Re-advance of the last British- Irish Ice Sheet during Greenland Interstade 1 (GI-1): the Wester Ross Re-advance, NW Scotland
The importance and nature of glacial records in terms of global climate and environmental situations has become apparent in recent years, and over a century’s worth of investigation has greatly aided the scientific community in their understanding, and thereafter in the reconstruction of paleoclimate, environmental influences and extent thereof (Broecker, 2002). Glacial records are commonly known as proxies: Proxy data is the indirect measure of past environmental and climatic conditions, usually gathered from natural recorders of climate variability (NGDC, 2008). Glacial Landforms such as drumlins, ribbed moraines and end moraines provide a record of the nature, scale and timing of ice-sheet oscillations.
There are many uncertainties surrounding the last British-Irish Ice-Sheet (BIIS), as extensive amounts of glacial records are hidden beneath the sea. Most evidence would suggest that two dynamic ice lobes came offshore across the shelf from both Ireland and Scotland, resulting in the entire North West shelf to be extensively glaciated during the ice age. Data shows that an extensive ice sheet margin extended offshore onto the shelf, and may have reached the shelf edge at the Last Glacial Maximum (LGM). The deglaciation of the Last BIIS was helped by the Minch paleo-ice stream at the Late Devensian glaciation, which drained the North-West sector of the last British ice sheet. This occurred just after the peak of glaciation around 22,000 years ago until approximately 17,000 years ago (BGS, 2007).
At the LGM much of Northern Britain was extensively covered by a 15km thick ice sheet (Seicgart, 2001). During the LGM, the British-Irish Ice Sheet extended westwards across the Wester Ross into the North Minch (Ballantyne et al, 2009). The Ice- sheet which was formed joined with the Outer Hebrides to create a major ice stream which moved North-Westwards towards the Atlantic Shelf.
Evidence extracted from the Garilough moraine in the North West Scotland provides the starting point of a site which demonstrates the best representative assemblage of landforms, a feature formed by the Wester Ross readvance (WRR) of the Late Devensian ice sheet (Ballantyne (1993). According to Robinson and Ballantyne, (1979) it .provides us with important geomorphological evidence that demonstrates a readvance of the Late Devensian ice-sheet in north-west Scotland, the WRR. Robinson and Ballantyne, (1979) also identified an area to which extends from Applecross to the north of Loch Broom. And in reference to any other relationships between the Wester Ross Moraine and former ice limits elsewhere in the area are uncertain (Ballantyne et al., 1993) But has since been reviewed and major developments are seen in the following. Ballantyne (1993) concludes that the Garilough Moraine demonstrates an end moraine created by the Late Devensian ice-sheet, around 13,500–13,000 years ago and is important to reference locality for the geomorphological expression of the event.
According to Ballanytne et al (2009) the chain of moraine ridges which were sculpted by the LGM and previously identified by Robinson and Ballantyne (1979) see fig 1, marks the limit of an extensive Ice-Sheet re-advance. Everest et al (2006) data suggests that the WRR may have been the result of glacio-climatic changes associated with Heinrich Event 1 in the North Atlantic (c. 16.5–18 ka BP). However, we cannot preclude the possibility that the ice-sheet oscillation may have post-dated this event and relates to ice-sheet equilibration in response to rising sea levels or topographic pinning in the aftermath of H1(Everest et al 2006).
Ballantyne et al (2009) suggests that the chain of moraine ridges which were sculpted by the LGM marks the limit of an extensive Ice-Sheet re-advance. Torridon Sandstone boulders were dated from four moraines; to provide us with further evidence that marks the limit of the Wester Ross Re-advance. The Fourteen samples that were dated using a cosmogenic dating technique, yielded average ages between 13.5 ± 1.2ka and 14.0 ± 1.7ka. This supports the proposal that there was a substantial cover of glacial ice over the low ground of the North-West of Scotland. Furthermore evidence supports that the cover of glacial ice persisted during the early part of the late glacial Interstade, Greenland Interstade 1 (GI-1). Research also suggests additional evidence that the regional re-advance was stimulated by renewed cooling and/or increased snow fall at sometime between 13.5 ± 1.2ka to 14.0 ± 1.7ka. Ballantyne et al (2009) assume that the WRR probably occurred in response to rapid short-lived cooling during the Older Dryas climatic reversal (Greenland Interstade 1). Although Ballantyne et al (2009) do not discard the possibilities that the WRR may represent an ice-margin response to a later climatic reversal during the Lateglacial Interstade or stabilization. And also that the readvance of the ice margin following rapid offshore calving is without a doubt not discounted either (Ballantyne et al 2009).
The data from Ballantyne et al. (2009) concludes that the GI-1 probably occurred during the Older Dryas event of c.14.1-13.9ka. The drastic cooling at this period of time correlates with the Older Dryas age and therefore resulted in the Wester Ross re-advance (WRR). Ballantyne et al. (2009), demonstrate that the WRR represents an ice margin oscillation as a result of the positive mass balance in an extensive ice sheet, with evidence that supports this suggestion.
Primary evidence into the notion as to whether there were substantial ice caps in the NW of Scotland during the Greenland Interstade 1 derives from Bradwell et al (2008). Bradwell et al (2008) mapped a continuous sequence of 40 recessional moraines that stretched from 10 km offshore to the Wester Ross Mountains. This finding demonstrates that there was a number of offshore and rather fewer onshore moraines, and suggest that there was an active oscillatory retreat of the last ice sheet in the North West of Scotland. From the collected samples eight of the ten that were dated using cosmogenic 10Be produced exposure ages for boulders on three onshore moraines that yielded an apparent mean exposure age of ca. 13.5 ka.
The work of Bradwell et al (2008) proposes that there was an active oscillatory retreat of the last ice sheet in NW Scotland. Eight samples obtained from three onshore moraine boulders in the north-west of Scotland produced exposure ages of between 13-14 ka BP as an estimate of the period of time where dynamic ice caps persisted in the north-west of Scotland.
The pattern seen throughout the moraines in the Summer Isles region (see Fig.2) strongly suggests that there was a dynamic oscillatory ice-front retreat after the WRR and before the Younger Dryas (ca. 15–11 ka BP). Bradwell et al. (2008) suggests that Lateglacial ice-cap oscillations in Scotland reflect the complex interplay between changing temperature and precipitation regimes during this climatically unstable period. This supports the notion of a readvance in the Wester Ross as previously suggested my Ballantyne et al (2009); Ballantyne and Ostone, (2011) and others who are mentioned later. Bradwell et al (2008), in their discussions concluded that firstly between 13 and 14 ka BP substantial, dynamic ice caps which existed in the North-West Scotland, secondly there were large active glaciers that probably survived throughout the Lateglacial Interstadial; and lastly during the Older Dryas period (ca. 14 ka BP) ice caps were thicker and considerably more extensive than in the subsequent Younger Dryas Stadial. Bradwell’s results produced cosmogenic-exposure ages to conclude that there was an existence of substantial ice caps in NW Scotland throughout the first half of Greenland Interstade 1 (GI-1). These findings show a significant relationship between both the moraine building phase and the Older Dryas event which is seen widely around the North Atlantic (Bradwell et al 2008). Thus evidence from geomorphological and cosmogenic-isotope dating in the North West of Scotland demonstrates that two substantial ice caps did indeed exist and remained active during the Lateglacial Interstadial (Bradwell et al 2008).
According to Ballantyne and Ostone (2011) the topic of debate to whether glacier ice actually persisted in the Scottish highlands throughout the Lateglacial Interstade [Greenland Interstade 1] has been ongoing for the past forty years. During the Greenland interstade 1 it was thought to be a period of time with a moderately warm climate which separated the Late Devensian Dimlington Stade (ca. 31-14.7 ka) from the Loch Lomond (Younger Dryas) Stade 12.9-11.7 ka). Ballantyne and Ostones’ (2011) paper revises the chronology suggested by Bradwell et al (2008), and disproves the contentions that firstly; there were substantial, dynamic ice caps which existed in the North West of Scotland between 13 and 14 ka BP, secondly; that large active glaciers probably survived throughout the Lateglacial Interstadial’; and lastly that during the Older Dryas period (ca. 14 ka) ice caps in NW Scotland were thicker and considerably more extensive than in the subsequent Younger Dryas Stadial’ as proposed by Bradwell et al. (2008). Ballantyne and Ostones’ (2011) recalibration of exposure ages using cosmogentic dating (10Be), has increased the original mean ages obtained by Bradwell et al (2008) by 6.5-12%. Thus implying that moraine deposition possibly occurred between 14.3 and 15.1 ka, to produce a probable age of ca.14.7 ka based on paleoclimatic considerations (Ballantyne and Ostone, 2011).
Ballantyne and Ostone (2011) do not fully disprove that there may have been a possibility that glacier ice could have survived the Lateglacial Interstade at varies favourable sites on high ground. Whereas Finlayson et al. (2011), who are cited in Ballantyne and Ostones’ (2011) research argues that this may not be case. The significant shift to 10Be exposure ages recalibrated by Ballantyne and Ostone (2011) through the use of locally derived 10Be production rates creates some implications for previous published 10Be exposure ages of that by Bradwell et al (2008) who represent the timing of deglaciation or glacial readvances elsewhere in the British Isles (Ballantyne and Ostone, 2011). Ballantyne and Ostone (2011) results after recalibration of 22 10Be ages showed an increase to the mean age from 13.471.30 ka to 14.340.84 ka. Evidence to which supports a regional readvance of an ice sheet margin or what is more commonly referred to as the WRR, primarily proposed by Ballantyne and Robinson (1979). Evidence according to Ballantyne and Ostone (2011) implicates that the readvance most likely occurred during the early part of the Lateglacial Interstade (GI-1e; ca. 14.7–14.1 ka) or towards the end of the Dimlington Stade (GS-2). A reasonable assumption for the timing of the readvance was the Dimlington Stade- Lateglacial Interstade transition at ca. 14.7 ka suggests Ballantyne and Ostone, (2011). There reason for this was that the northwards movement of the oceanic polar front had resulted in an enhanced cyclonic activity and the amplified snow fall across the North West of Scotland but before the Lateglacial Interstade. Thus implying that extensive ice masses were most definitely were present on lower ground and within fjords in North West Scotland immediately prior to the Lateglacial Interstade. But suggest that at this time there does not require persistence of extensive ice cover in these areas under the much warmer conditions of the interstade (Ballantyne and Ostone, 2011).
Findings in Stoker et al. (2009), confirmed by radiocarbon dating of marine shells, that suggest deglaciation at the seaboard of Western Scotland was on-going between 14-13ka BP, during the GI-1. Evidence from the seafloor moraine ridges also supports the idea of oscillatory retreat of the last Ice Sheet. The proposed research further supports the re-advance of the last BIIS during the GI-1.
Ballantyne and Ostone (2011), provide evidence of exposure ages of boulders on moraines suggests that the ice sheet was more extensive during the Older Dryas (c. 14. 0) than the Younger Dryas stade (12.9-11.7ka). These findings are in juxtaposition with that of those in Stoker et al. (2009) and the data proposed in Ballantyne et al. (2008). Thus, the investigation of past ice mass changes such as the WRR provides an analogue for how ice masses will behave in response to future warming.
McCabe et al (1998) proposed that there is direct stratigraphical and morphological evidence from four areas in Scotland and on the adjacent continental shelf for a widespread ice readvance after 15- 14 C kyr BP but pre-dating the Loch Lomond Stadial readvance. This phase of ice-sheet activity can be correlated broadly with the Killard Point Stadial in the north Irish Sea area because of the scale of ice-sheet activity and the fact that ice dispersal centres in Scotland were a major contributor to the overall ice system in the north ISB and on the coast of Northern Ireland.
According to the British Geological Survey (2007) new dating work, based on the length of time fresh boulder surfaces have been exposed to cosmic rays, has shown that the WRR occurred around 14-16 thousand years ago. The BGS through the integration of onshore mapping data with the newly acquired sea-floor imagery have established the stability and reliability of the ice-margin moraine system in the Wester Ross region, in association with the onshore WRR moraines onland (BGS, 2007). Evidence concludes that the WRR event wasn’t just simply an ice-sheet readvance but an oscillating ice-sheet margin as supported by Robinson and Ballantyne, (1979); Ballantyne (1993); Ballanytne et al (2009); Bradwell et al (2008) and Stoker et al. (2009). Occurring ca. 13-14 ka and supported by Ballanytne et al (2009) and Ballantyne and Ostone (2011). Confirming that the WRR probably occurred in response to rapid short-lived cooling during, the Older Dryas climatic reversal (Greenland Interstade 1) (Ballantyne et al, 2009). Research demonstrates that some time after the WRR, ice caps still persisted in the low grounds of Scotland at the loch lomand stadial (around 12,900 to 11,500 years ago, BGS, 2007) as suggested by Ballantyne and Ostone (2011) but is argued against by Finlayson et al. (2011). It can be seen through the evidence presented that there is a controversy between the limits of the WRR and its timing at the last BIIS and research still continuous to explore the past parameters and glacial systems that occurred during Greenland Interstade 1.
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Fig 1: A map showing the limits of the Wester Ross Readvance as cited in Ballantyne et al (2009) and previous adapted by Robinson and Ballantyne (1979). Bedrock samples from near Creag and Meall na h-Uamhaidh and the Achitibuie moraine are also indicated on the map (Ballantyne et al (2009).
Figure 2: A map of merged multi-beam and hill-shaded surface models, showing the topography and bathymetry of the Sumer Isles Region, NW Scotland. The white solid lines illustrate moraines mapped onshore and offshore: dashed white lines are inferred ice-front positions. WRR, Wester Ross Readvance; LLS, Loch Lomond Stadial (YD) limits. Previously dated Late Devensian sites are also shown. Map data collected by Intermap Technologies, 2005 (Bradwell et al 2008).
