an increase in crevasse extent, west greenland: hydrologic...

An increase in crevasse extent, West Greenland:Hydrologic implications

William Colgan,1 Konrad Steffen,1 W. Scott McLamb,1 Waleed Abdalati,1

Harihar Rajaram,2 Roman Motyka,3 Thomas Phillips,4 and Robert Anderson5

Received 10 June 2011; revised 11 August 2011; accepted 19 August 2011; published 16 September 2011.

[1] We compare high‐resolution 1985 and 2009 imagery toassess changes in crevasse extent in the Sermeq Avannarleqablation zone, West Greenland. The area occupied bycrevasses >2 m wide significantly increased (13 ± 4%)over the 24‐year period. This increase consists of anexpansion of existing crevasse fields, and is accompaniedby widespread changes in crevasse orientation (up to 45°).We suggest that a combination of ice sheet thinning andsteepening are responsible for the increase in crevasseextent. We examine the potential impact of this change onthe hydrology of the ice sheet. We provide a first‐orderdemonstration that moulin‐type drainage is more efficientin transferring meltwater fluctuations to the subglacialsystem than crevasse‐type drainage. As enhanced basalsliding is associated with meltwater “pulses”, an increasein crevasse extent can therefore be expected to result in anet decrease in basal sliding sensitivity. An increase increvasse extent may also accelerate cryo‐hydrologicwarming and enhance surface ablation. Citation: Colgan, W.,K. Steffen, W. S. McLamb, W. Abdalati, H. Rajaram, R. Motyka,T. Phillips, and R. Anderson (2011), An increase in crevasse extent,West Greenland: Hydrologic implications, Geophys. Res. Lett., 38,L18502, doi:10.1029/2011GL048491.

1. Introduction

[2] Recent studies have suggested that the Greenland IceSheet possesses a basal sliding mechanism similar to that ofan alpine glacier [e.g., Bartholomaus et al., 2008], wherebyenhanced basal sliding occurs when surface meltwater inputexceeds subglacial transmission capacity and pressurizesthe subglacial hydrology system [Shepherd et al., 2009;Bartholomew et al., 2010; Colgan et al., 2011]. Enhancedmelt season basal sliding constitutes a relatively largerfraction of annual displacement in land‐terminating glaciersthan in marine‐terminating glaciers [Joughin et al., 2008].While many studies suggest that relatively small increases infuture surface meltwater production may result in dispro-portionately large increases in basal sliding velocity [Zwally

et al., 2002; Shepherd et al., 2009; Bartholomew et al.,2010; Schoof, 2010], other studies suggest this will resultin a transition to more efficient subglacial drainage and a netdecrease in basal sliding velocity [van de Wal et al., 2008;Schoof, 2010; Sundal et al., 2011].[3] Meltwater is transferred from the supraglacial system

to the subglacial system via either moulins or crevasses(Figure 1). Moulins are near‐vertical conduits through theice that have been demonstrated to rapidly transmit anyfluctuations in surface meltwater production to the subgla-cial system [Shepherd et al., 2009]. Moulins concentratesurface meltwater production from relatively large areas for“point” delivery to the subglacial system. Crevasses collectsurface meltwater from comparatively small areas to drain viaa more “distributed” englacial network. Thus, the presence ofcrevasses influences both supra‐ and englacial water routing.Crevasse propagation can precondition ice for moulin for-mation at crevasse field boundaries [Holmlund, 1988;Phillips et al., 2011]. While crevasses and moulins occurthroughout a common elevation band referred to as the“runoff zone”, substantial supraglacial river/moulin systemsand lakes typically do not occur within crevassed areas inWest Greenland [Thomsen et al., 1988; Phillips et al., 2011].[4] We find that changes in marginal ice geometry (i.e.,

thinning and steepening), due to a combination of enhancedsurface ablation and the recent acceleration of nearbyJakobshavn Isbrae (JI), have increased the relative fractionof crevasse‐type drainage areas in the Sermeq Avannarleq(SA) ablation zone in West Greenland. We provide a first‐order demonstration that the characteristic transfer time forsupraglacial meltwater to reach the subglacial system isapproximately two orders of magnitude faster for moulin‐type drainage than for crevasse‐type drainage. Thus, wesuggest that moulin‐type drainage is more efficient in prop-agating surface meltwater fluctuations (diurnal and other-wise) to the subglacial system than crevasse‐type drainage(which attenuates meltwater fluctuations). As enhanced basalsliding is associated with meltwater “pulses”, rather thansustained meltwater input, a net transition from moulin tocrevasse‐type drainage may ultimately modify the basalsliding response of land‐terminating portions of the ice sheetthat are not presently crevassed to surface meltwater input.

2. Methods

[5] The SA ablation zone is comprised of predominantlyland‐terminating ice, with a relatively small tidewater gla-cier that calves into a sidearm of Jakobshavn Fiord. In 1985the Geological Survey of Greenland conducted an intensivesurvey of the SA ablation zone that included the acquisitionof high‐quality panchromatic aerial photographs on 10 and

1Cooperative Institute for Research in Environmental Sciences,University of Colorado at Boulder, Boulder, Colorado, USA.

2Department of Civil, Environmental, and ArchitecturalEngineering, University of Colorado at Boulder, Boulder, Colorado, USA.

3Geophysical Institute, University of Alaska Fairbanks, Fairbanks,Alaska, USA.

4Department of Aerospace Engineering Sciences, University ofColorado at Boulder, Boulder, Colorado, USA.

5Institute of Arctic and Alpine Research, University of Colorado atBoulder, Boulder, Colorado, USA.

This paper is not subject to U.S. copyright.Published in 2011 by the American Geophysical Union.

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L18502, doi:10.1029/2011GL048491, 2011

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http://dx.doi.org/10.1029/2011GL048491

24 July 1985 [Thomsen et al., 1988]. These photographswere recently digitized and aggregated into an orthomosaicwith the intent of studying changes in the ice geometry ofnearby JI (∼30 km southeast of SA [Motyka et al., 2010]).We compare this historical orthomosaic with a commer-cially acquired panchromatic 15 July 2009 WorldView‐1image of the SA ablation zone to assess changes in surfacemorphology over a 24‐year period. As the horizontalaccuracies of both the orthomosaic (∼2 m resolution) andWorldView‐1 image (resampled to 2.2 m resolution) areestimated to be ±5 m, the total positional uncertainty whencomparing a given location in the 1985 and 2009 images istaken as ±10 m.[6] We compared the distribution of supraglacial hydrol-

ogy features (i.e., rivers and lakes) and crevassed area extentin the 1985 and 2009 images. Supraglacial hydrology fea-tures were manually delineated from both imagery datasets.Crevassed area extent was delineated from both imagesusing a Roberts cross edge detector in ENVI. This methodinvolves convolving a gradient operator that enhances areasof the image with sharp changes in pixel value in a givendirection (which is characteristic of a crevasse field). Aftermanually digitizing the crevassed area boundaries identifiedby the Roberts operator results, we performed a manualinspection using the original imagery to verify the accuracyof polygons in representing crevassed regions. We delin-eated two sets of crevassed area extent polygons: crevasses>2 m wide (given the pixel size this is equivalent to “total”crevasse extent), and crevasses >10 m wide (this includesonly the most “severely” crevassed areas). We assess ouruncertainty in crevassed area extent as the total area occu-pied by 10 and 100 m buffers around each respective set ofcrevassed area polygons.[7] Inter‐annual variations in surface mass balance condi-

tions influence the identification of surface features. Cumu-lative melt intensity in both 1985 and 2009 was comparedusing positive degree days (PDDs) observed at the nearbyDanish Meteorological Institute (DMI) Ilulissat weather sta-tion (WMO 04216/04221; ∼35 km east of the ice sheetmargin). This comparison suggests that the 1985 melt seasonwas ∼12%more intense than the 2009 melt season at the timeof image acquisition (416 versus 370 PDD, respectively).

In order to minimize the potential differences in crevasseextent between 1985 and 2009 attributable to differences insnow cover and detection, we use the 1985 snowline position[Thomsen et al., 1988] as the upglacier limit of the compar-ison area. We note that the 2009 snowline also shares asimilar position (Figure 2). We use the 2009 margin as thedownglacier limit of the comparison area.

3. Results

[8] Supraglacial hydrology features and crevassed regionsexhibited high positional stability between 1985 and 2009.In both years it is apparent that there is little spatial overlapbetween the regions occupied by supraglacial hydrologyfeatures (i.e., rivers or lakes) and crevassed areas. The totalcrevassed area within the 608 km2 comparison areaincreased by 13 ± 4% over the 24‐year study period (from258 ± 5 km2 in 1985 to 291 ± 5 km2 in 2009; Figure 2).Similarly, the severely crevassed area increased by 20 ±60% over the same period (from 83 ± 26 km2 to 99 ±28 km2; not shown). The increase in total crevasse extent,as well as severely crevassed extent, may be generalized asan expansion of existing crevasse fields. While a qualitativeassessment suggests a corresponding decrease in the areaoccupied by supraglacial rivers, we do not assess this decreasein a quantitative fashion here. We simply note that crevassesand prominent supraglacial hydrologic features (e.g., severalkm lake and river systems) are typically mutually exclusive[cf. Thomsen et al., 1988]. Therefore, the net increase inice sheet area drained by crevasse systems between 1985 and2009 implies a net decrease in ice sheet area drained by river/moulin systems (cf. Figures 2c and 2d).

4. Discussion

4.1. Increased Crevasse Extent

[9] Previous research suggests that the land‐terminatingice within the study area has experienced widespread thin-ning in excess of 65 m between 1985 and 2007, with a localmaximum of 104 ± 8 m [Motyka et al., 2010]. The cause ofthis thinning is discussed in the next paragraph. Comparisonof a 1985 digital elevation model (DEM), derived from aGeological Survey of Greenland supraglacial topographymap [Thomsen et al., 1988], and a 2009 DEM, derived fromthe ASTER Global DEM (http://asterweb.jpl.nasa.gov/gdem.asp), indicates that mean ice surface slope in the SA ablationzone increased ∼0.10° (≈7%) over the 24‐year period. A two‐tailed paired t‐test indicates that this increase in mean iceslope from 1.45° (s.d. 0.98°) in 1985 to 1.55° (s.d. 0.91°) in2009 is significant at the p < 0.001 level (Figure 3). Thissteepening is an anticipated result of previously observed,and ongoing, low elevation thinning and high elevationthickening of the ice sheet [Parizek and Alley, 2004; Luthckeet al., 2006]. As the lithostatic stress opposing crevassepropagation is proportional to ice thickness and the tensilestress promoting crevasse propagation is proportional to localsurface slope [van der Veen, 1998], changes in ice thicknessand local surface slope influence crevasse propagation. Icethinning and steepening both act to enhance crevasse prop-agation. Application of the crevasse model proposed by vander Veen [1998] suggests that for an initial ice thicknesses of500 m and an crevasse depth of 20 m (values representativeof the SA ablation zone), the combination of a 65 m decrease

Figure 1. Schematic of two differing englacial meltwaterdrainage pathways (∼4 times vertically exaggerated). Ideal-ized hydrographs illustrate the increased attenuation of agiven surface meltwater input (i) by crevasse drainage(Qc) in comparison to moulin drainage (Qm) over the diurnaltime‐scale (t).

COLGAN ET AL.: INCREASED CREVASSE EXTENT WEST GREENLAND L18502L18502

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in ice thickness and a 0.10° increase in surface slope increasescrevasse depth by ∼30%. Given that crevasse propagation isgreatly enhanced by hydrofracture when crevasses are water‐filled [van der Veen, 1998], a substantial increase in surfacemeltwater production since 1985 is also expected to facilitatecrevasse propagation. As a crevasse field delineates the localextent of a critical ratio between tensile and lithostatic stresses,widespread thinning and steepening is expected to cause anexisting crevasse field to expand outwards.[10] The general continuity equation for ice suggests that

there are two possible causes of ice sheet thinning: a decreasein surface mass balance (b) and an increase in divergence ofice flux [Hooke, 2005]. In situ surface mass balance obser-

vations suggest that the maximum thinning observed byMotyka et al. [2010] cannot be attributed to surface massbalance alone, which would invoke b ≈ −4.7 m/a over thestudy period [cf. Fausto et al., 2009]. An increase in thedivergence of ice flux, either in the along or across‐flowdirections, must therefore be invoked to explain excessivethinning. A comparison of 1985 velocity vectors derivedfrom photogrammetry [Fastook et al., 1995] with 2005/06velocity vectors derived from InSAR observations [Joughinet al., 2010] suggests that the recent acceleration of JI hasincreased southbound ice flow at the expense of westboundice flow in the SA ablation zone (Figure 4). Surface cre-vasses, which typically form perpendicular to a glacier’s

Figure 2. (a) Supraglacial rivers and lakes overlaid on a panchromatic 24 July 1985 orthomosaic [Motyka et al., 2010].(b) Crevassed areas (width >2 m) overlaid on a panchromatic 15 July 2009 WorldView‐1 image. In both images, thelocations of features in 2009 and 1985 are denoted in red and yellow, respectively, with overlapping areas in orange.Crevassed area increased from 258 ± 5 km2 in 1985 to 291 ± 5 km2 in 2009. The 1985 snowline is denoted in cyan. The bluedetail box illustrates the transition of (c) a supraglacial catchment from moulin‐type drainage in 1985 to (d) crevasse‐typedrainage in 2009. The 1985 location of the moulin system is denoted with an arrow.


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longitudinal stress field (i.e., across‐flow under extensionalflow), have rotated by up to 45° throughout the study area indirections consistent with this notion (Figures 4b and 4c). Weexpect that the majority of this reorganization likely occurredafter 1998, rather than in a monotonic fashion, following the1997 breakup of the JI ice tongue and the consequent dou-bling of ice speed and dramatic changes in ice geometry[Motyka et al., 2010].[11] Previous studies indicate that recent changes at JI are

part of a widespread change in the low elevation velocity and

geometry of the Greenland Ice Sheet [Luthcke et al., 2006;Rignot and Kanagaratnam, 2006]. Around the Greenland IceSheet, the “runoff zone” elevation band, which is character-ized by meltwater runoff via crevasses and moulins [Phillipset al., 2011], resides below the higher elevation “dark zone”,the elevation band of maximum meltwater accumulation insupraglacial lakes [Greuell, 2000]. Thus, our observations inthe SA runoff zone may be part of an ice sheet‐wide responsein crevasse extent to changing ice velocity and geometrythroughout the low elevation runoff zone.

Figure 3. Change in ice surface slope derived from a comparison of 1985 [Thomsen et al., 1988] and 2009 (http://asterweb.jpl.nasa.gov/gdem.asp) DEMs at 500 m grid spacing. Values in parentheses indicate the percent area in each class.Inset: Scatter plot of 1985 and 2009 ice surface slopes at each DEM grid point with line y = x shown for reference.

Figure 4. (a) Comparison between 1985 [Fastook et al., 1995] and 2005/06 [Joughin et al., 2010] velocity vectors in theJakobshavn Isbrae region. Colored contours denote change in absolute velocity. The dashed magenta box denotes the extentof Figures 2a and 2b. The magenta detail box is a highlighted region in which crevasse rotation between (b) 1985 and(c) 2009 is shown. Arrows denote the approximate local crevasse orientation in each year.


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4.2. Hydrologic Implications

[12] Meltwater is transferred from the supraglacial systemto the subglacial system via either moulins or crevasses.Assuming that changes in englacial water volume due tointernal meltwater generation and deformational closure arenegligible, the rate of change of water volume (dV/dt) ineither type of englacial transfer system may be conceptu-alized as the difference between the rate of surface melt-water input (asA) and the rate of englacial discharge (whichwe represent as V/t):

dV

dt¼ asA� V

�ð1Þ

where as is the surface ablation rate, A is the supraglacialcatchment area, and t is a characteristic englacial transfertime. This characteristic transfer time represents the meantime required for supraglacial meltwater input to travel fromthe ice surface to the subglacial system. This formulationassumes that the englacial system behaves as a linear res-ervoir [Jansson et al., 2003]. The total englacial conduitvolume (V) may be estimated as:

V � H � Sc ð2Þ

where H is ice thickness and Sc is the depth‐averaged cross‐sectional area of the englacial conduit draining either acrevasse or a moulin. This formulation assumes that water andconduit volumes are equivalent (i.e., discounts open channelflow), and also implicitly takes ice thickness as a proxy forenglacial travel distance. This neglects the reality that theenglacial conduits draining moulins and crevasses may travelthrough the ice column at varying angles off‐vertical.[13] Assuming that englacial conduit volume evolves to

achieve a quasi‐steady‐state diurnal cycle (i.e.,R(dV/dt)·dt ≈ 0

over the diurnal cycle), the characteristic englacial transfer timemay be approximated as:

� � H � ScA � as

� �ð3Þ

This quasi‐steady‐state assumption is reasonable, giventhat empirical observations on alpine glaciers suggest thatthe 1/e timescale for conduit closure beneath ∼500 m of iceis expected to be ∼5 hr when water pressures are low[Bartholomaus et al., 2008]. Furthermore, assuming hightemporal resolution GPS observations near “K‐transect”(∼250 km south of SA) are broadly representative of theGreenland Ice Sheet, the “summer speedup” event is actuallycomprised of a series of diurnal events [Shepherd et al.,2009]. Additionally, at SA, there is observational evidenceof the persistence of englacial hydrologic features throughoutthe year [Catania and Neumann, 2010] and model inferencesthat englacial hydraulic head fluctuates annually within arelatively small range [Colgan et al., 2011].[14] To provide a first‐order demonstration of the funda-

mental difference in englacial transfer time between cre-vasse‐type and moulin‐type drainage systems, we evaluateequation (3) for both systems using order‐of‐magnitudeparameter estimates. For moulins, we assume that englacialconduits are generally circular in shape, with cross‐sectionalarea approximated as pr2 (where r is depth‐averaged eng-lacial conduit radius). For crevasses, we do not impose a cir-

cular cross‐sectional conduit geometry, but rather approximateSc as lw (where l is crevasse length and w is depth‐averagedwidth of the crevasse and underlying distributed englacialsystem). In both scenarios we take mean daily ablation rateas 4 cm/d and ice thickness as 500 m. In the moulin scenariowe assume that 1 km2 of ice sheet area drains into a singlemoulin with r = 1 m. In the crevasse scenario we assume thatthe same ice sheet area is drained by multiple crevasses, witheach crevasse draining an area of A = dl (where d is a meancrevasse spacing of 100 m) and assume w = 0.1 m. Usingthese specified values, the mean englacial transfer timesfor moulin‐type and crevasse‐type drainage are ∼1 hr and∼12 days, respectively. As H/as may be regarded as constantin a given region, Sc/A controls this ∼200‐fold differencein t. This suggests that drainage systems with relatively largeSc/A (i.e., crevasse‐type) will attenuate surface meltwaterfluctuations in comparison to those with relatively small Sc/Adrainage (i.e., moulin‐type). Decreasing the assumed w by anorder‐of‐magnitude yields t ≈ 1.25 days (i.e., crevasse‐typedrainage still remains an order‐of‐magnitude slower thanmoulin‐type drainage), while increasing the assumed w fur-ther increases the discrepancy between crevasse‐type andmoulin‐type drainages.[15] The attenuation of meltwater fluctuations can also be

demonstrated by analytically solving equation (1). Thisreveals an inverse relation between englacial transfer timeand englacial discharge variability. To facilitate an analyti-cal solution, we approximate surface ablation rate (as) as asinusoidal function of the form:

_as tð Þ ¼ as1

21� sin

2�t

T

� �� ð4Þ

where t is time and T is period of the surface ablation cycle(24 hr). Using this surface ablation forcing, the analyticalsolution of equation (1) after many cycles of forcing (i.e.,loss of memory of the initial water volume) may beexpressed as:

V tð Þ ¼ asA�

21þ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ 4�2�2=T 2p !

cos2�t

T� ’

� �" #ð5Þ

where V(t) is the total englacial water volume and ’ is aphase angle equal to:

’ ¼ tan�1 �T

2��

� �ð6Þ

Conduit discharge is obtained by dividing equation (5) by t.[16] Thus, it becomes evident that the amplitude of con-

duit discharge is inversely proportional to characteristicenglacial transfer time. For small t (i.e., ≤T/2p), as expectedin moulin‐type drainage, variations in conduit discharge pre-cisely follow those of surface ablation forcing and there islittle attenuation of the diurnal signal. For large t (i.e., >T/2p),as expected in crevasse‐type drainage, englacial dischargeexhibits smaller amplitude variations. As basal sliding requiresmeltwater inputs to overwhelm the transmission capacity ofthe subglacial system, it is reasonable to expect efficientmoulin‐type drainage (i.e., meltwater “pulses”) to enhancebasal sliding more than inefficient crevasse‐type drainage(i.e., sustained meltwater input). In addition to “englacialhydraulic adjustment” [van de Wal et al., 2008], decreased


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basal sliding could provide a complementary explanation forobservations of decreasing mean annual velocities of land‐terminating ice along K‐transect.[17] We note that the potential change from moulin to

crevasse‐type drainage is a one‐time transition limited toareas of the low elevation ice sheet runoff zone that are notpresently crevassed; ice sheet areas that are already crevassedwould not be expected to be sensitive to this mechanism.As enhanced melt season basal sliding contributes to a largerfraction of annual displacement in land‐terminating glaciersthan marine‐terminating glaciers [Joughin et al., 2008],potential decreases in basal sliding velocity would exert agreater effect on land‐terminating glaciers than on marine‐terminating glaciers.

4.3. Other Implications

[18] While a net increase in crevasse extent may result ina net decrease in basal sliding sensitivity, a net increasein crevasse extent may enhance ice flow through othermechanisms. As cryo‐hydrologic warming is highly sensi-tive to the mean spacing of englacial hydrologic pathways[Phillips et al., 2010], a net increase in crevasse extent isexpected to expose an increased area of the ice sheet toclosely‐spaced hydrologic pathways that facilitate cryo‐hydrologic warming. These pathways would be capable ofraising ice temperatures, thereby enhancing deformationalice velocities, in newly crevassed regions of the runoffzone [van der Veen et al., 2011]. A positive feedback couldbe possible if the subsequent change in the ice velocity fieldfurther expands crevassed areas. Thus, predicting the impli-cations of a change in crevasse extent on ice flow is com-plicated by counteracting processes. In addition to ice flow,the presence of crevasses has been shown to more thandouble the absorption of solar radiation (and hence enhancesurface ablation) in comparison to ice sheet areas withoutcrevasses [Pfeffer and Bretherton, 1987]. Thus, a spatiallywidespread increase in crevasse extent (i.e., an increase inextent characterized by the outward expansion of multipleexisting crevasse fields) is expected to result in a widespreadincrease in summer ablation.

5. Summary Remarks

[19] We suggest that the recent acceleration of JakobshavnIsbrae has resulted in an increase in crevasse extent in theSermeq Avannarleq ablation zone, due to a combinationof ice thinning and steepening surface slope. We argue thatthe resultant transition from moulin‐type to crevasse‐typedrainage is expected to result in a net dampening of basalsliding sensitivity to surface meltwater input. This damp-ening would be most significant in portions of the ice sheetthat are land‐terminating and not presently crevassed. Dueto the absence of an ice sheet‐wide time series of crevasseextent, the potential prevalence of this transition outside ourstudy area is not known at present. We expect, however, thatour observations at Sermeq Avannarleq are representative ofa widespread response to low elevation thinning and outletglacier acceleration throughout the low elevation runoffzone of the Greenland Ice Sheet. We acknowledge that anypotential reduction of ice discharge stemming from decreasedbasal sliding may be offset by other counteracting processes.In particular, increased crevasse extent may enhance sur-face ablation through increased absorption of solar radiation,

and increase deformational ice velocities by exposing anincreased ice sheet area to closely‐spaced hydrologic path-ways that facilitate cryo‐hydrologic warming.

[20] Acknowledgments. This work was supported by NationalAeronautics and Space Administration (NASA) Cryospheric ScienceProgram grants NNX08AT85G and NNX07AF15G to KS, and NationalScience Foundation (NSF) DDRI 0926911 to WC. WC thanks the NaturalScience and Engineering Research Council (NSERC) of Canada andCooperative Institute for Research in Environmental Sciences (CIRES)for fellowship support. We are grateful to Henry Brecher for providingan electronic copy of the 1985 velocity data and to Chuck Chaapelfor his effort in coordinating the WorldView‐1 imagery. We thank AndreasAhlstrøm, Thomas Neumann and Eric Rignot for their reviews ofthis paper.[21] The Editor thanks Andreas Ahlstrom and Thomas Neumann for

their assistance in evaluating this paper.

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