Major changes in ice stream dynamics during deglaciation of the north-western margin of the Laurentide Ice Sheet

Victoria Island lies at the north-western limit of the former North American (Laurentide) Ice Sheet in the Canadian Arctic Archipelago and displays numerous cross-cutting glacial lineations. Previous work suggests that several ice streams operated in this region during the last (Wisconsinan) glaciation and played a major role in ice sheet dynamics and the delivery of icebergs into the Arctic Ocean. This paper produces the first detailed synthesis of their behaviour from the Last Glacial Maximum through to deglaciation (～21–9.5 cal ka BP) based on new mapping and a previously published radiocarbon-constrained ice sheet margin chronology. Over 70 discrete ice flow events (flow-sets) are ‘fitted’ to the ice margin configuration to allow identification of several ice streams ranging in size from large and long-lived (thousands of years) to much smaller and short-lived (hundreds of years). The reconstruction depicts major ice streams in M'Clure Strait and Amundsen Gulf which underwent relatively rapid retreat from the continental shelf edge at some time between ～15.2 and 14.1 cal ka BP: a period which encompasses climatic warming and rapid sea level rise (meltwater pulse-1a). Following this, overall retreat was slower and the ice streams exhibited asynchronous behaviour. The Amundsen Gulf Ice Stream continued to operate during ice margin retreat, whereas the M'Clure Strait Ice Stream ceased operating and was replaced by an ice divide within ～1000 years. This ice divide was subsequently obliterated by another short-lived phase of ice streaming in M'Clintock Channel ～13 cal ka BP. The timing of this large ice discharge event coincides with the onset of the Younger Dryas. Subsequently, a minor ice divide developed once again in M'Clintock Channel, before final deglaciation of the island shortly after 9.5 cal ka BP. It is concluded that large ice streams at the NW margin of the Laurentide Ice Sheet, equivalent in size to the Hudson Strait Ice Stream, underwent major changes during deglaciation, resulting in punctuated delivery of icebergs into the Arctic Ocean. Published radiocarbon dates constrain this punctuated delivery, as far as is possible within the limits imposed by their precision, and we note their coincidence with pulses of meltwater delivery inferred from numerical modelling and ocean sediment cores.     

Sediment sources of northern Québec and Labrador glacial deposits and the northeastern sector of the Laurentide Ice Sheet during ice-rafting events of the last glacial cycle

Provenance studies of anomalously high-flux layers of ice-rafted detritus (IRD) in North Atlantic sediments of the last glacial cycle show evidence for massive iceberg discharges coming from the Hudson Strait region of the Laurentide Ice Sheet (LIS). Although these so-called Heinrich events (H events) are commonly thought to be associated with abrupt drawdown of the LIS interior, uncertainties remain regarding the sector(s) of this multi-domed ice sheet that conveyed ice through Hudson Strait. In Northern Québec and Labrador (NQL), large-scale patterns of glacial lineations indicate massive ice flows towards Ungava Bay and Hudson Strait that could reflect the participation of the Labrador–Québec ice dome in H events. Here we evaluate this hypothesis by constraining the source of NQL glacial deposits, which provide an estimate of the provenance characteristics of IRD originating from this sector. Specifically, we use ⁴⁰Ar/³⁹Ar ages of detrital hornblende grains in 25 till samples distributed along a latitudinal transect (lat. 58°) extending east and west of Ungava Bay. The data show that tills located west and southwest of the Ungava Bay region are largely dominated by hornblende grains with Archean ages (>2.6 Ga), while tills located east of Ungava Bay are characterized by grains with early Paleoproterozoic ages (2.0–1.8 Ga), although most samples contain a few Archean-age grains. IRD derived from the NQL region should thus be characterized by a large proportion of Archean-age detrital grains, which contrasts significantly with the predominant Paleoproterozoic ⁴⁰Ar/³⁹Ar ages (1.8–1.6 Ga) typically reported for the dominant age population of hornblende grains in H layers. Comparisons with IRD through the last glacial cycle from a western North Atlantic core off Newfoundland do not show evidence for any prominent ice-rafted event with the provenance characteristics of NQL glacial deposits, thereby suggesting that significant ice-calving event(s) from the Labrador–Québec sector may have been limited throughout that interval. Although these results tend to point towards a relative stability of this ice dome during H events, our study also indicates that further provenance work is required on IRD proximal to the Hudson Strait mouth in order to constrain with a greater confidence the sector(s) of the LIS that fed ice into Hudson Strait during H events. Alternatively, these results and other paleogeographic considerations tend to support models suggesting that part of the Ungava Bay glacial lineations could be associated with a Late-Glacial ice flow across Hudson Strait.     

Changes of environments and human activity at the Sannai-Maruyama ruins in Japan during the mid-Holocene Hypsithermal climatic interval

Sannai-Maruyama is one of the most famous and best-researched mid-Holocene (mid-Jomon) archaeological sites in Japan, because of a large community of people for a long period. Archaeological studies have shown that the Jomon people inhabi1ted the Sannai-Maruyama site from 5.9 to 4.2 ± 0.1 cal kyr BP However, a continuous record of the terrestrial and marine environments around the site has not been available. Core KT05-7 PC-02, was recovered from Mutsu Bay, only 20 km from the site, for the reconstruction of high-resolution time series of environmental records, including sea surface temperature (SST). C₃₇ alkenone SSTs showed clear fluctuations, with four periods of high (8.4–7.9, 7.0–5.9, 5.1–4.1, and 2.3–1.4 cal kyr BP) and four of low (?8.4, 7.9–7.0, 5.9–5.1, and 4.1–2.3 cal kyr BP) SST. Thus, each SST cycle lasted 1.0–2.0 kyr, and the amplitude of fluctuation was about 1.5–2.0 °C. Total organic carbon (TOC) and C₃₇ alkenone contents, and the TOC/total nitrogen ratio indicate that marine biogenic production was low before 7.0 cal kyr BP, but was clearly increased between 5.9 and 4.0 cal kyr BP, because of stronger vertical mixing. During the period when the community at the site prospered (between 5.9 and 4.2 ± 0.1 cal kyr BP), the terrestrial climate was relatively warm. The high relative abundance of pollen of both Castanea and Quercus subgen. Cyclobalanopsis supports the interpretation that the local climate was optimal for human habitation. Between 5.9 and 5.1 cal kyr BP, in spite of warm terrestrial climates, the C₃₇ alkenone SST was low; this apparent discrepancy may be attributed to the water column structure in the Tsugaru Strait, which differed from the modern condition. The evidence suggests that at about 5.9 cal kyr B.P, high productivity of marine resources such as fish and shellfish and a warm terrestrial climate led to the establishment of a human community at the Sannai-Maruyama site. Then, at about 4.1 ± 0.1 cal kyr BP, abrupt marine and terrestrial cooling, indicated by a decrease of about 2 °C in the C₃₇ alkenone SST and an increase in the pollen of taxa of cooler climates, led to a reduced terrestrial food supply, causing the people to abandon the site. The timing of the abandonment is consistent with the timing (around 4.0–4.3 cal kyr BP) of the decline of civilizations in north Mesopotamia and along the Yangtze River. These findings suggest that a temperature rise of ～2 °C in this century as a result of global warming could have a great impact on the human community and especially on agriculture, despite the advances of contemporary society.     

The palaeoglaciology of the central sector of the British and Irish Ice Sheet: reconciling glacial geomorphology and preliminary ice sheet modelling

Digital elevation models of the area around the Solway Lowlands reveal complex subglacial bedform imprints relating the central sector of the LGM British and Irish Ice Sheet. Drumlin and lineation mapping in four case studies show that glacier flow directions switched significantly through time. These are summarised in four major flow phases in the region: Phase I flow was from a dominant Scottish dispersal centre, which transported Criffel granite erratics to the Eden Valley and forced Lake District ice eastwards over the Pennines at Stainmore; Phase II involved easterly flow of Lake District and Scottish ice through the Tyne Gap and Stainmore Gap with an ice divide located over the Solway Firth; Phase III was a dominant westerly flow from upland dispersal centres into the Solway lowlands and along the Solway Firth due to draw down of ice into the Irish Sea basin; Phase IV was characterised by unconstrained advance of Scottish ice across the Solway Firth. Forcing of a numerical model of ice sheet inception and decay by the Greenland ice core record facilitates an assessment of the potential for rapid ice flow directional switching during one glacial cycle. The model indicates that, after fluctuations of smaller radially flowing ice caps prior to 30 ka BP, the ice sheet grows to produce an elongate, triangular-shaped dome over NW England and SW Scotland at the LGM at 19.5 ka BP. Recession after 18.5 ka BP displays a complex pattern of significant ice flow directional switches over relatively short timescales, complementing the geomorphologically-based assessments of palaeo-ice dynamics. The palaeoglaciological implications of this combined geomorphic and modelling approach are that: (a) the central sector of the BIIS was as a major dispersal centre for only ca 2.5 ka after the LGM; (b) the ice sheet had no real steady state and comprised constantly migrating dispersal centres and ice divides; (c) subglacial streamlining of flow sets was completed over short phases of fast flow activity, with some flow reversals taking place in less than 300 years.     

Revision of the NW Laurentide Ice Sheet: implications for paleoclimate,the northeast extremity of Beringia,and Arctic Ocean sedimentation

For the past half-century, reconstructions of North American ice cover during the Last Glacial Maximum have shown ice-free land distal to the Laurentide Ice Sheet, primarily on Melville and Banks islands in the western Canadian Arctic Archipelago. Both islands reputedly preserve at the surface multiple Laurentide till sheets, together with associated marine and lacustrine deposits, recording as many as three pre-Late Wisconsinan glaciations. The northwest corner of Banks Island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation (Banks Glaciation) considered to be of Matuyama age (>780 ka BP). Inside the limit of Banks Glaciation, younger till sheets are ascribed to the Thomsen Glaciation (pre-Sangamonian) and the Amundsen Glaciation (Early Wisconsinan Stade). The view that the western Canadian Arctic Archipelago remained largely ice-free during the Late Wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on Banks and Melville islands, both dated to ～22 ka BP. These dates imply that these islands constitute the northeast extremity of Beringia.A fundamental revision of this model is now warranted based on widespread fieldwork across the adjacent coastlines of Banks and Melville islands, including new dating of glacial and marine landforms and sediments. On Dundas Peninsula, southern Melville Island, AMS ¹⁴C dates on ice-transported marine molluscs within the most extensive Laurentide till yield ages of 25–49 ka BP. These dates require that Late Wisconsinan ice advanced northwestward from Visount Melville Sound, excavating fauna spanning Marine Isotope Stage 3. Laurentide ice that crossed Dundas Peninsula (300 m asl) coalesced with Melville Island ice occupying Liddon Gulf. Coalescent Laurentide and Melville ice continued to advance westward through M'Clure Strait depositing granite erratics at ≥235 m asl that require grounded ice in M'Clure Strait, as do streamlined bedforms on the channel floor. Deglaciation is recorded by widespread meltwater channels that show both the initial separation of Laurentide and Melvile ice, and the successive retreat of Laurentide ice southward across Dundas Peninsula into Viscount Melville Sound. Sedimentation from these channels deposited deltas marking deglacial marine limit. Forty dates on shells collected from associated glaciomarine rhythmites record near-synchronous ice retreat from M'Clure Strait and Dundas Peninsula to north-central Victoria Island ～11.5 ka BP. Along the adjacent coast of Banks Island, deglacial shorelines also record the retreat of Laurentide ice both eastward through M'Clure Strait and southward into the island's interior. The elevation and age (～11.5 ka BP) of deglacial marine limit there is fully compatible with the record of ice retreat on Melville Island. The last retreat of ice from Mercy Bay (northern Banks Island), previously assigned to northward retreat into M'Clure Strait during the Early Wisconsinan, is contradicted by geomorphic evidence for southward retreat into the island's interior during the Late Wisconsinan. This revision of the pattern and age of ice retreat across northern Banks Island results in a significant simplification of the previous Quaternary model. Our observations support the amalgamation of multiple till sheets – previously assigned to at least three pre-Late Wisconsinan glaciations – into the Late Wisconsinan. This revision also removes their formally named marine transgressions and proglacial lakes for which evidence is lacking. Erratics were also widely observed armouring meltwater channels originating on the previously proposed never-glaciated landscape. An extensive Late Wisconsinan Laurentide Ice Sheet across the western Canadian Arctic is compatible with similar evidence for extensive Laurentide ice entering the Richardson Mountains (Yukon) farther south and with the Innuitian Ice Sheet to the north. Widespread Late Wisconsinan ice, in a region previously thought to be too arid to sustain it, has important implications for paleoclimate, ice sheet modelling, Arctic Ocean ice and sediment delivery, and clarifying the northeast limit of Beringia.     

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.     

Chironomid and pollen evidence for climate fluctuations during the Last Glacial Termination in NW Patagonia

We present chironomid and pollen records from the Huelmo site (～41°30′S), NW Patagonia, to examine in detail the timing and structure of climate changes during the Last Glacial Termination in the southern mid-latitudes. The chironomid record has the highest temporal and taxonomic resolution for this critical interval, and constitutes the first account of midge faunas at the culmination of the Last Glacial Maximum (LGM) for the region. The chironomid record suggests cold and wet conditions during the LGM, followed by deglacial warming between 17.6 and 16.8 cal kyr BP. Relatively warm conditions prevailed between ～15–14 cal kyr BP, followed by a reversal in trend with cooling pulses at ～14 and 13.5 cal kyr BP, and warming at the beginning of the Holocene. Cool-temperate conditions prevailed during the Huelmo Mascardi Cold Reversal (HMCR) which, according to chironomid data, exhibits a wet phase (13.5–12.8 cal kyr BP) followed by a conspicuous drier phase (12.8–11.5 cal kyr BP). The chironomid and pollen records from the Huelmo site indicate step-wise deglacial warming beginning at 17.6 cal kyr BP, in agreement with other paleoclimate records from NW Patagonia and isotopic signals from Antarctic ice cores. Peak warmth during the Last Glacial Termination was achieved by ～14.5 cal kyr BP, followed by a cooling trend that commenced during the Antarctic Cold Reversal, which later intensified and persisted during the HMCR (13.5–11.5 cal kyr BP). We observe a shift toward drier conditions at ～12.8 cal kyr BP superimposed upon the HMCR, coeval with intense fire activity and vegetation disturbance during Younger Dryas time.     

Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27°S)

Easter Island (SE Pacific, 27°S) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Although the general climate evolution of the south Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial–interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by “El Niño-like” dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2–0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake.     

Global,regional and local scale factors determining glaciation extent in Eastern Siberia over the last 140,000 years

The aim of this work is to evaluate the effect of variations of greenhouse gas concentrations, orbital parameters, sea-surface conditions, vegetation and dust deposition on the extent of East Siberian mountain glaciations during the late Quaternary. An atmosphere-only general circulation model is used in a series of 16 sensitivity tests at high spatial resolution over the region of interest to systematically evaluate the relative importance of these different forcing parameters. No attempt was made to reproduce in detail the history of late Quaternary mountain glaciations in East Siberia, because, given (a) the temporal and spatial scarcity of available evidence of mountain glaciations in this region and (b) the large uncertainties concerning the boundary conditions to be prescribed in this model, such an exercise must necessarily remain incomplete and partially inconclusive. The results of this study suggest that moisture delivery from the Atlantic is an important factor determining mountain glacier mass balance in Eastern Siberia and is very sensitive to the geometry of the West Eurasian ice sheet. This means that variable moisture blocking by the West Eurasian ice sheet during the Weichselian is the most important single factor explaining the opposite history of glacier and ice sheet extent in West and East Eurasia during the Weichselian. This work confirms earlier results showing that the large 140 kyr BP West Eurasian ice sheet caused regional-scale cooling extending towards Eastern Eurasia. Nevertheless, the simulated response of the regional summer temperature (and thus glacier extent because of the strong dependency of glacier mass balance of summer melt rates) is to a very large extent directly determined by insolation. For the Early Weichselian, this leads to a clear maximum of local glacier extent at 70 kyr BP, which is in line with the variations of top-of-the-atmosphere insolation on orbital time scales, but to some degree at odds with geological evidence which suggests larger glacier extent at 115 and 90 kyr BP than at 70 kyr BP. Through snow feedbacks, the effects of changes in the prescribed vegetation distribution and dust deposition rate are also substantial. In summary, it appears that the broad features of late Quaternary glaciation history in Eastern Eurasia can be understood in terms of known forcings.     

Ice-rafted detritus evidence from 40Ar/39Ar ages of individual hornblende grains for evolution of the eastern margin of the Laurentide ice sheet since 43 14C ky

During the last glacial interval, the North Atlantic ice sheets expanded and contracted in approximate synchronicity with orbitally forced global climate change. Variation in ice rafted detritus content in North Atlantic marine sediment cores record the waxing and waning of glaciers, as well as the abrupt temperature changes at millennial time scales. The background variations of ice rafting are punctuated by Heinrich layers, which appear to record the catastrophic collapse of the Laurentide ice sheet through the Hudson Strait. The objective of this paper is to document the evolution of glaciation on Laurentia during the last 43 ¹⁴C kyr. We present a provenance study based on ⁴⁰Ar/³⁹Ar dates of individual hornblende grains from 57 samples taken at 2 cm spacing between 4 and 134 cm from core V23-14 (43.4°N, 45.25°W, 3177 m). Sedimentation rates outside of the Heinrich layers are very low in this core, but the Heinrich layers are easily identified. Laurentide glaciation did not extend into the ocean south of 55°N until about 26 ¹⁴C kyr, and retreated to the coastline or beyond by 14 ¹⁴C kyr. Documenting the history of this major ice sheet has significant implications for understanding ice rafting sources in more distal locations where mixing among different ice sheets is likely.     

Erratum to: Severnaya Zemlya,Arctic Russia: a nucleation area for Kara Sea ice sheets during the Middle to Late Quaternary: [Quaternary Science Reviews 25(21–22) (2006) 2894–2936]

Quaternary glacial stratigraphy and relative sea-level changes reveal at least four expansions of the Kara Sea ice sheet over the Severnaya Zemlya Archipelago at 79°N in the Russian Arctic, as indicated from tills interbedded with marine sediments, exposed in stratigraphic superposition, and from raised-beach sequences that occur at altitudes up to 140 m a.s.l. Chronologic control is provided by AMS ¹⁴C, electron-spin resonance, green-stimulated luminescence, and aspartic-acid geochronology. Major glaciations followed by deglaciation and marine inundation occurred during MIS 10-9, MIS 8-7, MIS 6-5e and MIS 5d-3. The MIS 6-5e event, associated with the high marine limit, implies ice-sheet thickness of >2000 m only 200 km from the deep Arctic Ocean, consistent with published evidence of ice grounding at ∼1000 m water depth in the central Arctic Ocean. Till fabrics and glacial tectonics record repeated expansions of local ice caps exclusively, suggesting wet-based ice cap advance followed by cold-based regional ice-sheet expansion. Local ice caps over highland sites along the perimeter of the shallow Kara Sea, including the Byrranga Mountains, appear to have repeatedly fostered initiation of a large Kara Sea ice sheet, with exception of the Last Glacial Maximum (MIS 2), when Kara Sea ice did not impact Severnaya Zemlya and barely graced northernmost Taymyr Peninsula.     

Advance,deglacial and sea‐level chronology for Foxe Peninsula,Baffin Island,Nunavut

The impact of the Laurentide Ice Sheet (LIS) deglaciation on Northern Hemisphere early Holocene climate can be evaluated only once a detailed chronology of ice history and sea‐level change is established. Foxe Peninsula is ideally situated on the northern boundary of Hudson Strait, and preserves a chronostratigraphy that provides important glaciological insights regarding changes in ice‐sheet position and relative sea level before and after the 8.2 ka cooling event. We utilized a combination of radiocarbon ages, adjusted with a new locally derived ΔR, and terrestrial in‐situ cosmogenic nuclide (TCN) exposure ages to develop a chronology for early‐Holocene events in the northern Hudson Strait. A marine limit at 192 m a.s.l., dated at 8.1–7.9 cal. ka BP, provides the timing of deglaciation following the 8.2 ka event, confirming that ice persisted at least north of Hudson Bay until then. A moraine complex and esker morphosequence, the Foxe Moraine, relates to glaciomarine outwash deltas and beaches at 160 m a.s.l., and is tightly dated at 7.6 cal. ka BP with a combination of shell dates and exposure ages on boulders. The final rapid collapse of Foxe Peninsula ice occurred by 7.1–6.9 cal. ka BP (radiocarbon dates and TCN depth profile age on an outwash delta), which supports the hypothesis that LIS melting contributed to the contemporaneous global sea‐level rise known as the Catastrophic Rise Event 3 (CRE‐3).     

Holocene environments and climate in the Mongolian Altai reconstructed from the Hoton-Nur pollen and diatom records: a step towards better understanding climate dynamics in Central Asia

This study presents the results of the palynological and diatom analyses of the sediment core recovered in Hoton-Nur Lake (48°37′18″N, 88°20′45″E, 2083 m) in 2004. Quantitative reconstruction of the Holocene vegetation and climate dynamics in the semiarid Mongolian Altai suggests that boreal woodland replaced the primarily open landscape of northwestern Mongolia at about 10 kyr BP (1 kyr = 1000 cal yr) in response to a noticeable increase in precipitation from 200–250 mm/yr to 450–550 mm/yr. A decline of the forest vegetation and a return to a predominance of open vegetation types occurred after 5 kyr BP when precipitation sums decreased to 250–300 mm/yr. Prior to 11.5 kyr BP diatom concentrations are relatively low and the lake is dominated by planktonic Cyclotella and small Fragilariaceae, suggesting the existence of a relatively deep and oligotrophic/mesotrophic lake. The great abundance of Staurosirella pinnata from the beginning of the record until 10.7 kyr BP might imply intensified erosion processes in the catchment and this is fully consistent with the presence of scarce and dry vegetation and the generally arid climate during this period. From about 10.7 kyr BP, more planktonic diatom taxa appeared and increased in abundance, indicating that the lake became more productive as diatom concentration increased. This change correlates well with the development of boreal woodland in the catchment. Decrease in precipitation and changes in the vegetation towards steppe are reflected by the rapid increase in Aulacoseira distans from about 5 kyr BP. The Holocene pollen and diatom records do not indicate soil and vegetation cover disturbances by the anthropogenic activities, implying that the main transformations of the regional vegetation occurred as a result of the natural climate change. Our reconstruction is in agreement with the paleomonsoon records from China, demonstrating an abrupt strengthening of the summer monsoon at 12 kyr BP and an associated increase in precipitation and in lake levels between 11 and 8 kyr BP, followed by the stepwise attenuation of the monsoon circulation and climate aridization towards the modern level. The records from the neighboring areas of Kazakhstan and Russia, situated west and north of Hoton-Nur, demonstrate spatially and temporally different Holocene vegetation and climate histories, indicating that the Altai Mountains as a climate boundary are of pivotal importance for the Holocene environmental and, possibly, habitation history of Central Asia.     

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120 s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15 km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1 km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2 km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150 km) beneath the Taiwan Strait is about 0.1–0.3 km/s lower than the AK135 model. The overall crustal thickness is approximately 30 km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000 m should take water layer into consideration except for the continental shelves.     

Chironomid record of Late Quaternary climatic and environmental changes from two sites in Central Asia (Tuva Republic,Russia)—local,regional or global causes?

Sediment cores from two mountain lakes (Lake Grusha at 2413 m a.s.l. and Ak-Khol at 2204 m a.s.l.) situated in the Tuva Republic (southern Siberia, Russia), just north of Mongolia, were studied for chironomid fossils in order to infer post-glacial climatic changes and to investigate responses of the lake ecosystems to these changes. The results show that chironomids are responding both to temperature and to changing lake depth, which is regarded as a sensitive proxy of regional effective moisture. The post-glacial history of this mountain region in Central Asia can be divided into seven successive climatic phases: the progressive warming during the last glacial–interglacial transition (ca 15.8–14.6 cal kyr BP), the warm and moist Bølling-Allerød-like interval (ca 14.6–13.1 cal kyr BP), the cool and dry Younger Dryas-like event (ca 13.1–12.1 cal kyr BP), warmer and wetter conditions during ca 12.1–8.5 cal kyr BP, a warm and dry phase ca 8.5–5.9 cal kyr BP, cold and wet conditions during ca 5.9–1.8 cal kyr BP, as well as cold and dry climate within the last 1800 years. The chironomid records reveal patterns of climatic variability during the Late-glacial and Holocene, which can be correlated with abrupt climatic events in the North Atlantic and the Asian monsoon-dominated regimes. Apparently, the water balance of the studied lakes is controlled by the interrelation between the dominant westerly system and the changing influence of the summer monsoon, as well as the influence of alpine glacier meltwater supply. It is possible that monsoon tracks could have reached the southwest Tuva, resulting in an increase in precipitation at ca 14.6–13.1 and ca 12.1–8.5 cal kyr BP, whereas cyclonic westerlies from the North Atlantic were likely responsible for considerable moisture transport accompanying the global Neoglacial cooling at ca 5.9–1.8 cal kyr BP. These events suggest the changes of the regional pattern of atmospheric circulation, which could be in turn induced by the global climatic shifts. Some discrepancies compared with other reconstructions from Central Asia may be associated with regional (spatial) differences between the changing predominant circulation mechanisms and with local differences in uplift and descent of air masses within the complicated mountain landscape. In this paper, we also discuss the possibilities and perspectives for using chironomids in reconstructions of past temperatures and climate-induced changes in water depth of lakes in Central Asia.     

Radiocarbon deglaciation chronology of the Thunder Bay,Ontario area and implications for ice sheet retreat patterns

The sensitivity of ice sheets to climate change influences the return of meltwater to the oceans. Here we track the Laurentide Ice Sheet along a ～400 km long transect spanning about 6000 yr of retreat during the major climate oscillations of the lateglacial. Thunder Bay, Ontario is near a major topographic drainage divide, thus terrestrial ablation processes are the primary forcers of ice margin recession in the study area. During deglaciation three major moraine sets were produced, and have been assigned minimum ages of 13.9 ± 0.2, 12.3 ± 0.2–12.1 ± 0.1, and 11.2 ± 0.2 cal ka BP from south to north. These define a slow retreat (～10–50 m/a) prior to major climate oscillations which was then followed by a factor of ～2 increase during the Bölling–Alleröd, and an additional increase during the early Holocene. When compared to retreat rates in other terrestrial settings of the ice sheet, nearly identical patterns emerge. However this becomes problematic because a key control on retreat rates is the surface slope of the ice sheet and this should vary considerably over areas of so-called hard and soft beds. Further these ice margin reconstructions would not allow meltwater sourced in the Hudson Basin to drain into the Atlantic basin until after Younger Dryas time.     

The deglacial history of NW Alexander Island,Antarctica, from surface exposure dating

Recent changes along the margins of the Antarctic Peninsula, such as the collapse of the Wilkins Ice Shelf, have highlighted the effects of climatic warming on the Antarctic Peninsula Ice Sheet (APIS). However, such changes must be viewed in a long-term (millennial-scale) context if we are to understand their significance for future stability of the Antarctic ice sheets. To address this, we present nine new cosmogenic ¹⁰Be exposure ages from sites on NW Alexander Island and Rothschild Island (adjacent to the Wilkins Ice Shelf) that provide constraints on the timing of thinning of the Alexander Island ice cap since the last glacial maximum. All but one of the ¹⁰Be ages are in the range 10.2–21.7 ka, showing a general trend of progressive ice-sheet thinning since at least 22 ka until 10 ka. The data also provide a minimum estimate (490 m) for ice-cap thickness on NW Alexander Island at the last glacial maximum. Cosmogenic ³He ages from a rare occurrence of mantle xenoliths on Rothschild Island yield variable ages up to 46 ka, probably reflecting exhumation by periglacial processes.     

Ice-flow evolution of the Labrador Sector of the Laurentide Ice Sheet: a review,with new evidence from northern Quebec

Outcrops bearing stained, striated facets indicative of north–northeastward moving ice, truncated by unstained, striated facets indicative of various younger flows occur in the Caniapiscau area of north–central Quebec. This is the first report of differential staining of striated facets in the region. We propose that the staining occurred in an ice-free interval of probable interglacial age. This early ice flow probably occurred during ice retreat toward the Quebec highlands. Ice flow and glacial transport data from the southern Hudson Bay and James Bay basins indicate that the next major regional ice flow was toward the northwest and resulted from the expansion of an Early Wisconsinan glacier in the Quebec highlands. The northern part of this flow was diverted northwestward through Hudson Bay, and the southern part southwestward across James Bay, following a progressive counterclockwise rotation of flow. A zone of intersection (ZI) of two major glacier bedform systems, often referred to as the horseshoe-shaped Labrador Ice Divide, represents the head of a large northward convergent ice-flow system that extended to Ungava Bay and beyond. The Ungava flow propagated southward and captured the head of the opposing flow from an outflow centre located east of Caniaspiscau reservoir. We propose that this capture event correlates with the Gold Cove Advance in Ungava Bay and on Baffin Island at about 9900 14C yr BP. It is the largest advance of Quebec-Labrador ice yet proposed for the region. This correlation is based on the relative ice-flow chronology, accommodation of glacial lakes Naskaupi and McLean in the deglaciation sequence, the constraints placed on Last Glacial Maximum (LGM) ice configuration by the postglacial uplift pattern and events in the deep-sea record. Therefore, the Ungava ice-flow pattern is not a relict pre-Wisconsinan glacial landscape as recently proposed.     

Glaciation and deglaciation of the SW Lake District,England: implications of cosmogenic 36Cl exposure dating

Exposure dating using cosmogenic ³⁶Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26–21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750–765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840–870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750–765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9–11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion.     

Hudson Strait ice streams: a review of stratigraphy, chronology and links with North Atlantic Heinrich events

We review the literature on the occupation of Hudson Strait (800 km long by 90 km wide) by late Quaternary ice streams, and the importance of Hudson Strait as the major source for sediments associated with the North Atlantic Heinrich (H-) events. Glacial erosion of the Paleozoic outcrop on the floor of Hudson Strait and Ungava Bay resulted in the export of detrital carbonate-rich sediments to ice-proximal locations on the slope and floor of the NW Labrador Sea, mainly in meltwater and turbidite plumes, and to ice distal sites thousands of kilometres away largely as iceberg-rafted detritus (IRD). Erosion of bedrock from the Precambrian Superior and Churchill provenances of the Canadian Shield is also indicated by the isotopic analyses of sediments. The major late Quaternary H-events (H-4, H-2 and H-1) are represented in southeast Baffin Island slope sediments as detrital carbonate-rich intervals up to 40 cm in thickness and appear to represent flow along the axis of the Strait. However, the late marine isotope stage #3 event, H-3 (∼27 ka), and a younger event (H-0, ∼11 ka), are not as dominant in the sedimentary record and probably represent a different glaciological regime with flow across Hudson Strait from eastern Ungava-Labrador. The freezing-on of sediments by supercooling in the rise from the 900 m deep Eastern Basin to the 400 m sill is proposed as the source of the abundant carbonate-rich glaciomarine sediments some 250 km from the outcrop in Eastern Basin. Sediment transport by meltwater and turbidity currents was the major process during H-events in ice-proximal settings. IRD was not a key diagnostic process at sites fronting Hudson Strait. A key feature in the instability of this ice stream might be the great depth (600 m) at the shelf break, and the deep basin, which lies seaward of the outer Hudson Strait sill.