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Nonequilihriurn Regolith Thickness in the Ouachita Mountains

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Nonequilihriurn Regolith Thickness in the Ouachita Mountains Nonequilihriurn Regolith Thickness in the Ouachita Mountains Tonothan D. IJlzilli~,s,Daniel A. Mrrrion,' r(enl2etl.1Lzlclto~r,' rzl2d Icristin R. Adllms ABSTRACT Intcrl~iet,~t~onsof iegol~th anel so11 th~ckncss111 the context of landscape evolutlon ;Ire typlc,llly hascd o~ithe liotion that thl~ki7ess1s co~~t~ollecIby the Iliterdctloll of wcatlier~ngrates anct erosion '111d tu11ed to topography On siJcslopcs oi the Otldchita Mount,ilns, Alltansas, how eve^, there 1s ,I high clcgrcc of local spat~~lvarlah~l~ty th,lt 1s ldrgcly u111c1,ttcd to topog~aphy T111i iriel~cates~~onccl~~il~l~r~ti In the sc~lsc that there 1s 110 cv~dcnccof '1 h,il'~ncc between ~atcsof weatl~crlngaid ~et~~ovdl,as 1s ~~o\t~~l~ited111 so111e co~~ce~tu'll17iodels 111 gco~norphologyalld pedology Johnson's sol1 tl~~clzne\srilodcl 1s ,~pplledas an ;~lternat~veto lnteiprct local v'1ii~tionsIn regol~thtl1icl<i1ess At the study sites, regol~thtl~icltness 1s not gcncr,illy ~cl~tcdto slope, culvature, clcvatlon, ol pedogcilic ctcvelopment 111 the solurn Th~slndlc,ites that va11ab111tyIn thickness is related chiefly to processes and controls dctil~gin the lower rcgolith, helow the solum The pru~liarycontrols of variabil~ty'Ire local litholog~calvar~atio~i, va~lablc structural rcslstancc assoelated w~thfract~ucs 'lnd bedd~ngplanes 111 strongly t~ltedI-'alcozoic sed~ment,lryparent n~~itcr~al,'III~ polnt- ccrltcrccl pedologicdl II~~~L~CI~CL'~of trees A steady state regolith ]nay be relC~ttvclyrale Rcstilts of th~sstudy suggest that 'In cq~111hri~1111~egollth thicl<~~css 1s most hltcly 111 ~ln~forml~tliology w~tli a 111gh clegrec of l~tliologicpuiity, lcss lllzely 111 i~ltcrhcddcclscd~rmcnt~~ry ~ocl<s, and more unl~lzclyst111 tf the I'itter ,Ire t~tled'11id fla~t~lredEqul11b11~1rn th~clznesswould also he mo~e11l<cly where the effects of h~ot~irh,lt~o~i,lie morc 'ileally un~form(as opposed to the point-centered effects of ~ndiviclualtrees) ~1ic1wlie~c the lxo~~idl~tle1s above the weCltheringftont Introduction Several models of landscape evolution postulate products, the wcatlzering- or tr,lnsport-limited na- the development of '1 steddy state ecl~~il~hri~~i~reg- ture of lanctscdpe evolution, and storage of water, olith thiclziicss, implyiilg '1 iuorc or lcss spc~tially carbon, ilutrlcnts, and other elements. Thiclzilcss uniform regolith cover withi11 reas as with siil~ilar is '11so a functamental property relevant to the use environillental controls 111 some situations, how- and man,lgement of land '111d soil resources. ever, rcgol~th,lnd soil thiclzness exhibit 2 high de- In th~sstudy, ccluilibriuill (a terin that is vari- gree of loc,ll spatial vari~hility,111l~lyi11g ilonequi- ously aild poorly defined in geology) refers to a lihriuill T11c purpose of this study is cxailliile the ste,~dystate in the case of rcgolith thiclzncss, 1111- spatial v~ri~lb~lityIII regolith thiclzness in the plyii~gthat the production of weathered debris is Ouachita Mountains, Arlz,ulsas. In adciition to 'lt- approxiin'~telybCllancec1 hy thinning processes such tempting to ~rilderst~lndrcgolith and I'lndscape evo- AS eros~oll~~liemov'~1 to illdiiltain a r~l~~tivelyCO~I- lut~on,as well as soil vL1riLlhilityat our sttes, we statlt thiclzi~e~~.111 the sl~tialdoin'~in, this wo~11~1 11opc to 'llso address morc gener'll theoretical clues- be manifesteci ;is iniiliill~llyv'1riC~ble regolith thielz- tioils of (noi~)ecluiI~b~i~ii~~regolith. The thiclzncss of ness w~thinany clr~aof negligibly vc~riablegeology soil ai~dregolith is ,I fund,lmeilt,ll property related ~ndtopogr'lphy that has heen subjected to the same to tlzc inass h'11,lnce c~i~clallo~ati~ii of ~e~~tlierii~ghistory of e~~viroi~i~~e~~t~llch~lilgesand dist~lrh~~nccs. Following Kenwiclz (1992),we distiilguish between disecluilihnum ,lnd nonequilihrium. A discquilih- 2004 rium systeill IS OIIC that IS MOVIII~ towarc1 ste,ldy ' Southcln licse,~r~li5t,1t1on, U5I)A rorest Serv~cc,1'0 lhx statc but that 112s not 11'1~1s~ifficiei~t tiille to achieve 1270, 1 lot 5prlnq5, AkCln5,ti 7ILjO2, U 5 A oLlde~ltJN3tlolldl Fole5t U51)A Selvlce Iic,t it-in this case, a regolith cover tll~tis thlclzen~ng \pl lnp, Arlz,~n\,~\71 902, (J ? A or thinning tow'lrd some steady state th~clzncss. jTlrc ~ourii.11ot (;i~~logy,LOO5, voitiiirc 11.3, 1,. ~12i-.34O]LOOi hy Tlic Uiiiver\lty 01 (:lilc:igo All rigl~t\reservcd 007L-17h/L005/l 1.30.3-OOOiSl5.00 Nonecluilibrium systems are inherciltly dyiiaimi- (1997, 1999, 2001), who present methods for testliig cally ~iiist~ihleor dominated by treclueiit cfistur- ecluil~hriumthiclzness. Tliey presciit radioiiucl~de bance and do iiot develop a steady state eclu~ltb- data that lndlcdte a11 expo~~ei~ti~~lcfecl~ne of soil rium. Noiiecluilihriuiii regol~thtli~clzncss would be proctuction with depth (consisteilt with the reriew'il ch~~racterizedby thiclzeiling or thinil~iigtliLit can of exposure concept) and show tli~tsoil thiclzness conrtinue until Iimitect by external f'ictors, with iio varies iiivcrsely wit11 slope curvature. Carter and particuldr tc~idencyto m'iintain constant thick- Ciollzosz (1991)touiic1 that the tliiclziiess of surfici,il ness. In the sp,iti,~l ciomarn, this is in'mifestcd 2s (0,A, met E) liorizons was inr related to slope 111 variable tliiclzness withln areas of s~mil'lrgeology, soils eleveloped on s'it~dstonein Pen~lsylv~~iiiamet topogr,zplly, aiid other environmental coiltrols concluded tli'it the rate of soil formation exceeds The concept of cc~uilihriumregolitli or soil thiclz- the rate of crosloil This 1s tinplicitly based on a ness is expl~citor ii~lplicit111 iilost of the best- 11otio11 of sol1 thiclzness as 'I function of rcites of lznown models of l'iiidscapc evolution. At ledst as foriiiatioi~versus removal, altliough the authors far b'iclz as Davis 11 892) ,inel Gilbert (1909), it has dlso speculate that effects of erosion oil steep slopes been suggestect thdt weathering r,itcs decline with are inaslzed by laterC1l movement of throughflow soil thiclziiess and that hillslope transport flux 1s and by the effects of tree throw. Siiliilar conclusions proportional to slope gradient. Penclz (1924)more were re'1chcd by L~sctslzi~(1999) on the basis of explicitly cfescrihed such ;I situation 111 111s discus- studies of dated surfaces in the Russ~~tnsteppes. sion of the "renewal of exposure" concept, whereby Heliiisath and others (1999)postulate that if the the production of weathered debris is inversely re- rate of bedrock conversion to iiiobile regolith is a lated to the th~clznessof the regolith cover. Where fuilction of local soil thiclzness, then on ~ui~forni conditions allow regolith to accumulate, eventu- bedroclz, a h~llslopeapproaching dynamic ecjuilib- ally a steacty state coilditioil will be reached wliere riuiil should have a un~forinso~l/regolltl~ mantle. debris proctuct~oii,lt the weatl~er~ngfront is hal- Conversely, they iilaiiitaiil that variations in soil 'iiiced by erosional rei-~~ovdlsat the surf'lce (Penclz depth would produce variations 111 so11 production 1924).Several hillslope dild regol~thevolutioi~ mod- and a non- or d~secluillhr~ui~~coi-~clition. The notion els arc baseel on the production of i~l~~terialhy of feedbCiclzsbetween regolith thiclzness and weath- weathering, a decline in wcatlier~ngratcs ,is rcgo- cilng rates produc~ng<I steady st'xte tli~clziiessa1lc1 11th gets th~clzer(sometimes ,is a siiilple ilegative spatial un~forii~ityis also supported hy S111all et '11. exponeiltidl f~iictioil,soinetimes with r'ites peak- J 1999)and Aiiderson 12002).In the ternporal doiiiaiii 11-11: ~indera relatively thin soil or regolith cover), Jor, inore re~listlcally,In stratigraphic interpreta- slope transport by soil creep, and fluv~altrailsport tions of the regolith), '1 stedcty state-weatheringpro- Je.g., Young 1963; Ahnert 1976; Armstrong 1980) file cl~ar~icterizedhy a balailce of croslon dllc1 Perhaps the best-lznowii expression of th~sconcep- weathcriiig rates liiay appear to he ~inchangiiig(01- tual frciineworlz is that of C;irsoii aiicl Kirlzby (1972); lier and Pal11 1996; Pdiii and Ollicr 1996).The feed- their frameworlz contii-~~iesto be eiliployed In va- h'ick between soil or regolith th~clzness and ious forins with numerous elaborat~oils'~ild wlth we'lthering rates, gencr'11ly ch'~r'~cterizedby an ex- ditferent i~~iner~cdlsol~tioil schemes up to the ponenti'll decline in weathering with thiclzcr reg- present (e.g., D~etrichct '11. 1995; He~ms~itliet al. ol~ths,has beell coilfirmed by sevcrcil studies using 1997, 1999, 2001; M111,lsiiy aiid McRratiiey 1999; cosmogenic radioiiucl~dcs(Hcii~zsatli ct '11. 1999, Furb~sliand F<igherazzl200 1). 2000; Rraun et al. 2001; Anderso~i2002), thougli Steady state soil thickness is cilso implicit in the results differ on whether a critical thresl~oldthiclz- coilcept of ix'lture zonal soils developed by Dolz- ness 1s rccluired before the rel~tionshipholds. ucliacv 11883) and expounded oil by suhsecluent WcCithc~iilgrates may also decline wltll soil dge generations of pcdologists. Th~sgeneral phenoinc- JMarlzewich et '11. 1989; T'iylor dild Klu1-11 1995; non is described '1s self-regulatloii by Lisctslzi~ Blrlzel'iiid 1999), c~ltliougli~t is iiot clear to what J 1999). Altliougli L~setslzi~'~work (1999, p. 109 1) extent the decline is rclatccf to so11 thiclziiess as focused on liumus accumul~ition,it glves evidence opposed to other factors such ,is dep1et1oi-1of weath- for "ii~orec~~tive SOII forillation 111 co~iditioi~sof crable minerals. den~idation" 'iild st'ites that "during the initial Although thcorct~c'illandsc,lpc cvo1utio1-1moctels stages of so11 development, soil-foriniiig processes and so11 zonalism concepts m,iy lillply tli'it ~ii1111- are iioiiecluilihriuin." Equilihriuin regolith/soil terruptcd pccfogeiiesis le'~dsto a steacfy state soil, thiclzness ~llar~icterizedby ,I b,il'~nce hetween soil disturbances ciilcf iiiterr~~ptioiisare ~~clz~~(~wleetg~d proctuction and erosion 'liid tu~iedto topography to occur, anct none of the thcortes or models ex- provldes the basis for the work of Hcimsatli ct al.
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