The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

Radiocarbon constraints on Antarctic Peninsula Ice Sheet retreat following the Last Glacial Maximum (LGM)

We present marine sedimentologic and radiocarbon data for the timing of retreat of the largely marine-based Antarctic Peninsula Ice Sheet since the Last Glacial Maximum (LGM). Our findings indicate minimum estimates of deglaciation between 18,000 and 9000 calibrated years before present (cal yr BP), roughly in phase with the Northern Hemisphere deglaciation and eustatic sea-level rise. Our findings show this retreat occurred progressively from the outer, middle, and inner continental shelf regions, as well as progressively from the north to the south. Retreat initiated on the outer shelf of the northern Peninsula by 18,000 cal yr BP and continued southward by 14,000 cal yr BP on the outer shelf of Marguerite Bay, several thousand years earlier than estimated by numeric models. While individual cores yield estimates of glacial retreat that may vary up to ±1100 years, we note steps in the data occur at 14,000 and possibly 11,000 cal yr BP, coincidental to rapidly rising (eustatic) sea level, including the well documented melt water pulses (MWP 1a and 1b). These data support the hypothesis that rapidly rising sea level is associated with marine ice sheet destabilization, although additional dates are necessary to substantiate this finding. This study highlights problems with radiocarbon dating acid insoluble organic (AIO) matter in proximal Lateglacial sediments as well as the need for more accurate dating techniques.     

Insights on the events surrounding the final drainage of Lake Ojibway based on James Bay stratigraphic sequences

Deglaciation of the James Bay region was highly dynamic, with the occurrence of ice (Cochrane) readvances into glacial Lake Ojibway around final deglaciation time, which culminated with the drainage of Ojibway waters into Hudson Bay and subsequent incursion of the Tyrrell Sea at ～8 ka. Renewed interest on these events comes from the possible link between the drainage of the ice-dammed Lake Agassiz-Ojibway and a major climate deterioration known as the 8.2-ka cooling event. Recent glaciological modeling suggests that this drainage may have occurred subglacially, a mechanism that can accommodate more than one lake discharge, as suggested by marine records. The exact number and timing of drainage events, as well as location of the lake discharge pathway(s) remain, however, largely unconstrained. Here we focus on the events that led to the drainage of Lake Ojibway by documenting late-glacial sedimentary sequences located east of James Bay. Our investigations indicate that the deglacial sequence consists of a readvance till, extensive Ojibway rhythmites, and thick marine sediments. The glaciolacustrine and marine units are separated by a 60 cm-thick horizon composed of laminated silt beds containing rounded clay balls and disseminated clasts resulting from the abrupt drainage of the lake. Radiocarbon dating of marine fossils lying above the drainage horizon indicates that the glaciolacustrine episode ended around 8128–8282 cal yr BP. Micropaleontological analyses reveal that freshwater ostracods (Candona sp.) and marine microfossils (foraminifers, dinocysts) occur together in the upper part of the Ojibway sediments. Analysis of oxygen isotopes (δ¹⁸O) of ostracods and foraminifers originating from the same stratigraphic position show highly contrasting values that suggest possible subglacial exchanges between Lake Ojibway and Tyrrell Sea waters prior to the final drainage event. The complexity of the deglacial events is further indicated by radiocarbon dating of marine shells retrieved from a Cochrane till that suggests that the last ice readvance occurred almost simultaneously with the final lake discharge. These results bring additional constraints on the drainage mechanism of the coalesced Lake Agassiz-Ojibway and indicate that the James Bay region formed an important drainage pathway for meltwaters at the end of the last deglaciation.     

Postglacial uplift and relative sea level changes in Finnmark,northern Norway

The outer coast of Finnmark in northern Norway is where the former Fennoscandian and Barents Sea ice sheets coalesced. This key area for isostatic modelling and deglaciation history of the ice sheets has abundant raised shorelines, but only a few existing radiocarbon dates constrain their chronology. Here we present three Holocene sea level curves based on radiocarbon dated deposits from isolation basins at the outermost coast of Finnmark; located at the islands Sørøya and Rolvsøya and at the Nordkinn peninsula. We analysed animal and plant remains in the basin deposits to identify the transitions between marine and lacustrine sediments. Terrestrial plant fragments from these transitions were then radiocarbon dated. Radiocarbon dated mollusk shells and marine macroalgae from the lowermost deposits in several basins suggest that the first land at the outer coast became ice free around 14,600 cal yr BP. We find that the gradients of the shorelines are much lower than elsewhere along the Norwegian coast because of substantial uplift of the Barents Sea. Also, the anomalously high elevation of the marine limit in the region can be attributed to uplift of the adjacent seafloor. After the Younger Dryas the coast emerged 1.6–1.0 cm per year until about 9500–9000 cal yr BP. Between 9000 and 7000 cal yr BP relative sea level rose 2–4 m and several of the studied lakes became submerged. At the outermost locality Rolvsøya, relative sea level was stable at the transgression highstand for more than 3000 years, between ca 8000 and 5000 cal yr BP. Deposits in five of the studied lakes were disturbed by the Storegga tsunami ca 8200–8100 cal yr BP.     

Late Weichselian deglacial history of Disko Bugt, West Greenland, and the dynamics of the Jakobshavns Isbrae ice stream

New relative sea-level (RSL) data from Disko Bugt, a large marine embayment in West Greenland, are used to examine the deglacial history of the Jakobshavns Isbrae ice stream. RSL data show rapid deglaciation after 10.3 ka cal. yr BP. Once deglaciation began, a bedrock high in the west of the bay exerted no discernible influence on the deglacial chronology. Following initial rapid retreat, ice stream recession slowed as it approached the eastern shores of the bay. Seabed elevations increase here and the ice stream terminus lingered for several thousand years before retreating into the narrow bedrock-confined Jakobshavns Isfjord. The seabed topography of Disko Bugt includes several deep channels which probably record the former course of the ice stream. Using a simple water depth/calving velocity relationship it is estimated that the maximum calving velocity on deglaciation was c. 4.8 km a^-1. This is less than the present rate (6–7 km a^-1), although ice discharge was two to four times that observed today. Initiation of rapid ice stream retreat was probably caused by ice stream thinning and increased surface melting. A critical point in time was the retreat of the ice stream from shallow continental shelf waters ( c. 400 m) into the deep bedrock trough (>800 m) which marks the entrance to Disko Bugt.     

AMS 14C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet

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.     

Evidence for Heinrich event 1 in the British Isles

In the north Irish Sea basin (ISB), sedimentary successions constrained by AMS ¹⁴C dates obtained from marine microfaunas record three major palaeoenvironmental shifts during the last deglacial cycle. (i) Marine muds (Cooley Point Interstadial) dated to between 16.7 and 14.7 ¹⁴C kyr BP record a major deglaciation of the ISB following the Late Glacial Maximum (LGM). (ii) Terminal outwash and ice-contact landforms (Killard Point Stadial) were deposited during an extensive ice readvance, which occurred after 14.7 ¹⁴C kyr BP and reached a maximum extent at ca.14 ¹⁴C kyr BP. At this time the lowlands surrounding the north ISB were drumlinised. Coeval flowlines reconstructed from these bedforms end at prominent moraines (Killard Point, Bride, St Bees) and indicate contemporaneity of drumlinisation from separate ice dispersal centres, substrate erosion by fast ice flow, and subglacial sediment transfer to ice-sheet margins. In north central Ireland bed reorganisation associated with this fast ice-flow phase involved overprinting and drumlinisation of earlier transverse ridges (Rogen-type moraines) by headward erosion along ice streams that exited through tidewater ice margins. This is the first direct terrestrial evidence that the British Ice Sheet (BIS) participated in Heinrich event 1 (H1). (iii) Regional mud drapes, directly overlying drumlins, record high relative sea-level (RSL) with stagnation zone retreat after 13.7 ¹⁴C kyr BP (Rough Island Interstadial). Elsewhere in lowland areas of northern Britain ice-marginal sediments and morainic belts record millennial-scale oscillations of the BIS, which post-date the LGM advance on to the continental shelf, and pre-date the Loch Lomond Stadial (Younger Dryas) advance in the highlands of western Scotland (ca. 11–10 ¹⁴C kyr BP). In western, northwestern and northern Ireland, Killard Point Stadial (H1) ice limits are reconstructed from ice-flow lines that are coeval with those in the north ISB and end at prominent moraines. On the Scottish continental shelf possible H1-age ice limits are reconstructed from dated marine muds and associated ice marginal moraines. It is argued that the last major offshore ice expansion from the Scottish mountains post-dated ca. 15 ¹⁴C kyr BP and is therefore part of the H1 event. In eastern England the stratigraphic significance of the Dimlington silts is re-evaluated because evidence shows that there was only one major ice oscillation post-dating ca.18 ¹⁴C kyr BP in these lowlands. In a wider context the sequence of deglacial events in the ISB (widespread deglaciation of southern part of the BIS → major readvance during H1 → ice sheet collapse) is similar to records of ice sheet variability from the southern margins of the Laurentide Ice Sheet (LIS). Well-dated ice-marginal records, however, show that during the Killard Point readvance the BIS was at its maximum position when retreat of the LIS was well underway. This phasing relationship supports the idea that the BIS readvance was a response to North Atlantic cooling induced by collapse of the LIS. © 1998 John Wiley & Sons, Ltd.     

Deglaciation and ice shelf development at the northeast margin of the Laurentide Ice Sheet during the Younger Dryas chronozone

Core 2011804‐0010 from easternmost Lancaster Sound provides important insights into deglacial timing and style at the marine margin of the NE Laurentide Ice Sheet (LIS). Spanning 13.2–11.0 cal. ka BP and investigated for ice‐rafted debris (IRD), foraminifera, biogenic silica and total organic carbon, the stratigraphy comprises a lithofacies progression from proximal grounding line and sub‐ice shelf environments to open glaciomarine deposition; a sequence similar to deposits from Antarctic ice shelves. These results are the first marine evidence of a former ice shelf in the eastern Northwest Passage and are consistent with a preceding phase of ice streaming in eastern Lancaster Sound. Initial glacial float‐off and retreat occurred >13.2 cal. ka BP, followed by formation of an extensive deglacial ice shelf during the Younger Dryas, which acted to stabilize the retreating margin of the NE LIS until 12.5 cal. ka BP. IRD analyses of sub‐ice shelf facies indicate initial high input from source areas on northern Baffin Island delivered to Lancaster Sound by a tributary ice stream in Admiralty Inlet. After ice shelf break‐up, Bylot Island became the dominant source area. Foraminifera are dominated by characteristic ice‐proximal glaciomarine benthics (Cassidulina reniforme, Elphidium excavatum f. clavata), complemented by advected Atlantic water (Cassidulina neoteretis, Neogloboquadrina pachyderma) and enhanced current indicators (Lobatula lobatula). The biostratigraphy further supports the ice shelf model, with advection of sparse faunas beneath the ice shelf, followed by increased productivity under open water glaciomarine conditions. The absence of Holocene sediments in the core suggests that the uppermost deposits were removed, most likely due to mass transport resulting from the site's proximity to modern tidewater glacier margins. Collectively, this study presents important new constraints on the deglacial behaviour of the NE Laurentide Ice Sheet, with implications for past ice sheet stability, ice‐rafted sediment delivery, and ice−ocean interactions in this complex archipelago setting.     

Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea

Graham, A.G.C., Lonergan, L. & Stoker, M.S. 2010: Depositional environments and chronology of Late Weichselian glaciation and deglaciation in the central North Sea. Boreas, Vol. 39, pp. 471–491. 10.1111/j.1502‐3885.2010.00144.x. ISSN 0300‐9483. Geological constraints on ice‐sheet deglaciation are essential for improving the modelling of ice masses and understanding their potential for future change. Here, we present a detailed interpretation of depositional environments from a new 30‐m‐long borehole in the central North Sea, with the aim of improving constraints on the history of the marine Late Pleistocene British–Fennoscandian Ice Sheet. Seven units characterize a sequence of compacted and distorted glaciomarine diamictons, which are overlain by interbedded glaciomarine diamictons and soft, bedded to homogeneous marine muds. Through correlation of borehole and 2D/3D seismic observations, we identify three palaeoregimes. These are: a period of advance and ice‐sheet overriding; a phase of deglaciation; and a phase of postglacial glaciomarine‐to‐marine sedimentation. Deformed subglacial sediments correlate with a buried suite of streamlined subglacial bedforms, and indicate overriding by the SE–NW‐flowing Witch Ground ice stream. AMS ¹⁴C dating confirms ice‐stream activity and extensive glaciation of the North Sea during the Last Glacial Maximum, between c. 30 and 16.2 ¹⁴C ka BP. Sediments overlying the ice‐compacted deposits have been reworked, but can be used to constrain initial deglaciation to no later than 16.2 ¹⁴C ka BP. A re‐advance of British ice during the last deglaciation, dated at 13.9 ¹⁴C ka BP, delivered ice‐proximal deposits to the core site and deposited glaciomarine sediments rapidly during the subsequent retreat. A transition to more temperate marine conditions is clear in lithostratigraphic and seismic records, marked by a regionally pervasive iceberg‐ploughmarked erosion surface. The iceberg discharges that formed this horizon are dated to between 13.9 and 12 ¹⁴C ka BP, and may correspond to oscillating ice‐sheet margins during final, dynamic ice‐sheet decay.     

Volume of Antarctic Ice at the Last Glacial Maximum, and its impact on global sea level change

The volume of Antarctic ice at the Last Glacial Maximum is a key factor for calculating the past contribution of melting ice sheets to Late Pleistocene global sea level change. At present, there are large uncertainties in our knowledge of the extent and thickness of the formerly expanded Antarctic ice sheets, and in the timing of their release as meltwater into the world’s oceans. This paper reviews the four main approaches to determining former Antarctic ice volume, namely glacial geology, glacio-isostatic studies, glaciological modelling, and ice core analysis and attempts to reconcile these to give a ‘best estimate’ for ice volume. In the Ross Sea there was a major expansion of grounded ice at the Last Glacial Maximum, accounting for 2.3–3.2 m of global sea level. At some time in the Weddell Sea a large grounded ice sheet corresponding to c. 2.7 m of global sea level extended to the shelf break. However, this ice expansion has not yet been confidently dated and may not relate to the Last Glacial Maximum. Around East Antarctica there was thickening and advance offshore of ice in coastal regions. Ice core evidence suggests that the interior of East Antarctica was either close to its present elevation or thinner during the last glacial so the effect of East Antarctica on sea level depends on the net balance between marginal thickening and interior thinning. Suggested East Antarctic contributions vary from a 3–5.5 m lowering to a 0.64 m rise in global sea level. The Antarctic Peninsula ice sheet thickened and extended offshore at the Last Glacial Maximum, with a sea level equivalent contribution of c. 1.7 m. Thus, the Antarctic ice sheets accounted for between 6.1 and 13.1 m of global sea level fall at the Last Glacial Maximum. This is substantially less than has been suggested by most previous studies but the maximum figure matches well with one modelling estimate. The timing of Antarctic deglaciation is not well known. In the Ross Sea, terrestrial evidence suggests deglaciation may have begun at c. 13,000 yr BP¹ but that grounded ice persisted until c. 6,500 yr BP. Marine evidence suggests the western Ross Sea was deglaciated by c. 11,500 yr BP. Deglaciation of the Weddell Sea is poorly constrained. Grounded ice in the northern Antarctic Peninsula had retreated by c. 13,000 yr BP, and further south deglaciation occurred sometime prior to c. 6,000 yr BP. Many parts of coastal East Antarctica apparently escaped glaciation at the LGM, but in those areas that were ice-covered deglaciation was underway by 10,000 yr BP. With existing data, the timing of deglaciation shows no firm relation to northern hemisphere-driven sea level rise. This is probably due partly to lack of Antarctic dating evidence but also to the combined influence of several forcing mechanisms acting during deglaciation.     

Use of the magnetic,geochemical, and sedimentary records in establishing paleoclimate change in the environment of Sebkha: Case of the Sebkha Mhabeul in southeastern Tunisia

In this study, we explored the evidence of late Holocene climate changes in southern Tunisia, examining extreme events of flood and sandstorm sedimentary succession records, in Sebkha Mhabeul. A sediment-derived climatic proxy was inferred from a 93.5-cm-deep core (Mh1), whose dating by tephrochronology has already been achieved in previous works. Multiple geochemical, sedimentological, magnetic susceptibility, and quartz grain microtexture proxies were used to determine the shifts in regional climatic conditions. The Sebkha core captured sensitive changes in the precipitation/evaporation (P/E) balance by adjustments in salinity, and is especially valuable for reconstructing variability over centennial timescales. The Sebkha Mhabeul area showed higher salinity during the Roman Warm Period (RWP: 2100–1400 cal yr BP), the Medieval Warm Period (MWP: 1000–600 cal yr BP) and the present era, and generally lower salinity during the Dark Ages (DA: 1400-1000 cal yr BP) and the Little Ice Age (LIA: 600-200 cal yr BP).In southeastern Tunisia, the hydrological behavior was sometimes consistent with the flood activities in the eastern Mediterranean, and sometimes contemporaneous with the flood pulsations of the western Mediterranean.     

Stratigraphic constraints on late Pleistocene glacial erosion and deglaciation of the Chukchi margin, Arctic Ocean

At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock. The diamictons are estimated to form during the last glacial maximum (LGM) and an earlier glacial event, possibly between OIS 4 to 5d. Both erosional events were presumably caused by the grounding of ice shelves originating from the Laurentide ice sheet. Broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor. Postglacial sedimentation evolved from iceberg-dominated environments to those controlled by sea-ice rafting and marine processes in the Holocene. A prominent minimum in planktonic foraminiferal δ¹⁸O is identified in deglacial sediments at an estimated age near 13,000 cal yr BP. This δ¹⁸O minimum, also reported elsewhere in the Amerasia Basin, is probably related to a major Laurentide meltwater pulse at the Younger Dryas onset. The Bering Strait opening is also marked in the composition of late deglacial Chukchi sediments.     

New relative sea-level curves for the southern Scott Coast,Antarctica: evidence for Holocene deglaciation of the western Ross Sea

Here we present new relative sea-level (RSL) curves developed from Holocene-aged raised beaches along the southern Scott Coast of the western Ross Sea, Antarctica. Fifty-four dates of marine shells, seal skin and elephant seal remains incorporated within raised beaches during storms afford a chronology for these curves. All of the curves show the same pattern and timing of RSL change within a small range of error. The best-dated curve suggests that final unloading of grounded Ross Sea ice from the southern Scott Coast and McMurdo Sound region occurred shortly before 6500 ¹⁴C yr BP. This age is consistent with glacial geological evidence that places deglaciation between 5730 and 8340 ¹⁴C yr BP. Our data strongly suggest that grounding-line retreat of the Ross Sea ice sheet southward through the McMurdo Sound region occurred in mid- and late Holocene time. If this is correct, then rising sea level could not have driven ice recession to the present-day grounding line on the Siple Coast, because global deglacial sea-level rise was essentially accomplished by mid-Holocene time. Copyright © 1999 John Wiley & Sons, Ltd.     

Ice-margin retreat and grounding-zone dynamics during initial deglaciation of the Storfjordrenna Ice Stream,western Barents Sea

Processes occurring at the grounding zone of marine terminating ice streams are crucial to marginal stability, influencing ice discharge over the grounding-line, and thereby regulating ice-sheet mass balance. We present new marine geophysical data sets over a ~30×40 km area from a former ice-stream grounding zone in Storfjordrenna, a large cross-shelf trough in the western Barents Sea, south of Svalbard. Mapped ice-marginal landforms on the outer shelf include a large accumulation of grounding-zone deposits and a diverse population of iceberg ploughmarks. Published minimum ages of deglaciation in this region indicate that the deposits relate to the deglaciation of the Late Weichselian Storfjordrenna Ice Stream, a major outlet of the Barents Sea–Svalbard Ice Sheet. Sea-floor geomorphology records initial ice-stream retreat from the continental shelf break, and subsequent stabilization of the ice margin in outer-Storfjordrenna. Clustering of distinct iceberg ploughmark sets suggests locally diverse controls on iceberg calving, producing multi-keeled, tabular icebergs at the southern sector of the former ice margin, and deep-drafted, single-keeled icebergs in the northern sector. Retreat of the palaeo-ice stream from the continental shelf break was characterized by ice-margin break-up via large calving events, evidenced by intensive iceberg scouring on the outer shelf. The retreating ice margin stabilized in outer-Storfjordrenna, where the southern tip of Spitsbergen and underlying bedrock ridges provide lateral and basal pinning points. Ice-proximal fans on the western flank of the grounding-zone deposits document subglacial meltwater conduit and meltwater plume activity at the ice margin during deglaciation. Along the length of the former ice margin, key environmental parameters probably impacted ice-margin stability and grounding-zone deposition, and should be taken into consideration when reconstructing recent changes or predicting future changes to the margins of modern ice streams.     

Conference on the last deglaciation: Timing and mechanism

For years paleoclimatologists have held the general view that the last deglaciation began around 17,000 to 15,000 yr ago, that the shape of the globally integrated deglacial curve was smoothly sigmoidal with the fastest rate of change centered around 11,000 yr ago, and that the deglaciation ended around 7000 to 5000 yr ago. Recent studies have challenged several aspects of this consensus and have suggested that the mechanisms responsible for the deglaciation are significantly different from those previously proposed. As a result, an international workshop was held at Airlie House in Virginia during May 2–6 of 1983 to evaluate a wide range of evidence relevant to this controversy. The conference results suggested that (1) the decrease in global ice volume occurred in two steps, with the dating of the earlier step still in doubt, but the later step occurring at about 10,000–7000 yr ago and (2) the most likely feedback mechanisms for accelerating the initial forcing by orbital variations are delayed bedrock rebound, marine downdraw/calving, and CO₂ heating.     

Holocene climate and glacial history of the northeastern Antarctic Peninsula: the marine sedimentary record from a long SHALDRIL core

A high-resolution record of Holocene deglacial and climate history was obtained from a 77 m sediment core from the Firth of Tay, Antarctic Peninsula, as part of the SHALDRIL initiative. This study provides a detailed sedimentological record of Holocene paleoclimate and glacial advance and retreat from the eastern side of the peninsula. A robust chronostratigraphy was derived from thirty-three radiocarbon dates on carbonate material. This chronostratigraphic framework was used to establish the timing of glacial and climate events derived from multiple proxies including: magnetic susceptibility, electric resistivity, porosity, ice-rafted debris content, organic carbon content, nitrogen content, biogenic silica content, and diatom and foraminiferal assemblages. The core bottomed-out in a stiff diamicton interpreted as till. Gravelly and sandy mud above the till is interpreted as proximal glaciomarine sediment that represents decoupling of the glacier from the seafloor circa 9400 cal. yr BP and its subsequent landward retreat. This was approximately 5000 yr later than in the Bransfield Basin and South Shetland Islands, on the western side of the peninsula. The Firth of Tay core site remained in a proximal glaciomarine setting until 8300 cal. yr BP, at which time significant glacial retreat took place. Deposition of diatomaceous glaciomarine sediments after 8300 cal. yr BP indicates that an ice shelf has not existed in the area since this time.The onset of seasonally open marine conditions between 7800 and 6000 cal. yr BP followed the deglacial period and is interpreted as the mid-Holocene Climatic Optimum. Open marine conditions lasted until present, with a minor cooling having occurred between 6000 and 4500 cal. yr BP and a period of minor glacial retreat and/or decreased sea ice coverage between 4500 and 3500 cal. yr BP. Finally, climatic cooling and variable sea ice cover occurred from 3500 cal. yr BP to near present and it is interpreted as being part of the Neoglacial. The onset of the Neoglacial appears to have occurred earlier in the Firth of Tay than on the western side of the Antarctic Peninsula. The Medieval Warm Period and Little Ice Age were not pronounced in the Firth of Tay. The breadth and synchroneity of the rapid regional warming and glacial retreat observed in the Antarctic Peninsula during the last century appear to be unprecedented during the Holocene epoch.     

Pattern,style and timing of British–Irish Ice Sheet retreat: Shetland and northern North Sea sector

The offshore sector around Shetland remains one of the least well-studied parts of the former British–Irish Ice Sheet with several long-standing scientific issues unresolved. These key issues include (i) the dominance of a locally sourced ‘Shetland ice cap’ vs an invasive Fennoscandian Ice Sheet; (ii) the flow configuration and style of glaciation at the Last Glacial Maximum (i.e. terrestrial vs marine glaciation); (iii) the nature of confluence between the British–Irish and Fennoscandian Ice Sheets; (iv) the cause, style and rate of ice sheet separation; and (v) the wider implications of ice sheet uncoupling on the tempo of subsequent deglaciation. As part of the Britice-Chrono project, we present new geological (seabed cores), geomorphological, marine geophysical and geochronological data from the northernmost sector of the last British–Irish Ice Sheet (north of 59.5°N) to address these questions. The study area covers ca. 95 000 km², an area approximately the size of Ireland, and includes the islands of Shetland and the surrounding continental shelf, some of the continental slope, and the western margin of the Norwegian Channel. We collect and analyse data from onshore in Shetland and along key transects offshore, to establish the most coherent picture, so far, of former ice-sheet deglaciation in this important sector. Alongside new seabed mapping and Quaternary sediment analysis, we use a multi-proxy suite of new isotopic age assessments, including 32 cosmogenic-nuclide exposure ages from glacially transported boulders and 35 radiocarbon dates from deglacial marine sediments, to develop a synoptic sector-wide reconstruction combining strong onshore and offshore geological evidence with Bayesian chronosequence modelling. The results show widespread and significant spatial fluctuations in size, shape and flow configuration of an ice sheet/ice cap centred on, or to the east of, the Orkney–Shetland Platform, between ~30 and ~15 ka BP. At its maximum extent ca. 26–25 ka BP , this ice sheet was coalescent with the Fennoscandian Ice Sheet to the east. Between ~25 and 23 ka BP the ice sheet in this sector underwent a significant size reduction from ca. 85 000 to <50 000 km², accompanied by several ice-margin oscillations. Soon after, connection was lost with the Fennoscandian Ice Sheet and a marine corridor opened to the east of Shetland. This triggered initial (and unstable) re-growth of a glaciologically independent Shetland Ice Cap ca. 21–20 ka BP with a strong east–west asymmetry with respect to topography. Ice mass growth was followed by rapid collapse, from an area of ca. 45 000 km² to ca. 15 000 km² between 19 and 18 ka BP , stabilizing at ca. 2000 km² by ~17 ka BP. Final deglaciation of Shetland occurred ca. 17–15 ka BP , and may have involved one or more subsidiary ice centres on now-submerged parts of the continental shelf. We suggest that the unusually dynamic behaviour of the northernmost sector of the British–Irish Ice Sheet between 21 and 18 ka BP – characterized by numerous extensive ice sheet/ice mass readvances, rapid loss and flow redistributions – was driven by significant changes in ice mass geometry, ice divide location and calving flux as the glaciologically independent ice cap adjusted to new boundary conditions. We propose that this dynamism was forced to a large degree by internal (glaciological) factors specific to the strongly marine-influenced Shetland Ice Cap.     

Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel,Antarctic Peninsula

The recent disintegration of Antarctic Peninsula ice shelves, and the associated accelerated discharge and retreat of continental glaciers, has highlighted the necessity of quantifying the current rate of Antarctic ice mass loss and the regional contributions to future sea-level rise. Observations of present day ice mass change need to be corrected for ongoing glacial isostatic adjustment, a process which must be constrained by geological data. However, there are relatively little geological data on the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene so the glacial isostatic correction remains poorly constrained. To address this we provide field constraints on the timing and rate of APIS deglaciation, and changes in relative sea-level (RSL) for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea-level fell from a maximum of c. 15 m above present at c. 8000 cal yr BP, at a rate of 3.91 mm yr^?1 declining to c. 2.11 mm yr^?1 between c. 6900–2900 cal yr BP, 1.63 mm yr^?1 between c. 2900–1800 cal yr BP, and finally to 0.29 mm yr^?1 during the last c. 1800 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic. It is in broad agreement with some glacio-isostatic adjustment models applied to this location, and with work undertaken elsewhere on the Antarctic Peninsula. These geological and RSL constraints from Beak Island imply significant thinning of the north-eastern APIS by the early Holocene. Further, they provide key data for the glacial isostatic correction required by satellite-derived gravity measurements of contemporary ice mass loss, which can be used to better assess the future contribution of the APIS to rising sea-levels.     

Lateglacial ice-cap dynamics in NW Scotland: evidence from the fjords of the Summer Isles region

The seaboard of western Scotland is a classic fjord landscape formed by glaciation over at least the last 0.5 Ma. We examine the glacial geology preserved in the fjords (or sea lochs) of the Summer Isles region of NW Scotland using high-resolution seismic data, multibeam swath bathymetry, seabed sediment cores, digital terrain models, aerial photographs, and field investigations. Detailed analyses include seismic facies and lithofacies interpretations; sedimentological and palaeoenvironmental analyses; and radiocarbon dating of selected microfauna. Our results indicate that the Pleistocene sediments of the Summer Isles region, on- and offshore, can be subdivided into several lithostratigraphic formations on the basis of seismic character, geomorphology and sedimentology. These are: subglacial tills; ice-distal and glacimarine facies; ice-proximal and ice-contact facies; moraine assemblages; and Holocene basin fill. The submarine landscape is also notable for its large-scale mass-movement events – the result of glaciodynamic, paraglacial or seismotectonic processes. Radiocarbon dating of marine shells indicate that deglaciation of this part of NW Scotland was ongoing between 14 and 13 ka BP – during the Lateglacial Interstadial (Greenland Interstadial 1) – consistent with cosmogenic surface-exposure ages from previous studies. A sequence of numerous seafloor moraine ridges charts oscillatory retreat of the last ice sheet from a buoyant calving margin in The Minch to a firmly grounded margin amongst the Summer Isles in the early part of Lateglacial Interstadial (GI-1) (pre-14 ka BP). Subsequent, punctuated, frontal retreat of the ice mass occurred in the following ～1000 years, during which time ice-cap outlet glaciers became topographically confined and restricted to the fjords. A late-stage readvance of glaciers into the inner fjords occurred soon after 13 ka BP, which calls into question the accepted limits of ice extent during the Younger Dryas Stadial (Greenland Stadial 1). We examine the wider implications of our chronostratigraphic model, discussing the implications for British Ice Sheet deglaciation, Lateglacial climate change, and the style and rates of fjord sedimentation.     

Thickness evolution of the Scandinavian Ice Sheet during the Late Weichselian in Nordfjord, western Norway: evidence from ice-flow modeling

Winguth, C., Mickelson, D. M., Larsen, E., Darter, J. R., Moeller, C. A. & Stalsberg, K. 2005 (May): Thickness evolution of the Scandinavian Ice Sheet during the Late Weichselian in Nordfjord, western Norway: evidence from ice-flow modeling. Boreas , Vol. 34, pp. 176–185. Oslo. ISSN 0300–9483.
Results from experiments with a two-dimensional ice-flow model, applied along a west-east transect in western Norway, provide new constraints on the thickness evolution of the Scandinavian Ice Sheet throughout the Late Weichselian glaciation and deglaciation. Investigations took place along an E-W flowline of the former ice sheet at c. 62N, from the modern glacier Jostedalsbreen, through the Nordfjord, and across the continental shelf. A paleoclimate record from Kråkenes, which is located directly at the flowline, provides temperature and precipitation information for the time between 13 800 and 9200 cal. yr BP. LGM climate conditions for the study area are estimated from various GCM studies. The GISP2 δ¹⁸O record has been tuned to the local data in order to provide a continuous temperature record as input for time-transgressive model runs. The results of all experiments suggest that the ice did not cover the highest mountain peaks in this area, and that nunataks persisted throughout the Late Weichselian glaciation. These findings are in contrast to results from many previous model studies and other ice-sheet reconstructions, but agree well with minimum thickness estimates from cosmogenic dating and with vertical ice limits inferred from lower block field boundaries and trimlines.