Reply on RC2

Abstract. The North Sea Last Interglacial sea level is sensitive to the fingerprint of mass loss from polar ice sheets. However, the signal is complicated by the influence of glacial isostatic adjustment driven by Penultimate Glacial Period ice-sheet changes, and yet these ice-sheet geometries remain significantly uncertain. Here, we produce new reconstructions of the Eurasian ice sheet during the Penultimate Glacial Maximum (PGM) by employing large ensemble experiments from a simple ice-sheet model that depends solely on basal shear stress, ice extent, and topography. To explore the range of uncertainty in possible ice geometries, we use a parameterised shear-stress map as input that has been developed to incorporate bedrock characteristics and the influence of ice-sheet basal processes. We perform Bayesian uncertainty quantification, utilising Gaussian process emulation, to calibrate against global ice-sheet reconstructions of the Last Deglaciation and rule out combinations of input parameters that produce unrealistic ice sheets. The refined parameter space is then applied to the PGM to create an ensemble of constrained 3D Eurasian ice-sheet geometries. Our reconstructed PGM Eurasian ice-sheet volume is 48±8 m sea-level equivalent (SLE). We find that the Barents–Kara Sea region displays both the largest mean volume and volume uncertainty of 24±8 m SLE while the British–Irish sector volume of 1.7±0.2 m SLE is the smallest. Our new workflow may be applied to other locations and periods where ice-sheet histories have limited empirical data.

Abstract. The configuration of the Northern Hemisphere ice sheets during the Penultimate Glacial Maximum differed to the Last Glacial Maximum. However, the reasons for this are not yet fully understood. These differences likely contributed to the varied deglaciation pathways experienced following the glacial maxima and may have had consequences for the interglacial sea level rise. To understand the differences between the North American Ice Sheet at the Last and Penultimate glacial maxima (21 and 140 ka), we perform two perturbed-physics ensembles of 62 simulations using a coupled atmosphere–ice sheet model, FAMOUS-ice, with prescribed surface ocean conditions, in which the North American and Greenland ice sheets are dynamically simulated with the Glimmer ice sheet model. We apply an implausibility metric to find ensemble members that match reconstructed ice extent and volumes at the Last and Penultimate glacial maxima. We use a resulting set of “plausible” parameters to perform sensitivity experiments to decompose the role of climate forcings (orbit, greenhouse gases) and initial conditions on the final ice sheet configurations. This confirms that the initial ice sheet conditions used in the model are extremely important in determining the difference in final ice volumes between both periods due to the large effect of the ice–albedo feedback. In contrast to evidence of a smaller Penultimate North American Ice Sheet, our results show that the climate boundary conditions at these glacial maxima, if considered in isolation, imply a larger Penultimate Glacial Maximum North American Ice Sheet than at the Last Glacial Maximum by around 6 m sea level equivalent. This supports the notion that the growth of the ice sheet prior to the glacial maxima is key in explaining the differences in North American ice volume.

During the last deglaciation large proglacial lakes formed at the base of the retreating northern hemisphere ice sheets. We assess the effect of these ice-contact lakes on regional climate and on the ice sheet surface mass balance components of the adjacent  Laurentide (LIS) and Fennoscandian (FIS) ice sheets,  using an atmosphere general circulation model with a novel extension for proglacial lakes in combination with a surface mass balance scheme for ice sheets, which, for the first time, allows to estimate the effect of the cold surface of these extensive lakes on the surface mass balance of the adjacent ice sheets. In a set of simulations inspired by the  Allerød interstadial around 13000 years before present, we demonstrate that the presence of proglacial lakes significantly reduces summer air temperatures in a larger area around the proglacial lakes and leads to reduced precipitation with increased snow/rain ratio. In consequence surface ablation reduces by 39% over the FIS and 28% over the LIS while accumulation only changes slightly by 1% and -3%  respectively. About one quarter of the response in surface ablation is related to the perennially cold surface of the proglacial lakes.

Numerical reconstructions of LGM climate across the Eurasian Arctic

The topography and bathymetry of Scandinavia have been molded by ice across numerous glacial cycles during the Quaternary. In this study, we explore the interplay between this changing morphology and the Scandinavian Ice Sheet (SIS). Using a higher-order ice-sheet model, we simulate the SIS over a glacial cycle on three different topographies, representing different stages of Quaternary glacial landscape evolution. By subjecting these simulations to identical climate conditions, we isolate the effects of landscape morphology on the evolution and dynamics of the ice sheet. Our findings indicate that early Quaternary glaciations in Scandinavia were restricted in both extent and volume by the pre-glacial bathymetry. It was only as glacial deposits filled a depression in the North Sea and expanded the Norwegian shelf that the ice sheet could expand further. This is illustrated by our middle to late Quaternary simulation (around 0.5 million years ago), where a filled bathymetry facilitated both a faster and further southward expansion, resulting in a relative increase in both ice-sheet volume and extent. Additionally, our study highlights that the formation of The Norwegian Channel acted as a barrier to southward ice-sheet expansion. This limitation only allowed the ice sheet to advance into the southern North Sea close to glacial maxima. Notably, our experiments suggest that distinct segments of The Norwegian Channel may have formed in different stages during glacial periods after the bathymetry was sufficiently filled with glacial sediments. These results underscore the importance of considering changes in landscape morphology over time when interpreting ice-sheet history based on ice-volume proxies and when interpreting climate variability from past ice-sheet extents.

Most numerical models of present ice-sheet dynamics predict basal thermal conditions for an assumed geothermal heat flux and measured ice thickness, surface temperature, and snow precipitation. These models are not ideally suited for reconstructing former ice sheets because what is known for present ice sheets is unknown for former ones, and vice versa. In particular, geothermal heat fluxes are immeasurable at an ice-sheet bed but can be measured after the ice sheet is gone, and the thermal conditions predicted at an ice-sheet bed can be inferred from the glacial-geological–topographic record after the ice sheet is gone. The Maine CLIMAP ice-sheet reconstruction model uses these inferred basal thermal conditions to compute ice thicknesses from basal shear stresses. Basal shear stress is assumed to reflect the degree of ice–bed coupling which, in turn, is assumed to reflect the amount and distribution of basal water under the ice sheet. Under the ice-sheet interior, basal water exists in a thin film of constant thickness covering the low places on the bed. This film expands for a melting bed and contracts for a freezing bed. Along the ice-sheet margin, basal water exists in narrow channels of varying thickness corresponding to troughs on the bed. These water channels become deeper for a melting bed and shallower for a freezing bed. In areas covered by the Laurentide and Scandinavian ice sheets, myriads of interconnected lakes in regions of greatest postglacial rebound are interpreted as evidence suggesting the interior basal water distribution, whereas eskers pointed toward terminal moraines and troughs across continental shelves are interpreted as evidence suggesting the basal water distribution toward the margins. Continental-shelf troughs were assumed to correspond to former ice streams, by analogy with observations in Greenland and Antarctica. Three modes of glacial erosion are considered to be responsible for the lakes, eskers, troughs, and associated topography. Quarrying is by a freeze-thaw mechanism which occurs where the melting-point isotherm intersects bedrock, so it is important only for freezing or melting beds because high places on the bed are frozen, low places are melted, and minor basal temperature fluctuations shift the isotherm separating them. Crushing results when rocks at the ice-bed interface are ground against each other and the bed by glacial sliding, so it occurs where the bed is melted and is most important when the entire bed is melted. Abrasion of bedrock occurs when rock cutting tools imbedded in the ice at the ice–rock interface are moved across the interface by glacial sliding, so it is also most important when the entire bed is melted. If basal melting continued after the entire bed is melted, abrasion-rates drop because the basal water layer thickens and drowns bedrock projections otherwise subjected to abrasion. Basal freezing reduces both crushing and abrasion-rates by coating quarried rocks with a sheath of relatively soft ice and transporting them upward from the ice–rock interface. An initially flat subglacial topography will develop depressions where glacial erosion is greatest and deposition is least, and ridges where the opposite conditions prevail. We interpret the central depressions represented today by Hudson Bay and the Gulf of Bothnia as caused by erosion on a melting bed under the Laurentide and Scandinavian ice sheets, respectively. The arc of lakes, gulfs, and shallow seas surrounding these depressions are interpreted as resulting from a freezing bed under the former ice sheets. The present watershed separating the depressions from the arcs marks the approximate former basal equilibrium line where the bed was melted. The Canadian and Baltic continental shields beyond these arcs are blanketed by material eroded from within the arcs, and represent areas having a frozen bed where evidence for abrasion is missing and a second zone having a melting bed where evidence for abrasion is present. This basic pattern was assumed to be imprinted on the bed during the steady-state period of maximum ice-sheet extent, and maintained in varying degrees during growth and shrinkage of these ice sheets.

The pattern, style, and timing of glaciation of the last Fennoscandian Ice Sheet (FIS) on the Kola Peninsula and Russian Lapland (northwest Arctic Russia) is widely debated. This is due, in part, to the lower-resolution empirical data used in previous investigations. In this paper, we present an ice margin reconstruction, an updated database of previously published numerical ages, and a new time-slice reconstruction. The reconstruction, which is presented across a series of 10 maps, documents the spatial evolution of the ice sheet every 1000 years between 16 and 11 ka, and for four selected time periods back to 29 ka (19–17, 21–20, 25–22, and 29–26 ka).Our reconstruction indicates that the Kola Peninsula and Russian Lapland was probably ice-free prior to the advance of the FIS c. 29 ka. The FIS reached its maximum lateral extent in northwest Arctic Russia c. 19–17 ka, later than many other sectors of the ice sheet. This disparity probably arises as a result of both the topography of Fennoscandia and the position of the Kola Peninsula and Russian Lapland in a precipitation shadow of the Scandinavian Mountains. Thus, FIS glaciation in northwest Arctic Russia was strongly influenced by a fluctuating climate. Most of the FIS in northwest Arctic Russia was terrestrial-based, although marine-terminating margins existed in the fjords of northern Russian Lapland. The retreat of the White Sea lobe was probably not influenced by marine transgression until c. 12 ka because palaeo-sea levels during most of the Late Weichselian were considerably lower than present, and a shallow sill in the Mouth of the White Sea would have inhibited ocean waters entering the White Sea basin. Instances of ice margin readvance during deglaciation are also apparent, including a significant readvance of the White Sea lobe c. 14 ka. This study presents the first reconstruction grounded in high-resolution empirical data for the Kola Peninsula and Russian Lapland, and is presented in a time-slice format that is of critical importance for testing and validating numerical ice sheet and climate models.

Provinciality of ice rafting in the North Atlantic: application of 40Ar/ 39Ar dating of individual ice rafted hornblende grains

A detailed reconstruction of palaeo-ice flow configuration is lacking for the Kola Peninsula and Russian Lapland, northwest Arctic Russia. This study presents, for the first time, a 14-stage reconstruction of the last Fennoscandian Ice Sheet (FIS) flow configuration on the Kola Peninsula and Russian Lapland from build-up to complete deglaciation. Flowsets (n = 102) and cross-cutting bedform assemblages identified from a high-resolution subglacial bedform record (subglacial lineations and subglacial ribs) are combined with subglacially streamlined bedrock data to define ice flow patterns, ice divides, ice margins, and glaciation styles of the last glaciation through relative time.The results demonstrate that the FIS flow configuration was not static. The FIS advanced eastwards across the region from Scandinavia, rather than expanding from local upland areas, and established a predominantly cold-based ice mass on the Kola Peninsula with an extensive adjacent warm-based ice lobe in the White Sea. An east-west aligned ice divide was located on the Kola Peninsula and Russian Lapland during the ice sheet build-up stages. However, this divide was short-lived as the White Sea lobe dominated ice flow on the peninsula during the local-Last Glacial Maximum, and ice predominantly flowed across the region from the main ice dispersal zone that was centred over the Gulf of Bothnia during deglaciation. Deglaciation on the peninsula was initially characterised by cold-based ice sheet retreat on the central eastern Kola Peninsula and warm-based ice lobe retreat in the White Sea. Continued deglaciation was characterised by ice sheet thinning, exposing mountain summits as nunataks behind the ice sheet margin, with topography constraining ice flow around upland areas. Ice streams in the region were drivers and consequences of continued ice sheet configuration change throughout glaciation. Four palaeo ice streams – (i) the Imandra Ice Stream; (ii) the Lovozero Ice Stream; (iii) the Kanozero Ice Stream; and (iv) the Kuusamo Ice Stream – and three possible palaeo-ice stream pathways are identified.We consider our ice flow configuration reconstruction to be the simplest palaeo-glaciological interpretation of the bedform record of the Kola Peninsula and Russian Lapland. The empirically-based reconstruction of ice flow geometry provides a regional framework and context for interpreting results from local-scale fieldwork, and is presented in a format that can be utilised by numerical ice sheet modellers to test and verify their models. The results underpin a new data-driven reconstruction of the last FIS in northwest Arctic Russia that we present in Part 2 of this study.

<p>Data-driven reconstructions of palaeo-ice sheets based on their landform records are required for validation and improvement of numerical ice sheet models. In turn, such models can be used to better predict the future responses of the Antarctic and Greenland ice sheets to climate change. We are exploiting the recent expansion in availability and coverage of very-high-resolution (1–2 m) digital elevation models (DEMs) within the domain of the former Fennoscandian Ice Sheet to reconstruct its flow pattern evolution from the glacial landform record.</p><p>The Fennoscandian Ice Sheet reached its maximum extent at 21–20 ka. Previous data-driven reconstructions over the whole ice sheet domain (encompassing Fennoscandia, northern continental Europe and western Russia) have necessarily relied upon landform mapping from relatively coarse-resolution (decametre-scale) data, predominantly from satellite images and aerial photographs. However, high-resolution (1–2 m/pixel resolution) LiDAR DEMs have recently become available over a large portion of the ice sheet domain above contemporary sea level. This reveals previously unobserved assemblages of landforms which record past ice sheet flow, including fine-scale cross-cutting and superposition relationships between landforms. These observations are likely to reveal previously unidentified complexity in the flow evolution of the ice sheet. However, the richness of the data available over such a large area amplifies labour-intensity challenges of data-driven whole-ice-sheet reconstructions; it is not possible to map every flow-related landform (or even a majority of the landforms) manually in a timely manner. We therefore present a new multi-scale sampling approach for systematic and comprehensive ice-sheet-scale mapping, which aims to overcome the data-richness challenge while maintaining rigor and providing informative data products for model-data comparisons.</p><p>We present in-progress mapping products covering Finland, Norway and Sweden produced using our new multi-scale sampling approach. The products include mapping of >200 000 subglacial bedforms and bedform fields, and a summary map of ‘landform linkages’. Landform linkages summarise the detailed landform mapping but do not extrapolate over large distances between observed landforms. Thus, they provide a reduced data product that is useful for regional-scale flow reconstruction and model-data comparisons and remains closely tied to landform observations. The landform linkages will be reduced further into longer interpretative flowlines, which we will then use to generate ‘flowsets’ describing discrete ice flow patterns within the ice sheet. We will use cross-cutting relationships observed in the detailed landform mapping to ascribe a relative chronology to overlapping flowsets where relevant. We will then combine the flowsets into a new reconstruction of the flow pattern evolution of the ice sheet.</p>

<p>It is challenging to model the last deglaciation, as it is characterized by abrupt millennial scale climate events, such as ice-sheet surges, that are superimposed on long-term climate changes, such as a global warming and the decay of a substantial part of the glacial ice sheets. Within PMIP, several groups have simulated the last deglaciation with CMIP-type models prescribing ice sheets from reconstructions. Whereas this type of simulations accounts for the effects of ice-sheet changes including meltwater release on climate, the prescribed ice sheet evolution is typically not consistent with the simulated climate evolution. Here we present a set of deglacial simulations that include fully interactive ice sheets that respond to changes in the climate. The setup also allows for feedbacks between ice sheets and climate and , hence, allows for a more realistic representation of the mechanisms of the last deglaciation, as the simulated climate and ice sheet changes are fully consistent..</p><p>The model consists of the coarse resolution set-up of MPI-ESM coupled to the ice sheet model mPISM (Northern Hemisphere and Antarctica) and the solid earth model VILMA. The model includes interactive icebergs and an automated calculation of the land-sea mask and river routing directions. A set of synchronously coupled simulations were started from an asynchronously coupled spin-up at 26ky and integrated throughout the deglaciation into the Holocene. The only prescribed external forcing are atmospheric concentrations of greenhouse gases and earth orbital parameters. One goal of this ensemble was to find the optimal combination of model parameters for the simulation of the deglaciation.</p><p>The model simulates the decay of the ice sheets, the rise of sea level, the flooding of shelf seas and the opening of passages. A large fraction of the ice sheet retreat is due to dynamical events (e.g. the final decay of the ice sheets on Barents Shelf or the Hudson Bay). Superimposed on the relatively slow glacial/interglacial transition are abrupt climate changes, triggered for example by recurrent ice sheet surges. These surges correspond to Heinrich Events tand result in a weakening of the AMOC. Three source regions for ice sheet surges occur during these simulations: from the Laurentide ice sheet through Hudson Strait, from the Laurentide ice sheet northward directly to the Arctic ocean, and from the Fennoscandian ice sheet into the Norwegian Sea. The characteristic climate response shows a large dependence on the surge location.</p><p>The simulated changes in strength of the AMOC are except for millennial-scale reduction events only moderate. However, during glacial periods, brine release is the central process for deep water formation in both hemispheres, in contrast to the Holocene. dDuring the deglaciation the ventilation of the deep ocean is strongly reduced, leading to a strong increase of the simulated deep water ages. This effect lasts longest in the deep North Pacific and extends in some simulations into the Holocene.</p>

Glaciology of the British Isles Ice Sheet during the last glacial cycle: form, flow, streams and lobes

Abstract. In this study we compare a recent reconstruction of the Weichselian Ice Sheet as simulated by the University of Maine ice sheet model (UMISM) to two reconstructions commonly used in glacial isostatic adjustment (GIA) modelling: ICE-5G and ANU (Australian National University, also known as RSES). The UMISM reconstruction is carried out on a regional scale based on thermo-mechanical modelling, whereas ANU and ICE-5G are global models based on the sea level equation. The three models of the Weichselian Ice Sheet are compared directly in terms of ice volume, extent and thickness, as well as in terms of predicted glacial isostatic adjustment in Fennoscandia. The three reconstructions display significant differences. Whereas UMISM and ANU includes phases of pronounced advance and retreat prior to the last glacial maximum (LGM), the thickness and areal extent of the ICE-5G ice sheet is more or less constant up until the LGM. During the post-LGM deglaciation phase ANU and ICE-5G melt relatively uniformly over the entire ice sheet in contrast to UMISM, which melts preferentially from the edges, thus reflecting the fundamental difference in the reconstruction scheme. We find that all three reconstructions fit the present-day uplift rates over Fennoscandia equally well, albeit with different optimal earth model parameters. Given identical earth models, ICE-5G predicts the fastest present-day uplift rates, and ANU the slowest. Moreover, only for ANU can a unique best-fit model be determined. For UMISM and ICE-5G there is a range of earth models that can reproduce the present-day uplift rates equally well. This is understood from the higher present-day uplift rates predicted by ICE-5G and UMISM, which result in bifurcations in the best-fit upper- and lower-mantle viscosities. We study the areal distributions of present-day residual surface velocities in Fennoscandia and show that all three reconstructions generally over-predict velocities in southwestern Fennoscandia and that there are large differences in the fit to the observational data in Finland and northernmost Sweden and Norway. These difference may provide input to further enhancements of the ice sheet reconstructions.

Antarctic glacial history from numerical models and continental margin sediments

<p>An interconnected complex of ice sheets, collectively referred to as the Eurasian ice sheets, covered north-westernmost Europe, Russia and the Barents Sea during the Last Glacial Maximum (around 21 ky BP), connecting to the Scandinavian Ice Sheet to the south. Due to common geological features, the Barents Sea component of this ice complex is seen as a paleo-analogue for the present-day West Antarctic Ice Sheet. Investigating key processes driving the last deglaciation of the Barents Sea Ice Sheet represents an important tool to interpret recent observations in Antarctica over the multi-millennial temporal scale of glaciological changes. We present results from a statistical ensemble of ice sheet model simulations of the last deglaciation of the Barents Sea Ice Sheet, all forced with transient atmospheric and oceanic conditions derived from AOGCM simulations. The ensemble of transient simulations is evaluated against the data-based DATED-1 reconstruction. We find that the simulated deglaciation of the Barents Sea Ice Sheet is primarily driven by the oceanic forcing, with sea level rise and surface melting amplifying the ice sheet sensitivity to ocean warming over relatively short intervals. Despite a large model/data mismatch at the western and eastern ice sheet margins, the simulated and DATED-1 deglaciation scenarios agree well on the timing of the deglaciation of the central and northern Barents Sea. The primary role played by ocean forcing in our simulations suggests that the long-term stability of the West Antarctic Ice Sheet could be at stake if the current trend in ocean warming will continue.</p>