a ete possible de determiner quelles sont les variables environnementales (parmi 49) qui exercent le plus d’influence sur la composition des communautes de diatomees. Ce sont les concentrations en magnesium, en sodium, en carbone organique dissous (COD) et en silice, ainsi que la profondeur des lacs qui semblent etre les variables qui influencent le plus la composition des communautes de diatomees dans cette region. Un modele de reconstitution du COD a ete cree. Ce modele pourra etre utilise dans des etudes paleolimnologiques afin d...

La presente etude a permis de determiner la flore diatomifere de quatre sources thermales situees au Canada (Iles de la Reine-Charlotte en Colombie Britannique et Takhini au Yukon), au sud-ouest de l'Islande et au Japon (mont Hakone). La composition en especes des assemblages de chaque echantillon est decrite et appuyee par des illustrations originales. Certains assemblages sont constitues d'especes dominantes, telles que Diadesmis gallica W. Smith pour l'echantillon de Takhini, Brachysira microcephala (Grunow) Compere et Achnanthes minutissima Kutzing pour l'Islande et Pinnularia sp. [aff. anglica (Cleve) Krammer] pour le Japon. La majorite des taxons repertories sont des formes alcaliphiles et circumneutrales selon leur affinite pour le pH, et de formes oligohalobes, mesohalobes et euryhalobes selon la salinite. La comparaison de la microflore entre les quatre sources echantillonnees nous amene a la conclusion qu'on ne peut ici etablir de flore diatomifere specifique aux sources thermales. L'eloignement...