Jasper Knight

Evidence for Heinrich event 1 in the British Isles

In the north Irish Sea basin (ISB), sedimentary successions constrained by AMS 14C dates obtained from marine microfaunas record three major palaeoenvironmental shifts during the last deglacial cycle. (i) Marine muds (Cooley Point Interstadial) dated to between 16.7 and 14.7 14C kyr BP record a major deglaciation of the ISB following the Late Glacial Maximum (LGM). (ii) Terminal outwash and ice-contact landforms (Killard Point Stadial) were deposited during an extensive ice readvance, which occurred after 14.7 14C kyr BP and reached a maximum extent at ca.14 14C kyr BP. At this time the lowlands surrounding the north ISB were drumlinised. Coeval flowlines reconstructed from these bedforms end at prominent moraines (Killard Point, Bride, St Bees) and indicate contemporaneity of drumlinisation from separate ice dispersal centres, substrate erosion by fast ice flow, and subglacial sediment transfer to ice-sheet margins. In north central Ireland bed reorganisation associated with this fast ice-flow phase involved overprinting and drumlinisation of earlier transverse ridges (Rogen-type moraines) by headward erosion along ice streams that exited through tidewater ice margins. This is the first direct terrestrial evidence that the British Ice Sheet (BIS) participated in Heinrich event 1 (H1). (iii) Regional mud drapes, directly overlying drumlins, record high relative sea-level (RSL) with stagnation zone retreat after 13.7 14C kyr BP (Rough Island Interstadial). Elsewhere in lowland areas of northern Britain ice-marginal sediments and morainic belts record millennial-scale oscillations of the BIS, which post-date the LGM advance on to the continental shelf, and pre-date the Loch Lomond Stadial (Younger Dryas) advance in the highlands of western Scotland (ca. 11–10 14C kyr BP). In western, northwestern and northern Ireland, Killard Point Stadial (H1) ice limits are reconstructed from ice-flow lines that are coeval with those in the north ISB and end at prominent moraines. On the Scottish continental shelf possible H1-age ice limits are reconstructed from dated marine muds and associated ice marginal moraines. It is argued that the last major offshore ice expansion from the Scottish mountains post-dated ca. 15 14C kyr BP and is therefore part of the H1 event. In eastern England the stratigraphic significance of the Dimlington silts is re-evaluated because evidence shows that there was only one major ice oscillation post-dating ca.18 14C kyr BP in these lowlands. In a wider context the sequence of deglacial events in the ISB (widespread deglaciation of southern part of the BIS major readvance during H1 ice sheet collapse) is similar to records of ice sheet variability from the southern margins of the Laurentide Ice Sheet (LIS). Well-dated ice-marginal records, however, show that during the Killard Point readvance the BIS was at its maximum position when retreat of the LIS was well underway. This phasing relationship supports the idea that the BIS readvance was a response to North Atlantic cooling induced by collapse of the LIS.

Identification and significance of ice-flow-transverse subglacial ridges (Rogen moraines) in northern central Ireland

In the Omagh Basin, north central Ireland, subglacial diamict ridges lie transverse to southwestward Late Devensian (ca. 23–13 ka) ice flow. These ridges (0.5–2.5 km long, 100–450 m wide, 15–35 m high), are similar morphologically to Rogen moraines, which have not been described previously from the British Isles. The crests of some transverse ridges are streamlined, cross-cut or overprinted by drumlins, whereas other ridges are unmodified and were not affected by later drumlinisation. At Kilskeery, west–east trending eskers overlying unmodified transverse ridges post-date drumlinisation (17–14 14C ka). Esker formation shows that the subglacial thermal regime changed from cold-based, favouring bedform preservation, to warm-based with meltwater flowing through enclosed subglacial channels. Patterns of flow-transverse-ridges and spatial variations in the degree of bedform modification record dynamic changes in regional subglacial environments during the last deglacial cycle. This ice-mass variability cannot be reconciled with current Irish glacial models, which are based on immobile ice centres and ordered stages of ice retreat. In a wider context, these changes in bedform patterns and basal ice regimes have a similar signature to millennial-scale ice-mass oscillations recorded by dated proxy evidence elsewhere in the amphi-North Atlantic.

Drumlin evolution and ice sheet oscillations along the NE Atlantic margin, Donegal Bay, western Ireland

Abstract Satellite imagery of Donegal Bay, northwestern Ireland, reveals two streamlined subglacial bedform sets produced during the late Devensian glaciation (ca. 22,000–15,000 yrs B.P.). The first bedform set trends northeast-southwest and records fast ice flow from inland ice domes onto the eastern Atlantic continental shelf. The second bedform set, trending east-west, crosscuts and partially reorientates the first. Morphological and sedimentary evidence show that the second set corresponds to the last phase of fast ice flow (drumlinization) in northwestern Ireland. Morphological characteristics of the Donegal Bay drumlin field, and sedimentary characteristics of the Mullinasole drumlin, Donegal Bay, support a two-stage interpretation of drumlin evolution. These are the following. (1) Deposition of glaciomarine mud and diamict facies at a tidewater glacier margin. Stratified diamicts record debris flow events and sediment reworking. This facies sequence is erosionally truncated. (2) Deposition of subglacial diamict and sand facies recording ice readvance and drumlinization. Drumlinization (sediment streamlining) reflects ice mass-balance destabilization, episodic: fast ice flow and ice-marginal oscillation, and may be correlated with millennial-time scale climate changes in the circum-North Atlantic.

Ice-flow stages and glacial bedforms in north central Ireland: a record of rapid environmental change during the last glacial termination

Satellite imagery is a useful tool to assess large-scale (>103km2) geological patterns. Satellite imagery of north central Ireland shows cross-cut and overprinted subglacial bedform patterns. Bedform lineations, supported by sedimentary evidence, are used to reconstruct four ice-flow stages (named A-D) during the termination of the last (Devensian) glaciation (22 000–13 000 radiocarbon (14C) years BP). Flow stage A relates to southwesterly flow of Scottish ice across eastern Ireland. Flow stage B, during the last glacial maximum (c. 25 000–22 000 14C years BP), records southeasterly ice-flow from dispersal centres in northern parts of Ireland. Transverse subglacial ridges (Rogen moraines) formed during stage B are the largest area of this ridge type in western Europe. Flow stage C records erosional headcutting in the Rogen moraine field and sediment streamlining (drumlinization) by ice streams drawn into the Irish Sea Basin. This flow stage is constrained by AMS radiocarbon dates from marine microfaunas to around 14 500 14C years BP. Flow stage D records drumlinization from an inland centre in the Lough Neagh Basin. Overall bedform patterns can be used to reconstruct ice mass history and evaluate controls on ice activity, including millennial-scale changes in amphi-North Atlantic climate. Field evidence of bedform relationships also casts doubt on current concepts of drumlinization and the role of pervasive sediment deformation in glacial systems.

Holocene coastal dune initiation in Northumberland and Norfolk, eastern UK: climate and sea-level changes as possible forcing agents for dune initiation

Abstract Relative sea-level (RSL) control on dune initiation during the Holocene is examined in the context of chronostratigraphies established from 43 vibracores through dunes and into sub-dune sediments taken from the Northumberland and Norfolk (UK) coasts. The chronology is based on 23 accelerated mass spectroscopy and conventional 14C dates, and 37 infra-red-stimulated luminescence dates. The oldest dunes in Northumberland are c. 4 cal. ka bp with phases of dune development at 2.8 and 1.5–1 ka bp. Most dune deposition is of last millennium age, with a concentration, especially in Norfolk, around 500–200 a bp. The initiation and survival of coastal dune sequences relate to macroscale RSL changes over the last 4 ka. Northumberland dunes reflect a gradient of RSL change from a northern RSL fall (forced regression) through to a southern RSL rise (normal regression through sediment supply). The north Norfolk coast has been dominated by a rising RSL through the Holocene, though associated with a sediment supply sufficient to offset the transgressive tendency and allow normal regressive deposition at numerous positions along the coast over the last 1 ka. It is suggested that the development of Little Ice Age (LIA) dunes in both Norfolk and Northumberland identifies the onset of specific conditions in which intertidal sediment sources were exposed (falling sea-level) to onshore winds (LIA circulation changes), which reflect a brief west North Sea period in dune initiation and deposition rates. A comparison of this consolidated dune chronology with statements of RSL elevation and climate conditions in the last 2.5 ka leads to some recognition of RSL fall preceding major dune building in two phases post 1.5 ka bp and post 0.6 ka bp.

Pleistocene glaciations in Ireland

Abstract A literature survey and data from recent investigations are used to reconstruct ice limits in Ireland during the last (Midlandian) and penultimate (Munsterian) cold periods which are correlated with Marine Isotope Stages (MIS) 2-5d (Weichselian) and 6-8 (Saalian) respectively. Evidence for Munsterian ice limits and flow directions is equivocal and based mainly on erratic carriage and the presence of striae and subdued glacial landforms found outside well-marked Midlandian end moraines. Ice extent and flow direction is known only from the late Midlandian (MIS 2; 24-10 kyr BP) although ice may well have been present in the early Midlandian (MIS 3-5d; 24-117 kyr BP). Six late Midlandian glacial stages are identified on the basis of morphosedimentary and dating evidence, and patterns of subglacial bedforms including drumlins and Rogen moraines. Previous late Midlandian glacial models are well-established but are generally based on incomplete and/or erroneous datasets, are not age-constrained, and do not consider time-transgressive sedimentation and landform-shaping events. Recent work shows that repeated ice advance-retreat cycles (oscillations) occurred during the late Midlandian. Oscillations resulted in stratigraphically superimposed, overprinted and cross-cut landform and sediment patterns that record ice activity throughout the glacial cycle. Additionally, subglacial bedforms previously unrecorded in the British Isles, such as flow-transverse ridges (Rogen moraines), are also present. Late Midlandian ice oscillations in Ireland occurred in tempo with millennial-scale changes in North Atlantic climate, suggesting connection to hemispheric shifts of the ice-ocean-atmosphere system.

Glacial sedimentary evidence supporting stick-slip basal ice flow

Last Glacial (Late Devensian) subglacial sediments in the southern Sperrin Mountains, north central Ireland, comprise tabular bedrock rafts which are interbedded with diamicton (till) and brecciated bedrock and separated by glaciotectonic shears. Bedrock rafts and diamicton beds alternate laterally and vertically in the profile, suggesting the ice-bed interface was chequered with both high-strength (rock rafts) and low-strength (diamicton) patches during accumulation of the sediment pile. The overall facies arrangement is interpreted qualitatively as reflecting stick-slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice-bed interface. It is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower-pressure regions, infilling shallow cavities on the surrounding diamicton surface. Slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces, causing ice-bed uncoupling. The presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the Late Devensian mid-latitude ice sheets.

Geological evidence for neotectonic activity during deglaciation of the southern Sperrin Mountains, Northern Ireland

In the southern Sperrin Mountains, Northern Ireland, stacked glacigenic sequences that accumulated during deglaciation (ca. 17000–13000 yr BP) overlie a basement of jointed and mascerated bedrock. The glacigenic sequences comprise interbedded glaciotectonic shears, diamictic breccias and rock rafts. At one site a normal fault with a metre-scale vertical displacement cuts through part of the sequence and is overlain by a glacial diamict. Sediments at an adjacent site show that faulting and associated hydrothermal activity was related to neotectonic reactivation of pre-existing Caledonian lineaments caused by ice unloading. From stratigraphical and directional evidence, fault reactivation occurred early in the deglaciation after north central Ireland ice had retreated southwards into lowland areas, but before Sperrin Mountain ice readvanced from the north. This relationship provides evidence for the relative timing of neotectonic activity in Northern Ireland, and demonstrates the effects of glacio-isostatic unloading near ice-sheet centres. Copyright

Bedform patterns, subglacial meltwater events, and Late Devensian ice sheet dynamics in north-central Ireland

Abstract Glacial bedform patterns and sediments deposited by the temperate and polythermal Late Devensian ice sheet in north-central Ireland record changes in the processes, location, and magnitude of subglacial meltwater throughout the last full glacial cycle (∼21–14 14C kyear BP). Meltwater characteristics are related directly to basal ice thermal regime and ice dynamics, including ice velocity and shifts in the location of ice centres. Therefore, reconstructed meltwater characteristics may provide insight into wider controls on dynamic ice behaviour. A range of meltwater-related features are present across north-central Ireland. These include tunnel valleys, drumlin leeside sequences, eskers, and boulder lags. Other bedforms including Rogen moraines were modified by meltwater activity along ice streams. Meltwater was stored subglacially in two contrasting regions located beneath or near ice centres in north-central Ireland. (1) The Lough Erne Basin is developed in a lowland depression occupied partly by subglacial Rogen moraine ridges which were formed around the time of the last glacial maximum. Meltwater was stored between Rogen ridge crests and released by hydraulic jacking associated with drumlinisation (∼16.6 14C kyear BP) and ice streaming (∼13.8 14C kyear BP). (2) The Lough Neagh Basin occupies a similar lowland depression and was the location of an ice sheet centre throughout the last glacial cycle. No bedforms are present beneath or immediately surrounding Lough Neagh. A larger, more continuous meltwater lake existed in the Lough Neagh depression, probably sealed by a region of cold-based ice outside lake margins. Water escaped through regional-scale tunnel valleys, particularly the Poyntzpass channel which was active during the Carlingford ice readvance (Killard Stadial, correlated with Heinrich event 1 at ∼14.5 14C kyear BP). Overall, reconstructed subglacial lake characteristics and drainage mechanisms are related closely to basal ice thermal regime and substrate relief (controlling lake geometry), and provide insight into controls on overall ice sheet dynamics.

Problems of Irish drumlins and Late Devensian ice sheet reconstructions

Reconstructions of the Late Devensian ice sheet in Ireland are strongly biased towards explaining data obtained by the Geological Survey of Ireland during the last century, and observations undertaken by key workers in this century (i.e. J. K. Charlesworth). Modern field observation and analytical techniques, however, show that many of these data, such as the distribution of drumlins, are erroneous. Furthermore, models derived from these data, dealing with the significance and distribution of different bedform types, the location of ice centres and the direction of ice flow lines, can no longer be supported. New observations based on mapping from satellite imagery, and validated by detailed regional-scale field studies, show the complexity of subglacial bedform patterns which record several phases of subglacial activity during the Late Devensian. The presence of ice flow-transverse Rogen moraines and patterns of overprinted bedforms in north central Ireland suggest that subglacial environments and the location of ice centres changed dramatically during the overall glacial cycle.