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.
•New geomorphology and chronology constrain ice-margin retreat offshore western Ireland.•Early deglaciation from a maximum position was underway by 24.4 ka BP.•Retreat was not continuous with evidence of an oscillating ice-margin.•Internal glaciodynamics mechanism related to sediment delivery at the grounding-line was a key control on retreat/advance.
A Holocene sand/silt-filled tidal creek, locally called a roddon, excavated at Must Farm near Whittlesey, Cambridgeshire, within the English Fenland, preserves an unprecedented record of Fenland ...tidal creek evolution. The tidal creek was formed sometime between c. 4735 BP and c. 3645 BP. Its infill comprised centimetre-scale, laminated, tidal sand-mud couplets, numbering some 600 in total, with microfossils that show marine and coastal derivation of the sediment. The nature of the infill suggests rapid sedimentation that choked the creek system, perhaps in as little as a few years. At least three successive generations of roddons are present in the Fenland, suggesting at least three distinct episodes of tidal creek formation. The Must Farm roddon belongs to the first generation. A later, mud-filled channel was subsequently incised into the roddon, perhaps initiated by a marine surge. It quickly transformed into a slowly flowing river, as suggested by molluscs, ostracods and diatoms. Infill of this late-stage channel was slow, perhaps over about 1200 years between c. 3250 and 2050 BP (c. 1300-100 cal BC), as shown by archaeological finds. The mud-filled channel superficially resembles late-stage, organic-rich channel infills visible on aerial photographs, but is wider and deeper and might have been part of a more substantial long-lived "river", perhaps a tributary of the proto-Nene.
Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast‐flowing glaciers or ice streams, buffered by ...ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice‐stream‐dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km2 of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic‐nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector‐wide ice‐sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice‐sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice‐sheet sector undergoing large size variations in response to sub‐millennial‐scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine‐terminating ice‐sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice‐sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice‐sheet build‐up in NW Scotland, with the Minch ice‐stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding‐zone wedges and moraines on the mid‐ and inner continental shelf attest to significant stabilizations of the ice‐sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present‐day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea‐level c. 26 kabp, followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice‐shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp, as the grounding‐line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding‐line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp, ice‐sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine‐instability‐triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid‐Greenland Stadial 2, with an ice‐accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice‐sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp, before significant ice‐mass losses thereafter. At the glacial termination, c. 14.5 kabp, glaciers fed outwash sediment to now‐abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice‐sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice‐sheet glaciation dominated by ice‐streaming were interspersed with periods of much more restricted ice‐cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice‐sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice‐sheet health but also to the acceleration of ice‐sheet demise via marine ice‐sheet instabilities.
Lake sediments have the potential to preserve proxy records of past climate change. Organic material suitable for radiocarbon dating often provides age control of such proxy records. Six shallow ...freshwater lakes on the sub-Antarctic island of South Georgia were investigated for carbon reservoir effects that may influence age-depth profiles from lake sediment records in this important region. Paired samples of particulate organic matter (POM) from the water column and surface sediment (bulk organic carbon) were analyzed by accelerator mass spectrometry 14C. POM in 4 lakes was found to be in equilibrium with the atmosphere (~107% modern), whereas 2 lakes showed significant depletion of 14C. In each lake, the surface sediment ages were older than the paired POM age. Surface sediment ages showed a much greater range of ages compared to the equivalent POM ages, even for lakes located in close proximity. We conclude that sediment disturbance during coring, bioturbation, and periodic resuspension of sediments are likely factors causing the difference in the apparent age of surface sediments.
Kongsfjorden and Krossfjorden are two ice‐proximal fjords on the western coast of Spitsbergen which have been surveyed using multibeam bathymetry, sub‐bottom profiling and gravity coring. Central and ...outer Kongsfjorden is dominated by a 30 km2 outcrop of bedrock, with a thin (<10m) sediment cover. The bedrock displays a relict sub‐glacial, ice‐scoured topography produced during the glacial re‐advances of the Weichselian (20 Ky BP) and again during the last major Holocene re‐advance of the Little Ice Age (550‐200 yrs BP). Drumlins and glacial flutes are common across the floor of Kongsfjorden, with lengths of 1.5‐2.5 km and widths of <100 m, rising up to 10 m in water depths of <100 m. This topography is smoothed by bottom currents from the wind‐driven forcing of surface waters. The flow is counter‐clockwise, matching boundary layer movement under the influence of Coriolis force. Both fjords are characterized by a variable acoustic character, based on sub‐bottom profile data. The deepest basins are dominated by parallel, well‐laminated reflectors and an irregular‐transparent acoustic character indicating the presence of Holocene‐age fine‐grained sediments up to 30 m thick. A parallel, irregular‐transparent acoustic character with waveform morphology in inner Kongsfjorden is interpreted as moraines, originating from the 1948 and 1869 surges of Kronebreen glacier. Mass‐flows are common on the flanks of topographic highs as acoustically chaotic‐transparent lensoid and wedge‐shaped reflectors. The sediments of outer and central Kongsfjorden are characterized by bioturbated, gas‐rich homogeneous muds interpreted as being the result of the settling of fine‐grained sediment and particulate suspensions.
Kongsfjorden and Krossfjorden are two ice-proximal fjords on the western coast of Spitsbergen which have been surveyed using multibeam bathymetry, sub-bottom profi ling and gravity coring. Central ...and outer Kongsfjorden is dominated by a 30 km
2
outcrop of bedrock, with a thin (< 10 m) sediment cover. The bedrock displays a relict sub-glacial, ice-scoured topography produced during the glacial re-advances of the Weichselian (20 Ky BP) and again during the last major Holocene re-advance of the Little Ice Age (550 - 200 yrs BP). Drumlins and glacial fl utes are common across the fl oor of Kongsfjorden, with lengths of 1.5 - 2.5 km and widths of < 100 m, rising up to 10 m in water depths of < 100 m. This topography is smoothed by bottom currents from the wind-driven forcing of surface waters. The fl ow is counter-clockwise, matching boundary layer movement under the infl uence of Coriolis force. Both fjords are characterized by a variable acoustic character, based on sub-bottom profi le data. The deepest basins are dominated by parallel, well-laminated refl ectors and an irregular-transparent acoustic character indicating the presence of Holocene-age fi ne-grained sediments up to 30 m thick. A parallel, irregular-transparent acoustic character with waveform morphology in inner Kongsfjorden is interpreted as moraines, originating from the 1948 and 1869 surges of Kronebreen glacier. Mass-fl ows are common on the fl anks of topographic highs as acoustically chaotic-transparent lensoid and wedge-shaped refl ectors. The sediments of outer and central Kongsfjorden are characterized by bioturbated, gas-rich homogeneous muds interpreted as being the result of the settling of fi ne-grained sediment and particulate suspensions.
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