14C Kyr Research Articles

Improving the reliability of bulk sediment radiocarbon dating

Radiocarbon dated chronologies are the most extensively applied dating technique when investigating the last ∼45,000 years. In lake and marine sediments, a single macrofossil or several microfossils are the preferred sample material because they have a 14C age that accurately reflects the time of deposition. However, absence of macro- or microfossils are a recurring challenge. Often, it is necessary to use bulk sediment samples for radiocarbon dating, but they frequently yield ages clearly exceeding the depositional timeframe due to occurrence of organic material of older origin. Until now, it has not been possible to assess if the dating result of a bulk sediment sample is reliable, or to adequately explain the mechanisms behind age disagreement between bulk and macro- and microfossils. In this study, we investigate the age offset between paired bulk and macrofossil ages in sediment cores from three lakes in SE and E Greenland. Furthermore, we distinguish three quantifiable main carbon sources in the samples: a) recent terrestrial organic fragments, b) recent aquatic organic matter, and c) reworked land-derived old carbon, through pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization. Our results show that the offset between bulk (humic fraction) and macrofossil radiocarbon ages in three similar lakes range from 9 14C yr to 20.1 14C kyr. We observe a reduced age offset between bulk- and macrofossil ages, and thereby a higher reliability of bulk (humic fraction) ages, correlated with increased values of TOC, C/N ratio, and high CO2 release in pyrolysis (S3 value). We recommend that future studies presenting bulk sediment chronologies apply pyrolysis organic geochemistry, organic petrographic microscopy, and isotopic fractionization as demonstrated here, to select reliable sediment intervals for bulk dating and retrieve more robust results.

Aragonite Fraction Dating of Vermetids in the Context of Paleo Sea-Level Curves Reconstruction

ABSTRACTIdentifying and tackling recrystallization is a critical factor in the reliable radiocarbon (14C) dating of carbonates, since exogenous carbon can be incorporated and thus mask the real age of the samples. Vermetids are among the most important bioindicators used for paleo sea-level reconstruction, and the accuracy of their chronology can significantly impact sea-level curves. Age differences larger than 1 14C kyr before and after acid etching, combined with X-ray diffraction (XRD) analysis that indicates a significant amount of calcite still remains in the shell, led us to apply the previously developed carbonate density separation protocol (CarDS). Using a solution of sodium polytungstate, with density of 2.80 g/cm3, we successfully separated different carbonate fractions for a set of 10 vermetid samples from the coast of Rio de Janeiro, southeast of Brazil. Each separation was verified by XRD analysis and the 14C concentrations of different fractions were compared. The results show that the calcite fraction in the studied vermetid samples varied from 12 to 63% and aragonite fraction ages are up to 2 14C kyr older than the raw samples, thus confirming the efficacy of CarDS in removing young carbonates and the importance of density separation to vermetids prior to accelerator mass spectrometry (AMS) dating.

Geochemistry of Carbonates in Small Lakes of Southern West Siberia Exampled from the Holocene Sediments of Lake Itkul’

Abstract —A 1.8 m thick core of the Holocene (7.9 14C kyr BP) sediments of Lake Itkul’ (Novosibirsk Region) has been studied. Based on the geochemical and lithostratigraphic properties of the bottom sediments, we have established the following stages of the lake evolution: (1) the beginning of sedimentation, 7.8–7.0 14С kyr BP; (2) extreme shallowing with a probable complete drying, ~7.0–5.5 14С ky BP; (3) rise in the water level, ~5.5–4.3 14С kyr BP; (4) repeated shallowing, 4.3–2.8 14С kyr BP; and (5) subsequent watering, 2.8–0 14С kyr BP. At present, the lake again tends to shallowing. We have established that Lake Itkul’ has mineral sediments with a high content of carbonates (up to 64%). The high concentrations of HCO3– and Ca2+ and stable saturation (S/St > 1) of the surface water cause a shift of the carbonate– calcium equilibrium toward the carbonate formation. The studied authigenic carbonates are aragonite and fine-grained aggregates of poorly crystallized calcite particles with different Mg contents. Aragonite is both biogenic (mollusk and ostracode shells) and chemogenic (formed during the lake shallowing). In addition to carbonates, the lacustrine sediments contain mixed-layer aluminosilicates, feldspars, and quartz. The presence of pyrite throughout the sediment section indicates reducing conditions and the activity of sulfate-reducing microorganisms. Carbonates (especially aragonite) are significantly enriched in strontium. Manganese does not form its own minerals but is present as an isomorphic impurity in authigenic carbonates.

Quantifying regional vegetation changes in China during three contrasting temperature intervals since the last glacial maximum

The need to reduce the uncertainty of predictions of vegetation change under global warming highlights the importance of understanding the vegetation patterns and possible mechanisms of vegetation response during past warming intervals. Here, we present quantitative regional vegetation reconstructions for China during the Last Glacial Maximum (LGM, 18 ± 2 14C kyr B.P.), early Holocene (EH, 8.5 ± 0.5 14C kyr B.P.), and mid-Holocene (MH, 6 ± 0.5 14C kyr B.P.), using the biomization method and based on 249 pollen records. In addition, we used an inverse vegetation modeling approach to investigate the effect of climate change and CO2 concentration on the observed vegetation changes. The results demonstrate that during the LGM, steppe expanded southeastwards, reaching the present-day temperate deciduous forest (TEDE) zone; in contrast, the forest in eastern China underwent a substantial southward retreat and its percentage reached a minimum. The warm mixed forest (WAMF) and TEDE shifted southwards of ∼10° N relative to today, and tropical seasonal rain forest (TSFO) was almost absent. In addition, the forest-steppe boundary shifted southwards to near the middle and lower reaches of the Yangtze River. During the EH and MH, the TSFO, WAMF, and TEDE exhibited respective northward shifts of 2°, 4° and 5° relative to today. In MH, the percentage of forest sites increased and reached a maximum, and the forest-steppe boundary had shifted northwestwards to near the present-day 300 mm isohyet. Our palaeovegetation reconstructions and model sensitivity experiments suggest that temperature was the dominant factor controlling the vegetation distribution during the LGM, while precipitation became increasingly more important during the Holocene throughout China. We further show that precipitation was the primary factor controlling palaeovegetation distribution in northern China, while temperature became increasingly more important in southern China. Our findings are potentially important for understanding the evolution of vegetation in China in response to ongoing global warming.

Drumlinized tunnel valleys in south-central Ontario

A network of Late Wisconsin valleys deeply incise thick successions (up to 200 m) of Quaternary sediment and Paleozoic bedrock across south-central Ontario. The valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the Laurentide Ice Sheet (LIS). Recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping, continuously-cored boreholes that penetrate the entire Quaternary sediment succession in valleys as well as intervening uplands, and both water-borne and land-based geophysical surveys. Together, these data provide new insights and a refined interpretation of the genesis, timing and paleoglaciological significance of these valleys.Valleys within the study area are some of the largest observed within the regional network – up to 30 km long, 7 km wide and >175 m deep. They are oriented both north-south and in a radial pattern, varying from NE-SW in the southern part of the study area, to ESE-WNW in the north. An undulating Late Wisconsin till sheet (Newmarket Till) caps the regional uplands and is commonly observed along the flanks and floors of the valleys – all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys. At least two generations of valleys are interpreted from cross-cutting relationships, with abrupt heads and locally perched downflow ends of some valley forms. Radiocarbon age determinations of organic material from both below and above the Newmarket Till constrain the timing of valley excavation to between 28.05 and 12.81 14C kyr BP. Borehole and outcrop data within the valleys reveal four stratigraphic units (SU1-SU4) that form the floor and sediment infill of the valleys. The Newmarket Till (SU1) is commonly underlain by deformed substrates. The till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse-grained gravels (SU2) that pass upwards into sands (SU3). The bulk of the sediment infill consists of glaciolacustrine silt-clay rhythmites (SU4) that pass upwards into organic- and mollusk-bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation.It is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during Late Wisconsin ice cover. Meltwater surpluses would be generated at the ice-bed interface due to the low-transmissivity substrate in the study area with flow preferentially routed into lows on the bed. Downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping. Waning meltwater flow velocities/discharges allowed ice creep to re-occupy newly-formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys. The valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice-proximal subaquatic fans in the Oak Ridges Moraine and within proglacial lakes that developed during ice retreat. However, there is no evidence for regional-scale catastrophic release of subglacial meltwater in the valley infills. This integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in North America.

New Dates for the Solutrean and Magdalenian of Cantabrian Spain: El Miron and La Riera Caves

ABSTRACTThis sixth date list for the prehistoric site of El Mirón Cave (Cantabria, Spain) reports on new age determinations for the earliest and last Solutrean occupations (20.4 and 18.0 14C kyr BP) and for a Lower/Initial Magdalenian level with a possible rock wall (16.75 14C kyr BP). The site has now been dated by 92 radiocarbon (14C) assays. In addition, to help resolve inconsistencies in the 14C chronology of La Riera Cave (Asturias)—the first Paleolithic site in Spain to be extensively 14C-dated back in the 1970s—two AMS assays were done on bones from the Lower and Upper Magdalenian collections (15.1 and 13.5 14C kyr BP).

High-precision 14C and 40Ar/39Ar dating of the Campanian Ignimbrite (Y-5) reconciles the time-scales of climatic-cultural processes at 40 ka

The Late Pleistocene Campanian Ignimbrite (CI) super-eruption (Southern Italy) is the largest known volcanic event in the Mediterranean area. The CI tephra is widely dispersed through western Eurasia and occurs in close stratigraphic association with significant palaeoclimatic and Palaeolithic cultural events. Here we present new high-precision 14C (34.29 ± 0.09 14C kyr BP, 1σ) and 40Ar/39Ar (39.85 ± 0.14 ka, 95% confidence level) dating results for the age of the CI eruption, which substantially improve upon or augment previous age determinations and permit fuller exploitation of the chronological potential of the CI tephra marker. These results provide a robust pair of 14C and 40Ar/39Ar ages for refining both the radiocarbon calibration curve and the Late Pleistocene time-scale at ca. 40 ka. In addition, these new age constraints provide compelling chronological evidence for the significance of the combined influence of the CI eruption and Heinrich Event 4 on European climate and potentially evolutionary processes of the Early Upper Palaeolithic.

LATE GLACIATION AND HOLOCENE DEVELOPMENT OF THE EROSION- ACCUMULATION PROCESSES IN THE SMALL BASINS OF THE WESTERN TRANSBAIKALIA

Detailed investigations of soil-sediment sequences of three sections (Nizhnyaya Bulanka, Nadeino and Kuytun) in the Kuytunka river basin (Western Transbaikalia) was carried out. Three stages of activation of erosion- accumulation processes in small drainage basins of Western Transbaikalia are determined: Pre Holocene (13.0– 10.7), Early- (10.3–8.0) and Late Holocene (last 2.5 14C kyr). During these stages several phases of sedimentation and soil-forming process are distinguished. High intensity of aeolian processes together with washout and sedimentation during time spans of 10.7–10.0 and 9.5–9.0 kyr were determined on erosion landforms and alluvial fans. The main causes of deposition processes intensification have been revealed and quantitative data about their intensity were obtained. Intensification of soil-forming process in conditions of mountain-forest-steppe landscapes of Western Transbaikalia begins when sedimentation rates reach 0.06–0.37 mm/yr. Within the sedimentation rate interval 0.3–0.6 mm/yr humus interlayers alternate with layers of colluvial and aeolian deposits. At the rate above 0.6 mm/yr sedimentation of sandy loam and sands with inclusions of rubble are observed. Duration of soil development is 300–3500 years. Sediment accumulation phases have duration from 70 to 2000 years. Development of erosion-accumulation processes are characterized by polychronicity related to different response of small drainages basins to global, regional and local environmental changes. Rhythmical development of the washout, erosion, accumulation and soil-forming processes provides long-term activity of erosional landforms.

The last ice sheet in Western Scotland during Greenland Interstade 1: Integrating the onshore and offshore records

Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred ⩾18.3 kyr 14C BP along the northwestern Irish coast, in agreement with a deglacial age of ∼22 36Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m ∼16.7 14C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL ∼15.0 14C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr 14C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 14C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level.

Origin of the Herman–Norcross–Tintah sequence of Lake Agassiz beaches in Manitoba, Canada

The giant glacial Lake Agassiz basin is fringed by many strandlines, which have long been used to trace the paleogeography of the former lake over its 5000year history. The oldest and highest of these strandlines were placed into three groups by Warren Upham in the 1890s – the Herman, Norcross, and Tintah – and form a staircase of small landforms. The formation of these old strandlines began as early as ~13.9 cal (12.0 14C) kyr BP and ended ~12.8 cal (10.8 14C) kyr BP, based on OSL dates and the history of lake level in the Agassiz basin. New mapping and augering of beach ridges in southern Manitoba, Canada, associated with the earliest phase of the lake, indicate that there are a series of up to 28 small discontinuous beach ridges that are generally only a few metres high and a few tens of metres wide. These beaches mainly consist of weakly defined beds of poorly sorted sediments; in many cases a central sandy diamicton unit lies stratigraphically between overlying beach sediments and clay diamicton (till) below. Spatially, ridges are separated by silty or sandy units or gravel lags; inter-beach lagoonal organics were not found. We discuss the possible origin of these Lake Agassiz beaches, concluding that they were deposited over a few centuries by episodic storm events, as lake level slowly declined. We base this conclusion on the nature of the sediments in the beach ridges and the regional geomorphology, as well theoretical considerations about sedimentation along a regressing shoreline of a large lake. Other origins are rejected, although some other factors may have contributed to formation of some of the beaches, such as temporary increases in sediment supply, variable rate of outlet erosion, and short increases in lake-level that reworked sediment upslope into ridges. Using the time frame of 13.9 to 12.8 cal kyr BP for the formation of the beaches, the average interval between formation of each beach is ~39 years.

Exploring preserved fossil dinoflagellate and haptophyte DNA signatures to infer ecological and environmental changes during deposition of sapropel S1 in the eastern Mediterranean

In this study we used a comparative multiproxy survey (fossil DNA, calcareous nannofossils, and lipid biomarkers) to test whether preserved genetic signatures provide an accurate view of haptophyte and dinoflagellate populations during deposition of the eastern Mediterranean sapropel S1 and the organic carbon‐depleted oxidized marls flanking the S1 and to see if we could identify important environmental indicator species that did not fossilize and escaped previous microscopic identification. The marls above and below the S1 contained low concentrations of lipid biomarkers diagnostic for dinoflagellates and haptophytes (i.e., dinosterol and long‐chain alkenones), but 500 base pair long ribosomal DNA fragments of these protists were below the detection limit. In contrast, dinoflagellate and haptophyte DNA could be recovered from the organic carbon‐rich S1, but the most abundant sequences did not represent species that were part of the nannofossil (this study) or previously described dinocyst composition. The oldest section of S1 (9.8 to ∼8 14C kyr B.P.) revealed a predominance of dinoflagellate phylotypes, which were previously only detected in anoxic Black Sea sediments. In the same section of the core, the most abundant haptophyte sequence showed highest similarity with uncultivated haptophytes that were previously shown to grow mixotrophically as predators of picocyanobacteria, an adaptation that promotes growth in oligotrophic marine waters. Sequences with highest similarities to clones found in marine surface waters predominated in the S1 after ∼8 14C kyr B.P. We discuss whether the shifts in haptophyte and dinoflagellate populations inferred from the preserved DNA reflect known environmental changes that occurred during the formation of sapropel S1.

Submarine landforms and ice-sheet flow in the Kvitøya Trough, northwestern Barents Sea

High-resolution geophysical and sediment core data are used to investigate the pattern and dynamics of former ice flow in Kvitøya Trough, northwestern Barents Sea. A new swath-bathymetric dataset identifies three types of submarine landform in the study area (streamlined landforms, meltwater channels and cavities, iceberg scours). Subglacially produced streamlined landforms provide a record of ice flow through Kvitøya Trough during the last glaciation. Flow directions are inferred from the orientations of streamlined landforms (drumlins, crag-and-tail features). Ice flowed northward for at least 135 km from an ice divide at the southern end of Kvitøya Trough. A large channel-cavity system incised into bedrock in the southern trough indicates that subglacial meltwater was present at the former ice-sheet base. Modest landform elongation ratios and a lack of mega-scale glacial lineations suggest that, although ice in Kvitøya Trough was melting at the bed and flowed faster than the likely thin and cold-based ice on adjacent banks, a major ice stream probably did not occupy the trough. Retreat was relatively rapid after 14–13.5 14C kyr B.P. and probably progressed via ice sheet-bed decoupling in response to rising sea level. There is little evidence for still stands during ice retreat or of ice-proximal deglacial sediments. Relict iceberg scours in present-day water depths of more than 350 m in the northern trough indicate that calving was an important mass loss mechanism during retreat.

14C‐dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bølling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland

Mangerud, J., Gulliksen, S. & Larsen, E. 2009: 14C‐dated fluctuations of the western flank of the Scandinavian Ice Sheet 45–25 kyr BP compared with Bølling–Younger Dryas fluctuations and Dansgaard–Oeschger events in Greenland. Boreas, 10.1111/j.1502‐3885.2009.00127.x. ISSN 0300‐9483.We present 32 accelerator mass spectrometry (AMS) 14C dates obtained on well‐preserved bones from caves in western Norway. The resulting ages of 34–28 14C kyr BP demonstrate that the coast was ice‐free during the so‐called Ålesund Interstadial. New AMS 14C dates on shells aged 41–38 14C kyr BP are evidence of an earlier (Austnes) ice‐free period. The Ålesund Interstadial correlates with Greenland interstadials 8–7 and the Austnes Interstadial with Greenland interstadials 12–11. Between and after the two interstadials, the ice margin reached onto the continental shelf west of Norway. These events can be closely correlated with the Greenland ice core stratigraphy, partly based on identification of the Laschamp and Mono Lake palaeomagnetic excursions. We found that the pattern of the NGRIP δ18O curves for the two periods Greenland Interstadial (GI) 8 to Greenland Stadial (GS) 8 and GI 1–GS 1 (Bølling–Younger Dryas) were strikingly similar, which leads us to suggest that the underlying causes of these climate shifts could have been the same. We therefore discuss some aspects of glacial fluctuations during the Bølling–Younger Dryas in order to elucidate processes during Dansgaard–Oeschger events.

MIS 3 marine and lacustrine sediments at Kriegers Flak, southwestern Baltic Sea

Anjar, J., Larsen, N. K., Björck, S., Adrielsson, L. & Filipsson, H. L. 2010: MIS 3 marine and lacustrine sediments at Kriegers Flak, southwestern Baltic Sea. Boreas, 10.1111/j.1502‐3885.2010.00139.x. ISSN 0300‐9483.Sediment cores from the Kriegers Flak area in the southwestern Baltic Sea show a distinct lithological succession, starting with a lower diamict that is overlain by a c. 10 m thick clay unit that contains peat, gyttja and other organic remains. On top follows an upper diamict that is inter‐layered with sorted sediments and overlain by an upward‐coarsening sequence with molluscs. In this paper we focus on the clay unit, which has been subdivided into three subunits: (A) lower clay with benthic foraminifera and with diamict beds in the lower part; (B) thin beds of gyttja and peat, which have been radiocarbon‐dated to 31–35 14C kyr BP (c. 36–41 cal. kyr BP); and (C) upper clay unit. Based on the preliminary results we suggest the following depositional model: fine‐grained sediments interbedded with diamict in the lower part (subunit A) were deposited in a brackish basin during a retreat of the Scandinavian Ice Sheet, probably during the Middle Weichselian. Around 40 kyr BP the area turned into a wetland with small ponds (subunit B). A transgression, possibly caused by the damming of the Baltic Basin during the Kattegat advance at 29 kyr BP, led to the deposition of massive clay (subunit C). The data presented here provide new information about the paleoenvironmental changes occurring in the Baltic Basin following the Middle Weichselian glaciation.

Geochemical record of Late Quaternary paleoclimate from lacustrine sediments of paleo-lake San Felipe, western Sonora Desert, Mexico

Abstract Stratigraphy, mineralogy, major and trace elements, organic carbon, carbonate, sulfate and AMS 14C dates are used to infer Late Quaternary depositional environments and paleo-hydrological conditions in the paleo-lake San Felipe located in the western part of Sonora Desert. Sediment stratigraphy divides the depth profile into aeolian and pluvio-lacustrine regimes. Aeolian regime is constrained to >44.5 14C kyr BP. The pluvio-lacustrine regime consists of two stratigraphic units with characteristic geochemical proxies indicating changing chemical weathering, clastic input, salinity and provenance and provides a measure of varying climatic conditions between ca. 37 and 3 14C kyr BP. Lower catchment erosion and inflow into the basin, higher lake productivity, precipitation of Na-sulfate mineral and higher clastic input from the proximal aeolian deposits during ca. 37–14 14C kyr BP are comparable to the regional registers of dominant winter rainfall related to westerly storm tracks and colder conditions. In the last 12 14C kyr BP, higher sedimentation and inflow and lower productivity are comparable to dominant summer rainfall. Higher humidity and lake productivity during ca. 37–29 14C kyr BP is possibly due to the position of westerly storm tracks at 31°N and the gradually reducing humidity till ca. 14 14C kyr BP is related to northerly migration of westerly storm tracks. Regional arid conditions during ca. 11 14C kyr BP and ca. 6 14C kyr BP are characterized by influx of coarser quartz and feldspars into the basin.

Palaeo‐ice streams and the subglacial landscape mosaic of the North American mid‐continental prairies

The analysis of the glacial landscape of southern Saskatchewan (Canada) through multiple data sets (e.g. digital elevation model, till compositional data) has revealed previously unrecognized subglacial sediment–landform assemblages. A southwest‐trending corridor of mega‐scale till lineations (Maskwa corridor) bounded on each side by hummocky terrain extends from the Canadian Shield to southwestern Saskatchewan. This landform assemblage is clearly cross‐cut by a broad south to southeast trending corridor (Buffalo corridor) consisting of subparallel curvilinear till ridges. The carbonate content of the surface till is spatially consistent within these assemblages, suggesting a strong sediment–landform relationship. The two corridors are interpreted as the product of palaeo‐ice streams. The Maskwa palaeo‐ice stream flowed up the regional slope and across preglacial valleys, indicating it was thick and stable. Narrow dispersal trains extending across as well as down‐glacier from the Athabasca Basin suggest that the Maskwa palaeo‐ice stream extended far into the ice sheet across contrasting shield and platform terrains. In comparison, the Buffalo palaeo‐ice stream was thinner and largely controlled by subglacial geology and topography. Its catchments were located at the Canadian Shield boundary and the system was oriented along‐slope. It experienced lateral shifts and it was fed by a network of tributaries. The glacial dynamics shift from the Maskwa to the Buffalo system occurred at about 13.5 14C kyr BP. The Buffalo system later evolved into thin outlet lobes until final deglaciation of the area. The proposed model has implications for ice‐sheet reconstruction and the assessment of till properties in the prairies and in similar terrains.

Late Pleistocene and Holocene vegetation and climate records from Lake Kotokel, central Baikal region

Climatically driven Late Pleistocene and Holocene vegetation changes were reconstructed based on pollen records from the sediments of Lake Kotokel and Cheremushka Bog, located on the eastern shore of Lake Baikal. The described paleoenvironmental record has higher resolution than records collected from Lake Baikal and unites individual events identified in prior studies of bottom and onshore cores. Remarkable shifts in landscapes and expansions of index plants are as follows. Forest tundra and/or forest steppe landscape with birch, spruce, Artemisia, and Poaceae prevailed at ca. 50–25 14C kyr BP. Tundra and/or steppe vegetation dominated by Artemisia and Poaceae was typical for the Last Glacial Maximum. The expansion of shrub birch and willow occurred at ca. 15.5 14C kyr BP. Two peaks of spruce expansion at ca. 47.5–42.4 14C kyr BP (Karginian time) and at ca. 14.5–13 ka (Bølling-Allerød warm intervals) suggest that the condition were more humid than today. A slight increase in Artemisia at ca. 11–10.5 14C kyr BP (13–12 ka) was indicative of the Younger Dryas event. An expansion of birch forests with fir at ca. 12–6.4 ka suggests higher humidity. The currently dominant Scots and Siberian pine forests with birch expanded since 6.4 ka.

Weichselian and Holocene palaeoenvironmental history of the Bol'shoy Lyakhovsky Island, New Siberian Archipelago, Arctic Siberia

Cryolithological, ground ice and fossil bioindicator (pollen, diatoms, plant macrofossils, rhizopods, insects, mammal bones) records from Bol'shoy Lyakhovsky Island permafrost sequences (73°20′N, 141°30′E) document the environmental history in the region for the past c. 115 kyr. Vegetation similar to modern subarctic tundra communities prevailed during the Eemian/Early Weichselian transition with a climate warmer than the present. Sparse tundra‐like vegetation and harsher climate conditions were predominant during the Early Weichselian. The Middle Weichselian deposits contain peat and peaty soil horizons with bioindicators documenting climate amelioration. Although dwarf willows grew in more protected places, tundra and steppe vegetation prevailed. Climate conditions became colder and drier c. 30 kyr BP. No sediments dated between c. 28.5 and 12.05 14C kyr BP were found, which may reflect active erosion during that time. Herb and shrubby vegetation were predominant 11.6–11.3 14C kyr BP. Summer temperatures were c. 4 °C higher than today. Typical arctic environments prevailed around 10.5 14C kyr BP. Shrub alder and dwarf birch tundra were predominant between c. 9 and 7.6 kyr BP. Reconstructed summer temperatures were at least 4 °C higher than present. However, insect remains reflect that steppe‐like habitats existed until c. 8 kyr BP. After 7.6 kyr BP, shrubs gradually disappeared and the vegetation cover became similar to that of modern tundra. Pollen and beetles indicate a severe arctic environment c. 3.7 kyr BP. However, Betula nana, absent on the island today, was still present. Together with our previous study on Bol'shoy Lyakhovsky Island covering the period between about 200 and 115 kyr, a comprehensive terrestrial palaeoenvironmental data set from this area in western Beringia is now available for the past two glacial–interglacial cycles.

Continental palaeoenvironments during MIS 2 and 3 in southwestern France: the La Ferrassie rockshelter record

Reappraisal of La Ferrassie rockshelter sequence provides novel information on the continental palaeoenvironments in southwestern France during the end of Marine Isotope Stage 3 and the beginning of MIS 2. Cycles of sedimentation and soil development have been recognised, and are tentatively interpreted as reflecting the Dansgaard/Oeschger climatic cycles. From bottom to top, the sequence shows a trend to progressive reduction in length (and/or intensity) of the soil-forming periods and conversely, an increase in frost-related processes and sedimentation. Cambisols, associated with significant clay weathering and decalcification, developed at the base of the sequence (≥35 to ∼30 14C kyr BP). They are then replaced by simple carbonate-rich humus horizons during the interval ∼30 to ∼28 14C kyr BP. At the beginning of MIS 2, the talus dynamics shifts from rockfall accumulation with intense snow-induced washing to stone-banked solifluction lobes in connection to a semi-desert-like periglacial environment.

Cryptotephra detection using high-resolution trace-element analysis of Holocene marine sediments, southwest Japan

Detection techniques for invisible tephra, known as cryptotephra, have been exploited to construct precise and high-resolution correlations for a broad range of sedimentary sequences. We demonstrate that continuous trace-element profiles are an effective means for detecting probable positions of distal cryptotephra in Holocene hemipelagic sediments. Instrumental neutron activation analyses were performed on specimens of bulk sediments from five piston and gravity cores (water depths: 300–1500 m) taken from the southern Japan/East Sea. The down-core variations in the Ta/Sc ratio identify the positions of one to three alkaline cryptotephra in four of these cores. The Cr/Sc profiles show the position of one rhyolitic cryptotephra in three of the cores. The existence of tephra-derived components (glass ± crystals) was confirmed by microscopic observation, SEM-EPMA analysis and refractive index measurement on grains extracted from these layers. Based on microscopic observation and the stratigraphic correlations between cores, we identified eruption ages of the cryptotephras at 6.3, 7.5 and 9.3 14C kyr BP, and two source volcanoes around 800 and 400 km from the study area. The tephra layers visible to the naked eye contained volcanic grains coarser than 200 μm, and the alkaline and rhyolitic tephra component comprised >20% and >33% of the sediment on weight basis, respectively. In contrast, the range of particle sizes of the cryptotephras detected in this study is finer than 125 μm, and almost all of the glass shards were finer than 40 μm. The alkaline and rhyolitic cryptotephras made up only 2–17% and 22–24%, respectively, of the sediment on weigh basis. The high sensitivity of this method stems from the significant difference in trace-element contents between the tephras and enclosing hemipelagic sediments in the core. Alkaline U-Oki tephra was enriched in Ta by one order of magnitude over that of the sediment, and depleted in Sc by one order. The rhyolitic tephra, K-Ah, was depleted by about one order in Cr relative to that of enclosing the sediment. The differences in chemical composition between within-plate alkaline tephras and hemipelagic sediments are usually so large that trace-element geochemical method is likely to be useful for alkaline cryptotephra detection in other areas with similar tectonic characteristics.