Aug 10, 1996 - enhanced plastic flow over obstacles IN!re, 1969, 1970;. Kamb, 1970]. ...... drews, and Niels Reeh have strengthened this paper. Chris.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. B8, PAGES 17,827-17,839, AUGUST 10, 1996
Geologic and topographic controls on fast flow in the
Laurentide
and
Cordilleran
Ice
Sheets
Shawn J. Marshall and Garry K. C. Clarke Department of Earth and Ocean Sciences,University of British Columbia, Vancouver
Art S. Dyke and David A. Fisher Terrain SciencesDivision, Geological Survey of Canada, Ottawa, Ontario
Abstract. Ice streams are fast flowing arteries which play a vital role in the dynamicsand massbalanceof present-dayice sheets.Althoughnot fully understood, fast flow dynamicsare intimately coupledwith geologic,topographic,thermal, and hydrologicconditionsof the underlyingbed. Theseare difficult observablesbeneath contemporaryice sheets,hinderingelucidationof the processes which governice streaxnbehavior.For past ice sheetsthe problemis antithetic. Geologicevidenceof formerice streamsexists,but spatiMand temporalhistoriesare uncertain;however, detailedknowledgeof bed geologyand topographyis availablein many places.We take advantageof this informationto compileterrain characteristicsrelevant to fast
flowdynamicsin the Laurentideand CordilleranIce Sheets.Usingseedpointswhere fast flowingWisconsinan ice hasbeengeologically inferred,discriminantanalysis of a suiteof North Americangeologicand topographicpropertiesyieldsa concise measureof ice-bedcouplingstrength.Our analysissuggests that the interiorplains andcontinental shelfregions of NorthAmericahavelowbasalcoupling relativeto areas of variable relief or exposedbedrock in the Cordillera and on the Canadian Shield. We concludethat the interior plains and continentalshelvesare both
topographically and geologically predisposed to large-scale basalflows(i.e., ice
streaxns or surgelobes). This resultholdsindependent of whetherthe mechanism of fastflowis sedimentdeformation or decoupled slidingoverthe bed. Introduction
By introducingweakice-bedcoupling(low basalshear stress)in areasof deformablesediment,Fisher et al. Glacier and ice sheet behavior is greatly influenced producedLast Glacial Maximum (LGM) reconstruc-
by the topography and geologyof the underlying surface. Bed topography is important to the initiation and the dynamics of an ice mass. Topographic variability on scalesof lessthan 10 m affects the basal flow of ice through the processesof regelation and stress-
tions of the Laurentide
Ice Sheet which
are dramat-
pears to be concentratedat isolated pinning points on
coupledsliding at the ice-bed contact, with the exact
ically thinned. These reconstructionsimplicitly invoke the role of basal conditions in the fast flow of ice, as observedin surgingglaciersand Antarctic ice streams
[Blankenshipet al., 1986; Alle•l et al., 1986, 1987; enhancedplasticflow over obstaclesIN!re, 1969, 1970; Clarke,1987;Kamb, 1987,1991]. Fast flow is linked to sediment deformation or deKamb, 1970]. Basaldrag of Antarctic ice streamsap-
scalesof 10•-104m [MacA•leale• al., 1995]. These nature and partitioning of these mechanismsunclear. "sticky spots" may be geologicor topographicin na- We believe that differentiation between these processes ture, with enhanced ice-bed coupling resulting from is lessimportant than the facilitatory conditions involvo.rographicobstaclesor from areasof bedrockoutcrop, ing basalthermal regime,subglacialhydrology,and ice-
bedcoupling,because(1) eachflowmechanism requires Fisl•eret al. [1985]andBoultoneZal. [1985]explored a temperate bed which permits free water; (2) each
frozen, or well-consolidated sediment.
the potential role of bed geology on ice sheet form.
mechanism is encouragedby broad, continuousareas
and ($) eachmechaFisl•er et al. [1985] divided the bed of the Lauren- of highsubglacialwaterpressure; tide Ice Sheet into yield stressregimesbasedon geo- nism benefitsfrom low geologicand topographiccouplogic province and estimated snow accumulationrates. ling with the ice (minimalbasalpinningpointsor sticky spots). Fast flow is not uncommonin contemporaryice massCopyright 1996 by the American Geophysical Union. es, but basal conditionsare very difficult to monitor in detail. On the contrary• the beds of former ice sheets Paper number 96JB01180. 0148-0227/ 96/ 96JB-01180509.00
are accessiblein many places but the extent of former 17,827
17,828
MARSHALL
ice streams
is uncertain.
ET AL.: TERRAIN There
is little
CONTROLS
doubt
that
ice streams acted as critical arteries for drainage of the Laurentide
Ice Sheet.
Lobes of the southern
mar-
gins left evidenceof dramatic and repeated flow ex-
ON LAURENTIDE
ICE STREAMS
cellresolutions of order1ø or 50-100km [e.g.,Deblonde and Peltier, 1991, 1993; Marstat, 1994; Hu!/brechtsand T'Siobbel,1995]. To analyzeterrain dispositionat this resolution,we divided glaciated North America into
cursions(surges)with averageice velocitiesof order 1ø x 1ø longitude-latitude cellsin the region(37øN 100myr-1 [MornerandDreimanis,1973;Cla!/tonand 80øN)and (165øW-45øW).Nineteensubgridgeologic Moran, 1982; Mickelson et al., 1983; Clattton et al., and topographicparameterswerecompiledfor eachcell. 1985; Fullerton and Colton, 198(}; Be!l•t, 198(}, 1987; We usedmultivariatefactor analysisto isolatelinearly Clark, 1994]. independent attributes. Subsets of the data set were Debris dispersalpatterns suggestiveof ice streams then selected to characterize areas where ice streams have been identified over a range of physiographicre- wereknownto exist and areaswherethey likely did not. gions[D!tke,1984;D!tkeandPrest,1987;Hicock,1988]. Discriminantanalysisapplied to the remainingdata The deep-seasedimentrecordtells of quasi-periodicand set yieldsobjectivenumericalestimatesof ice-streaming profligateexportationsof ice to the North Atlantic from probability. the Laurentide'snortheastmargin [Heinrich,1988; All cellsare referencedon a sphericalCartesiangrid, dreiosand Tedesco,1992; Bond et al., 1992]. These with •i and 8• indicatinglongitudeand latitude. We eventshave been linked to surgingbehavioror flow os- denote synoptic-scale1ø cells with longitude-latitude cillations in ice streamsissuingfrom the Hudson and indices i and j and use indices m and n for subCabot Straits [Andrelosand Tedesco,1992;MacA!leal, grid data points. Synopticgrid cells have dimensions 1993a,b;Bondandœotti,1995;Doiodesioell et al., 1995]. _R•cos8iAA x R•AS, where_R• is Earth'sradiusand We assumein this paper that large-scalesurgingand AA = A8 = 1ø. ice-streamingbehavior have similar flow mechanisms Geologic Characterization and experiencesimilar terrain controls. This analysis therefore applies only to ice streams such as those on We compiledsurficialmaterialsinformationfrom conAntarctica's $iple Coast whose flow is basally driven. tinent-scale surfacegeologymaps. Canadian coverage The
fast
flow
of ice streams
such as J acobshavn
Is-
brm in Greenland has been attributed to thermally enhancedcreep deformation rather than basal motion
wasdigitizedfrom the synthesis of Fulton [1993].The number of geologicunits per 1ø finite differencecell rangesfrom 1-8. Table 1 lists the surfaceunits, along
[Echelmetter et al., 1991;/kenet al., 1993].Ice streams with estimatesof typical thickness,porosity,grain size, of this class are certainly plausible in the Wisconsinan ice complex in North America, but we do not consider them here. With little or no basal flow they would leave little geologicalsignature of their history. Further, elevated ice deformation
rates such as in J acobshavn Is-
brm are only feasible under large topographicmotivation and will be topographicallycontrainedin a deep channelor fjord. Their positioningmight be readily predictedfrom detailed topographicinformation. Given sufficientresolution,standardglaciologicalmodelsof ice creep dynamicsshould readily capture this classof ice streams.
To addressthe questionof fast basal flows, we have developed a numerical ice sheet model which has the
capacityto describesubgridice streamsand surgelobe dynamics[Marshalland Clarke,1996]. Objectivepositioningof ice streamsin the modelis a challenge in applications with the Laurentide Ice Sheet. While the
and hydraulic conductivity. We subdividedFulton'still blanket into six regionalsubclasses basedon the divi-
sionsof Fulton [1989]. Material characterizations were specifiedwith the assistance of L. Dyke (personalcommunication,1994) and from Fulton [1989]. Porosities are estimated to within 10%. Hydraulic conductivities
are calculatedfrom the empiricalformula k = Cd•, wherek is in ms-1, C = 1.1,andd isthe 10thpercentfie on a "cumulativepercent finer than" grain size curve,
measuredin mm [Freezeand Cherr!l,1979,p. 350]. Americancoveragewasdigitizedfrom Heath [1988], translatedinto geologicunits whichare roughlyequivalent to thoseon the Canadian surficialmap. The U.S. information
is less detailed
than the Canadian
counter-
part, and "equivalence"is unclear for some units. As a
result, the British Columbia-Washingtonborder occa. sionallyshowsup in the full compilation, but the border
is elsewheretransparent. Figure 1 illustratesaverage temporalevolutionof fast flowdynamicsis controlledby sedimentthicknessand areal fraction of bedrockexposbasalthermaland hydrological regimes,terraindispo- ure in each cell. The latter includes contributions from sition can be expectedto overlayand regulatetime- bedrock outcrops,alpine and volcaniccomplexes,and dependentinternaldynamics.This paperconcentrates till veneer(thin anddiscontinuous till coverpunctuated onspatialvariationsin ice-bedcouplingimposed by bed by outcrops). geologyand topography. Present-dayNorth American terrain is quantifiedon a scaleof 3-10 km, and we ex- Topographic Characterization plore terrain attributes which predispose certain areas Rapid improvementsin digital terrain modelshave to large-scalebasal flows. openedthe door for detailed topographicanalysisat subgridscalesfor ice sheetmodels. We make useof the Bed Characterization National GeophysicalData Center's TerrainBase comSynoptic-scalefinite differencemodels of the Lauren- pilation, which includesglobal 5 arc min data and a tide Ice Sheetsolveicesheetdynamicsat horizontalgrid numberof regionalmodelsat higherresolution[Roio
MARSHALL ET AL.: TERRAIN CONTROLS ON LAURENTIDE ICE STREAMS Table
1. Sediment
Characterization
Material
Unit
Till blanket Shield Arctic Cordillera
Depth,m Porosity d,mm k, m/s x106
TS TA TC
1 2 3
3. 3. 3.
0.1 0.1 0.1
0.004 0.004 0.002
0.18 0.18 0.04
Interior plains
TP
4
20.
0.1
0.002
0.04
Hudson
TH
5
10.
0.1
0.002
0.04
TG
6
20.
0.1
0.002
0.04
7
1.
0.3
0.004
0.18
8
100.
lowlands
Great Lakes/St. Lawrence Till
veneer
Tv
Bedrock outcrops
R
Water (unmapped)
1.
I 10
Glaciolacustrine, fine Glaciolacustrine, coarse
0.1 0.3
0.002 0.5
0.04 2 750.
fL 11 cL 12
0.1 0.3 0.3
0.002 0.2 1.
0.04 440. 11 000.
10. 2.
0.1 0.3
0.002 1.
0.04 11 000.
18 19 20 21 22 fC 23
2. 2. 1. 1. 1. 1.
0.2 0.3 0.3 0.1 0.2 0.1
0.002 0.2 0.004 0.002 0.05 0.002
0.04 440. 0.18 0.04 27.5 0.04
Gp 24 Gx 25 Ra 26
10. 10. 100.
0.3 0.3 0.01
1. 1.
Alluvium
A 27
10.
0.3
0.1
110.
Organics Eolian
O 28 E 29
2. 2.
0.5 0.3
0.5 0.2
2 750. 440.
Lacustrine mud Lacustrine sand Marine veneer
mL 13 sL 14 Mv 15
Glaciomarine, fine Glaciomarine, coarse
i'M 16 cM 17
Marine mud Marine sand Colluvial blocks Colluvial rubble Colluvial sand Colluvial frees
mM sM bC rC sC
Gl•ciofluvial, plain Glaciofiuvial, complex Alpine complexes
5. 10. 2. 3. 0.5
11 000. 11 000. 1.
,
and Hastings,1994]. The most detailed information available
0.01
W 9
Ice masses
,,
17,829
for the entire
North
American
bed is 5 arc
Attributes of the second class in Table 2 examine each
subgridpoint h,,,, and its eightneighbors.Definelo-
min coverage,which gives 144 data points within a 10 cal length scales• - R• cosS,•SAand 5y - Rz:58 as cell. Higher-resolutiondata are available in southern shownin Figure2. SlopeamplitudeIIVhm,,,11 is derived sectorsof the domain and we use this where appropri- from
ate. Eachpoint (Ai, 8j) is madeup of an M x N array of subgridelevationsh,,•,,•. M = N - 12 in this study, and subgridcellshave dimensionsRr cos8,•5), x Rr58,
where58 - 5A - 1/ 12ø. We collecteda suite of subgrid topographicattrib-
(3) where
utes which we deemed relevant to ice mechanics. Table
2 summarizesthese as well as the geologicattributes availablefor each cell. The first classof topographic information includesdirectly determinedsubgrid statistics: maxima, minima, mean, range, and standard deviation. Roughness H/L measuresthe heightof a typical "bump" above the background. Subcell elevations The meanslopeamplitudefor cell (i, j) is then are ranked from low to high and H is the difference 1 betweenthe 75th percenttieelevation and the mean, H = h.v5- h. L is the horizontal length scale correspondingto the terrain resolution,calculatedfrom 1/4
IlVh[I,, -
of thesubcell perimeter:L - Rs (258+cos8,_•/25• + (oulr l0 km).
(4)
m
Standard deviation of this measure is
I is a dimensionless measureof topographicdistribution, calculated from
Ii,:i-
1
.-ij , am,.,
(1) Slope•pect ffm,• defineslocaldownslope direction,
whereAij is cellareaandam,,,is subcellarea. Volume V is a measure of terrain
volume within
a cell above a
reference plane,'-•,$ h.mi." in our case.It is calculated from the cell range and hypsometry,
-
a
-
-
'-*,s' a,•,. . (2)
' Mean and standard deviationsof cell slope•pect followaccordingly.The motivationfor compilings•andard deviationsof slopeamplitudeand aspectis to givesome
17,830
MARSHALL
ET AL.: TERRAIN
•- •...••
CONTROLS
Attribute
........
:a• ............. •'"
Description
m m
minimum maximum
0.63
m
average cell elevation
m
standard
0.5
m
range in cell elevation roughness
0.75
..
Units
Topographic Attributes: First Class
0.88 •:::•. :.:.. •...•...:..::..:'. • "...,.•'"•'•:.;.
ICE STREAMS
Table 2. List of Topographicand GeologicAttributes
Bedrock re 1.
......
ON LAURENTIDE
cell elevation cell elevation
deviation
in cell elevation
hypsometric integral km •
.... :•::•:•:•:•::::::::•.:.::: •a•:•;•;•:•:•:•:•
•:•:::•:•:• ................... "'.-•:::':•:: • '•
terrain volume
Topographic Attributes: Second Class mean slope amplitude standard deviation in slope amplitude degrees mean slope aspect degrees standard deviation in slope aspect
'
water
m -z
mean downslopecurvature
Topographic Attributes: Third Class upstream
area
maximum upstream drainagepath length
_
•-..--'•'•'-••
Sediment
(m)20.0
ka
ms -z
Geologic Attributes fractional bedrock exposure average sediment depth water storage capacity average hydraulic conductivity
17.5 15.0 12.5
i
•
5.0 2.5
'
0.0
the amount
of terrain
within
the full model area which
is upflow of a given point. Following the approach of Zevenbergenand Thorne, we calculated this from a cumulative four-way sweep across the data set, to flag upstream points from all possiblepaths. Upstream area maps of present-day and Last Glacial
Maximum (LGM) surfacetopographiesare shownin Figure 4. Hudsonand Mississippiandrainageroutesare evident in the present-daycase,while the LGM map accentuatesdrainageon the northern and southernflanks of the ice sheet, in Laurentide-Cordilleran confluences
:,"•:;•...:J•i land water
(both north and south), and via a saddlein Hudson
Bay. LGM paleotopographyis estimated from the re-
construction of Peltier [1994]for both bed and ice surFigure 1. Resultsof surfacegeologyanalysis.All fields faces. This reconstructionwas basedon high-resolution are contouredfrom 1ø cell averagesfor the Laurentide
bed. (a) Arealfractionof bedrock exposure. (b) Thick- isostatic modeling in a viscoelasticEarth, constrained ness of sediment
cover.
by globally distributed relative sea level histories. The initial ice surface which we approximated from Peltier
[1994]has been integratedforwardfor 8 kyr in the ice descriptionof second-orderterrain variability within sheetmodelof Marshall and Clarke[1996]to smooth the cell: e.g., to differentiatehummockyterrain from an initially blocky ice distribution.
monotonic continental shelf.Terraincurvature IIV2• hll is a similar measure,calculatedfrom the secondderivative in the downslopedirection at eachsubgridpoint. Mean cell curvature
is found from
On Figure3 depictssyntheses of selectedtopographicfields for the entire region. Note the closesimilarity between the differentroughnessmeasuresplotted in Figures3a, 3c, and 3d.
In addition to direct subgridtopographicattributes, we calculated upstream catchment area and maximum
m ,m+l Figure 2. Notation for the subgriddigital terrain data.
drainagepath lengthfor the full modeldomain[after Elevationh,,,,• corresponds to longitudeAm and latiZevenberõen and Thorne,1987]. Theseare measuresof tude 8,•.
El
--.-
_
Rangeinsub-cell ,_
•
Deviation in
:ography 2400 •
pe aspect 48
•/•-'- •'"•'"•'•""•"••'•••••••..:...::•.,.•:• • ...1800 :'"•::i• ....... ii...'..• • ..1500
":
•
/
•.•.., \
•
'.•
•_
•'
•:' 6
•••• ...........•iIwa:tder •
C
/
'•......•
•. 36 "•'" •::...-:?.:. 30
Logofterrain
,.•
•""'""'--" "' -'"•"::••'•'••••••.•" • curvature
"'
•
•
•'••::' "•:•:• ............
_ iIwa:td • •
LOg ofdeviation in slopeamplitude
/,....•.•:f.•:•::.•!..:.•...:..•.•::•:;•....:.:,..........•.•......•.•::•.•: .... •-..• /,.. •..•.•.:.f.:..:•?::.:::•:.•..•;.:•..•...•.•:•.•.•.•.• • 0.65 •.._...,.,.!. .....•i•i[•.•:. ?•_.. .....:•ii•...•. ...•`•,•....... •`...•••••i•i•:`•:... • 0.6 //•.,.:.: .... ' •`•:??:)•;i•i.•ii:•!!i!•i•i•i?•!: •!•.``..:. ...::i!•`i!!`•iii::•:::•:•i•:•:• • • 0.3
•
.... -:'"""•'""":';';'"' ":•'"•!iii'i',',ii:ii?,iii::•:• '•" ............... '.-':'..."...-'•:•.-:...-..-:;:• iwater• '""'"'""•'•:::•'"'••',ii•iii',ii'• ................ .:•"""'?":'":•:'•"• iwater
Figure $. Resultsof subgridtopographicanalysis.All fieldsare contouredfrom 1ø cell averages
for the Laurentidebed. Each 1ø cell cont•ns 144 subgridpoints. (a) Logarithmof meancell slopeamplitudes.(b) Deviationin subgridslopeaspect.(c) Logarithmof meancelldown•lope curvatures.(d) Logarithmof deviationin subgridslopeamplitudes. Square root of upstream area (km) 26OO
2275
1950 ,.:: :..1625 ...
::::::::::':
"'•'?':•i 975 :•-•:650
::.::..:::• 325 .....
.... ......
I water Figure 4. Upstreamareacompilations contoured from1ø cellvaluesfor the Laurentide bed. Present-day bedtopography.(b) Last GlacialMaximumsurfacetopography[afterPeltier, 1994].
17,832
MARSHALL ET AL.: TERRAIN CONTROLS ON LAURENTIDE ICE STREAMS
Elevation (m)
a
b
Elevation (m)
-:,:-:-:,:.:-:-:-:-:-.->:-:-•,•,-•-.:,:-:,:.,:.>> .:...>•,,.. ``.i:i``.:!:•.::i:::•i:::::•:•*:•`::i:i:i:!::::i:•:i:!:i.•::. '•.v.. .i:i:.::!i:•:•::.•.:•, ::..?:i•!•ii!: :.:•;..> :•:.:',::'.•::::!:!:::!:::i:::!:!::':i:!:!:!:i:!:!:!:i:::!:::i:!:i:::!:i ':,. .. '"'"'""'
?::?:?:i•i:•:? ' ?:?:,,-,..,,, ,"•-.-.
