Anna L.C. Hughes

Subglacial bedforms of the last British Ice Sheet

Abstract Please click here to download the map associated with this article. Subglacial bedforms are a characteristic feature of formerly glaciated landscapes. Formed parallel (e.g. drumlins) or transverse (ribbed moraine) to ice flow their preserved distribution is a valuable record of ice sheet configuration. To date these landforms have been underused in Britain such that we have only a simple static view of the flow patterns of the last British Ice Sheet, with glimpses of complexity recorded in just a few locations. Arguably, this is because of the lack of a suitable, ice-sheet scale, map of subglacial bedform distribution. We present the first consistent and countrywide map of subglacial bedforms for Britain produced from systematic mapping. Individual landforms were mapped from relief-shaded renditions of high-resolution elevation data and Landsat TM imagery. The outline or crest-line of each bedform was manually captured by onscreen digitisation directly into a Geographic Information System (GIS) database. Over 39,000 landforms were identified revealing new lineation patterns, multiple instances of cross-cutting/superimposition, and adding additional detail to the known distribution established by local field surveys. A near complete representation of the British subglacial bedform population, the map is a major step towards a detailed model of British Ice Sheet configuration by providing a suitable basis for reconstructing the flow evolution at the ice sheet scale, and a geomorphological framework for the interpretation of sedimentological and stratigraphical data. The map is presented at a scale of 1:525,000.

Rapid response of Helheim Glacier, southeast Greenland, to early Holocene climate warming

Recent changes in speed, thinning, and retreat rates of marine-terminating outlet glaciers have raised concerns about the future stability of the Greenland Ice Sheet. Establishing a longer term record of outlet glacier retreat rates is essential to provide a context for present-day observations and to improve and constrain numerical models of outlet glacier behavior. New exposure dating (10Be) of streamlined bedrock surfaces and glacial erratic boulders of Sermilik Fjord, southeast Greenland, the present-day drainage route of Helheim Glacier, documents rapid retreat (∼80 m a−1) of this major marine-terminating outlet glacier at the close of the last glaciation. The glacier front retreated ∼80 km to within 20 km of the present-day (2010) position of Helheim Glacier in <1 ka, ca. 10.8 ± 0.3 ka ago. Retreat followed rapidly rising air temperatures at the start of the Holocene, and at this temporal resolution there is no evidence that fjord geometry influenced glacier behavior. The significant response to climatic amelioration at the end of the last glacial suggests a high sensitivity to abrupt temperature increases, which has major implications for the future stability of present-day Greenlandic outlet glaciers in a warming climate.

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.

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.

Extensive Retreat of Greenland Tidewater Glaciers, 2000–2010

Abstract Overall mass loss from the Greenland ice sheet nearly doubled during the early 2000s resulting in an increased contribution to sea-level rise, with this step-change being mainly attributed to the widespread frontal retreat and accompanying dynamic thinning of tidewater glaciers. Changes in glacier calving-front positions are easily derived from remotely sensed imagery and provide a record of dynamic change. However, ice-sheet-wide studies of calving fronts have been either spatially or temporally limited. In this study multiple calving-front positions were derived for 199 Greenland marine-terminating outlet glaciers with width greater than 1 km using Landsat imagery for the 11-year period 2000–2010 in order to identify regional seasonal and inter-annual variations. During this period, outlet glaciers were characterized by sustained and substantial retreat summing to more than 267 km, with only 11 glaciers showing overall advance. In general, the pattern of mass loss detected by GRACE (Gravity Recovery and Climate Experiment) and other measurements is reflected in the calving record of Greenland glaciers. Our results suggest several regions in the south and east of the ice sheet likely share controls on their dynamic changes, but no simple single control is apparent.

Spatial organisation of drumlins

Ice-sheets flowing over soft sediments produce undulations in the bed, typically of metres in relief, of which drumlins are the most abundant and widely investigated. Consensus regarding their mechanism of formation has yet to be achieved. In this paper we examine the spatial organisation of drumlins in order to provide an improved description of the phenomenon and to guide hypotheses of their formation. We review the literature highlighting contradictory findings regarding drumlin spatial organisation and then use this to motivate our study based on a large sample (42,488) of drumlins from Canada, Britain and Norway. Are there typical arrangements in drumlin positioning and are they organised in a regular spatial manner (patterned) or are they distributed randomly? We recognise that drumlin fields are inherently patchy and therefore apply inhomogeneous spatial statistics in order to study their distribution. This shows that whilst drumlins are occasionally randomly placed, their main state is non- random. They exhibit a strong and statistically significant signal of regularity across lengths scales of 100 – 1200 m. We conclude that patterning is a near ubiquitous property of drumlins. This finding of regularity demonstrates spatial self-organisation in the bedforming process with drumlins as an emergent manifestation of subglacial sediment mobility. Kilometre-scale interactions between drumlins must occur as they evolve, or interactions may arise as a consequence of growth or migration. Hypotheses or models are required that can explain the regular spacing of drumlins. We highlight three suggestions for such self-organisation: instability in the coupling of ice flow - sediment flux - bed shape; local feedback between sediment mobility and relief; and coarsening by growth or migration.

Using the size and position of drumlins to understand how they grow, interact and evolve

Drumlins are subglacial bedforms streamlined in the direction of ice flow. Common in deglaciated landscapes, they have been widely studied providing rich information on their internal geology, size, shape, and spacing. In contrast with bedform investigations elsewhere in geomorphology (aeolian and fluvial dunes and ripples for example) most drumlin studies derive observations from relict, and thus static features. This has made it difficult to gain information and insights about their evolution over time, which likely hampers our understanding of the process(es) of drumlin formation. Here we take a morphological approach, studying drumlin size and spacing metrics. Unlike previous studies which have focussed on databases derived from entire ice sheet beds, we adopt a space-for-time substitution approach using individual drumlin flow-sets distributed in space as proxies for different development times/periods. Framed and assisted by insights from aeolian and fluvial geomorphology, we use our metric data to explore possible scenarios of drumlin growth, evolution and interaction. We study the metrics of the size and spacing of 36,222 drumlins, distributed amongst 71 flow-sets, left behind by the former British-Irish Ice Sheet, and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis. Through characterising and analysing the shape of the probability distribution functions of size and spacing metrics for each flow-set we argue that drumlins grow, and potentially migrate, as they evolve leading to pattern coarsening. Furthermore, our findings add support to the notion that no upper limit to drumlin size exists, and to the idea that perpetual coarsening could occur if given sufficient time. We propose that the framework of process and patterning commonly applied to non-glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes.

Scottish Landform Examples 43: Glacifluvial Landforms of Strathallan, Perthshire

A wide range of landforms and sediments is created by the melting of glacier ice, and commonly constitute the most prominent evidence of former glaciation. In upland landscapes, the concentration of meltwater activity in valley settings results in the development of complex assemblages of ice-contact glacifluvial erosional and depositional forms. An excellent example of such an assemblage exists in Strathallan and the tributary River Knaik, documenting the intensity and changing nature of ice-contact glacifluvial processes during the recession of the last British–Irish Ice Sheet into the Highlands of Scotland.

Minimal Holocene retreat of large tidewater glaciers in Køge Bugt, southeast Greenland

Køge Bugt, in southeast Greenland, hosts three of the largest glaciers of the Greenland Ice Sheet; these have been major contributors to ice loss in the last two decades. Despite its importance, the Holocene history of this area has not been investigated. We present a 9100 year sediment core record of glaciological and oceanographic changes from analysis of foraminiferal assemblages, the abundance of ice-rafted debris, and sortable silt grain size data. Results show that ice-rafted debris accumulated constantly throughout the core; this demonstrates that glaciers in Køge Bugt remained in tidewater settings throughout the last 9100 years. This observation constrains maximum Holocene glacier retreat here to less than 6 km from present-day positions. Retreat was minimal despite oceanic and climatic conditions during the early-Holocene that were at least as warm as the present-day. The limited Holocene retreat of glaciers in Køge Bugt was controlled by the subglacial topography of the area; the steeply sloping bed allowed glaciers here to stabilise during retreat. These findings underscore the need to account for individual glacier geometry when predicting future behaviour. We anticipate that glaciers in Køge Bugt will remain in stable configurations in the near-future, despite the predicted continuation of atmospheric and oceanic warming.

The deglaciation of coastal areas of southeast Greenland.

Large marine-terminating glaciers around the margins of the Greenland Ice Sheet have retreated, accelerated and thinned over the last two decades. Relatively little is known about the longer term behaviour of the Greenland Ice Sheet, yet this information is valuable for assessing the significance of modern changes. We address this by reporting 11 new beryllium-10 (10Be) exposure ages from previously uninvestigated coastal areas across southeast Greenland. The new ages are combined with existing data from the region to assess the timing of glacier retreat after the Last Glacial Maximum. The results show that deglaciation occurred first in the north of the region (~68°N) and progressed southwards. This north–south progression is attributed to the influence of the warm Irminger Current on the ice margin. Areas in the south of the region were isolated from the warm waters by the shallow bathymetry of the continental shelf. This demonstrates that oceanographic forcing paced the deglaciation of southeast Greenland through the Younger Dryas and early Holocene. In most areas of southeast Greenland bedrock ages are systematically older than their counterpart boulder samples; this offset is likely the result of inherited 10Be content in bedrock surfaces. This suggests that subglacial erosion during the last glacial cycle was insufficient to completely remove pre-existing 10Be content. Alternatively, this pattern may be the signature of a substantial retreat and advance cycle prior to final Holocene deglaciation.