Description: The BRITICE-CHRONO project measured 639 new geochronometric ages that constrain the timing of advance and retreat of the last British-Irish Ice Sheet between 31,000 and 15,000 years ago, including across the North Sea. These ages (optically stimulated luminescence, radiocarbon and terrestrial cosmogenic nuclide methods) are made available in an excel spreadsheet, along with all sample and laboratory metadata and calibrations. Together with other published information, the ages were used to build an empirical ice sheet reconstruction at one thousand year time-steps. A poster-map and slideshow (PDF) of the reconstruction (31 to 15 ka) and the underlying GIS data (ArcGIS shapefiles) of ice extents (min, max and optimum) are made available here. An ice sheet model was nudged to fit these ice limits and the ensuing model-reconstruction is made available as a poster-map, slideshow (PDF) and movie (GIF) of the reconstruction (31 to 15 ka). The GIS data is also available including grounded ice extent and ice shelves, ice thickness, ice surface elevation, and ice velocity (as ArcGIS grids). From glacio-isostatic adjustment modelling we also provide digital elevation models of the palaeotopography of the British Isles and surrounding sea floors and coastline positions from 36 to 1 ka (ArcGIS grids and shapefiles). Full methods, descriptions, caveats and interpretations are available in the parent paper to this dataset: Clark. C.D. et al. (2022) Growth and retreat of the last British-Irish Ice Sheet, 31,000 to 15,000 years ago: the BRITICE-CHRONO reconstruction, Boreas.

Description: The reconstruction of past ice sheet dynamics can inform on long-term ice stream activity, and in turn provide constraints on the response of modern ice sheets to climate change. The Hebrides Ice Stream (HIS) flowed across part of the western Scottish shelf to the shelf-break during the last glacial cycle. To investigate the deglacial dynamics of the HIS following the Last Glacial Maximum (LGM), lead (Pb) isotope records were extracted from the FeMn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of HIS during its retreat. These provide, respectively, relative timing of glacially weathered inputs to the marine environment and some source information on the eroded sediments. The FeMn oxyhydroxide fraction is dominated by pre-formed particles and shows a marked decrease from radiogenic at ~ 21 cal ka to less radiogenic Pb isotope compositions towards 15.4–13 ka. This decrease represents a reduction in the flux of subglacially-derived radiogenic Pb to the continental shelf, and it is interpreted as the result of the break-up of the ice-stream in western Scotland around that time. The Pb, Sr and Nd isotopic signatures of the detrital fraction indicate a preponderance of fine sediments originated from the NW Highlands throughout the period studied (~ 21 to 15 cal ka BP), most likely dictated by the orientation of tidal and oceanic current directions and sediment delivery. Both fractions show inversion of the 208Pb/204Pb ratio relative to the other Pb isotope ratios. This is observed only in one core site in the detrital fraction, and extended to all cores in the FeMn oxyhydroxide fraction. This behaviour highlights the influence of ocean currents in restricting the detrital but encouraging dispersal of the FeMn oxyhydroxide signal. Periodic increased contributions from a high Th/U source, potentially the neighbouring Archaean amphibolitic Lewisian basement in the Outer Hebrides, are proposed as the source of these 208Pb/204Pb inversions. This study demonstrates how geochemical investigation on continental shelves can be used to constrain the activity and flow sources of palaeo-ice streams, and the utility of combining detrital and FeMn oxyhydroxides to determine the combined influence of the continental sources of material and their dispersal in the marine environment.

Description: In four rivers in western Scotland for which there is a well-constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantified by measuring the concentration of in situ cosmogenic 10Be on strath terraces downstream of headward-retreating knickpoints. Along-channel gradients in 10Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a deficiency in "tools" for bedrock erosion. Our results imply that episodes of major fluvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.

Description: Paleoglaciological reconstructions based on glacial geological and geomorphological traces are the basis for testing and constraining numerical models of ice sheet extent and dynamics. In MAGIC-DML (“Mapping, Measuring and Modelling Antarctic Geomorphology and Ice Change, in Dronning Maud Land”) we aim to reconstruct the timing and pattern of ice surface elevation changes since the mid-Pliocene across western Dronning Maud Land, East Antarctica. The choice of study area is motivated by an absence of empirical field data and considerable ice sheet model uncertainties. We present a remote sensing-based mapping of glacial geomorphology and structures on the ice sheet surface for a coastal-inland transect including Ahlmannryggen-Borgmassivet-Kirwanveggen using high-resolution WorldView imagery. The primary aim of the study is to map traces of a thicker ice sheet on nunatak slopes that were formerly partly or entirely covered during surface highstands. Panchromatic and multispectral images were analysed in a multi-step procedure using ArcGIS, including image processing and mosaicking, visual feature recognition, and mapping. The identification of crucial landforms (such as till veneers and erratic boulders) required the adoption of some assumptions to differentiate, for example, till from regolith. Where patterned ground was mapped, we infer a presence of till rather than regolith since subglacial erosion is more likely to produce fine material than subaerial weathering. Huge boulders on plateau surfaces are mapped as erratics because they could not have been delivered by slope processes to local highpoints. Sediment veneers with ridges were mapped as till because the ridges are inferred to originate from reworking by ice. However, because of the necessity to invoke assumptions and the absence of crosscutting relationships between landforms, we are unsure of the reliability of derived paleo-ice sheet reconstructions. At face value, the data indicates thicker ice in the past from the altitude above present ice surface of till cover and erratics on some nunataks. Mapping and landform interpretations will be verified during the MAGIC-DML 2017-18 field season. The mapping of structures on the ice sheet surface is used to (i) infer ice flow characteristics; This was possible by assessing the distribution of primarily blue ice areas, crevasse fields, and supraglacial moraines, and by analysing their connection to wind directions and ablation rates; and (ii) yield target field routes to potential cosmogenic nuclide (CN) sampling locations along vertical transects on nunataks. The timing derived from CN dating will permit the delineation of ice sheet surface elevations as targets for ice sheet modeling.

Description: Constraining numerical ice sheet models by comparison with observational data is crucial to address the interactions between cryosphere and climate at a wide range of scales. Such models are tested and refined by comparing model predictions of past ice geometries with field-based reconstructions from geological, geomorphological, and ice core data. However, for the East Antarctic Ice sheet, there is a critical gap in the empirical data necessary to reconstruct changes in ice sheet geometry in the Dronning Maud Land (DML) region. In addition, there is poor control on the regional climate history of the ice sheet margin, because ice-core locations, where detailed reconstructions of climate history exist, are located on high inland domes. This leaves numerical models ofregional glaciation history in this near-coastal area largely unconstrained. MAGIC-DML is an ongoing Swedish-US-Norwegian-German-UK collaboration with a focus on improvingice sheet models of the western DML margin by combining advances in modeling with filling critical data gaps regarding the timing and pattern of ice-surface changes. A combination of geomorphological mapping using remote sensing data, field observations, cosmogenic nuclide surface exposure dating, and numerical ice sheetmodeling are being used in an iterative manner to produce a comprehensive reconstruction of the glacial historyof western DML. Here, we present an overview of the project, field evidence for formerly higher ice surfaces and in-situ cosmogenic nuclide measurements from the 2016/17 expedition. Preliminary field evidence indicate that interior sectors of DML have experienced a general decrease in ice sheet thickness since the late Miocene, with potential episodes of increasing thickness in the late Pleistocene (700-300 ka, 250-75 ka). To aid in interpreting these field data, new high-resolution ice sheet model reconstructions, constraining ice sheet configurations during key episodes, are presented.

Description: The East Antarctic Ice Sheet (EAIS) is generally assumed to have been relatively insensitive to Quaternary climat echange. However, recent studies have shown potential instabilities in coastal, marine sectors of the EAIS. In addition, long-term climate reconstructions and modelling experiments indicate the potential for significant changes in ice volume and ice sheet configuration since the Pliocene. Hence, more empirical evidence for ice surface and ice volume changes is required to discriminate between contrasting inferences. MAGIC-DML is an ongoing Swedish-US-Norwegian-German-UK collaboration focused on improving ice sheetm odels by filling critical data gaps that exist in our knowledge of the timing and pattern of ice surface changes along the western Dronning Maud Land (DML) margin and combining this with advances in numerical techniques. As part of the project, field studies in the 2016/17 and 2017/18 austral summers targeted selected sites spanning accessible altitudes in the Heimefrontfjella, Vestfjella, Ahlmannryggen, Borgmassivet, and Kirwanveggen nunatakranges for in situcosmogenic nuclide sampling. Comparing concentrations of nuclides with widely differing half-lives in bedrock and erratics from a range of altitudes above modern ice surfaces can provide information on ice sheet fluctuations and complex burial and exposure histories, and thus, past configurations of non-erosive ice. Quartz-bearing rock types were sampled and analyzed for 10Be (t1/21.4 My),14C (t1/25.7 ky),26Al (t1/2705ky), and 21Ne (stable), and mafic lithologies for36Cl (t1/2301 ky). Results thus far for 3210Be and 26Al isotope pairs complemented with seven21Ne measurements have yielded some consistent patterns of paleoglaciation for the western DML margin. Eight out of fourteen bedrock samples from high-elevation (1700-2238 m a.s.l.) ridges and summits return some of the oldest exposure ages in Antarctica and have consistent 10Be,26Al, and 21Ne minimum apparent exposure ages of 1.8-4.1 Ma. Initial results therefore indicate that parts of the ice sheet marginal to the Antarctic plateau, along the Heimefrontfjella range, generally have experienced a decrease in ice thickness since the late Miocene. Another six bedrock samples (1556-1732 ma.s.l.) fall in the 300-700 ka range, and they all show significant burial. At face value, perhaps this indicates aregional ice sheet surface above 1700 m a.s.l. for much of the Plio-early Pleistocene. All other samples analyzedto date are erratics from lower elevation and more coastal sites (10 from nunataks at 553-1400 m a.s.l., and 6 froma surface moraine at 1385 m a.s.l.), exhibiting ages between 59 and 275 ka, save for two (4 and 6 ka). Whereas almost all of the nunatak erratics (including the young ones) show significant burial durations, five of the six surface moraine samples do not. These 2016/17 field samples are not yet leading to conclusive age constraints but already start to paint a picture of the western DML margin being relatively stable although there was possibly one or more episodes of relatively limited ice thickening during the last 700 ka.