A stratigraphic investigation of the Celtic Sea megaridges based on seismic and core data from the Irish-UK sectors

The Celtic Sea contains the world's largest continental shelf sediment ridges. These megaridges were initially interpreted as tidal features formed during post-glacial marine transgression, but glacigenic sediments have been recovered from their flanks. We examine the stratigraphy of the megaridges using new decimetric-resolution geophysical data correlated to sediment cores to test hypothetical tidal vs glacial modes of formation. The megaridges comprise three main units, 1) a superficial fining-upward drape that extends across the shelf above an unconformity. Underlying this drape is 2), the Melville Formation (MFm) which comprises the upper bulk of the megaridges, sometimes displaying dipping internal acoustic reflections and consisting of medium to coarse sand and shell fragments; characteristics consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously interpreted to be of late Pliocene to early Pleistocene age, but here shown to correlate to Late Pleistocene glacigenic sediments forming a precursor topography. The superficial drape is interpreted as a product of prolonged wave energy as tidal currents diminished during the final stages of post-glacial marine transgression. We argue that the stratigraphy constrains the age of the MFm to between 24.3 and 14 ka BP, based on published dates, coeval with deglaciation and a modelled period of megatidal conditions during post-glacial marine transgression. Stratigraphically and sedimentologically, the megaridges could represent preserved glacifluvial features, but we suggest that they comprise post-glacial tidal deposits (MFm) mantling a partially-eroded glacial topography (ULSFm). The observed stratigraphy suggests that ice extended to the continental shelf-edge

The advance and retreat of the Irish Sea Ice Stream in the Celtic Sea and its influence on shelf evolution

International audienceThe reconstruction of the largest ice stream to drain the British-Irish Ice Sheet at the Last Glacial Maximum (LGM) can provide essential palaeoglacial observations required for constraining numerical ice sheet models. The Irish Sea Ice Stream (ISIS) was long considered to have terminated on the mid-shelf of the Celtic Sea, based on sediment cores and seismic data collected in the 1970s. Here we summarise findings from sediment cores and geophysical data acquired since 2009, and multi-beam bathymetric data acquired since 2001, which permit an updated evolution and palaeoglacial reconstruction of the Irish and UK sectors of the Celtic Sea shelf. In near-shore areas, multi-beam data reveal over 2000 glacial features, including moraine ridges, streamlined bedrock and meltwater channels, recording the southwest advance of the ISIS towards the shelf-edge and its subsequent retreat. The mid- to outer-shelf is characterised by the largest known linear shelf sediment ridges. These vary from long and linear features, the megaridges, in the northwest to sinuous and shorter ridges in the southeast. This ridge field was initially interpreted as tidal in origin, but glacigenic sediments have been recovered from the flanks of the megaridges. Correlating decimetric-resolution geophysical data to sediment cores, the megaridges comprise three main units. 1) A superficial fining-upward drape above an unconformity, inferred to record decreasing ocean energy during marine transgression. Underlying this drape is 2), the Melville Formation (MFm), which comprises the upper bulk of the megaridges, displaying dipping internal acoustic reflections and consisting of medium to coarse sand and gravel, characteristics that could be consistent with either a tidal or glacifluvial origin. The MFm unconformably overlies 3), the Upper Little Sole Formation (ULSFm), previously proposed to be of late Pliocene to early Pleistocene age, but is here shown to contain glacigenic sediments dated to the LGM. This stratigraphy constrains the age of the MFm to between 24-14 ka BP, coeval with deglaciation and a modelled period of megatidal conditions during transgression. Stratigraphically and sedimentologically these megaridges could represent glacifluvial features eroded during the post-glacial marine transgression. However, it is argued that they comprise a partially-eroded glacial topography (ULSFm) mantled by post-glacial tidal deposits (MFm), both subsequently eroded by a proposed mechanism of enhanced wave energy during decreasing tidal energy in the later stages of transgression. Regardless of the origin of the ridges, the evidence shows that the ISIS extended to the shelf-edge of the Irish and UK sectors during the LGM

Oscillating retreat of the last British-Irish Ice Sheet on the continental shelf offshore Galway Bay, western Ireland

During the Last Glacial Maximum, the British-Irish Ice Sheet extended across the continental shelf offshore of Galway Bay, western Ireland, and reached a maximum westward extent on the Porcupine Bank. New marine geophysical data, sediment cores and radiocarbon dates are used to constrain the style and timing of ice-sheet retreat across the mid to inner-shelf. Radiocarbon dated shell fragments in subglacial till on the mid-shelf constrains ice advance to after 26.4 ka BP. Initial retreat was underway before 24.4 ka BP, significantly earlier than previous reconstructions. Grounding-line retreat was accompanied by stillstands and/or localised readvances of the grounding-line. A large composite Mid-Shelf Grounding Zone Complex marks a major grounding-line position, with the ice grounded and the margin oscillating at this position by, and probably after, 23 ka BP. The continental shelf was ice-free by 17.1 cal. ka BP, but the ice sheet may have retained a marine margin until c. 15.3 ka BP. Retreat occurred in a glacimarine setting and the ice sheet was fringed by a floating ice-shelf. Collectively, this evidence indicates a dynamic and oscillatory marine-terminating ice sheet offshore of western Ireland during the last deglaciation

Advance and retreat of the marine-terminating Irish Sea Ice Stream into the Celtic Sea during the last glacial: Timing and maximum extent

The dynamics of the British-Irish Ice Sheet (BIIS) during the Last Glacial were conditioned by marine-based ice streams, the largest of which by far was the Irish Sea Ice Stream (ISIS) which drained southwest across the Celtic shelf. The maximum extent and timing of the ISIS have been constrained by onshore evidence from the UK and Ireland, and by glacigenic sediments encountered in a small suite of vibrocores from the UK-Irish continental shelf, from which a single radiocarbon date is available. These data have long supported ice advance to at least the mid-shelf, while recent results suggest the ISIS may have extended 150 km farther seaward to the shelf edge. The glacigenic sequences have not been placed within a secure seismic-stratigraphic context and the relationship between glaciation and the linear sediment megaridges observed on the outer shelf of the Celtic Sea has remained uncertain. Here we report results of sedimentological, geochemical, geochronological and micropalaeontological analyses combined with a seismic-stratigraphic investigation of the glacigenic sequences of the Celtic Sea with the aims of establishing maximum extent, depositional context, timing and retreat chronology of ISIS. Eight lithofacies packages are identified, six of which correlate with seismic facies. Lithofacies LF1 and LF2 correlate to a seafloor seismic facies (SF1) that we interpret to record the postglacial and Holocene transgressive flooding of the shelf. Lithofacies LF10 (till), LF3, LF4 and LF8 (glacimarine) correlate to different seismic facies that we interpret to be of glacigenic origin based on sedimentological, geotechnical and micropalaeontological evidence, and their distribution, supported by geochemical evidence from lithofacies LF8 and LF10 indicate extension of ISIS as far as the Celtic Sea shelf break. New radiocarbon ages on calcareous micro- and macrofauana constrain this advance to be between 24 and 27 cal ka BP, consistent with pre-existing geochronological constraints. Glacimarine lithofacies LF8 is in places glacitectonically contorted and deformed, indicating ice readvance, but the nature and timing of this readvance is unclear. Retreat out of the Celtic Sea was initially rapid and may have been triggered by high relative sea-levels driven by significant glacio-isostatic depression, consistent with greater ice loads over Britain and Ireland than previously considered

Maximum extent and readvance dynamics of the Irish Sea Ice Stream since the Last Glacial Maximum

International audienceThe Irish Sea Ice Stream (ISIS) has long had one of the best documented retreat histories of the British-Irish Ice Sheet (BIIS) and was the first ice stream to be constrained by Bayesian analysis of geochronological data. These attributes made it a model system for the BRITICE-CHRONO research project, which aims to produce the best constrained retreat record of any palaeo-ice sheet contributing key observational constraints for ice sheet modelling. The project has generated a suite of new radiocarbon ages from deglacial sequences offshore in the Celtic and Irish seas and terrestrial cosmogenic nuclide and optically-stimulated luminescence ages from ice-marginal sites in the Isles of Scilly, Ireland, Wales and NW England. The ISIS was unusual within the former BIIS, in that it was a compound ice stream with two outlets, one marine terminating that flowed through the Irish Sea Basin into the Celtic Sea, and a terrestrial terminus that flowed southwards through Cheshire-Shropshire lowlands into the English Midlands around 25.5 ka. Here we assess the retreat dynamics across the entirety of the ISIS, integrating the new chronology in a revised Bayesian analysis that constrains the pattern and timing ice marginal fluctuations. The retreat chronology in the Irish Sea is better constrained than in the Celtic Sea, where the ISIS is now recognised to have extended as far as the continental shelf break to the SW of Britain and Ireland between 24 and 27 ka; this advance was synchronous with independently-dated ice-rafted detritus from ISIS in adjacent deep-sea cores. The ISIS then retreated rapidly northwards through the Celtic Sea, with evidence for readvance phases, deglaciating the Isles of Scilly at 25.5 ka, reaching St Georges Channel by 24.3 ka and the Llŷn Peninsula by 23.9 ka. The initiation of retreat from both the eastern (terrestrial) and western (marine) components of ISIS was synchronous. The eastern terrestrial lobe had vacated the Cheshire-Shropshire lowlands by 22-21 ka. The complex readvance sequences identified on the Llŷn (24-20ka) and in eastern Ireland have now been tightly constrained to register centennial-scale oscillations of the ice front driven by internal ice dynamics over topographic pinning points and constrictions of the ice-stream. Retreat northwards into the northern Irish Sea then accelerated, first evacuating the deeper water of the western Irish Sea, and developing pronounced ice margins across the northern Isle of Man by 19.1 ka. The final retreat phase, with ice margins pulling back onto terrestrial settings in the English Lake District, the north of Ireland and SW Scotland around 17 ka, was a deglaciation accomplished in a fully marine context evidenced by the preservation on the seabed of subglacial landforms and by increasing influence of local ice sources with flow realignment during draw-down and ice margin retreat

Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE‐CHRONO reconstruction

The BRITICE-CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British–Irish Ice Sheet between 31 000 and 15 000 years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500 days of field investigation yielding 18 000 km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet-wide reconstructions consistent with these findings and using retreat patterns and dates for the inter-transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand-year timesteps are presented. The maximum ice volume of 1.8 m sea level equivalent occurred at 23 ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22 ka. The tipping point of deglaciation at 22 ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine-influenced sectors collapsed rapidly. First order controls on ice-sheet demise were glacio-isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector-scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British–Irish Ice Sheet is now the world’s most well-constrained and a valuable data-rich environment for improving ice-sheet modelling