Showing papers in "Journal of Quaternary Science in 2001"

Oxygen isotope and palaeotemperature records from six Greenland ice‐core stations: Camp Century, Dye‐3, GRIP, GISP2, Renland and NorthGRIP

Abstract: Oxygen isotope variations spanning the last glacial cycle and the Holocene derived from ice-core records for six sites in Greenland (Camp Century, Dye-3, GRIP, GISP2, Renland and NorthGRIP) show strong similarities. This suggests that the dominant influence on oxygen isotope variations reflected in the ice-sheet records was regional climatic change. Differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios. Palaeotemperature estimates have been obtained from the records using three approaches: (i) inferences based on the measured relationship between mean annual δ18O of snow and of mean annual surface temperature over Greenland; (ii) modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile, or (iii) by using Monte Carlo simulation techniques. The third of these approaches was adopted to reconstruct Holocene temperature variations for the Dye 3 and GRIP temperature profiles, which yields remarkably compatible results. A new record of Holocene isotope variations obtained from the NorthGRIP ice-core matches the GRIP short-term isotope record, and also shows similar long-term trends to the Dye-3 and GRIP inverted temperature data. The NorthGRIP isotope record reflects: (i) a generally stronger isotopic signal than is found in the GRIP record; (ii) several short-lived temperature fluctuations during the first 1500 yr of the Holocene; (iii) a marked cold event at ca. 8.2 ka (the ‘8.2 ka event’); (iv) optimum temperatures for the Holocene between ca. 8.6 and 4.3 ka, a signal that is 0.6‰ stronger than for the GRIP profile; (v) a clear signal for the Little Ice Age; and (vi) a clear signal of climate warming during the last century. These data suggest that the NorthGRIP stable isotope record responded in a sensitive manner to temperature fluctuations during the Holocene. Copyright © 2001 John Wiley & Sons, Ltd.

Dinoflagellate cyst assemblages as tracers of sea‐surface conditions in the northern North Atlantic, Arctic and sub‐Arctic seas: the new ‘n = 677’ data base and its application for quantitative palaeoceanographic reconstruction

Abstract: The distribution of dinoflagellate cyst (dinocyst) assemblages in surface sediment samples from 677 sites of the northern North Atlantic, Arctic and sub-Arctic seas is discussed with emphasis on the relationships with sea-surface parameters, including sea-ice cover, salinity and temperature of the coldest and warmest months. Difficulties in developing a circum-Arctic data base include the morphological variation within taxa (e.g. Operculodinium centrocarpum, Islandinium? cezare and Polykrikos sp.), which probably relate to phenotypic adaptations to cold and/or low salinity environments. Sparse hydrographical data, together with large interannual variations of temperature and salinity in surface waters of Arctic seas constitute additional limitations. Nevertheless, the use of the best-analogue technique with this new dinocyst data base including 677 samples permits quantitative reconstruction of sea-surface conditions at the scale of the northern North Atlantic and the Arctic domain. The error of prediction calculated from modern assemblages is ±1.3 ° Ca nd±1.8 °C for the temperature of February and August, respectively, ±1.8 for the salinity, and ±1.5 months yr −1 for the sea-ice cover. Application to late Quaternary sequences from the western and eastern subpolar North Atlantic (Labrador Sea and Barents Sea) provide reconstructions compatible with those obtained using the previous dinocyst data base (n = 371), which mainly included modern data from the northern North Atlantic. Copyright  2001 John Wiley & Sons, Ltd. Journal of Quaternary Science

Foraminifera, testate amoebae and diatoms as sea-level indicators in UK saltmarshes: a quantitative multiproxy approach

Abstract: The vertical distribution of foraminifera, testate amoebae and diatoms was investigated in saltmarshes in the Taf estuary (south Wales), the Erme estuary (south Devon) and the Brancaster marshes (north Norfolk), to assess the use of multiproxy indicators in sea-level reconstructions. A total of 116 samples were subjected to regression analyses, using the program calibrate, with duration of tidal flooding as the dependent variable. We found that the relationship between flooding duration and taxa was strongest for diatoms and testate amoebae and weakest for foraminifera. The vertical range of testate amoebae in saltmarshes is small. Their lower tolerance limit in present-day saltmarshes occurs where tides cover the marsh less than a combined total of 7 days (1.9%) in a year. However, they are important sea-level indicators because information for sea-level reconstruction is best derived from sediments that originate in the highest part of the intertidal zone. Diatoms span the entire sampled range in intertidal and supratidal areas, whereas the upper limit of foraminifera is found very close to the highest astronomical tide level. Local training sets provide reconstructions with higher accuracy and precision than combined training sets, but their use is limited if they do not represent adequate modern analogues for fossil assemblages. Although analyses are time consuming, a regional training set of all three groups of micro-organisms yields highly accurate (r2 = 0.80) and precise (low value of root mean square error) predictions of tidal level. This approach therefore could improve the accuracy and precision of Holocene sea-level reconstructions. Copyright © 2001 John Wiley & Sons, Ltd.

Cold marine indicators of the late Quaternary: the new dinoflagellate cyst genus Islandinium and related morphotypes

Abstract: Round, brown, spiny dinoflagellate cysts characterise many modern high-latitude assemblages. Abundance is often highest where summer sea-surface temperatures seldom exceed 7 °C and where winter sea-surface temperatures are around 0 °C, making this morphological group important for reconstructing cold intervals within marine Quaternary sequences. Our analysis of modern sediments from the Beaufort Sea of Arctic Canada, the Kara and Laptev seas of Arctic Russia, and across the Arctic Ocean, allows us to recognise the new cyst genus Islandinium along with the extant species Islandinium minutum (Harland and Reid in Harland et al., 1980) new combination (basionym: Multispinula? minuta), Islandinium? cezare (de Vernal et al., 1989 ex de Vernal in Rochon et al., 1999) new status and combination (basionym: Multispinula? minuta var. cezare) sensu lato ,a ndEchinidinium karaense new species. Of these, the generotype Islandinium minutum is a well-known but morphologically problematic species. We have re-examined the type material from the Beaufort Sea and studied specimens from across the Arctic, and our observations clarify ambiguities in the original description of this species. The archeopyle of Islandinium minutum results from the loss of three apical plates, an unusual style among peridiniphycidean dinoflagellates. The asymmetrical location of these plates around the apex is distinctive, and probably contributed to earlier misunderstandings of the archeopyle. Previous attributions to Multispinula? and Algidasphaeridium? are unsustainable. Maps showing the distribution of Islandinium minutum are given for the Northern Hemisphere and show this species to be polar to north-temperate, whereas Islandinium? cezare s.l. and Echinidinium karaense appear to be more restricted to polar environments. Copyright  2001 John Wiley & Sons, Ltd.

The Greenland 8200 cal. yr BP event detected in loss‐on‐ignition profiles in Norwegian lacustrine sediment sequences

Abstract: High-resolution loss-on-ignition analyses of lacustrine sediment cores from both proglacial and non-glacial lakes in southern Norway have revealed a specific pattern characterised by a significant, two-peaked reduction of the loss-on-ignition values in the basal half of the cores. In non-glacier-fed lakes, the loss-on-ignition variations are interpreted to reflect mainly lake productivity and hence variability in surface summer air temperature. Sediments deposited in proglacial lakes, however, reflect mainly the glacier activity in the lake catchment. Bulk AMS radiocarbon dates from the core sequences and the loss-on-ignition curve pattern suggest that this event correlates with the ‘8200 cal. yr BP event’ recorded in the GRIP and GISP2 Greenland ice-cores, termed the Finse event in southern Norway. Copyright © 2001 John Wiley & Sons, Ltd.

Cosmogenic radionuclide dating of glacial landforms in the Lahul Himalaya, northern India: defining the timing of Late Quaternary glaciation

Abstract: The timing of glaciation in the Lahul Himalaya of northern India was ascertained using the concentrations of cosmogenic 10Be and 26Al from boulders on moraines and drumlins, and from glacially polished bedrock surfaces. Five glacial stages were identified: Sonapani I and II, Kulti, Batal and Chandra. Of these, cosmogenic exposure ages were obtained on samples representative of the Batal and Kulti glacial cycles. Stratigraphical relationships indicate that the Sonapani I and II are younger. No age was obtained for the Chandra glacial advance. Batal Glacial Stage deposits are found throughout the valley, indicating the presence of an extensive valley glacial system. During the Kulti Stage, glaciers advanced ca. 10 km beyond their current positions. Moraines produced during the Batal Stage, ca. 12–15.5 ka, are coeval with the Northern Hemisphere Late-glacial Interstadial (Bolling/Allerod). Deglaciation of the Batal Glacial Stage was completed by ca. 12 ka and was followed by the Kulti Glacial Stage during the early Holocene, at ca. 10–11.4 ka. On millennial time-scales, glacier oscillations in the Lahul Himalaya apparently reflect periods of positive mass-balance coincident with times of increased insolation. During these periods the South Asian summer monsoon strengthened and/or extended its influence further north and west, thereby enhancing high-altitude summer snowfall. Copyright © 2001 John Wiley & Sons, Ltd.

How sensitive are river systems to climate and land‐use changes? A model‐based evaluation

Abstract: Coulthard, T. J., Macklin, M. G. (2001). How sensitive are river systems to climate and land-use changes? A model-based evaluation. Journal of Quarternary Science, 16(4), 347-351.

Sea-surface conditions in northernmost Baffin Bay during the Holocene: palynological evidence

Abstract: The analysis of cores collected in northernmost Baffin Bay, from within the area of the North Water Polynya, permits definition of a composite sedimentary sequence ca. 12 m thick spanning the last 10 000 14 C yr, with only a few discontinuities. Palynological analyses were performed in order to reconstruct changes in surface water conditions and biogenic production. Transfer functions, using dinocyst assemblages, were applied to estimate sea-surface temperature (SST) and salinity, as well as the seasonal duration of sea ice cover. At the base of the record, prior to 9300 14 C yr BP, dinocysts and organic linings of benthic foraminifers are sparse, indicating harsh conditions and low productivity. After ca. 9300 14 C yr BP, the increased concentration of benthic foraminifers (up to 10 3 linings cm -3 ) and dinocyst fluxes (10 2 -10 3 cysts cm -2 yr -1 ) reveals high biological productivity related to open-water conditions. The early to middle Holocene, from ca. 9000 to ca. 3600 14 C yr BP, is marked by relatively high species diversity in dinocyst assemblages and the significant occurrence of autotrophic taxa such as Spiniferites elongatus, Pentapharsodinium dalei and Impagidinium pallidum. This assemblage suggests conditions at least as warm as at present. From ca. 6400 to ca. 3600 14 C yr BP, transfer functions indicate warmer conditions than at present, with SST in August fluctuating up to 5.5°C. After 3600 14 C yr BP, the dinocyst record suggests a trend of decreasing temperature toward modern values, marked by recurrent cooling events. Copyright © 2001 John Wiley & Sons, Ltd.

Geochemistry, dispersal, volumes and chronology of Holocene silicic tephra layers from the Katla volcanic system, Iceland

Abstract: At least 12 silicic tephra layers (SILK tephras) erupted between ca 6600 and ca 1675 yr BP from the Katla volcanic system, have been identified in southern Iceland In addition to providing significant new knowledge on the Holocene volcanism of the Katla system which typically produces basaltic tephra, the SILK tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts With one exception the SILK tephras have a narrow compositional range, with SiO2 between 63 and 67% Geochemically they are indistinguishable from ocean transported pumice found on beaches in the North Atlantic region, although they differ significantly from the silicic component of the North Atlantic Ash Zone One (NAAZO) Volumes of airborne SILK tephra range from 005 to 03 km3 We present new isopach maps of the six largest layers and demonstrate that they originate within the Katla caldera The apparently stable magma system conditions that produced the SILK tephras may have been established as a consequence of the eruption of the silicic component of NAAZO (ca 103 ka) and disrupted by another large-scale event, the tenth century ad Eldgja eruption (ca 1 ka) Despite the current long repose, silicic activity of this type may occur again in the future, presenting hitherto unknown hazards Copyright © 2001 John Wiley & Sons, Ltd

Subglacial bedform geomorphology of the Irish Ice Sheet reveals major configuration changes during growth and decay

Abstract: The belated realisation that ribbed (Rogen) moraines form such an integral part of Irish geomorphology, and the piecemeal approach to previous drumlin mapping, is probably responsible for the highly contrasting views of palaeoflow patterns of the Irish Ice Sheet. Using a high resolution (25 m) digital elevation model we present morphological maps of a large part (100 × 100 km) of the so-called ‘Drumlin Belt’ of north central Ireland. The landforms comprise mostly ribbed moraine much larger than found elsewhere (up to 16 km in length), which in places are superimposed on each other. Contrary to most prior assessments we find the bedform record to contain numerous and overlapping episodes of bed formation (ribbed moraine, drumlins and crag-and-tails) that provide a palimpsest record of changing flow geometries. These demonstrate an ice sheet with a centre of mass and flow geometry that changed during growth and decay. Using distinctive flow patterns and relative age relationships between them we reconstruct ice sheet evolution into four phases during a single glacial cycle. In phase 1 (early in the glacial cycle), Scottish and local ice coalesced to form a northeast-centred Irish Ice Sheet. As it grew its centre of mass migrated southwards, culminating in a major N–S divide positioned down the east of Ireland (phase 2, ca. Last Glacial Maximum). During retreat, the centre of mass migrated at least 120 km northwards and became established in northwest Ireland and at this point a dramatic bedforming event produced one of the world's largest and most contiguous ribbed moraine fields (phase 3). Final deglaciation is thought to be by fragmentation into many topographically controlled minor ice-caps (phase 4). Rather than any dramatic or unexpected behaviour, the reconstructed phases indicate a relatively predictable pattern of ice sheet growth and decay with changes in centres of mass, and does not require major readvances or ice-stream events. Copyright © 2001 John Wiley & Sons, Ltd.

Redating the onset of burning at Lynch's Crater (North Queensland): implications for human settlement in Australia

Abstract: Lynch's Crater preserves a continuous, high-resolution record of environmental changes in north Queensland. This record suggests a marked increase in burning that appears to be independent of any known major climatic boundaries. This increase is accompanied, or closely followed, by the virtually complete replacement of rainforest by sclerophyll vegetation. The absence of any major climatic shift associated with this increase in fire frequency therefore has been interpreted as a result of early human impact in the area. The age for this increase in burning, on the basis of conventional radiocarbon dating, was previously thought to be approximately 38000 C-14 yr BP, supporting the traditional model for human arrival in Australia at 40 000 C-14 yr BP Here we have applied a more rigorous pre-treatment and graphitisation procedure for radiocarbon dating samples from the Lynch's Crater sequence. These new dates suggest that the increase in fire frequency occurred at 45 000 C-14 yr BP, supporting the alternative view that human occupation of Australia occurred by at least 45 000-55 000 cal. yr BP. Copyright (C) 2001 John Wiley & Sons, Ltd.

Tephrological implications of beam size—sample-size effects in electron microprobe analysis of glass shards

Abstract: This paper concerns the potential consequences of varying procedures for the determination of tephra geochemistry by electron microprobe. Application of electron probe microanalysis to tephrostratigraphical methods has increasingly facilitated the resolution and refinement of Quaternary chronology associated with records of proxy-environmental or proxy-climatic change. The geographical range over which tephras are recovered has expanded significantly with the identification and analysis of crypto (or hidden) tephras in areas far removed from tephra sources. These tephras are dominated by glass shards, which, in many distal environments, may be either small in size (μm) or may be highly pumiceous with low glass:void ratios and thin (<10 μm) shard walls. We demonstrate that reducing the size of the electron beam used to analyse shard geochemistry cannot be used reliably to permit analysis of thin glass walls. This approach distorts the geochemical data, creating analytical differences that may generate inappropriate tephrogeochemical fingerprints. Additional distortion of the geochemical fingerprint in the form of hybrid analyses may be encountered in glass fragments containing micron-sized crystalline phases such as feldspar. Copyright © 2001 John Wiley & Sons, Ltd.

Aeolian origin and palaeoclimatic implications of the 'Red Clay' (north China) as evidenced by grain-size distribution.

Abstract: The ‘Red Clay’ is an important deposit underlying the Quaternary loess–palaeosol sequence in the Chinese Loess Plateau, being regarded as an excellent record of palaeoclimate changes in the late Tertiary. Several properties of the ‘Red Clay’ have been measured previously in order to derive climatic information. However, the sedimentary processes involved and the origin of the materials remain controversial. Here we present results of grain-size analyses of the ‘Red Clay’ from four representative sites in the Chinese Loess Plateau. In particular their grain-size distribution is compared with that of typical Quaternary aeolian loess–palaeosol, as well as lacustrine and fluvial sediments. It appears from the sedimentological evidence that the major part of the ‘Red Clay’ is of aeolian origin. It is rather similar in some of its properties to the Quaternary loessic palaeosols. The dust forming the ‘Red Clay’ was transported by a wind system that was weaker than that involved in the accretion of the Quaternary loess. Furthermore, the ‘Red Clay’ sediment has been modified by post-depositional weathering. Copyright © 2001 John Wiley & Sons, Ltd.

The geomorphological impact of Loch Lomond (Younger Dryas) Stadial plateau icefields in the central Lake District, northwest England

Abstract: Detailed geomorphological mapping has revealed evidence for the development of plateau icefields in the central fells of the English Lake District during the Loch Lomond (Younger Dryas) Stadial (ca. 12.9–11.5 ka). The largest plateau icefield system, which covered an area of approximately 55 km2 (including outlet glaciers), was centred on High Raise. To the west, smaller plateau icefields developed on Grey Knotts/Brandreth and Dale Head, covering areas of 7 km2 and 3 km2 respectively. The geomorphological impact of these plateau icefields appears to have been minimal on the summits, where the survival of blockfields and other frost-weathered debris (mostly peat-covered) implies the existence of at least patches of protective, cold-based ice. Ice-moulded bedrock at some plateau edges, however, documents a transition to wet-based, erosive conditions. Prominent moraine systems were produced by outlet glaciers, which descended into the surrounding valleys where their margins became sediment traps for supraglacial debris and inwash. In some valleys, ice-marginal moraines record successive positions of outlet glaciers, which actively back-wasted towards their plateau source. This interpretation differs from that of previous workers,who assumed an alpine style of glaciation, with reconstructed glaciers emanating from corries and valley heads. It is likely that plateau icefields were more common at this time in upland Britain than hitherto has been appreciated.

A critical review of the glaciomarine model for Irish sea deglaciation: evidence from southern Britain, the Celtic shelf and adjacent continental slope

Abstract: In support of their ‘glaciomarine’ model for the deglaciation of the Irish Sea basin, Eyles and McCabe cited the occurrence of distal glaciomarine mud drapes onshore in the Isles of Scilly and North Devon, and of arctic beach-face gravels and sands around the shores of the Celtic Sea. Glacial and sea-level data from the southern part of the Irish Sea in the terminal zone of the ice stream and the adjacent continental slope are reviewed here to test this aspect of the model. The suggestion that the glacial sequences of both the Isles of Scilly and Fremington in North Devon are glaciomarine mud drapes is rejected. An actively calving tidewater margin only occurred early in the deglacial sequence close to the terminal zone in the south-central Celtic Sea. Relative sea-levels were lower, and therefore glacio-isostatic depression less, than envisaged in the glaciomarine model. Geochronological, sedimentological and biostratigraphical data indicate that the raised beach sequences around the shores of the Celtic Sea and English Channel were deposited at, or during regression soon after, interglacial eustatic highstands. Evidence for ice-rafting at a time of high relative sea-levels is restricted to a phase(s) earlier than the Late Devensian. These data indicate that the raised beach sequences have no bearing on the style of Irish Sea deglaciation. Copyright © 2001 John Wiley & Sons, Ltd.

Distribution of dinoflagellate cysts in the Canadian Arctic marine region

Abstract: The Canadian Arctic is a major gateway for transport of freshwater from the Arctic Ocean to the North Atlantic. This region comprises the Beaufort Sea, the Canadian Arctic Archipelago (CAA) and northern sections of Baffin and Hudson bays. Subregional differences include major freshwater runoff to the Beaufort Sea and Hudson Bay, presence of Pacific and Atlantic Intermediate water in the west, and Atlantic Water in Baffin and Hudson bays. Principal component analysis of 50 core-top samples shows four subregional dinoflagellate cyst assemblages. Outer Beaufort Shelf Assemblage I is co-dominated by Operculodinium centrocarpum s.l. and Brigantedinium spp., with minor cysts of Pentapharsodinium dalei, Algidasphaeridium? minutum s.l. and cysts of Polykrikos spp. Assemblage II in the Canadian Arctic Archipelago is co-dominated by Brigantedinium spp., A.? minutum s.l. and cysts of Polykrikos spp., including two Arctic morphotypes. Assemblage III in the North Water polynya exclusively comprises A.? minutum s.l. and Brigantedinium spp. Assemblage IV in Baffin Bay is dominated by O. centrocarpum s.l. and Spiniferites spp., with Brigantedinium spp. on the shelves. The ratio of gonyaulacoid to protoperidinioid cysts (G : P) generally decreases with increased sea-ice cover, but it may also decrease in river plumes and in polynyas. Copyright  2001 John Wiley & Sons, Ltd. Journal of Quaternary Science

Geomorphological investigations and luminescence dating in the southern part of the Khangay and the Valley of the Gobi Lakes (Central Mongolia)

Abstract: Geomorphological investigations in the catchment area of the Baydragiyn river along the southern slope of the Khangay and in the Valley of the Gobi Lakes in western Mongolia provide evidence for Late Quaternary glaciations and lake-level changes. Thermoluminescence (TL) and infrared optically stimulated luminescence (IRSL) dating of aeolian and colluvial sediments from the central Khangay place the sediments that overlie fluvial and glaciofluvial terraces in the Holocene. An age of 21 ka is determined for a sand deposit overlying the terrace that is related to the last glacial ice-margin. Lacustrine sediments from higher beach lines in the Valley of the Gobi Lakes provide evidence for a slightly more humid period around 1.5 ka, and a larger extent of the lakes in the Early Holocene at about 8.5 ka, as also reported from other parts of Central Asia. Remnants of lacustrine sediments buried by alluvial gravel, and indicating a huge palaeolake in the basin of the Orog Nuur, however, date to the early stage of the last glaciation around 70 ka. Copyright © 2001 John Wiley & Sons, Ltd.

Huge Ice‐age lakes in Russia

Abstract: During an early phase of the Last Ice Age (Weichselian, Valdaian), about 90 000 yr ago, an ice sheet formed over the shallow Barents and Kara seas. The ice front advanced on to mainland Russia and blocked the north-flowing rivers (Yenissei, Ob, Pechora, Dvina and others) that supply most of the freshwater to the Arctic Ocean. The result was that large ice-dammed lakes were formed between the ice sheet in the north and the continental water divides to the south. Here we present reconstructions and calculations of the areas and volumes of these lakes. The lake on the West Siberian Plain was nearly twice as large as the largest lake on Earth today. The well-mapped Lake Komi in northeast Europe and a postulated lake in the White Sea Basin would also rank before the present-day third largest lake. The lakes overflowed towards the south and thus the drainage of much of the Eurasian continent was reversed. The result was a major change in the water balance on the continent, decreased freshwater supply to the Arctic Ocean, and increased freshwater flow to the Aral, Caspian, Black and Baltic seas. A sudden outburst of the lakes' water to the Arctic Ocean when the ice sheet thinned is postulated. Copyright © 2001 John Wiley & Sons, Ltd.

Holocene variations of sea-surface conditions in the southeastern Barents Sea, reconstructed from dinoflagellate cyst assemblages

Abstract: Palynomorphs were analysed in two sediment cores from the southeastern Barents Sea representing the past 8.3 and 4.4 kyr. High dinocyst contents and species diversity enabled the application of the best analogue method to quantitatively reconstruct sea-surface salinities, temperatures and ice cover using 677 modern reference sites from the North Atlantic and Arctic seas, including new data from the Barents Sea reported here. At the southern core site, where waters are affected by the Atlantic inflow, sea-surface conditions were relatively warm and stable between ca. 8000 and 5000 calendar yr BP. In contrast, the past 5 kyr had periods with cooler temperatures and extended ice cover, fluctuating mostly at 1–1.5 kyr frequencies at both sites. Most pronounced coolings occurred around 8.1, 5, 3.5–3.2 and 2.5 ka. The northern site additionally shows younger cooling events, tentatively dated to 1.4, 0.3 and 0.1 ka. Identified variations in sea-surface conditions indicate changes in Atlantic water inputs to the Barents Sea. Our results generally correlate to palaeoclimatic reconstructions from northwestern Eurasia, exemplified by palynological records from Karelia. This correlation suggests that sea-surface variations in the Barents Sea reflect large-scale changes in atmospheric and oceanic interactions between the North Atlantic and the Arctic. Copyright © 2001 John Wiley & Sons, Ltd.

Sedimentary evidence for deforming bed conditions associated with a grounded Irish Sea glacier, southern Ireland

Abstract: Along the south coast of Ireland, a shelly diamict facies, the Irish Sea Till, has been variously ascribed to subglacial deposition by a grounded Irish Sea glacier or to glacimarine sedimentation by suspension settling and iceberg rafting. Observations are presented here from five sites along the south coast to directly address this question. At these sites, sedimentary evidence is preserved for the onshore advance of a grounded Irish Sea glacier, which glacitectonically disturbed and eroded pre-existing sediments and redeposited them as deformation till. Recession of this Irish Sea glacier resulted in the damming of ice-marginal lakes in embayments along the south coast, into which glacilacustrine sedimentation then took place. These lake sediments were subsequently glacitectonised and reworked by overriding glacier ice of inland origin, which deposited deformation till on top of the succession. There is no evidence for deposition of the Irish Sea diamicts by glacimarine sedimentation at these sites. The widespread development of subglacial deforming bed conditions reflected the abundance of fine-grained marine and lacustrine sediments available for subglacial erosion and reworking. Stratigraphical and chronological data suggest that the advance of a grounded Irish Sea glacier along the south coast occurred during the last glaciation, and this is regionally consistent with marine geological data from the Celtic Sea. These observations demonstrate extension of glacier ice far beyond its traditional limits in the Celtic Sea and on-land in southern Ireland during the last glaciation, and remove the stratigraphical basis for chronological differentiation of surficial glacial drifts, and thus the Munsterian Glaciation, in southern Ireland. Copyright © 2001 John Wiley & Sons, Ltd.

Relationships between dinoflagellate cyst assemblages in surface sediment and hydrographic conditions in the Bering and Chukchi seas

Abstract: Palynological analyses were performed on 52 surface sediment samples from the eastern part of the Bering Sea and Chukchi Sea in order to document the regional distribution of dinoflagellate cyst assemblages and their relationship with sea-surface conditions. The assemblages present a relatively high species diversity (20 taxa are recovered routinely), especially in the Bering Sea, where they are dominated by Operculodinium centrocarpum and the cyst of Pentapharsodinium dalei accompanied mainly by Spiniferites elongatus s.l., Spiniferites ramosus, Impagidinium pallidum, Brigantedinium spp., Islandinium minutum, Selenopemphix quanta, Selenopemphix nephroides, Quinquecuspis concreta and the cyst of Polykrikos kofoidii. The percentages of the main taxa vary with latitude, and principal component analysis shows that the distribution of assemblages is closely related to hydrographic conditions, notably the seasonal duration of sea-ice cover and the sea-surface temperature in February. The dinoflagellate cyst assemblages from the Bering Sea differ significantly from those of subarctic seas of the North Atlantic, with respect to their species composition and relationships with sea-surface conditions. In particular, the occurrence of the cyst of Polykrikos kofoidii and Quinquecuspis concreta and the positive correlation between the percentages of Operculodinium centrocarpum and the extent of sea-ice, constitute peculiar features in the Bering Sea assemblages. Copyright  2001 John Wiley & Sons, Ltd. Journal of Quaternary Science

Vegetation and climate in the Early- and Pleni-Weichselian in northern Central Europe

Abstract: Analysis of numerous pollen diagrams from north and central Germany and from the adjacent lowlands of The Netherlands, Denmark, Poland and Belarus have facilitated a regional comparison of the vegetation and climatic evolution from the beginning of the Early Weichselian glaciation to the Weichselian pleniglacial Data from geological studies and analysis of fossil beetles and plant macroremains have been used to supplement the palaeoclimatic reconstruction Up to the end of the Oerel Interstadial the palaeoclimate was characterised by increasing continentality The winter temperatures in particular tended to fall continuously during the stadials and the interstadials of the Early Weichselian and early Pleniglacial In the Brorup and Odderade Interstadials summer temperatures where sufficiently high to enable boreal forests to grow, whereas in the Oerel Interstadial summer temperatures were such that tree growth was inhibited It is probable that falling sea-level and the consequent extension northwards of the North Sea coast were the main causes of increasing continentality In the latter part of the Pleniglacial, peat accumulation frequently took place and humic silts were deposited under lacustrine conditions Correlation between various sites is extraordinarily difficult Pollen diagrams from the so-called interstadials of the Glinde, Moershoofd Complex, Hengelo and Denekamp are similar to those of the known Early Weichselian stadials It is proposed therefore that these interstadials should be called intervals, and that the term interstadial be reserved for climatic variations that result in distinctive pollen assemblages and which, in turn, reflect distinctive vegetation dynamics Interstadials should be capable of being characterised on a biostratigraphical basis and it should be possible to establish correlations over considerable distances According to this definition, the first three warm oscillations of the Weichselian glaciation in the central European lowlands are the Brorup, Odderade and Oerel Interstadials All other pleniglacial peat layers should be assigned the rank of interval Copyright © 2001 John Wiley & Sons, Ltd

Identification and significance of a visible, basalt‐rich Vedde Ash layer in a Late‐glacial sequence on the Isle of Skye, Inner Hebrides, Scotland

Abstract: This paper reports the discovery of a visible, tephra horizon of Late-glacial age from the site of Loch Ashik in the Isle of Skye, the Inner Hebrides, Scotland. Although the tephra shards have a bimodal geochemical composition identical to that of the Vedde Ash (a well known marker horizon within Late-glacial sequences. The horizon at Ashik is dominated by basaltic shards and devitrified tephra shards, giving the layer its characteristic black colour. Only rhyolitic shards have previously been reported from Vedde Ash horizons in the British Isles. This new evidence raises some important questions about the factors that govern the distribution and accumulation of basaltic tephra, and about the methods used to detect ash shards in basins distal to centres of volcanic activity. Copyright © 2001 John Wiley & Sons, Ltd.

A proxy record of late Holocene climate change from May Moss, northeast England

Abstract: Quantitative reconstruction of changes in mire surface wetness has been used to reconstruct proxy climate from an upland ombrotrophic blanket mire on the North York Moors in northeast England (May Moss). Testate amoebae, plant macrofossil and humification analyses were carried out for six peat profiles. Transfer functions are used to generate estimates of water table levels from the testate amoebae stratigraphy, which complement the semi-quantitative indications of changing surface wetness provided by plant macrofossil and humification analysis. 14C dates provide the chronology for the stratigraphy. Differences were encountered between AMS 14C dates on pure Sphagnum remains and radiometric dates on bulk peat from the same horizon, which perhaps arise from the heterogeneity of peat. Replicate palaeoecological analysis of adjacent cores identifies consistency within testate amoebae and plant macrofossil stratigraphies, and reveals a strong agreement between the water table level proxies. The record of hydrological changes at sites across May Moss are in synchrony, and so climate change is the most likely cause of the moisture fluctuations. Changes to a wetter or cooler climate were identified cal. ad 260–540, ca. ad 550–650, cal. ad 670–980, ca. ad 1350–1450, cal. ad 1400–1620 and ca. ad 1700–1800. Periods with a drier or warmer climate precede all of these wet shifts, with particularly dry periods between cal. ad 650–860 and 690–980 and between cal. ad 1290–1410 and 1400–1620. Copyright © 2001 John Wiley & Sons, Ltd.

Moraine development at a small High‐Arctic valley glacier: Rieperbreen, Svalbard

Abstract: Ice-cored lateral and frontal moraine complexes, formed at the margin of the small, land-based Rieperbreen glacier, central Svalbard, have been investigated through field observations and interpretations of aerial photographs (1936, 1961 and 1990). The main focus has been on the stratigraphical and dynamic development of these moraines as well as the disintegration processes. The glacier has been wasting down since the ‘Little Ice Age’ (LIA) maximum, and between 1936 and 1990 the glacier surface was lowered by 50–60 m and the front retreated by approximately 900 m. As the glacier wasted, three moraine ridges developed at the front, mainly as melting out of sediments from debris-rich foliation and debris-bands formed when the glacier was polythermal, probably during the LIA maximum. The disintegration of the moraines is dominated by wastage of buried ice, sediment gravity-flows, meltwater activity and some frost weathering. A transverse glacier profile with a northward sloping surface has developed owing to the higher insolation along the south-facing ice margin. This asymmetric geometry also strongly affects the supraglacial drainage pattern. Lateral moraines have formed along both sides of the glacier, although the insolation aspect of the glacier has resulted in the development of a moraine 60 m high along its northern margin. Copyright © 2001 John Wiley & Sons, Ltd.

Sea‐level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling

Abstract: Models of glacio-hydroisostatic sea-level change have been published for the British Isles that are broadly consistent with the observational evidence, as well as with glaciological constraints. It has been argued, however, that the models fail to represent sea-level change along the Irish Sea margins and in southern Ireland for the post-deglaciation period. The argument rests on the interpretation of the depositional environment of the elevated ‘Irish Sea Drift’ on both sides of the Irish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the British Isles are consistent with the former interpretation in that sea-levels on either side of the Irish Sea, south of about the Isle of Man, are not predicted to have risen above present sea-level at any time since the deglaciation of the Irish Sea. This implies that ice over both the Irish Sea and Ireland was relatively thin (ca. 600–700 m over Ireland). If the glaciomarine interpretation of the elevated Irish Sea Drift is correct, then the maximum ice thickness over central and southern Ireland would have to reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea-level change to be consistent with the Holocene evidence, this thick ice sheet could not have extended to the eastern side of the Irish Sea. Nor could it have been very thick at its northern and western limits. If such an ice model is extreme and incompatible with glaciological observations then the alternative is to accept the interpretation of the Irish Sea Drift as terrestrial in origin. Copyright © 2001 John Wiley & Sons, Ltd.

Atlantic Ocean heat piracy and the bipolar climate see-saw during Heinrich and Dansgaard-Oeschger events

Abstract: The millennial-scale asynchrony of Antarctic and Greenland climate records during the last glacial period implies that the global climate system acts as a bipolar see-saw driven by either high-latitudinal and/or near-equatorial sea-surface perturbations. Based on the results of recent modelling of generic Heinrich and Dansgaard-Oeschger scenarios, we discuss the possibility that oscillations of the deep-ocean conveyor may have been sufficient to cause this bipolar see-saw. The bipolar climate asynchrony in our scenarios is caused by the toggle between North Atlantic heat piracy and South Atlantic counter heat piracy. Ocean circulation has an enhanced sensitivity to the northern deep-water source as the North Atlantic Deep Water (NADW) cannot enter the Southern Ocean at depths shallower than the bottom of the Drake Passage. Any shoaling of the NADW can, therefore, increase the northward incursion of Antarctic Bottom Water (AABW), and trigger an interhemispheric climate oscillation. As hundreds of years are required to warm the respective high latitudes, the observed climate lead and lags between the two hemispheres can be explained entirely by the variability of the meridional overturning and by the corresponding change in the oceanic heat transport. Accordingly, it is entirely feasible for the global climate to work like a pendulum, which theoretically could be controlled by pushing at either of the deep-water sources. Our model scenarios suggest that it is entirely feasible for the bipolar climate see-saw to be controlled solely by variations in NADW formation. Copyright (C) 2001 John Wiley & Sons, Ltd.

Vegetation and climate during the Weichselian Early Glacial and Pleniglacial in the Niederlausitz, eastern Germany — macrofossil and pollen evidence

Abstract: Cryoturbated organic beds and channel fills, intercalated with sandy and gravelly fluvial units, have been studied in an opencast brown-coal mine near Nochten (Niederlausitz), eastern Germany. The fluvial–aeolian sequence covers parts of the Early, Pleni- and Late-glacial. The detailed chronology is based on 11 radiocarbon and 12 OSL dates, covering the period between ca. 100 kyr and 11 kyr BP. Basal peat deposits are correlated with an Early Weichselian interstadial. During this period boreal forests were present and minimum mean summer temperatures were > 13°C. Early Pleniglacial deposits are absent. The Middle and Late Pleniglacial environments were treeless and different types of tundra vegetation can be recognised. Minimum mean summer temperatures varied between 10 and 15°C. Vegetation and climate is reconstructed in detail for the periods around 34–38 kyr BP and 24–25 kyr BP. Around 34–38 ka, a mixture between a low shrub tundra and a cottongrass tussock–subshrub tundra was present. The botanical and sedimentological data suggest that from the Middle to the Late Pleniglacial, the climate became more continental, aridity and wind strength increased, and the role of a protecting winter snow cover decreased. A sedge–grass–moss tundra dominated around 24 and 25 kyr BP. Copyright © 2001 John Wiley & Sons, Ltd.

Integrated marine and terrestrial evidence for abrupt Congo River palaeodischarge fluctuations during the last deglaciation

Abstract: We present a high-resolution reconstruction of tropical palaeoenvironmental changes for the last deglacial transition (18 to 9 cal. kyr BP) based on integrated oceanic and terrestrial proxies from a Congo fan core. Pollen, grass cuticle, Pediastrum and dinoflagellate cyst fluxes, sedimentation rates and planktonic foraminiferal δ18O ratios, u37K′ sea-surface temperature and alkane/alkenone ratio data highlight a series of abrupt changes in Congo River palaeodischarge. A major discharge pulse is registered at around 13.0 cal. kyr BP which we attribute to latitudinal migration of the Intertropical Convergence Zone (ITCZ) during deglaciation. The data indicate abrupt and short-lived changes in the equatorial precipitation regime within a system of monsoonal dynamics forced by precessional cycles. The phases of enhanced Congo discharge stimulated river-induced upwelling and enhanced productivity in the adjacent ocean. Copyright © 2001 John Wiley & Sons, Ltd.

Dinoflagellate cyst assemblages in surface sediments of the Laptev Sea region (Arctic Ocean) and their relationship to hydrographic conditions

Abstract: The occurrence and distribution of dinoflagellate cysts in surface sediments from the Laptev Sea shelf and the adjacent continental margin has been studied in relation to surface water conditions. Assemblages were interpreted by visual inspection and Q-mode factor analysis. The inner Laptev shelf is a type area for polar environments because of near absence of relatively warm waters from the Pacific or Atlantic oceans and an extensive seasonal sea-ice cover. Assemblages are of low diversity and are dominated by the cold water taxon Islandinium minutum and related morphotypes. The common occurrence of distinctive polykrikoid cyst morphotypes is an indicator of polar environments. Furthermore, strong supply of freshwater in summer influences the surface water conditions, and is a major factor controlling the occurrence and distribution of dinoflagellate cysts. The dinoflagellate cysts Nematosphaeropsis labyrinthus and Operculodinium centrocarpum are restricted to the continental margin suggesting a relation to the inflow of relatively warm Atlantic waters along the Eurasian continental margin. An abundance maximum of Brigantedinium spp. at the shelf break is related to the mean position of the marginal ice zone.