Showing papers in "Geographie Physique Et Quaternaire in 2002"
TL;DR: Growth data from precise surveys have been obtained for 11 pingos for periods ranging from 20 to 26 years as mentioned in this paper, and some of the data derived from the long-term study of pingo growth are relevant to the identification of collapse features, interpreted as paleo pingos, in areas now without permafrost.
Abstract: Growth data from precise surveys have been obtained for 11 pingos for periods ranging from 20 to 26 years. Most of the 1350 pingos, perhaps one quarter of the world's total, have grown up in the bottoms of drained lakes underlain by sands. Permafrost aggradation on the drained lake bottoms has resulted in pore water expulsion, solute rejection below the freezing front, a freezing point depression, and groundwater flow at below 0° C to one or more residual ponds, the sites of pingo growth. Sub-pingo water lenses underlie many growing pingos.The pure ice which grows by downward freezing in a sub-pingo water lens may be composed of seasonal growth bands which, like tree rings, are of potential use in the study of past climates. Growing pingos underlain by sub-pingo water lenses can often be identified by features such as peripheral pingo rupture, spring flow, frost mound growth, normal faulting, and oscillations in pingo height. Such features, and others, are associated with hydrofracturing and water loss from a sub-pingo water lens. Some of the data derived from the long-term study of pingo growth are relevant to the identification of collapse features, interpreted as paleo pingos, in areas now without permafrost.
179 citations
TL;DR: In this article, the authors measured seasonal movements of the active layer and subjacent permafrost in numerous ice-wedge polygons that have varied in age, type, crack frequency, and topographic location.
Abstract: Thermally induced seasonal movements of the active layer and subjacent permafrost have been measured in numerous ice-wedge polygons that have varied in age, type, crack frequency, and topographic location. The field observations show that, in winter, thermal contraction, which is inward, is constrained or vanishes at the polygon centres but, in summer, thermal expansion, which is outward, is unconstrained at the ice-wedge troughs. Therefore, there tends to be a small net summer transport of the active layer, to varying depths, into the ice-wedge troughs. The movement has been observed in all polygons studied. The slow net transport of material into the ice-wedge troughs has implications for: permafrost aggradation and the growth of syngenetic wedges in some troughs; the palaeoclimatic reconstruction of some ice- wedge casts; and the interpretation of polygon stratigraphy based upon the assumption that the polygon material has accumulated in situ .
171 citations
TL;DR: In this paper, a methode paleoenvironnementale is defined for the vegetation of Pinus cembra in the Alpes francaises du Sud region, based on the identification of charbons de bois macrofossiles.
Abstract: Cet article a pour objectif de montrer comment l'emploi de la pedoanthracologie permet de contribuer a l'etude de l'histoire de la vegetation ligneuse de stations situees a des altitudes elevees. Cette methode paleoenvironnementale est fondee sur l'identification microscopique et la datation (au 14 C par SMA) des charbons de bois macrofossiles enfouis dans les sols mineraux et extraits par tamisage sur un tamis de 400 μm. Dans les Alpes francaises du Sud, le determinisme climatique de l'asylvatisme actuel de l'etage alpin est remis en cause par la decouverte de charbons de bois dans tous les prelevements realises dans cet etage. Les resultats revelent l'importance geographique de Pinus cembra , dont l'aire de repartition etait bien plus etendue entre 3 800 et 1 400 ans BP que de nos jours. La precision spatiale de la methode est soulignee par l'etude de plusieurs profils pedoanthracologiques d'une station dans une vallee des Alpes francaises du Nord ; cette analyse presente les changements locaux de la limite superieure d' Alnus glutinosa/A. incana depuis environ 6 000 ans. Des hypotheses impliquant les pratiques agro-sylvo-pastorales couplees a d'eventuelles pejorations climatiques expliquent les changements survenus dans la vegetation forestiere de cette station.
84 citations
TL;DR: In this article, the age of the earliest moraine, the Frontier Moraine, is estimated to be about 12 550 years BP, while the ages of the subsequent Dixville, Cherry River-East-Angus, Mont Ham and Ulverton-Tingwick moraines are estimated at 12 500, 12 325, 12 200 et 12 100 years BP.
Abstract: Late Wisconsinan deglaciation in southeastern Quebec was preceded by a northward ice-flow reversal that was recorded in the northeastern part of the region. The reversal event was generated by flow convergence toward the St. Lawrence Ice Stream, a northeastward-flowing ice stream which formed in the St. Lawrence estuary prior to 13 000 years BP and lasted until at least 12 400 years BP. In the Bois-Francs uplands, the flow reversal event led to the formation of a semi-detached ice mass that underwent widespread stagnation and downwasting. In the southwestern region, northward retreat of the margin of the Laurentide Ice Sheet was marked by the formation of a series of discontinuous recessional moraines and by the development of ice-dammed lakes in the main valleys. The level of these lakes fell as progressively lower outlets became ice-free. The main episodes are (1) the Sherbrooke Phase of Glacial Lake Memphremagog, (2) an unnamed transitional lake and (3) Glacial Lake Candona, a large lake which had expanded northeastward from the deglaciated regions of the Upper St. Lawrence (Lake Iroquois) and Ottawa valleys to the Lake Champlain (Glacial Lake Vermont) basin. As recorded by the Danville Varves, Lake Candona lasted about 100 years following deposition of the Ulverton-Tingwick Moraine. Subsequent ice retreat along the Appalachian piedmont led to final drainage of Lake Candona and allowed Champlain Sea waters to invade much of these glaciolacustrine terrains about 12 000 years BP. On the basis of the Danville Varves record, a regional rate of ice retreat of about 200 m·a -1 is inferred. The age of the earliest moraine, the Frontier Moraine, is thus about 12 550 years BP, while the ages of the subsequent Dixville, Cherry River-East-Angus, Mont Ham and Ulverton-Tingwick moraines are estimated at 12 500, 12 325, 12 200 et 12 100 years BP, respectively.
66 citations
TL;DR: In this paper, a deglacial chronology for northern New England has been formulated using an atmospheric 14 C calibration of the New England Varve Chronology and paleomagnetic records.
Abstract: A deglacial chronology for northern New England has been formulated using an atmospheric 14 C calibration of the New England Varve Chronology and paleomagnetic records. This 14 C chronology is based on 14 C ages from macrofossils of non-aquatic plants and is about 1 500 yr younger than existing chronologies that are based primarily on 14 C ages of bulk organic samples. The lower and upper Connecticut Valley varve sequences of Ernst Antevs (NE varves 2 701-6 352 and 6 601-8 500) overlap (lower 6 012 = upper 6 601) based on their crudely matching varve records and their similar paleomagnetic records. Three 14 C ages at Canoe Brook, Vermont (NE varve 6 150 = 12.3 14C ka) calibrate the lower Con necticut Valley sequence. New AMS and con ventional 14 C ages on woody twigs from Newbury, Vermont calibrate the upper se quence from 11.6-10.4 14 C ka (NE varves 7 440-8 660) and are consistent with the over lapping varve and paleomagnetic records, and the Canoe Brook 14 C ages. Deglaciation of the Connecticut Valley in southern Vermont began at 12.6 14 C ka (15.2 cal ka) and the Littleton-Bethlehem Readvance in northern New Hampshire and Vermont reached its maximum at11.9-11.8 14 C ka (14.0-13.9 cal ka) followed by recession of ice into Quebec at about 11.5 14 C ka (13.4 cal ka). A lake persisted in the upper Connecticut Valley until at least 10.4 14 C ka (12.3 cal ka) and may have been seen by the first humans in the area.
59 citations
TL;DR: In this paper, a global vision of the Somme basin valleys evolution between the end of the Weichselian Upper Pleniglacial and the middle of the Holocene is presented.
Abstract: Recent research on Late Glacial and Holocene sequences of the Somme basin are based on large sections of alluvial plains completed by investigation of numerous archeological profiles. Field and bioclimatic datas confrontation result in a global vision of the Somme basin valleys evolution between the end of the Weichselian Upper Pleniglacial and the middle of the Holocene. Within this evolution it is possible to identify important changes in fluvial morphology and sedimentation linked to climatic modifications. The first major modification of fluvial morphology is dated from the the Upper Pleniglacial/Lateglacial transition (13 000 BP): valley response to the climatic improvement is marked by an incision and the passage from a braided channel pattern to a transitional system (multi-channel). The first organic deposits infilling these channels are dated from the Bolling interstadial. During the Allerod we observe the deposition of organic silts in a large single channel system. The end of the Lateglacial, is characterized by a very important infilling of the entire valley by fine calcareous silts attributed to the Younger Dryas. These overbank deposits are linked to the overflowing of a single meandering channel. A second major incision phase, in a single channel, is related at the beginning of the Hololcene at about 10 000 BP. Then, between the Preboreal and the middle of the Atlantic (10 300 to about 5 500 BP), the entire valley is progressively infilled by a peat bog with two lateral channels where organic silts were deposited. Finally, a last phase of erosion and incision, whose climatic origin is not obvious, appears in the Atlantic between about 6000 and 5500 BP. This last fluvial pattern modification is followed by the deposition of laminated organic silts in a large single channel and by an important calcareous tufa development.
51 citations
TL;DR: This paper reconstructed the deglaciation of Nova Scotia using the AMS-dated chronology of lake sediments and buried organic sections exposed in the basins of former glacial lakes.
Abstract: The deglaciation of Nova Scotia is reconstructed using the AMS-dated chronology of lake sediments and buried organic sections exposed in the basins of former glacial lakes. Ice cleared out of the Bay of Fundy around 13.5 ka, punctuated by a brief read- vance ca. 13-12.5 ka (Ice Flow Phase 4). Glacial Lake Shubenacadie (1) formed in central Nova Scotia, impounded by a lobe of ice covering the northern Bay of Fundy outlet. Drainage was re-routed to the Atlantic Ocean until the Fundy outlet became ice free after 12 ka. When this lake drained, bogs and fens formed on the lake plain during climatic warming. Organic sediment (gyttja) began to accumulate in lake basins throughout Nova Scotia. Glacierization during the Younger Dryas period (ca. 10.8 ka) resulted in the inundation of lakes and lake plains with mineral sediment. The nature and intensity of this mineral sediment flux or "oscillation" varies from south to northern regions. Southern lakes simply record changes in total organic content whereas northern lakes, where most buried peat sections are found, feature a thick inorganic sediment layer. Glacial ice or permanent snow cover and seasonal melting are essential in the formation of this mineral sediment layer; both to provide the water source for erosion, and to prevent plant re- colonization and landscape stabilization. Some northern lakes do not appear to record the Younger Dryas event, with organic accumulation starting around 10 ka. During the Younger Dryas, fine and coarse-grained deposits were deposited in Glacial Lake Shubenacadie (2) and other lowland areas at elevations similar to former (12 ka) lake levels, impounded by re-invigorated residual ice caps and permanent snow/aufeis.
46 citations
TL;DR: In this paper, the Acerno lacustrine basin represents a tectonic palaeolake formed in the southern Apennines during the Middle Pleistocene, and Pollen analysis performed on the Lacustrine sequence (about 100 m thick) allows the vegetation history to be reconstructed and correlated with a late middle-paleistocene interglacial-glacial cycle.
Abstract: The Acerno lacustrine basin represents a tectonic palaeolake formed in the southern Apennines during the Middle Pleistocene. Pollen analysis performed on the lacustrine sequence (about 100 m thick) allows the vegetation history to be reconstructed and correlated with a late Middle Pleistocene interglacial-glacial cycle. Interglacial conditions are evidenced by a rich oak forest phase while the glacial period is highlighted by a sharp decrease in all arboreal taxa and by the contemporaneous increase in herbaceous and steppe elements. The absence of subtropical elements such as cf. Taxodium and Carya throughout the sequence suggests a late Middle Pleistocene age on the basis of comparison with the nearby Vallo di Diano palaeolacustrine basin where these elements survived until the end of isotopic stage 13. Analysis of the thickest tephra layers identified throughout the sequence provide further constraints to better define its chronological position which is correlated with isotopic stages 9 and 8 by the presence of the Lower WTT (White Trachytic Tuff) marker level, dated 297 000 yr BP.
46 citations
TL;DR: In this paper, the authors describe licheno- metric and dendroglaciological investigations of Little Ice Age deposits at Tzeetsaytsul Glacier, Tweedsmuir Provincial Park, British Columbia.
Abstract: This paper describes licheno- metric and dendroglaciological investigations of Little Ice Age (LIA) deposits at Tzeetsaytsul Glacier, Tweedsmuir Provincial Park, British Columbia. The glacier originates from an ice- field on the northeast flank of Tzeetsaytsul Peak and terminates in a moraine-dammed lake. A stream draining the lake has incised the moraine dam and flows through nested moraines into a second lake. Two end moraines near the lower lake record separate advances, with numerous morainic ridges found between the two lakes. A locally calibrated Rhizocarpon geographicum growth curve was constructed and provides relative ages for all the moraines. Absolute dates from wood fragments collected from within the morainic debris were determined by matching their annual growth ring patterns to a local Abies lasiocarpa tree-ring chronology. The outermost terminal moraine was deposited by a 17th century advance that culminated in ca. 1700. Following subsequent recession, Tzeetsaytsul Glacier readvanced to build a second terminal moraine by the mid-1800s. Recession of this glacier occurred within 40 years and by 1935 the glacier was beginning to calve into the uppermost lake. The research shows that the most recent LIA advance of Tzeetsaytsul Glacier was not the most extensive, and that multiple events characterized the late-LIA. Application of the Rhizocarpon growth curve indicates a previously unreported 17th century advance at other glaciers in the region. These findings serve to reinforce the synchroneity of late-LIA glacier fluctuations within the coastal cordillera of NW North America suggesting that they record regional climate forcing.
42 citations
TL;DR: In this article, the development of the forest vegetation on a scree slope located in the valley of Riviere a Pierre, Gaspe Peninsula, was reconstructed over the last two centuries.
Abstract: The development of the forest vegetation on a scree slope located in the valley of Riviere a Pierre, Gaspe Peninsula, was reconstructed over the last two centuries. It was established from dendroecological data that the forest limit was lower than its present position around 1850 and that the forest cover expanded until the middle of the 20th century, in spite of evidence for slope instability at the end of the 19th century. After 1950, the forest edge retreated to its present position. Annual sedimentation rates were calculated by using dendrogeomorphological methods. They were lowest (< 0.1 cm/year) during the second half of the 19th century, and increased to 0.5 cm/year during the 20th century, and up to 1.0 cm/year after 1975. Increased slope activity was likely associated with a higher frequency of frost-coated clast flows.
41 citations
TL;DR: In this article, the authors re-examined the Bethlehem Moraine complex and support the original interpretation of a series of moraines deposited by active ice, and found other moraine clusters of similar age to the northeast in the Johns River and Israel River basins.
Abstract: The mode of deglaciation in the northwestern White Mountains of New Hampshire has been controversial since the mid 1800's. Early workers believed that active ice deposited the Bethlehem Moraine complex in the Ammonoosuc River basin during recession of the last ice sheet. In the 1930's this deglaciation model was replaced by the concept of widespread simultaneous stagnation and downwastage of Late Wisconsinan ice. The present authors reexamined the Bethlehem Moraine complex and support the original interpretation of a series of moraines deposited by active ice. We found other moraine clusters of similar age to the northeast in the Johns River and Israel River basins. Ice-marginal deposits that probably correlate with the Bethlehem Moraine also occur west of Littleton. The Bethlehem Moraine complex and equivalent deposits in adjacent areas were formed by readvance and oscillatory retreat of the Connecticut Valley lobe of the Laurentide Ice Sheet. This event is called the Littleton-Bethlehem Readvance. Throughout the study area, sequences of glaciolacustrine deposits and meltwater drainage channels indicate progressive northward recession of the glacier margin. Radiocarbon dates from nearby New England and Quebec suggest that the ice sheet withdrew from this part of the White Mountains between about 12 500 and 12 000 14 C yr BP. We attribute the Littleton- Bethlehem Readvance to a brief climatic cooling during Older Dyas time, close to 12,000 BP.
TL;DR: The Chaos Crags, a group of dacite domes in the Cascades Mtns (California), were affected by volcanic debris avalanches ca. 1675 A.D., these left a sizable deposit and a scar on the north mountain flank, now covered by talus as discussed by the authors.
Abstract: The Chaos Crags, a group of dacite domes in the Cascades Mtns (California), were affected by volcanic debris avalanches ca. 1675 A.D.; these left a sizable deposit and a scar on the north mountain flank, now covered by talus. This report examines the fabric and morphology of talus debris, their spatial variation, and the geomorphic processes presently affecting the slope. The talus presents a bi-segmented profile with a steep upper rectilinear segment and a shorter concave, basal zone. Debris are sorted by size both along (larger clasts downslope) and across the talus (larger particles below the cliffs). Shape sorting is weaker, but clast sphericity increases, and elongation decreases, toward the footslope. Upper-talus fabrics (long axes parallel to talus plane and slope) show that clasts there move by sliding, while basal blocks are deposited by rockfall, which causes more iso- tropic fabrics. Field observations and botanical evidence indicate the overall significance of grain flows, which are prevalent duetoanabundantsupplyofrubbleonthe upper talus. Comparison with similar recent slopes and repeat photography suggest the Chaos talus formed swiftly following dome collapse, when much debris may have collected below the unstable rockwalls. The talus has experienced only modest sedimentation during the 20 th century, and is currently affected by 'normal' mass-wasting processes, which also include snow avalanches and debris flows.
TL;DR: In this paper, the changes in chironomid communities and vegetation after deposition of the Mazama ash do not reflect a rapid shift to warmer or drier climate and evaporation, but rather an increased ionic concentration due to solutes derived from the freshly deposited tephra and perhaps in- washed silts and clays.
Abstract: Salinity fluctuations in lakes of semi-arid regions have been recognised as indicators of paleoclimatic change and have provided a valuable line of evidence in paleoclimatic reconstruction. However, factors other than climate, including sedimentologic events, may also affect salinity. At Kilpoola Lake, early postglacial freshwater chironomids (Microtendipes, Sergentia, and Heterotrissocladius) occur in the basal sediments and yield a chironomid-inferred salinity of <0.03 g/l. Higher salinities, ranging from 1.0 to 3.5 g/l, with Cricotopus/Orthocladius and Tanypus (chironomids typical of saline environments) follow and, persist for most of the remainder of the Holocene. An inferred 450% salinity increase (from 1.6 to 7.3 g/l) occurred in the sediment above the Mount Mazama tephra, followed by a return to the pre-Mount Mazama salinity. The early Holocene pollen spectra are typical of open steppe, but the post-Mazama Artemisia pollen percentages are exceptionally high and are associated with silty clays. Pollen spectra following this Artemisia peak represent steppe communities and are consistent with regional trends. We suggest that the changes in chironomid communities and vegetation after deposition of the Mazama ash do not reflect a rapid shift to warmer or drier climate and evaporation, but rather an increased ionic concentration due to solutes derived from the freshly deposited tephra and perhaps in- washed silts and clays.
TL;DR: In this article, a model for estimating peatland sensitivity to climate warming was developed using published information on the current state of climate, vegetation, and permafrost together with the changes expected with a doubling of CO 2.
Abstract: Under current scenarios of increasing greenhouse gases, the expected increases in global temperatures have the potential to affect, in many areas, the peat- lands that now cover 14 % of the soil area of Canada. A model for estimating peatland sensitivity to climate warming was developed using published information on the current state of climate, vegetation, and permafrost together with the changes expected with a doubling of CO 2 . Calculations based on this sensitivity model and data for the areal extent and carbon content of organic soils in Canada, show that approximately 60 % of the area of Canadian peatlands is expected to be severely to extremely severely affected by climate warming. These peatlands, which are deemed most sensitive to climate warming, also contain 53 % of the 154 Gt of carbon found in organic soils.
TL;DR: In this article, the materials and methods used in dendroglaciology with a series of examples derived from studies of Little Ice Age glacier fluctuations in the Canadian cordillera.
Abstract: Dendroglaciology is the use of absolutely dated tree-ring series to reconstruct the history of glacier fluctuations. Tree-ring series may be used (a) to provide minimum ages for glacial landforms by dating the germination of the oldest tree growing on the surface; (b) to provide exact dates from living trees damaged (tilted or scarred) at former glacial margins and, (c) to provide, by crossdating with long living-tree chronologies, calendar "kill" dates for trees within the glacier forefield that were overridden by the glacier. Although in-situ wood is preferred for these studies useful results can also be obtained from detrital material of limited but known provenance. This paper outlines the materials and methods used in dendroglaciology with a series of examples derived from studies of Little Ice Age glacier fluctuations in the Canadian cordillera.
TL;DR: In this article, the diatom assemblages of a sediment core were analysed to retrace the successive postglacial isolation phases of a coastal lake located near Kuujjuaraapik-Whapmagoostui (eastern Hudson Bay) in order to understand the evolution of the landscape in the southeastern Hudson Bay area following the retreat of the post-glacial Tyrrell Sea about 5000 years ago.
Abstract: In order to retrace the successive postglacial isolation phases of a coastal lake located near Kuujjuaraapik-Whapmagoostui (eastern Hudson Bay), the diatom assemblages of a sediment core were analysed The sedimentary sequence collected from Kachishayoot Lake shows three zones corresponding to paleoenvironmental changes brought about by isostatic rebound Each zone is characterized by a contrasting sedimentology and fossil diatom assemblages The transition from marine clays to gyttja and the evolution of the diatom assemblages, successively dominated by poly-mesohalobian, oligohalobian and finally halophobous species, documents the overall trend of decreasing salinity in the lake Diatominferred alkalinity also decreased, most likely reflecting the change from a nutrient-rich to a nutrient-poor environment as the basin was isolated from the waters of the postglacial Tyrrell Sea and gradually purged of nutrients and major ions (dissolved inorganic carbon, Ca, Mg) from the carbonate weathering of glacial tills and marine clays Chronological references obtained through AMS dating made it possible to evaluate sedimentation rates and the duration of the three different stages (marine conditions from 5400-4500 cal yr BP, isolation phase between 4500-1600 cal yr BP and modern lake phase since 1600 cal yr BP) of lake succession in the Kachishayoot Lake basin This biostratigraphic study yields information about the evolution of the landscape in the southeastern Hudson Bay area following the retreat of the postglacial Tyrrell Sea about 5000 years ago
TL;DR: In this paper, 40 radiocarbon dates have been obtained on foraminifera from nine, 2.5 to 11 m piston cores from the slope of SE Baffin Island.
Abstract: In order to describe ice sheet/ocean interactions at the NE margin of the Laurentide Ice Sheet over 40 radiocarbon dates have been obtained on foraminifera from nine, 2.5 to 11 m piston cores from the slope of SE Baffin Island. The cores were collected off Cumberland Sound and north of Hudson Straitfrom 750 to 1 510 m waterdepth. Rates of sediment accumulation varied between 20 and 40 cm/ky. Six cores contain high-resolution records of events during Marine Isotope Stage (MIS) 2 and parts of 1 and 3, whereas three cores have core top dates of ≥30 ka and thus provide information on MIS 3 and possibly 4/5. The cores include three main facies: Lithofacies A - yellowish/buff detrital carbonate-rich sediment, also referred to as Detrital Carbonate (DC-) events; Lithofacies B - olive-green detrital carbonate-poor sediment; and Lithofacies C - a black sedimentary unit. The lithofacies represent changes in glacial sources, ice sheet proximity, and processes of deposition. We conclude that there are DC- events correlative with Heinrich events H-1, H- 2, and H-4 in the North Atlantic; however, we find no compelling evidence for a DC-layer during H-3 (ca. 27 ± ka). There are three to four distinct DC- events after H-4 (ca.35 ± ka) but their exact ages are difficult to determine. Grain-size spectra and X-radiographs show that the DC-sediments are stratified to massive silty-clays with little sand, but generally have higher sand percentages at the base. DC-layers were deposited in part from turbidity currents, melting of icebergs, and rain-out of fine-grained silts and clays. In contrast, ice distal sediments in the eastern North Atlantic recorded H-events as an abrupt increase in ice rafted sand-size particles. In our study area, H-layer thicknesses vary from 0 and 70 cm for H-1 and 20 to 90 cm for H-2; H-4 is ≥60 to 100 cm thick. Over the total length of our records, the sedimentary conditions have been dominated by hemipelagic deposition (lithofacies B), implying that those times when ice reached the shelf (lithofacies A and C) have been short.
TL;DR: In this article, the dissolved inorganic carbon effect in the Champlain Sea is assessed on the basis of a comparison of 14 C ages obtained from marine shells and wood, which are at least 350 y.p. older than stratigraphically equivalent wood ages.
Abstract: At Saint- Nicolas, in the Quebec City Strait, tidal current sands of the Pleistocene-Holocene transition preserve an exceptional marine fossil fauna: 34 types of invertebrates in addition to the microfossils, three species of marine mammals, three species of seabirds and five species of fish, whose 14 C age varies between 10 060 and 9810 BP (δ 13 C = 0 ‰). These sands are overlain by tidalites which correspond to the last phase of the Champlain Sea, until ca. 9750 BP. The lithological, geochemical, and paleontological data gathered in the area suggest that sedimentation took place in an archipelago which was located at the entrance of the Champlain Sea and was subjected to tides that reached levels on the order of 5 to 10 metres. The strong currents of the rising tide favored the late occurrence of salt water influx in the area. The archipelago protected the sedimentary units from erosion during ebb tide as well as from later fluvial erosion resulting from glacioisostatic rebound. Faunal diversity is associated with high marine productivity which is tied to the mixing of fresh and salt waters. The food web was composed of molluscs which were abundant at low tide, of fishes, walrus, seals, and birds that nested on the small rocky islands. This sedimentary and ecological system implies a stabilization of the relative sea level attributed to the early Holocene eustatic rise. The dissolved inorganic carbon effect in the Champlain Sea is assessed on the basis of a comparison of 14 C ages obtained from marine shells and wood. Ages from marine shells (δ 13 C= 0 ‰) are at least 350 y. older than stratigraphically equivalent wood ages (δ 13 C=25 ‰).
TL;DR: The evolution history of lits fluviaux de la moyenne Durance (Sud-Est de la France) is studied in this paper, a partir de documents d'archives (textes et cartes anciennes), de series diachroniques de photographies aeriennes and d'analyses sedimentologiques des depots de la plaine alluviale.
Abstract: L'evolution historique des lits fluviaux de la moyenne Durance (Sud-Est de la France) est etudiee a partir de documents d'archives (textes et cartes anciennes), de series diachroniques de photographies aeriennes et d'analyses sedimentologiques des depots de la plaine alluviale. La richesse de l'information recueillie a permis de retracer l'evolution de la riviere au XIX e et au XX e siecle : diminution de la frequence des crues, contraction des lits fluviaux, incision. Lors des periodes anterieures, il semble que des phases de forte activite morphodynamique et hydrologiques (fin XIV e siecle, XVI-XIX e siecles) alternent avec des phases d'accalmie (XV e siecle), sans que disparaisse le style fluvial en tresses. L'etude souligne les particularites d'une riviere soumise a des influences mediterraneennes et montagnardes et l'importance, dans ce contexte, des fluctuations climatiques sur l'evolution de la morphogenese fluviale.
TL;DR: A distinctive flora of 73 species of vascular plants and numerous bryophytes occurs in the ca. 20 km 2 of alpine tundra in the White Mountains, New Hampshire as discussed by the authors.
Abstract: A distinctive flora of 73 species of vascular plants and numerous bryophytes occurs in the ca. 20 km 2 of alpine tundra in the White Mountains, New Hampshire. The late- Quaternary distribution of these plants, many of which are disjuncts, was investigated by studies of pollen and plant macrofossils from lower Lakes of the Clouds (1 542 m) in the alpine zone of Mount Washington. Results were compared with pollen and macrofossils from lowland late-glacial deposits in western New England. Lowland paleofloras contained fossils of 43 species of vascular plants, 13 of which occur in the contemporary alpine flora of the White Mountains. A majority of species in the paleoflora has geographic affinities to Labrador, northern Quebec, and Greenland, a pattern also apparent for mosses in the lowland deposits. The first macrofossils in lower Lakes of the Clouds were arctic-alpine mosses of acid soils. Although open-ground mosses and vascular plants continued to occur throughout the Holocene, indicating that alpine tundra persisted, fossils of a low-elevation moss Hylocomiastrum umbratum are evidence that forest (perhaps as krummholz) covered a greater area near the basin from 7 500 to 3 500 yBP. No calcicolous plants were recovered from sediments at lower Lakes of the Clouds. Climatic constraints on the alpine flora during the Younger Dryas oscillation and perhaps during other cold-climate events and intervening periods of higher temperature may have led to the loss of plant species in the White Mountain alpine zone. Late-glacial floras of lowland western New England were much richer than floras of areas above treeline during late-glacial time and at the present.
TL;DR: The internal structure of a 5.7 m high palsa was studied through a pattern of closely spaced drill holes in permafrost along two orthogonal section lines.
Abstract: The internal structure of a 5.7 m high palsa was studied through a pattern of closely spaced drill holes in permafrost along two orthogonal section lines. Holes were also drilled on a 1.3 m high peat plateau along a topographic transect for comparison purposes. The morphology of the palsa closely reflects the shape of the ice-rich core heaved by the growth of thick ice lenses in thick marine clay silts of the Tyrrell Sea. During and since palsa growth, the sand and peat covering was deformed by gelifluction and sliding and was also partly eroded by overland flow and wind. Palsa growth was accompanied by the formation of numerous ice-filled fault planes in the frozen sediments. The peat plateau was heaved to a lower height through the formation of thin ice lenses in an underlying layer of sandy silt only 1.4 m thick; this sediment is believed to be of intertidal origin. Therefore, the local Quaternary geomorphological settings are at the origin of differences in morphology and size between the palsa and the peat plateau. General inferences for the development of palsas and like landforms are made from the findings of the study.
TL;DR: The direction of Wisconsinan glacial dispersion of distinctive Proterozoic erratics derived from the Belcher Group in southeastern Hudson Bay is shown to have been northwestward, westward and southward for hundreds of kilometres across Hudson Bay, Northern Ontario, western Canada, and several adjoining northern States.
Abstract: The direction of Wisconsinan glacial dispersion of distinctive Proterozoic erratics derived from the Belcher Group in southeastern Hudson Bay is shown to have been northwestward, westward and southward for hundreds of kilometres across Hudson Bay, Northern Ontario, western Canada, and several adjoining northern States. The most distinctive of these erratics, termed "omars", are composed of massive siliceous wacke characterized by buff-weathering calcareous concretions; these erratics were derived from the Omarolluk Formation of the Belcher Group, exposed in the Belcher Islands of eastern Hudson Bay, and probably underlying much of the southern part of this inland sea. Far less common but equally distinctive are erratics of red oolitic jasper that were derived from the Kipalu Formation of the Belcher Group. In parallel with the now widely accepted field term "omar", we introduce the term "kipalu" for such erratics of oolitic jasper. A map showing the distribution of the distinctive erratics, in relation to indicators of Wisconsinan glacier movement, provides the basis for inferring at least two discrete glaciations that produced several major ice lobes. This paper summarizes the field observations of numerous Canadian and American earth scientists, traces the evolution of thought on provenance of the distinctive erratics, and outlines the criteria for distinguishing "true" omars from erratics derived from other bedrock sources of concretion-bearing wackes.
TL;DR: In this article, the authors used dendrochronological analysis to date mass movements that affected two unstable sites within a watershed (Boulc valley) and found that the triggering conditions for landslides seem to result from long-term slope evolution.
Abstract: Landslides represent major hazards in southern Alps where the plastic character of superficial deposits and regional climate create triggering conditions. They also provide an important sediment source for streambed dynamics. This article aims to date mass movements that affected two unstable sites within a watershed (Boulc valley). The reactions of 41 pines ( Pinus sylvestris L.) to the micro-movements on the Ravel-et-Ferriers site show that the January 1994 landslide is the result of a precarious stability pertaining to local factors since the end of the 1980's. The triggering conditions for landslides seem to result from long-term slope evolution (at least eight unstable periods occurred since the beginning of the century). The study of the mortality of 35 pines and 47 firs ( Abies alba Mill.) partially buried under sediments allowed to reconstruct the chronology of a dammed-lake development following the debris-flows of Bonneval-en-Diois. These events seem to be linked with some exceptional pluviometric conditions that dated recent movements. Results given by the dendrochronological analysis lead to set up the typological property of the two landslides, and to know the episods of water- course-sediment recharge, downstream to the sites. The Bonneval-en-Diois fossilization processes besides show analogies with the South Alps conditions during Holocene when part of thick fillings of valleys excavated in marlaceous formations should be the consequence of localized mass movements.
TL;DR: In this article, microscopic (pollen, rhizopods, charcoal) and plant macrofossil analyses were carried out on three cores for a peatland in the Laurentian Highlands (Quebec).
Abstract: Microscopic (pollen, rhizopods, charcoal) and plant macrofossil analyses were carried out on three cores for a peatland in the Laurentian Highlands (Quebec). The objectives were to reconstruct the developmental history and the past hydrological conditions of the peatland, and to infer paleoclimatic conditions, notably for the moisture balance. The peatland was characterized by contrasting hydrological conditions between the sampling points during its history. Except for the late Holocene, no clear regional hydrological control was revealed by paleobotanical analyses. Peat inception began in a small pond around 10 300 cal. years BP in a shrub tundra, more than 2000 years after the ice retreat. The peat- land dynamics during the early Holocene likely reflects increased biological productivity caused by the continuous warming of the climate after initial harsh conditions. From 8000 to 3000 BP, a general decrease of the net peat accumulation rate for all cores is in part attributed to the activity of nitrogen fixing cyano- bacteria which promotes an important decomposition of the organic matter. An important vertical peat increment occurred during the last 3000 years probably in response to wetter and colder climatic conditions.
TL;DR: Pollen and plant macrofossil analysis was performed on two small peatlands, situated on the facing slopes of the Guisane Valley (internal Southern French Alps).
Abstract: Pollen and plant macrofossil analysis were undertaken on two small peatlands, situated on the facing slopes of the Guisane Valley (internal Southern French Alps) Their comparative study allowed for the determination of the role played by exposition and human activity in the Holocene forest dynamic of the valley The vegetation distribution between 9000 and 4000 cal BP was influenced primarily by the microclimatic context related to exposition During the period around 8500 cal BP, the northern flank was already covered by Pinus forests, while the non-forested southern side was colonised by Pinus and Betula woodlands After 6500 cal BP, Abies woodlands and Pinus woodlands became dominant on the northern and southern sides, respectively The onset of human perturbation around 4000 cal BP caused a retreat of the Abies forests on the northern side and an opening of the plant cover on the southern side The latter is associated to an important increase in pollen abundance, which appears, in this study, to be a good indicator of forested and non-forested phases in the surrounding vegetation Human activities, which radically modified the vegetation of the valley for 4000 years, masked the influence of the exposition; Larix forests expanded on the northern side, while the southern side was invaded by grasslands and Pinus forests Finally, the reconstruction of local vegetation history stresses the major influence of the surrounding environment on sites development
TL;DR: The pattern of glaciation on the Avalon Peninsula has been established through study of geomorphic features, striations, and erratic provenance, and three phases in a continuum of glaciations are recognized.
Abstract: The pattern of glaciation on the Avalon Peninsula has been established through study of geomorphic features, striations, and erratic provenance. Three phases in a continuum of glaciation are recognized. The initial phase involved the expansion of ice from several centres. Ice thickness and extent reached a maximum during the subsequent Phase 2 event, correlated with the Late Wisconsinan. Lowered sea level permitted the development of the St. Mary's Bay ice centre. Ice from the Newfoundland mainland coalesced with Avalon Peninsula ice inPlacentia Bay, on the Isthmus, and in Trinity Bay. Rising sea level, triggered by the retreat of Laurentide ice in Labrador, resulted in destabilization of the St. Mary's Bay ice cap, marking Phase 3. Final deglaciation of theAvalon Peninsula began before 10,100 ± 250 BP. The Avalon Peninsula ice caps were controlled by regional and hemispheric events, and by the response of the Lauren- tide glaciers.
TL;DR: In this paper, the authors present a synthesis of the research made on fire frequency and forest composition in relation to climate change in the southwestern boreal forest of Quebec and show that warming leads to an important decrease in fire activity in the southern boreal forests of Quebec.
Abstract: We present a synthesis of the research made on fire frequency and forest composition in relation to climate change in the southwestern boreal forest of Quebec Dendrochronological studies, for the last 300 years, showed a decrease of fire frequency since the end of the Little Ice Age (~1850) Simulations using the Canadian General Atmospheric Circulation Model predict a decrease in forest fire activity with future warming The decrease in fire frequency is responsible for an increase in balsam fir ( Abies balsamea L) and white cedar ( Thuja occidentalis L) abundance during the twentieth century A low fire frequency could also explain the high abundance of white cedar during the warmest period of the Holocene (~6000 BP) while its subsequent decrease, together with a significant increase in jack pine ( Pinus banksiana Lamb), may be explained by an increase in fire frequency following the cooling of the climate that has occurred through the Neoglacial period The agreements between studies and at different time scales suggest that warming leads to an important decrease in fire activity in the southern boreal forest These results may apply to a large part of the boreal forest
TL;DR: The radiocarbon ages from lake sediment cores suggest that a low-altitude valley head, located up- valley of a series of cross-valley moraines, was ice-free by 11,940 14 C yrs BP.
Abstract: Evidence for rejuvenation of cirque glaciers following wastage of continental ice remains elusive for the Presidential Range and Mount Moosilauke of New Hampshire, Mount Katahdin and the Longfellow Mountains of Maine, and the Adirondack Mountains of New York. At Ritterbush Pond in the northern Green Mountains of Vermont, radiocarbon ages from lake sediment cores suggest that a low-altitude valley head, located up- valley of a series of cross-valley moraines, was ice-free by 11,940 14 C yrs BP (Bierman et al. , 1997). Although some workers argue that these moraines in Vermont are evidence for cirque glaciation, the moraines could have been formed by a tongue of continental ice during deglaciation. At Johnson Hollow Brook valley in the Catskill Mountains of New York, a radiocarbon age from basal sediments in a pond dammed by a moraine suggests that glacier ice may have persisted until 10,860 14 C yrs BP (Lederer and Rodbell, 1998). Because this moraine appears to have been deposited by a cirque glacier, the radiocarbon age provides the best evidence in the northeastern United States for cirque glaciation post-dating recession of continental ice. Cirque morphometric data, compiled from newly available topographic maps, add to the conundrum that these two sites in the Green and Catskill Mountains should not be nearly as favorable for maintaining local glaciers post- dating icesheet recession as higher-altitude and better-developed cirques in the Presidential Range and Mount Katahdin, where evidence for post-icesheet cirque glaciers is lacking.
TL;DR: Using a systematic and diachronic method, measurements of slopes and granulometry were conducted on twenty snow avalanche deposits in the Massif des Ecrins (southern French Alps). The main results show some specific morphosedimentary features: 1) an apical inclination lower than 34°, 2) a concavity index "C" higher than 2,7, 3) a break point φ with relatively constant coordinates (X = 90 % of the profile ; Y = 14°± 2°), 4) two types of granulo- metric sorting along the longitudinal
Abstract: Using a systematic and diachronic method, measurements of slopes and granulometry were conducted on twenty snow avalanche deposits in the Massif des Ecrins (southern French Alps). The main results show some specific morphosedimentary features: 1) an apical inclination lower than 34°, 2) a concavity index "C" higher than 2,7, 3) a break point φ with relatively constant coordinates (X = 90 % of the profile ; Y = 14°± 2°), 4) two types of granulo- metric sorting along the longitudinal profile (slight increase until the breaking point then high increase afterwards), and 5) isotropical fabrics. The analysis of the spatial and diachronic evolutions of avalanches frequency enabled us to better explain these morphometrical and sedimentological features. The largest avalanches reaching the base of slopes have decreased brutaly. The pronounced and sudden variation in the run-out frequency could be linked essentially to the brutal decrease in the amount of removed snow by the avalanches, which could be climatic in origin. According to lichenometrical observations, this dynamic variation of the avalanches could have occured at the end of the Little Ice Age.
TL;DR: In this paper, the torus surface of bordered pits along the longitudinal tracheids of the tracheid rays was used to distinguish woody macro-remains of Picea mariana, picea glauca and Larix laricina.
Abstract: Eighty-four specimens of wood collected in northeastern North America from pre-identified conifers were used to define new anatomical criteria to distinguish woody macroremains of Picea mariana, Picea glauca and Larix laricina. In addition to characters already described in the literature, the torus surface of bordered pits along the longitudinal tracheids were demonstrated to be a very good distinction criterion. The distinction between Picea glauca and P. mariana is based on the seriation of bordered pits on the tracheids of the rays. As a method of species distinction, we suggest a dichotomous classification based on the dominance of uniseriate or biseriate pits. In the wood collection studied, 69 % of radial tracheids were predominantly biseriated in Picea mariana, compared with only 28 % in Picea glauca. These simple distinction methods are only applicable in cases where no other Picea species could cause confusion in batches of woody and charred sub-fossils. They add a practical tool for further paleoecological reconstructions in northeastern North America.