Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 sets from six ff , and J. Kleman 2 , M. Margold 2 4147 4148 , A. P. Stroeven 3 , I. Rogozhina 1,2,3 shore, where it calved into the Pacific Ocean, and merged with the western margin ff This discussion paper is/has beenPlease under refer review to for the the corresponding journal final The paper Cryosphere in (TC). TC if available. Laboratory of Hydraulics, Hydrology andDepartment Glaciology, ETH of Zürich, Physical Zürich, Geography and Switzerland the Bolin Centre for ClimateHelmholtz Research, Centre Stockholm Potsdam, GFZ German Research Centre for Geosciences, Abstract Despite more than aNorth century America remains of poorly geological understoodnamics. in observations, terms Although the of geomorphological its Cordilleran evidence former icewhole is extent, volume ice-sheet sheet abundant, and reconstructions of its dy- of complexityuse advance is and a such retreat numerical that patterns icequantitative are sheet reconstruction lacking. model Here of calibrated we the againstcycle. Cordilleran field-based A ice evidence series to sheet of attempt history simulations a is through driven the by last time-dependent glacial temperature o 1 2 University, Stockholm, Sweden 3 Potsdam, Germany Received: 21 June 2015 – Accepted: 25Correspondence June to: 2015 J. – Seguinot Published: ([email protected]) 7 August 2015 Published by Copernicus Publications on behalf of the European Geosciences Union. proxy records located aroundtions the in model globe. response Although toglaciations this during evolving marine approach climate oxygen forcing, isotope reveals stages all largeThe 4 simulations (61.9–56.5 timing varia- ka) produce of and two 2 glaciation major (23.2–16.8Greenland ka). is and better Antarctic ice reproduced coresmost using than of temperature from the reconstructions last regional from glacial oceanichigh cycle, sediment mountain the cores. areas. modelled During ice However,favours widespread cover the is precipitation discontinuous persistence over and of the restricteda a to Skeena nucleation central Mountains centre ice before domeCordilleran ice the throughout until the Last the glacial middle Glacial Holocene cycle. Maximum (6.6–6.2 It ka). and acts hosts as the last1 remains of Introduction During the last glacialhave been cycle, more glaciers extensive and thanous today. ice blanket At of caps the ice, Last of the Glacial CordilleranRange the Maximum ice in North sheet (LGM), the, (Dawson a north American1888), continu- to stretchedo Cordillera the from North the Alaska Cascades inof the its south much (Fig.1). larger In neighbour, addition, the it Laurentide extended ice sheet, east of the Rocky Mountains. Numerical simulations of the Cordilleran ice sheet through the last glacialJ. Seguinot cycle The Cryosphere Discuss., 9, 4147–4203,www.the-cryosphere-discuss.net/9/4147/2015/ 2015 doi:10.5194/tcd-9-4147-2015 © Author(s) 2015. CC Attribution 3.0 License. 5 10 15 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ect ff shore, in ff 4150 4149 shore sedimentary records (Cosma et al., 2008; Davies ff How much ice was locked in the CordilleranWhat ice was sheet the during scale the of LGM? glaciation prior toWhich the were LGM? the primaryconfigurations? dispersal centres? Do they reflectHow stable rapid or was the ephemeral last deglaciation?vances? Did it include Late Glacial standstills or read- – – – – In general, the topographic complexity of the North American Cordillera and its e More than a century of exploration and geological investigation of the Cordilleran Our understanding of the Cordilleran glaciation history prior to the LGM is even more In contrast, evidence for the deglaciation history of the Cordilleran ice sheet since the zone of confluencesoula with floods the (Carrara Laurentide et icewestern al., sheet, margins1996), and provide key moraines in constraints that areas thatimum demarcate allow ice swept reasonable sheet the by reconstructions extents the northern of (Prest Mis- max- Booth and et al., et south- 1968; , al. , Clague 2003 ; 1989,ing, Dyke Fig. (Clague 1.12; 2004).Duk-Rodkin, et As; 1999 , al. indicatedet1980; by al., fieldClague, 2008 ), evidence, 1985 cosmogenic from exposure; 1986 2014), dating radiocarbonPorter bedrockStroeven ( dat- deformation and et in al., , Swanson response2010, 1998; Clague to2014; formerMenounos etMargold ice et al., loads al., (Clague),2005 and and, James 2002; o fragmentary (Barendregt and Irving, ; 1998 thoughKleman it et is al., clear; 2010 thatRutterdates the the et Pleistocene last al., maximum glacial2012 ), extent cycle al- (Hidy ofand et the in al. , Cordilleran).2013 the ice In Puget sheet parts Lowland, pre- ofdated the the distribution glacial Yukon Territory of and erratics Alaska, tills (Ward (Turneret et et al., al., al., ; 2013 2010, , Troost 2014)2007, )2014 indicate and tion.2008 ; an LandformsBriner extensive Marine in and Oxygen the Kaufman, IsotopeLGM interior Stage2008; (Kleman (MIS) et regionsStroeven 4 al., include glacia- 2010, flow Fig. 2), sets but that their are absolutethe LGM age likely is remains older considerable, uncertain. albeit than mostlyfrom the at south-central a British regional scale. Columbiaemergence Geomorphological indicates of evidence elevated a areas rapid whilelands thin, deglaciation, stagnant (Fulton, including ice an, 1967 still covered early 1991; theMargold surrounding low- et al. , , 2011 2013b). This model, although cred- et, al. 2011), the LGM Cordilleranthicknesses ice and, sheet therefore, extent the was ice short-lived.(Carlson However, sheet’s and former contribution ice Clark, to2012; theClark LGM and sea Mix, )2002 level lowstand remain uncertain. prove our understanding of the Cordilleran ice sheet (Jackson and Clague , 1991, Although numerical ice sheet modelling has been established as a useful tool to im- on glacial history haveand inhibited retreat the patterns such reconstruction assheets of those (Boulton ice available for et sheet-wide the al., glacial Fennoscandian1997, 2001 ; and advance Laurentide2010; Dyke ice andStroeven Prest, etPISM1987; authors, al., 2015 ),Dyke2015). calibrated et against Here, al., field-basedreconstruction evidence, we2003; of to theKleman use perform Cordilleran a et a quantitative iceanalyse al., numerical sheet some evolution of ice through the sheet the long-standing last questions model glacial related (the to cycle, its and evolution: ible, may not applyAlthough in solid all evidence areas forCoast, of Columbia late-glacial and the Rocky glacier mountains Cordilleran (Clague re-advancesb; ice, etKovanen have al., ; 2002 sheet; 1997 beenKovanen (MargoldFriele and found andet Easterbrook, et Clague , al., in2002; , 2002a al., ),2008 Lakeman the 2013a). it etthe al., appears2008 ; North toMenounos be Atlantic sparser (e.g., et thanSissons, al., for).2011 1979; formerly Nevertheless,Lundqvist, glaciated recentsediments; 1987 regions oxygen reveal a surrounding isotopeIvy-Ochs climatic measurements et evolution fromperiod, al. , highly Gulf including correlated; 1999 of a toStea Alaska distinct that Latetorius of Glacial Greenland and cold Mix, during2014). reversal this between 14.1 and 11.7 ka (Prae- mountains have led toson many and observations Clague, in1991). support Despite of the the lack former of ice documented sheet end (Jack- moraines o 5 5 25 15 20 10 10 20 25 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 − set time ff sets and lapse-rate tempera- ff erence, shallow ice sheet model ff erent palaeo-temperature reconstruc- ff erent records to modelling the history of ff 4152 4151 7cbe), an open source, finite di ff Each simulation starts from assumed ice-free conditions at 12 0000 years ago Basal topography is derived from the ETOPO1 combined topography and bathymetry Our palaeo-climate forcing therefore includes spatial temperature and precipitation simulations were run on twoand distinct a grids, higher using a horizontal lower resolutionto horizontal of 128 resolution 5 computing of km. cores 10 These km, at computations the were Swedish performed National on Supercomputing2.2 16 Centre. Ice thermodynamics Ice sheet dynamics are typicallyand modelled basal using sliding. a PISM combination of is internal a deformation shallow ice sheet model, which implies that the balance (120 ka), and runs topasses the the present. entire Our area modelling covered domain by of the 1500 Cordilleran by ice 3000 sheet km at encom- the LGM (Fig.1). The 2 Model setup 2.1 Overview The simulations presented here were runvelopment using version the 8 Parallel Ice Sheet Model (PISM, de- ture corrections (Sect. 2.4). tions from proxy records aroundGreenland the ice globe, including cores twoisotope (Dansgaard oxygen records isotope et records from from al., Antarctictwo alkenone ice1993; unsaturation cores indexAndersen records (Petit from(Herbert et et Northwest et Pacific al., al., al., ocean2001).1999; sediment2004), We cores Jouzeldence then two and proceed et discuss to oxygen al., the compare2007), timing thebased and model and on output extent which of to we geological glaciation use evi- and the the applicability patterns of of di deglaciation, as a function of time1989) based time on scale. the Spectral Geothermalat Mapping heat 3 Project km flux (SPECMAP, Imbrie depth is (Sect. etday applied2.2). al., Surface as (PDD) mass a model balance constant isaveraged (Sect. value computed from2.3). of using 1979 a 70 Climate mWm to positiveMesinger forcing degree- 2000 et is al., from2006), provided the perturbated North by by time-dependent American a o Regional monthly Reanalysis climatology (NARR, dataset with a resolution of 1 arc-min (Amante and Eakins, 2009). Sea level is lowered the Cordilleran ice sheet. (the PISM authors, 2015).geothermal The heat model flux requires andthickness input climate over forcing. on time, the It basal thermalated computes topography, lithospheric and sea the response. dynamic level, evolution states of of ice the extent ice and sheet, and the associ- (Seguinot et al., 2014). Togrids mimic are climate evolution simply through supplemented theseries. by last The lapse-rate glacial latter cycle, corrections are these and obtained temperature by scaling o six di grids derived from aincludes present-day the atmospheric steep reanalysis precipitation (Mesingerthe gradients et previously LGM identified al., extent as2006) of necessary that the to Cordilleran model ice sheet in agreement with its geological imprint p. 227; Robert, ; 1991 phy ofMarshall et the al., recent region developments2000), has in the presented numerical ubiquitously iceing two tools sheet mountainous major (Bueler models topogra- and challenges and, Brown numerical underlying to; 2009 modelling scientificBalay its of et comput- application. glaciers al., 2015)time and First, have scales ice allowed only (e.g., sheets forGolledge on high-resolution etAmerican mountainous al., Cordillera terrain2012). also over Second, induces millennial the strong complexcipitation, geographic topography variations thus of in requiring the temperature the North and usean pre- of ice high-resolution sheet climate model forcingthe (Seguinot fields et last as al., an glacial).2014 input cyclecomputational However, to evolving reach are climate of subject conditions atmosphere circulation to over models. considerable uncertainty and still lie beyond the 5 5 25 20 20 15 10 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ◦ , and σ , varies W is a mea- . A subgrid c ff C Pas is used for the 1).(Table The yield , and the modelled b 19 , varies from 15 to 45 v ρgh φ 10 × 1 = . Although this uniform value 2 m − ν , to the sliding velocity, 4154 4153 b τ is a measured reference void ratio and 0 , (3) e )) cient1). (Table The amount of water at the base, max ffi W/W ( , is related to the ice overburden stress, 2 m, a threshold above which additional melt water is assumed to − , (1) N , is assumed to be zero. The friction angle, , monthly standard deviation of near-surface air temperature, 0 = q )(1 m c , (2) − c T 1 | /C φ b max 0 b e v ( v W | tan q 10 th is chosen as 0.02, N , is modelled using the Mohr–Coulomb criterion, instantaneously. Finally, the bedrock topography responds to ice load follow- v c c + τ ff δ τ 0 − c δρgh = ective pressure, = = Ice shelf calving is computed using a double criterion. First, a physically-realistic calv- A pseudo-plastic sliding law, Ice rheology depends on temperature and water content through an enthalpy for- ff b c from zero to ing a bedrock deformation model thatand includes viscous local astenosphere isostasy, deformation elastic in lithosphere anBueler flexure infinite et half-space (Lingle al., and2007). Clark, A1985; astenosphere relatively1) low (Table viscosity in value accordance of adjustment with modelling the at the results northern from Cascadia regional subduction zone glacial (James isostatic ing et flux al. , is2009). computed basedmann on et eigenvalues al., of2011 ; theLevermannfined horizontal et strain embayments, al., rate2012). but tensor This (Winkel- preventsocean. allows the floating Second, ice formation floating to ice of advancescheme in thinner by extensive con- Albrecht than ice et 50 al.(2011) shelves mThis allows formulation in is for of a the systematically calving continuous calved open has migrationto o been of produce applied the to calving a the front. (Martin realistic Antarctic et ice calving al., sheet2011). front and positon has shown for many2.3 of the Surface present-day mass ice balance shelves Ice surface accumulation and ablationair are temperature, computed from monthly mean near-surface drain o sured compressibility coe τ where cohesion, where stress, as a piecewise-linear function ofbelow modern modern bed elevation, sea with levelalised the (0 lowest elevation ma.s.l.) value of occuring and the highest thecounting shorelines for highest a (200 value ma.s.l., weakening occuring ofClague, Martin above till1981, et the associated Fig. al., with 5), gener- 2011; the thusAschwanden presence ac- et of al., marine sediments,2013 (cf. Supplement; the PISM authors, 2015). amount of subglacial water, usingtill a extracted formula from derived the fromBueler base laboratory and of van experiments Ice, Pelt with 2015), Stream B in West AntarcticaN (Tulaczyk et al., 2000; relates the bed-parallel shear stresses, E τ does not accountand for Richards, the2004), high it spatialments. is, on In geothermal average, the variability representative low-resolution of inlayers simulations, available in the heat the the region flow vertical bedrock measure- (Blackwell a and grid vertical up consists resolution to of of 51 31up 100 enthalpy to m. temperature layers 101 The in ice high-resolution layers the with simulations ice a 61 sheet, vertical bedrock resolution corresponding layers of to and 50 m. mulation (Aschwanden et al., 2012).forcing Surface (Sect. air2.4) temperature provides derivedTemperature the from is the upper computed climate boundary subglaciallya condition to lower to a the boundary depth ice of geothermal enthalpy 3 km, heat model. where flux it of is 70 conditioned mWm by of stresses isShelf approximated Approximation based (SSA) is on used(SIA) as their by a adding predominant “sliding velocity law” components. solutions forWinkelmann the of The et Shallow the al., Ice Shallow two2011, Approximation approximations Eqs. (Bueler 7–9 and and, Brown 15). ; 2009 5 5 10 15 20 25 20 25 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the ff , and TS T ∆ C, and decreases ◦ , and the NARR ref- b O) measurements using 18 , on the one hand, and for set time series, aroung the expected value δ b ff σ − s , (4) = # C. Ablation is computed from PDD, ◦ h  σ C in one year. m ◦ 2 T √ ), (5) − ], (9) y  , ref 4156 4155 x , are derived from palaeo-temperature proxy , b ], (7) t , are computed as a function of ice surface el- 2 erfc TS ( − T ) LR m LR 2 T ∆ y T T , O(0) ∆ x + ] (8) 18 , O(0)] t + ∆ δ ( ref ! ) 18 t 2 b b − δ ( 2 10 2.4 Jouzel.etal.2007, the Vostok ice core (Petit et al., : m σ 2 + − T = ) − 2 TS LR ) ) t ) , using a temperature-index model (e.g., , Hock 2003). Accu- T T y − y t m , , on the other hand. All simulations use an annual temperature , ∆ O( P

x x O( ref , + ∆ , 18 is computed from NARR daily temperature values from 1979 to t ). (6) t ) b 18 ( δ ( y y σ set time-series, exp δ h s , , [ [ ects of spatial and seasonal variations of temperature variability ff , where temperatures are modified by o x x π ff } γ γ ( : ( 2 σ − − m0 ref m0 √ m0 P = = b P T 11.88[ 0.1925[ , " ) t = = − − , using the NARR surface geopotential height invariant field as a reference y m0 d , ) ) s 2 T = t 1 x y Z y { t ) , , , t t = x x ( ( , , t t erences between the basal topography of the ice flow model, TS LR ( ( . It is expressed by an error-function formulation (Calov and, Greve 2005), Temperature o The PDD computation accounts for stochastic temperature variations by assuming Finally, lapse-rate corrections, ff T T m m m T lapse-rate corrections, erence topography, coast of California (Herbert et al. , and2001). NGRIP Palaeo-temperature records anomalies were from calculated the GRIP from oxygen isotope ( while temperature reconstructions from Antarcticsuch. and All oceanic records cores were wereextents provided at scaled as the linearly LGM3)2) (Table (Table and in realistic outlines order (Fig.4). to simulate comparable ice thus accounting for thedi evolution of ice thickness, a quadratic equation (Johnsen et, al. 1995), ∆ topography, ∆ climate of the Northtions American in Cordillera temperature is seasonality, timing characterised oftemperature by the variability maximum strong (Fig.2). annual geographic The precipitation, varia- curate, and use high-resolution daily of precipitation NARR forcing, is as(Seguinot motivated identified et in by al., a the). 2014 previous need sensitivity for study an ac- records from the Greenland IceGreenland Core Project Ice (GRIP, Dansgaard Core et Project al., Ice (NGRIP, 1993),Andersen the Coring et North in al., 2004), Antarctica the ‘ European Project for evation, P The present-day monthly climatologyture and was precipitation computed rate fields from from near-surface the NARR, air averaged tempera- from 1979 to 2000. Modern 1999), and Ocean Drilling Program (ODP) sites 1012 and 1020, both located o 2.4 Climate forcing Climate forcing driving icetology, sheet simulations consists of a present-day monthly clima- monthly precipitation, which is numericallycount approximated for using the(Seguinot, week-long e 2013), sub-intervals. In2000 (Mesinger order et to al., (Fig.2).),2006 Degree-day ac- factors including for snow variability andsurements associated ice on melt with are contemporary the derived glaciers from seasonal fromin mass-balance cycle the mea- British Coast Columbia1; Mountains (Table Shea and et Rocky al., Mountains 2009). mulation is equal to precipitation when air temperatures are below 0 to zero linearly with temperatures betweendefined 0 as and an 2 integral of temperatures above 0 a normal temperature distribution of standard deviation T PDD 5 5 25 10 15 20 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent model runs by ff erent palaeo-temperature proxy ff 4158 4157 . In the rest of this paper, we refer to di 1 − 6Kkm erences in the input climate forcing (Fig.3, upper panel), model out- erences in the timing of attained volume extrema3), (Table all model ff ff = γ er significantly between model runs, and vary between 3.84 and 8.84 m sea level Modelled ice sheet geometries during the LGM (MIS 2; Fig.4, lower panels) invari- In the MIS 3 ice volume minimum reconstructions, a central ice cap persists over While simulations driven by the GRIP and the two Antarctic palaeo-temperature Simulations forced by the Greenland ice core palaeo-temperature records (GRIP, ff using the other palaeo-temperature records.Antarctic Finally, whereas palaeo-temperature model records runs forced result byafter in the the a LGM, rapid the andsults simulation uninterrupted in driven deglaciation a by rapid theregrowth deglaciation GRIP (Fig.3). but palaeo-temperature in record three also steps, re- separated by3.2 two periods of Extreme ice configurations sheet Despite large di rect result of the choice of scaling factors applied to di ably include a ca.spine 1500 of km-long the central Rocky divide Mountains. Although above the 3000 ma.s.l. similarity of located modelled along ice the extents is a di- runs show relativelyproduce consistent an patterns extensive of iceing sheet, glaciation. MIS covering During an 23; MIS area (Table 4, Fig. di of 4, upper at all panels). least simulations Corresponding half maximum of ice that volumes attained also dur- temperature record. Modelled ice1.69 volume and minima 2.88 m spread s.l.e.3). (Table over a wide range between equivalents (m s.l.e.;3). Table the Skeena Mountains (Fig.4,simulations, middle its panels). dimensions Although depend this ice sensitively cap on is the present choice in of all the applied palaeo- records2), (Table it is interestingwithin to a note tight that range modelled of maximum 8.40 ice to volumes 8.91 also m s.l.e. fall 3). (Table early maximum in ice volume at 23.2 ka, followed by slower deglaciation than modelled records attain a lastby ice the volume NGRIP maximum and betweenice the 19.1 two volumes and oceanic thousands 16.8 palaeo-temperature ka, ofdeglaciation records those of years attain informed the earlier. their modelled maximum Moreover, area the prior to ODP 17 1012 ka. The run ODP yields 1020 simulation a predicts rapid an lapse rate of glacial cycle. In contrast,Antarctic simulations (EPICA, Vostok) driven palaeo-temperature by records generally oceanicume result (ODP variability in 1012, lower throughout ice ODP the vol- umes 1020) during simulation and MIS length, 4 and resultingthe larger in only ice one volumes lower that during modelled results MIS in ice 3. a The vol- larger NGRIP ice climate volume forcing is during MIS 4 than during MIS 2. NGRIP) produce the highest variability in modelled ice volume throughout the last the name of the proxy record used for the palaeo-temperature forcing. 3 Sensitivity to climate forcing time-series 3.1 Evolution of ice volume Despite large di put presents consistent features thatused. can be In observed all across thethe simulations, range glacial of modelled forcing cycle, ice data exceptand volumes 56.5 during remain ka two during relatively major MISpanel). low An 4, glacial during ice and events volume most between minimum whiching of is 23.2 occur consistently MIS and reached 3. between 16.8 between ka However, 61.9 53.0significantly the during and magnitude MIS on 41.3 ka and 2 the dur- precise (Fig.3, choiceforcing3). timing lower (Table of of proxy these record three events used depend to derive a time-dependent climate 5 5 20 10 15 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff Ccalka (Cosma et al., 2008 ; Tay- 14 Beka (Stroeven et al. , 2010, 2014). A sharp 4159 4160 10 Ccalka (Davies et al., 2011). 14 ected by variations in the California current, it probably shore sedimentary record, which reveals an increase of ff ff ect, corresponding to a weakening of the California current (Herbert ff Ccalka (Porter and, Swanson ).1998 These dates are consistent with ra- 14 The exact timing of modelled MIS 2 maximum ice volume depends strongly on Among the simulations presented here, only those forced with the GRIP, EPICA and Because most of the marine margin of the Cordilleran ice sheet terminated in a sec- comparison with geological evidence for the timing of maximum Cordilleran ice sheet the choice of applied palaeo-temperature record, which allows for a more in-depth glaciomarine sedimentation between 19.5 and 16.2 timing to the Gladstonegeological (MIS evidence for 4) the andet northern al., McConnell1996; sectorWard (MIS et of al., 2)(MIS2007 ; the 2) glaciationsStroeven et Cordilleran documented, al. documented for ice2010 , its by 2014), southern sheet2014). and sector (Duk-Rodkin to There (Porter the and Fraser is Glaciation , Swanson (Clague stratigraphical1998; andMargold evidence, Ward et2011) al., for and antiming in the MIS are Puget 4 still Lowland, ( Troost highly),2014 glaciation2008). but conjectural in their (perhaps extent British and MIS Columbia 4 or early MIS 3; e.g., Cosma et al., extent. In the Pugetice Lowland sheet (Fig.1), margin has theand been LGM 16.4 constrained advance by of radiocarbondiocarbon the dating dates southern on from wood Cordilleran between the 17.4 o 4.1.1 Timing of glaciation Independently of the palaeo-temperature records usedsimulations to force consistently the produce ice sheet two model, glacialfirst our maxima maximum during configuration the is lastduring obtained glacial MIS during cycle. 2 The MIS (23.2–16.8 ka; 43,4; Figs. (61.9–56.53). ka) Table These and events the broadly second correspond in lor et al., 2014).much less Radiocarbon constrained dating but ofHowever, straddles cosmogenic the presented exposure northern constraints dating from places Cordilleraning the the ice the southern timing McConnell sheet margin. of glaciation margin close maximum to CIS is 17 extent dur- records, geological evidence of formerruns, glaciations while provide the a basis resultsof for from the validation numerical complexity of our of modellinggeologic this can record, evidence. perhaps in In helption terms this to and section, of analyse lifetime we of timing some major comparedeglaciation. and nucleation model centres, configuration outputs and of to patterns the of the ice maximum retreat stages, during4.1 loca- the last Glacial maxima transition in the sedimentoutlet record glaciers onto of land the at Gulf 14.8 of Alaska indicates a retreat of regional 4 Comparison with the geologic record Large variations in the modelity responses to to the evolving choice climate of forcing palaeo-temperature reveal proxy its record. sensitiv- To distinguish between di on the contrary, yieldfield-based constraints MIS by 2tively). several maximum Concerning thousands Cordilleran the of ice simulations yearsis sheet driven (about caused volumes by 6, by oceanic thatstructions 6 an records, pre-date (Fig.3, this and early upper early 4 panel; warming ka deglaciation Herberting present respec- et is al., in a2001 , the Fig. localet 3). alkenone e al., However, palaeo-temperature this2001). early recon- The warm- western California coast current, of North driving America, coldof has global waters been glaciation southwards shown (in to along SPECMAP) havein during the weakened the paradoxically during south- past each warmer 550 peak ka, sea-surfaceand including temperatures the ODP LGM, 1020 at resulting sites the (Herbert locations et al., of2001). the ODPtor of 1012 the Pacificremained Ocean insensitive to una this local phenomenum. However, the above paradox illustrates Vostok palaeo-temperature records yieldmay Cordilleran be ice sheet compatible maximum withlations extents these driven that by field the constraints NGRIP, (Fig.3, ODP 1012 lower and panel;3). ODP Table Simu- 1020 palaeo-temperature records, 5 5 25 20 15 10 10 20 25 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erences be- ff erence of being slightly ff erences are not subject to ff 4162 4161 During MIS 2, the modelled total ice volume peaks at 8.01 m s.l.e. (19.1 ka) in the However, field-based palaeoglaciological reconstructions have struggled to reconcile Such deviation from the geological inferences could reflect the fact that our model ing to ice volume maximaulation during (Fig.6, MIS left panel) 24, results (Figs. resembles in upper that a panels obtained modelled and 6). during maximum The MIS ice GRIP 2, sheet sim- with extent the that only closely major di GRIP simulation and at 8.77 m s.l.e. (17.2 ka) in4.1.3 the EPICA simulation. Ice configuration during MIS 4 The modelled iceMIS sheet 4 configurations are corresponding more sensitive to to ice the choice volume of maxima atmospheric forcing during than those correspond- Interior Plains, where itsheet merged and with was the2007; deflected southwesternKleman to et margin the al., ofruns south; 2010 the doMargold (Jackson Laurentide not et et ice have al., ensure al., significant2013a,b). this ice; 1997 Ice problem, drainage geometriesBednarski because across the from the and Rocky our position Mountains Smith, model at and the elevation LGM (Fig.5). of the ice divide the more westerly-centred ice dividethe in Rocky south-central British Mountains Columbia and with beyond, evidence that in the Cordilleran ice sheet invaded the western margin of the ice sheetbiased and towards the the position east of in the our main simulations (Seguinot meridional et ice al., divide2014). are certainly does not includegional feedback climate. mechanisms Firstly, between duringlation ice the over sheet build-up the phase topography Coastmountain preceding and Mountains ranges, the the which enhanced LGM, re- likely thea rapid resulted decrease topographic accumu- in of barrier accumulation a formeddepleted in decrease air by the of parcels these interior. flowing precipitation Secondly,inflow over latent and, of this warming therefore, potentially enhanced of warmer topography air thealong could over moisture- have the the resulted advancing eastern in margin flankboth an (cf. of withLangen the a et ice tendency sheet, al., to increasing limit).2012 melt ice-sheet Because growth, these are two absent processes, from our model, the eastern the complexity of ice-sheetthough feedbacks on located regional in climate,ODP and the 1020 demonstrates neighbourhood palaeo-temperature that, records of al- cannotthe the be Cordilleran used modelling ice as sheet domain, athe through the realistic NGRIP the forcing ODP palaeo-temperature last to record glacial 1012 model following depicts cycle. its and an Similarly, last the early glacial simulation onsetlier using volume than of maximum dated deglaciation (22.9 evidence (Fig.3) ka,EPICA of3) Table and the attained Vostok LGM palaeo-temperature about advance. records, 6 There resulting ka is in ear- a only small fair di agreement between the tween the simulations driven by those records. These di margin of the Interior2010; PlateauClagueRyder ( and, et Ward 2011; al., saddleMargold1991; connected etStumpf al., ice et2013b). dispersal Thesedivide al., centres (Ryder indicate2000 ; in et thatKleman al., a the et1991; latitudinal Columbia2013b).Kleman al., et Mountains A al., with latitudinal2010; saddle theClaguean and does main inverse, Ward configuration indeed ice ; 2011 between featureMargold the et in maina al., ice our secondary divide modelling divide over the results, over Columbia the however, Mountains southern in and Coast Mountains (Fig.5). 4.1.2 Ice configuration during MIS 2 During maximum glaciation, bothover simulations the position western flank thesult of main appears the meridional to Rocky contrast ice Mountains with divide ern4, (Figs. palaeoglaciological lower reconstructions British panels for Columbia central and5). and with This south- re- ice divides in a more westerly position, over the western further analysis further; instead werecords focus from on the simulations GRIP forced and byreconstructions palaeo-temperature of EPICA regional ice glaciation cores history, yet thatoutput. bearing appear significant To disparities to allow in produce for model thetwo a most simulations more realistic were detailed re-run using comparisondotted a against lines). higher-resolution the grid (Sect.2; geological Fig.3, record, lower these panel, 5 5 25 20 15 10 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Ccalka (Cosma 14 4164 4163 shore Vancouver Island at ca. 47 ff shore sedimentary records (Cosma et al., 2008; Davies et al., ff A notable exception to the transient character of the maximum extent of Cordilleran The resulting maps show that, during most of the glacial cycle, modelled ice cover is For the Cordilleran ice sheet, geological evidence from radiocarbon dating (Clague During MIS 4, the modelled total ice volume peaks at 7.19 m s.l.e. (57.3 ka) in the of low precipitation, hightion summer (PDD temperature SD) and in largethe the temperature foothills plains standard of of devia- the the mountains.of Alaska This glacial Interior result deposits, could (Fig.2) potentially which whichAlaska explain confines indicates Range the glaciation have that local remained to distribution glaciers small throughout2004). flowing the on Pleistocene the (Kaufman and northern, Manley slope of the 4.2.2 Major ice-dispersal centres It is generally believedseveral mountain-centred that ice caps the (Davis and Cordilleranjor, Mathews ice-dispersal ice1944). centres, In sheet visible our simulations, formed onlocated ma- the over by modelled the the ice Coast coalescence coverthe Mountains duration Skeena of (CM), maps Mountains the (Fig.7), (SM), and are ColumbiaWrangell the and and Selwyn Saint Rocky and Elias mountains Mackenzie mountains, mountains (CRM), heavily (SMKM). glacierized The at present, host an ice cap for ice sheet is the northernfined slope to its of foothills the during Alaska thesitivity Range, entire of where simulation modelled period modelled glacial (Fig.7, glaciers extent AR). are to This temperature con- apparent fluctuations insen- results from a combination contemporary climate. restricted to disjoint ice capsCordillera centred (Fig.7, on major blue mountain areas). rangesfrom of A the the 2500 North km-long Alaska American Range continuousonly expanse in in of the operation ice, northeast for extending glacial to at the cycle most Rocky (Fig.7, 32 ka, MountainsOcean hatched which in and areas). is the in about However, southwest, the except aice is northern third for sheet foothills of its is of the margins(Fig.7, the attained timespan red in Alaska for of areas). Range, the This a the the result Pacific last sheet much illustrates maximum during that shorter extent MIS the of 4 period maximum the and extents of of MIS the 2 time modelled were of ice both only short-lived and few therefore thousand out of years balance with et, al. 1980; , Clague 1985, cosmogenic; 1986 exposurePorter and dating, Swanson bedrock; 1998 deformation (Stroeven inMenounos response et et to al., formeret al., ice2008), loads al., 2010, (Clague and2005 ),, James 2014; and2002; Clague Margold o et al., ), 2014 to respectively 90 and 57 % of modelled MIS4.2 2 ice volumes. Nucleation centres 4.2.1 Transient ice sheet states Palaeo-glaciological reconstructions are generally moreextents robust and for maximum late ice ice sheet extents sheet and configurations older ice than sheet formum configurations stages intermediate (Kleman are, et or by al., nature, minimum extreme2010).the configurations, ice However, these which dominant sheet do maxi- patterns not necessarily of represent Kleman glaciation and, Stroeven throughout1997; theKleman et period al., of2008, ice2010). cover, (Porter ; 1989 2011) indicate that theing LGM to our maximum simulations, extent wecover was use throughout the short-lived. numerical last modelling To glacial output compare cycle to this (Fig.7). compute find- durations of ice GRIP simulation and at 5.04 m s.l.e. (61.9 ka) in the EPICA simulation, corresponding et al., 2008). less extensive across northern andproduces eastern a sectors. lower In ice contrast,sheet volume the geometry EPICA maximum into simulation (Fig.3), a whichin significantly translates reduced northern in southern and the sector, eastern moreterior modelled sectors, restricted6, (Fig. ice and right ice panel). generally cover Thus,MIS lower only 4 ice the glacial surface deposits GRIP elevations in simulationin the in can Puget the the explain Lowland the marine, in- (Troost 2014) presence sediment and of record that of o ice-rafted debris 5 5 20 25 15 10 20 15 25 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the Skeena Mountains7 (Figs. and8; Kle- ff 4166 4165 The modelled total ice volume corresponding to these persistent ice-dispersal The presence of a Skeena Mountains ice cap during most of the last glacial cycle Perhaps the most striking feature displayed by the distributions of modelled ice cover (Fig.8) and the degreethe development of of glacial deep glacial modificationslopes valleys of of and the troughs). the Coast We landscape Mountains,and find Saint the evidence (mainly Elias for the in mountains, this Columbia and on terms andman radiating the Rocky o et of mountains, al., the2010 , Wrangell glacial Fig. imprint 2). that The indicatesnating Skeena ice in Mountains, drainage a for in radial example, aFig. pattern indeed system 2). from bear of the We a distinct centre suggest strong peratures glacial of that at troughs the the persistent ema- mountain pressure ice range meltingerosional cover (Kleman point imprint et (Fig.7) on8 (Figs. theand9) al. , associated landscape. explains, 2010 to A with the well-developed the basal network large-scale north-west of glacial ice on glacial tem- valleys theet running west al., slope of2010, thedominantly Fig. Selwyn warm-based and 2; ice MackenzieStroeven (Fig.9). Mountainsfraction However, et (Kleman because of al., it the is last2010,the only glacial Fig. observed glaciated cycle 8) landscape for in a pattern our isan short originates simulations modelled increased from (Fig.7), relative to multiple this importance glacial perhaps havesal of cycles indicates centre hosted the and that (Fig.7, Selwyn pre- witnesses SMKM), and prioret Mackenzie to al., mountains the2012). ice Late Pleistocene disper- (cf. Ward et al. , 2008; Demuro centres attains a2.68 m minimum s.l.e. of (51.8 ka) 1.52 in31 m % s.l.e. the of (42.9 the EPICA ka) MIS simulation, in 2 corresponding ice the volumes. to GRIP respectively4.2.3 19 simulation and and Erosional imprint of on the landscape A correlation is observed between the modelled duration of warm based ice cover the centre of ourallowing westerly modelling winds domain, to thistensive carry barrier Skeena moisture is Mountains further in less inland,widespread north-central pronounced until distribution British it of than winter Columbia, is elsewhere, precipitation captured thus (Fig.2). by resulting the in ex- a more also occur further inlandmost of than the along north-western coast other ofnounced east-west North topographic transects America, barrier coastal for (Fig.2). mountain westerly In rangesform winds, form of fact, capturing a orographic pro- along atmospheric precipitation, moisture in and the resulting in arid interior lowlands. However, near can be explained bythan elsewhere. meteorological In conditions fact, more reanalysedbalance atmospheric favourable model fields for show used that ice to highslope growth force winter of the precipitations there the surface are Coast mass mainly Mountains, except confined in to the the centre western of the modelling domain where they readvance towards the LGM (MISLaurentide 2). This ice situation sheet, appear for similarleading up to which to the the the neighbouring LGM importanceand has been eastern of illustrated Canada residual by as the iceration MIS nucleation (Kleman 3 for centres residual et for the ice al., a bodies glacial2010). much in history northern more extensive MIS 2 configu- is the persistence of the Skeenariod Mountains (ca. ice cap 100–10 ka) throughout the and entireand7, its last predominance SM). glacial over pe- Regardless the other of(Fig.4, ice-dispersal the centres middle applied4 (Figs. panels), forcing, and this serves ice as cap a appears nucleation to centre survive at MIS the 3 onset of the glacial the entire length offeed both to simulations, the but that CordilleranColumbia and ice ice Rocky cap sheet mountains does (CM, (Fig.7,most not SM, of WSEM). CRM) appear the are last Although to covered glacial be theis by cycle, not mountain providing a Coast, durable glaciers the major nucleation Skeena for case centres for and the for the an highest Selwyn ice peaks and sheet, is this Mackenzie limitedthe mountains to Selwyn (SMKM), a and where small ice Mackenzie fractiontre cover mountains of on during only the the appear last coldest as glacial periods(Fig.7, a cycle. of NRM) secondary In the do ice-dispersal other last not cen- words, glacialCordilleran act cycle. ice as sheet The margin Northern a coming nucleation Rocky from Mountains centre, the but west. rather as a pinning point for the 5 5 15 20 25 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | shore Vancouver Island, where ff geothermal heat flux at 3 km depth 2 − 4168 4167 Ccalka (Taylor et, al. 2014). 14 er in detail. On the one hand, the EPICA run depicts peak (Blackwell and Richards, ).2004 We can therefore expect ff 2 − Hence, the two model runs, while similar in overall timing compared to the runs In our simulations, the timing of peak ice volume during the LGM and the pacing of Modelled patterns of ice sheet retreat are relatively consistent between the two sim- The modelled distribution of warm-based ice cover8 (Figs. and9) is inevitably af- In the North American Cordillera, the presence of lateral meltwater channels at high (Sect. 2.2). However, the Skeena Mountains andtains the experience area west higher-than-average of the geothermalca. Mackenzie Moun- heat 80 and flux ca. witheven 100 mWm measured longer values durations of of spatially warm-based variable ice geothermal cover forcing in for our these Cordilleran areas ice if sheet4.3 simulations. we were to The include last deglaciation 4.3.1 Pace and patterns of deglaciation Similarly to other glaciated regions, mostrelate glacial traces to in the the North last Americanbeen few Cordillera overprinted by millennia warm-based of ice retreat glaciation,Kleman during et because the al., last most2010). deglaciation From of (Kleman, a; 1994 are the numerical more modelling older challenging perspective, evidence phases than of has phasestions glacier of retreat of growth, the because ice they margin,poorly involve increased more understood flow rapid hydrological velocities processes. fluctua- and Theels longitudinal latter stress are through gradients, typically simple and included parametrisations in, Pelt the (e.g. 2015 ), mod- ifClason included etextents at al. , during all.2012, MIS However,2014; next 2reconstructions afterBueler and of the and patterns MIS mapping van of 4 of icesecond (Sects. maximum sheet best 4.1.2 ice retreat source sheet and of during evidence the),4.1.3 for last geomorphologically-based the deglaciation validation provide of the ourelevation simulations. (Margold et, al. 2011, vation2013b, (Burke2014), et and al., abundant,b; 2012a eskermeltwaterPerkins systems was et at produced al., low over ele- 2013; largetion.Margold The portions et southern, al. of and2013a) the northern indicateglacial ice margins that maximum sheet of extent the surface around Cordilleran during 17 ka ice deglacia- (Sect. sheet reached4.1.1; theirPorter last and, Swanson 1998; Cosma with other climate drivers, di at 19.3 ka andThe a first deglaciation interruption interrupted occurs byand between two 11.6 16.6 ka phases and (Fig. of 14.510, ka, regrowth lower and panel, until the blue about curve). second 8 between ka. 13.1 glaciation about 2 ka later thanextents, the and GRIP shows run, a in faster, closer uninterruptedmore agreement deglaciation than with which dated 1 yields ka maximum sporadic earlier.steps, ice On compatible cover the with other marine hand, sediment the sequences GRIP o run yields a deglaciation in three deglaciation depend critically on theAdopting choice the of climate EPICA forcing climate3; (Table 3 rupted Figs. forcingand deglaciation yields10). until peak about ice 9the volume ka at (Fig. simulation 17.2,10 driven ka lower by and panel, the an red GRIP curves). uninter- On palaeo-temperature the record contrary, yields peak ice volume et al., 2008 ; Stroevenset et of al., ice2010, 2014), retreat.cosmogenic which The dates we timing from take north-central of as Britishemanating final a Columbia from deglaciation limiting indicate the is that age central less athe for Coast seizable Younger well the Dryas Mountains ice constrained, on- chronozone, or cap at but the least recent Skeena until Mountains 12.4 ka persisted (Margold into et al., 2014). the distribution of ice-raftedStrait debris in two indicates phases an17.2 that to ice are 16.5 margin and contemporary retreat from with 15.5 from warming to the 14.0 oceanic Georgia temperatures from the Interior Plateauwhereas of a British significant ice Columbia, coverand becomes remains the completely over Wrangell the deglaciated Skeena, anddomain. by the Saint After Selwyn 10 10 Elias ka, and ka, mountains Mackenzie, deglaciation in continues to the proceed northern across sector the Liard of Lowland the with modelling ulations (Figs. 11 and 12).Puget The Lowland, southern the sector of Coast the Mountains, modelling the domain, including Columbia the and and Rocky mountains, and fected by our assumption of a constant, 70 mWm 5 5 10 15 20 25 10 15 20 25 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er- ff , 1997), and locally these have been recon- ff 4170 4169 the southern tip of the Coast Mountains, indicate consecutive glacier maxima, , 1997; Menounos et al., 2008). ff ff On the Interior Plateau of south-central British Columbia, both simulations produce Patterns of glacial lineations formed in the northern and southern sectors of the Although further work is needed to constrain the timing of the late-glacial readvance, a retreat of thedated by ice the margin geomorphological towards and the stratigraphical record north-east for (Fig. ancient pro-glacial12 ), lakes a pattern which is vali- (Fig. 13). In thetions northern depict half an active of downhill the flowmountain as modelling ranges. the Converging domain, last deglacial modelled remnants flow of deglacial patterns(Fig. the in flow ice13), the direc- sheet closely Liard retreat Lowland, towards resemble2013a, for the instance Fig. pattern 2). indicated by glacial lineations (Margold et al. , Cordilleran ice sheet (Prest etFig., al. 2) show1968; similaritiesClague , with,1989 the Fig. patterns of 1.12; deglacialKleman ice et flow al., from2010, numerical modelling contrast, the EPICA-driven simulation producesonly a a tightly nearly-continuous restricted deglaciation late-glacial with and readvance the on Coast the mountains western (Fig. 12, slopes right of panel). the4.3.3 Saint Elias Deglacial flow directions Because a general tenant(lineations in and glacial eskers) geomorphology areet is part that al., of the),2006 the majority(Boulton having deglacial of and envelope been Clark landforms , (terminology1990 ; formeddirections from Kleman close immediately etKleman preceding al., inside deglaciation1997, theside or),2010 a at retreating we cold-based the present retreating margin time margin mapsmostly of (Fig. of of consistent13). cessation basal between ice The of flow the modelled sliding sheets GRIP deglacialsheet in- and flow retreat patterns EPICA in simulations. are the They peripheral depict areas,rior an followed regions. by active stagnant Several ice parts ice decay of inthroughout the some the modelling of deglaciation domain the phase do inte- (Fig. not13,of experience hatched the any areas). basal Interior This sliding notably Plateau includesice in parts sheet, British and Columbia, a major tortuousSkeena portions and ribbon of Selwyn running the Mountains from Alaskan and the sector into Northern of the the Rocky Mackenzie Mountains. Mountains over the Columbia and Rocky Mountains (Fig. ,12 of left panel). some In local addition to readvances, matchingthe the the decaying location ice GRIP-driven sheet simulation may still shows have that existed at a the time large of remnant this late-glacial of readvance. In to assess its extents andtriggers geographical distribution, (Menounos and et to al., identifythe the2008), GRIP potential it climatic record is produces interesting a to late-glacial note readvance in that the the Coast simulation Mountains driven and by in the ent kind of evidence. There,of sharp-crested local moraines alpine indicate aremnants glaciers late-glacial of and, readvance the more main2008). body importantly, Additional of their evidence the for interaction Cordilleran late-glacial alpine icein with glacier the sheet larger, readvances eastern in includes Coast lingering the moraines Mountains, valleysGerlo and (Lakeman the et Columbia al., and Rocky mountains (Osborn and a radial ice marginthe retreat regional towards the melt surroundingmaining water mountain ice record continues ranges, to of consistent decaySkeena with the by mountains. retreating last towards The deglaciation the lastfrom Selwyn (Margold the remnants and Skeena et MacKenzie, of Mountains and al., at the 6.52013a). ka Cordilleran (GRIP) Re- ice and4.3.2 6.1 sheet ka (EPICA). finally disappear Late-glacial readvance The possibility of latedebated glacial for some readvances time in (Osbornstructed and the and Gerlo North dated. Radiocarbon-dated American endleys, Cordillera moraines o in has the been Fraser andor Squamish standstills val- while inchronozone overall (Clague retreat, et one al. , ofnen; 1997 which andFriele corresponds, Easterbrook and to2002). Clague, dent the Although2002a,b; valley, Younger mostKovanen glaciers, Dryas ; 2002 of thatKova- these maysheet, have moraines the been characterise Finlay disconnected indepen- River from area the in waning the Cordilleran Omenica ice Mountains (Fig. ,12 OM) presents a di 5 5 25 20 10 15 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent deglacial flow pattern over ff 4172 4171 erent topographic and climatic conditions, our simula- ff erent deglaciation patterns in the northern half of the model domain, ff er in the mode of retreat. The GRIP simulation yields an active retreat ff ected by the weakening of the California current during the LGM. ff During deglaciation, none of the climate records used can be selected as produc- In all simulations presented here, ice cover is limited to disjoint mountain ice caps The modelled deglaciation of the Interior Plateau of British Columbia consists of ing an optimal agreement between the model results and the geological evidence. indicating that this conceptualcovered by model the may Cordilleran not ice sheet. be applied to the entire5 area formerly Conclusions Numerical simulations of thesented Cordilleran in ice this sheet56.5 through study ka, 3.8–8.8 the m consistently s.l.e.) last and produce glacialresponding MIS two 2 cycle to (23.2–16.8 pre- glacial ka, documented 8.40–8.91 m maximapalaeo-temperature s.l.e.), extensive record two during used glaciations. periods among cor- MIS the Thisregarded six 4 selected result as for a (61.9– this is robust study, model and independentever, the thus output, timing can which of of be broadly the the matches twois glaciation geological used peaks evidence. to depends How- sensitively drive on the which model.Vostok climate Antarctic The record ice timing core of records. the It LGMby occurs is the about best GRIP 2 reproduced ka ice by too the corematch early EPICA record, is in and and largest the occurs for simulation even the forced are earlier two a in Northwest all Pacific ODP other palaeo-temperature simulations. records, The which mis- the moisture intruding inland fromMountains. the The west Skeena through ice a capvance topographic acts towards breach as the in the the LGM mainthe Coast configuration. nucleation model, centre As this for indicated ice the cap byon glacial perhaps persistent, the rea- explains landscape warm-based the of distinct ice the glacial in Skeena erosional Mountains. imprint observed during most of the glacialthe cycle. Coast The Mountains, the most Columbia persistentmountains, and nucleation and Rocky centres most mountains, are importantly, the located inlast Selwyn in the and glacial Skeena Mackenzie cycle, Mountains. the Throughout Skeena the Mountains modelled host an ice cap which appears to be fed by Columbia, in which mountain1991, ranges Fig. emerge 7). However, from due to the di ice before the plateau (Fulton, a rapid northwards retreat (Fig. 12)positioned of southwards-flowing in-between non-sliding deglaciated ice lobes mountain (Fig. pears ranges13 ) compatible (Figs. with 14 theand prevailing conceptual15). model This of result deglaciation ap- of central British the Interior Plateau. However,tainly a be more associated westerly-positioned with LGMthickness a ice would thinner divide not ice promote would sheet warm-basedregion cer- of conditions than negligible but, that basal on sliding modelled (Fig. theent here.13). results contrary, Thus, Decreased enlarge could a be ice the second that explanationflow, the for as the Interior perhaps incongru- Plateau indicated lineation by systemlineations some predates (Margold eskers deglaciation et that ice al., appear2013b, incompatiblegeothermal Fig. with 9). heat these Finally, associated glacial a third withtriggered explanation the volcanic could basal be activity sliding that on (cf. local Greenland the ice Interior sheet; Fahnestock Plateau et could al., 2001). have tions produce di dammed by the retreatingsimulations ice di sheet (Perkins andwith, Brennand basal2014). sliding However, the towards two theproduces negligible ice basal margin sliding to onInterior the Plateau the south, also plateau whereas hosts during the an deglaciationstantial EPICA impressive eastwards (Fig. simulation set flow).13 of component Yet, glacial of the lineationsman the et which Cordilleran al., indicate ice).2010 a sheet This sub- (Prestgun Interior et for Plateau al., lineation the1968; setKle- reliability couldgruent therefore of results present could the a be presented smoking pography that model and the results. regional missing climate One feedback resultedbeingtoo mechanisms explanation in far between for a to ice the modelled the sheet incon- located ice to- east LGM divide (Sect. ice of4.1.2; divide the Fig.5; would LGMSeguinot certainly ice et sheet result al. , in a).2014 di A more westerly- 5 5 10 25 15 20 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4174 4173 , M., Winkelmann, R., and Levermann, A.: Parameterization for ff ensen, J. P., Stocker, T., Sveinbjörnsdóttir, A. E., Svensson, A., ff J. Seguinot ran the simulations; I. Rogozhina guided experiment design; Foremost, we would like to thank Shawn Marshall for providing a de- Takata, M., Tison, J.-L., Thorsteinsson,High-resolution T., Watanabe, record O., of Wilhelms, Northern F.,period, and Nature, Hemisphere White, J. 431, climate W. 147–151,Center extending C.: for into, doi:10.1038/nature02805 Paleoclimatology, Boulder, the data Colorado, USA, last archived 2004. interglacial at4151, the4156, 4187 Worldand ice Data sheets, J. Glaciol., 58, 441–457, doi:10.3189/2012JoG11J088, 2012. model4153 sensitivity to2013, initial 2013. states,4153 The Cryosphere, 7, 1083–1093,Gropp, doi:10.5194/tc-7-1083- W. D.,Zhang, Kaushik, H.: D., PETSc Web Knepley, page, available M.2015. at: 4151 http://www.mcs.anl.gov/petsc G.,(last access: McInnes, 2015), L. C., Rupp,the late K., Cenozoic, Smith, Can. J. B. Earth Sci., F., 35, and 504–509,Foothills doi:10.1139/e97-126, of 1998. northeastern4149 British Columbia,095, Can. 2007. J. Earth4162 Sci., 44, 445–457, doi:10.1139/e06- subgrid-scale motion of ice-shelf5-35-2011, calving 2011. fronts,4154 The Cryosphere, 5, 35–44, doi:10.5194/tc- sources and analysis,Data NOAA Center, technical NOAA, Boulder, memorandum CO, doi:10.7289/V5C8276M, NESDIS 2009. NGDC-24,4152, Natl.4189 laz, Geophys. J., Clausen,Goto-Azuma, H. K., B., Grønvold, K., Dahl-Jensen, Gundestrup,Johnsen, D., N. S. Fischer, J., S., Jonsell, Hansson, H., U., M., Jouzel, Flückiger,Masson-Delmotte, Huber, J., J., Kipfstuhl, C., V., S., Hvidberg, Fritzsche, Miller, Landais, C. A., D.,Raynaud, H., S., Leuenberger, Fujii, M., D., Motoyama, Lorrain, Y., R., Rothlisberger, H.,Andersen, R., Narita, Ruth, M.-L., H., U., Popp, Ste Samyn, T., D., Rasmussen, Schwander, S. 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W.: ETOPO1 1 arc-minute globalAndersen, relief K. model: K., procedures, data Azuma, N., Barnola, J.-M., Bigler, M., Biscaye, P., Caillon, N., Chappel- Although the EPICA-driven simulation yieldstherefore, the start most of realistic deglaciation, timingreadvances only of in the the areas LGM GRIP-driven where and, simulationof these produces have ice late been sheet glacial documented.deglaciation retreat Nonetheless, of the are the patterns consistent southerntreat sector between across of the the the Interior twotion ice Plateau simulations, then sheet, of produces central and including a British a showthe late-glacial Columbia. rapid decaying a readvance The northwards Cordilleran of GRIP-driven rapid ice re- local simula- sheetMountains. ice primarily In caps both in simulations, the and this Coast of isa and followed the by final the main an Columbia retreat opening body and of of of Rocky mountains, the the Liard which Lowland, remaining hosts and the ice(6.6–6.2 last caps ka). remnant Our towards results of the identify the Selwyn theglacial ice Skeena and, dynamics Mountains sheet of finally, as during the a the the key Cordillerancal Skeena area middle ice investigation to Holocene of sheet, understanding this highlighting region. the need for further geologi- The Supplement related to this. doi:10.5194/tcd-9-4147-2015-supplement article is available online at Author contributions. 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J.: TheSeguinot, Cordilleran ice J.: sheet and the Spatial glacial geo- and seasonal e 5 5 10 15 20 25 30 25 30 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1 1 1 − 1 − − s − 3 3 3 1 1 1 2 K − − − − 10 ka) constrains − − − day day 1 K N 1 1 − ± 1 Pas − − Km − mK mK 3 3 − − 3 24 19 − 10 10 10 10 10 × × × × × 4186 4185 cient 0.12 – cient 0.02 – ffi ffi , M., Albrecht, T., Bueler, E., Khroulev, C., and Lever- ff ective pressure coe ff Bedrock densityBedrock thermal conductivity 3.0Temperature of 3300 snow precipitation JmTemperature of rain precipitationDegree-day factor 273.15 for snow kgm 275.15 3.04 K K Ice densityPseudo-plastic sliding exponentPseudo-plastic threshold velocity 0.25 100.0 910 myr – Bedrock kgm specific heat capacityLithosphere density 1000 Jkg 3300 kgm Standard gravityTill cohesionE 9.81 ms Astenosphere 0.0 viscosityLithosphere flexural rigidity Pa 1 5.0 Degree-day factor for ice 4.59 Till reference void ratioTill compressibility coe 0.69 – Air temperature lapse rate 6 Maximal till water thickness 2.0 m max c b l 0 b 0 b s i s r th m Surface and atmosphere T T F Not. Nameρ Basal sliding q v Bedrock Value and lithosphere ρ c Unit k ρ g c δ ν D F e C γ W Parameter values used in the ice sheet model. mann, A.: The PotsdamThe Parallel Cryosphere, Ice 5, 715–726, Sheet, doi:10.5194/tc-5-715-2011 Model 2011. (PISM-PIK)4153, –4154 Part 1: Model description, penultimate Reid glaciation,doi:10.1016/j.quascirev.2008.07.012 2008. in4149, central4166 Yukon Territory, Quaternary Res., 27, 1909–1915, Table 1. Ward, B. C., Bond, J. D., Froese, D., and Jensen, B.: Old Crow tephra (140 Winkelmann, R., Martin, M. A., Haselo 5 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | set ff corre- f Ice volume (m s.l.e.) 3.843.84 1.758.84 1.69 8.81 2.88 8.40 8.91 7.358.84 1.695.11 1.825.15 8.71 2.744.39 8.76 2.88 8.91 2.35 8.40 8.62 MIS 4 MIS 3 MIS 2 Reference ) 5.8 Dansgaard et al.(1993) 6.5 Andersen et al.(2004) 5.9 Jouzel et al.(2007) 5.9 Petit et al.(1999) 6.1 Herbert et al.(2001) 6.0 Herbert et al.(2001) 2 [32,22] − − − − − − km 6 f T 1.61 1.20 0 0 O 0.37 O 0.24 O 0.64 O 0.74 K 37 K 37 18 18 18 18 δ δ δ δ 4188 4187 Ice extent (10 1.872.11 0.691.51 0.741.51 2.07 1.011.38 2.10 1.031.25 2.09 0.90 2.01 1.25 0.72 2.07 2.11 0.69 2.09 1.03 2.01 2.10 MIS 4 MIS 3 MIS 2 1772 U 3038 U − − (ma.s.l.) (K) refers to the resulting mean temperature anomaly during the E 3233 E 3488 W W W 3238 W 2917 0 0 0 0 0 0 21 50 23 26 ◦ ◦ 38 19 ◦ ◦ ◦ ◦ [32,22] T Age (ka) S 123 S 106 N 37 N 42 N 118 N 126 0 0 0 0 0 0 06 28 35 06 17 00 ◦ ◦ ◦ ◦ ◦ ◦ 57.4560.25 42.9361.89 45.86 19.14 60.86 45.30 22.85 56.46 41.25 17.23 60.23 47.39 16.84 52.97 23.19 61.89 20.56 56.46 52.97 41.25 23.19 16.84 MIS 4 MIS 3 MIS 2 22 ka after scaling. − Extremes in Cordilleran ice sheet volume and extent corresponding to MIS 4, 3 and 2 Palaeo-temperature proxy records and scaling parameters yielding temperature o 32 to − Record LatitudeGRIP Longitude Elev. 72 Proxy NGRIP 75 EPICA 75 Vostok 78 ODP 1012 32 ODP 1020 41 Maximum GRIP NGRIP EPICA Vostok ODP 1012 ODP 1020 Minimum Record period Table 3. for each of the six low-resolution simulations (Fig.3). Table 2. time-series used to forcesponds the to ice the sheet scalingmapped model factor end moraines, through adopted and the to last yield last glacial glacial cycle maximum3). (Fig. ice limits in the vicinity of Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 12 July 9 January 6 PDD SD (°C) GIS 3 1000 LIS 215 46 IIS 4190 4189 10 Precipitation (mm) 2 24 12 CRM LL 0 SMKM IP NC 12 SM PL CIS CM Temperature (°C) AR 24 WSEM Cka (21.4 cal ka, , Dyke ).2004 The rectangular box denotes the lo- 14 Relief map of northern North America showing a reconstruction of the areas once Monthly mean near-surface air temperature, precipitation, and standard deviation of daily mean temperatureReanalysis (PDD SD) (NARR; forMesinger Januarycomponent et of and, al. the July ice2006), fromcipitation sheet used model. peak, the to Note and North the force temperatureand American strong the inland variability contrasts Regional regions. surface over in seasonality, the mass timing model balance of domain, the (PDD) pre- notably between coastal Figure 2. Figure 1. covered by the Cordilleransheets during (CIS), the Laurentide last (LIS),cation 18 of Innuitian the (IIS), modellingsheet and domain include Greenland used the (GIS) in Alaska thisSelwyn ice Range and study. Mackenzie Major (AR), mountains mountain the (SMKM),(CM), ranges the Wrangell Skeena the covered and Mountains by Columbia (SM), Saint the and thepressions Elias ice Coast include Rocky mountains Mountains the Mountains (WSEM), Liard (CRM), Lowland the Puget and (LL), Lowland the the (PL). Interior North Plateau Theand Cascades of background Natural British (NC). Earth map Columbia Data Major consists (IP), (Patterson de- and of and, the Kelso ETOPO12015). (Amante and, Eakins 2009)

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | surface elevation (km) elevation surface 0 2 1 0 4 3 MIS 1 20 erences in timing. 20.6 ka 60.2 ka 53.0 ka MIS 2 ff ODP 1020 40 56.5 ka 47.4 ka 23.2 ka ODP 1012 MIS 3 O records (Lisiecki and, Raymo 2005). 18 δ 60 60.9 ka 41.3 ka 16.8 ka Vostok MIS 4 4192 4191 model age (ka) SM 45.3 ka 17.2 ka 61.9 ka 80 EPICA Vostok ODP 1012 ODP 1020 MIS 5 set time-series from ice core and ocean records2) (Table used as 60.3 ka 45.9 ka 22.8 ka ff NGRIP 100 GRIP NGRIP EPICA GRIP 42.9 ka 19.1 ka 57.4 ka 120 2 8 2 2 6 4 8 6 0 0 4

10

Snapshots of modelled surface topography (500 m contours) corresponding to the Temperature o MIS 3 MIS 2 MIS MIS 4 MIS ice volume (m s.l.e.) (m volume ice temperature offset (K) offset temperature ice volume extremes indicated onduring Fig.3. MIS An 3. ice Note cap the persists occurence over of the spatial Skeena similarities Mountains despite (SM) large di Figure 4. palaeo-climate forcing for thepanel) ice through sheet the model last(m (top s.l.e.). 120 panel), Gray ka. and fields Ice indicate modelled volumesMIS Marine ice are 4 volume Oxygen according expressed (bottom Isotope to in Stage a meters global (MIS) of compilation boundaries sea of for level benthic MIS equivalent 2 and Figure 3. Hatched rectangles highlight theMIS time-volume 4 span (61.9–56.5 for ka), MIS icerespond 3 volume to (53.0–41.3 extremes GRIP- ka), corresponding and and EPICA-driven to MIS 5 2 km-resolution (LGM, runs. 23.2–16.8 ka). Dotted lines cor-

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) yr (m velocity surface ) yr (m velocity surface

-1 -1 100 31 10 1000 316 100 31 10 1000 316 EPICA, 17.2 ka EPICA, 61.9 ka 4194 4193 GRIP, 19.1 ka GRIP, 57.3 ka Modelled surface topography (200 m contours) and surface velocity (colour mapping) Modelled surface topography (200 m contours) and surface velocity (colour mapping) Figure 6. corresponding to the maximum ice volumesimulations. during MIS 4 in the GRIP and EPICA high-resolution Figure 5. corresponding to the maximum ice volumesimulations. during MIS 2 in the GRIP and EPICA high-resolution

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Duration of ice cover (ka) cover ice of Duration Duration of warm-based ice cover (ka) cover ice warm-based of Duration 60 40 20 0 120 100 80 40 20 10 0 120 80 EPICA EPICA CRM NRM MKM SMKM

WSEM 26 ka 26 AR SM NC SM CM 4196 4195 GRIP

GRIP 32 ka 32 Modelled duration of warm-based ice cover during the last 120 ka. Long ice cover Modelled duration of ice cover during the last 120 ka using GRIP and EPICA climate durations combined withstrong basal glacial temperatures erosional atareas imprint the that of were pressure covered the melting by Skeena cold point ice Mountains only. may (SM) explain landscape. the Hatches indicate Figure 8. Figure 7. forcing. Note the irregular colour scale.(AR) A to continuous ice the cover spanning Coastabout from Mountains the 32 Alaska ka (CM) Range in andof the the the GRIP Columbia ice simulation and sheet and Rockypersists generally 26 mountains over ka corresponds the (CRM) in Skeena to exists the Mountains relativelyabbreviations. for (SM) EPICA short during simulation. durations most The of of maximum ice the extent simulation. cover, See but Fig.1 icefor cover a list of

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fraction of warm-based ice cover ice warm-based of Fraction 5 0.90 0.50 0.00 1.00 0.99 EPICA MKM GRIP 10km GRIP 5km EPICA 10km EPICA 5km 10 15 SM 4198 4197 model age (ka) GRIP 20 set time-series from the GRIP and EPICA ice core records2) (Table ff 25 2 8 2 2 6 4 8 6 0 0 4 10

Temperature o ice volume (m s.-l. eq.) s.-l. (m volume ice Modelled fraction of warm-based ice cover during the ice-covered period. Note the (K) offset temperature (top panel), and modelled iceequivalent volume (bottom during panel). the deglaciation, expressed in meters of sea-level Figure 10. Figure 9. dominance of warm-based conditions oncoastal the ranges. Hatches continental indicate shelf areas and that major were glacial covered troughs by of cold the ice only. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

indicate

Deglaciation age (ka) age Deglaciation ) yr (m velocity surface -1 16 15 14 13 12 11 10 9 8 22 21 20 19 18 17 100 31 10 1000 316 (A–D) A B C D A B C D A B C D GRIP EPICA EPICA 10.0 ka 10.0 ka indicate the location of profiles used in 12.0 ka 12.0 ka (A–D) 4200 4199 A B C D GRIP 14.0 ka 14.0 ka OM 16.0 ka 16.0 ka Snapshots of modelled surface topography (200 m contours) and surface velocity Modelled age of the last deglaciation. Areas that have been covered only before the Figs. 14 and 15. last glacial maximum areice marked caps and in the green. decaying Hatches ice denote sheet between re-advance 14 of and mountain-centred 10 ka. Dashed segments Figure 12. the location of profiles used in Figs. 14 and 15. Figure 11. (colour mapping) during thepanels) last 5 deglaciation km from simulations. the Dashed GRIP (top segments panels) and EPICA (bottom Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . 1

− Age of last basal sliding (ka) sliding basal last of Age 16 14 12 10 8 22 20 18 IP LL EPICA 1400 1600 1800 4202 4201 2000 projection x-coordinate (km) GRIP 2200 D A B C

1 1 1 1 2 2 2 2 3 3 3 3 0 0 0 0 elevation (km) elevation Modelled deglacial basal ice velocities. Hatches indicate areas that remain non- Modelled bedrock (black) and ice surface (blue) topography profiles during deglacia- Figure 14. tion (22.0–8.0 ka) in the Figs. GRIP 11 5 kmand simulation,. 12 corresponding to the four transects indicated in Figure 13. sliding throughout deglaciation(IP). (22.0–8.0 ka), Note the notably concentric including patternswere of parts distinguished from deglacial of non-sliding flow grid in the cells the Interior using Liard a Lowland Plateau basal (LL). velocity Sliding threshold grid of cells 1 myr Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 1400 1600 1800 4203 2000 projection x-coordinate (km) 2200 D A B C

1 1 1 1 2 2 2 2 3 3 3 3 0 0 0 0 elevation (km) elevation Modelled bedrock (black) and ice surface (red) topography profiles during deglacia- Figure 15. tion (22.0–8.0 ka) in thein EPICA Figs. 11 5 kmand simulation,12. corresponding to the four transects indicated