Palaeozoic co-evolution of rivers and vegetation: a synthesis of current knowledge
M.R. Gibling
a,*, N.S. Davies
b,1, H.J. Falcon-Lang
c, A.R. Bashforth
d, W.A. DiMichele
d, M.C. Rygel
e, A. Ielpi
aaDepartmentofEarthSciences,DalhousieUniversity,Halifax,NovaScotiaB3H4R2,Canada
bDepartmentofGeologyandSoilScience,GhentUniversity,Krijgslaan281s.8,GhentB-9000,Belgium
cDepartmentofEarthSciences,RoyalHolloway,UniversityofLondon,Egham,SurreyTW200EX,UK
dDepartmentofPaleobiology,NMNHSmithsonianInstitution,Washington,DC20560,USA
eDepartmentofGeology,StateUniversityofNewYork,CollegeatPotsdam,NY13676,USA
1. Introduction
The term ‘‘evolution’’ has generally been applied to living organisms,followingtheworkofCharlesDarwinandthegrowthof evolutionarybiology.Inanessaywrittenlateinhislifeandbased onacontemporaryunderstandingofbiology,thefamousfluvial geomorphologistLuna B. Leopold (1915–2006) arguedthat the behaviourandmorphologyofrivershaveparallelswithbiological systems(Leopold,1994).Hepointedoutthatalthoughriversasa wholeshowpredictableresponsestoexternalfactors,eachriver system is unique and its individual reaches have a natural variability.Thus,ariversystemisanalogoustoaspeciescomposed ofapopulationofreaches,thevariabilityofwhichreflectscurrent processesand thesystem’sgeologicalhistory. Theriver system mightevolvegraduallyorthroughabruptevents.
The parallel with biological systems as set out by Leopold largelyrepresentsthewayinwhichorganisedsystemsrespondto externalstimuli.Heenvisagedriverevolutionmainlyasagradual
responsetochangesinfactorssuchasclimate.Ofcourse,inthe absenceofgeneticmaterialinfluvialsystems,theparallelshould notbetakentoofar.However,Leopold’sessayimplies thatthe term‘‘evolution’’maybeappliedlegitimatelytolandformsand other essentially physical systems, raising some interesting questions. Might radicalchanges totheEarth systeminfluence virtuallyallriver‘‘species’’worldwide,promotingahigherorderof river evolution? Individual rivers might presumably become
‘‘extinct’’, but might entire styles of river behaviour and morphologyalsobecomeextinct?In contrasttotheirreversible nature of biological evolution, might ‘‘primitive’’ fluvial styles reappearifradicalchangestookplaceinthephysicalenvironment and thebiosphere? Andoverwhat timescalesmightgradual or suddenriverevolutiontakeplace?
Since the late 1960s, research by numerous authors has establishedthat,asaconsequenceoftheevolutionofterrestrial vegetation,riversystemsasawholeevolveddramaticallythrough the240millionyearsofthePalaeozoicEra,especiallyduringthe 120million years of theDevonian and Carboniferous periods (Schumm,1968;Cotter,1978;Went,2005;CorenblitandSteiger, 2009; Davies and Gibling, 2010a,b, 2013; Gibling and Davies, 2012).Amongmanyeffects,plantsinfluencedrockandsediment weathering,thegrain-sizespectrumsuppliedtorivers,landscape stability,theroughnessofsedimentsurfaces,andthecapacityfor ARTICLE INFO
Articlehistory:
Received23March2013
Receivedinrevisedform15November2013 Accepted5December2013
Availableonline30December2013
Keywords:
Braided Meandering Anabranching Blackwater Riparian Ecospace
ABSTRACT
AsvegetationevolvedduringthePalaeozoicEra,terrestriallandscapesweresubstantiallytransformed, especiallyduringthe120millionyearintervalfromtheDevonianthroughtheCarboniferous.Early Palaeozoicriversystemswereofsheet-braidedstyle–broad,shallow,sandbedriverswithnon-cohesive andreadilyerodedbanks.Undertheinfluenceofevolvingrootsandtreesthatstabilisedbanksandadded largewoodydebristo channels, arangeofnew fluvial planformandarchitecturalstylescame to prominence,includingchannelled-andisland-braidedsystems,meanderingandanabranchingsystems, andstablemuddyfloodplains. Riversystemsco-evolvedwith plantsandanimals, generatingnew ecospacethatweinferwouldhavepromotedbiologicalevolution.BytheendoftheCarboniferous,most landformscharacteristicofmodernfluvialsystemswereinexistence.
ß2013TheGeologists’Association.PublishedbyElsevierLtd.Allrightsreserved.
* Correspondingauthor.Tel.:+19024942355.
E-mailaddresses:[email protected],[email protected](M.R.Gibling).
1Current address: Department of EarthSciences, University of Cambridge, DowningStreet,CambridgeCB23EQ,UK.
ContentslistsavailableatScienceDirect
Proceedings of the Geologists’ Association
j ou rna l h ome pa ge : w w w. e l se v i e r. co m/ l oc a te / pge ol a
0016-7878/$–seefrontmatterß2013TheGeologists’Association.PublishedbyElsevierLtd.Allrightsreserved.
http://dx.doi.org/10.1016/j.pgeola.2013.12.003
sediment storage,as wellasthe compositionof theocean and atmosphere(AlgeoandScheckler,1998;Berner,2006;Daviesand Gibling,2010a).
We provide here a synthesis of current knowledge of the Palaeozoicevolution of riversystems, firstquantifiedby Cotter (1978).Theaccountisbasedonaworldwideliteraturecompilation of330alluvialformationsofCambriantoPennsylvanianage,set outinthreepapers(DaviesandGibling,2010a,2011,2013)and referredtobelowas‘‘thecompilation’’.Thereaderisreferredto thesepapersforapproachesusedandthelimitationsoftheresults.
Althoughitisprobablynotpossibletodeterminepreciselywhen andwhereparticularfluvialstylesfirstappearedordisappeared, thegeologicalrecordprovidesabroadrepresentationofeventsin river evolution,which are setout following a summary of key eventsintheevolutionofterrestrialvegetation(Fig.1).
2. Palaeozoicvegetation
FromtheearlyPrecambrian onwards,cryptobiotic filmsand crusts inhabited theterrestrial realm and wereprobably more widespreadthanrecordsindicate(HorodyskiandKnauth,1994;
Prave,2002;Labandeira,2005).EukaryoticmaterialfromtheStoer GroupandDiabaigFormation(TorridonianSandstone)ofScotland indicates that, about one billion years ago, these organisms inhabiteda range of freshwatersettings (Strother etal., 2011).
Althoughitisdifficulttogaugetheireffectonlandscapes,thissuite oforganismsprobablycouldnotpreventbederosion(Long,2011) but may have provided sufficient stabilisation to promote weathering (Dott, 2003), which may have beenmore active in thelaterProterozoicthanearlier (Kennedyet al.,2006).In arid regionstodaythatlacksubstantialvegetation,suchcryptobiotic films and crusts play a significant role in stabilising surfaces (Belnap et al., 2001), and cryptobiotic growths flourished in Palaeozoicterrestrialecosystems(EdwardsandRichardson,2004;
Tomescuetal.,2006;Marriottetal.,2012).
Thefirstundoubtedindicationsoflandplants(embryophytes) are cryptospores from Middle Ordovician strata (Gray, 1985;
Rubinsteinetal.,2010).Thenatureofthesecryptosporesandthe isotopiccomposition of later Ordovicianplantmaterial suggest
that they mark the origination of bryophyte-grade plants (Tomescuetal.,2009).Bryophyteshaveconsiderableweathering capability andmayhave beensufficientlyabundantintheLate Ordovician to reduce atmospheric CO2 and promote glaciation (Lenton et al., 2012). Trilete spores indicative of tracheophyte- gradeplantsareknownfromtheLateOrdovician(Steemansetal., 2009)andbecamemorewidespreadintheearlytomiddleSilurian.
Molecularclockssuggestearlieroriginsformanygroups(Clarke etal.,2011),butitremainsunclearwhenalandscapethresholdin plantcoverwascrossed(Wellman,2003).
BythelaterSilurian,acomplexofprimitivelandplantswasin existence, including bryophytic and tracheophytic elements (Edwards and Wellman, 2001; Genseland Berry, 2001; Boyce, 2008;GerrienneandGonez,2010;Kenricketal.,2012).Isotopic evidence suggests that plant cover expanded at the Silurian- Devonianboundary(MałkowskiandRacki,2009).Diverseplants arewellpreservedintheRhynieChertofEarlyDevonian(Pragian) age,whichhasalsoyieldedtheearliestdirectevidenceofrhizoid- basedrootsystems,mycorrhizalfungi,andfungal–algalsymbiosis (Tayloretal.,1992;BatemanandDiMichele,1994;Genseletal., 2001;Kerpetal.,2001;Kenricketal.,2012).Woodfirstappearedin the Early Devonian (Gerrienne et al., 2011). Through the Early Devonian,plantsbecamediverseand widespreadincoastaland alluvial environments, building up coal-like accumulations in places(Wehrmannetal.,2005;Kennedyetal.,2012a,2013;Morris et al.,2012). Charcoalis known fromlatestSilurian and Lower Devoniantidalandalluvialdeposits(Glasspooletal.,2004,2006;
Daviesand Gibling,2010a;GlasspoolandScott,2010),showing thatsufficientbiomasscoveredthelandsurface,atleastlocally,for wildfirestojointhespectrumofterrestrialprocesses.
Prototaxites,upto8mtallandprobablyafungus(Hueber,2001;
Boyceetal.,2007),constitutesthefirstarborescentorganism.Early Devonian (Lochkovian) rooting structures may belong to this genus(Hillieretal.,2008),whichalsocontributedthefirstlogsto fluvialsystems(DineleyandWilliams,1968;Daviesetal.,2011).A major development in the Middle and Late Devonian was the appearanceofforestswithlargeand diversetrees(Mintzetal., 2010; Steinet al.,2012)including theprogymnospermArchae- opteriswithdense,conifer-likewoodandanarchitecturesimilarto
Fig.1.TimelineforPalaeozoicchangesinvegetationandriversystems.ProportionsofriversystemswithparticularfluvialstylesarebasedonthecompilationsofDaviesand Gibling(2010a,2011,2013).Asdiscussedinthetext,meanderingriverswereidentifiedonthebasisofheterolithiclateral-accretionsets.Anastomosingriversrepresent
‘‘fixed-channelfloodplain’’depositswithsand-filledchannelbodiesencasedinmudstone;manywereattributedtoanastomosingriversbytheoriginalauthors.Variedstyles ofbraidedsystemhavenotbeenquantified,andbothisland-braidedandanastomosingsystemsareanabranching.Seetextforsourcesofinformationforplantevolution.
Miss.=Mississippian,Penn.=Pennsylvanian.TimescalefromOgg(2010).
moderntrees(Meyer-Berthaudetal.,1999).FortheLateDevonian, isotopic data suggest an expanded plant cover (Godderis and Joachimski,2004)thatwascapableofinfluencingglobalclimate throughitseffectsonsurfacealbedo,landscapethermalproperties, andevapotranspiration(LeHiretal.,2011).
The Carboniferous sawa progressivediversification ofplant groups. Of particular importance was the spread of woody gymnosperms,whichappearintheLateDevonianbutproliferated fromtheearly Mississippianonwards(Decombeixet al., 2006;
GaltierandMeyer-Berthaud,2006;Decombeixetal.,2011).Some ofthesewereoflargesize,exceeding2mindiameter(Long,1979), and patterns of occurrence suggest that some were living in habitatsoutsideswampylowlands.Incombinationwiththesizeof theseplants,suchhabitatinferencessuggesttheexistenceofwell developed root systems that penetrated deeply into the soil (Vanstone, 1993). By thelatestMississippian, the cordaitaleans hadappearedonthelandsurface,withdeep,laterallyextensive rootsystems,thickdensewood,andheightsof50m(Falcon-Lang andBashforth,2004,2005).BytheEarlyPennsylvanian,theywere abundant in seasonally dry alluvial settings with periodically loweredwatertables,andarerepresentedbyfragmentaryrecords fromredbedsthatoncewereconsideredlargelybarrenofplantlife (Falcon-Lang,2003,2006;Falcon-Langetal.,2009;DiMicheleetal., 2010;Bashforthetal.,2014).Theserecordsimplythatvegetation was able to colonise moisture-stressed settings across alluvial plainsand,probably,uplandareas.
The Carboniferous also witnessed an expansion of the sphenopsids,agroupcharacterisedbyclonalgrowththatproduced densethicketsofaerialshootsarisingfromundergroundrhizomes, andanecologicalpreferenceforshiftingsubstratesindisturbance- proneriparianhabitats(Gastaldo,1992;Pfefferkornetal.,2001;
Calderetal.,2006).Alsoduringthisperiod,wetlandecosystems dominatedby lycopsidsexpandedacrossequatorial Euramerica (Grebetal.,2003),andmirescoveredlarge,high-latitudeareasof GondwanaandnorthernAsia,especiallyinPermiantimes(Diessel, 1992;HiltonandCleal,2007).
The later Pennsylvanian (Kasimovian-Gzhelian)and Permian weremarked by increasing dryness across muchof equatorial Pangaea.Althoughmanyofthelargewetlandtreetypesdeclined, includingmostof thelycopsidspecies (DiMichele etal., 2009), there was a progressive spread and associated evolutionary radiation of many other kinds of plants (especially conifers) capableoflivinginmoisture-stressedhabitats(e.g.,Broutinetal., 1990;Kerp,1996,2000;DiMichele etal.,2008;vanHoofetal., 2013). Many of theseconiferopsids weresubstantial trees with pronouncedtree-ringsthatdemonstrategrowthunderconditions ofstrongwet–dryseasonality(Falcon-Langetal.,2011,2014).
The following sections document fluvial styles from the Cambrianthrough theCarboniferous (Fig. 1), drawing parallels withtheevolutionofvegetation.
3. CambriantoSilurianrivers:sheet-braidedsystems
Trunk riversystems of this ageareuniversally preservedas sandstone bodies composed of relatively thin sheets (Fig. 2A), promptingCotter(1978)totermthem‘‘sheet-braided’’.Inoneof thebestexposedexamples,theCambrianAlderneySandstoneof theChannel Islands,thesheetsare afew decimetrestometres thick and are predominantly trough cross-bedded with some accretionary bar forms but with minimal mudstone beds, fragmentsormatrix(ToddandWent,1991; Daviesetal.,2011;
Went,2013).Barformsarealsoprominentinsheetsandstonesof theCambrian Guarda Velha Formation of Brazil (Santos et al., 2013).Somesheetsrestonflat-lyingerosionalsurfaces,themore prominentofwhichmaymarkmajorchannels,andafewinclined erosionsurfacesmarkchannelmarginsoflowrelief.Intheabsence
ofrootedvegetationtostrengthensandybanks,channelmargins wouldhavehadlittlecohesion,andalluvialchannelswouldhave adjusted to discharge variation, widening or accreting readily (Daviesetal.,2011).
With outcrop limitations and the difficulty of establishing channel margins, the dimensions of sheet-braided channels remain unknown. For theCambrian-Ordovician fluvial deposits ofTableMountain,SouthAfrica,Fuller(1985)speculatedthatthe channelwidth:depthratiomayhaveexceeded1000:1,althoughno firm data werepresented. Associatedalluvial-fan and tributary channelsmayhavegeneratedmorecomplexdepositsmarkedby deeperincision(Went,2005;Santosetal.,2013).
Another problem in workingwith sheet-braided systems is distinguishingfluvial from deltaic and marine deposits(Davies etal.,2010;Went,2013).River-bornesandwasdepositedacross low-gradient coastal platforms where transgressions could ad- vanceconsiderabledistanceslandward, leavinga marinefaunal record without a distinctive marine sedimentological record (Daviesetal.,2010).
Itisnotclearwhethersuchriversexist onEarthtoday.Pre- vegetationalfluvialsystemswouldhavehaddistinctivedischarge, runoff and flood patterns(see summary in Daviesand Gibling, 2010a), and Schumm (1968) compared pre-vegetational rivers withthoseinmodernaridzoneswithsparsevegetationandwhere pollution hadcaused vegetationtodieback.Riverswithsheet- braided characteristics may exist in such settings. A possible humid-zone example is the William Riverof northern Canada, whichshowsatenfoldincreaseinwidth:thicknessratiotoabout 300whereitpassesthrougheoliandunes(SmithandSmith,1984).
OtherhumidexamplesmayexistinIcelandwherevegetationis limited(Cotter,1983;Daviesetal.,2011).
The minimal degree of upland weathering at this time is illustratedbytheCambrianRozelConglomerateofJerseyinthe ChannelIslands(Went,2005;Daviesetal.,2011).Thesealluvial- fanconglomeratescontainlittlematrixandtheformationcontains fewsandstoneandmudstonebeds,despitetheproximityofthis humid-zonefantoanuplandarea.However,moremudstonehas beenreportedfromotherlatestPrecambrianandCambrianfluvial deposits(Avigadetal.,2005).Eolianandfluvialbedsarecommonly intercalatedinCambrianandOrdovicianformations(Dottetal., 1986; Clemmensen and Dam, 1993) and, in the absence of vegetation,theplainswouldhaveexperiencedstrongdeflationand oceanwarddusttransport(Dalrympleetal.,1985).
4. DevoniantoMississippianrivers:theriseofchannelled- braidedandmeanderingsystems
BythelateSilurian,theinternalarchitectureof manyfluvial depositshadbecomemorecomplex,withmorelensoidunitsand anincreasedproportionofmudstonebedsandclasts(Fig.2B)–the
‘‘channelled-braided’’ systems described by Cotter (1978). The compilationcontainsfewSilurianformationsanditisuncertain whenchannelled-braidedriversbecamecommon,althoughthey arecertainlypresentwithinsomeearlyOldRedSandstone-type successions,suchastheLudlovianStubdalFormationofNorway (Daviesetal.,2005a,2011).Channelled-braidedriversrecordedin the Stubdal Formation may have coexisted with sheet-braided systems,asrecordedinthecontemporaneousstrataoftheStore Arøya(Skien)Formation(Daviesetal.,2005b),withthechannel geometries in these co-existing systems being partially deter- minedbytectoniccontrolsandpalaeoslope(Daviesetal.,2005a).
A channelled-braided architectural style characterises many Devonian and Mississippian formations (Cotter’s type example wastheMississippianPoconoFormationofthenortheastU.S.A.).
GoodexamplesarepresentintheLowerDevonianBatteryPoint FormationofeasternCanada(Cant,1978;Daviesetal.,2011)and
theUpperDevonianBulgeriFormationofAustralia,whichhasdeep scoursandarangeofarchitecturalelements(LangandFielding, 1991).
Devonian alluvial-fan deposits contain prominent mudstone beds,indicatingincreaseduplandweatheringandsuggestingan expansionofvegetationcover.Correspondingly,theriseofmuddy floodplainandcoastal-plaindepositsindicatesincreasedstorageof
fines in lowlands(Daviesand Gibling, 2010a). The incomingof channelled-braided architectureprobablyrepresentsthegreater availabilityofmud,resultinginsufficientmudmatrixandbedsto influencethecohesivepropertiesofriversediment(Daviesetal., 2011).AcoveroflandplantsfromtheMiddleOrdovicianonwards would have begun to increase surface plant litter and organic matter in soils, enhancing weathering of alluvium and early Fig.2.Palaeozoicfluvialstylesandplantmaterial.(A)Sheet-braidedstyle,CambrianAlderneySandstoneFormation,Alderney,ChannelIslands.Stackedsandstonesheets(14m visible)arecomposedmainlyoftroughcross-beds,typically30–50cmthickinthepinkstrata,whicharemedium-tocoarse-grained.(B)Channelled-braidedstyle,Lower DevonianBatteryPointFormation,Gaspe´,Que´bec,Canada.Noteseveralerosionalsurfaces(arrowed),lensoidformofseveralbedsets,andpresenceofmudstone(m).Outcropis 10mhigh.(C)Lateral-accretionset(toparrowed),attributedtomeandering-fluvialstylewithpointbars,LowerDevonianBatteryPointFormation.Setis2mthickandcomposed ofinterbeddedsandstoneandmudstone,witherosionalongsomebedsurfaces.Noteabundanceofmudstonebedsinoverlyingstrata,includingpoorlydevelopedpaleosols.(D) Fixedchannels,LowerPennsylvanianTynemouthCreekFormation,NewBrunswick,Canada(seeBashforthetal.,2014).Cliffis12mhigh.Twochannelbodiesofsandstoneand mudstone(basesarrowed)exhibitchannelmarginscutintoredpaleosolsandgreysandstonesplays.Theupperbody,2mthick,hasasteeplefthandmargin.(E)Logaccumulation atbaseofbraided-fluvialchannel,LowerPennsylvanianBossPointFormation,NovaScotia,Canada(seeIelpietal.,inpress).Logsarestackedtoformalayerofsandstoneand organicmaterial3mthick,andaremainlycordaitalean,someatleast2mlong.Partsoftheformationareinterpretedasanisland-braidedsystem.(F)Uprightlycopsidtree,1.5m tall,originallyhollowandfilledwithmudstone.Thetreeoriginallyprojectedupthroughthebaseoftheoverlyingchannel,andinclinedlayers(a)inthechannel-sandstonebodyat thetopofthephotorepresentasandbarformedaroundthetrunk.LowerPennsylvanianJogginsFormation,NovaScotia,Canada.
diagenesisofbankmaterial.Althoughthisfluvialstylepredatesthe adventofrootedvegetation intheEarlyDevonian,rootsystems wouldhaveenhancedbankstrengthoncetheyoriginated.TheLate Devonianwitnessedtheriseofthesphenopsidsand,withtheir propensity to inhabit disturbed settings and form pioneer communities,thisplantgroupinparticularmayhavecolonised and stabilised bars and banks in sandbed rivers in the later Palaeozoic.
Based on the compilation, meandering rivers appear in the terrestrialrockrecordclosetotheSilurian-Devonianboundaryas determinedbyheterolithiclateral-accretionsetswithinterbedded sandstoneandmudstonethatrepresentthesystematicmigration ofpointbars(Fig.2C).Smallexampleslessthan1.5mthickare seen in Lochkovian strata of the Old Red Sandstone in Wales, although not in coeval trunk systems (Allen et al., 1981). In contrast,lateral-accretion sets, commonly several metres thick, have been reported from some 40% of Upper Devonian fluvial formations.Thisradicalgeomorphicchangetookplaceoverabout 50millionyears(DaviesandGibling,2010a,b).
Sandy braidbarsmay accretelaterally (Bristow, 1987;Horn et al., 2012), and good examples are present in the Cambrian Alderney Sandstone (Todd and Went, 1991). However, the compilationyieldednoCambriantoSilurianexamplesofhetero- lithiclateral-accretiondepositsoffluvialorigin.Thismayinpartbe apreservationalhappenstance.Lateral-accretionsurfacesmaybe poorlypreservedinsinuoussandbedriverswherelargebedforms reworkthedippingsurfaces,aswellasin coarse-grainedrivers suchasthemodernColorado(McGowenandGarner,1970),where laterallyaccretedpointbarsaredissectedandlocallydestroyedby chutecutoffs.Nevertheless,therecognitionbymanyresearchersof heterolithicpoint-bardepositsinDevonianstrataconfirmstherise ofmeanderingriversintheDevoniancomparedwiththeirvirtual absenceearlierinthePalaeozoic.
AlthoughFig.1suggestsageneralaccordbetweentheincoming ofrootedvegetation andtheriseofmeanderingpointbars, the incoming of lateral-accretion sets predates the earliest deep rootingstructures.Flumemodelssuggestthatrootedvegetation strongly promotes meandering by strengthening banks, thus mediatinga balancebetween therates of cutbank erosion and point-baradvance(TalandPaola,2007).However,finesediment also assists in healing cutoffs and maintaining systematic migration (Braudricket al., 2009), and near-surface induration ofalluviumstrengthensbanksinsomeseasonalsettings(Gibling etal.,1998;Nansonet al.,2005). Asnotedabove,diagenesisof alluviummayhavebeenenhancedbyanincipientplantcoverprior totheDevonian.
5. Pennsylvanianrivers:theriseofanabranchingsystems
TheEarlyPennsylvanianmarkstheriseoftwotypesofmulti- channelsystem.Inredbedformationslaiddownunderconditions of seasonal flow and lowered water tables, relatively narrow channel-sandstonebodiesencasedinmudstoneareprominentfor thefirst time(Daviesand Gibling,2011)(Fig.2D). Thechannel bodieshavealowwidth:thicknessratio,typicallylessthanabout 30:1(ribbonsandnarrowsheets:Gibling,2006).Characterisedby stablebanksandpredominantverticalaccretion,theyweretermed
‘‘fixed channels’’ by Friend (1983). Although a few small fixed channelsarepresentin Devonian and Mississippianformations (Graham, 1983; Tunbridge, 1984; Melvin, 1993; James and Graham,1995;Astinetal.,2010),numerousLowerPennsylvanian formationsarecharacterisedalmostentirelybythisfluvialstyle.
The fixed-channelstylereflectsa circumstancein which the rateofsedimentsupplyoutpacestherateofbankerosion,resulting in vertical accretion and eventually abandonment without significantwideningofthechannels.Manymodernchannelswith
this geometry and mode of accretion are anastomosing and associatedwithabundantvegetation(Makaskeetal.,2002;Tooth etal.,2008).Theincomingofthefixed-channelstylecorresponds broadlywiththeriseofcordaitaleantrees(Fig.1),whichhaddeep and laterallyextensive roots thatwereable toreachthewater tableindrylandsettings.
Also in the Early Pennsylvanian, the abundance of logs in braided-fluvialformationsdramaticallyincreased(Fig.2E).Such formationshave deep channels,upright trees preservedwithin channel deposits, inclined surfaces that reflect lateral and downstreamaccretion,andevidenceofrootedzonesthatsuggest thepresenceofvegetatedislands(RustandGibling,1990;Fielding etal.,2009;Giblingetal.,2010;Ielpietal.,inpress).Cordaitalean logs are especially abundant in these systems, and are locally stackedtothicknessesofseveralmetresinsmallchannels,with otheraccumulationsformingthecoreofbars.Suchaccumulations arecommonlyoverlainbyabandoned-channelfillsandmounded sandbodies, and may have triggered channel obstruction and avulsion(Giblingetal.,2010).
Thissuiteoffeaturessuggestscomparisonwithwanderingor island-braided rivers suchas theTagliamento, Bella Coolaand Platte,which arevariouslygravellyand sandyand locallyhave densevegetationalongbanksandonislands(DeslogesandChurch, 1987; Gurnell et al., 2001; Tockner et al., 2003; Joeckel and Henebry, 2008; Horn et al., 2012). In these and other modern rivers, strandedlogspromote barandislandformation through sediment accretion and the growth of pioneer communities (Francisetal.,2009; Collinsetal., 2011). Theislandsareshort- livedandthechannelsunderminebankandfloodplainvegetation as they migrate laterally, supplying the river with logs and promotingbiologicalfeedbackloops.
Directevidenceforanabranchingisdifficult,ifnotimpossible, toestablishintherockrecord,requiringevidencethatmultiple channels were active simultaneously. However, modern fixed- channel and island-braided styles are commonly anabranching and,appearingtogetherintheEarlyPennsylvanian,theyprobably markthefirstappearanceofanabranchingacrossalluvialwetland and dryland areas. The distinctive fixed-channeldeposits were quantifiedinthecompilation(10%ofPennsylvanianformations:
Fig.1),butisland-braidedsystemscouldrarelybeidentifiedfrom literatureexamples. Thus, Pennsylvanian anabranchingsystems are probably much more abundant than Fig. 1 suggests.
MeanderingriversremainedprominentinPennsylvanianwetland settings (Greb et al., 2006), commonly associated withupright trees(Fig.2F).
ThePennsylvanianisalsonotedforthickandextensivecoals, someofhugearealextentandtraversedbychannelsystems(Greb et al., 2003). These vast peatlands would have yielded large volumesoforganicmattertoriversand,inturn,toestuariesand shallowseas,asinmodernsettings(Mooreetal.,2011).Modern
‘‘blackwater rivers’’ have little clastic sediment but transport humicacidsthatdarkenthewater(Winemilleretal.,2008;Nanson etal.,2010;Mooreetal.,2011).Becausethetermrepresentswater composition,blackwaterrivers cannotbeidentified in therock recordonthebasisoffluvialstyle,althoughtheirpresencemaybe inferredonpetrographicgrounds(Ielpi,2012).Inaddition,some channel bodies associated with Pennsylvanian coals, especially mud-filledchannels,mayhavebeensediment-deficientblackwa- terriversduringtheperiodofpeataccumulationbutlaterfilled with sediment, thereby allowing their recognition as channels (Forgeronetal.,1986;Nelsonetal.,2008;Davies-Vollumetal., 2012).
By theLatePennsylvanian,vegetation had colonisedalluvial settingsfrommountainfrontstotheoceanandpossiblyupland areas (Bashforth et al., 2010,2011; Davies and Gibling, 2013).
Many plantsprobably spreadfromcoastaland lowland regions
acrossinlandalluvialplainsanduplands(DiMicheleandAronson, 1992),but someadaptable uplandplantsmayhavespreadinto lowland areasduringtheLatePaleozoic, aswellaslater in the Mesozoic (DiMichele et al., 2001; DiMichele et al., 2006;
Decombeix et al., 2011). Bryophytes and lycopsids have been documented from Pennsylvanian near-glacial settings in South America(Ricardi-Brancoetal.,2013),andvegetationisprominent inthehigh-latitude Permiansettingof Australiaand Antarctica (Diessel,1992;Birgenheieretal.,2009),withevidencelocallyfor anassociationwithpermafrost(Krull,1999).Thus,virtuallyallLate Pennsylvanianriverswouldhavebeeninfluencedbyvegetation,as inthemodernworld.
6. Discussion
Fig1suggestsacausativelinkbetweenthePalaeozoicriseof vegetation and the evolution of fluvial style. The evidence is circumstantialanddependsonanunderstandingoftheinfluence ofvegetation inmodernsystems andanaloguemodels(Gibling andDavies,2012).Nevertheless,thestepwiseandunidirectional Palaeozoicevolutionofriverscannotentirelybeattributedtosuch extrabasinal factors as tectonism, climate and sea-level, all of whichmusthaveinfluencedriverssincetheArchaean.Although theprecisetimingoffluvialeventsis difficulttodetermine,the appearanceorincreaseinabundanceofseveralfluvialstylesover timescales as short as a few million years, as well as their subsequent persistence, requires an explanation in biological evolution.
Thecompilationonwhichthisstudyandothersarebaseddoes notcoverthe3.5billionyearsofPrecambrianhistoryrepresented intherockrecord.Wemakenoattemptheretoextrapolateour results to the Precambrian record, apart from noting the prominenceofsheet-braidedsystemsandthemanystudiesthat have emphasised pre-vegetational conditions (see Long, 2011).
Raremuddyfloodplaindepositsandlateral-accretiondepositsare knowninPrecambrianformations(FralickandZaniewski,2011;
Long, 2011; Eriksson et al., 2012), and periods of aggressive chemicalweatheringmayhaveinfluencedmudsupply(Donaldson and deKemp, 1998). Recentdiscoveriesof microbiallyinduced sedimentarystructuresandeukaryotefossils intheTorridonian Sandstone(Prave,2002;Strotheretal.,2011)suggestthatearly organismswererelativelyabundantonsomeProterozoicterres- triallandscapes.However,itisunclearhowmuchinfluencethese organismshadonsubstratestrengthandonthedegreetowhich they, directly or indirectly through an increase in soil organic matter,contributedtochemicalweathering.
Numerousmodernbraided-and meandering-riverstylesare representedinarangeoffaciesmodels(Miall,1996).Itisunclear whether these models can be applied to Palaeozoic and Precambrian systems, which eitherpredated vascular plants or coexisted with substantially different taxa than at present.
Furthermore, mostfacies modelsdo notconsidertheeffects of vegetation(seeFieldingetal.,2009,foranexception),andrecent anthropogeniceffects on riversand their associated vegetation havebeensoseverethatweoftenhavelittleideawhatphysical andbiological frameworksare‘‘normal’’fortheseriversystems (Walter and Merritts, 2008). In studying Palaeozoic and older alluvium,we currentlyprefertousethethreegeneral typesof braided system and one meandering type (Fig. 1), without attemptingtodefinethemintermsofexistingfaciesmodels.
Thegeneration of newecospace throughniche construction (Day etal.,2003)andecosystemengineering(Erwin,2008)isa significant driver of evolution and generally operates through adaptationsthatenableorganismstocolonisepreviouslyinacces- siblehabitats (Benton, 2010)or touse newkinds of resources (Erwin,2008).ThePalaeozoicevolutionofriversystemsgenerated
landformsthathadpreviouslybeenrareornon-existent,including muddy floodplains bordered by stable channels, levees, and crevasse splays. In particular, the rise of avulsive meandering and anabranching systems, culminating in the Pennsylvanian, would have greatly expanded the area and importance of basinwideriparian(riverbank)corridors(Grebetal.,2006;Davies andGibling,2013).Suchcorridorswouldhaveconnectedcoastal wetlands with dryland alluvial plains and upland valleys, enhancingthedispersaloforganisms.Theripariansystemswould haveprogressivelyacquiredactivetrunkandsidechannelswith varied energy levels suitable for a range of aquatic organisms, including abandoned channels from meander cutoff and ana- branches.Theywouldalsohaveyieldedlargevolumesoflogsand wouldhavepromotedriver/groundwaterinteraction,encouraging interchangebetweenriparianandaquaticorganisms(Daviesand Gibling,2013).
There is considerable evidence that the creation of new ecospace influenced further biological evolution in Palaeozoic terrestrialplantandanimalcommunities.Modernriparianzones sufferfrequentflooddisturbance,resultinginpatchyhabitatsand highbiodiversity,andasimilarpatchydistributionofvegetation hasbeen documentedin Pennsylvanianalluvial systems (Scott, 1978;Bashforthetal.,2010,2011).ManyimportantDevonianand Carboniferous fossilaccumulationsoccupyabandonedchannels, variouslyreflectingenhancedpreservationin meanderingchan- nelsorthepresenceofwaterholesindrylandchannels(Hookand Ferm,1988;Behrensmeyeretal.,1992;Falcon-Langetal.,2004;
Astin et al., 2010; Bashforth et al., 2014). Early Devonian assemblages of vertebrates, invertebrates and plants represent theriseofcomplexecosystems(Batemanetal.,1998; Habgood etal.,2003;Labandeira,2005;Kennedyetal.,2012b),andtrace- fossil diversity expanded greatly in alluvial deposits after the Silurian(BuatoisandMa´ngano,2011;DaviesandGibling,2013).
We infer that Palaeozoic evolutionary trends representthe co- evolutionofterrestrialplants,landformsandanimals,withcomplex feedbackloopsandlinkstosoils,theocean,andtheatmosphere.
Such a concept is in accord with the recent biogeochemical understanding oflandscape(Fisheretal., 2007;Corenblitetal., 2011). Corenblit and Steiger (2009) noted that Silurian plant colonisationrepresentsacriticalperiodofbiogeomorphicevolu- tion.
By the end of the Pennsylvanian, terrestrial vegetation was widespreadand diverse,and mostmodern terrestriallandforms wereinexistence.MesozoicandCenozoicterrestrialsystemsshould haveexperiencedmoresubtlechanges, forexamplethroughthe evolutionofangiospermplants,includinggrasses(Retallack,1990).
Vegetationbecametemporarilylessdiverseandabundantfollowing major extinctions (McElwain and Punyasena, 2007), and the Permian-Triassicboundary extinction event may haveincreased the prominence of braided systems(Cotter,1978; Smith, 1995;
SmithandWard,2001),althoughaspectsofthisinterpretationhave beenchallenged(Gastaldoetal.,2005,2009).Periodsofextremely arid climate,asexperiencedacrosstropicalwesternpartsofthe Pangeansupercontinent latein the EarlyPermian (Zambito and Benison, 2013), stronglyreduced vegetationcoverand diversity (Tabor,2013)andmayhaveinfluencedfluvialsystemsregionally.
Foremanetal.(2012)documentedachangeinfluvialstylerelatedto the Paleocene-Eocene Thermal Maximum, linked to change in climateandvegetationduringamajorgreenhouseperiod,andmany other similar changesprobablyawaitidentification.Thus,major extinctions and periodsof vegetation diebackmayhave caused fluvialsystemstoreverttoanEarlyPalaeozoicmodeofoperation until vegetationcover was re-established fromrefugial areas.If surfacetemperatures continuetoincreaseon Earth throughthe presentcentury,someregionsmayexperienceareversiontomore
‘‘primitive’’fluvialsystems.
7. Conclusions
The Palaeozoic evolution of vegetation forced a profound evolutionof river systems and terrestriallandscapes over little morethan120millionyears–aremarkablyrapiddevelopmentfor oneofthemostprofoundeventsinEarthhistory.Arangeofnew fluvial planforms, floodplain elements, and physical processes emergedasriversystemsco-evolvedwithplantsandanimals,and thegenerationofnewecospacemayhavebeenamajorfactorin promoting biological evolution. Entirely new classes of river system evolved during the Palaeozoic while others may have becomeextinct.Thisemergingevidenceforarapidchangeinrivers duringthegreeningof thecontinentsextendsLeopold’s(1994) insightthat,insomesenses,fluvialsystemsmaybeconsideredto evolve.Manyaspectsofthis broadaccountneedfurthertesting, especially the evolution of Precambrian fluvial systems, of Palaeozoicbraided systems, and of fluvial systems in response tomajorextinctionsandclimaticevents.
Acknowledgments
We are grateful to the Geologists’ Association and The DevonshireAssociationforaninvitationtopresenttheseresults ataconferenceinExeter,U.K.in2012,andparticularlythankDavid Bridgland, Jenny Bennett and Sarah Stafford for organising the conference. Two anonymous reviewers made suggestions that greatlyimprovedthemanuscript.Theresearchwasfundedmainly froma DiscoveryGrant toMRGfromthe NaturalSciences and Engineering Research Council of Canada. We thank many colleaguesfordiscussionandinsights,especiallyChrisBerry,Luis Buatois,BlaineCecil,BobGastaldo,PatGensel,SteveGreb,Gerald Nanson,andBillStein.
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