Volcanism-induced, local wet-based glacial conditions recorded in the Late Amazonian Arsia Mons

14 Pages·2014·6.49 MB·English

by  Kathleen E. Scanlon

Icarus250(2015)18–31
ContentslistsavailableatScienceDirect
Icarus
journal homepage: www.elsevier.com/locate/icarus
Volcanism-induced, local wet-based glacial conditions recorded in the
Late Amazonian Arsia Mons tropical mountain glacier deposits
Kathleen E. Scanlona,⇑, James W. Heada, David R. Marchantb
aDepartmentofGeologicalSciences,BrownUniversity,Providence,RI02912,USA
bDepartmentofEarthandEnvironment,BostonUniversity,Boston,MA02215,USA
a r t i c l e i n f o a b s t r a c t
Articlehistory: Thetropicalmountainglacialfan-shapeddeposit(FSD)tothenorthwestoftheArsiaMonsvolcanoon
Received26August2014 Mars contains numerous glacial and volcanic landforms. While most of the glacial landforms are
Revised22October2014 interpretedtohaveformedbycold-basedglacialprocesses,severalglaciallandformsnearglaciovolcanic
Accepted16November2014
edificesaremoreconsistentwithlocalizedwet-basedglacialprocesses.Theselandformsincluderibbed
Availableonline22November2014
moraines, which suggest local, thermal transitions between wet- and cold-based ice; thrust-block
moraines,whoseformationistypicallyassistedbythepresenceofsubglacialwater;streamlinedknobs
Keywords:
that we interpret to have been sculpted by ice sliding along its base; and a braided outflow channel.
Mars
Thepresenceandassociationofthesefeatures,togetherwithevidenceofbothsubglacialvolcanicerup-
Mars,climate
tionsandlocalice-marginaladvances,favorpolythermalglacierswithlocalizedwet-basedconditions.
Mars,surface
Volcanism Weproposethatlava-to-iceheattransferduringtheeruptionoftheglaciovolcanicedificescausedthe
ArsiaMonspaleoglaciertomeltatitsbaseinsomeareas,resultingintheselocallywet-basedglacialcon-
ditions.Apolythermalglacierprovidesmorepotentialmicrobialhabitatsandmoreconnectivitybetween
habitatsthandoesacold-basedglacier,andwereviewglacialandglaciovolcanichabitatsonEarththat
may provide insight into thelikelihood of potentialmicrobialhabitats withintheArsia Mons FSDon
Mars.
(cid:2)2014ElsevierInc.Allrightsreserved.
1.Introduction thewesternflanksoftheTharsisMonteswhenmartianspin-axis
obliquity is high (Forget et al., 2006), and output from coupled
TothewestornorthwestofeachoftheTharsisMontesliearcu- ice-sheet climate models closely reproduce the footprints of the
atedeposits,delineatedbyconcentricridges,knownasthelobe-or deposits(Fastooketal.,2008).Threegeomorphologicfaciescom-
fan-shapeddeposits(e.g.ZimbelmanandEdgett,1992;Scottand prisethemajorityofthedepositatArsiaMons:aRidged,Knobby,
Zimbelman,1995).Whilethefan-shapeddeposits(FSDs)wereini- and Smooth Facies (Zimbelman and Edgett, 1992; Scott and
tially interpreted as having formed by either mass-wasting pro- Zimbelman, 1995), interpreted, respectively, as drop moraines,
cesses,volcanicprocesses,ortectonicdeformation(seereviewin sublimation till, and remnant debris-covered glaciers (Head and
Shean et al., 2005), recent work (Head and Marchant, 2003; Marchant,2003).
Shean et al., 2005, 2007; Forget et al., 2006; Kadish et al., 2008; LiketheotherTharsisMontesFSDs(Sheanetal.,2005;Kadish
Fastook et al., 2008; Scanlon et al., 2014) strongly supports the etal.,2008),theArsiaMonsfan-shapeddepositisassociatedwith
hypothesis(Williams,1978;Lucchitta,1981)thattheFSDsaregla- landformsthathavebeeninterpretedassubglaciallyeruptedvol-
cialinorigin.Specifically,themorphologyoflandformsintheFSDs canicedifices(Fig.1).Theselandforms,outlinedinblackinFig.1,
(HeadandMarchant,2003;Sheanetal.,2005,2007;Kadishetal., include four plateaus interpreted as pillow mounds topped by
2008;Scanlonetal.,2014)stronglyresemble,andshowthesame hyaloclastite mounds; a flat-topped, steep-sided plateau inter-
spatial and stratigraphic relationships as, characteristic glacial pretedasatuya;asteep-sidedridgeinterpretedasanice-confined
landforms deposited from cold-based ice on Earth (e.g. Atkins, lavaflow;aseriesofdigitatecliffsinterpretedasice-contactfea-
2013). The term ‘‘cold-based’’ refers to glaciers whose basal ice tures; ridges grading into chains of pits, interpreted as eruptions
temperatures are below the pressure melting point.Results from spanning the boundary between permeable and impermeable
Mars global climate models show that ice should accumulate on ice; steep-sided, sunken-centered edifices interpreted as having
formed by lava extrusions that chilled at the margins but whose
⇑ still-molten centers receded into the vent; and numerous low,
Correspondingauthor.
http://dx.doi.org/10.1016/j.icarus.2014.11.016
0019-1035/(cid:2)2014ElsevierInc.Allrightsreserved.
K.E.Scanlonetal./Icarus250(2015)18–31 19
steep-sided, areally extensive mounds interpreted as pillow 1998; Shean et al., 2007) suggest that volcano–ice interactions
mounds(Scanlonetal.,2014). may have allowed local basal melting in the Pavonis tropical
Amazonian-aged relict glacial deposits, including concentric mountain glacier, though it is unclear whether the stratigraphic
crater fill, lobate debris aprons and lineated valley fill (e.g. positionofthestreamlinedknobsisconsistentwiththehypothesis
Squyres, 1979; Squyres and Carr, 1986; Mangold, 2003; Milliken thattheyaredrumlins.Noevidenceforfluvialfloworwet-based
et al., 2003; Head et al., 2003; Garvin et al., 2006; Head et al., glacialconditionshasbeendocumented,however,intheOlympus
2006,2010;Dicksonetal.2008)arefoundfrompolartoequatorial MonsglacialdepositsortheAscraeusMonsFSD(Milkovichetal.,
regionsofMars.Alackoferosivefeatures(e.g.groovesandmolded 2006;Kadishetal.,2008).IntheArsiaMonsFSD,wefindseveral
forms)orevidenceforassociatedfluvialactivity,however,suggests characteristic classes of wet-based glacial landforms. The spatial
thatmostofthesedepositswereformedbycold-basedglaciers.In relationships of these landforms to each other and to the docu-
theNoachianandHesperianeras,thereisevidencethaticesheets mentedglaciovolcanicedificesinthedepositsuggestthattheArsia
atthesouthpole(e.g.HeadandPratt,2001;Fastooketal.,2012),in glacier was locally wet-based (i.e., polythermal), with volcanism
Valles Marineris (e.g. Gourronc et al., 2014), in Argyre basin (e.g. ratherthanclimatecausingthewet-basedconditions.
Banksetal.,2009;Bernhardtetal.,2013),andinoutflowchannels
(e.g.Lucchittaetal.,1981)werewet-based,andthatglaciofluvial 2.Data
activity may have occurred in Gale crater in the Late Hesperian
or more recently (Fairén et al., 2014), but as the martian climate UsingArcMap10.0,weconstructeda(cid:2)5mperpixelbasemap
became colder, cold-based glaciation became dominant. To our ofallReconnaissanceOrbiter(MRO)ContextCamera(CTX)images
knowledge, outside of the Tharsis Montes FSDs, all conclusively (Malinetal.,2007)intheArsiaMonsfan-shaped deposit.Where
Amazonian candidate polythermal or wet-based glacial deposits CTXcoveragewasabsentorunclear,weconsultedHighResolution
proposed thus far have been restricted to small regions within Stereo Camera (HRSC) images with 10–30m per pixel resolution
impactcraters(Marchantetal.,2006;Hubbardetal.,2011),where (NeukumandJaumann,2004).Theglacialdebrisandatmospheric
basalmeltingmayhavebeenassistedbyremnantthermalgradi- dustcoveringmuchoftheArsiaMonsFSDobscurefine-scalefea-
entsafterimpact(Marchantetal.,2006). tures and dominate spectroscopic data, respectively, limiting the
On Earth, heat transfer from intrusive or extrusive volcanism usefulness of HiRISE and CRISM data in the vicinity of the land-
underneathanotherwisecold-basedicesheetcanbesufficientto forms we describe here. We obtained topographic information
allow melting and locally uncouple the glacier from the ground from 128pixel per degree ((cid:2)463m per pixel) resolution MOLA
(e.g. Fahnestock et al., 2001; Fox Maule et al., 2005; Head and (Zuberetal.,1992;Smithetal.,1999)dataandHRSC-derivedDig-
Wilson,2007;Hambreyetal.,2008).OnMars,fluvialoutflowchan- ital Elevation Maps (DEMs) with (cid:2)100m per pixel resolution
nelsandaligned,streamlinedknobsthathavebeeninterpretedas (Dumkeetal.,2008).ThesedatawereprocessedusingtheSpatial
drumlins in the Pavonis Mons fan-shaped deposit (Scott et al., AnalysttoolkitinArcMap10.0.
Fig.1. GeomorphologicalunitmapoftheArsiaMonsfan-shapeddeposit(FSD),afterZimbelmanandEdgett(1992)andScottandZimbelman(1995).Legendforcolorswaths
isprovidedinfigure.Elsewhere,thinredlinesdenotelargevolcanicgrabens,whitelinesdenotecontactsbetweenunits(dashedwhereinferred),andblacklinesdenotethe
outlinesofglaciovolcaniclandformsandmodifiedmoraineridges.Blueandwhitedotsinthesmoothfaciesdenotepitsandknobs,respectively.THEMIS100m/pixeldaytime
imagemosaic;modifiedfromScanlonetal.(2014).(Forinterpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
20 K.E.Scanlonetal./Icarus250(2015)18–31
3.Glaciallandforms 3.1.2.Evidenceofwet-basedglacialactivity
On the basis of their braided planform and their emergence
3.1.Landformassociationsneartheglaciovolcanic‘‘Northwest from glacial debris, we have interpreted the braided channels
Plateau’’ downslopeoftheNorthwestPlateauasfluvialchannels,indicating
the production and flow of a substantial volume of water at the
A plateau towards the northwest of the Arsia Mons FSD (the baseoftheglacier(Scanlonetal.,2014).Thestreamlinedplanform
‘‘NorthwestPlateau’’),firstdescribedbyWilsonandHead(2007a, and consistent northwest-to-southeast axis of elongation shared
2007b) and later by Scanlon et al. (2014), is interpreted to have bythefieldofelongatedknobssuggestsformationbylocalglacial
formedinasubglacialeruption(Fig.2).Thebroadlower-elevation scour,possiblyin aprocessanalogoustotheformationofterres-
mass of the plateau is interpreted as an effusive pillow eruption. trialdrumlins.Onthebasisoftheirmorphology,size,anddown-
The steep, central mound superimposed upon it is interpreted in slope transition from sharp-crested to broad-crested (consistent
Scanlonetal.(2014)tohaveformedinalater,explosivephaseof withtheexpectedbehaviorofsubglacialchannelswithincreasing
the eruption that began after subglacial meltwater drained from distance from a heat source; e.g. Shreve, 1985), Scanlon et al.
the edge of the glacier and the ice above the vent melted and (2014)interpretedthesinuousridgesasaccumulationsofglacio-
thinned.Herewebrieflyreviewhowthisassociationoflandforms, fluvial debris (i.e. eskers), which are characteristic of wet-based
describedingreaterdetailbyScanlonetal.(2014),exemplifiesthe glacialenvironments.Thenon-destructivesuperimpositionofthe
interactions between glaciovolcanic activity and glacier thermal arcuateridgesdownslopeoftheplateauuponunderlyingfeatures
regimerecordedintheArsiaMonsFSD. and their morphological similarities to members of the Ridged
Faciessuggestthatthearcuateridges,liketheRidgedFaciesridges
(HeadandMarchant,2003),aredropmoraines.Thetighterradius
3.1.1.Observations ofcurvatureshownbytheridgesdownslopeoftheplateau,rela-
Directlydownslopeoftheplateau,andintheinferreddirection tive to nearby members of the Ridged Facies, suggests that they
of the paleo ice-surface slope, several shallow channels emerge were formed at the edges of a lobe of ice elongated in the same
from underneath mounds of glacial debris and coalesce into a directionimpliedbytheother3landformclasses,consistentwith
braidedchannel(cid:2)35kmlongand(cid:2)35mdeepwheretopographic a local increase in longitudinal ice-flow velocity and local ice-
dataisavailable.Betweentheplateauandtheglacio-fluvialchan- marginadvance(Fig.4).
nelsliesafieldofknobsthatareelongatedandstreamlinedinthe
downslope and inferred down-ice direction. Two sharp-crested 3.2.Ridgestransversetoinferredglacialflow
ridges,gradingdownslopetobroad-crestedridgesintheinferred
ice-flowdirection,emergefromthesteepuppermoundofthepla- TheLobateFacies(ZimbelmanandEdgett,1992;Fig.1)isaunit
teauandsuperimposethelowerplateau.Thelongaxesofthecon- intheArsiaMonsFSDcharacterizedbynumerous,denselydistrib-
centric ridges immediately downslope of the plateau are highly uted steep-sided, lobate scarps and ridges, some of which grade
arcuate in the inferred down-ice direction compared to nearby upslope into chains of pit craters. The steep-sided features are
members of the Ridged Facies. These ridges are superimposed interpretedassubglaciallavaflows,andthepitcratersasthesites
non-destructively upon other ridges with similar morphology of phreatomagmatic eruptions under thinner ice, suggesting that
(Fig.3),andaresimilarinsizeandshapetoRidgedFaciesmembers volcano–ice interactions were common in this region of the FSD
atthesameradiusfromArsiaMons. (Scanlonetal.,2014).
Fig.2. TheNorthwestPlateau:(a)CTXimagemosaic.(b)Sketchmap.TheRidgedFaciesisshadedinredandtheKnobbyFaciesisshadedinblue.Fluvialchannelsaredenoted
bylightbluelinesandstreamlinedknobsaredenotedbygreenlines.BoxedinsetsshowthelocationsofFigs.3and7.ModifiedfromScanlonetal.(2014).(Forinterpretation
ofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
K.E.Scanlonetal./Icarus250(2015)18–31 21
Fig.3. CloseviewofdropmorainesdownslopeoftheNorthwestPlateau.Dropmorainesreflectinglocallyenhancedglacialflow(inthedirectionindicatedbycyanarrow)are
non-erosivelysuperimposed(redarrows)uponearlierdropmorainesconcentricwiththeboundariesoftheRidgedFacies.CTXimageP19_008605_1772_XI_02S129W.(For
interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
3.2.1.Observations downsloperegionofaglacierflowsmorerapidlythanacold-based
At the contact between the Lobate and Knobby Facies (Fig. 5) interior region (Hättestrand and Kleman, 1999); (2) ice-cored
liesa(cid:2)17kmwidefieldofsharplydefined,commonlyasymmetric, moraines left by a cold-based glacier are re-sculpted into ribbed
regularlyspaced,concentricoranastomosingridges.Theridgesare moraines when subsequently overrun by wet-based ice (Möller,
oriented transverse to inferred ice-flow directions, and are up to 2006, 2010); (3) the crests of the moraines represent the crests
30kmlongandtypically500mto1kmwide(Fig.6).WhereHRSC of former waves produced by a fluid dynamical instability in the
DEMcoverageisavailable,theridgesaretypically10–50mhigh. flowofglacialiceandsubglacialtill(Dunlopetal.,2008;Fowler,
Smaller ridges ((cid:2)200m wide and not high enough to be 2009; Chapwanya et al., 2011); or (4) moraines form when sea-
resolved bythe HRSCDEM),ofsimilar shapeand similar spacing sonalorclimaticvariationsinsubglacialmeltwatercontentcause
relative to their size, occur superimposed upon the Northwest theglacierbedtotransitionlocallyfromanextensionaltoacom-
Plateau(Section3.1;Fig.7).Theorientationoftheseridges,south- pressional deformation regime (Lindén et al., 2008; Stokes et al.,
west-to-northeast, is also transverse to the direction of inferred 2008).Whetherallsubtypesofribbedmoraineformbythesame
glacial flow based on local topographic slopes and the landforms processisdebated(e.g.HättestrandandKleman,1999;Finlayson
describedinSection3.1. andBradwell,2008;Lindénetal.,2008;Chapwanyaetal.,2011),
but most researchers acknowledge that marked changes in hori-
3.2.2.Evidenceofwet-basedglacialactivity zontal ice-flow velocity, most easily accomplished beneath poly-
On the basis of their asymmetric and sharp-crested morphol- thermalglaciers,areacrucialfactorintheirformation(Finlayson
ogy,theirconcentricorbranchingarrangement,theirregularsize andBradwell,2008).
andspacing,theirorientationtransversetoinferredpaleoiceflow, Weproposethattheheatproducedduringepisodesofextrusive
their alignment with moraines of unusual morphology farther subglacial volcanism in the Lobate Facies led to basal meltwater
downslope (Section 3.3), and the fact that they do not appear to production, resulting in spatial and temporal gradients in basal
gently drape underlying topography as do the drop moraines in slidingandlongitudinalice-flowvelocity(Fig.8).Theseconditions
theRidgedFacies(HeadandMarchant,2003;Sheanetal.,2007), would have led to the development of the population of large
we interpret both the transverse ridges between the Lobate and transverse ridges at the contact between the Lobate and Knobby
Knobby Facies and the transverse ridges superimposed on the Facies by one of the formation mechanisms for ribbed moraines
NorthwestPlateauasribbedmorainesformedinassociationwith discussedabove.
glaciericethatlocallyachievedwet-basedconditions(cf.Dunlop We assert that the volcanic construction and subsequent
andClark,2006). cooling of the Northwest Plateau affected the thermal regime of
Ribbed moraines are unusual and complex glacial landforms. adjacentglacierice,producingpolythermalconditionsthatledto
Theydo notform alongsideice margins,but ratherare produced thedevelopmentofthepopulationofsmallribbedmorainesatop
locallyatthebaseofpolythermalglaciers(e.g.DunlopandClark, theplateau.Thesuperpositionofridgesontopoftheplateausug-
2006 and sources therein; Möller, 2006). The precise mechanism gestsanoriginviatemporal,aswellasspatial,transitionsbetween
bywhichtheyformisincompletelyunderstood,butcandidateori- wet-based and cold-based ice conditions. Specifically, the ice
ginsincludethoseinwhich(1)themorainesaretheremnantsofa directly above the Northwest Plateau would have remainedwet-
frozen till sheet that extends and fractures when a wet-based, basedlongerthantheicefartherdownslopefromthevolcanicheat
22 K.E.Scanlonetal./Icarus250(2015)18–31
Fig.4. Mapviewofvolcaniceruption(brightorange/gray)undericesheet(white)onflankofvolcano(darkbrown),andresultingmoraines(lightbrownlines):(aandb)
Localeruptiongeneratesheatandinitiatesiceaccelerationandice-marginadvance.(c)Asvolcanismwanes,basalicecoolsbelowthepressuremeltingpointandsubglacial
materialsmaybeentrained;theentraineddebris,aswellasanypreviouslydepositedsupraglacialdebris,eventuallyfallspassivelyatstationaryicemargins,producingdrop
moraines.(dande)Astheglacierrecedes,dropmorainesleftbehindwillcontinuetodelineatelocalstillstandsinoverallicerecession.Basedonvariableicemargin
geometry,thesesuperposedmorainesmaycross-cut,atobliqueangles,dropmorainesdepositedduringearliertimes(Fig.3).(Forinterpretationofthereferencestocolorin
thisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
source,causingcompressivestressandribbedmoraineformation 3.3.1.Observations
abovetheplateau. Downslope and to the west of the ridge (Fig. 9), the terminal
moraine of the FSD differs dramatically in size and appearance
from elsewhere in the deposit (Fig. 10). The moraine is up to
3.3.Enlarged,ridgedmoraine
3kmwidehere,whereasitistypicallyonlyafewhundredmeters
wide, and is taller here (up to (cid:2)125m) than elsewhere in the
AtthenorthernedgeoftheFSD(Figs.1and9)liesanL-shaped
deposit(Fig.11).Themorainehasseveralsmallparallelridgesat
ridge(cid:2)50kmlong,upto(cid:2)450mhigh,andupto(cid:2)4kmwide.Due
itstopanditsMOLAprofileresolvesonebroadridgesuperimposed
toitsheight,steepsides,andthechanneledmorphologyatitsdis-
onthemoraine.
talend,thisridgeisinterpretedtohaveformedasanice-confined
volcanic flow; heat transfer from this volcanic flow to the sur-
rounding ice is conservatively estimated to have generated 3.3.2.Evidenceofwet-basedglacialactivity
(cid:2)38km3ofliquidwater(Scanlonetal.,2014).Furtherupslopeof Based on its size, surface appearance, and topographicprofile,
this ridge lies the glaciovolcanic Lobate Facies (Fig. 5). The mor- aswellasitsexclusiveoccurrencedownslopeoftheregionofthe
phology of the outermost moraine of the FSD in this region FSD containing the largest glaciovolcanic edifice and the highest
suggests that the subglacial volcanism that formed the L-shaped densityofglaciovolcaniclandforms,weinterpretthisfeatureasa
ridge and/ortheLobate Faciescausedwet-basedbehaviorin this thrust-block moraine (also known as a composite moraine, or
partofthepaleoglacier. sometimes as a push moraine; e.g. Benn and Evans, 2010). The
K.E.Scanlonetal./Icarus250(2015)18–31 23
Fig.5. RegionalcontextmapshowingcontactsbetweentheLobate,Ridged,andKnobbyfacies.TheareashowninFig.6ishighlightedbyawhitebox.
Fig.6. Concentric,sharp,evenlyspacedridges,interpretedasribbedmoraines,atthecontactbetweentheLobateFaciesandtheKnobbyFacies(locationindicatedbywhite
boxinFig.5).CTXimagemosaic.
24 K.E.Scanlonetal./Icarus250(2015)18–31
Fig.7. Ridgesinterpretedasribbedmoraines(highlightedbywhitearrows)ontheNorthwestPlateau(locationindicatedinFig.2b).CTXimagemosaic.
basal deformation and entrainment (e.g. Boulton et al., 1999;
Bennett, 2001; Benn and Evans, 2010), leading to the formation
ofthrust-blockmoraines.Inadditiontobeinginfluencedbyrheo-
logical properties of the deforming substrate, the size of thrust-
block moraines scale with the magnitude and rate of glacial
advance(Bennett,2001).
Withinpurelycold-basedglaciers,thrust-blockmoraineshave
only been documented where ice advances into marginal lakes,
wherewetlacustrinesedimentmayfreezeontobasaliceandmove
upwardalonglocalthrustplanes(Fitzsimons,2003;Hambreyand
Fitzsimons,2010);thisprocess,however,isunlikelytohavebeen
thecaseatArsiaMons.Instead,weproposethatthelargemoraine
ridgeintheArsiaMonsFSDreflectspolythermalthermalvariation
near the ice margin. Local, wet-based conditions associated with
periodsof subglacial volcanism werelikely associated with basal
entrainment and increased sediment transport downglacier
(Fig.12).Astheiceflowedfromthevolcanicheatsourceitcooled,
Fig.8. Ribbedmorainesmayformwhereaglaciertransitionsfromwet-basedto
cold-basedconditions,eitherovertimeoralongaspatialgradient.Thefieldoflarge and saturated basal debris would have frozen onto the cold ice;
ribbedmorainesintheArsiaMonsfan-shapeddepositmayhaveformedwhere compressive stresses near the ice margin could then have pro-
subglacial eruptions in the Lobate Facies caused a transition from wet-based duced imbricate thrusts, bringing up frozen basal sediment and
conditionsupglaciertocold-basedconditionsdownglacier.
producingtheobservedthrust-blockmoraine.Thisexplanationis
bolstered by the fact that the hypothesized thrust-blockmoraine
multi-ridgedappearance(Fig.10)andprofile(Fig.11aandb)ofthe lies directly downslope of a large field of ribbed moraines
moraineisconsistentwithanimbricatestackofthrustslabs. (Section3.3),whichformdirectlyoverregionspostulatedtoexpe-
OnEarth,suchthrust-blockmorainesformwherehighlongitu- riencetransitionsfromwet-basedtocold-basedglacierice.
dinal stresses act on weak subglacial sediments or bedrock, e.g.
adjacenttoasteeplyslopingicemarginoracrossathermalbound-
arybetweenwet-basedandcold-basediceconditions.Basaldebris, 4.Discussion
entrained by regelation, may be advected upward along thrust
planes in the ice, potentially producing an imbricated moraine Fourcharacteristicclassesofwet-basedorpolythermalglacial
ridge. Locally, elevated pore pressures that arise from perma- landforms are observed near glaciovolcanic edifices in the Arsia
frost-confinedaquifersbeneathpolythermalglaciersmayenhance MonsFSD:(1)outflowchannels,whichrecordthereleaseoflarge
K.E.Scanlonetal./Icarus250(2015)18–31 25
Fig.9. TheL-shapedridgeandsurroundinglandforms.(a)THEMISdaytimeIRimagemosaic.(b)Sketchmapoftheregion.
volumes of meltwater; (2) streamlined knobs, which record the large glaciotectonic stresses typically associated with either fast-
directionofglacialmotionaswellasitserosivepower;(3)ribbed flowing glacier ice (surging) or transitions from wet-based to
moraines,whicharethoughttoindicatetransitionsbetweenwet- cold-basedglacierice.Together,thelandformsprovideacoherent
basedandcold-basedglacierice;and(4)alargethrust-blockmor- historyateachglaciovolcanicsite(Fig.13):uniformice-flowdirec-
aine, which records the local deformation of basal sediment by tionsarerecorded bothbythestreamlinedknobsandtheribbed
26 K.E.Scanlonetal./Icarus250(2015)18–31
Fig.10. TerminalmoraineneartheL-shapedridge,showingribbedsurfaceappearance(redarrows).CTXimagemosaic.(Forinterpretationofthereferencestocolorinthis
figurelegend,thereaderisreferredtothewebversionofthisarticle.)
moraines at the Northwest Plateau. Furthermore, the outflow 5.1.Englacialandbasallakes
channels emerge directly beyond these landforms, as would be
expectediftheenhancedflowwasstimulatedbywet-basedcondi- TheglaciovolcaniclandformsintheArsiaMonsFSDarelikelyto
tions.Similarly, the ribbedmorainesdownslopeofthenumerous havegeneratedseveralenglaciallakeswithvolumesontheorder
glaciovolcanic edifices in the Lobate Facies and the thrust-block oftensofcubickilometersandlongevityofhundredstothousands
moraine farther downslope at the terminus of the deposit both ofyears(Scanlonetal.,2014).Thefewterrestrialvolcanicallygen-
point to a transition from wet-based to cold-based ice, which is erated englacial lakes whose ecosystems have been surveyed
consistent with the placement of glaciovolcanic landforms in the (Gaidos et al., 2004, 2008; Marteinsson et al., 2013) contained
FSD (Fig. 13). These consistent landform associations imply that microbial communities capable of fixing carbon and nitrogen,
glaciovolcanic activity was sufficient to alter the local thermal and including organisms related to known acetogens, sulfate
regime of the Arsia Mons tropical mountain glacier, and their reducersand ironreducers.These communitiesare supportedby
exclusiveassociationwithglaciovolcanicedificesimpliesthatcli- sedimentsandbottomwatersrichinvolcanicCO ,H ,ferriciron,
2 2
mateandpressureeffectswereneverbythemselvessufficientto andsulfurcompounds,aswellasoxygenfromthemeltedglacial
causebasalmeltingintheglacier. ice(Jóhannessonetal.,2007;Marteinssonetal.,2013).
Atleastonebasallakebeneath800mthickiceintheAntarctic
5.PotentialmicrobialhabitatsintheArsiaMonsFSD Ice Sheet has been shown to host a microbial community sup-
ported by chemoautotrophic production (Christner et al., 2014).
On Earth, glaciovolcanic environments provide a range of SincetheArsiaMonspaleoglacierexperiencedtransientwet-based
microbial habitats (e.g. Cousins and Crawford, 2011). While the conditions, analogous basal lakes within the FSD may have pro-
chemicalconditionswouldhavedifferedfromthoseinterrestrial videdadditionalhabitatsformicrobiallife.Otherice-coveredlakes,
glaciers due to the lack of an oxygen atmosphere and a vigorous suchasthoseintheAntarcticDryValleys(e.g.Robertsetal.,2000;
biosphere,plausibleredoxcouplesandnutrientsourcescouldhave Murray et al., 2012), are poorer analogs to the Arsia Mons lakes
madewetglacialenvironmentsonMarshospitabletolife(Cockell becausetheyoriginatedassubaeriallakes.Subaeriallakescanbe
etal.,2011).Sincesuchenvironmentswouldhaveoriginatedfrom colonized more easily than englacial lakes due to their exposure
volcano–iceinteractioninthecaseofthisdeposit,volcanicgasesin to surface runoff and dormant, windborne microorganisms, and
some of the habitats would have further strengthened redox after they develop an ice cover their productivity can still be
gradients and provided an additional carbon source in the form boosted by legacy carbon from the earlier lake ecosystem and
of volcanic CO (Skidmore, 2011). The Amazonian aqueous envi- othernearbyenvironments(Priscuetal.,1999).
2
ronmentsinducedbyglaciovolcanismmayhavebeenshorter-lived Phylogeneticevidencesuggeststhatthemicrobesthatcolonize
thanthosearisingduetoimpactheatinlargecraters(e.g.Abramov terrestrialsubglacialvolcaniclakesoriginateina varietyofother
and Kring, 2005; Barnhart et al., 2010). Taking the (cid:2)102m3s(cid:3)1 environments, including glacial ice and snow, dust and marine
effusion rates estimated for flows at Ascraeus Mons (Hiesinger aerosolsthatworktheirwayintothelakeslongafterbeingdepos-
et al., 2007) as plausible rates for flows in the Arsia Mons FSD, itedontheglaciersurface,drainageofsurfacemeltwaterintothe
the volumes of FSD landforms like the Northwest Plateau and lake (Gaidos et al., 2004), or subsurface aquifers in contact with
L-shapedridge(ontheorderof1010m3;Scanlonetal.,2014)sug- thesubglacialmelt(Marteinssonetal.,2013).Neithersurfacerun-
gestadurationofyearstotensofyearsforedificeconstructionand off nor iced-over subaerial lakes are likely to have been present
thereforeatleastthatdurationforwet-basedconditions.Englacial nearthesubglaciallakesintheArsiaMonsFSD.Thedikeintrusions
water bodies formed in response to these eruptions, however, intheArsiaMonsFSDcouldpotentiallyhavewarmedthesubsur-
would likely have persisted for hundreds to thousands of years faceenoughandmeltedenoughgroundiceforaliquidaquiferto
beforefreezing(Scanlonetal.,2014). persist (e.g. Head and Wilson, 2002). Such an aquifer could have
Thefollowingenvironments,whichgeomorphologicalevidence hydrologically connected some habitats within the deposit,
indicates were present in the Arsia Mons FSD paleoglacier, are enabling microorganisms to move between them. However, due
inhabitedonEarth. to the regional dust cover mantling the deposits, no spectral
K.E.Scanlonetal./Icarus250(2015)18–31 27
Fig.11. (a)TopographicprofileacrossthelineA–A0inFig.9b,showingtheincreasedheightandridgedsurfaceassociatedwithinferredglaciotectonism.(b)Topographic
profileacrossthelineB–B0inFig.9b,showingtheincreasedheightandridgedsurfaceassociatedwithinferredglaciotectonism.(c)TopographicprofileacrossthelineC–C0in
Fig.9b,showatypicalprofileacrosstheterminalmoraineoutsidetheregionofwet-basedglacialthrusting.Notethedifferenceiny-axisscalebetweenthethreeplots.
TopographicdatafromHRSC-derivedDEM.
evidenceforhydrothermalalterationmineralshasyetbeenfound theRidged,KnobbyandSmoothFacies.Ifcirculatinghydrothermal
inthedeposit,andanyfine-scalemorphologicfeaturesofhydro- fluid was not present, putative martian life could still have colo-
thermal systems would have been obscured by glacial debris in nizedthelakesifairbornepropagules(perhapsthelong-dormant