Sarah L. Greenwood

Subglacial bedforms of the Irish Ice Sheet

Abstract Please click here to download the map associated with this article. The last Irish Ice Sheet has a long history of investigation, but its most basic properties are still debated. A palaeo-glaciological ‘inversion’ of the glacial geomorphological record has elsewhere proven an effective means to reconstruct former ice sheets at ice sheet scale. To this end, this paper describes new glacial geomorphological mapping of the Irish landscape, using Digital Elevation Models, Landsat ETM+ and SPOT satellite images, with the objective of achieving a thorough and systematic documentation of the form, spatial distribution and arrangement of glacial landforms in Ireland. A resulting map, presented here at a scale of 1:425,000, identi-es ~33,000 subglacial bedforms. Glacial lineations (drumlins, mega-scale glacial lineations and crag and tails) and ribbed moraine comprise the bulk of the bedform population, although a wide spectrum of bedform types, shapes and sizes is observed. In particular, a suite of enigmatic bedforms were mapped which were not classi-ed either as streamlined lineations or transverse ridges; rather, they are quasi-circular in form. This map is the first to document the subglacial bedform record of Ireland at the scale of individual landforms, and countrywide. In so doing, it reveals the extent and complexity of superimposed and cross-cutting bedform relationships, which demand an ice sheet reconstruction depicting marked changes in ice sheet geometry, con-guration and behaviour through its evolution. This map forms the basis for a palaeo-glaciological reconstruction which addresses and attempts to incorporate the complexity of ice ow history revealed by the subglacial bedforms of Ireland.

Ice-flow switching and East/West Antarctic Ice Sheet roles in glaciation of the western Ross Sea

The long-term behavior of the East and West Antarctic Ice Sheets, and their respective responses to forcing provide essential context for assessment of modern dynamic changes in ice-flow regimes and ice-sheet and shelf margins. The western Ross Sea discharges ice from both the East and West Antarctic Ice Sheets, and the paleoglacial record from this region is therefore valuable in unraveling their long-term behavior. New, high-resolution multibeam bathymetric data reveal snapshots of well-preserved glacial landforms on the seafloor around Ross Island and McMurdo Sound. Glacial lineations, grounding zone wedges, draped recessional moraines, and meltwater channels record a series of different ice-flow events in the region, contradictions between which require major phases of ice-flow reorganization. From the glacial geomorphology, we reconstruct a four-stage model of ice-flow evolution for the last glacial cycle, consisting of: (1) northeastward flow into the Ross Sea from McMurdo Sound; (2) westward flow from the Ross Sea, around Ross Island, and onto the Victoria Land coast and coastal seafloor trough; (3) a deglacial phase of ice-sheet thinning, minor shifts in flow, and grounding line retreat into McMurdo Sound; and (4) grounding line pinning on Ross Island during regional retreat, uncoupling of a remnant Ross Island ice cap, and local oscillation of Victoria Land outlet glaciers. We find that East Antarctic Ice Sheet ice discharge had a strong influence on ice-flow geometry in this part of the Ross Sea during the last glacial stage, but that it was not necessarily in phase with the behavior of the West Antarctic Ice Sheet. It is similarly evident that the ice streams that drained the Ross Sea over the continental shelf at the Last Glacial Maximum did not all operate synchronously, and exerted different drawdown power at different times. Finally, we conclude that Ross Island acts as an important pinning point in the Ross Sea ice-sheet-shelf system, stabilizing grounding line retreat and encouraging lasting ice-shelf development.

Dating constraints on the last British-Irish Ice Sheet: a map and database

Abstract Please click here to download the map associated with this article. We present a collation of 975 published dates relevant to the timing of build-up and retreat of ice over the British Isles during the most recent glacial stage. The spatial distribution of dates is essential to interpreting the evolution of the ice sheet over time and space. For this reason the dates are presented as a map showing the spatial distribution across the UK and Ireland. Full documentation for each date, attributed to its source publication and detailing geographic location, the material dated, its stratigraphic position or setting, the dating technique, the dating result, errors and calibration, and comments pertinent to its interpretation, are included in the accompanying table. It is anticipated that this dataset will be auseful resource for Quaternary research in the UK and Ireland and is therefore available to download as a shapefile and/or PDF file. The map is presented at a scale of 1: 2,700,000, designed to be printed at A2 size.

Major earthquake at the Pleistocene-Holocene transition in Lake Vättern, southern Sweden

Lake Vattern, Sweden, is within a graben that formed through rifting along the boundary between two Precambrian terrains. Geophysical mapping and geological coring show that substantial tectonic movements along the Lake Vattern graben occurred at the very onset of the Holocene. This is evident from deformation structures in the soft sediment accumulated on the lake floor. Our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a >80-km-long fault system. If these large displacements are from one tectonic event, Lake Vattern must have had an earthquake with seismic moment magnitudes to 7.5. In addition, our geophysical mapping shows large landslides along sections of the steep lake shores. Pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the Younger Dryas-Preboreal transition, ca. 11.5 ka. We suggest that this event is mainly related to the rapid release of ice-sheet load following the deglaciation. This paleoseismic event in Lake Vattern ranks among the larger known intraplate tectonic events in Scandinavia and attests to the significance of glacio-isostatic unloading.

Manual mapping of drumlins in synthetic landscapes to assess operator effectiveness

Mapped topographic features are important for understanding processes that sculpt the Earths surface. This paper presents maps that are the primary product of an exercise that brought together 27 researchers with an interest in landform mapping wherein the efficacy and causes of variation in mapping were tested using novel synthetic DEMs containing drumlins. The variation between interpreters (e.g. mapping philosophy, experience) and across the study region (e.g. woodland prevalence) opens these factors up to assessment. A priori known answers in the synthetics increase the number and strength of conclusions that may be drawn with respect to a traditional comparative study. Initial results suggest that overall detection rates are relatively low (34–40%), but reliability of mapping is higher (72–86%). The maps form a reference dataset.

Integrated use of LiDAR and multibeam bathymetry reveals onset of ice streaming in the northern Bothnian Sea

Abstract Geomorphological mapping from the new LiDAR (Light Detection and Ranging)-derived digital elevation model for Sweden and a high-resolution multibeam bathymetry data-set for the Gulf of Bothnia reveals a continuous system of glacial landforms crossing the transition between the modern terrestrial and marine environments. A palaeo-ice stream in the northern Bothnian Sea is reconstructed, with an onset tributary over the present-day Ångermanland–Västerbotten coastline. Systematic contrasts in landform morphology and lineation length indicate that this ice stream comprised a relatively narrow (∼40 km) corridor of fast flow, flowing first SW then S, and likely fed by converging flow around the upper Bothnian Sea. The geometry and landform associations of this system imply that ice, at the time period represented here, did not flow across the Gulf of Bothnia: SSE-ward ice flow indicators on the northern Swedish coast do not correspond directly with landform assemblages of the large SE-oriented Finnish deglacial lobes. Instead, we suggest they may contribute to a late-stage fast-flow event to the S and SW. Multibeam bathymetry data offer entirely new access into the rich, landform-scale geomorphological record on the seafloor of the Gulf of Bothnia. The combination of offshore multibeam with the new terrestrial LiDAR data provides unprecedented insight into and renewed understanding of the glacial dynamics of the Bothnian Sea sector of the Fennoscandian Ice Sheet, hitherto interpreted over large areas of unmapped ice sheet bed.

Regional deglaciation and postglacial lake development as reflected in a 74 m sedimentary record from Lake Vättern, southern Sweden

The withdrawal of the Late Weichselian ice sheet and rapid isostatic uplift in southern Scandinavia led to the entrainment of large volumes of melt water within the proglacial Baltic Ice Lake (BIL). The eventual western outpost of BIL, Lake Vättern, has been a focal point for studying the dynamic retreat history of the Late Weichselian ice sheet in south central Sweden. This part of the deglacial history is described from an abundance of terrestrial studies, but, to date, no complimentary long sediment cores from Lake Vättern have been available. Here, we present the results from a unique, 74 m borehole in southern Lake Vättern that recovered a Late Pleistocene to Holocene sedimentary sequence. Physical and chemical analyses of the sediment and pore water, together with geophysical mapping, reveal glacial as well as postglacial imprints implying an oscillating ice sheet margin, evidence for neotectonic activity and one or more marine incursions into the lake during deglaciation. We attribute the glaciotectonic deformation of the sediments at 54 m below the lake floor to an ice readvance that likely occurred at the same time or before the advance that formed the Levene moraine (∼13.8–13.4 cal. ka BP). After this event, potential readvances were likely restricted to a more northerly position in the basin. We identify the final drainage of the BIL, but find evidence for an earlier marine incursion into the Vättern basin (∼13.0 cal. ka BP), indicating water exchange between the North Atlantic and the Baltic Ice Lake during the late Alleröd.

Glacial landform assemblage reveals complex retreat of grounded ice in the Ross Sea, Antarctica

The Ross Sea Embayment is the largest drainage basin of the Antarctic Ice Sheet. Multiple ice streams sourced from both the East Antarctic and West Antarctic ice sheets merged in the Ross Sea during the Last Glacial Maximum (LGM). The continental shelf record is important for understanding processes that influence retreat, constraining deglacial patterns and assessing Antarcticas contribution to eustatic sea-level rise since the LGM. Newly acquired, high-resolution multibeam bathymetry provides a detailed record of the complex deglacial history of a sector in the western Ross Sea (Fig. 1a). Retreat of grounded ice is marked by small, ice-marginal retreat landforms that were poorly resolved prior to the use of an upgraded multibeam system. Dynamic grounding zone behaviour is influenced by physiography and the presence of subglacial meltwater. Fig. 1. ( a ) Multibeam survey south of Crary Bank between Franklin Island and the Central Basin, western Ross Sea shelf (basemap modified by L. Prothro from GeoMapApp). Acquisition system Kongsberg EM122. Frequency 12 kHz. Grid-cell size 20 m. ( b ) Ross Sea showing the location of the survey area ((a) in red box; map from IBCSO v. 1.0). LAB, Little American Basin; GCB, Glomar-Challenger Basin; RB, Ross Bank; PT, Pennell Trough; PB, Pennell Bank; JT, JOIDES Trough; MB, Mawson Bank; CB, Central Bank; CrB Crary Bank; FrIS, Franklin Island; DT, Drygalski Ice Tongue; VLB, Victoria Land Basin; McM, McMurdo Sound. ( c ) Two sets of linear features interpreted as glacial lineations separated by an abrupt boundary (grey dashed line). Inset shows seismic profile across glacial lineations. Acquisition system Knudsen CHIRP. Frequency 3.5 kHz. ( d ) Asymmetrical ridges, interpreted as GZWs, deposited along the banks of a channel and on top of a volcanic landform with lineated topsets. ( e ) Profile of 4 m GZW deposited on volcanic landform showing underlying surface reflection. ( f ) Example of channel cross-sectional profile. …

Geotechnical and sedimentary evidence for thick-grounded ice in southern Lake Vättern during deglaciation

A 74-meter Late Pleistocene to Holocene sedimentary sequence was recovered from southern Lake Vättern in the autumn of 2012. At ∼54 m below the lake floor, shear strength and high-resolution bulk density measurements suggest the presence of an unconformity in the varved proglacial clays. Incremental load consolidation tests reveal highly overconsolidated sediments below this level. Preconsolidation pressures for the underlying sediments are between 1250 and 2100 kPa, up to ∼1700 kPa more than the current in-situ effective stress. The highly overconsolidated sediments indicate either substantial erosion (the removal of 215–360 m of sediment), or consolidation under a large grounded ice mass sitting up to 230 m above paleo-lake level. Glaciotectonic deformation in underlying sediments supports the interpretation of a grounded ice mass. It is likely that this horizon is either contemporaneous with or older than the Levene moraine, formed between 13.4 and 13.8 ka. In the ∼30 m of overlying proglacial clays, there is no further evidence for grounded ice, indicating that any ice advance to southern Lake Vättern during the Younger Dryas would have been limited to an extremely thin ice tongue.

Glacial landforms in a hard bedrock terrain, Melville Bay, northwestern Greenland

Glaciated landscapes in gneissose or granitic bedrock often show a scoured ‘hilly relief’ (Bonow et al. 2006) or ‘knock-and-lochan’ morphology (Linton 1963), characterized by a highly fractured landscape with strong relief of knolls and basins. This topography closely follows old deep-weathering fronts from previous sub-aerial exposure, and was probably formed through a multistage process including deep weathering, glacial erosion and stripping of saprolites, followed by further glacial modification (Krabbendam & Bradwell 2014). Multibeam-bathymetric data and high-resolution onshore digital elevation models (DEM) from southern Melville Bay, NW Greenland, show similar glacially modified knock-and-lochan landscapes from the inner shelf and nearby coast. Over a bathymetric range of 150–400 m on the inner Melville Bay shelf, c. 30 km from the West Greenland coast, the seafloor shows a rugose, bedrock-dominated landscape with limited sediment cover (Fig. 1a). The bedrock onshore, to the east of the study area, consists of Precambrian metaturbidites and orthogneisses (Kokfelt et al. 2013). The submarine landscape is dominated by criss-crossing incisions, numerous knolls, depressions, angular junctions and steep scarps. Minor landforms include U-shaped troughs, streamlined features and roche moutonee-type features often with stoss- and lee-side forms that have a clear …