Le till pollinifère de la péninsule du Nunavik, Québec septentrionnal

Des concentrations polliniques étonnamment élevées ont été notées dans le till en périphérie du cratère du Nouveau-Québec (Richard et al., 1991; Frechette, 1994). Nous avons posé l'hypothèse selon laquelle le till situé près de la ligne de partage glaciaire est plus riche en pollen que les tills qui en sont éloignés. L'hypothèse est ici vérifiée. En effet, le till du centre du Nunavik, près de la ligne de partage glaciaire, recèle des concentrations polliniques de l'ordre de ~31 000 grains/g, alors que ceux plus à l'est et plus à l'ouest ont des valeurs moyennes respectives de ~8500 grains/g et de ~6800 grains/g. Les sédiments du centre du Nunavik ont pu être préservés d'une érosion glaciaire intense, de sorte que le pollen accumulé au cours des interstadiaires et interglaciaires antérieurs a pu être conservé puis recyclé. À première vue, les spectres polliniques du till de la péninsule du Nunavik reproduisent à peu près la biogéographie contemporaine. Les assemblages polliniques du till plus au nord semblent être le reflet d'une toundra herbacée, alors que ceux du till plus au sud s'approcheraient plutôt d'une représentation de toundra arbustive. L'analyse des composantes principales a permis de comparer les assemblages polliniques du till à ceux des sédiments de surface des lacs environnants. Les résultats soulignent l'absence d'analogues contemporains de même qu'ils laissent voir des différences dans la répartition ou l'étendue des zones bioclimatiques pré-wisconsiniennes.Exceptionally high pollen concentrations have been noted in till in the vicinity of the Nouveau-Québec crater (Richard et al., 1991; Frechette, 1994). We have proposed as an explanation the hypothesis that till near the ice divide is polliniferous. This present hypothesis is tested in parts of the Nunavik crossed by the Payne ice divide. Close to the ice divide, till shows pollen concentrations in the order of ~31 000 grains/g whereas tills at a distance eastward and westward have mean values of 8500 grains/g and 6800 grains/g respectively. Deposits near the ice divide zone of Nunavik may have escaped significant glacial erosion and thus pollen that had accumulated during previous interstadials and interglacials may have been preserved and recycled in till. Pollen spectra of till throughout Nunavik reflects only approximately the modern biogeography, ranging from herb-tundra in the north to shrub-tundra in the south. Main components analysis has allowed comparison between till samples and neighbouring lacustrine surface sediments; based on that comparison the pollen assemblages of the till samples have no modern analogs which suggests some shifting in the distribution of the pre-Wisconsinan bioclimatic zones.Erstaunlich hohe Pollen-Konzentrationen wurdem im Till in der Nâhe des Kraters von Nouveau-Québec festgestellt (Richard et al., 1991; Frechette 1994). Wir haben die Hypothèse aufgestellt, daB das nahe bei der glazialen Eisscheide gelegene Till reicher an Pollen ist, als die von ihr entfemten Tills. Die Hypothèse wird in diesem Beitrag ûberprùft. In der Tat enthâlt das Till aus dem Zentrum von Nunavik nahe bei der glazialen Eisscheide Pollen-Konzentrationen der GrôBenordung -31 000 Korner/g, wogegen die mehr Ôstlich und westlich gelegenen mittlere Werte von -8500 Kôrner/ g bzw. -6800 Korner/g haben. Die Sedimente aus dem Zentrum von Nunavik konnten vor einer intensiven glazialen Erosion bewahrt werden, so daB der wâhrend der vorher-gehenden lnterstadiale und Interglaziale angesammelte Pollen erhalten und wieder-verwendet werden konnte. Auf den ersten Blick entsprechen die Pollen-Spektren vom Till der Halbinsel Nunavik ungefàhr der heutigen Biogeographie. Die Pollen-Einheiten vom mehr nôrdlichen Till scheinen eine Pflanzen-Tundra zu spiegeln, wogegen diejenigen vom sudlicheren Till eher einer Busch-Tundra gleichen. Die Analyse der Hauptbestandteile erlaubte die Polleneinheiten des Tills mit denen der Oberflàchensedimente der umliegenden Seen zu vergleichen. Die Ergebnisse heben das Fehlen gegenwàrtiger Entsprechungen hervor und lassen auch Unterschiede in der Verteilung oder Ausbreitung der bioklimatischen Zonen im Vor-Wisconsinium erkennen

The last interglacial in the northern North Atlantic and adjacent areas: evidence for a more zonal climate than during the Holocene

We document climate conditions from the last interglacial optimum (LIO) or marine isotope stage 5e (MIS 5e) from terrestrial and oceanic sedimentary archives. Terrestrial climate conditions are reconstructed from pollen assemblages, whereas sea-surface temperature and salinity conditions are estimated from dinocyst assemblages and foraminiferal data (both assemblages and stable isotope composition of carbonate shells). LIO data from the eastern Canadian Arctic and northern Labrador Sea led to reconstruct much higher summer air temperature and seasurface temperature than at present by about 5°C. Data from southeastern Canada and southern Labrador Sea also suggest more thermophilic vegetation and warmer conditions although the contrast between LIO and the Holocene is of lesser amplitude. On the whole, the terrestrial and marine data sets from the northwest North Atlantic and adjacent lands suggest limited influence of southward flow from Arctic waters through the east Greenland and Labrador Currents as compared to the modern situation. The compilation of sea-surface reconstructions from the northwest and northeast North Atlantic indicate much reduced longitudinal contrasts of temperatures than at present, thus a more zonal pattern of circulation. The reconstructions also indicate a lower sea-surface salinity than at present, thus stronger stratification of upper water masses, which would be compatible with a reduced North Atlantic deep-water formation

Middle to late Wisconsinan climate and ecological changes in northern Alaska: Evidences from the Itkillik River Yedoma

Continuous paleoenvironmental records covering the period prior to the Last Glacial Maximum in northeastern Beringia are sparse. This study presents a multi-proxy analysis of a 35-m-high yedoma exposure located on the right bank of the Itkillik River in Alaska. The exposure accumulated over 39 thousand years (kyr) during the Middle Wisconsinan Interstadial and the Late Wisconsinan glacial advance. We identified five stratigraphic units based on pollen, carbon and ice content, and isotopic composition (δ18O) of the sediments. Middle Wisconsinan climate in northern Alaska promoted peat accumulation prior to 33.6 cal kyr BP (calibrated kyr before present). Reconstructed July air temperatures were 1–2 °C lower than current at 34.8 cal kyr BP, consistent with the timing of the interstadial climatic optimum in interior Alaska and Yukon. Colder (by up to 4 °C) and drier conditions characterize the transition from interstadial to glacial conditions between 33.6 and 29.8 cal kyr BP. Late Wisconsinan (29.8–17.9 cal kyr BP) July air temperatures were 2–3 °C lower than today, with grassland vegetation dominated by Poaceae, Artemisia and forbs, in contrast to the modern Cyperaceae dominance. Moister and warmer environmental conditions after 17.9 cal kyr BP correspond to the Late Glacial to Early Holocene interva

Impacts of stronger winds and less sea ice on Canadian Beaufort Sea shelf ecosystems since the late 1990s

Continuous and multi-decadal records of faunal abundance and diversity helping to identify the impacts of ongoing global warming on aquatic ecosystems are rare in the coastal Arctic. Here, we used a 50-year-long microfaunal record from a sediment core collected in the Herschel Basin (YC18-HB-GC01; 18 m water depth) to document some aspects of the environmental responses of the southern Canadian coastal Beaufort Sea to climate change. The microfaunal indicators include benthic foraminiferal assemblages, ostracods and tintinnids. The carbonate shells of two foraminiferal species were also analyzed for their stable isotope signatures (δ13C and δ18O). We compiled environmental parameters from 1970 to 2019 for the coastal region, including sea ice data (break-up date, freeze-up date, open season length and mean summer concentration), the wind regime (mean speed, direction of strong winds and the number of storms), hydrological data (freshet date, freshet discharge and mean summer discharge of the Firth and the Mackenzie rivers), and air temperature. Large-scale atmospheric patterns were also taken into consideration. Time-constrained hierarchal clustering analysis of foraminiferal assemblages and environmental parameters revealed a near-synchronous shift around the late 1990s. The microfaunal shift corresponds to an increased abundance of taxa tolerant to variable salinity, turbulent bottom water conditions, and turbid waters towards the present. The same time interval is marked by stronger easterly winds, more frequent storms, reduced sea-ice cover, and a pervasive anticyclonic circulation in the Arctic Ocean (positive Arctic Ocean Oscillation; AOO+). Deeper vertical mixing in the water column in response to intensified winds was fostered by increased open surface waters in summer leading to turbulence, increased particle loading and less saline bottom waters at the study site. Stronger easterly winds probably also resulted in enhanced resuspension events and coastal erosion in addition to a westward spreading of the Mackenzie River plume, altogether contributing to high particulate-matter transport. Increase food availability since ∼2000 was probably linked to enhanced degradation of terrestrial organic carbon, which also implies higher oxygen consumption. The sensitivity of microfaunal communities to environmental variations allowed capturing consequences of climate change on a marine Arctic shelf ecosystem over the last 50 years

Histoire postglaciaire de la végétation de la forêt boréale du Québec et du Labrador

L’histoire postglaciaire des domaines bioclimatiques actuels des pessières et des sapinières du Québec boréal et du sud du Labrador (Canada) fut reconstituée à partir de 61 diagrammes polliniques de sédiments lacustres. L’ époque, la géographie de la déglaciation et le climat associé ont déterminé la présence ou non d’une végétation initiale de toundra selon les endroits. La migration des arbres et le développement ultérieur du couvert forestier se firent surtout en fonction des conditions climatiques changeantes et de l’incidence inégale des incendies naturels au fil du temps. L’afforestation fut très diversifiée, comportant de nombreux paysages végétaux sans analogue moderne. Leur durée fut variable, avec des cas extrêmes de coincement ou d’effusion temporelle ou spatiale de certaines espèces. Des forêts semblables aux couverts modernes se sont établies progressivement. Avec l’amélioration climatique, la progression a culminé entre 8 000 et 4 000 ans avant nos jours par l’abondance accrue d’essences relativement thermophiles. Une régression de la végétation liée à une détérioration climatique s’ensuivit, ce qui mena aux domaines bioclimatiques actuels.The postglacial history of the present-day black spruce and balsam fir-dominated bioclimatic domains of boreal Québec and southern Labrador (Canada) was reconstructed using 61 pollen diagrams from lake sediments. The period, deglaciation geography and climate determined whether or not there was an initial tundra vegetation. The establishment of trees and the subsequent development of forest cover were largely a function of climatic changes and the varied occurrence of fire over time. Afforestation was diverse, with numerous vegetational landscapes lacking modern analogues. Their duration was varied, with certain species showing extreme cases of cornering and effusion, both in space and time. The establishment of forests similar to those found today was gradual. As the climate warmed, there was an increase in the abundance of relatively thermophilous species. This progression culminated between 8,000 and 4,000 years ago. Climatic deterioration then caused a regression of the vegetation cover, giving rise to the present bioclimatic domains

Regional paleofire regimes affected by non-uniform climate, vegetation and human drivers

International audienceClimate, vegetation and humans act on biomass burning at different spatial and temporal scales. In this study, we used a dense network of sedimentary charcoal records from eastern Canada to reconstruct regional biomass burning history over the last 7000 years at the scale of four potential vegetation types: open coniferous forest/tundra, boreal coniferous forest, boreal mixedwood forest and temperate forest. The biomass burning trajectories were compared with regional climate trends reconstructed from general circulation models, tree biomass reconstructed from pollen series, and human population densities. We found that non-uniform climate, vegetation and human drivers acted on regional biomass burning history. In the open coniferous forest/tundra and dense coniferous forest, the regional biomass burning was primarily shaped by gradual establishment of less climate-conducive burning conditions over 5000 years. In the mixed boreal forest an increasing relative proportion of flammable conifers in landscapes since 2000 BP contributed to maintaining biomass burning constant despite climatic conditions less favourable to fires. In the temperate forest, biomass burning was uncoupled with climatic conditions and the main driver was seemingly vegetation until European colonization, i.e. 300 BP. Tree biomass and thus fuel accumulation modulated fire activity, an indication that biomass burning is fuel-dependent and notably upon long-term co-dominance shifts between conifers and broadleaf trees

Timing of MIS 5e marine optimum and implications for the stratigraphic context of last interglacial sediments in the (sub)arctic North Atlantic

Marine sediment records of dinoflagellate cyst, foraminiferal and ice rafted debris content and stable isotopes from the Nordic seas and Labrador Sea were used to reconstruct the evolution of the surface circulation in the (sub)arctic North Atlantic during the Last Interglacial. Average global temperatures of this time interval, known as Marine Isotope subStage (MIS) 5e, are believed to have been higher than those of the present Holocene interglacial period. However, the abundance peak of warm dinoflagellate cyst taxa and subpolar planktic foraminifera in the eastern Nordic seas during late MIS 5e would suggest that the development of upper ocean interglacial conditions in that area was delayed with respect to the temperate latitudes, and the marine optimum in the eastern Nordic seas with a surface circulation comparable to the modern one was not reached until late MIS 5e. While the lack of a modern type of surface circulation during much of the early MIS 5e probably prevented the formation of Labrador Sea Water (Hillaire-Marcel et al., 2001), our data furthermore suggest that only with the establishment of this intensified modern-type of northward heat transport, an interglacial surface ocean environment also developed in the northern Nordic seas. Hence, our findings illustrate the importance of a correct (stratigraphic) context placement of those last interglacial records from the high Arctic pointing out overall warmer conditions with respect to the Holocene, as these might represent only specific phases of MIS 5e. Hillaire-Marcel, C., de Vernal, A., Bilodeau, G., Weaver, A.J., 2001. Absence of deep-water formation in the Labrador Sea during the last interglacial period. Nature 410, 1073-1077

Last interglacial (MIS 5e) surface circulation in the North Atlantic and Nordic seas, based on dinoflagellate cysts

Sediments from the last interglacial (MIS 5e) were studied for their dinoflagellate cyst (dinocyst) content from cores across the North Atlantic and Nordic seas, in order to trace changes in the interaction between the warm water masses of the northward flowing Gulf Stream/North Atlantic Drift current system and the cold water masses of the East Greenland/Labrador Current. Supported by stable isotope, IRD and planktic foraminiferal data, the qualitative and quantitative analysis of the dinocyst assemblages illustrates the stepwise development of interglacial conditions from late MIS 6 and Termination II towards a MIS 5e optimum and back to the colder MIS 5d stadial. It is shown that the development of the MIS 5e optimum occurred only late during MIS 5e in the eastern Nordic seas, as a consequence of a delayed intensification of the northernmost limb of the Gulf Stream/North Atlantic Drift. While prolonged deglacial processes presumably had hindered this northward protrusion of warm surface waters during early MIS 5e, its intensification during late MIS 5e in turn caused a reorganisation of the cold surface current system

Multi-sensor global compilation of mid-Holocene sea surface temperatures, based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst

We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface temperatures (SST), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations