Preboreal Period Research Articles

Deglaciation and Human Colonization of Northern Europe

Few places worldwide experienced Late Glacial ecological shifts as drastic as those seen in the areas covered by, or adjacent to, the massive ice sheets that blanketed much of the northern hemisphere. Among the most heavily glaciated regions, northern Europe underwent substantial ecological shifts during and after the Last Glacial Maximum. The climatically unstable Pleistocene–Holocene transition repeatedly transformed far-northern Europe, placing it among the last regions to be colonized by Paleolithic societies. As such, it shares paleoenvironmental and archaeological analogues with other once glaciated areas where human populations, entrenched in periglacial environments prior to glacier retreat, spread into newly deglaciated territories. Perhaps most significant for northern Europeans were post-glacial effects of the Younger Dryas and Preboreal periods, as shifts in climate, plant, and animal communities elicited several adaptive responses including innovation, exploration, and the eventual settlement of once glaciated landscapes. This paper is a detailed review of existing archaeological and paleoecological evidence pertaining to the Late Upper Paleolithic of northern Europe, and offers theoretical observations on human colonization models and ecological responses to large-scale stadial and interstadial events.

Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea

As a source region of North Pacific Intermediate Water, the Okhotsk Sea plays an important role in the ventilation of the North Pacific. To understand the detailed oceanographic changes in this marginal sea since the last glaciation, we studied decadal to centennial scale proxy records from new sediment cores from the southwestern Okhotsk Sea. Glacial to Holocene Δ14C records of benthic foraminiferal shells suggested enhanced ventilation in the Okhotsk Sea during the early deglacial period between 18 and 15ka, corresponding to Heinrich Event 1. Although the Δ14C reconstruction has considerable uncertainties, the Okhotsk Sea may have acted as a source for vigorous ventilation of the subarctic Pacific during this period. CaCO3 preservation events appear to be better explained by the ventilation history of the Okhotsk Sea than by coccolithophores and foraminifera production. CaCO3 preservation started to improve during 18 to 15ka, and pronounced peaks in the CaCO3 content corresponded to the Bølling–Allerød (15 to 13ka) and Preboreal (11.5 to 10ka) warm periods. Diatom and coccolithophore productivity remained low in the Okhotsk Sea throughout the glacial to deglacial periods, different from the situation in the open subarctic Pacific, where high productivity was observed during the Bølling–Allerød period. After the Preboreal period, biogenic opal gradually increased and δ15N decreased in the southern Okhotsk Sea, suggesting that productivity was enhanced by a relaxation of the nitrate limitation.

Late Glacial–early Holocene environmental changes in Charzykowskie Lake (northern Poland) based on oxygen and carbon isotopes and Cladocera data

Based on isotopic and cladoceran investigations of a 6-m-long sediment core, a reconstruction of paleoenvironmental dynamics in Charzykowskie Lake (northern Poland) is proposed. The sediments consist of sandy silts at the bottom, followed (in upward succession) by gyttja characterized by increasing CaCO3 content. The measured δ18O values oscillate from −9.3 to −5‰ and δ13C from – 5.7 to +0.4‰. We identify 24 taxa of subfossil Cladocera and six phases (CAZ) of faunal development. Isotopic and cladoceran data together with detrended correspondence analyses (DCA) allow reconstruction of the environmental conditions, particularly changes in water level, temperature, and trophic status, during the Late Glacial and early Holocene. The lowest water temperature, connected with the inflow of cold melt water, occurred in the Late Glacial and Preboreal period. Since the Boreal period, gradual warming is observed and expressed through a positive trend in both δ18O and δ13C values and changes in Cladocera species assemblages. The lake was deepest at the beginning of the Holocene as a result of dead ice melting (supply of melt water; lowering of lake bottom) and/or precipitation increase (change in atmospheric circulation). During the Boreal period, the fall in water level takes place despite the opposite trend observed at nearby sites. In addition, the cladoceran data indicate an initial decrease in trophic conditions from α-mesotrophic to oligotrophic and then an increase to the β-mesotrophic state.

Isotope record of environmental changes at the Skaliska Basin during the Late Glacial and Holocene

ABSTRACT The results of isotopic investigations of the Skaliska Basin sediments are presented. Stable isotope analyses were done for authigenic carbonates from three profiles: W1 - Piotrowo-Ławniki, W2 - Sąkieły Małe, and W4 - Budzewo. The profiles contain carbonate silts at the bottom, then calcareous detritus gyttja and organic silts and peat at the top. Palynological data indicate that sediment were accumulated in the Skaliska Basin from the final phase of the Younger Dryas to the Subboreal period. The values δ18O change from ca −9.4 to −4.5‰, and δ13C values varies from −5.1 to +0.1‰. Such a large range of isotopic data reflects changing conditions in the basin during accumulation of deposits (different water levels, water temperatures, and bioproduction). Based on the results of stable isotope analyses of the carbonates, isotopic zones (Is) were defined and characterized for each profile. The results of isotopic analysis enabled reconstruction of varying environmental conditions connected with lake deepening and/or influx of water enriched in light isotopes, climatic warming, and increase of biological activity. The Late Glacial deposits are characterized by δ18O values of ca −6.5-6‰ and δ13C of ca −1‰. In the transitional time between the Late Glacial and the Holocene, δ18O systematically falls below −8‰, reflecting a substantial change in the isotopic composition of the lake water, likely caused by to inflow of melt water. During the Preboreal period the δ18O as well as the δ13C values systematically decrease and reach minima (−8.4-9.4‰ for δ18O and ca −3.5-4.7‰ for δ13C). At the beginning of the Boreal period, an increase of ca 2‰ in δ18O is noted and is associated with climatic warming. During the Atlantic period the varying trends in the δ18O record are likely connected with changing precipitation/evaporation ratios, causing changes in the isotope composition of the water. The fluctuations of the isotopic values in the upper parts of the successions probably point to shallowing of the lake due to sedimentary infill.

Tracing the Nd isotope evolution of North Pacific Intermediate and Deep Waters through the last deglaciation from South China Sea sediments

The intermediate and deep waters of the Pacific Ocean play a crucial role for regulating global climate changes on the millennium timescale. However, due to poor preservation of carbonate sediments in the deep Pacific, little attention has been received to better understand deep ocean circulation and its relationship with abrupt climate events. Here we present the first authigenic Nd isotope (εNd) record extracted from Fe–Mn oxyhydroxides of the South China Sea (SCS) sediments (SO17940-2, 1727m water depth) in order to investigate changes of intermediate and deep-water circulations in the Pacific Ocean since the Last Glacial Maximum (LGM).In general, the seawater εNd record extracted from the SCS sediments paralleled the existing Nd isotopes of fish teeth/debris from the eastern North Pacific core during the last deglaciation, demonstrating that our authigenic εNd can faithfully record the seawater εNd evolution in the deep Pacific Ocean. At the onset of the deglacial period, our data exhibit a clear negative shift in εNd towards a gradually increased component of Antarctica Intermediate Water (AAIW) and reached a maximal influx during the mid Heinrich Stadial 1 (HS1, ∼16kyr BP). Between the HS1 and Younger Dryas, deep-water circulation gradually shifted back to the modern condition during the Bølling-Allerød warm interval. A pronounced negative εNd excursion had occurred during the Pre-Boreal (PB), suggesting a rapid reorganization of deep-water ventilation in the Pacific Ocean and was predominated by AAIW. This is most likely due to poor ventilation in the western North Pacific associated with an intensified summer monsoon during the PB-early Holocene warm period. After the PB period, the seawater εNd values were once again dominated by Pacific Deep Water at the study site.

Using the deuterium isotope composition of permafrost meltwater to constrain thermokarst lake contributions to atmospheric CH4 during the last deglaciation

Thermokarst lakes are thought to have been an important source of methane (CH4) during the last deglaciation when atmospheric CH4 concentrations increased rapidly. Here we demonstrate that meltwater from permafrost ice serves as an H source to CH4production in thermokarst lakes, allowing for region‐specific reconstructions ofδDCH4 emissions from Siberian and North American lakes. δDCH4 reflects regionally varying δD values of precipitation incorporated into ground ice at the time of its formation. Late Pleistocene‐aged permafrost ground ice was the dominant H source to CH4production in primary thermokarst lakes, whereas Holocene‐aged permafrost ground ice contributed H to CH4 production in later generation lakes. We found that Alaskan thermokarst lake δDCH4 was higher (−334 ± 17‰) than Siberian lake δDCH4 (−381 ± 18‰). Weighted mean δDCH4values for Beringian lakes ranged from −385‰ to −382‰ over the deglacial period. Bottom‐up estimates suggest that Beringian thermokarst lakes contributed 15 ± 4 Tg CH4 yr−1 to the atmosphere during the Younger Dryas and 25 ± 5 Tg CH4 yr−1during the Preboreal period. These estimates are supported by independent, top‐down isotope mass balance calculations based on ice coreδDCH4 and δ13CCH4 records. Both approaches suggest that thermokarst lakes and boreal wetlands together were important sources of deglacial CH4.

Deglaciation pattern during the Lateglacial/Holocene transition in the southern French Alps. Chronological data and geographical reconstruction from the Clarée Valley (upper Durance catchment, southeastern France)

In this paper, we make an inventory of geomorphic remnants of past glaciations, especially glacial stages that occurred just after the Last Glacial Maximum (LGM). More than 35 terrestrial cosmogenic radionuclide (TCR) ages were obtained, providing new chronological benchmarks for defining the transition between the LGM and the Holocene, thus answering some remaining questions in the southern French Alps (upper Durance catchment). TCR ages fit very well with the scenario defined in other parts of the European Alps: two main generations of post-LGM moraines were identified, highlighting a synchronicity of glacial responses during the Younger Dryas and the Preboreal. Indeed, the first generation we identified corresponds to the Younger Dryas period, while the second generation was shaped during the Preboreal period. They suggest that a relatively thick glacier tongue (300 m thick) occupied the upper catchment until the end of the Lateglacial period. The glacier tongue disappeared at the very beginning of the Holocene, before a short glacial re-advance during the Preboreal period. This scenario is linked to a very low equilibrium line altitude (ELA) during the Younger Dryas and the Preboreal: located respectively 340 and 150 m below the Little Ice Age (LIA) ELA. Collectively, the results highlight a significant west–east gradient in ELA, probably in relation to westerly and northwesterly winds. Finally, the deglaciation pattern in this area is similar to that of the western and northern Alps, and appears significantly different from the patterns found in the inner and eastern Alps (such as the Ubaye and Maritime Alps patterns).

Atlantic surface water inflow to the Nordic seas during the Pleistocene–Holocene transition (mid–late Younger Dryas and Pre‐Boreal periods, 12 450–10 000 a BP)

AbstractThe inflow of Atlantic Water to the Nordic seas from mid–late Younger Dryas to earliest Holocene (12 450–10 000 a BP) is reconstructed on the basis of a high‐resolution core (LINK14) from 346 m water depth on the east Faroe shelf. We have analysed the distribution of planktic and benthic foraminifera, stable isotopes and ice‐rafted debris (IRD), and calculated absolute temperatures and salinities by transfer functions. During the investigated time period there was almost continuous inflow of Atlantic Water to the Nordic seas. Deposition of IRD during the mid–late Younger Dryas and Pre‐Boreal coolings indicates the presence of melting icebergs and that summer sea surface temperatures were low. The east–west temperature gradient across the Faroe–Shetland Channel was much steeper than today. The cold conditions around the Faroe Islands are attributed to stronger East Greenland and East Icelandic currents than at present. The near‐continuous inflow of Atlantic Water is consistent with published evidence suggesting that deep convection took place in the Nordic seas, although the convection sites probably had shifted to a more easterly position than at present. Around the time of deposition of the Saksunarvatn Tephra c. 10 350 a BP, sea surface temperatures increased to the present level. Copyright © 2011 John Wiley & Sons, Ltd.

Climate and vegetation change during the late Pleistocene and early Holocene in Sakhalin and Hokkaido, northeast Asia

Pollen data from marine isotope stage (MIS) 3 to the early Holocene in Kenbuchi, central Hokkaido, and Khoe, central-west Sakhalin, were reviewed and the climate and vegetation compared. Changes in climate and vegetation during the examined time period in the two regions resembled each other, except for latitudinal differences in forest composition. During MIS 3, cold evergreen needle-leaf forests expanded under a cool, moist climate. The climate then rapidly changed to cool and dry around the MIS 3/MIS 2 boundary. MIS 2 was characterized by millennial-scale variability in climate. Heinrich event 2 (H2, 24–21 ka), the last glacial maximum (LGM), and H1 (between 17 and 13 ka) were characterized by the dominance of Larix and Pinus in cold deciduous forests. In Kenbuchi, grassland composed of alpine elements developed. In both regions, climate was coldest and driest during H1. The Bölling-Allerød interstadial (14–12 ka) was characterized by a maritime climate with an abundance of Betula in the coastal region. The Younger Dryas (11.5 ka) was a minor event in East Asia. During the Pre-Boreal period (around 12 ka), cold evergreen needle-leaf ( Picea– Betula) forest and mixed ( Quercus– Abies) forest developed in two different regions under a warm, moist climate.

A preboreal Elk (Alces alces L., 1758) antler from south-eastern Norway

In 1895 a shed elk antler was found in a mire on a farm near Fluberg, in Søndre Land municipality in south-eastern Norway. The antler was first radiocarbon dated in 2008 and yielded the age 9,100 ± 50 BP (8,340 – 8,250 BC), which is the oldest dated elk remain from Norway. Elk (Alces alces L., 1758) are a pioneer colonising species; they were already established south of the ice front in Denmark and southern Sweden in the Late Glacial period. This antler shows that the species had arrived in south-eastern Norway in the late Preboreal period. This could tie in with the earliest arrival of elk once the colonizing routes from southern Sweden were established 9,300-9,200 BP. The antler is clearly of the palmate morph, and strongly resembles elk antlers found in Denmark and southern Sweden from the Late Glacial and Early Holocene periods. This find also reveals that the vegetation at the end of the Preboreal period suited large herbivores such as elk.

Late Pleistocene–early Holocene transition recorded in the sediments of a former shallow lake in the Czech Republic

Quaternary limnic sediments from Velanska Cesta, a former lake in the Czech Republic, were investigated to document the response of an aquatic environment to global climatic shifts during the late Pleistocene–early Holocene transition. The obtained multi-proxy record, including subfossil diatoms, diatom-inferred variables (index of trophic state, halobic index, TP, and pH), plant macrofossils, and geochemical data, was compared with the δ18O GISP2 curve. Diatom accumulation zones and subzones established on the basis of a ConSLink cluster analysis corresponded well with the boundaries of the Bolling, Older Dryas, Allerod, Younger Dryas, and Preboreal periods. The diatom-inferred trophic state and salinity data showed increased values during the colder epochs, which were supported by the findings of low-temperature macrofossil indicators in the corresponding samples. The results of this study document a long-term forcing of a shallow lacustrine ecosystem in central Europe by climatic events of a global scale.

Makroszczątki roślinne w torfie w analizie paleośrodowiskowych uwarunkowań osadnictwa na przykładzie stanowiska Łane Małe w dolinie Kłodnicy

Results of archaeological studies from the territory of Poland show that prehistorical settlements were often founded on the banks of lakes and mires. Botanical composition of organic deposits allows reconstructing historical plant communities and palaeohydrological conditions of the settlement, the subject of research was a peatland situated in the neighborhood of the archaeological site at Łany Małe. The latter is located on hillslopes of the Kłodnica valley (Upper Silesia, southern Poland). Peat formation at the Łany Małe site took place in the Pre-Boreal Period as an effect of the valley floor paludification. In the Mesolithic and Neolithic Periods as well as the Early Bronze Age, swampy alder forests occupied the entire area of the peatland. At that time, the fen surface was relatively easily accessible for humans, especially for people of the Funnel Beaker Culture in the Middle Neolithic Period. It was then when, probably, the ground water level dropped down and peat decay processes took over those of peat production. In the Hallstatt Period, during the presence of the Lusatian Culture settlement, there was subsequent increase in biotope moisture. Forest communities became more open and trees were gradually displaced by grasslike plants, especially of sedge (Carex) genus. In the Fate Roman Period (the Przeworsk Culture settlement) and in the early Middle Ages, the peatland was subject to inundation due to frequent flooding, the peat accumulation was then halted by the delivery of fine-grained products of soil erosion.

Evolution of vegetation landscapes since the Late Mesolithic on the French West Atlantic coast

This article presents a review of the data collected from the palynological analysis of six sites situated on the southern edge of the Armorican Massif (coastal Vendée, Western France). A multi and interdisciplinary approach, involving branches as diverse as archaeology, history, geography, geomorphology, sedimentology, climatology and the natural sciences, enabled us to gain an understanding of the evolution of vegetation and coastal ecosystems from the end of the Preboreal period to the present, and to see whether this evolution was due to the combined influence of climate and geomorphology of the environment or due to the influence of natural factors, human-induced processes or interaction between these components. Since the Preboreal, the coastal area has undergone significant geomorphological changes through a succession of marine transgressions and regressions. Coastal valleys filled up rapidly to the Atlantic and the progressive development of dune ridges led to the clogging of lagoons, resulting in the present day tidal marshes. On the first hillsides of the coastal area, and with improvement in climate conditions, a woodland environment rapidly appeared which had distinctive features related to the proximity of the ocean. These data reinforce the research already carried out on the subject and help in an understanding of the evolution of coastal and continental ecosystems. From the end of the Mesolithic period this woodland ecosystem saw clearance activities which were sometimes followed by the cultivation of such plants as cereals or walnut. The discovery of this type of occurrence is not an isolated case in continental Europe. However, the date of these occurrences raises numerous questions about the earliest stages of neolithization, about the diffusion of agricultural knowledge and the factors leading to this diffusion.

Isotopic records of tropical vegetation and climate change from terrestrial vascular plant biomarkers preserved in Cariaco Basin sediments

Records of vascular plant leaf wax δ 13C and δD were generated from terrestrial biomarkers preserved in Cariaco Basin marine sediments, and were found to parallel known regional millennial-scale climate changes between the late Glacial and Preboreal periods. The terrestrial biomarkers used consisted of C 24–C 32 n-alkanoic acids, which originate from vascular plant leaf waxes. Differences in δD between C 16–C 18 and C 24–C 30 n-alkanoic acid homologues suggest a marine source for the shorter chain lengths and a terrestrial source for the longer chains. Both the δ 13C and δD leaf wax records exhibited enrichment during the late Glacial and Younger Dryas and relative depletion during the Bølling/Allerød and Preboreal periods. δD likely records the hydrogen isotopic composition of plant growth water, and thus may act as a proxy for local aridity. In agreement with Cariaco Basin sediment grey scale records, the δD record suggests that the late Glacial and Younger Dryas were more arid than the Bølling/Allerød and Preboreal periods. n-Alkanoic acid δ 13C, which is a proxy for C 3 versus C 4 plant type, indicates that C 3 plants predominated in this area of the tropics during warm and wet periods, such as the Bølling/Allerød and the early Holocene, and C 4 plants proliferated during cooler and more arid periods, such as the Glacial and Younger Dryas. The biomarker δ 13C record agrees with pollen data from Cariaco Basin sediments, confirming that leaf wax compounds preserved in marine sediments can accurately record terrestrial vegetation changes.

Vegetation history and palaeoclimate of the past 30 kyr in Pakistan as inferred from the palynology of continental margin sediments off the Indus Delta

Palynological investigations on a continuous uppermost Pleistocene to Holocene high-resolution gravity core collected from the continental margin off the Indus Delta provide the first, uninterrupted, 30-kyr-long record of vegetation history and palaeoclimate of the Pakistan hinterland. Core analysis and interpretation allows the recognition of (1) greater glacial aridity than during the following postglacial period up to the present day; (2) a significant southward extension of the Himalayan sub-alpine and deciduous hardwood tree zone during the time interval from the Last Glacial Maximum (LGM) to the Younger Dryas cold interval; (3) extreme aridity and climatic deterioration during the LGM and the equivalent to Heinrich event 1; (4) a marked change to early Holocene humid conditions, starting already during the Bölling/Alleröd and increasing abruptly during the Preboreal period; and (5) a gradual reduction of humidity from the middle Holocene to the present. These results are in general agreement with palaeoclimatic reconstructions based on multiple proxies from the Indian subcontinent as well as the Arabian Sea, and especially confirm the occurrence of a clear precipitation maximum in Pakistan between ca. 9 and 7 cal. kyr BP which probably resulted from a combination of increased and prolonged summer (SW) monsoonal strength; both processes ultimately being controlled by orbital changes in northern hemisphere summer insolation and shifts in the position of the Intertropical Convergence Zone.

Holocene subfossil chironomid stratigraphy (Diptera: Chironomidae) in the sediment of Plešné Lake (the Bohemian Forest, Czech Republic): Palaeoenvironmental implications

A faunal record of chironomid remains was analyzed in the upper 280 cm of a 543 cm long sediment core from Plesne jezero (Plesne Lake), the Bohemian Forest (Sumava, Bohmerwald), Czech Republic. The chronology of the sediment was established by means of 5 AMS-dated plant macroremains. The resolution of individual 3-cm sediment layers is ∼115 years and the analyzed upper 280 cm of the sediment core represent 10.4 cal. ka BP. As the results of DCA show, two marked changes were recorded in the otherwise relatively stable Holocene chironomid composition: (1) at the beginning of the Holocene (ca. 10.4-10.1 cal. ka BP) only oligotrophic and cold-adapted taxa (Diamesa sp., M. insignilobus-type, H. grimshawi-type) were present in the chironomid assemblages, clearly reflecting a cool climate oscillation during the Preboreal period, and (2) during an event dated in the interval 1540–1771 AD, when most taxa vanished entirely and only Zavrelimyia sp. and Procladius sp. were alternately present accompanied by Tanytarsus sp. Although, the age of this event is in agreement with the dating of the Little Ice Age, the most probable reason for the elimination of many chironomid taxa was very low sums recorded in this part of the sediment, rather than cool conditions connected with the LIA. Variations in the chironomid fauna after the Preboreal period were reflected mainly by changes in abundances of dominant taxa rather than by changes in species composition. These variations could be explained by: (1) climatic changes, namely temperature and amount of rainfall resulting in oscillations in lake level, with changes in the occurrence of macrophytes in the littoral and (2) increasingly dense afforestation which led to a considerable input of organic material into the lake and a subsequent increase in the trophic status of the lake water.

Climate change in lowland Central America during the late deglacial and early Holocene

AbstractThe transition from arid glacial to moist early Holocene conditions represented a profound change in northern lowland Neotropical climate. Here we report a detailed record of changes in moisture availability during the latter part of this transition (∼11 250 to 7500 cal. yr BP) inferred from sediment cores retrieved in Lake Petén Itzá, northern Guatemala. Pollen assemblages demonstrate that a mesic forest had been largely established by ∼11 250 cal. yr BP, but sediment properties indicate that lake level was more than 35 m below modern stage. From 11 250 to 10 350 cal. yr BP, during the Preboreal period, lithologic changes in sediments from deep‐water cores (>50 m below modern water level) indicate several wet–dry cycles that suggest distinct changes in effective moisture. Four dry events (designated PBE1‐4) occurred centred at 11 200, 10 900, 10 700 and 10 400 cal. yr BP and correlate with similar variability observed in the Cariaco Basin titanium record and glacial meltwater pulses into the Gulf of Mexico. After 10 350 cal. yr BP, multiple sediment proxies suggest a shift to a more persistently moist early Holocene climate. Comparison of results from Lake Petén Itzá with other records from the circum‐Caribbean demonstrates a coherent climate response during the entire span of our record. Furthermore, lowland Neotropical climate during the late deglacial and early Holocene period appears to be tightly linked to climate change in the high‐latitude North Atlantic. We speculate that the observed changes in lowland Neotropical precipitation were related to the intensity of the annual cycle and associated displacements in the mean latitudinal position of the Intertropical Convergence Zone and Azores–Bermuda high‐pressure system. This mechanism operated on millennial‐to‐submillennial timescales and may have responded to changes in solar radiation, glacial meltwater, North Atlantic sea ice, and the Atlantic meridional overturning circulation (MOC). Copyright © 2005 John Wiley & Sons, Ltd.

Linking continental-slope failures and climate change: Testing the clathrate gun hypothesis

It has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice-core atmospheric methane record during the Pleistocene. Because submarine sediment failures can involve as much as 5000 Gt of sediment and have the capacity to release vast quantities of methane hydrates, one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental-slope failure and atmospheric methane correlate. To test the clathrate gun hypothesis, we have collated published dates for submarine sediment failures in the North Atlantic sector and correlated them with climatic change for the past 45 k.y. More than 70% by volume of continental-slope failures during the past 45 k.y. was displaced in two periods, between 15 and 13 ka and between 11 and 8 ka. Both these intervals correlate with rising sea level and peaks in the methane record during the Bolling-Allerod and Preboreal periods. These data support the clathrate gun hypothesis for glacial-interglacial transitions. The data do not, however, support the clathrate gun hypothesis for glacial millennial-scale climate cycles, because the occurrence of sediment failures correlates with Heinrich events, i.e., lows in sea level and atmospheric methane. A secondary use of this data set is the insight into the possible cause of continental-slope failures. Glacial-period slope failures occur mainly in the low latitudes and are associated with lowering sea level. This finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause. The Bolling-Allerod sediment failures were predominantly low latitude, suggesting an early tropical response to deglaciation, e.g., enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters. In contrast, sediment failures during the Preboreal period and the majority of the Holocene occurred in the high latitudes, suggesting either isostatic rebound–related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound, causing destabilization of gas hydrates.

Environmental changes on Yugorski Peninsula, Kara Sea, Russia, during the last 12,800 radiocarbon years

New pollen and radiocarbon data from an 8.6-m coastal section, Cape Shpindler (69°43′N; 62°48′E), Yugorski Peninsula, document the latest Pleistocene and Holocene environmental history of this low Arctic region. Twelve AMS 14C dates indicate that the deposits accumulated since about 13,000 until 2000 radiocarbon years BP. A thermokarst lake formed ca. 13,000–12,800 years BP, when scarce arctic tundra vegetation dominated the area. By 12,500 years BP, a shallow lake existed at the site, and Arctic tundra with Poaceae, Cyperaceae, Salix, Saxifraga, and Artemisia dominated nearby vegetation. Climate was colder than today. Betula nana became dominant during the Early Preboreal period about 9500 years BP, responding to a warm event, which was one of the warmest during the Holocene. Decline in B. nana and Salix after 9500 years BP reflects a brief event of Preboreal cooling. A subsequent increase in Betula and Alnus fruticosa pollen percentages reflects amelioration of environmental conditions at the end of Preboreal period (ca. 9300 years BP). A decline in arboreal taxa later, with a dramatic increase in herb taxa, reflects a short cold event at about 9200 years BP. The pollen data reflect a northward movement of tree birch, peaking at the middle Boreal period, around 8500 years BP. Open Betula forest existed on the Kara Sea coast of the Yugorski Peninsula during the Atlantic period (8000–4500 years BP), indicating that climate was significantly warmer than today. Deteriorating climate around the Atlantic–Subboreal boundary (ca. 4500 years BP) is recorded by a decline in Betula percentages. Sedimentation slowed at the site, and processes of denudation and/or soil formation started at the beginning of the Subatlantic period, when vegetation cover on Yugorski Peninsula shifted to near-modern assemblages.

La station mésolithique des Vergères à Boulogne (Vendée)

The Vergères site, located on a table-land widely overhanging la Boulogne has revealed a profuse Mesolithic industry from the three rocks traditionnally used in the Bocage in Vendée : quartzite, phtanite and flint. Phtanite is the only local rock but it is not widespread over the site ; studying and interpreting what it was intended for could enable to define more accurately the economy of the people who went hunting and picking. The hollovv-based point seems to be the prevailing framework in the analysis of the different groups from the Early Mesolithic period of the Loire river. We are trying to state its local type in the hereby article. Nearly the whole of the gathered material chronologically seems to belong to the second third of the Preboreal period.