Bayesian modelling the retreat of the Irish Sea Ice Stream, Chiverrell, R. et al


ABSTRACT: Accurate reconstructions of ice sheet evolution through time depend on the quality of the available   geochronological data. Increasingly, data compilations are being used to build such reconstructions,   necessitating data quality judgements to be made upon the reliability and suitability of any given age(s)   for constraining deglaciation. This is likely a complex and controversial process and, as such, requires a   high degree of transparency. It is also a process that has received little attention in the literature and, to   our knowledge, has not been formalised before. This contribution outlines the quality assurance   undertaken as part of the BRITICE-CHRONO project. We develop protocols for assessing the suitability   of data for a given research question and assign criteria that make an objective assessment of the   likelihood that an age is influenced by the main and technique specific sources of geological uncertainty.   These criteria were applied to an extant database of all geochronological data relating to the last British-  Irish Ice Sheet and resulted in a significant reduction in data considered suitable for synthesis. A sample   of this reduced data set is used to test a Bayesian approach to modelling ice stream retreat and shows   good agreement with previously published results. This highlights the value of a Bayesian approach to   focus efforts in subsequent collection of geochronological data.

Glacial Lake Pickering: stratigraphy and chronology of a proglacial lake dammed by the North Sea Lobe ofthe British–Irish Ice Sheet, Evans, D. et al


ABSTRACT:  We report the first chronology, using four new optically stimulated luminescence dates, on the sedimentary record of Glacial Lake Pickering, dammed by the North Sea Lobe of the British–Irish Ice Sheet during the Dimlington Stadial (24–11 ka cal BP). Dates range from 17.6± 1.0 to 15.8 ±0.9 ka for the sedimentation of the Sherburn Sands at East Heslerton, which were formed by multiple coalescing alluvial fans prograding into the falling water levels of the lake and fed by progressively larger volumes of debris from the Wolds. Fan formation ceased ~ 15.8 ka, at a time when permafrost was degrading and nival-fed streams were no longer capable of supplying sediment to the fans. A further age of 10.1 ±0.7 ka dates the reworking of coversand into the early part of the Holocene, immediately post-dating Younger Dryas periglacial structures. A 45-m lake level dates to ~ 17.6 ka, when the North Sea Lobe was already in retreat, having moved eastward of the Wykeham Moraine; it stood further east at the Flamborough Moraine by ~ 17.3 ka. The highest (70m) lake level and the occupation of the Wykeham Moraine date to an earlier phase of the North Sea Lobe occupation of the Vale of Pickering.

Rapid ice sheet retreat triggered by ice stream debuttressing:  Evidence from the North Sea, Sejrup, H-P. et al

Geological Society of America

ABSTRACT:  Using high-resolution bathymetric and shallow seismic data from the North Sea, we have mapped hitherto unknown glacial landforms that connect and resolve longstanding gaps in the Quaternary geological history of the basin. We use these data combined with published information and dates from sediment cores to reconstruct the extent of the Fennoscandian and British Ice Sheets (FIS and BIS) in the North Sea during the last phases of the last glacial stage. It is concluded that the BIS occupied a much larger part of the North Sea than previously suggested and that North Sea ice underwent a dramatic disintegration ~18,500 yr ago. This was triggered by grounding-line retreat of the Norwegian Channel Ice Stream, which debuttressed adjacent ice masses, and led to an unzipping of the BIS and FIS accompanied by drainage of a large ice-dammed lake. Our reconstruction of events provides an opportunity to improve understanding and modeling of the disintegration of marine-based ice sheets, and the complex interplay between ocean circulation and the cryosphere.

 Morphological and sedimentary responses to ice mass interaction during the last deglaciation, Chiverrell, R. et al


ABSTRACT:  During decline of the last British–Irish Ice Sheet (BIIS) down-wasting of ice meant that local sources played a larger role in regulating ice flow dynamics and driving the sediment and landform record. At the Last Glacial Maximum, glaciers in north-western England interacted with an Irish Sea Ice Stream (ISIS) occupying the eastern Irish Sea basin (ISB) and advanced as a unified ice-mass. During a retreat constrained to 21–17.3 ka, the sediment landform assemblages lain down reflect the progressive unzipping of the ice masses, oscillations of the ice margin during retreat, and then rapid wastage and disintegration. Evacuation of ice from the Ribble valley and Lancashire occurred first while the ISIS occupied the ISB to the west, creating ice-dammed lakes. Deglaciation, complete after 18.6– 17.3 ka, was rapid (50–25ma_1), but slower than rates identified for the western ISIS (550–100ma_1). The slower pace is interpreted as reflecting the lack of a calving margin and the decline of a terrestrial, grounded glacier. Ice marginal oscillations during retreat were probably forced by ice-sheet dynamics rather than climatic variation. These data demonstrate that large grounded glaciers can display complex uncoupling and realignment during deglaciation, with asynchronous behaviour between adjacent ice lobes generating complex landform records.

Submarine sediment and landform record of a palaeo-ice stream within the British−Irish Ice Sheet, Bradwell T. & Stoker, M.


ABSTRACT:  This paper examines marine geophysical and geological data, and new multibeam bathymetry data to describe the Pleistocene sediment and landform record of a large ice-stream system that drained ∼3% of the entire British−Irish Ice Sheet at its maximum extent. Starting on the outer continental shelf NW of Scotland we describe: the ice-stream terminus environment and depocentre on the outer shelf and continental slope; sediment architecture and subglacial landforms on the mid-shelf and in a large marine embayment (the Minch); moraines and grounding line features on the inner shelf and in the fjordic zone. We identify new soft-bed (sediment) and hard-bed (bedrock) subglacial landform assemblages in the central and inner parts of the Minch that confirm the spatial distribution, coherence and trajectory of a grounded fast-flowing ice-sheet corridor. These include strongly streamlined bedrock forms and megagrooves indicating a high degree of ice-bed coupling in a zone of flow convergence associated with ice-stream onset; and a downstream bedform evolution (short drumlins to km-scale glacial lineations) suggesting an ice-flow velocity transition associated with a bed substrate and roughness change in the ice-stream trunk. Chronology is still lacking for the timing of ice-stream demise; however, the seismic stratigraphy, absence of moraines or grounding-line features, and presence of well-preserved subglacial bedforms and iceberg scours, combined with the landward deepening bathymetry, all suggest that frontal retreat in the Minch was probably rapid, via widespread calving, before stabilization in the nearshore zone. Large moraine complexes recording a coherent, apparently long-lived, ice-sheet margin position only 5–15 km offshore strongly support this model. Reconstructed ice-discharge values for the Minch ice stream (12–20 Gt a−1) are comparable to high mass-flux ice streams today, underlining it as an excellent palaeo-analogue for recent rapid change at the margins of the Greenland and West Antarctic Ice Sheets.

Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland, Peters, J et al

Quaternary Science Reviews (2016) 140, 101-124

.Abstract:  The last British-Irish Ice Sheet (BIIS) had extensive marine-terminating margins and was drained by multiple large ice streams and is thus a useful analogue for marine-based areas of modern ice sheets. However, despite recent advances from investigating the offshore record of the BIIS, the dynamic history of its marine margins, which would have been sensitive to external forcing(s), remain inadequately understood. This study is the first reconstruction of the retreat dynamics and chronology of the western, marine-terminating, margin of the last (Late Midlandian) BIIS. Analyses of shelf geomorphology and core sedimentology and chronology enable a reconstruction of the Late Midlandian history of the BIIS west of Ireland, from initial advance to final retreat onshore. Five AMS radiocarbon dates from marine cores constrain the timing of retreat and associated readvances during deglaciation. The BIIS advanced without streaming or surging, depositing a bed of highly consolidated subglacial traction till, and reached to within ~20 km of the shelf break by ~24,000 Cal BP. Ice margin retreat was likely preceded by thinning, grounding zone retreat and ice shelf formation on the outer shelf by ~22,000 Cal BP. This ice shelf persisted for _2500 years, while retreating at a minimum rate of ~24 m/yr and buttressing a >150-km long, 20-km wide, bathymetrically-controlled grounding zone. A large (~150 km long), arcuate, flattopped grounding-zone wedge, termed here the Galway Lobe Grounding-Zone Wedge (GLGZW), was deposited below this ice shelf and records a significant stillstand in BIIS retreat. Geomorphic relationships indicate that the BIIS experienced continued thinning during its retreat across the shelf, which led to increased topographic influence on its flow dynamics following ice shelf break up and grounding zone retreat past the GLGZW. At this stage of retreat the western BIIS was comprised of several discrete, asynchronous lobes that underwent several readvances. Sedimentary evidence of dilatant till deposition suggests that the readvances may have been rapid and possibly associated with ice streaming or surging. The largest lobe extended offshore from Galway Bay and deposited the Galway Lobe Readvance Moraine by <18,500 Cal BP. Further to the north, an ice lobe readvanced at least 50 km offshore from Killary Harbour, possibly by _15,100 Cal BP. The existing chronology currently does not allow us to determine conclusively whether these readvances were a glaciodynamic (internally-driven) response of the ice sheet during deglaciation or were climatically-driven. Following the <18,500 Cal BP readvance, the Galway Lobe experienced accelerated eastward retreat at an estimated rate of ~113 m/yr.

Colour Atlas of Glacial Phenomena, Hambey, M. & Alean, J.

CRC Press (2016)

In a world where human civilization is increasingly impacted by the effects of changing glacial activity, Colour Atlas of Glacial Phenomena presents itself as an indispensable guide for students, professionals, and researchers who want to be better informed while studying and tracking the future influences of glaciers and ice sheets on the global environment. While stressing both the beauty and utility of glaciers, the authors cover critical features of glaciers and their landforms and provide useful explanations of the key concepts in glaciology and glacial geology.

New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly, Smedley, R. et al


ABSTRACT: The southernmost terrestrial extent of the Irish Sea Ice Stream (ISIS), which drained a large proportion of the last British–Irish Ice Sheet, impinged on to the Isles of Scilly during Marine Isotope Stage 2. However, the age of this ice limit has been contested and the interpretation that this occurred during the Last Glacial Maximum (LGM) remains controversial. This study reports new ages using optically stimulated luminescence (OSL) dating of outwash sediments at Battery, Tresco (25.5± 1.5 ka), and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on Scilly Rock (25.9± 1.6 ka), which confirm that the ISIS reached the Isles of Scilly during the LGM. The ages demonstrate this ice advance on to the northern Isles of Scilly occurred at ~ 26 ka around the time of increased ice-rafted debris in the adjacent marine record from the continental margin, which coincided with Heinrich Event 2 at ~24 ka. OSL dating (19.6± 1.5 ka) of the post-glacial Hell Bay Gravel at Battery suggests there was then an ~5-ka delay between primary deposition and aeolian reworking of the glacigenic sediment, during a time when the ISIS ice front was oscillating on and around the Ll^yn Peninsula, ~390km to the north.