Timing of glacier response to Younger Dryas climatic cooling in Scotland

Global and Planetary Change 79 (2011) 264–274

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Global and Planetary Change j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g l o p l a c h a

Timing of glacier response to Younger Dryas climatic cooling in Scotland Alison MacLeod a,⁎, Adrian Palmer a, John Lowe a, James Rose a,b, Charlotte Bryant c, Jonathan Merritt d a

Centre for Quaternary Research, Department of Geography, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, UK c NERC Radiocarbon Facility (Environment), Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, Glasgow, G75 0QF, UK d British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH10 6HR, UK b

a r t i c l e

i n f o

Article history: Received 3 March 2010 Accepted 5 July 2010 Available online 14 July 2010 Keywords: radiocarbon dating glaciolacustrine varves Bayesian-based age modelling Younger Dryas Scotland

a b s t r a c t Much speculation surrounds the ‘Younger Dryas’ (YD) event, a cold interval with abrupt thermal transitions that, on evidence from Greenland ice cores, lasted from 12.85 until 11.65 ka cal BP (GS-1: Greenland Stadial 1). Stratigraphic records for this interval are often well resolved and fall within the range of a number of dating methods, yet its cause, propagation and regional environmental effects remain unclear. In Scotland this climatic downturn led to a readvance of glacial ice, the precise timing of which has proved difficult to determine. Here we present new varve and radiocarbon evidence that indicates that the last glacier to occupy the Loch Lomond area, the type locality for the YD in Scotland, achieved its maximum extent very late in the YD, after c. 12.0 ka cal BP. This accords with evidence obtained for another former major glacial system in the Scottish Highlands, in the Lochaber area, and with the maximum advance of some large YD ice masses in Norway. The new empirical data from Scotland provide robust chronological constraints for validating numerical simulations of ice growth during the YD, and for assessing the links between climate change and glacier response. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The YD is a short-lived but intensely cold climatic reversal, first described in Scandinavia (Mangerud et al., 1974; Mangerud, 1987), that occurred towards the end of the Last Glacial Stage (MIS-2), between c. 12,850 and 11,650 years ago (12,900 and 11,700 GICC05 yr b2k) (before 2000; see Rasmussen et al., 2006). Evidence for a cold climatic downturn which dates approximately to this interval has been reported from widely-scattered locations, leading some to conclude that the YD had a global impact (Mangerud, 1987; Gosse et al., 1995; Peteet, 1995; Andres et al., 2003). The event appears to have been most pronounced in the northern hemisphere during a time of high summer insolation, when glaciers that had previously been in progressive decline experienced significant readvance, while some deglaciated areas saw the re-establishment of glacier ice. A number of forcing factors have been proposed for this climatic reversal. The current majority view seems to be that it was driven by a slowing down or cessation of the North Atlantic thermohaline circulation (THC; Clark et al., 2001), though alternative factors have been suggested, such as reduced solar activity, extraterrestrial impact or shifts in surface wind patterns, acting either singly or in combination (Renssen et al., 2000; Firestone et al., 2007; Brauer et al., 2008).

⁎ Corresponding author. Tel.: + 44 1784 443563; fax: + 44 1784 472836. E-mail address: [email protected] (A. MacLeod). 0921-8181/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2010.07.006

Testing these hypotheses is difficult for several reasons. Firstly, annually-resolved ice-core records show that the thermal transitions at the start and end of the YD took place within a few decades (Rasmussen et al., 2006), a level of chronological precision that is difficult to achieve in most other geological contexts (Lowe et al., 2007). Secondly, climatic reconstructions are based on proxy records, some of which may reflect climate indirectly, while others relate to environmental responses that lagged behind climatic shifts. Third, the YD may have been climatically more complex than previously assumed, even within the confines of the North Atlantic region. For example, the Greenland ice-core records contain evidence of a slight but progressive warming throughout the latter part of the YD (Rasmussen et al., 2006) while a lake sediment record from southern Greenland suggests anomalously mild summer temperatures throughout the YD (Björck et al., 2002). Ebbesen and Hald (2004) and Bakke et al. (2009) have also concluded that climatic conditions in the Nordic Seas and Norway were quite variable during the second half of the period, leading to retreat of the margins of YD cirque glaciers in Norway well before the onset of the Holocene (Mangerud, 1980). Recent attempts to simulate the growth of YD ice in Scotland using numerical models have generated equivocal results that do not precisely match reconstructions based on empirical data; this may reflect assumptions or simplifications in the models, such as the lack of quantified bathymetric information (e.g. Golledge et al., 2008; Golledge, 2010). Here we report new evidence that provides more precise chronological constraints for growth of YD ice in the Loch Lomond

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area of west-central Scotland, the type area for this event in Britain. It is based on the first published Lateglacial varve chronology for the UK which is specifically tied by robust radiocarbon analysis and Bayesian age modelling to the calendar timescale. The results indicate that maximum expansion of the YD Loch Lomond Glacier took place about, or later than, 12.0 ka cal BP, a finding that agrees with evidence reported from the Lochaber region, further north in Scotland (Palmer et al., 2010) and with some evidence for growth of the Main Inland Ice Sheet in Norway during the YD (Mangerud et al., 2010). 2. Geological and site context There is general consensus that much of the western Highlands of Scotland was covered by an ice cap with radiating outlet glaciers during the YD (Fig. 1), though opinions differ on the precise maximal limits achieved by the ice in some regions (Sissons, 1979; Benn, 1997). The stratotype evidence for this event in the UK is to be found around the southeastern margin of Loch Lomond where a conspicuous set of glacigenic landforms and sediments mark the terminus of a former piedmont glacier (Rose, 1981; Fig. 2). Marine shells and foraminiferal tests found within these deposits demonstrate that ice readvanced from the Highlands to erode fossiliferous marine sediments deposited on lower ground after the retreat of the Late Devensian (MIS-2; LGM) ice sheet, evidence first noted by Jack (1876) and elaborated by Simpson (1933), who introduced the local term Loch Lomond Readvance. Subsequent research has generated abundant evidence to show that ice readvanced throughout many other parts of the Scottish Highlands as a result of climatic cooling at around the time of the YD (e.g. Sissons, 1979; Benn, 1997). Here we focus on new evidence obtained from two sites, Croftamie and Bogwood (Fig. 2), that enables the timing and duration of the maximum extent of the YD Loch Lomond Glacier to be established more precisely than was possible hitherto.

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At Croftamie, south east of Loch Lomond (UK National Grid Reference NS 4718 8604; Lat: 56°2′34.23″N Long: 4°27′15.71″W; Figs. 2 and 3), a 2.5 m sediment sequence consists, from the base upwards, of a lower till deposited during the LGM, upon which a thin organic bed has been deposited (Rose, 1981; Rose et al., 1988; Rose and Lloyd-Davies, 2003). The organic layer, which contains seeds and other well preserved plant debris (Salix leaves and Polytrichum fragments; Fig. 4) as well as disarticulated arthropod remains (Coope and Rose, 2008), was initially radiocarbon-dated to 12,782–11,846 cal yr BP (sample Q-2673; Table 1) using a bulk sediment sample (Rose et al., 1988). It is overlain by laminated silts and clays which are buried and partly deformed by an upper till of YD age. The laminated silts were investigated from a number of exposures by Rose et al. (1988) and Rose and Lloyd-Davies (2003). Examination and measurement of the coarse and fine-grained sediment couplets at the macro-scale revealed large variations in the number present (between 50 and 153). This approach is considered unreliable as it does not allow in-depth analysis of the depositional processes operating and has been superseded in this research with detailed thin section micromorphology which enables microscopic examination of sedimentary features and hence provides greater analytical precision. Bogwood (Fig. 2) lies outside the Loch Lomond Readvance terminal moraine (UK National Grid Reference NS 5169 8367; Lat: 56°1′22.57″N Long: 4°22′51.87″W), in the lower catchment of the Blane River, which today flows into Loch Lomond. In this vicinity a 12 m succession of laminated silts and clays is preserved, which have not been over-ridden by ice and thus remain largely undisturbed (Fig. 5). These accumulated in a lake (Proglacial Lake Blane) which formed in the lower Blane and Endrick valleys when the YD ice margin advanced to block drainage to Loch Lomond (Rose, 1981). Critically, therefore, the glacier margin over-rode the site of Croftamie but did not extend as far as Bogwood. 3. Methods 3.1. Sampling The Croftamie site was re-excavated in 2007 to obtain new material for radiocarbon dating and to establish a more robust radiocarbon chronology. At the same time, samples of the overlying laminated sediments were collected for detailed sedimentological and micromorphological analysis (Fig. 5A). The samples for thin-section micromorphological analysis were collected using overlapping Kubiena tins. The Bogwood site was sampled in June 2006 using an ultrasonic coring device which recovered 15 m of cores that collectively encompass the full sequence of laminated sediment, with overlaps between cores to ensure 100% sequence recovery. 3.2. Lithological examination

Fig. 1. Location of Proglacial Lake Blane at the margin of the Loch Lomond outlet glacier at the southern margin of the Younger Dryas ice cap. The map shows the extent of glacier ice cover in Scotland during the Younger Dryas (Loch Lomond Readvance) based on a composite figure of mapped limits collated by Golledge et al. (2008) and the numerically-modelled maximal YD limits reported by Hubbard (1999).

The Bogwood cores were analysed for variation in chemical components using an ITRAX™ XRF Corescanner (Croudace et al., 2006) operating at an analytical resolution of 500 μm. Block samples of the laminated sediment units in both sites were prepared for thin section production using standard impregnation methods developed for clay-rich samples (Palmer et al., 2008b). The full laminated sequence at Croftamie was prepared in this way. For the Bogwood sequence, selected samples were extracted from those parts of the record that exhibited laminations with a thickness of a centimetre or less. These were used to calibrate interpretations of the same deposits based on ITRAX™ analysis (see below), and especially to confirm depositional processes and the position and nature of lithological boundaries (Fig. 5B). The laminae in both sequences were examined in thin section for description of micro-structural features using an Olympus™ BX51 microscope, with digital archive images captured and measured using a Pixera Penguin 600es Camera™ linked to Image Pro-Express™ software.

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Fig. 2. Location of Croftamie and Bogwood, sampled for radiocarbon dating and the study of varves. Also shown is: i) the maximal extent of the glacier in the southeastern part of the Loch Lomond basin during the Younger Dryas; ii) the extent of Proglacial Lake Blane while the Loch Lomond glacier was at this maximal extent; iii) the location of the overflow channel at Ballat through which Lake Blane drained into the Forth valley and iv) the Younger Dryas corrie glacier at Balglass. The location of the region within western central Scotland is shown on the inset figure along with the ice-flow directions during the LGM and Loch Lomond (YD) Readvance.

3.3. Radiocarbon measurement Radiocarbon measurements were obtained from bulk organic material and from plant macrofossil samples extracted from the

Croftamie organic layer in order to establish a chronology based on modern methodologies and to test the validity of the determination reported by Rose et al. (1988). The thickness of the layer is generally about 2 cm, though in places it separates into two less distinct thinner

Fig. 3. Temporary exposure at Croftamie. A) The sediments sampled for this study. B) Schematic sediment log. C) Description of the sequence. D) Interpretation summarising the results arising from this study.

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constituent organic components, nine discrete samples were analysed (Table 1). Of these, seven consisted of single-species plant macrofossil fragments, one comprised heterogeneous seeds, with one further sample composed of the fine fraction (c. 200–63 μm) of undifferentiated organic material. The fine-grained organic material was extracted from the bulk sediment by heavy liquid density separation using Sodium polytungstate (SPT). To test whether SPT had an adverse effect on radiocarbon content, two aliquots of IAEA-C5 standard (23.05 pMC ±0.02; Rozanski et al., 1992) were processed with all pre-treatment protocols used for samples. Both results were within 2 sigma confidence limits of the IAEA-C5 consensus value. All other samples were processed following the protocols advised by the NERC Radiocarbon Facility (Environment) and sieved to retain the fraction N200 μm. The single-species plant macrofossil remains were hand-picked from bulk samples of the organic layer using a low powered stereozoom microscope and stored in deionised water in glass vials before transfer to the NERC Radiocarbon Facility (Environment), where they were digested with 2 M hydrochloric acid, combusted to CO2 and reduced to graphite before AMS 14C analysis at the SUERC AMS Facility. Calibration to the calendar timescale was carried out using the IntCal09 data-set (Reimer et al., 2009) and the OxCal v.4.1 calibration software (Bronk Ramsey, 2001; 2009). The calibrated dates are presented as ranges with 95.4% confidence limits and converted to the GICC05 timescale (b2k) by addition of 50 years (see Lowe et al., 2008). A Bayesian-based Sequence algorithm (Bronk Ramsey, 2008) was used to integrate the radiocarbon and varve data and to construct age models for the events represented at both sites. 4. Results 4.1. Varve analysis

Fig. 4. Plant macrofossil remains from the layer of organic detritus at Croftamie showing the excellent preservation of Salix leaves and Polytrichum moss tips.

bands. The organic material consists of a heterogeneous mix of small plant and arthropod fragments contained within a fine organic matrix. To test for significant variation in ‘age’ (i.e. 14C activity) between

The sediments consist of couplets of alternating coarser and finergrained layers, the latter comprising clay layers which exhibit sharp upper boundaries (Fig. 5), resulting from the settling out of suspension in the water column during winter when the lake surface was frozen. The coarser layers commonly comprise multiple laminations formed by varying sediment supply to the lake basin and transported as underflows generated by thermal/density stratification within the lake basin during the spring–summer months. These couplets display diagnostic features that enable them to be differentiated from other fine-grained rhythmite deposits (c.f. Ringberg and Erlström, 1999; Palmer et al., 2008a; 2010), and allow their interpretation as annuallylaminated varves. Key features include densely packed winter clay layers with high birefringence under cross-polarised light, sharp winter to summer contacts and well-defined transitions between summer and winter components with multiple laminations within the summer components. The base of the sequence shows greatest affinity to the distal varves described by Ringberg and Erlström (1999),

Table 1 Radiocarbon determinations. A) Rose et al. (1988). B) This study (B–J). Laboratory code

Dated material

14 C age determination (1σ)

δ13CVPDB (‰)

Calendar age Range yrs BP (2σ)

Q-2673 (A) SUERC-20164 SUERC-20165 SUERC-20166 SUERC-22255 SUERC-22256 SUERC-22257 SUERC-22258 SUERC-22259 SUERC-22260

Bulk organic Single-species leaf Single-species moss tip Wood fragments Single-species leaf Single-species moss tip Single-species moss tip Wood fragments Mixed seeds Bulk organic fines

10,560 ± 160 10,226 ± 43 10,165 ± 45 10,236 ± 42 10,197 ± 45 10,212 ± 41 10,232 ± 44 10,344 ± 45 9919 ± 42 10,350 ± 42

– − 28.5 − 27.6 − 29.5 − 28.3 − 27.2 − 26.6 − 28.5 − 28.5 − 27.9

12,782–11,846 12,092–11,766 12,039–11,627 12,115–11,769 12,074–11,717 12,074–11,763 12,104–11,768 12,398–12,030 11,601–11,227 12,399–12,039

(B) (C) (D) (E) (F) (G) (H) (I) (J)

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Fig. 5. Varves from the Croftamie and Bogwood sequences. A) Core sections analysed by X-radiography. B) XRF data (Fe/Ca k counts per second) superimposed onto the X-radiograph image of Core 7. This is used for measurement and counting of varves. C) Photomicrographs of thin section images of varves: silt component (yellow bar) and clay component (red bar). These show sharp contacts between the clay and silt components and the complex structure of some silt components.

although winter and summer components are of approximately equal thickness. Higher up in the Bogwood sequence, varves more closely resemble those described as proximal varves by Ringberg and Erlström (1999). When using ITRAX data, the most appropriate elements for analysis are thought to be highly dependent on the catchment geology. The area occupied by the Loch Lomond glacier during the LLS comprises iron and magnesium rich carbonates from the Old Red Sandstone bedrock, while deposition of the Clyde Beds as a result of marine incursion following retreat of Dimlington Stadial ice generated sediments with a much greater variety of minerals including clays, coal and calcium carbonate from the Carboniferous and metamorphic minerals from the Dalradian rocks of the Highlands. Fe and Ca are relatively enriched in the varves and show the greatest variation in a Principal Components Analysis (PCA) plot of the quantitative ICP-AES data (MacLeod, 2010). Clear variations also characterise Ti values (lower values of Ti occurring in the clays), which enabled varve layers to be counted and their thicknesses measured precisely (Fig. 6D). The results of micromorphological examination indicate a total of 94 ± 4 varve years in the Croftamie varve series studied here, though this is considered a minimum number as glaciotectonic deformation of the upper layers is apparent (Fig. 3). In the case of the Bogwood sequence, a total of 259 ± 3 undisturbed varves were counted using a combination of thin-section micromophology and analysis of ITRAX scans, which can be considered a minimum estimate for the duration of Glacial Lake Blane (Fig. 6). It is thought that the lowest varves in the Bogwood sequence were deposited contemporaneously with those preserved at Croftamie, the latter subsequently over-ridden by the advancing ice front. This assumption was tested by comparing variations in varve thicknesses between the two sequences, for which purpose only the basal 107 varves from Bogwood are examined here; the rest of the Bogwood

series is described in detail elsewhere (MacLeod, 2010). Fig. 6C and D shows that despite significant differences in mean varve thickness between the two series (varve thickness ranges are 0.7 mm–10.0 mm for Croftamie, and 1.4 mm–43.0 mm for the bottom 107 Bogwood varves), and compilation of varve counts by different analysts, parallel trends in thickness variations are evident in the lower part of each series. This is as illustrated by the common marker events picked out by the four symbols shown in Fig. 6. 4.2. Radiocarbon dates Of the ten radiocarbon dates now available for the Croftamie sequence, six are consistent in age range (Fig. 7A and B). One of these is based on a wood fragment and the others on leaf and moss remains. The fragile nature and excellent preservation of the leaf and moss fragments (Fig. 4) suggest limited transport distance and rapid burial, and perhaps also close proximity to the site of some Salix (willow) trees, scrub or dwarf bushes, which may also be the source of the wood which gave a contemporaneous age estimate. Calibration of these six results (Table 1, Samples B, C, D, E, F, G) provides a calendar age range estimate for the deposition of the organic layer of between 12,115 and 11,627 cal yr BP at the 95.4% confidence interval, prior to application of age modelling procedures (see below). The two results based on bulk, undifferentiated organic detritus (Table 1, A and J) and a second wood fragment date (Table 1, H) overlapped with the calibrated ages of the other six samples at the 2 sigma confidence limit, but the ranges extend to significantly older ages (as does the previously published bulk organic calibrated age range), perhaps reflecting the inclusion of small amounts of reworked older material. Since the original age determination obtained from a bulk sample (Rose et al., 1988) corresponds with these older estimates, this too is

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Fig. 6. Correlation and counting of the varves from the Blane Valley Silts at Croftamie and Bogwood, Central Scotland. A and B) Total varve thickness count by thin section analysis and digital image analysis from Croftamie undertaken by two different operators (AM and AP). C) Varve thickness count from Bogwood using thin section analysis and digital image analysis. D) Varve thickness count from Bogwood using μ-XRF Ti data. Four marker varves common to all sequences are highlighted by symbols and the number of varves between each is given.

likely to have been contaminated by some older carbon. The sample based on heterogeneous seeds (most not identified to genera) yielded an anonymously young age estimate (Table 1, I) for reasons not presently clear. The six convergent dates are considered the most reliable for age model construction. 4.3. Age models The chronological data reported above can be integrated using Bayesian-based age models to assess the most likely ages for events. The models used here are Sequence algorithms developed by Bronk Ramsey (2001, 2009), which are constrained by rules of the order of superposition of dated samples, the probability data (error ranges) of all age estimates employed, and statistical matching between the chronological data-series and the IntCal09 radiocarbon calibration curve (Reimer et al., 2009). The most coherent and best constrained data-set is evaluated using the Agreement Index (AI), a statistical measure of the degree of fit between the raw data and the prior model, with AI values in excess of 60% regarded as indicating good agreement (Bronk Ramsey, 2008). The higher the AI value, the more coherent the data-set within the statistical rules employed. Close scrutiny shows the organic layer to be comprised of a lower component of compacted leaf matter and an upper component of organic material disseminated in sand, but these elements are very thin and difficult to separate. Consequently, the six radiocarbon dates are considered to have been derived from replicate samples from a single horizon, and hence treated as a single Phase in the model (Table 2). The individual dates were modelled independently however, because different organic components could have been derived from more than one source and hence be of varying age (Bronk Ramsey, 2001). The varve data provide estimates of the duration of an overlying Interval of time (Table 2). When integrated using the Sequence model algorithm, the highest AI generated by these data is 123.7%, generously exceeding the recommended

acceptance value of 60% (Fig. 8). The results suggest modelled age ranges at the 95.4% confidence limits of between 12,046 and 11,808 cal yr BP for deposition of the Croftamie organic layer, between 11,927 and 11,638 cal yr BP for the time when glacial ice over-rode the Croftamie site, and between 11,762 and 11,474 cal yr BP for cessation of varve deposition in the Bogwood sequence. The age model could, in theory, be further constrained by integration of other chronological information available for the site and from adjacent localities. The Clyde Beds are shell-rich marine deposits found throughout the area. These pre-date the advance of Loch Lomond Stadial ice across the region (Rose, 1981) and could therefore provide additional chronological constraint for the earliest ice advance into the area. While a number of radiocarbon dates are available from shells extracted from these deposits, there are uncertainties concerning some of the samples and, more generally, over the magnitude of the marine reservoir correction to apply (Ascough et al., 2005). Radiocarbon dates are also available from nearby sites, such as Muir Park Reservoir near Drymen, for the YD/Holocene boundary (Vasari, 1977), which would further constrain the age model. Again, there are questions over the reliability of the results as they are based on bulk sediment samples, which frequently contain older detrital material and can be subject to a mineral carbon error (Lowe, 1991; Walker et al., 2001). Furthermore, the precise correlation of these sites with the Croftamie sequence is not established. Various Sequence models were trialled using some or all these additional data, and these appear to generate more precise age estimates for events at Croftamie (MacLeod, 2010). For reasons given above, however, the results derived when incorporating such evidence could be illusory, and therefore preference is given to what is considered to be the best constrained model which is based solely on the new radiocarbon and varve data from Croftamie, the integrity and robustness of which are defined with a higher degree of confidence. A point to note, however, is that none of the experimental models generated age estimates for the onset of varved sediments in Glacial Lake Blane older than c. 12.1 cal ka BP.

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Fig. 7. Calibration of radiocarbon determinations using OxCal v4.1.3 (Bronk Ramsey, 2009) and IntCal09 atmospheric curve (Reimer et al., 2009). (A) Radiocarbon determinations of all the dates generated at Croftamie (Table 1). B) The six radiocarbon determinations that have been used to generate the age model.

5. Implications of the new data 5.1. Timing of YD ice maximum and formation of Glacial Lake Blane The new data and age model from Croftamie suggest the following sequence and chronology of events. Deposition in Glacial Lake Blane was initiated when the ice margin advanced to the southern shores of Loch Lomond to block the drainage channel at its south western edge, which exits through the Vale of Leven to the Clyde Estuary. This drainage exit point lies 10 km WSW of Croftamie (Fig. 2). A combined process of accumulation of leaf matter from annual autumn fall and rising lake levels concentrated surface vegetal remains into depressions on the ground surface. Material was then rapidly buried by the first glaciolacustrine layers (varves), which continued to be deposited uninterrupted, the most likely explanation for the preservation of the buried organic material at Croftamie.

The radiocarbon data used for the age model suggest that the sampled organic material was pene-contemporaneous with the time of deposition, and that little or no older, recycled plant material was incorporated. Burial occurred at around 12.0 ka BP. While Glacial Lake Blane was in existence, seasonal freezing of the lake surface led to formation of glaciolacustrine varves that provide an annuallyresolved estimate of the duration of the lake. At least 94 years after the lake was formed, the ice over-rode the site at Croftamie. Glacial Lake Blane persisted for a minimum of 260 years, after which the ice receded and the lake drained at some time between ca. 11.76 and 11.47 cal ka BP (Fig. 9). This estimate is close to that for the onset of Holocene warming based on Greenland ice-core and other highresolution stratigraphical records (Walker et al., 2009). There is no evidence of any significant oscillation of the ice front or of interruption in varve deposition in the Loch Lomond area: the evidence points to a single phase of glacier expansion, varve deposition, and ice

A. MacLeod et al. / Global and Planetary Change 79 (2011) 264–274 Table 2 Age model for the radiocarbon determinations and the combined Croftamie and Bogwood varve thickness records. The model also includes a boundary between the deposition of the organic detritus and the onset of the varve deposition. The age model is generated using OxCal v4 1.3. Name Boundary End 1 Top of Glacial Lake Blane varves N(165,1.5) Interval Ice over-rides Croftamie N(94,2) Interval Boundary contact organics and varves R_Date SUERC-22257 (G) R_Date SUERC-22256 (F) R_Date SUERC-22255 (E) R_Date SUERC-20166 (D) R_Date SUERC-20165 (C) R_Date SUERC-20164 (B) Phase of organic deposition Boundary start sequence Sequence 1 Agreement indices: A model 125; A

Unmodelled (%) (cal yrs BP 2σ)

162–168 162–168

95.4 95.4

90–98 90–98

95.4 95.4

12,104–11,768 12,074–11,763 12,074–11,717 12,115–11,769 12,039–11,627 12,092–11,766

95.4 95.4 95.4 95.4 95.4 95.4

Modelled (%) (cal yrs BP 2σ) 11,749–10,979 11,762–11,474

95.4 95.4

162–168 11,927–11,638

95.4 95.4

90–98 12,021–11,732 12,046–11,822 12,041–11,820 12,040–11,815 12,046–11,823 12,036–11,808 12,045–11,821

95.4 95.4 95.4 95.4 95.4 95.4 95.4 95.4

12,117–11,835

95.4

overall 123.7

withdrawal, the demise of the ice probably terminated by the marked thermal amelioration at the YD/Holocene transition. Any oscillations of the ice front that may have occurred during the period 12.1 to 11.7 ka BP must have been very minor, as Glacial Lake Blane remained in existence throughout this interval while the ice front never reached the Bogwood site. On the assumption that lake drainage occurred close to the time of Holocene warming as recorded in Greenland, the chronology of events reported in this paper can be linked to the GICC05 timescale (Rasmussen et al., 2006), as shown in Fig. 10.

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6. Wider significance: timing of Younger Dryas glacier expansion in Scotland and Norway These conclusions concur with those pertaining to another major Loch Lomond Stadial glacier, that which expanded into Glen Spean in the Lochaber region to block adjacent Highland ice-free valleys, creating ice-dammed lakes and, in the case of Glens Gloy and Roy, leading to the formation of distinctive lake shorelines — the famous ‘Parallel Roads of Glen Roy’ (Palmer et al., 2010). Although the chronology of the evidence is not yet quite as secure as that from the Loch Lomond area, nevertheless the data suggest that the YD glaciolacustrine lakes of Lochaber began to form after ca. 12.16 ka BP, implying that ice advance in that area also did not achieve its maximum limit until near the end of the YD. Whether these results are typical of the behaviour of YD glaciers in Scotland is difficult to ascertain however, because other glacial limits have not been dated so precisely. Evidence from Norway suggests differential timing and amplitude of the response of YD ice masses to the YD climatic downturn (Mangerud et al., 2010). For example, while some parts of the margins of the Main Inland Ice Sheet experienced a standstill or minimal readvance (Andersen et al., 1995), other parts, such as in the Bergen area (Lohne et al., 2007), experienced a major readvance which lasted until late in the YD, dated to between 11.6 and 11.7 ka BP (Bondevik and Mangerud, 2002; Fig. 10). Cirque glaciers and ice caps, on the other hand, such as Ålfotbreen, reached their maximum extents much earlier in the YD, and experienced negative mass balance while parts of the Main Inland Ice Sheet were still advancing (Mangerud, 1980). This suggests that glacier responses to climate forcing during the YD may have varied significantly throughout the North Atlantic region, depending on glacier volume, topographical constraints and regional patterns of snow supply (Mangerud et al., 2010). The evidence from Scotland, presented above, closely matches that for the Main Inland Ice Sheet in Norway, in achieving maximum glacier growth very late in the YD (cf. Bondevik and Mangerud, 2002; Fig. 10).

Fig. 8. Bayesian-based age model based on the calibrated radiocarbon dates and varve data from Croftamie using the Sequence model approach of Bronk Ramsey (2008).

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Fig. 9. Schematic representation of the relationship between the sites at Croftamie and Bogwood. A. Phase one, recording varve deposition at both Croftamie and Bogwood and B. Phase two, Croftamie is over-ridden by ice and varve sedimentation continues undisturbed at Bogwood. The diagram includes vertical exaggeration, especially at the sites, but an approximate vertical scale is given to indicate the true altitudinal positions of feature such as the maximum level of lake waters (65 m OD) controlled by an overflow channel at Ballat. All geological relationships and contacts are correct. It should be noted that: i) organic detritus is observed between the Wilderness Till and Blane Valley Silts (varves) at Croftamie (the material used for radiocarbon dating), and ii) there is no evidence for substantial ice calving into the lake at any time during advance or retreat of the Loch Lomond Glacier into Lake Blane.

The glaciers that terminated near the southern shores of Loch Lomond and which led to the formation of the ice-dammed lakes in Glen Roy were the products of two of the largest YD glacier catchments in Scotland. The Loch Lomond ice lobe, for example, was fed by a large number of cirques and plateaux surfaces, forming a glacier catchment with an estimated minimum surface area in the region of 1000 km2. Ice masses of this size require more time for mass balance adjustment than small independent cirque glaciers (Johannesson et al., 1989; Arendt et al., 2002). Since the Loch Lomond and Lochaber YD glaciers attained their maximal positions during a period of amelioration in Greenland (Rasmussen et al., 2008), a lagged response to climatic forcing is implied (Fig. 10). This might reflect a slow adjustment of large glacier budgets to increasing summer temperatures, but another possibility is an increase in local snow supply towards the end of the YD. Bakke et al. (2009) and Brauer et al. (2008) have concluded that periodic thawing of sea ice affected the North-east Atlantic during the latter half of the YD, which permitted influxes of warm North Atlantic waters and a northward movement of westerly wind systems. This could have increased snow supply in some areas, reinvigorating some glaciers, but not necessarily in all districts. A northward migration of the wind systems might also explain why some reconstructions of ice mass growth in Scotland

during the YD appear to reflect the influence of more intense snowfall from a southerly direction (Sissons, 1974, 1979).

7. Comparison with numerical models of glacier generation during the Younger Dryas Simulations of glacier growth in Scotland during the Younger Dryas based on numerical models tend to underestimate the maximum expansion of YD ice in the Loch Lomond region but overestimate its growth in the Lochaber region, when compared with glacial limits based on empirical data (Hubbard, 1999; Golledge and Hubbard, 2005; Golledge, 2010) (Figs. 1B and 8). Some model outputs also suggest that maximal YD ice growth in Scotland occurred during the first half of the YD (e.g. Golledge et al., 2008). This is contrary to the empirical evidence regarding the timing of YD ice advance presented here, which suggests that: (1) The YD ice cap reached its maximum position very late in the Stadial (after 12.0 cal ka BP). (2) Ice remained at its maximum position for a minimum of 259 years and subsequently decayed at approximately the

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Fig. 10. The relationship of the new chronostratigraphy from Croftamie and Bogwood to the NGRIP chronostratigraphy of Rasmussen et al. (2006) with ages presented in GICC05 yr b2k. A) NGRIP ice-core oxygen isotope chronostratigraphy and comparison to published data which records the timing of YD ice mass maxima in Scotland and Norway (arrows with associated references provided); B) chronological model of events recorded within Proglacial Lake Blane with solid vertical black bars and grey shading to the right of the NGRIP record reflecting the modelled 2σ age range and the narrow dashes reflecting the midpoint values; C) Loch Lomond event stratigraphy.

same time (within errors) as the onset of the Holocene as recorded in the Greenland ice cores. (3) There is little evidence of significant oscillation of the Loch Lomond and Lochaber ice fronts in the vicinity of their maximum positions; they appear to have reached their maximal positions towards the end of the YD, and then progressively decayed. (4) The Scottish evidence accords most closely with the inferred behaviour of the Norwegian Main Inland Ice Sheet during the YD, though smaller ice caps and cirque glaciers in Norway were more variable in behaviour depending on their mass, topographic context and regional snow supply (Mangerud et al., 2010); whether this was also true of smaller YD ice masses in Scotland is difficult to judge on current evidence, due to inadequate chronological control. Numerical models may well hold the key to the quantification of climate–glacier lag effects during the YD, but well-resolved, empirical data, of the type presented in this paper, are necessary for testing their output. Despite decades of detailed empirical research however, such precise pinning points remain quite rare, and greater efforts are necessary to develop methods for refining the chronology of events in Scotland during the YD. One key requirement for improved understanding in the future is a closer collaboration between the palaeo-data and modelling communities, in order to resolve the differences between empirical and model outputs. To meet this objective, however, a clearer distinction will need to be made between the onset of abrupt thermal climatic changes on the one hand, which is usually the key variable used in stratigraphical and palaeoenvironmental schemes, and the timing of responses to those changes by environmental agencies on the other hand. Some YD

glacier systems appear either to have lagged climatic effects significantly, or to have experienced rejuvenation through enhanced precipitation from incursions of Atlantic air masses during milder interludes. Quantifying the effects of these different environmental influences is vital for establishing the relative importance of proposed causal mechanisms, among which is the role of North Atlantic thermohaline circulation, which many assume to have orchestrated overall climatic shifts in the North Atlantic region during the Younger Dryas. Acknowledgements This research was conducted within the NERC Rapid climate change programme and funded by NERC grant NE/C509158/1. Thanks to Scottish Natural Heritage and the Loch Lomond and The Trossachs National Park for permission to excavate at the Croftamie site, and to the NERC's British Ocean Sediment Core Research Facility at the National Oceanography Centre, Southampton, for access to down-core analytical apparatus. Jon Merritt and James Rose (Honorary Research Associate) publish with the permission of the Executive Director of the British Geological Survey, NERC. We are grateful to Jan Mangerud and Tom Bradwell for providing insightful comments on an earlier draft of this paper. References Andersen, B.G., Mangerud, J., Sørensen, R., Reite, A., Sveian, H., Thoresen, M., Bergstrøm, B., 1995. Younger Dryas ice-marginal deposits in Norway. Quaternary International 28, 147–169. Andres, M.S., Bernasconi, S.M., McKenzie, J.A., Röhl, U., 2003. Southern Ocean deglacial record supports global Younger Dryas. Earth and Planetary Science Letters 216, 515–524.

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