stream surface, or floods from subglacial lakes, may have
temporarily facilitated faster ice flow and ‘over-
extension’ of the ice margin by locally increasing basal
lubrication. For the Irish Sea Ice Stream an important
challenge remains, to develop new ingredients in process-
based numerical modelling so that simulations can
reconcile with the geological record.

Small mismatches (50 km) occur at 25 and 18 ka for marine-based ice surrounding Shetland. The modelled grounding lines underrun those defined empirically and we suggest that this may arise due to choices of marine-induced ice melt rates in the modelling. Model-data mismatches provide useful opportunities to improve ice-sheet modelling, and further work using hundreds of ensemble model runs quantitatively scored against the BRITICE-CHRONO ice margins is underway to address the mismatches identified here.

Rapid and ‘early’ ice-sheet growth

Constraints on growth of the ice sheet are much more
uncertain than retreat because evidence is more difficult
to find and most dating is deliberately focused on retreat
phases. This is reflected in the wider levels of uncertainty
expressed in the minimum and maximum extents in the
empirical reconstruction (Fig. 5). In Scotland, little is
known regarding the existence and extent of ice masses
prior to our time focus of between 31 to 15 ka. On
Transect 8 however, new luminescence ages on the island
of Lewis led Bradwell et al. (2021b) to suggest a major
Scottish Ice Sheet existed between ~44 and 38 ka, in MIS
3, nourished in the Highlands of mainland Scotland and
with a functioning Minch Ice Stream. This ice sheet is
interpreted to have retreated from Lewis and from much
of the Minch during the ‘Tolsta Interstadial’ probably
between 38 and 35 ka and prior to the main Late
Devensian advance of the ice sheet.

By 31 ka, a Scottish Ice Sheet existed, with small
satellite ice caps on uplands elsewhere. Subsequent
growth was remarkably rapid, with the ice sheet expand-
ing from a Scottish Highlands centred ice sheet to a fully
coalesced BIIS (apart from the Welsh Ice Sheet) and
reaching the continental shelf edge in places by 30 ka
(Fig. 6). This fivefold expansion in size in only a
thousand years demonstrates that ice advance can be
nearly as rapid as retreat (see later) and that calving
icebergs as the margin advanced across the shelf did little
to slow down ice sheet growth. This growth is both earlier
and faster than suggested in the Hughes et al. (2016)
DATED reconstruction. The ‘Late Devensian BIIS’ has
usually been ascribed to MIS 2, but as noted above our
new ages demonstrate that the Scottish Ice Sheet started
advancing in the latter parts of MIS 3, advancing through
the north Minch at 32 to 31 ka and reaching as far as the
continental shelf break by around 30 ka (Bradwell
et al. 2021b).

Asynchrony of maximum ice extents and in the onset of deglaciation

The ice sheet is found to be larger than previously
thought and extending as far west as the Porcupine Bank
(Peters et al. 2016; Ó Cofaigh et al. 2021), and as far
south as the continental shelf break in the Celtic Sea
(Scourse et al. 2021). This southernmost limit reaches
beyond 49°, the latitude of Paris! The maximum areal
extent reaches 900 000 km², including Irish, British and
North Sea ice up to the western edge of the Norwegian
Channel. The maximum limits on the continental shelf
are now based on robust evidence dated to the last glacial
(MIS 2–3). This confirms earlier inferences about MIS 2–
3 (Late Weichselian/Devensian) ice mostly reaching the
continental shelf edge (e.g. Sejrup et al. 2005; Chiverrell
& Thomas 2010; Clark et al. 2012) and rules out the
possibility that these limits were only attained during
more extensive prior glaciations such as in MIS 6
(Wolstonian) or MIS 12 (Anglian). There is no single
time at which the ice sheet was everywhere at its
maximum extent. As Fig. 10 demonstrates, maximum
extents in the north and west were attained as early as 30
to 26 ka and as late as 22 ka along parts of the southern
margin. This asynchrony in build-up is explained by
simple geography in that the continental shelf west and
northwest of the British Isles is relatively narrow (c.
100 km) and most of the snow-nourishing mountains lie
close to these westernmost coasts. Ice masses grew from
the mountains up to 31 ka and then rapidly became
marine-based, advancing to the shelf break by 30–26 ka,
beyond which the ice could grow no further due to
excessive water depths that force flotation and calving.
From a mass balance perspective, if the continental shelf
was wider it would have taken longer for ice to expand
across it and would have reached its maximum here much
later. In contrast, the southern margin spent much longer
advancing across England and the southern North Sea,
reaching maximum limits here at 22 ka in response to the
continued glacial cooling. It did not run out of ice0sheet
bed on which to grow.

With no uplands to nourish ice caps, the North Sea
depended on ice fed from adjacent source areas. Growth
of British ice into the North Sea preceded that of
Scandinavian ice and with lobes from each finally
meeting around two thirds of the distance eastwards
across the North Sea, at an area NE of Dogger Bank at
around 27 ka. This led to the establishment of a distinct
North Sea ice dome by 22 ka (see earlier and Fig. 7). At
22 ka North Sea ice was at its maximum, some
3000 years subsequent to the maximum of the Irish
and British Ice Sheets. This is because the 200-m-deep
Norwegian Channel steered much Scandinavian ice
northwards, thus delaying ice confluence over the North
Sea and delaying the timing of maximum extent here. We
speculate that in earlier glaciations and prior to the
existence of this important topographic channel, which
<figure><img src="image_29.png" /></figure>

Fig. 11. Variation in ice margin behaviour across ice-sheet sectors demonstrating considerable asynchrony in the timing of maximum limits, and the onset of retreat. It is notable that maximum limits were only short-lived (1–3 thousand years) before the onset of retreat. To aid comparisons, margin position data are normalized to the percentage distance to the maximum extent per sector; ISIS is the Irish Sea Ice Stream and NCIS, the Norwegian Channel Ice Stream. Actual retreat rates in metres per year are discussed later and in Figs 12 and 13.

purged ice away, a North Sea ice dome would have been centred further west and with a much greater influence of Scandinavian ice over the North Sea. In the last glaciation the channel acted as a gutter diverting Norwegian ice northwards.

Clearly, a large ice sheet in steady state with long periods of neutral mass balance is not found in the reconstruction. Rather we have an always rapidly adjusting ice sheet, asynchronous in advance and retreat around

its perimeter and one that started recession in most places shortly after reaching its maximum. In Fig. 11 this asynchrony is illustrated by plots of ice margin advance and retreat for north, west and southwest, and North Sea sectors of the ice sheet. The northern sector of the ice sheet (Minch and Orkney–Shetland) experienced fast growth toward the continental shelf break reaching a maximum extent as early as 30 ka, followed by slow retreat relative to other sectors (Fig. 11). In general, western and southwestern sectors of the ice sheet experienced fast growth and slower retreat respectively, reaching maximum extents at 27 and 26 ka respectively. Exceptions to this were retreat of the Irish Sea Ice Stream and across Donegal Bay, both of which collapsed back rapidly. North Sea ice experienced the slowest growth of all sectors, because it had to wait for confluence to occur between British and Norwegian ice, finally reaching a maximum extent at 22 ka. Relative to other areas, the North Sea sector underwent the most rapid retreat phase of all comprising a dramatic collapse event that halved the ice-sheet area here in less than 1000 years.

Our reconstruction does not support Hypothesis 2, articulated in the Introduction, that the main ice catchments draining the ice sheet retreated synchronously in simple response to external climatic and sea level controls. This might be expected if external drivers (e.g. warming or sea level rise) outweigh the local controls on ice retreat. Rather we find that deglaciation is heavily influenced by local factors and especially the geography of the region, which preconditioned timing of coalescence and separation of ice masses emanating from uplands in Ireland, Scotland and Norway. Topographic complexity clearly exerts a large influence on the (a) synchrony of ice-sheet behaviour. A lesson here is that if one is seeking climate inferences from ice-sheet variations it might be best to choose ice sheets with simple

<figure><img src="image_30.png" /></figure>

Fig. 12. Large changes occurred in the centres of ice mass over time, shown here in terms of modelled migrations of the main ice divides and flow geometry between 24 and 21 ka. Mainly as a consequence of ice drawdown through the Irish Sea Ice Stream, the divide across Ireland and southern Scotland migrated north by 70 km, and the North Sea divide moved northwards by 125 km during the later build-up of ice volume here, subsequent to the confluence between British and Scandinavian ice.
<figure><img src="image_31.png" /><figcaption>Fig. 13. Left panel: net rates of retreat calculated at 1-ka timesteps, demonstrating large variations between terrestrial (50 m a⁻¹) and marine sectors (>50 and up to 451 m a⁻¹). Note that these time-averaged values likely mask faster shorter-lived rates that are below our dating resolution. Inset panel shows frequency distribution of rates of retreat and advance. Right panel: flotation factor for marine-terminating margins (advancing and retreating) calculated using ice thickness behind grounding lines and water depths from the relative sea level modelling (black indicates terrestrial-terminating ice margins). Comparing both maps indicates a general correspondence between faster retreat for margins that were marine-based and for those that were more lightly grounded (high flotation factors).</figcaption></figure>

radial structures and underlying topography, such as in Iceland. The surprise in the data for the BIIS is both the abundance of asynchronous variations, and that retreat in some sectors commenced prior to proxy records of warming. Why did ice retreat prior to the warming? Specific explanations for this ‘early retreat’, and stepped and oscillating margin retreat in relation to bed topographic and geological variations have been explored in the BRITICE-CHRONO transect papers (Benetti et al. 2021; Bradwell et al. 2021a, b; Chiverrell et al. 2021; Evans et al. 2021; Ó Cofaigh et al. 2021; Scourse et al. 2021). At the ice-sheet scale we return to these issues later.

## Ice divide migrations

The spatial asymmetry of ice-sheet growth and decay shown by variations in ice extent over time (Fig. 6) is also reflected in migrations in the centres of mass, as defined by the main ice divides (Fig. 12). A notable example is that between 24 and 21 ka large changes in ice extent and flow geometry occurred, with ice volume reducing over Ireland and Britain but still expanding over the North Sea. During this time, ice drawdown from the Irish Sea Ice Stream helped drive the Irish-Scottish ice divide northwards by 70 km (Fig. 12) and the North Sea divide moved northwards by 125 km as a consequence of continued growth following confluence of British and Scandinavian ice. Changes in the flow geometry (e.g. streamlines in Figs 7 and 12) were less at the ice-sheet

periphery and with larger angular changes and increasing complexity inwards, especially near locations of divide migration. Some of the divide shifts we note here are consistent with those defined from an empirical reconstruction of the Irish Ice Sheet (Greenwood & Clark 2009) and which were built from flowsets of ice geometry from mapped drumlins and subglacial ribs. A fuller examination of how well the modelled flow geometric changes match with those defined by subglacial bedforms would be fruitful in terms of quantifying how well the model performs against observations of ice flow (e.g. Ely et al. 2021) and might also be useful for progressing knowledge about glaciological conditions required for drumlin formation.

## Variations in pace of ice margin retreat

The pace of retreat (Fig. 13) varied between slow (<50 m a⁻¹) at mostly terrestrial margins, faster (up to 80 m a⁻¹) at marine-calving margins mostly on the west and northern continental shelves, and much faster (80–451 m a⁻¹) collapse events in the Celtic, Irish and North Seas. All really fast retreat rates (>200 m a⁻¹) were marine, but marine retreat can also be slow. Note that these time-averaged or net retreat rates at 1-ka timesteps likely mask faster short-lived (decadal) rates that are below our dating resolution. Retreat rates of up to 50 m a⁻¹ were the most frequent (Fig. 13 inset panel), with 10 m a⁻¹ as the modal value across the whole ice sheet. A notable exception to the generally slower
<figure><img src="image_32.png" /><figcaption>Fig. 14. Analysis of ice-marginal retreat rates (right panel) demonstrates faster retreat for margins with higher flotation factors. Zero flotation is fully grounded (terrestrial), and approaching 1.0 for buoyancy at the margin. Data are from 6954 measurements of flotation factor averaged from flowlines 0–5 km behind the ice margins from 31 to 15 ka. Median (red line), interquartile range (blue box) and 2.7 sigma (whiskers) are indicated. Modal flotation factor was 0.5. In the schematic cartoon (left panel) we suggest that retreat rate variation with flotation factor is not just a consequence of buoyancy-induced calving, but also arises from increased ice melting from contact with warm water (red annotation), noting that most (70%) of the marine margins were heavily grounded (<0.5 flotation) due to the mostly shallow seas.</figcaption></figure>

terrestrial rates of retreat (<50 m a⁻¹) was in the southern North Sea, which was terrestrially exposed at this time (22–21 ka), and in places experienced retreat faster than 160 m a⁻¹. This might be a consequence of over-extension of this lobe by a flow instability or surge that increased the ablation area, promoting rapid surface melting and margin retreat (e.g. Evans & Thomson 2010). To explore the extent to which reverse-bed slopes hosted retreat rates faster than on normal slopes, in accordance with the marine ice-sheet instability theory (Schoof 2007), ice sheet-wide retreat rates were compared to the gradient of bed slope (averaged along 5-km flowlines) behind grounding lines. No relationship was found, and the fastest retreat rates in fact occurred across fairly flat parts of the sea floor, arising from a saddle collapse over the North Sea and retreat from over-extended ice lobes or ice streams. For some parts of the ice sheet however, notably in the Minch and in the Norwegian Channel (Gandy et al. 2018; Bradwell et al. 2021 b), which host reverse bed slopes in waters deeper than is typical across the domain, marine ice-sheet instability is predicted to have occurred using ice-sheet modelling experiments aimed at replicating the BRITICE-CHRONO glacial histories (Gandy et al. 2018, 2021). That the influence of marine ice-sheet instability was found to be largely absent across the ice-sheet domain

may indicate that its operation was the exception rather than the rule, consistent with some findings elsewhere (Greenwood et al. 2021), or in the case of the BIIS might simply arise from the relatively shallow waters (100–300 m) and heavily grounded margins that predominated. This contrasts with larger ice sheets such as in Antarctica where the ice mass is great enough to depress continental shelves to depths exceeding 500 m, thereby encouraging higher flotation factors and instability.

To examine the influence of ice-marginal buoyancy on retreat rates we calculated flotation factors (FF) along flowlines behind grounding lines using:

$$ \text{Flotation Factor} = P_w \times g(D-1)/P_i \times g \times H \quad 1 $$

where Pw and Pi are the density of water and ice, respectively, g is gravitational acceleration, D is water depth and H is ice thickness at grounding line, averaged along the final 5-km flowline leading to the grounding line. The modal flotation factor was 0.5, with around 70% of marine margins heavily grounded (<0.5; thick ice in shallow water). Comparison of maps (Fig. 13) and graphs (Fig. 14) of retreat rates against flotation factor demonstrates that faster retreat is typical for marine margins compared with terrestrial (FF = 0) and that retreat was faster for more lightly grounded margins
<figure><img src="image_33.png" /></figure>

Fig. 15. Growth of the British–Irish Ice Sheet to its maximum volume of 717 112 km³ (1.8 m sea level equivalent) at 23 ka, and its rapid collapse starting at 22 ka, at rates of ice loss of up to 194 km³ a⁻¹. Four stages of development are shown in the lower panels, which plot the mass losses and gains computed by subtracting across these timesteps. Between 26 and 22 ka there was a dramatic west–east asymmetry in behaviour, the Irish and British sectors were losing mass at the same time as ice over the North Sea was gaining. The time–space plot of ice-sheet extents (top right panel) shows 1-ka ‘time slabs’ from the empirical reconstruction, illustrating the regional asymmetry in growth and decay and the large collapse events over the Celtic and North Seas.

(higher flotation factors). Variability in retreat rates is highest for mid-range flotation factors (0.45–0.65; Fig. 14), which might be explained by increased sensitivity to sea level perturbations in cases undampened by buttressing ice shelves. In addition to expected buoyancy effects, we suggest that margins with higher flotation factors will typically have greater surface areas of ice in contact with warm water (Fig. 14), increasing the loss by melting, and that submarine melting promotes calving by creating weak points (notches) for structural failure (Benn et al. 2007).

From our reconstruction of the ice sheet and analysis of retreat rates we therefore accept the earlier stated Hypothesis 1 (see Introduction), that the main marine-influenced sectors (e.g. North and Irish Seas) collapsed rapidly and that once onshore the ice sheet stabilized and retreated more slowly. The new data reject the alternative interpretation speculated in Clark *et al.* (2012), that widespread purging of ice by fast flow happened while marine-margins retreated slowly, dynamically thinning interior (land-based) ice, and preconditioning the ice sheet to more rapid retreat once it was terrestrially based. Activation of marine ice-sheet instability (MISI) was not found to be the primary cause for this marine–terrestrial

difference, because conditions where MISI arises are the exception across the area due to the mostly shallow seawater depths.

### *Ice volume changes*

Changes in the volume of the ice sheet are plotted in Fig. 15, with values including all ice grounded over the North Sea up to the western edge of the Norwegian Channel. The maximum volume occurred at 23 ka with 717 112 km³ of grounded ice, with an estimated mass of 657 000 Giga tonnes, enough to raise sea level by 1.8 m once it all melted. This is about half the volume of the present-day West Antarctic Ice Sheet and a quarter of the Greenland Ice Sheet. A notable feature of the volume graph (Fig. 15) is the asymmetry in the peak, with faster rates of loss than for growth, and especially in the period 22–20 ka when the ice sheet collapsed at a rate of 194 km³ a⁻¹. That equates to a mass loss of 178 Gt a⁻¹ at this time, which proportionally is around twice as fast as the current rates of ice loss observed for the Greenland Ice Sheet (345 Gt a⁻¹ in 2011; IMBIE Team 2020) when scaled by the relative size of the ice sheets by volume. Four stages of development are evident from the plot of
<figure><img src="image_34.png" /></figure>

Fig. 16. Comparison of fluctuations in ice volume of the British-Irish Ice Sheet against potential climate-related drivers of change. Ice volume loss initiated at 22 ka and the ice sheet reduced to half of its maximum volume in just 2000 years (time interval marked by red rectangle). This collapse in ice volume and extent occurred thousands of years prior to deglacial warming and CO₂ rise (they are flat-lining in the 22–20 ka interval), but at a time when June insolation (W m⁻² at 60°N) was rising from its minima. Ice volume graph (this study); insolation values from Berger & Loutre (1991) and Berger (1992); CO₂ values from Köhler et al. (2017) and the global temperature proxy from Helsen et al. (2012) and Bradley et al. (2018), which is a transfer of δ¹⁸O to temperature using the glacial index method.

volume. Steady growth (120 km³ a⁻¹) from 31 to 26 ka occurred as ice built up over Ireland and Britain and extended out across the continental shelves. This was followed by a period (26–22 ka) of slower overall growth in ice volume (20 km³ a⁻¹) but characterized by dramatic west to east asymmetry in behaviour (Fig. 15). Large losses from the Irish and British sectors were compensated and exceeded by gains over the North Sea as an ice divide grew following the confluence of British and Scandinavian ice. The third and most dramatic phase was from 22 to 17 ka when saddle-collapses occurred over the North and Irish Seas and the ice sheet lost its marine-based sectors (125 km³ a⁻¹). The final demise of the much-reduced terrestrial ice sheet occurred more slowly (30 km³ a⁻¹) from 17 ka onwards.

## Controls on ice-sheet demise

The pace of ice margin retreat within each of the eight transects was analysed relative to atmospheric and ocean warming, and local sea level rise, and against topographical, geological factors and ice shelves as potential controls (Benetti et al. 2021; Bradwell et al. 2021a, b; Chiverrell et al. 2021; Evans et al. 2021; Ó Cofaigh et al. 2021; Scourse et al. 2021). Here we compare the change in volume of the whole ice sheet and reflect on

how well this fits with climate and sea level drivers (Fig. 16). Ice volume loss initiated at 22 ka and the ice sheet reduced to half of its maximum volume in just 2000 years. This collapse in volume and extent occurred thousands of years prior to deglacial warming and CO₂ rise; they can be seen ‘flat-lining’ in the 22–20 ka interval in Fig. 16, ruling out these as the primary triggers. The ice volume curve, however, closely mirrors the variation in June insolation (W m⁻² at 60°N), with peak ice volume at 23 ka matching the insolation minimum and with volume decreasing as insolation rises (Fig. 16). This close relationship opens up the possibility that deglaciation might have been triggered by orbitally induced increasing levels of summer solar radiation producing more surface melting, and with associated positive feedbacks, for example on albedo (e.g. Roberts et al. 2013a, b; Ryan et al. 2019). Melt in ice-sheet modelling is usually computed via positive degree days of air temperature, with insolation not directly computed and so the hypothesized insolation trigger highlighted here may go undetected in modelling investigations. Interestingly, in modelling the Greenland Ice Sheet during the last interglacial (Eemian), Van de Berg et al. (2011) found that it was necessary to introduce an insolation term and that, along with its positive feedback effects, insolation contributed to 45% of the melt required to
<figure><img src="image_35.png" /><figcaption>Fig. 17. Variation in sea floor depths (converted from modelled relative sea level computations) for marine sectors of the ice sheet, plotted for six locations close behind the maximal ice extents (stars on inset map of palaeotopography at 24 ka). In these places, water depths at the ice sheet's grounding line increased by around 50 m as a consequence of lithospheric downwarping by mass loading of the British-Irish Ice Sheet between around 30 and 25 ka. For most locations, the start of grounding line retreat (whose timing is marked by stars) occurred after deepening of the water column. We infer a causal link of glacio-isostatic deepening of the water column triggering deglaciation due to increased mass loss by iceberg calving. Glacio-isostatic adjustment modelling used BRITICE-CHRONO reconstructions of the mass of the British-Irish-Scandinavian-Barents ice sheets.</figcaption></figure>

yield the smaller ice sheet at that time. The BRITICE-CHRONO reconstruction of the ice sheet along with the findings from Greenland therefore suggest that ice-sheet modelling investigations should assess the effects of insolation on surface melting directly in their computations.

The BIIS is now known to have had extensive marine margins (~50% of perimeter summed over growth and decay) and so ice loss by calving and submarine melting must have been a significant control on the ice sheet's mass balance. The rate of ice loss at marine margins by the calving of icebergs is known to increase with increasing water depths (e.g. Pelto & Warren 1991). Did increasing water depth trigger deglaciation? Figure 17 plots relative sea level calculations, converted to palaeo depths, from our glacio-isostatic adjustment modelling. This modelling includes loading of the lithosphere from local ice sheets from the BRITICE-CHRONO and DATED (Hughes et al. 2016) Scandinavian ice-sheet reconstructions (see Method of model reconstruction), and also accounts for the effect of far-field ice-sheet fluctuations on sea level. The results show that, superimposed upon sea level variations arising from ice-sheet fluctuations elsewhere in the world, the isostatic loading from the British-Irish Ice Sheet locally depressed the sea floor by over 140 m in interior regions where ice thickness was greatest (not illustrated) and in the order of

50 m close behind the ice sheet's maximum extents (Fig. 17). Using one of the most influential ice streams on mass balance as a case study, the ice stream draining through the Malin Sea, the present-day water depth on the continental shelf here is 145 m. At 30 ka the depth was only 75 m due to global lowering of sea level, but by 26 ka had increased to 130 m as a consequence of bed depression from the mass of the BIIS, which by now had advanced out across the shelf. Although calving rates are difficult to understand and model, a simple empirical relationship (Pelto & Warren 1991) suggests that this increase in depth, from local ice loading, increased the calving velocity by 66% to a value of 1153 m a⁻¹. For the marine sectors of the ice sheet, in four of the five cases the onset of retreat occurred following increases in water depths in the order of 50 m (plotted in Fig. 17). We infer a causal link between this lowering of the sea floor by ice mass loading and the onset of grounding line retreat across these sectors due to increased calving fluxes. This link is strengthened by the relationship reported earlier (Figs 13, 14) showing that more lightly grounded margins (in deeper water) experienced faster rates of retreat. The causal link is appealing because it can both explain why retreat commenced in the marine sectors thousands of years prior to rising temperatures, and why retreat in all sectors commenced shortly (1–2 ka) after reaching its maximum extent, this delay being the time-lag for
<figure><img src="image_36.png" /><figcaption>Fig. 18. Hypothesis of calving-induced ice stream–ELA feedback, and runaway retreat. In blue, an ice stream advancing across the continental shelf experiences an increase in ice flux (represented by size of arrows) once it reaches the deeper water at the shelf edge, which lowers interior ice surface elevations (green) increasing the area below the equilibrium line altitude (ELA in red) sufficient in some cases we suspect, to trigger widespread and irreversible ice-sheet retreat from the continental shelf edge. The ice surface lowering also steepens the surface slope at ice stream onset zones (black arrow), which locally increases ice velocity and enlarges ice stream networks (see text).</figcaption></figure>

isostatic loading. The plots of palaeo-depth show that the seabed rebounded just a few thousand years after ice margin withdrawal.

Interactions between ice-sheet loading, ice fluxes and margin retreat have been previously suggested by Eyles & McCabe (1989) and in many of the BRITICE-CHRONO transect reconstructions (Callard et al. 2018; Ó Cofaigh et al. 2019, 2021, Benetti et al. 2021), and explored elsewhere in ice-sheet modelling, for example of the Antarctic Ice Sheet (Whitehouse et al. 2019). We note that Whitehouse et al. (2019) suggested that the unloading part of the cycle, when the sea floor rebounds upwards, might provide a stabilizing feedback on ongoing ice-sheet retreat by decreasing the water depth and ice flux. We speculate that such a stabilizing feedback might explain the slow rates of retreat on the continental shelf of Ireland (Benetti et al. 2021; Ó Cofaigh et al. 2021) and prominent reconstructed stillstands of the ice margin, such as at the mid-shelf grounding zone complex in Galway Bay (Roberts et al. 2020; Ó Cofaigh et al. 2021), and at large grounding zone wedges in the North Sea (Roberts et al. 2019). Specific GIA and ice-sheet modelling experiments could be usefully aimed at assessing these inferences and would need to account for strength of the lithosphere, because this value will determine the response time scale.

We explore two further possible reasons for why the ice sheet commenced deglaciation prior to the warming signal. The first stems from lack of knowledge of the actual NW European regional climate under which the ice sheet operated. Our comparisons of ‘early retreat in relation to climate forcing’ are in relation to global signals of air temperature and carbon dioxide. Perhaps the actual climate that developed over the region encompassed notable warming of air or ocean temperatures from 30 ka onwards, and the ice sheet simply responded to these. This could be investigated by developing regional climate model simulations for NW Europe (e.g. Strandberg et al. 2011) to drive ice-sheet

models rather than using climate-forcing from Greenland ice-cores that are over 2000 km distant and in a different regional climate system. We used an index approach to scale climate oscillations from the Greenland ice-core record to values more appropriate over NW Europe, the point being that it might not be just values that are different (e.g. Britain warmer than Greenland) but the timing of oscillations might vary. Furthermore, during the last glacial, the Atlantic Polar Front is known to have migrated from north of the British Isles to positions substantially to the south (e.g. Ruddiman & McIntyre 1981; Lowe et al. 1994; Scourse et al. 2009; Hall et al. 2011) profoundly affecting the ice sheet’s upwind air and sea temperature, and the incidence of sea ice and moisture availability, all of which would have affected the mass balance of the ice sheet. Despite apparent synchrony between sea surface temperatures and millennial-scale flux of ice rafted debris from the ice sheet (Scourse et al. 2009) it is likely that north–south shifts in the locus of snow accumulation and melting would have influenced mass balance across different ice drainage basins that might help explain the asynchrony of ice advance and retreat, as discussed earlier and in Fig. 11. Establishing if such ice-sheet asymmetric response arose from the Polar Front migrations is not easy, and ideally would require regional climate model simulations that encapsulate the effect of the Polar Front on spatial fields of temperature and precipitation.

The ice sheet did not stay at its maximum extent for long; shortly after (1 or 2 ka) reaching the continental shelf edge it commenced retreat, with different sectors doing this at different times, which rules out a simple climate explanation. Earlier we suggested that this was because such maximum extents were not sustainable (not enough ice being fed by outflow from the ice sheet) once glacio-isostatic loading increased water depths and iceberg calving, thereby flipping the drainage basin into negative mass balance. Although this explanation appears reasonable in that the time delay of a few
<figure><img src="image_37.png" /><figcaption>Fig. 19. The duration of time that has elapsed since land has been exposed from beneath ice sheets. Relevant for biological, hydrogeological and archaeological investigations this is based on the BRITICE-CHRONO reconstruction presented earlier, along with younger (Loch Lomond and Nahanagan Stadial) and older (Wolstonian and Anglian) ice cover estimates from Bickerdike et al. (2016), and Gibbard & Clark (2011), respectively.</figcaption></figure>

thousand years matches the GIA-modelled time-lag for lithospheric down-warping, we here propose an alternative based on ice stream dynamics. In Fig. 18 we consider an ice stream advancing out across the continental shelf in a cool climate, its grounding line advancing because ice flux from the accumulation area exceeds mass losses by surface melting and by iceberg discharge, which is controlled by water depth at the grounding line. Once the ice margin reaches the continental shelf edge, ice flux dramatically increases because of the much deeper waters encountered, halting further advance and producing upstream ice thinning. For an ice margin in a climate cold enough that surface melting is not happening (e.g. pre-Anthropocene Antarctica) then the grounding line would simply adjust position to balance the

incoming and outgoing fluxes. But for ice margins in more marginal conditions of glaciation, such as a mid-latitude ice sheet advancing into elevations low enough for surface melting (i.e. below the equilibrium line altitude, ELA) then we propose that an important negative feedback occurs in ice dynamics. Upon reaching the continental shelf edge, the increased calving flux produces upstream thinning of the ice stream surface, lowering elevations below the ELA and yielding more widespread surface melting, sufficient in some cases we suspect to trigger widespread and irreversible ice-sheet retreat from the continental shelf edge. For a network of ice streams whose surface elevations are now below the ELA then negative mass balance will force grounding line retreat, noting that earlier on, advance across this
area was only possible because the advancing ice streams had thicker surface profiles prior to reaching the continental shelf edge. This calving-induced ice stream-ELA feedback that we sketch out here requires investigation by modelling because of the large number of coupled factors that are involved. Is there a 'sweet spot' that exists for marine sectors of ice sheets and their ELAs such that this feedback or instability occurs?

A version of this destabilizing feedback has already been physically demonstrated in numerical ice-sheet modelling (Robel & Tziperman 2016; Gandy et al. 2019) but in cases not from increased calving on reaching the continental shelf edge, but from a lowering of the ELA (warming) during deglaciation. An additional accelerant to the feedback, first identified in Robel & Tziperman (2016) and then in Gandy et al. (2019), is that the increase in surface slopes in ice stream onset zones (Fig. 18) increases ice velocity here and promotes upstream growth of ice streaming and expansion of fast flow networks. Further feedbacks could also arise from increased surface meltwater reaching the bed and increasing basal lubrication (Zwally et al. 2002). We therefore speculate that for the BIIS its anomalous 'early retreat' from marine sectors might have occurred through a cascade of effects primarily driven by calving-induced ice stream-ELA feedback and runaway retreat. In this argument the ice sheet was sensitive to this feedback because it was already close to its climatically controlled maximum extent when it ran out of continental shelf to advance across. It explains why the grounding line did not persist at the continental shelf edge for long and why it retreated prior to climate warming. Much of our understanding about the mechanics and dynamics of ice streams comes from observations and modelling of Antarctic examples, which are not typical of mid-latitude ice sheets (or future Polar ice sheets) in that they (currently) experience little to no surface melting to influence their dynamics, and yet important instabilities between melt, ELA and dynamics might exist.

## Duration of land exposure from beneath ice

The presence of a kilometre thick ice sheet flowing over the landscape resulted in erosion of bedrock and a widespread redistribution of sediment as glacial deposits. After deglaciation these freshly exposed surfaces became available for the production of soil and a recolonization by biota, including humans, and with a refreshed and renewed hydrological and groundwater system. To help in biological and archaeological studies we plot the duration of time that has elapsed since land has been exposed from beneath ice, based on our reconstruction of the last glacial period and those of earlier glacials (Fig. 19). Reintroductions of biota from distant refugia were affected by pathways available for inward migration and our reconstructed maps of sea level change and ice cover at various times (Figs 5, 6)

might also help in these studies. The length of time that the seabed has been exposed from beneath ice can be visualized in Fig. 5 and in Data S3 and S4.

## Palaeoglaciological lessons for the interpretation and modelling of existing and future ice sheets

The main lesson for interpreting short-term (10–100 years) ice-marginal fluctuations of existing ice sheets is that such retreat rate measurements may not be representative of longer term trends. The markedly stepped retreat of the British-Irish Ice Sheet and the characteristic that individual margins were often not acting in concert with their neighbours highlight the perils of such thinking. Rather, specific topographical, geographical and geological factors of a region can lead to ice-marginal changes out of synchrony with climate or sea level forcing, and out of synchrony with nearby catchments due to ice piracy and wider flow adjustments. We propose that such stepped and knock-on ice geometrical readjustments are widespread in the behaviour of palaeo-, existing and future ice sheets and yet are difficult to diagnose, are easily ignored, or relegated in significance in favour of more newsworthy interpretations of (over) simplified climate teleconnections or instabilities. Our quantification of retreat rates and how these varied across the marine to terrestrial transition and across different topographical and geographical settings might provide useful contexts for framing and interpreting retreat rates from modern ice sheets.

We show that glacio-isostatic depression of the bed of the ice sheet triggered retreat, and suggest that bed relaxation upwards during the course of deglaciation likely also influenced rates of ice loss, with possible stabilizing effects on the mid continental shelf. These interactions highlight the importance of improved knowledge of glacio-isostasy for the modelling of future ice sheets and sea level rise.

In addition to glacio-isostatic and warming controls on deglaciation, our reconstruction underlines the importance of second order regional bed topographic and bed strength controls on modulating the rate of retreat of marine-based sectors of ice sheets. The lessons from this are firstly that we should not over-interpret or extrapolate from observed grounding line changes, say in Antarctica, without good knowledge of subglacial topography and bed properties, and secondly, that efforts to improve the resolution and accuracy of subglacial topography (e.g. Fretwell et al. 2013) will be vital for improving predictions of ice mass loss and seal level rise.

Are ice streams merely slaves to mass balance (arising from, and governed as part of, a wider self-adjusting network), or do individual ice streams profoundly alter the course of deglaciation by key activations? For the Laurentide Ice Sheet, Stokes et al. (2016) found that ice stream activity simply scaled with ice-sheet volume during deglaciation, pointing to the former interpreta-
tion. For the BIIS however, we suspect that its smaller
size and increased sensitivity to change meant that
whilst most ice streams likely operated in a regulating
role, some underwent changes large enough to signif-
icantly alter the pace of whole ice-sheet deglaciation
(tipping points), notably those that triggered knock-on
ice geometric effects, such as the collapse of North Sea
ice. The question could be usefully addressed by a series
of ice-sheet modelling experiments that try to elucidate
the extent to which individual ice streams and their
knock-on effects control whole ice-sheet volume
change, rather than just a regional redistribution of
ice loss that other ice streams then balance out.

Although the early loss of marine sectors of the ice
sheet was primarily concluded to be due to ice loading,
which increased water depths and therefore ice calving,
we suggested two alternatives that might have acted in
concert or have been dominant factors. These deserve
further exploration, namely the calving-induced ice
stream-ELA feedback, and whether the close relation-
ship found between timing of early deglaciation and
orbitally induced increase in summer insolation, pro-
ducing more surface melting, is causative or coincidental.
Does the direct inclusion of these in ice-sheet models
improve their predictability, and hence reliability for
making forecasts?

We note that the time resolution of our analysis is of the
order of 100–1000 years (due mostly to dating precision)
and that satellite observations of modern ice sheets are of
the order of 10 years, but we suggest that the 100s-year
time scale is of most relevance to society and might
actually be the most important glaciological time scale
for dynamics. We suggest that improvements in the
precision of dating are therefore worthwhile and that
palaeo-records of deglaciation at 100-year scales should
be sought.

Although we nudged the PISM model reconstruction
of the ice sheet to the empirically defined limits, we found
that for four timeslices (Fig. 6) we were unable to
reasonably get the modelled ice extent close enough to
the empirical limits (see Discussion). From this we infer
that there are ingredients missing in the ice-sheet
modelling and deserving of further attention to improve
model formulation. That we initially struggled to get the
model to retreat from the shelf edge fast enough along the
western and northern margins is most likely explained by
local ocean warming effects that were not sufficiently
included in the modelling, or perhaps from flexure of ice
shelves from the large tidal ranges forecast to exist but not
applied in the modelling. The 150-km model underrun in
the Celtic Sea points to a process, forcing, or feedback
that is likely missing in the numerical modelling. We
suggest that a thermal or basal water instability might be
required to produce a non-steady oscillation (surge), or
as an alternative we speculate that the drainage of lakes
on, or under the ice stream, may have temporarily
facilitated faster ice flow by locally increasing basal

lubrication. In common with our own efforts, no
numerical model has been able to simulate the full
extension of the Irish Sea Ice Stream and so an important
challenge remains; to develop new ingredients in process-
based numerical modelling so that simulations can
reconcile with the geological record.

Given the BIIS’s small size and topographic complex-
ity, along with its well-constrained history of retreat and
relative sea level variations, we suggest that it is an
excellent and data-rich target for future exploration,
testing and improving of ice-sheet modelling formula-
tions. Specific examples so far that use BRITICE-
CHRONO reconstructions include ice-sheet modelling
simulations using BICICLES (Cornford et al. 2013) on
exploring the role of marine ice-sheet instability and ice
shelves in the retreat of the Minch Ice Stream (Gandy
et al. 2018), deciphering the model ingredients required
to correctly predict ice streaming within the ice sheet
(Gandy et al. 2019) and on understanding the role of ice
saddle collapse and marine ice-sheet instabilities in
deglaciating the North Sea (Gandy et al. 2021). The
development of tools for assessing model simulations
against empirical data have been reported in Ely
et al. (2021) and have been used with BRITICE-
CHRONO data to assess (quantitatively score) the
performance of ensemble runs of the evolution of the
ice sheet, using the PISM (Winkelmann et al. 2011). Our
reconstructions have also been used in GIA modelling
and sea level investigations, such as on fingerprinting of
Meltwater Pulse 1A (Lin et al. 2021), forecasting 21st
century sea levels around Britain and NW Europe, the
UKCP18 Marine Report (Palmer et al. 2018), and the
finding of marine ice-sheet instability (Gandy et
al. 2018; Bradwell et al. 2019) used in consideration of
ice-sheet instabilities in relation to sea level rise in the 6th
Assessment Report of the Intergovernmental Panel on
Climate Change (IPCC) (Fox-Kemper et al. 2021). The
glaciological meaning of records of ice rafted debris has
been investigated using ice flux data from the model
reconstruction; to what extent do variations in IRD flux
match with changes in ice-sheet mass balance? Wilton
et al. (2021) explored this comparing by results from an
iceberg-ocean model tracking the trajectory of icebergs
discharged from the ice sheet (using fluxes from the ice-
sheet modelling), and computing the likely fluxes of
debris delivered to nearby observations of ice rafted
debris.

Conclusions

The BRITICE-CHRONO project has tripled the num-
ber of dated sites for constraining the timing of ice-sheet
retreat and has extended the spatial footprint of infor-
mation on timing to the marine sectors. Samples were
deliberately positioned orthogonal to the known pattern
of ice retreat and within a transect scheme at an ice sheet-
wide scale (Fig. 2) in order to maximize the constraining
value of each age. The BIIS is now the world's most well-
constrained retreating ice sheet, contemporary or
palaeo, at time scales of 10⁻² to 10⁻⁴ years, and with
ages well spread across its former extent.

In summary, the BRITICE-CHRONO project (logis-
tics and participants described in Clark et al. 2021)
sampled material for dating at 914 sites, on- and offshore,
producing 690 new age assessments (338 radiocarbon;
162 luminescence and 190 cosmogenic) related to the
advance or retreat of the ice sheet. A quality control
exercise (Small et al. 2017b) reported 1189 legacy ages,
reducing those deemed useful to 221. Together, these
legacy and new age constraints have been reported along
with their stratigraphical and glaciological contexts to
build local and regional reconstructions of ice advance
and retreat in a series of 23 publications (Peters
et al. 2016; Sejrup et al. 2016; Evans et al. 2017; Small
et al. 2017a, 2018; Smedley et al. 2017a, b; Arosio
et al. 2018; Bateman et al. 2018; Callard et al. 2018,
2020; Chiverrell et al. 2018; Evans et al. 2018a, b;
Lockhart et al. 2018; Roberts et al. 2018, 2019, 2020;
Bradwell et al. 2019; Ó Cofaigh et al. 2019; Scourse
et al. 2019; Wilson et al. 2019; Tarlati et al. 2020).
These findings were then synthesized into coherent
map-based reconstructions of ice advance and retreat
across the marine to terrestrial transition for each of the
eight transects investigated. This was achieved with the
help of Bayesian age sequence modelling to deal with age
outliers and uncertainties regarding marine reservoir
correction, and to improve dating precision. These
benchmark reconstructions are reported in a special
issue of the *Journal of Quaternary Science* (BRITICE-
CHRONO reconstructions of the last British-Irish Ice
Sheet, 2021; Benetti et al. 2021; Bradwell et al. 2021a,
b; Chiverrell et al. 2021; Evans et al. 2021; Ó Cofaigh
et al. 2021; Scourse et al. 2021) and include assessments
of the hierarchical controls on ice-sheet retreat, including
climatic, sea level, topographical, geological, ice shelf
buttressing and ice streaming, and with some findings of
unforced oscillations (glaciological instabilities) includ-
ing ice-marginal re-advances.

Here we used all assembled information on the timing
constraints to make whole ice-sheet reconstructions
from 31 ka, including from the eight transects and for
all areas in between, and from the continental shelf edge
until the final remaining small ice caps at 15 ka. An
empirical reconstruction (lines drawn on maps that best
fit the evidence) was presented as a series of palaeogeо-
graphical maps at 1-ka intervals, and which include error
assessment on margin positions. Using a novel method of
climate forcing and glacio-isostatic adjustment, a numer-
ical ice-sheet model (PISM; Winkelmann et al. 2011)
was nudged to fit within the error bounds of the empirical
reconstruction to produce a model reconstruction that
includes ice thickness, velocity, flow geometry and basal
thermal regime. We consider this as our preferred
reconstruction and it constitutes the main conclusion

of this paper. It is presented as a series of palaeoglaciо-
logical and palaeotopographical maps and data (includ-
ing relative sea level change) at 1-ka intervals between 31
and 15 ka. This reconstruction of the evolution of the ice
sheet through time has been analysed and described
earlier and yields the following headline findings:

* Initiation and rapid early growth of the ice sheet pre-dates its previously assumed timing (i.e. Late Deven-sian, MIS 2), with the Scottish Ice Sheet advancing in the later parts of MIS 3 through the North Minch at 32–31 ka and reaching as far as the continental shelf break by around 30 ka (Bradwell et al. 2021b). Growth was remarkably rapid with the ice sheet expanding from a Scottish Highlands Ice Sheet, to a fully coalesced BIIS, although not yet joined with the Welsh Ice Sheet. This fivefold expansion in size in 1000 years demonstrates that ice advance can be rapid and that calving icebergs along an ice margin advanc-ing across the shelf did little to slow down ice-sheet growth.

* Robust confirmation is made of earlier inferences that ice reached the continental shelf edge during the last glacial around most of its perimeter, rather than solely during previous glacial stages, and that confluence between British and Scandinavian ice led to estab-lishment of a thick (>1 km) North Sea ice divide.

* The ice sheet is found to be larger than previously thought and extending as far west as the Porcupine Bank (Peters et al. 2016; Ó Cofaigh et al. 2021), and as far south as the continental shelf break in the Celtic Sea (Scourse et al. 2019). This southernmost limit reaches beyond 49°, the latitude of Paris. The maximum areal extent reached 900 000 km² at 26 ka, and the maximum volume occurred at 23 ka with 717 112 km³ of grounded ice and an estimated mass of 657 377 Giga tonnes, enough to raise sea level by 1.8 m once it melted. This is about half the volume of the present-day West Antarctic Ice Sheet and a quarter of the Greenland Ice Sheet.

* At 24 ka over half of the ice sheet rested below sea level, which emphasizes its marine-based properties. The lithosphere was depressed by some 60 m relative to today in central Ireland and 130 m in parts of Scotland. The ice sheet had marine margins of around 50% summed over the whole cycle of growth and decay, and so ice loss by calving and submarine melting was a significant control on the ice sheet's mass balance.

* The existence of fringing ice shelves in some locations such as the Minch and west of Ireland (Bradwell et al. 2019; Ó Cofaigh et al. 2021) has been estab-lished through interpretation of submarine glacial landforms and sediments and we indicate their widespread occurrence in simulations by ice-sheet/ shelf modelling. Ice shelves are shown to influence retreat rates through the effects on buttressing upstream ice flow (Gandy et al. 2018, 2021).
* Maximum ice extents were asynchronous between sectors and there is no single time at which the ice-sheet margins were everywhere at their maximum extent. Overall, the maximum areal extent of the ice sheet was at 26 ka. In the north and west, maximum limits were attained as early as 30–26 ka, and as late as 22 ka along parts of the southern margin. This asynchrony is explained by simple geography in that the continental shelf west of the British Isles is narrow (~100 km) and most of the snow-nourishing mountains lay close to the western coasts, such that ice growth here rapidly reached the limit imposed by the shelf width. In contrast, the southern margin spent longer advancing across England in response to continued glacial cooling, and the southern North Sea reached its maximum limit as late as 22 ka, arising from the later coalescence with Scandinavian ice, delayed because much Scandinavian ice was lost down the NCIS. The Channel acted as an overspill of Scandinavian ice with its western margin being an important suture between the ice-sheet complexes, permitting them to behave quasi-independently, for example with British ice around Shetland retreating at a time that the NCIS was still advancing.

* Given the above-mentioned asynchrony, then the concept of a Local LGM for the ice sheet breaks down, or at least does not exist in the sense of a narrowly defined window of time. An ice sheet in steady state (neutral mass balance) is not found in the reconstruction; rather we have a constantly rapidly adjusting ice sheet.

* The hypothesis that margins of the main ice catchments retreated synchronously in simple response to external climate forcing is rejected because we find asynchrony in the onset of retreat across the sectors, ranging between 30 and 22 ka. This rules out a simple climate explanation for the onset of retreat, especially as retreat was occurring thousands of years prior to climate warming.

* From analyses of retreat rates we accept the hypothesis that the marine-influenced sectors collapsed rapidly (<1000 years) and that once onshore the ice sheet stabilized and retreated more slowly. The pace of retreat was found to vary between slow (<50 m a⁻¹) at mostly terrestrial margins, faster (up to 80 m a⁻¹) at marine-calving margins mostly on the west and northern continental shelves, and with much faster (80–451 m a⁻¹) collapse events in the Celtic, Irish and North Seas. All very fast retreat rates (>200 m a⁻¹) were marine, but marine retreat can also be slow. Retreat rates of up to 50 m a⁻¹ were the most frequent with 10 m a⁻¹ as the modal value across the whole ice sheet. Note that these time-averaged or net retreat rates at 1-ka timesteps likely mask faster short-lived (decadal) rates that are below our dating resolution.

* Quantitative comparison of rates of grounding line retreat against their computed flotation factors demonstrate that retreat was faster for more lightly grounded margins (higher flotation factors) than heavily grounded cases.

* Marine ice-sheet instability (MISI) was not found to be the primary driver of deglaciation, because MISI was the exception rather than the rule across the region due to the mostly shallow seawater depths. It very likely occurred, however, in the Minch and Norwegian Channel Ice Steams (Gandy et al. 2018, 2021; Bradwell et al. 2019), which have greater water depths than is typical across the domain. This is a crucial discovery because in the case of the Minch Ice Stream, rapid retreat occurred across reverse bed slopes when the climate was cooling, thereby providing rare and valuable empirical demonstration that an instability occurred, rather than grounding line retreat simply accompanying climate-forced retreat.

* A major finding is that we reject our Hypothesis 2 (see Introduction) because the ice sheet responded differently to climate and sea level forcing across sectors and scales. Controls on ice-sheet demise are a mixture of forcings that are best considered in a scale hierarchy from ice-sheet scale, down through outlet scale (transect), and then more locally, as described in the following.

The first order controls on deglaciation were threefold: glacio-isostatic loading of continental shelves, increased water depths and ice calving, all of which triggered retreat in marine-based areas ahead of climate warming. This was staggered between sectors according to their loading history, with Scotland first (30 ka) and then Ireland (27 ka). Then the collapse of North Sea ice (21–19 ka) was a consequence of retreat of the NCIS (see below), which unzipped British and Scandinavian ice permitting widespread ingress of marine waters and expansion of calving margins. Final deglaciation was by climate warming from around 19 ka, which increased surface melting, leading to fairly even ice margin retreat of the remaining, terrestrially grounded ice sheet. A further complicating factor was collapse events, where margin retreat was a consequence of glaciological instabilities that had over-extended the margin (i.e. went further than predicted by modelling of mass balance), such as in the Irish Sea Ice Stream (Celtic–Irish Sea) and in the southern North Sea. More simply, the first order controls on demise were that glacio-isostatic loading killed the marine sectors aided by glaciological instabilities, and then climate warming finished off the smaller, now terrestrial ice sheet.

Operating at smaller spatial and temporal scales, the second order controls are explored and documented in our transect and modelling papers (Gandy et al. 2018, 2021; Benetti et al. 2021; Bradwell et al. 2021a, b; Chiverrell et al. 2021; Evans et al. 2021; Ó Cofaigh
et al. 2021; Scourse et al. 2021). They include large
fluctuations in rates of retreat arising from marine ice-
sheet instability and from the loss of ice shelf buttressing,
on for example the Minch Ice Stream (Bradwell
et al. 2019; Gandy et al. 2019). The trigger for retreat
of the NCIS, suggested by modelling simulations that
broadly fit the empirical data (Gandy et al. 2020), was by
calving-induced ice stream-ELA feedback (Fig. 18)
along with episodes of MISI. Topography has been
shown to modulate rates of retreat due to variations in
trough geometry, with pinning points (shallowings and
narrowings) producing stillstands of ice margins,
overdeepenings accelerating retreat, and with variations
in the width of calving margins influencing the rate of
grounding line retreat (Smedley et al. 2017a; Callard
et al. 2018; Chiverrell et al. 2018; Small et al. 2018;
Bradwell et al. 2019). Such variations in trough geom-
etry were often found to have a greater influence in
regulating retreat than climate-ocean forcing on time
scales of thousands of years. Relief development of large
grounding zone wedges may also have stabilized retreat
in places (Batchelor & Dowdeswell 2015). Ice dammed
lakes were found to frequently accompany ice-marginal
retreat in some sectors (Roberts et al. 2019; Chiverrell
et al. 2021) and for the larger examples, such as Dogger
Lake, their effect on ice advance, flow dynamics and
retreat might have been substantial. Such influences of
ice dammed lakes deserve future ice-sheet modelling
experiments which include water ponding and ice calving
on mass balance and flow speeds.

Related to bed topography, the wider geography was
also found to exert a strong influence on ice dynamics
and retreat rates. Large (10–100 km) adjustments to ice
margins were frequently encountered across the tran-
sect reconstructions as direct consequences of nearby
changes in ice mass geometry due to the disposition and
pattern of, for example, coastal shape, and relative
positioning of uplands, large troughs and marine
embayments. Examples include interaction between
retreat of the Malin Sea Ice Stream and ice flowing
from adjacent uplands in Ireland (Wilson et al. 2019;
Benetti et al. 2021), flow and ice-marginal readjust-
ments, including re-advances, of Shetland ice as it felt
changes in North Sea ice masses (Bradwell et al. 2021
a), an East Minch re-advance in the aftermath of ice
shelf loss (Bradwell et al. 2021b), and expansion of
Welsh ice into the accommodation space made available
as Irish Sea ice withdrew from the Cheshire-Shropshire
Plain (Chiverrell et al. 2021). Additionally, oscillations
were found to arise from ice piracy, for example with
increased outflow from the Irish Sea Ice Stream
capturing ice accumulation away from its adjacent
catchment and affecting retreat there and leading to
widespread flow stagnation (Scourse et al. 2021).
Together these geographical effects can be thought of
as knock-on ice geometric readjustments, again seem-
ingly independent of direct climate forcings.

In summary, retreat of the BIIS had first order controls on its demise by glacio-isostatic loading triggering retreat of the marine sectors and aided by glaciological instabilities (marine ice-sheet instability, calving-induced ice steam-ELA feedback, and saddle-collapse); climate warming then finished off the smaller, terrestrial ice sheet. Rapid ice-sheet collapse was initiated (22 ka) by ice stream retreat and saddle-collapse over the North Sea and at rates of ice loss of up to 194 km³ a⁻¹. However, overprinted on the ice sheet-scale signal were second order controls exerting large (10–100 s km, over 1–4 thousands years) changes in the pace of retreat modulated by topographic variations of key troughs and with sector-scale ice geometrical readjustments arising from variations in the geography of the landscape. These second order controls produced a strongly stepped nature to deglaciation in specific sectors.

*Acknowledgements.* – This paper is dedicated to Richard Hindmarsh with much love, he died while the paper was in review. Without him the BRITICE-CHRONO project would not have happened. This work was funded by the Natural Environment Research Council consortium grant BRITICE-CHRONO NE/J009768/1 and by the NERC Radio-carbon Facility and the NERC Cosmogenic Isotope Analysis Facility. Thanks are due to the staff at the SUERC AMS Laboratory, East Kilbride for carbon and beryllium isotope measurements. Thanks are due to the technical staff at the Aberystwyth Luminescence Research Laboratory and the Sheffield Luminescence Laboratory. The project benefited from the PalGlac team of researchers with funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme to CDC (Grant Agreement No. 787263) and supporting SB and AI. JCE acknowledges support from a NERC independent fellowship award (NE/R014574/1). Many researchers are thanked for their generous discussions or help on fieldwork on land or at sea including: Arosio R., Cotterill, C., Davies, S., Gales, J., Greenwood, S., Grimoldi, E., Mellett, C., Morgan, S., Purcell, C., Roseby, Z., Schiele, K., Stewart, H., Tarlati, S., Weilbach, K., Wilton, D. For our research cruises, we are grateful to the crew of RRS *James Cook* and the technical and engineering support from the Marine Operations team of the British Geological Survey. We thank the PISM team of ice-sheet modellers and Evan Gowan for ICESHEET, and note that the development of PISM was supported by NSF grants PLR-1603799 and PLR-1644277 and NASA grant NNX17AG65G. DP acknowledges funding from the Italian PNRA project IPY GLAMAR (grant number 2009/A2.15), and from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 656821 (project SEAGAS). We are indebted to the numerous pubs and bars that lubricated our conversations of science and gave sustenance during fieldwork. Bethan Davies and Martin Margold are thanked for their helpful comments on a version of this paper, and Jan A. Piotrowski for his work as editor.

*Author contributions.* – CDC wrote the research proposal, with ideas and help from many, and was PI of the BRITICE-CHRONO project. He led the analysis and writing of this paper with input from all authors. In all these activities he was supported by the Steering Group of RCC, DF, RCAH, COC and JS. Responsible for planning and conducting fieldwork, data collection and interpretation were the transect teams led by TB, DJAE, DHR, RCC, JS, COC and S. Benetti. DF led on geochronology, COC on marine geology, RCC on terrestrial geology, JS on ice rafted debris and RCAH on ice-sheet modelling. JD was the project administrator, conducted GIS work and edited this paper. CDC and JCE built the ice sheet-wide reconstruction, JCE conducted the ice sheet modelling, and S. Bradley the glacio-isostatic adjustment modelling. AI compiled flotation, retreat rate and flowline data. Dating laboratory analyses and interpretation were conducted by SM, GATD, RKS, GTHJ, MDB, AM and others. Fieldwork on- and off-
shore were conducted by most of the authors including MB, SLC, MS, DS, DD, SLL, D. Praeg, KJJVL, CB and PW. Scientific guidance and varied inputs on modelling, data compilation and interpretation were provided by LG, NG, EG, ALCH, GRB, SMC, D. Pollard and HPS.

*Data availability statement.* – GIS shapefiles of the empirical reconstruction, the master spreadsheet of chronology v2.1, modelling files of ice thickness and velocity and the GIA-modelled palaeotopographies and coastline position are available for download in Clark *et al.* (2022).

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## Supporting Information

Additional Supporting Information to this article is available at http://www.boreas.dk.

*Data S1.* Luminescence dating laboratory inter-comparison results.

*Data S2.* BRITICE-CHRONO master spreadsheet of chronology v2.1.

*Data S3.* BRITICE-CHRONO empirical reconstruction (poster and slideshow).

*Data S4.* BRITICE-CHRONO model reconstruction (poster, slideshow, and movie).

*Data S5.* Palaeotopographies and sea level 14–6 ka slideshow.