The Irish Ice Sheet

By Dr Sam Roberson, British Geological Survey, Belfast

Glaciation of Ireland during the Devensian

The island of Ireland has been glaciated many times during the Quaternary period.  The last glaciation in Ireland is referred to historically as the ‘Midlandian’, but now, the British-Irish term ‘Devensian’ is more commonly used. During the Devensian, the British-Irish Ice Sheet was confluent, with an ice stream flowing south down the Irish Sea.

This change in terminology reflects a dramatic revision of the views held about the maximum extent of the Irish sector of the British-Irish ice sheet (Hegarty, 2004; Clark et al., 2012).  Prior to the acquisition of high-resolution sea bed bathymetry, evidence for the last glaciation in Ireland was strictly limited to onshore geological records. The advent of marine geological data (such as submarine landforms and sediment cores taken from the bed of the Irish Sea) has changed our views about the glaciation of Ireland.

LGM of the British-Irish Ice Sheet
The Last Glacial Maximum. A confluent ice sheet covered both Britain and Ireland. The Irish Sea Ice Stream flowed southwards in the Irish Sea.

The GIF below, from Hughes et al. (2016) shows the evolution of the European Ice Sheets through time. You can see how the British-Irish Ice Sheet remained confluent with ice flowing down the Irish Sea until around 16,000 years ago!

DATED Database, by Hughes et al. 2016
The evolution of the Eurasian Ice Sheet. DATED-1 reconstruction, from Hughes et al., 2016.

During the Last Glacial Maximum, the British-Irish Ice Sheet expanded onto the continental shelf, west of Ireland and Britain (Roberts et al., 2020). There were a number of fast-flowing ice streams that delivered ice and sediment to the continental shelf edge during these periods of maximum ice extent.

Irish Ice Sheet at the Last Glacial Maximum
The configuration of the BIIS at the Last Glacial Maximum (grey shading). Maximum ice extent offshore from western Ireland is likely to have been reached at ~26,000 to 24,000 years ago. Ice retreat and re‐advance across the mid‐continental shelf is dated to between 21,000 and 18,500 years ago, and is marked by the Galway Lobe Grounding Zone Wedge (GLGZW) and the Galway Lobe Re‐advance Moraine (GLRM). Deglaciation of the inner shelf back to the Galway and County Clare coast is poorly constrained. From Roberts et al., 2020.

Onshore geological record of the glaciation of Ireland

The onshore geological record takes the form of (i) landforms and (ii) stratigraphy. Glacial landforms include: moraines, drumlins, subglacial ribs, eskers, meltwater channels, and streamlined bedrock features (Greenwood and Clark, 2008).  Click here to see a map of the glacial landforms in Ireland.

In fact, drumlins are so common in Ireland, that the word ‘drumlin’ comes from the Irish word droimnín (“littlest ridge”)!

Stratigraphic evidence of glaciation includes: widespread till, glaciofluvial outwash, ice-marginal lake deposits, erratics, and organic remains. Erratics (glacially transported boulders of a different lithology to where they now lie) record the passage of ice and can be used to reconstruct ice-flow directions.

image
Carboniferous limestone pavement on Inis Meáin. Perched granite erratics on the pavement were transported from the Galway mainland. From Roberts et al., 2020.

Moraines at the modern Irish coast (e.g. Dundalk Bay and Galway Bay) are consistent with ice flowing offshore, but until comparatively recently the full extent of this ice was mainly speculation (Haflidason et al., 1997; Ballantyne et al., 2007).

image
Geomorphological map of SW County Clare and the Shannon Estuary, showing the Kilkee-Kilrush moraine complex and the Scattery Island Moraine. The top panel also shows mapped drumlins (long axis mapped as a black line). From: Roberts et al., 2020.

You can explore all of the landforms yourself using the BRITICE Glacial Map. Click the image below. Once the map loads, press shift and use the mouse to draw a box over Ireland. This will zoom in over Ireland and the landforms will appear. You can also view the BRITICE map (Ireland Sheet).

Bedrock lineations and drumlins in the central sector of Ireland in the BRITICE Glacial Map.

Offshore geological record of the Irish Ice Sheet

Landforms on the sea bed around Ireland has provided clear geomorphological evidence that ice extended onto the continental shelf during the Quaternary (Benetti et al., 2010; Clark et al., 2012; Ó Cofaigh and Ballantyne, 2017, Ó Cofaigh et al., 2019, Roberts et al. 2020).  

https://ars.els-cdn.com/content/image/1-s2.0-S0277379117306157-gr1.jpg
Continental shelf offshore NW Ireland showing core locations and seismic profiles taken as part of the BRITICE-CHRONO project (From: Ó Cofaigh et al., 2019). The blue shows mapped moraines on the continental shelf.

These data were collected by (i) processing navigational data from boats (e.g. Olex), as well as (ii) via bespoke sea bed surveys, (e.g. the Irish National Sea Bed Survey and INFOMAR). 

As part of the BRITICE-CHRONO consortium, research cruises have also collected sub-bottom data using seismic surveys and borehole corers (Peters et al., 2016; Callard et al., 2018; Ó Cofaigh et al., 2019). These studies have documented the nature and timing of the glacial processes that led to the formation of moraines, drumlins, grounding-zone wedges, meltwater channels, iceberg plough marks on the continental shelf.

Beyond the continental shelf itself, trough-mouth fans and debris rafted by icebergs provide further evidence of the nature of the last glaciations.  Importantly, the majority of the data gathered indicate that the ice arrived at the continental shelf edge after 27,000 years ago and had departed by 21,000 years ago.

Timing of glaciation

Within this general pattern is a considerable amount of asynchronicity, both in terms of ice advance onto the shelf, and retreat back onshore.

The most northerly Donegal and Malin Sea ice streams were heavily influenced by ice flowing westwards from Scotland, while the western and south-western ice streams were more heavily influenced by oceanic forcing.

Dating of shell remains from offshore cores indicates that the most northerly ice streams advanced to the shelf edge around 27,000 years ago, while the more southerly Galway ice stream and ice from the Cork-Kerry ice centre advanced later, 24,000 years ago (Callard et al., 2020).

Irish Sea Ice Stream

On the eastern and southern side of Ireland, ice advance and retreat of the ice sheet was dominated by the behaviour of the Irish Sea ice stream; a monster of an outlet glacier that at one point drained the majority of the British-Irish Ice Sheet.

This Irish Sea Ice Stream coupled the eastern part of the Irish Ice sheet with western part of the British Ice Sheet, reaching the edge of the continental shelf and the Isles of Scilly ~25,000 years ago (Smedley et al., 2017). 

https://ars.els-cdn.com/content/image/1-s2.0-S0037073815002651-gr2.jpg
British–Irish Ice Sheet with reported palaeo-ice streams, extent at Last Glacial Maximum (LGM) and simplified bedrock geology. Named palaeo-ice streams: 1. Norwegian Channel; 2. Orkney; 3. Minch; 4. Moray Firth; 5. Hebrides, 6. North Channel—Malin Shelf; 7. Irish Sea; 8. Tyne Gap; 9. North Sea lobe. Locations described in this paper: Ul = Ullapool; SJ = Sound of Jura; TG = Tyne Gap. From: Krabbendam et al., 2015.

The huge extent of the Irish Sea ice stream made it inherently unstable, leading to rapid retreat.  By ~23,000 years ago the ice stream had retreated as far north as the St George’s Channel, while ~1000 years later it was level with Brae, Co. Wicklow and the Llyn Peninsula in north Wales (Chiverell et al., 2013).  The final collapse of the Irish Sea ice stream is not well understood and remains a point of contention.

The Younger Dryas in Ireland

The Younger Dryas was an abrupt period of intense cold that drove the readvance of glaciers from 12,900 to 11,700 years ago. This was the Younger Dryas Stadial; in Ireland, this is commonly referred to as the Nahanagan (Colhoun and Synge, 1980). 

The work by Eric Colhoun and Francis Synge remains the only one to have successfully dated this period of glacial activity in Ireland. However, other researchers have argued on the basis of stratigraphic and geomorphological evidence, that mountains across Ireland hosted small cirque glaciers during this time (Synge, 1968; Rae et al., 2004; Barr et al., 2017).

About the Author

Sam Roberson is a Quaternary geologist at the Geological Survey of Northern Ireland. He has a PhD in glaciology and is interested in the impact of Pleistocene ice sheets in the UK and Ireland. Sam helped to create the first Quaternary Geological Map of Ireland and is currently working to update of the UK Superficial Deposits map.  He is involved in field mapping and geological modelling for the survey and likes using geostatistics and scientific programming as part of his applied research into the subsurface. Sam is an avid cyclist and in 2019 rode the length of Ireland to promote awareness of Quaternary geology.

Key Publications

  • S. Roberson and Weltje, G.J. 2014. Inter-instrument analysis of particle-size analysers. Sedimentology. 61, 1157-1174.
  • Merritt, J.W., Roberson, S. and Cooper, M.R., 2018. A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel, Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin. Proceedings of the Geologists’ Association, 129, 583-609.
  • S. Roberson, Hubbard, B., Coulson, H. and Boomer, I. 2011. Physical properties and formation of flutes at a polythermal valley glacier: Midre Lovénbreen, Svalbard. Geografiska Annaler Series A, Physical Geography. 93, 71-88.
  • Barr, I.D., Roberson, S., Flood, R. and Dortch, J., 2017. Younger Dryas glaciers and climate in the Mourne Mountains, Northern Ireland. Journal of Quaternary Science, 32, 104-115.
  • Roberson, S. and Hubbard, B., 2010. Application of borehole optical televiewing to investigating the 3-D structure of glaciers: implications for the formation of longitudinal debris ridges, midre Lovenbreen, Svalbard. Journal of Glaciology, 56(195), pp.143-156.

Further Reading

Videos

Glacial deposits at Killiney Beach, Co. Dublin, Ireland

https://youtu.be/JxHR1HvCgGs

References

Ballantyne, C.K., McCarroll, D. and Stone, J.O., 2007. The Donegal ice dome, northwest Ireland: dimensions and chronology. Journal of Quaternary Science22(8), pp.773-783.

Ballantyne, C.K. and Ó Cofaigh, C., 2017. The last Irish Ice Sheet: extent and chronology. In Advances in Irish Quaternary Studies (pp. 101-149). Atlantis Press, Paris.

Barr, I.D., Roberson, S., Flood, R. and Dortch, J., 2017. Younger Dryas glaciers and climate in the Mourne Mountains, Northern IrelandJournal of Quaternary Science32(1), pp.104-115.

Benetti, S., Dunlop, P. and Ó Cofaigh, C., 2010. Glacial and glacially-related features on the continental margin of northwest Ireland mapped from marine geophysical data. Journal of Maps6(1), 14-29.

Callard, S.L., Ó Cofaigh, C., Benetti, S., Chiverrell, R.C., Van Landeghem, K.J., Saher, M.H., Gales, J.A., Small, D., Clark, C.D., Stephen, J.L. and Fabel, D., 2018. Extent and retreat history of the Barra Fan Ice Stream offshore western Scotland and northern Ireland during the last glaciation. Quaternary Science Reviews201, pp.280-302.

Callard, S.L., Ó Cofaigh, C., Benetti, S., Chiverrell, R.C., Van Landeghem, K.J., Saher, M.H., Livingstone, S.J., Clark, C.D., Small, D., Fabel, D. and Moreton, S.G., 2020. Oscillating retreat of the last British-Irish Ice Sheet on the continental shelf offshore Galway Bay, western IrelandMarine Geology420, p.106087.

Chiverrell, R.C., Thrasher, I.M., Thomas, G.S., Lang, A., Scourse, J.D., van Landeghem, K.J., Mccarroll, D., Clark, C.D., Ó Cofaigh, C., Evans, D.J. and Ballantyne, C.K., 2013. Bayesian modelling the retreat of the Irish Sea Ice Stream. Journal of Quaternary Science28(2), pp.200-209.

Clark, C.D., Hughes, A.L., Greenwood, S.L., Jordan, C. and Sejrup, H.P., 2012. Pattern and timing of retreat of the last British-Irish Ice Sheet. Quaternary Science Reviews44, pp.112-146.

Colhoun, E.A. and Synge, F.M., 1980, January. The cirque moraines at Lough Nahanagan, County Wicklow, Ireland. In Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science (pp. 25-45). Royal Irish Academy.

Greenwood, S. L., & Clark, C. D. (2008). Subglacial bedforms of the Irish Ice Sheet. Journal of Maps, 4(1), 332–357. https://doi.org/10.4113/jom.2008.1030

Haflidason, H., King, E.L., Kristensen, D.K., Helland, E., Duffy, M., Scourse, J.D., Austin, W.E.N. and Sejrup, H.P., 1997. Marine geological/geophysical cruise report on the western Irish margin: Donegal Bay, Clew Bay, Galway Bay, Irish Shelf and Rockall Trough. University of Bergen, Bergen.

Hegarty, S., 2004. Limits of Midlandian glaciation in south‐eastern Ireland. Irish Geography37(1), pp.60-76.

Hughes, A. L. C., Gyllencreutz, R., Lohne, Ø. S., Mangerud, J., & Svendsen, J. I. (2016). The last Eurasian ice sheets–a chronological database and time‐slice reconstruction, DATED‐1. Boreas, 45(1), 1–45.

Krabbendam, M., Eyles, N., Putkinen, N., Bradwell, T., & Arbelaez-Moreno, L. (2016). Streamlined hard beds formed by palaeo-ice streams: A review. Sedimentary Geology, 338, 24–50. https://doi.org/https://doi.org/10.1016/j.sedgeo.2015.12.007

Ó Cofaigh, C., Weilbach, K., Lloyd, J.M., Benetti, S., Callard, S.L., Purcell, C., Chiverrell, R.C., Dunlop, P., Saher, M., Livingstone, S.J. and Van Landeghem, K.J., 2019. Early deglaciation of the British-Irish Ice Sheet on the Atlantic shelf northwest of Ireland driven by glacioisostatic depression and high relative sea level. Quaternary Science Reviews208, pp.76-96.

Peters, J.L., Benetti, S., Dunlop, P., Ó Cofaigh, C., Moreton, S.G., Wheeler, A.J. and Clark, C.D., 2016. Sedimentology and chronology of the advance and retreat of the last British-Irish Ice Sheet on the continental shelf west of Ireland. Quaternary Science Reviews140, pp.101-124.

Rae, A.C., Harrison, S., Mighall, T. and Dawson, A.G., 2004. Periglacial trimlines and nunataks of the Last Glacial Maximum: the Gap of Dunloe, southwest Ireland. Journal of Quaternary Science: Published for the Quaternary Research Association19(1), pp.87-97.

Roberts, D. H., Ó Cofaigh, C., Ballantyne, C. K., Burke, M., Chiverrell, R. C., Evans, D. J. A., … Callard, S. L. (2020). The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin. Boreas, 49(3), 438–460. https://doi.org/10.1111/bor.12448

Smedley, R.K., Scourse, J.D., Small, D., Hiemstra, J.F., Duller, G.A.T., Bateman, M.D., Burke, M.J., Chiverrell, R.C., Clark, C.D., Davies, S.M. and Fabel, D., 2017. New age constraints for the limit of the British–Irish Ice Sheet on the Isles of Scilly. Journal of Quaternary Science32(1), pp.48-62.

Synge, F.M., 1970. The Irish Quaternary: current views 1969. Irish Geographical Studies in honour of E. Estyn Evans (Eds., Stephens, N. and Glasscock, R.), Queens University Belfast, pp.34-48.

Last Glacial Maximum

The British-Irish Ice Sheet reached its most recent maximum extent around 27,000 years ago. It was drained by many fast-flowing ice streams, and reached the edge of the continental shelf around its northern border. In the south, it terminated near the Vale of York, with a lobe penetrating down the eastern coastline as far as north Norfolk. By 18,000 years ago, it was rapidly declining and shrinking.

The Global Last Glacial Maximum

Around 27,000 years ago, ice sheets reached their maximum across the world, after a period of global cooling caused by variations in the Earth’s orbit around the sun. There was a massive ice sheet in North America (the Laurentide Ice Sheet)[1, 2], a large Eurasian Ice Sheet covering Britain, Ireland and Scandinavia as well as northern Europe[3], an ice sheet in Antarctica[4], the Himalaya and Patagonia[5, 6]. Land near the ice sheets that escaped glaciation was cold, with tundra vegetation. Northern Europe was frequented by ice-age animals such as mammoth, reindeer and arctic hare. There was a landbridge betweeb Britain and Europe, and animals could walk freely across it. Numerous human artefacts from this time are scattered across the landscape.

Ice sheets at the Last Glacial Maximum worldwide, around 27,000 to 21,000 years ago. From data in Ehlers et al., 2011.

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Just published: most comprehensive review ever of the glaciation of Antartica

A major new review of the last glaciation of the entire Antarctic Ice Sheet has just been published by Quaternary Science ReviewsThe special issue of the journal includes a suite of review papers involving an international team of experts regarding the last glaciation of the entire Antarctic Ice Sheet. This review, which comprises six review papers and an overview paper in a special issue of Quaternary Science Reviews, is now complete and all papers have been accepted for publication. As this is the most important, up to date and inclusive review ever to be attempted for the glaciation and recession of the Antarctic Ice Sheet, it represents a major step forward in our understanding of palaeo ice-sheet dynamics, provides a benchmark against which future research needs can be identified and highlighted, and provides a compilation of data unlike anything seen before, which can be used to test and calibrate numerical ice-sheet models.

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Antarctic Peninsula Ice Sheet evolution

Pre-Quaternary Antarctic Peninsula Ice Sheet evolution | Quaternary glaciation of the Antarctic Peninsula | Last Glacial Maximum | Antarctic Peninsula Ice streams | References | Comments |

This section is largely taken from Davies et al. 2012 (Quaternary Science Reviews)[1], and summarises Antarctic Peninsula Ice Sheet evolution throughout the Cenozoic, Last Glacial Maximum and into the Holocene.

Pre-Quaternary Antarctic Peninsula Ice Sheet evolution

Palaeogene (65.5 to 23.03 Ma)

Evidence for pre-Quaternary (see Table 1) glaciations of the Antarctic Peninsula mostly come from offshore seismic and drilling campaigns. There are some terrestrial records from King George Island, South Shetland Islands, and on James Ross Island.  

The continental shelf and slope, extending beyond the reach of later Quaternary ice sheets, preserves thick sedimentary strata deposited during glacials and interglacials over the last 35-40 million years (Ma)[2]. Pre-Quaternary sediments have been dated using biostratigraphy (dinoflagellate cysts)[2], isotopic dating of volcanic rocks[3-5] and strontium isotopes analysis on shells[6, 7].

Map of the Antarctic Peninsula, after Davies et al., 2012 (Quaternary Science Reviews)

The earliest ice sheets began to develop around the Palaeogene-Neogene boundary (see Table 1), circa 35 Ma[8]. Mean global temperatures were around ~4°C higher than today[9, 10].

Mountain glaciation around the Antarctic Peninsula was initiated 37-34 Ma, coinciding with the opening of the Drake Passage, the separation of the Andes and the Antarctic Peninsula, and the development of the Antarctic Circumpolar Current[11]. The Antarctic Circumpolar Current isolated Antarctica from other regimes, resulting in the development of a cooler polar climate[12].

Westerly Winds and ocean fronts around Antarctica. The Antarctic Cirumpolar Current flows around the Antarctic Continent, driven by the Southern Hemisphere Westerly winds.

These early ice sheets were thin and dynamic, fluctuating at 40,000 year cycles in response to variations in the earth’s orbit around the sun (Milankovitch cycles)[13, 14].

The relatively high mountains of the Antarctic Peninsula probably acted as a nucleus for glaciation, with cooler temperatures at higher altitudes encouraging glacierisation[1, 11].

The longest terrestrial record of glaciation comes from King George Island, South Shetland Islands, with glaciers developing from the Miocene.

Neogene (23.03 to 2.54 Ma)

Sediments from the Pacific continental margin, ~9 Ma in age, have yielded a high-resolution history of multiple ice advances and erosional episodes, indicating a persistent Antarctic Peninsula Ice Sheet[1, 15, 16].

Sedimentary evidence suggests that Pliocene ice (<3 Ma) was probably relatively thin and did not inundate the topography[17].

During this period, the West Antarctic Ice Sheet and Antarctic Peninsula Ice Sheets together grew successively larger, with periodic collapses during interglacials.

Table 1. Timescale in the Antarctic Peninsula, showing glacial events from the Cenozoic to the present day. APIS – Antarctic Peninsula Ice Sheet. JRI – James Ross Island. WAIS – West Antarctic Ice Sheet. Small ice caps began to develop in the area about5 million years ago. Large continental wide ice sheets began to develop during the Quaternary, with oscillations at 100,000 year periodicities after about 400,000 years ago.

During periods of West Antarctic Ice Sheet absence, the Antarctic Peninsula Ice Sheet remained as a series of island ice caps, and was also a refuge for plants and animals[2, 18, 19]. The East Antarctic Ice Sheet remained relatively stable during this time.

James Ross Island has yielded an excellent record of Neogene glaciations, preserved in glaciovolcanic sediments. The volcanic rocks are dateable, and the sequence provides an excellent record of glacial activity[1]. The volcanic sequences were formed by repeated volcanic eruptions (>50) beneath glacier ice from 9.9 to 2.6 Ma, forming pillow lavas and hyaloclastites[4, 5, 20].

They form the Hobbs Glacier Formation, which lies between Cretaceous marine sediments and the younger James Ross Island Volcanic Group. See Subglacial Volcanoes for more information on this.

Neogene glaciovolcanic outcrops on James Ross Island. From: Davies et al. 2012

These sediments indicate that Antarctic Peninsula ice expanded as far as James Ross and Seymour islands, with a polythermal regime[4, 20, 21]. Sedimentary facies indicate a climatic regime similar to  that in Svalbard today[22].

Ice thickness data can be deduced from these glaciovolcanic rocks[23].  Maximum ice thicknesses are  now well known for the Antarctic Peninsula since 7.5 Ma[4, 5, 20, 24].

Ice thicknesses were generally around 250-300 m around James Ross Island, but occasionally reached 850 m. Ice thicknesses were increasing towards the end of the Pliocene. This is covered in more detail under Subglacial Volcanoes.

Quaternary glaciation of the Antarctic Peninsula

Early Pleistocene (2.54 to 1 Ma)

Excepting a few glaciovolcanic sequences on James Ross Island, there is little terrestrial evidence of Early Pleistocene glaciation. Most of the data is from seismic profiling and coring of sediment drifts on the continental rise[17].

These sediments are dated by magnetic stratigraphy, tuned to the marine isotope record. These continental rise sediments indicate an ice-sheet dominated environment developing through the Pliocene into the Pleistocene, with increasing grounding on the continental shelf[25].

Middle Pleistocene (1Ma to 200 ka)

Simplified cartoon of a tributary glacier feeding into an ice shelf, showing the grounding line (where the glacier begins to float).

After the Mid-Pleistocene Transition at 1 Ma, ice streams began to develop on the continental shelf during this period, leading to the development of trough-mouth fans at their termini[17].

After 1 Ma, lower global sea levels encouraged grounding line advance well into the Bellingshausen Sea continental shelf. A positive feedback loop was established, with cooling leading to ice sheet growth and sea level lowering, which encouraged further ice sheet growth and cooling[26, 27].

During the Middle Pleistocene, ice sheets were reaching the continental shelf for longer, with more distinct glacial-interglacial cyclicity[28]. Marine sediments from the continental slope have less ice-rafted debris, which suggests that the ice sheet was bound by sea ice and ice shelves, which inhibited iceberg transport of glacial debris[17].

Terrapin Hill, a tuff cone on James Ross Island

The tuff cone Terrapin Hill on James Ross Island has been dated to 0.66 Ma, with a base resting on glacial material. The tuff cone morphology and sedimentology, however, indicates open marine, interglacial conditions[4, 21].

Late Pleistocene (200 ka to Last Glacial Maximum)

Late Pleistocene interglacials were characterised by ice shelves in the Larsen embayment. During the last interglacial, a smaller-than-present or absent West Antarctic Ice Sheet may explain globally higher sea levels[29-31].

Ice-rafted debris occurs in greater abundances on the continental slope and rise during interglacial periods during the Late Pleistocene, with more varied stone lithologies[32-34]. This is because warmer conditions during interglacials encouraged the collapse of ice shelves.

Combined with reduced sea ice, this allowed icebergs to transport debris to the continental shelf and slope. Warmer conditions would also have encouraged faster movement and increased bedrock erosion. This is therefore analogous to the situation during the Holocene.

During Late Pleistocene glacial periods, ice volumes increased markedly along the Antarctic Peninsula[1], possibly reaching 2350 m at Mount Jackson. Glacial cycles now had a dominant periodicity of 100,000 years, with around 120 m eustatic sea level change.

The Antarctic Polar Front was located further north than during earlier periods, with enhanced sea ice and reduced iceberg transport of debris[17, 34]. Thick ice streams were abundant on the continental shelf, with warm-based ice grounded on the continental shelf during glacials[35].

Last Glacial Maximum (~18,000 years ago)

There was a significant increase in ice volume during the Last Glacial Maximum[36, 37], which peaked prior to 18,000 years ago (18 ka BP). The Antarctic Peninsula Ice Sheet had an increased volume relative to today of 1.7 m eustatic sea level equivalent.

Schematic figure of geomorphology on the continental shelf around the Antarctic Peninsula. From Davies et al., 2012.

Geomorphological landforms on the continental shelf are typified by irregular, short erosional forms on the inner shelf, drumlins on the middle shelf, and elongate forms on the outer shelf[38-43].

These elongated “Mega Scale Glacial Lineations are formed in thick off lapping sequences of deformable sedimentary strata, which were deposited on the continental shelf from the Miocene onwards[17].

These landforms indicate that, at the Last Glacial Maximum, ice streams occupied bathymetric troughs and flowed out across the continental shelf around the entire Antarctic continent[44].  

These topographically-controlled ice streams scoured out their bathymetric troughs throughout Pleistocene glacials, each time leaving a record of their occurrence in the trough-mouth fans on the continental rise. These ice streams drained the Antarctic Peninsula Ice Sheet and confined its thickness to less than 400 m[45].

Schematic reconstruction of the Antarctic Peninsula Ice Sheet during the Last Glacial Maximum. From: Davies et al., 2012

The figure above, from Davies et al. (2012), is a schematic map with the likely extent, disposition and behaviour of the ice sheet around 18 ka BP.

Antarctic Peninsula ice streams

Reconstructing past (palaeo) ice streams provides an important context for understanding their recent behaviour, controls on this behaviour, and how ice streams might behave in the future[44].

Studying the basal characteristics of Antarctic palaeo ice streams means that the role of basal topography, bedrock geology and sediment erosion, transportation and deposition can be better understood.

The diagnostic sediment-landform assemblages left behind by ice streams[46] has meant that a large number of ice streams have been identified around the Antarctic continent, from both marine and terrestrial settings.

Palaeo-ice streams around the Antarctic Peninsula during and after the LGM, showing isochrones of recession. From: Davies et al., 2012 (Quaternary Science Reviews).

At the Last Glacial Maximum, palaeo-ice streams extended to the shelf edge in West Antarctica and in the Antarctic Peninsula, but in East Antarctica they usually were restricted to the mid-outer shelf[44].

These palaeo-ice streams occupied bathymetric troughs, and are identified by the glacial bedforms (such as mega-scale glacial lineations) in these troughs, and trough-mouth fans at their termini.

The outer-shelf zones of these cross-shelf troughs are characterised by soft, unconsolidated sediments, in which mega-scale glacial lineations and grounding zone wedges are preserved[44]. The inner shelf, instead, is generally composed of crystalline bedrock and has a higher bed roughness. Drumlins, grooved bedrock and meltwater channels are often observed here.

Where there is detailed geomorphological data available, the retreat styles of various Antarctic ice streams can be better understood. Three styles of retreat have been identified around the Antarctic Peninsula.

Rapid retreat with floatation and calving results in well-preserved subglacial bedforms on the continental shelf. These include mega-scale glacial lineations[47]. Marguerite Trough Ice stream is an example of an ice stream characterised by rapid recession.

Episodic retreat is recorded by mega-scale glacial lineations that are overprinted by transverse grounding-zone wedges, each recording a pause in ice stream retreat with a stationary grounding line. An example of this would be the ice stream that extended out of the Larsen A embayment on the Antarctic Peninsula[47].

Finally, slow and steady retreat is recorded by numerous closely-spaced moraines and intermittent grounding-zone wedges[47]. In the Western Ross Sea, there are six bathymetric troughs on the continental shelf. The palaeo-ice stream was about 370 km long, with a zone of glacial deposition on the outer shelf, and erosional landforms on the inner shelf.

Transverse sedimentary ridges overprint mega-scale glacial lineations throughout.  They are grounding zone wedges, and are 3-12 m high, 180 m to 8 km apart. There are also smaller moraines, 1-2 m high and 10-100 m apart[47, 48].

The geomorphological record therefore suggests that retreat varies strongly between different troughs, with three principle styles of retreat recognised. This suggests that individual ice streams respond differently to external forcings during deglaciation[47], and instead are regulated by local factors, such as drainage basin size, bathymetry and sediment supply.

The western sector of the Ross Sea is fed from two drainage basins in East Antarctica measuring 1.6 million km2 and 265,000 km2. This huge drainage basin may have meant that the outlet glaciers may have responded more slowly to external forcing.

The Marguerite Bay drainage basin, in contrast, would have been of the order of 10,000 to 100,000 km2 during the LGM, and the ice streams draining this basin would have responded more rapidly to changes in external forcing[47]. Constraints such as these are important for numerical models that attempt to replicate and predict the past and future behaviour of the Antarctic Ice Sheet.

By analysing retreat styles and rates of retreat around Antarctica, we can put more recent variations into context and determine their significance. The individual characteristics of each ice stream modulates its recession. Even under the same changes in environmental conditions and external forcings, ice streams will retreat at individual rates. Ice-stream behaviour and grounding line retreat is therefore unique to every ice stream. In order to constrain future ice stream behaviour, a detailed understanding of subglacial bed properties and bed geometry is required[44].

Deglaciation and recession of the Antarctic Peninsula Ice Sheet

Around the Antarctic Peninsula, recession from the outer shelf began at about 17.5 ka BP, from the middle shelf around 14 ka BP and the inner shelf around 11 ka BP. Ice streams receded both rapidly and episodically, depositing grounding-line wedges during periods of stand-still[40].

Radiocarbon dates from marine sediment cores across the continental shelf provide an indication of ice marginal positions during recession.

Schematic map showing isochrones of ice sheet and ice stream recession around the Antarctic Peninsula. From: Davies et al. 2012

Further reading

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References


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4.            Smellie, J.L., Johnson, J.S., McIntosh, W.C., Esser, R., Gudmundsson, M.T., Hambrey, M.J., and van Wyk de Vries, B., 2008. Six million years of glacial history recorded in volcanic lithofacies of the James Ross Island Volcanic Group, Antarctic Peninsula. Palaeogeography, Palaeoclimatology, Palaeoecology, 2008. 260(1-2): p. 122-148.

5.            Smellie, J.L., McArthur, J.M., McIntosh, W.C., and Esser, R., 2006. Late Neogene interglacial events in the James Ross Island region, northern Antarctic Peninsula, dated by Ar/Ar and Sr-isotope stratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology, 2006. 242(3-4): p. 169-187.

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11.          Siegert, M.J., 2008. Antarctic subglacial topography and ice-sheet evolution. Earth Surface Processes and Landforms, 2008. 33: p. 646-660.

12.          Eagles, G. and Livermore, R.A., 2002. Opening history of Powell Basin, Antarctic Peninsula. Marine Geology, 2002. 185(3-4): p. 195-205.

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15.          Bart, P.J. and Anderson, J.B., 2000. Relative temporal stability of the Antarctic ice sheets during the late Neogene based on the minimum frequency of outer shelf grounding events. Earth and Planetary Science Letters, 2000. 182(3-4): p. 259-272.

16.          Pudsey, C.J., 2002. Neogene record of Antarctic Peninsula glaciation in continental rise sediments: ODP Leg 178, Site 1095, in Ocean Drilling Program Scientific Results, Vol. 178, P.F. Barker, et al., Editors. Texas A&M University: College Station, Texas. p. 1-25 (CD-ROM).

17.          Cowan, E.A., Hillenbrand, C.-D., Hassler, L.E., and Ake, M.T., 2008. Coarse-grained terrigenous sediment deposition on continental rise drifts: A record of Plio-Pleistocene glaciation on the Antarctic Peninsula. Palaeogeography, Palaeoclimatology, Palaeoecology, 2008. 265(3-4): p. 275-291.

18.          Convey, P., Gibson, J.A.E., Hillenbrand, C.-D., Hodgson, D.A., Pugh, P.J.A., Smellie, J.L., and Stevens, M.I., 2008. Antarctic terrestrial life – challenging the history of the frozen continent? Biological Reviews, 2008. 83(2): p. 103-117.

19.          Convey, P., Stevens, M.I., Hodgson, D.A., Smellie, J.L., Hillenbrand, C.-D., Barnes, D.K.A., Clarke, A., Pugh, P.J.A., Linse, K., and Cary, S.C., 2009. Exploring biological constraints on the glacial history of Antarctica. Quaternary Science Reviews, 2009. 28(27-28): p. 3035-3048.

20.          Smellie, J.L., Haywood, A.M., Hillenbrand, C.-D., Lunt, D.J., and Valdes, P.J., 2009. Nature of the Antarctic Peninsula Ice Sheet during the Pliocene: Geological evidence and modelling results compared. Earth-Science Reviews, 2009. 94(1-4): p. 79-94.

21.          Hambrey, M.J., Smellie, J.L., Nelson, A.E., and Johnson, J.S., 2008. Late Cenozoic glacier-volcano interaction on James Ross Island and adjacent areas, Antarctic Peninsula region. Geological Society of America Bulletin, 2008. 120(5-6): p. 709-731.

22.          Glasser, N.F. and Hambrey, M.J., 2001. Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes. Journal of the Geological Society, London, 2001. 158: p. 697-707.

23.          Smellie, J.L., Rocchi, S., and Armienti, P., 2011. Late Miocene volcanic sequences in northern Victoria Land, Antarctica: products of glaciovolcanic eruptions under different thermal regimes. Bulletin of Volcanology, 2011. 73(1): p. 1-25.

24.          Smellie, J.L., McIntosh, W.C., and Esser, R., 2006. Eruptive environment of volcanism on Brabant Island: Evidence for thin wet-based ice in northern Antarctic Peninsula during the Late Quaternary. Palaeogeography, Palaeoclimatology, Palaeoecology, 2006. 231(1-2): p. 233-252.

25.          Smith, T.R. and Anderson, J.B., 2010. Ice-sheet evolution in James Ross Basin, Weddell Sea margin of the Antarctic Peninsula: The seismic stratigraphic record. GSA Bulletin, 2010. 122(5/6): p. 830-842.

26.          Huybrechts, P., 1990. A 3-D model for the Antarctic ice sheet: a sensitivity study on the glacial-interglacial contrast. Climate Dynamics, 1990. 5: p. 79-92.

27.          Barker, P.F., Barrett, P.J., Cooper, A.K., and Huybrechts, P., 1999. Antarctic glacial history from numerical models and continental margin sediments. Palaeogeography Palaeoclimatology Palaeoecology, 1999. 150(3-4): p. 247-267.

28.          Hillenbrand, C.-D. and Ehrmann, W., 2005. Late Neogene to Quaternary environmental changes in the Antarctic Peninsula region: evidence from drift sediments. Global and Planetary Change, 2005. 45(1-3): p. 165-191.

29.          Birkenmajer, K., 1982. Pliocene tillite-bearing sucession of King George Island (South Shetland Islands, Antarctica). Studia Geologica Polonica, 1982: p. 77-72.

30.          Mercer, J.H., 1978. West Antarctic Ice Sheet and CO2 Greenhouse effect – threat of disaster. Nature, 1978. 271(5643): p. 321-325.

31.          Overpeck, J.T., Otto-Bliesner, B., Miller, G.H., Muhs, D.R., Alley, R.B., and Kiehl, J.T., 2006. Palaeoclimatic evidence for future ice sheet instability and rapid sea level rise. Science, 2006. 311(no. 5768): p. 1747-1750.

32.          Pudsey, C.J., Barker, P.F., and Larter, R.D., 1994. Ice sheet retreat from the Antarctic Peninsula shelf. Continental Shelf Research, 1994. 14(15): p. 1647-1675.

33.          Pudsey, C.J., 2000. Sedimentation on the continental rise west of the Antarctic Peninsula over the last three glacial cycles. Marine Geology, 2000. 167: p. 313-338.

34.          Ó Cofaigh, C., Dowdeswell, J.A., and Pudsey, C.J., 2001. Late Quaternary Iceberg Rafting along the Antarctic Peninsula Continental Rise and in the Weddell and Scotia Seas. Quaternary Research, 2001. 56: p. 308-321.

35.          Reinardy, B.T.I., Pudsey, C.J., Hillenbrand, C.-D., Murray, T., and Evans, J., 2009. Contrasting sources for glacial and interglacial shelf sediments used to interpret changing ice flow directions in the Larsen Basin, Northern Antarctic Peninsula. Marine Geology, 2009. 266(1-4): p. 156-171.

36.          Huybrechts, P., 2009. GLOBAL CHANGE West-side story of Antarctic ice. Nature, 2009. 458(7236): p. 295-296.

37.          Huybrechts, P., 2002. Sea-level changes at the LGM from ice-dynamic reconstructions of the Greenland and Antarctic ice sheets during the glacial cycles. Quaternary Science Reviews, 2002. 21(1-3): p. 203-231.

38.          Wellner, J.S., Heroy, D.C., and Andersen, J.B., 2006. The death mask of the Antarctic ice sheet: comparison of glacial geomorphic features across the continental shelf. Geomorphology, 2006. 75: p. 157-171.

39.          Ó Cofaigh, C., Dowdeswell, J.A., Allen, C.S., Hiemstra, J.F., Pudsey, C.J., Evans, J., and Evans, D.J.A., 2005. Flow dynamics and till genesis associated with a marine-based Antarctic palaeo-ice stream. Quaternary Science Reviews, 2005. 24(5-6): p. 709-740.

40.          Ó Cofaigh, C., Dowdeswell, J.A., Evans, J., and Larter, R.D., 2008. Geological constraints on Antarctic palaeo-ice-stream retreat. Earth Surface Processes and Landforms, 2008. 33(4): p. 513-525.

41.          Ó Cofaigh, C., Justin, T., Julian A, D., and Carol J, P., 2003. Palaeo-ice streams, trough mouth fans and high-latitude continental slope sedimentation. Boreas, 2003. 32: p. 37-55.

42.          Ó Cofaigh, C., Larter, R.D., Dowdeswel, J.A., Hillenbrand, C.-D., Pudsey, C.J., Evans, J., and Morris, P., 2005. Flow of the West Antarctic Ice Sheet on the continental margin of the Bellingshausen Sea at the Last Glacial Maximum. Journal of Geophysical Research, 2005. 110: p. B11103.

43.          Ó Cofaigh, C., Pudsey, C.J., Dowdeswel, J.A., and Morris, P., 2002. Evolution of subglacial bedforms along a palaeo-ice stream, Antarctic Peninsula continental shelf. Geophysical Research Letters, 2002. 29: p. 41-1 – 41-4.

44.          Livingstone, S.J., O Cofaigh, C., Stokes, C.R., Hillenbrand, C.-D., Vieli, A., and Jamieson, S.S.R., 2012. Antarctic palaeo-ice streams. Earth-Science Reviews, 2012. 111(1-2): p. 90-128.

45.          Bindschadler, R., 2006. The environment and evolution of the West Antarctic ice sheet: setting the stage. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2006. 364(1844): p. 1583-1605.

46            Stokes, C.R. and C.D. Clark, 1999. Geomorphological criteria for identifying Pleistocene ice streams. Annals of Glaciology, 28: 67-74.

47.           Ó Cofaigh, C., J.A. Dowdeswell, J. Evans, and R.D. Larter, 2008. Geological constraints on Antarctic palaeo-ice-stream retreat. Earth Surface Processes and Landforms, 33(4): 513-525.

48.          Shipp, S.S., J.S. Wellner, and J.B. Anderson, 2002. Retreat signature of a polar ice stream: subglacial geomorphic features and sediments from the Ross Sea, Antarctica. Geological Society of America Bulletin, 111: 1486-1516.