5.18 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime G.J.Retallack University ofOregon,Eugene, OR, USA

5.18.1 INTRODUCTION 581 5.18.2 APPROACHESTOTHE STUDY OF PALEOSOLS 582 5.18.2.1 MolecularWeatheringRatios 583 5.18.2.2 StrainandMass TransferAnalysis 584 5.18.2.3 AnalysesofStableIsotopesofCarbon andOxygen 584 5.18.3 RECORD OF PAST SOIL AND GLOBAL CHANGE 586 5.18.3.1 Origins ofSoil 587 5.18.3.2 Archean–Paleoproterozoic GreenhousePaleosols 588 5.18.3.3 Proterozoic IcehousePaleosols 589 5.18.3.4Cambro-OrdovicianGreenhousePaleosols 590 5.18.3.5TerminalOrdovicianIcehousePaleosols 591 5.18.3.6 Siluro-DevonianGreenhousePaleosols 591 5.18.3.7 LateDevoniantoPermianIcehousePaleosols 593 5.18.3.8Triassic–Jurassic GreenhousePaleosols 594 5.18.3.9 Early CretaceousIcehousePaleosols 595 5.18.3.10 Cretaceous–PaleogeneGreenhousePaleosols 595 5.18.3.11 NeogeneIcehousePaleosols 597 5.18.3.12 PleistoceneGlacialandInterglacialPaleosols 597 5.18.4SOILS AND GLOBAL CARBON CYCLE CHANGES 599 ACKNOWLEDGMENTS 600 REFERENCES 600

5.18.1 INTRODUCTION fixation andchemicalreduction ofatmospheric CO2 into plants andplantlike microbes,which are Soils playanimportant roleinthe carbon cycle atthe baseofthe foodchain. Plants andphoto- asthe nutrition ofphotosynthesizedbiomass. synthetic microbesareconsumed andoxidized by Nitrogenfixed bymicrobesfrom airisalimiting animals,fungi,andotherrespiringmicrobes, nutrient for ecosystems withinthe firstflush which releaseCO 2 ,methane, andwatervapor to ofecologicalsuccession ofnewground, andsulfur the air. Thesegreenhousegasesabsorbsolar canlimitsomecomponents ofwetlandecosystems. radiation moreeffectively thanatmospheric oxy- But overthe longterm,the limitingsoilnutrient genandnitrogen,andareimportant regulators of isphosphorus extracted byweatheringfrom planetary temperatureandalbedo(Kasting, 1992). minerals such asapatite(Vitousek etal.,1997a; Variations insolarinsolation (Kasting, 1992), Chadwick etal.,1999). Life hasanespecially mountainous topography(Raymo andRuddiman, voracious appetitefor common alkali(Naþ and 1992),andoceancurrents (Ramstein etal.,1997) 2 2 K þ )andalkalineearth(Ca þ andMg þ )cations, also playaroleinclimate, but thisreviewfocuses supplied byhydrolytic weathering, which isin on the carbon cycle.The carbon cycleisdiscussed turn amplified bybiologicalacidification indetailinVolume8ofthisTreatise. (Schwartzmann andVolk, 1991;see Chapter5.06). The greenhousemodelfor globalpaleoclimate Thesemineralnutrientsfuelphotosynthetic hasprovenremarkably robust (Retallack, 2002),

581 582 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime despitenewchallenges(Veizer etal.,2000). The hemisphere(Keeling etal.,1982; Siegenthaler balance ofproducers andconsumers isoneofa andSarmiento,1993; Stallard, 1998). Soilorganic numberofcontrols on atmospheric greenhouse matterisabig, rapidly cyclingreservoir,likely to gasbalance, becauseCO 2 isadded to the air include much ofthismissingsink. from fumaroles,volcanic eruptions,andother Duringthe geologicalpast,the sizesof, and forms ofmantledegassing(Holland, 1984). fluxesbetween,thesereservoirs havevaried Carbon dioxide isalso consumed byburialas enormously asthe worldhasalternated between carbonateandorganic matterwithinlimestones greenhousetimesofhigh carbon content ofthe andothersedimentary rocks; organic matter atmosphere, andicehousetimesoflow carbon burialisanimportant long-term control on CO2 content ofthe atmosphere.Oscillations inthe levels inthe atmosphere(BernerandKothavala, atmospheric content ofgreenhousegasescanbe 2001). Themagnitudesofcarbon pools and measured, estimated, or modeled on all timescales fluxesinvolved provide aperspectiveon the from annualtoeonal(Figure2). Theactively importance ofsoils compared withothercarbon cyclingsurficialcarbon reservoirs arebiomass, reservoirs (Figure1). surface oceans,air,andsoils,so itisnosurprise Beforeindustrialization,therewasonly 600 Gt thatthe fossilrecordoflife on Earthshows strong 15 linkage to globalclimatechange (Berner,1997; (1Gt 10 g)ofcarbon inCO2 andmethanein the atmosp¼ here, which isabout the sameamount AlgeoandScheckler,1998;Retallack, 2000a). asinall terrestrialbiomass,but less thanhalfof Thereisanadditionallineofevidence for past the reservoirofsoilorganic carbon. Theocean climatic andatmospheric historyinthe form of contained only , 3Gtofbiomass carbon. The fossilsoils,or paleosols,nowknowntobe deepoceanandsediments comprised the largest abundant throughoutthe geologicalrecord reservoirofbicarbonateandorganic matter,but (Retallack, 1997a, 2001a). Thischapteraddresses thatcarbon hasbeenkept out ofcirculation from evidence fromfossilsoils for globalclimate the atmospherefor geologicallysignificant change inthe past,andattemptstoassess the periodsoftime(Schidlowski andAharon,1992). roleofsoils incarbon cyclefluctuations through the longhistory ofour planet. Humans havetapped undergroundreservoirs of fossilfuels,andour otherperturbations ofthe carbon cyclehavealso beensignificant (Vitousek etal.,1997b;see Chapter8.10). Atmospheric increaseofcarbon inCO to 2 5.18.2 APPROACHES TO THE STUDY 750GtCbydeforestation andfossilfuelburning OF PALEOSOLS hasdrivenongoingglobalwarming, but isnot quitebalanced bychangesinthe othercarbon Many approachestothe studyofpaleosolsare reservoirs leadingto search for a“missingsink” unlike thoseofsoilscience, andmorelike soil ofsome1.8 ^ 1.3 GtC, probably interrestrial geochemistry prior to the earlierpart ofthe organisms,soils,andsediments ofthe northern twentiethcentury (Thaer,1857; Marbut,1935).

Figure1 Pools andfluxesofreduced carbon (bold)andoxidized carbon (regular) inGtinthe pre-industrialcarbon cycle(sourcesSchidlowski andAharon,1992; SiegenthalerandSarmiento,1993; Stallard, 1998). Approachestothe StudyofPaleosols 583 5.18.2.1 MolecularWeatheringRatios Soilformation (see Chapter5.01)isnot only a biologicalandphysicalalteration ofrocks,but a slow chemicaltransformation followingafew kindsofreactions thatseldom reach chemical equilibrium. Inmany soils,the most important of thesereactionsishydrolysis:the incongruent dissolution ofminerals such asfeldspars to yield clays andalkaliandalkalineearthcations in solution. Auseful proxy for the progress ofthis reaction insoils andpaleosolsisthe molarratioof alumina(representingclay) to the sum oflime, magnesia,soda, andpotash(representingmajor cationic nutrients lost intosoilsolution). Alarge databaseofNorthAmericansoils (Sheldon etal., 2002)hasshownthatthisratioisusually less than 2for fertilesoils (Alfisols andMollisols ofSoil SurveyStaff, 1999),but morethan2inless fertile soils (Ultisols). Insoils thathavebeendeeply weathered inhumid tropicalregions for geologi- cally significant periodsoftime(Oxisols ofSoil SurveyStaff, 1999),the molarratioofaluminato basescanreach100 or more, indicatingthatthe slow progress ofhydrolysishasalmost goneto completion. Application ofthisapproach to aPrecambrian (1,000 Ma)paleosol from Scotland(Figure3) showed the expected decreaseofhydrolytic weatheringdownfromthe surface, andan overall degree ofhydrolytic alteration thatis modest compared withdeeplyweathered modern soils (Figure4). Effects ofhydrolysisofthis Precambrianpaleosol canalsobe seeninpetro- graphicthinsections andelectron microprobe analyses,which document conversion offeldspar into clay(Retallack andMindszenty,1994). Othermolarweatheringratios canbe devised to reflectleaching(Ba/Sr),oxidation (FeO/Fe2 O 3 ), calcification (CaO MgO/Al 2 O 3 ),andsaliniza- tion (Na O/K O). þ Two oftheseratios reflect Figure2 Variation inatmospheric CO2 composition 2 2 on avariety oftimescalesrangingfrom annualtoeonal differentialsolubility ofchemicallycomparable ((a)sourceKeeling etal.,1982;reproducedbypermission elements,but calcification ratioquantifiesthe of(b)MacmillanJournals from Nature , 1999, 399, accumulation ofpedogenic calciteanddolomite, 429–436; (c)MacmillanJournals from J.Geol., 2001d , andthe ratioofironofdifferent valence gives 109,407–426; (d)AmericanJournalofScience from reactant andproductofironoxidation reactions. In Am. J.Sci., 2001, 301,182–204). the Precambrianpaleosol illustrated (Figure4), thesemolarratios indicatethatthe profilewas oxidized andwell drained, but littleleached, Such measuresofsoilfertility ascation exchange calcified or salinized. capacity andbasesaturation thatareused for Advantagesofusingmolarweatheringratios characterizingsurface soils (Buol etal.,1997)are aretheirsimplicity andprecision,free ofassump- inappropriatefor the studyofpaleosolsbecauseof tions concerningparent materialcomposition and profoundmodification ofthe cation exchange changesinvolumeduringweatheringandburial complexduringburialandlithification ofpaleo- compaction. Smoothdepthfunctions ofmolar sols (Retallack, 1991). Many paleosols arenow weatheringratios (Figure4)arecharacteristic of lithified andamenableto studyusingpetrographic soils andpaleosols,whereasparent material thinsections,X-raydiffraction,electron micro- heterogeneity isrevealed byerratic swingsin probe,andbulkchemicalanalysis(Holland, 1984; weatheringratios.Whole-rock chemicalanalyses Ohmoto,1996; Retallack, 1997a). arecommonly used to calculatemolarweathering 584 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime changesrelativeto zirconium duringsoil development andburialofthispaleosol,but is not atvariance withthe simplermolarweathering ratioapproach, which includesapartialnormali- zation to alumina. Limitationsoncalculatingstrainandmass transfercomemainlyfrom the identification and characterization ofthe parent materialofsoils and paleosols. Theactualmaterials from which they weathered no longerexist (Jenny,1941).The natureofparent materials canbe reconstructed by studyingthe rock or sediment lowerwithinsoilor Figure3 Sheigrapaleosol (bleached andreduced paleosol profiles. Parent materialreconstruction zone , 1mthick to right) underTorridonian(1,000 Ma) canbe checked chemically andpetrographically alluvialfandeposits,andStaca paleosolatsame for degree ofweatheringinigneous or meta- unfoncormity but on amphibolite(left-handside)near morphic rocksbelow asoil(Figures3and4),but the hamletofSheigra, northwest Scotland(photo courtesy ofG.E.Williams; geologicalage revised by isnot so easily assessed insediments or colluvium Williams andSchmidt,1997). belowasoil. Itisdifficult to ruleout soilformation from athinsedimentary or colluvialcaptoan igneous rock, although large influxesofnew ratios,andthus conflateweathered parts ofthe soil materialfrom wind, floods,or landslide will be withunweathered interiorsofsoilgrains. This revealed bypositivestrainvalues. problemcanbe circumvented bycalculating Kinetic modelingapproaches(Merino etal., molarweatheringratios from electron microprobe 1993)canbe applied to isovolumetric weatheringif spot analysesofweathered andunweathered conservation ofvolumeissupported bytextural grains withinpaleosol samples,which canillus- evidence (Delvigne, 1998). Computer-aided tratereaction paths(BestlandandRetallack, thermodynamic modelingofancient weathering 1993). hasalsoprovenuseful,especially for Precambrian paleosols (Schmitt,1999).

5.18.2.2 StrainandMass TransferAnalysis Afull accountingofvolumeandchemical 5.18.2.3 AnalysesofStableIsotopesofCarbon changesduringweatheringandburialcanbe made andOxygen byassumingthatonecomponent ofapaleosol has remained stablefrom the parent material. This Three isotopesofcarbon arecommonly assayed 12 methodrequiresmeasurement ofbulkdensity and bymass spectrometer:the common isotope C, the rareisotope 13C, andthe radiogenic isotope identification ofaparent material. Alumina, 14 titania,andzirconium arecommonly used as C.Radiocarbon isformed inthe atmosphere, is stableconstituents,withzirconium favored incorporated withinplantsandanimals,andis becauseits mobility canbe checked bymicro- thenfossilized asaconstituent ofcarbonatesand scopic examination ofpittingofgrains ofzircon, organic carbon. The progressiveradioactivedecay which isthe mainsoilmineralcontaining ofradiocarbon isused for isotopic dating, but zirconium (Brimhall etal.,1991). The assumption unfortunately its abundance decreasestoundetect- 5 ofgeochemicalstability allowsoneto calculate ableamounts after , 10 yr. Incontrast,the stable 12 13 volumelossesorgains (i.e.,strain) ofsamples isotopes Cand Carefoundinrocksand from aparent composition andmateriallosses paleosols ofall geologicalages. Theirrelative or gains (i.e.,mass transfer) ofindividual abundance iscommonly reported on ascaleof chemicalelements from asoilorpaleosol (see permilthatreflects theirratios normalized to a Chapter5.01). Thisformulation ofstrainis standard, afossilbelemnitefrom the Peedee especially useful for paleosols,becausesome Formation ofNorthCarolina(PDB),or the mean component ofstrainisdueto burialcompaction, valueofmodern oceanwater(SMOW). These which canbe expressed visually (Figure5). carbon isotopic values(d 13C)areaffected bya ThePrecambrianpaleosol illustrated asan variety ofphysical,chemical,andbiological exampleofthisapproach shows moderateweath- processes. The keyphotosynthetic enzymeof eringandvolumeloss withweatheringandburial plants,Rubisco,selects the lightisotope( 12C) compaction. Most elements werelost from the preferentially to the heavy isotope( 13C),so that profile, except potassium,andinone(but not plant organic matterisisotopicallymuch lighter anotheradjacent) paleosol,iron (Figure5). This ( d 13Cmorenegative)thanthe atmospheric or represents athorough geochemicalaccountingof oceanic CO2 from which itwasderived. Approachestothe StudyofPaleosols 585

Figure4 Columnarsection (measured infield),petrographic composition (from point counting),andmolecular weatheringratios (from major element chemicalanalyses) ofthe Sheigrapaleosol (reproduced bypermission of Society for Sedimentary Geologyfrom J.Sedim. Res ., 1994 , A64 ,264–281).

Someplants employ aphotosynthetic pathway Jahren etal.,2001). Todaymost C 4 plants are creatingatfirst athree-carbon phosphoglyceric tropicalgrasses,andmost CAMplantsare acid (C 3 or Calvin–Benson photosynthesis). submergedaquatic plants anddesert succulents. Theseplants fractionateisotopesmoreintensely, Most otherkindsofplants usethe C 3 photo- andso havemorenegative d 13Cvalues(2 33‰ synthetic pathway. Thereisthe potentialto to 2 22‰ PDB)thanplantswhich usea recognizethesevarious metabolic pathways photosynthetic pathwaycreatingatfirst afour- from the isotopic composition oforganic carbon carbon malic andaspartic acid(C 4 or Hatch– inpaleosols andinfossilplants,andinthe Slackphotosynthesis: 2 16‰ to 2 9‰ PDB). fossils ofanimals which atethe plants(Cerling Crassulaceanacid metabolism (CAM)isyet etal.,1997; MacFadden etal.,1999; Krull and anotherphotosynthetic pathway,which creates Retallack, 2000). organic matterofintermediateisotopic compo- The isotopic composition ofcarbon incarbonate sition ( 2 35‰to 2 11‰ PDB). Methanogenic inpaleosols canalso be used asaCO 2 paleo- microbesareevenmoreextremeintheir barometer(Cerling, 1991). Underhigh atmos- 13 fractionation ofthe lightisotope( d C pheric CO2 levels isotopically heavy CO2 intrudes down to 2 110‰ andtypically 2 60‰ PDB; into soilpores,andcanbe fixed therebythe 586 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime

Figure5 Mass transferandstrainofthe Sheigrapaleosol. The stippled bars arearange ofvaluesfor likely strain dueto burialcompaction,asopposed to pedogenic strain(reproduced bypermission ofSociety for Sedimentary Geologyfrom J.Sedim. Res ., 1994 , A64 ,264–281). precipitation ofpedogenic carbonate.Incontrast, mighthavebeencontaminated byoverlying underlow atmospheric CO2 levels,CO 2 ofsoilair marinerocks. isisotopicallylight,becauseitisrespired with Oxygenisotopes, 16Oand 18 O, areusually relatively minorisotopic fractionation from reported inpermil(d 18 O)relativeto the same isotopicallylightsoilplant material,andmay, standardsused for carbon isotopes(PDB and therefore, generateisotopicallylightpedogenic SMOW). Oxygenisotopesarealsofractionated carbonate.Itisalso prudent to measurethe differently byC 3 andC4 plants becausethey isotopic composition oforganic carbon inthe contributeto the mass ofCO 2 takeninfor samepaleosol,asaguide to the isotopic com- photosynthesis(Farquhar etal.,1993). Oxygen position ofCO 2 inthe ancient atmosphere, because isotopic valuesarealsodetermined bythe thiscanvary substantially (Mora etal.,1996; composition ofwaterinsoil,cominginasrain, Jahren etal.,2001). APhanerozoic atmospheric andlaterflowingout asgroundwaterthrough buriedpaleosols (Amundson etal.,1998). Tem- CO2 curveconstructed from acompilation ofsuch data(Ekart etal.,1999)isconsistent with perature, degree ofevaporation,andsalinity strongly affectthe isotopic composition ofoxygen independent evidence ofCO 2 levels from the stomatalindexoffossilleavesduringall but a insurface water,andcanpotentially be inferred fewepisodesofcatastrophicmethane-clathrate fromthe isotopic composition ofoxygenin dissociation (Retallack, 2001b, 2002). paleosol carbonates(Mora etal.,1998),paleosol The attenuation ofatmospheric isotopic clays (BirdandChivas,1993),andfossils in valueswithinpaleosol profilescanalso be used paleosols (JahrenandSternberg, 2002). to estimateformersoilrespiration (Yapp and Poths,1994),sometimeswithsurprisingresults, such asthe near-modern soilrespiration rates 5.18.3 RECORD OF PAST SOIL AND inferred from the dramatic attenuation ofisotopic GLOBAL CHANGE values(d 13C)inaLateOrdovicianpaleosol (Figure6).Inthiscase, carbonateoccluded Paleosols havelongbeenrecognized inthe withinpedogenic goethitewasanalyzed,rather geologicalrecord(Hutton,1795;Webster,1826; thanpedogenic carbonateitself, becausethis Buckland, 1837),but theirgreatabundance in RecordofPast SoilandGlobalChange 587 terrestrialsedimentary sequenceswasnot appreci- The author prefers to follow the US National ated untilthe 1970s (Allen,1974;Retallack, AeronauticalandSpace Administration (NASA) 1976). Many variegated red beds,such asthe inusingthe widely understoodwordsoilfor OligoceneBig BadlandsofSouthDakota, are nonsedimentary modified surfacesofthe Moon volumetrically dominated bypaleosols (Retallack, andthe Mars. 1983).Almostall coalseams arepaleosols Whethertherewasorislife on Mars remains (Histosols),andthesearenot the only paleosols uncertain(McSween,1997). Thereisnodiscern- inthick coalmeasuresequences(Retallack, iblelife inlunarormartiansoils atthe timeofthis 1994a). Thousandsofpaleosolsofall geological writing, but thatmaychange withfuturehuman ageshavebeendescribed since the early 1980s, discoveriesandcolonization ofspace.If the Moon andthereisnow the prospectofusingthemto andthe Mars areconsidered to havesoils,thensoil interpretlong-term patternsofenvironmentaland formation goesback to the first alterations of biotic change. planetismalandplanetary surfaceswhich occurred inplace, asopposed to thosetransported to form sediments,which aredistinctandantithetic to soil formation. Thus defined, bothsoils andsediments 5.18.3.1 Origins ofSoil arevery ancient. Soils,like loveandhome, aredifficult to define Hydrolytic alteration ofmafic minerals (pyrox- precisely. If onefollowssomesoilscientists in eneandolivine)toclays (iron-rich smectite), definingsoilasamedium ofplant growth(Buol oxides(magnetite),carbonates(gypsum,calcite), etal.,1997),thenthe formation ofsoils began andsalts(kieserite)hasbeendocumented in eitheratthe Silurianadvent ofvascularlandplants carbonaceous chondritic meteorites(Bunch and (Gray,1993),or atthe Cambrianadvent of Chang, 1980; Volume1ofthisTreatise). Carbo- nonvascularlandplants (Strother,2000),or at naceouschondritesalsoshow opaqueweathering the LatePrecambrianadvent ofeukaryotic soil rindsaroundmafic grains,cross-cuttingveins algaeor algalphycobiontsoflichens (Retallack, filled withcarbonate, clayskins,anddistinctive 1994b;SteinerandReitner,2001). Ageological clayeybirefringence fabrics(sepic plasmic fabric; viewofsoils,however,wouldinclude rocksand Retallack, 2001a). Carbonateveins havebeen soils altered byhydrolytic weathering, which has dated radiometrically atnomorethan50Ma beenwell documented atleast asfarback asthe youngerthanenclosingclayeymeteoritesdated at Archean(2,800 Ma;RyeandHolland, 1998). 4,566 Ma (Birck andAlle`gre, 1988;Endress etal., Hydrolytic weatheringhasalso beenproposed for 1996). Carbonaceous chondritesaresimilartothe rocksasoldas3,500 Ma (Buick etal.,1995),and surface ofsomeasteroids(Veverka etal.,1997). meteoritesasoldas4,566 Ma (Retallack, 2001a). Oneinterpretation ofcarbonaceous chondritesis

Figure6 Isotopic composition ofcarbon incarbonateandmolefraction CO3 ingoethiteofaLateOrdovician paleosol from Iron Knob, Wisconsin,showingsurprisingly high soilrespiration from shallow distance ofadjustment ofsoiltoatmospheric values(source Retallack, 2000b;reproduced bypermission ofPaleontologicalSociety). 588 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime asfragments ofpaleosols from asteroid-sized Precambrianpaleosolsreviewed byRyeand planetesimals formed early duringthe formation Holland(1998),thesepaleosols revealthe anti- ofthe solarsystem. Asprimevalsoils ofthe solar quity andthoroughness ofhydrolytic weathering systemtheyweresimilarintheirsmectites,salts, duringthe Precambrian. Eventhen,rock and andcarbonatestothe soils ofMars,which are sediment wereunderrelentless acid attack,which 2 2 probably paleosols relictfrom atimeoffree water leached basecations (especially Ca þ ,Mg þ ,and atthe martiansurface untilatleast 2,500 Ma Naþ ),andleftthick, clayeysoil. (Retallack, 2001a). Itislikely thatatleast back to 3,500 Ma the Asecondinterpretation ofcarbonaceous chon- principalenvironmentalacid drivingthishydro- dritesisasprimary condensatesofthe solar lytic reaction wascarbonic aciddissolved inrain nebula.Bythisview,theirhydrolytic alteration is waterandgroundwater(Holland, 1984),asisthe dueto meltingincometary nuclei duringclose caseinsoils today(Nahon,1991). Much soilCO2 passeswiththe Sun,or dueto transient heating mayalso havecomefrom respiringorganisms, events byshock wavesorcollisions(McSween, which alsocouldhavecontributed organic acids. 1999).Othercarbonaceous chondritesshow Nitric andsulfuric acid mayhavebeenlocally metamorphic alteration withminerals similarto important insoils developed on particularparent thoseinEarthformed duringdeepburialunder materials,but nitrogenandsulfur salts areso far elevated temperaturesandpressures(Brearley, unreported inPrecambrianpaleosols,unlike 1999). Like soils andpaleosols on EarthandMars, modern soils ofminedumps (Borden,2001), carbonaceous chondritesdemonstratethe great andhypothesized modern soils on Mars (Bell, antiquity ofhydrolytic weatheringindiluteacidic 1996; Farquhar etal.,2002),andVenus(Barsukov solutions,presumably ofcarbonic acid derived etal.,1982; Basilevsky etal.,1985). from watervapor andCO 2 .Theseremainthe Thisviewofthe likely acidsinvolved in principalgasesreleased from volcanoes,andsoils creatingPrecambriansoils on Earthissupported remainimportant buffers for thisenvironmental bythe isotopic composition ofcarbon,nitrogen, acid. andsulfur insedimentary organic matter,carbon- ates,sulfates,andsulfides,which aresurprisingly similartotheirmodern counterpartsback to 3,500 Ma, andunlike meteoritic or mantlevalues 5.18.3.2 Archean–Paleoproterozoic Greenhouse (Schidlowski etal.,1983; DesMarais,1997; Paleosols CanfieldandTeske, 1996). DespitepredictionsthatPrecambrianpaleosols Evidence for life inPrecambriansoils comes wouldbethin,rocky,anddominated byphysical from isotopic studiesoforganic carbon within weathering(Schumm,1956),hundredsofPre- paleosols. Microlaminated chips inthe 2,765Ma cambrianpaleosols now described havebeen Mt. Roepaleosol ofWestern Australia have foundto be thick, clayey,deeply weathered, and extremely depleted carbon isotopic compositions 13 insomecaseswithpossibletracesoflife, so that ( d C org 2 40‰). Isotopic fractionation ofcarbon chemicalandbiologicalweatheringcanbe traced to thisdegree isonly known inmethanogens and back almost to the beginningofthe suitably methanotrophs(RyeandHolland, 2000). These preserved sedimentary rock recordon Earth(Rye chips couldbefragmentsofpondscum ratherthan andHolland, 2000). The oldest knownprofile atruesoilmicrobiota.Organic matterinthe interpreted to be apaleosol showsalteration to 2,560 Ma SchagenpaleosolsofSouthAfrica isnot 13 depthsof50m on granitesunconformably nearlyasdepleted ( 2 16‰ to 2 14‰ d C org )as underlyingthe 3,500 Ma sedimentary succession organic matterinoverlyingmarinesediments 13 ofthe WarrawoonaGroup innorthwestern ( 2 35‰to 2 30‰ d C org ). Interpretation ofthe Western Australia (Buick etal.,1995). Corundum carbon inthispaleosol asthe signatureofa oresinthe 3,500 Ma AldanSchists ofSiberia have hypersalinemicrobialsoilcommunity iscompa- beeninterpreted asmetamorphosed,deeply tiblewithshallow dolocretesandotherfeaturesof weathered bauxites(Serdyuchenko,1968). the paleosols (Watanabe etal.,2000). TheJericoDampaleosol ofSouthAfrica Normalisotopic valuesfor soilorganic matter 13 (3,000 Ma;Grandstaff etal.,1986),the Pronto ( 2 25‰to 2 27‰ d C org )havebeenreported paleosol ofCanada (2,450Ma;Mossmanand from Precambrianpaleosols aswell (Mossman Farrow,1992),the Hokkalampipaleosol of andFarrow,1992; Retallack andMindszenty, Finland(2,200Ma;Marmo,1992),avariety 1994). Virtuallyall Precambrianpaleosols havea ofpaleosols associated withthe Hekpoort Basalt very low content oforganic carbon comparable ofSouthAfrica(2,100 Ma;YangandHolland, withto thatofwell-drained paleosolsofthe 2003),andthe Sheigrapaleosol ofScotland Phanerozoic.If life had beenpresent inthe Early (1,000 Ma;Figures3and4)havebeensubjected Precambrianpaleosols,theywouldhavebecome to exceptionallydetailedgeochemicaland carbonaceous inthe absence ofadecomposing petrographic analyses. Alongwithmany other microbiotaofactinobacteria andoffungi and RecordofPast SoilandGlobalChange 589 metazoans duringthe laterPrecambrian. Isotopic 5.18.3.3 Proterozoic IcehousePaleosols evidence thus suggeststhatmethanogenic, hyper- saline, normal,anddecompositionalmicrobes The oldest knownperiglacialpaleosolsare werepresent inPrecambrianpaleosols. Other from the 2,300 Ma to 2,400 Ma RamsayLake evidence for life inPrecambrianpaleosols Formation ofOntario,Canada (YoungandLong, includesmicrofossils (1,300Ma;Horodyski 1976; SchmidtandWilliams,1999). Theyhave andKnauth, 1994),microbialtrace fossils prominent ice wedges,which arestrongly taper- (2,200Ma;Retallack andKrinsley,1993), ingcracksfilled originally withice,but now chemofossils (2,900Ma;Prashnowskyand withmassiveor horizontally layered sandand Schidlowski,1967),plausiblemegafossils claystonebreccia.Modern ice wedgesform in (2,900Ma;Hallbauer etal.,1977;Retallack, climateswithameanannualtemperatureof 1994b),andthe impressivethickness andsoil 2 4 8 Cto 2 8 8 C, coldest monthtemperaturesof structureofPrecambrianpaleosols (3,500 Ma; 2 25 8 Cto 2 40 8 C, warmest monthtemperatures Retallack,1986,2001a;Buick etal.,1995; of10–20 8 C, andameanannualprecipitation of GutzmerandBeukes,1998;Beukes etal., 50–500 mm (Williams,1986; Bockheim,1995). 2002). Life andits by-products such aspoly- Periglacialpaleosols ofthe LatePrecambrian saccharidesmayhavebeensoil-binders like (600–1,000 Ma)inScotland, Norway,andSouth molassesapplied to acornfield(Foster,1981), Australia include sandwedges(Figure7),which protectingsoils from physicalweatheringso that indicateanevendrierandmorefrigid climate: a biochemicalweatheringcouldproceed. meanannualtemperatureof 2 12 8 Cto 2 20 8 C, a The likely existence ofmicrobialmats atthe meancold-monthtemperatureof 2 35 8 C, amean soilsurface considerably complicatesthe useof warm-monthtemperatureof48 C, andmean paleosols asindicators ofancient atmospheres annualprecipitation of100mm(Williams,1986). (Ohmoto,1996). Tropicalrainforest soils now Someofthe LatePrecambrianglaciations were remarkableinextendingto very low latitudes,as havesoilCO2 levels up to 110 timesthatofthe atmosphere, becauseofhighlevels ofsoil indicated bythe paleomagnetic inclination of respiration bytermitesandmicrobesofan glaciogenesediments,andhavebeendubbed abundant supply ofsoilorganic matterthat Snowball Earthevents(Kirschvink, 1992; forms inalivingmembraneseparatingthe subsoil Hoffman etal.,1998;SchmidtandWilliams, from the atmosphere(Brook etal.,1983; Colin 1999). BetweenandbeforethesePrecambrian etal.,1992). Nevertheless,modelingbyPinto and episodesofperiglacialpaleosols andassociated Holland(1988)makesitunlikely thatmicrobial glaciogenesediments thereisnoevidence offrigid scumsofthe Precambrianwereasproductiveand conditions,so thatthe alternation ofglobal effectivemembranesasrainforests. The observation thatPrecambrianpaleosols werechemically weathered to anextent compar- ablewithrainforest soils todayprobably indicates much higherlevels ofCO 2 inthe atmosphereat thattime(Holland, 1984). The extent ofthis greenhouseispoorlyconstrained, but the appar- ent lack ofsideriteinpaleosols such asthe Hekpoort andMt. Roepaleosols hasbeenused by Rye etal. (1995)toarguethatCO2 concen- trations couldnot havebeenmorethanca.100 timespresent levels beforethe riseofoxygenat ca.2,100 Ma. Sideriteiscommon inPhanerozoic wetland paleosols (Ludvigsen etal.,1998)inwhich respired soilCO2 exceeded thislevel. Thus,the estimateofRye etal. (1995)ofno morethan100 timespresent levels ofsoilCO2 also isacapon soilrespiration andbiologicalproductivity during the Precambrian(Sheldon etal.,2001).The contribution ofCH 4 to the atmospheric green- houseeffectwasprobably alsomuch higherthan atpresent,becauseitwasnecessary to maintain Figure7 Near-verticalsandstonewedge remaining planetary temperaturesabovethatofthe freezing from fill ofice wedge penetratingthe CattleGrid ofwateratthe timeofafaint youngSun (Kasting, Breccia (680Ma),inthe Mt. Gunson Mine, South 1992; Pavlov etal.,2000). Australia (photo courtesy ofG.E.Williams). 590 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime icehouseandgreenhousepaleoclimatesisancient (Retallack, 1994b). LatePrecambrian(600 Ma) indeed. enigmatic fossils,widely called “Twityadisks” Theseclimatic fluctuations couldbeattributed aftertheiroriginalnorthwest Canadian to changesinsolarluminosity,volcanic degassing, discovery site, areprobably microbialcolonies or oceancurrent reorganization withcontinental (Grazdhankin,2001),andsomehavebeenfound drift(Barley etal.,1997; Dalziel,1997),but inferruginized paleosols (Retallack andStoraasli, paleosolsrevealthattheseice ageswerealso times 1999). Latest Precambrian(550–540Ma)inter- ofchange inthe atmosphereandlife on land. glacialandpostglacialcircularfossils,widely Highly ferruginized pisolitic lateritic paleosolsfirst interpreted ascnidarianmedusae, havealso been appearinthe geologicalrecordat1,920–2,200 Ma reinterpreted aslichenized microbialcolonies inSouthAfrica(GutzmerandBeukes,1998; andarefoundinpaleosols(Retallack, 1994b; Beukes etal.,2002). The lateritic paleosols are Grazdhankin,2001; SteinerandReitner,2001). part ofacomplexerosionallandscapewitha Theappearance oflichens withtheirdeeply variety ofpaleosolsofsignificantly different reachingrhizinesinaworldofcyanobacterial geologicalages,includingmildly oxidized mats couldhavegreatly increased the rateof (Retallack, 1986; Maynard, 1992)andchemically biochemicalweathering, carbon sequestration, reduced paleosols(RyeandHolland, 1998). oxygenation ofthe atmosphere, andglobal Opinions differonthe natureandtimingof cooling(Schwartzmann andVolk, 1991). thisapparent oxygenation event.Holland(1984), HollandandBeukes(1990),andYangandHolland (2003) proposed anabruptrisefrom less than0.1% 5.18.3.4Cambro-OrdovicianGreenhouse byvolumeto morethan3%O 2 at , 2,100 Ma. Incontrast,Ohmoto(1996,1997) andBeukes Paleosols etal. (2002) arguethatthe GreatOxidation Event The most obviouswayinwhich Ordovician interpretation doesnot takeintoaccount the paleosols differfrom thoseofthe Precambrianis reducingpowerofbiologicalactivity within inthe localabundance ofanimalburrows. Precambrianpaleosols,andthatO 2 levels were Becauseburrows areknown inLatePrecambrian closeto present levels from3,000Ma to marinerocks,the mainprobleminestablishingthe 1,800 Ma.Anintermediateviewofrising, but presence ofanimals on landduringthe Ordovician fluctuatingatmospheric oxidation also iscompa- wastoprovethatthe burrows wereformed atthe tiblewithavailablepaleosol data(Retallack, sametimeasthe paleosols,andnotduring 2001a),andwithlimited evidence from mass- inundation beforeor aftersoilformation. This independent fractionation ofsulfur isotopes evidence cameinpart from petrographicstudies (Farquhar etal .,2002). ofsoilcarbonateinthe paleosols,which iscut by Oxidation ofthe atmosphereandsoils could someburrows andcuts across otherburrows havecomefrom lichens,possibly actinolichens, (Figure8). Thiscarbonateisalargely micritic consideringthe small diameteroftheirfilaments, mixtureofcalciteanddolomite, asiscommon in reported from the 2,900 Ma Carbon Leaderof pedogenic carbonates(Retallack, 1985). Compel- SouthAfrica(Hallbauer etal.,1977). Their lingevidence alsocamefromthe isotopic organic geochemicalandisotopic composition composition ofcarbon inthiscarbonate, which givesclearevidence ofaphotosynthetic com- wasisotopically too lighttohaveformed inaqua- ponent (PrashnowskyandSchidlowski, 1967). tic or marineenvironments(Retallack, 2001c). The potent greenhousegasCH4 wasproduced by Comparableburrows andtracksofmillipede- methanogens,detected isotopically inapaleosol like creatureshavenow beenreported inseveral dated at2,765Ma(RyeandHolland, 2000). Ordovicianpaleosol sequences(Johnson etal., Later,plausiblylichenlike andcarbon-sequestering 1994;TrewinandMcNamara, 1995;Retallack, organisms arerepresented byenigmatic, small 2000a),but thesewereprobably only asmall part (1 by0.5mm),encrusted, andellipsoidalobjects ofthe overallsoilrespiration ofOrdovician inthe 2,200 Ma WatervalOnderandcorrelative paleosols. Glomaleanfungi discovered inOrdo- paleosols(Retallack andKrinsley,1993; Gutzmer vicianmarinerocksofWisconsin(Redecker etal., andBeukes,1998). Alaterswingto greenhouse 2000)werealso part ofanactivecommunity of conditions couldbeinferred frommolecular microbialsoilrespirers. Burrowsarenot obvious sequence datafor aMid- to LatePrecambrian inthe LateOrdovicianIron Knobpaleosol of (1,458–966 Ma)originofascomycetefungi,after Wisconsin,but the short distance ofattenuation to 13 the originofalgae andbeforethe originof atmospheric valuesofCO 2 molefraction and d C metazoans (Heckman etal.,2001). Thisquestion valuesofcarbon ingoethiteofthatpaleosol isalso discussed inChapter5.06. (Figure6) indicatesoilrespiration ratescompar- Therehaslongbeenadebateabout plausible ableto thoseofmodern savannagrasslandsoils permineralized ascomycetesinthe 770Ma (Yapp andPoths,1994). Thisisremarkable, Skillogallee DolomiteofSouthAustralia becausethereareno clearroottracesin RecordofPast SoilandGlobalChange 591 inlatest Ordovician(Hirnantian) rocks. Periglacial paleosolsarebest documented inSouthAfrica, wherepatterned groundandsandwedgesare common inred bedsofthe PakhuisFormation (Daily andCooper,1976). The ice sheets extended overmuch ofAfrica (Ghienne, 2003). Thecausesofthisice ageareespecially enigmatic, becausevolcanic activity increased through the Ordovicianandthe continents were dispersed (BluthandKump,1991),thus working against coldLateOrdovicianpoles. Mass balance models makethe Ordovicianiceage seem particularlyenigmatic,becausetheypredict atmospheric CO2 levels 16 timesPAL (Berner andKothavala, 2001). Thismaybe anartifactof the 10 Ma spacingofdatapoints inthe model, blurringthe less than10Ma duration ofthe ice age thatisderived from carbon isotopic data (Brenchley etal.,1994). Studiesofcarbonate isotopic compositions from paleosolswithinthe glacialintervalareneeded to re-examinethis question. Also neededisanexamination ofpaleosols withinthisintervalfor evidence offossilmosses, which wouldhavebeenmoredeeply rooted than Figure8 Reconstructed ecosystemofthe Late liverworts andso haveaccelerated weatheringand OrdovicianPotters Mills paleosol from centralPenn- sylvania (reproduced bypermission ofPalaeontological carbon sequestration. RareLateOrdovicianmoss- Association from Phanerozoic TerrestrialEcosystems, like megafossils (Snigirevskaya etal.,1992)and 2000a , 6 ,pp. 21–45). spores(Nøhr-HansenandKoppelhus,1988) supportindicationsfromcladisticanalysis (Kenrick andCrane, 1997)for alatest Ordovician Ordovicianpaleosols,andpalynologicalevidence originofmosses. indicatesnomorethanacoverofliverwort-like plantstofeed such soilrespiration (Strother etal., 1996; Strother,2000). Primary carbon fixation by thesethinthalliwithshort root hairs couldnot 5.18.3.6 Siluro-DevonianGreenhousePaleosols havecreated aquantity ofbiomass or humus comparableto thatofmodern grasslands. Root tracesofvascularlandplants appearin Furthermore, organic-lean,red Ordovician Silurianpaleosols,but untilthe EarlyDevonian, paleosols containonly sparsereduction spotsand root tracesaresmall andshallow withinthe soilcarbonatenodules(Retallack, 1985;Driese profiles(Figure9(b)).The earliest known vas- andForeman,1992),indicatingmodest carbon cularlandplants ofthe MiddleandLateSilurian storageinsoilorganic matterandcarbonate lacked trueroots. Instead, theyhad stems thatran compared, e.g.,withmodern savannagrassland alongthe surface andjust beneaththe surface of soils (de Wit,1978). TheOrdovicianpaleosols the soilasrunners andrhizomesfurnished with studied so farshow unusually high soilrespiration, thinunicellularroot hairs (KenrickandCrane, consideringtheirprobablelow levels ofprimary 1997). Plant bioturbation insoils only extended productivity. Theyalsoformed atatimeestimated downtoafewcentimeters,but burrowsof from sedimentary mass balance models asthe millipedesreached moredeeply,andinsome steamiest greenhouseperiodofall Phanerozoic soils weremoreabundant thanplant traces time, with , 16 timesthe present atmospheric (Retallack, 1985). Inaddition to detritivorous levels ofCO 2 (BernerandKothavala, 2001). The andperhaps also herbivorous millipedes carbon budgetofknownOrdovicianpaleosols (Retallack, 2001c),LateSiluriansoilfaunas wouldhavecontributed to thisgreenhouse. includedpredatory centipedesandspiderlike trigonotarbids(Jeram etal.,1990).Fungal hyphae andsporesinSilurianandDevonian 5.18.3.5TerminalOrdovicianIcehouse rocksindicateproliferation ofchytridsandother Paleosols fungi (Sherwood-Pike andGray,1985; T.N.Taylor andE.L.Taylor,2000). Periglacialpaleosols,unknowninCambrianand Early Devonianpaleosols haveabundant traces EarlytoMiddleOrdovicianrocks,arefoundagain oftrueroots,includingwoodytaproots ofavariety 592 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime

Figure9 Early Paleozoic changesin(a)soildifferentiation asindicated byclaycontent (volumepercent) and alumina/bases(molarratio) ofthe most weathered horizon ofcalcareous red paleosols; (b)soilbioturbation as indicated byproportion oftransectinpaleosols occupied byroots or burrows (percent) andbymeasured rootingdepth (m); (c)atmospheric CO2 levels (PAL)calculated from asedimentary mass balance model; (d)maximum coalseam thickness andaverage thickness ofatleast 10 consecutiveseams (m); (e)diameteroffossilplant stems androots (m); (f)diversity offossillandplants (numberofspecies); (g)diversity ofsoilanimals (numberoffamilies) (reproduced by permission ofthe AmericanAssociation for the Advancement ofScience from Dinofest, 1997,pp. 345–359).

oflandplants (Elick etal.,1998). Root traces haveahigherclaycontent andaremoredeeply reached tens ofcentimeters down intopaleosols, weathered ofbasesthanSilurianorOrdovician extendinggreatly the depthofthe activerhizo- soils (Figure9(a)).Theyhaveisotopically lighter sphereandits associated mucigelofmicrobes. pedogenic carbon,closertothe isotopic compo- Amongthe numerous roots ofEarlyandMiddle sition ofcoexistingorganic carbon,thanSilurian Devonianpaleosols,the burrows ofsoilfaunaare andOrdovicianpaleosols(Mora etal.,1996; Mora less prominent (Figure9(b)).Devoniansoils also andDriese, 1999). RecordofPast SoilandGlobalChange 593 Withinthe parameters ofthe pedogenic car- Kiama, also insoutheastern Australia (Retallack, bonatepalaeobarometerofCerling(1991),these 1999b). dataindicatedecliningatmospheric levels ofCO 2 Milankovitch-scaletemporalvariation incli- from the Silurianinto the Devonian(Figure9(c)). mateandsea levelhaslongbeenrecognized in Consumption ofatmospheric CO2 byincreased cyclothemic sedimentation inNorthAmerican hydrolytic weathering, andburialofcarbon in paleotropicalCarboniferous marginalmarine limestoneandorganic matterduringthe Silurian sequences,andthisinturnhasbeenrelated andDevonianhasbeenwidely interpreted asan to ice-volumefluctuations on the southpolar instance ofatmospheric globalchange inducedby Gondwanasupercontinent (Rasbury etal.,1998; the evolution oflife (Retallack, 1997b;Berner, MillerandWest,1998). Full glacialcoalseams 1997; AlgeoandScheckler,1998). (Histosols) alternatingwithinterglacialmarine rocksareaclearindication ofthesechanges. Environmentalalternations offull-glacial,dry, calcareous,swelling-claysoils (Vertisols),and 5.18.3.7 LateDevoniantoPermianIcehouse interglacial,wet,decalcified, forest soils Paleosols (Alfisols) indicateaterrestrialcontribution to Periglacialpaleosolsandglaciogenesedimen- multimillenial-scalechange inatmospheric green- tary faciesunknowninSilurianandEarlyto housegasesandpaleoclimate(Retallack, 1995). MiddleDevonianappearinthe latest Devonian, ByMiddleDevoniantimethe evolution of andremainlocally common inCarboniferous and increasingly largerplantsculminated inthe Permianrocks,especially withinthe Gondwana evolution oftreeswithtrunksupto1.5mindia- supercontinent,thenpositioned nearthe south meter,which leaveobviouslarge root tracesin pole(Figure10; Krull,1999). Unlikeperiglacial paleosols(Driese etal.,1997),aswell asabundant paleosols ofthe OrdovicianandPrecambrian permineralized stumps andlogs(Meyer-Berthaud however,theseLatePaleozoic profilesinclude etal.,1999). MiddleDevonianpaleosolsarealso root tracesofwhatmusthavebeenfrost-hardy the oldest knownwithclay-enriched subsurface woodyplants. The earliest documented examples horizons (argillic horizons ofSoilSurveyStaff, oftundra(polarshrubland)vegetation havebeen 1999). The clayinmodern forest soils ispartly foundinpaleosolswithfreeze–thawbandingand formed byweatheringinplace, andispartly thufur moundsinCarboniferous glacigenesedi- washed down root holes,which taperstrongly mentary rocksnearLochinvarinsoutheastern downwardinforest trees. Evidence ofboth Australia (Retallack, 1999a). Taiga (polarforest) neoformation andilluviation ofclaycanbe seen paleosols withdiscontinuouspermafrost defor- inthinsectionsofDevonianforested paleosols mation arefoundinEarly Permianred bedsnear (Retallack, 1997b).

Figure10 Deepclastic dike inacoalofthe WellerCoalMeasuresofthe AllanHills,Antarctica, interpreted asinfill ofperiglacialpolygonalpatterned ground(E.S.Krull andhammerfor scale). 594 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime Latest Devonianpaleosols also include coals 5.18.3.8Triassic–Jurassic GreenhousePaleosols fromthe oldest woodypeats.Thinpeats of herbaceousplant remains such asthe Rhynie Greenhousepaleoclimatesrightfromthe Chert ofScotland(Rice etal.,1995)andthe very beginningofthe Mesozoic havebeen Barzass coalofSiberia (Krassilov,1981) are revealed bydiscovery ofdeeplyweathered foundinEarlyDevonianrocks,but bythe latest paleosols inearliest Triassic rocksofAntarctica Devonian(AlgeoandScheckler,1998)andinto (Figure11),which evenatthattimewasat the Carboniferous,woodycoals becamewide- paleolatitudesof65–778 S(Retallack andKrull, spread andthick (Figure9(d)). Carbon consump- 1999). Comparablemodern soils areUltisols(of tion byaccelerated weatheringinforest soils and SoilSurveyStaff, 1999)andAcrisols (ofFAO, carbon burialincoals arewidely acknowledged as 1988),which arenot foundeithernorthof48 8 N the likely causefor mass balance estimatesofLate latitude or southof40 8 S, andarerareoutside Paleozoic high atmospheric oxygenlevels (per- subtropicalregions. Greenhouseconditions atthis haps 35vol.%)andnear-modern CO2 levels timearealso indicated bystomatalindexstudies (Berner etal.,2000). Low Permianatmospheric offossilseed ferns (Retallack, 2001b)andbythe CO2 levels arealso confirmed bystomatalindex isotopic composition ofcarbon andoxygenin studies(Retallack, 2001b). Theseatmospheric marineandnonmarinecarbonateandorganic trendsandcoevalchangesinoceanic Mg/Ca ratio matter(HolserandScho¨nlaub, 1991). couldbeattributed to changesinvolcanic and The timingandmagnitude ofthisgreenhouse hydrothermalactivity,particularly atmid-ocean andisotopic excursion immediately atandafter ridges(StanleyandHardie, 1999). However,the the greatest mass extinction ofall timehas abundance ofEarlyPaleozoic pedogenic dolo- suggested acatastrophicreleaseofmethanefrom mite, but LatePaleozoic andNeogenepedogenic permafrost or marineclathratedeposits (Krull and calcite(Retallack, 1985, 1993),suggestsarolefor Retallack, 2000; Krull etal.,2000). Thereisno soils inthesechangesinoceanic ionic chemistry, othersource ofcarbon thatissufficiently large and aswell asinchangingatmospheric CO2 levels. isotopically depleted to createthe observed

Figure11 Geochemical(isotopic andmajor andtrace element) andpetrographic (point count) datafor adeeply weathered paleosol which formed atalatitude of708 Sinthe AllanHills ofAntarctica duringthe Early Triassic.This isthe Johnpaleosol,anUltisol (Sombrihumult),asindicated especially byits high ratios ofaluminato basesand barium to strontium,andits strongsubsurface enrichment inclay. Extremely lightcarbon isotopic valuesdeepinthe profileimply arolefor methanogenic methaneinthispostapocalyptic greenhouse(reproduced bypermission of GeologicalSociety ofAmerica from Geol. Soc.Am. Bull., 2000, 112,1459–1472). RecordofPast SoilandGlobalChange 595 negativecarbon isotopic anomaly. Releasemech- resins andothertoxins,andso rotted morerapidly anismsfor methanecouldhaveincludedmeteorite to createarichersoilhumusthanleavesof impacts,SiberianTraps volcanism,or continental conifers andcycadlike plants (Knoll andJames, shelfcollapse, which alsohavebeeninvokedas 1987).Erosion control andsoilhumification from causesfor extinctions atthistime(Hallamand newly evolved angiosperms mayhaveplayed a Wignall,1997). Really large life criseswerealso roleinEarly Cretaceouschilling. timesoftransient globalgreenhousesindicated bystomatalindexdata(Retallack, 2001b)at the earliest Jurassic (Pliensbachian),Early Jurassic (Toarcian),Mid-Jurassic (Bathonian), 5.18.3.10 Cretaceous–PaleogeneGreenhouse Early Cretaceous(Aptian),Mid-Cretaceous Paleosols (Cenomanian-Turonian),earliest Paleocene Anotherlongperiodofgenerallywarmer (Danian),andearliest Eocene(Ypresian). planetary climateswithoutevidence ofpolar Duringthe Early Mesozoic, atmospheric CO2 ice caps or periglacialpaleosols lasted from the minimaalso werehigh (atleast twice thatofthe Mid-Cretaceous to the latest Eocene.Mid- present),andthisgeneralandlong-term green- Cretaceous(Cenomanian) tropicalpaleosols housecalls for adifferent andnoncatastrophic (Ultisols andOxisols) areknownfrom South explanation.Paleosols andpermineralized wood Australia, thenat60 8 S(Firman,1994),andthe offorest ecosystems athigh latitudesprovide US,thenat458 N(ThorpandReed, 1949; evidence for thislong-term greenhouseduring Joeckel,1987; Mack,1992). The Mid-Cretaceous which no periglacialpaleosolsarerecorded greenhousewasunusually longandprofound, (OllierandPain,1996; Retallack, 2001a). The judgingfrom the stomatalindexoffossilginkgo Triassic appearance oflarge sauropoddinosaurs leaves(Retallack, 2001b). The volcanic activity such as Massospondylus and Plateosaurus, thatcreated the enormous Ontong-JavaPlateau togetherwithfootprintsandotherdinoturbation hasbeencited asacausefor thislong-term (Lockley,1991),andofavariety oftermiteand greenhouse(Larson,1991),but thereisanother ant nestsinpaleosols(HasiotisandDubiel, plausibleexplanation inthe co-evolution with 1995),wouldhaveeffectively increased the angiospermsofornithopoddinosaurs such as destruction ofwoodytissuesinandon soils Iguanodon,withtheirimpressivedentalbatteries (Olsen,1993). The effectofsuch evolutionary for processinglarge amounts offoliage.The innovation mayhavebeentodecreasecarbon feedingandtramplingefficiencyoftheselarge sequestration bylignininswamps,forests,and newlyevolved dinosaursmayhavefurther theirsoils. promoted the spread ofearly angiosperms with theirability to toleratehigherlevels ofdisturb- ance thanotherplants (Bakker,1985). Newly 5.18.3.9 Early Cretaceous IcehousePaleosols evolved ornithopoddinosaursandtheirtrackways areassociated withcarbonaceousandearly Fossilpatterned andhummockygroundreveal successionalpaleosols (Entisols,Inceptisols,and permafrost conditions duringthe Early Cretaceous Histosols),whereasarchaic sauropoddinosaurs (Aptian) sedimentsofsoutheastern Australia, andtheirtrackways remained associated withless which atthattimewasat66–768 Sandattached fertileandless carbonaceous paleosols (Aridi- to the Antarctic portion ofthe Gondwanasuper- sols)throughout the Cretaceous(Retallack, continent (Rich andVickers-Rich, 2000). Thisice 1997c). age doesnot appeartohavebeenasextensiveor Othertimesofunusually extensivetropical severeasthe Permo-Carboniferous or modern ice paleosolswerethe latest Paleocene(55 Ma; ages. Thisepisode ofplanetary coolingcoincides Taylor etal.,1992),latestEocene(35Ma; withadramatic evolutionary radiation offlower- Bestland etal.,1996; Retallack etal.,2000),and ingplants(Retallack andDilcher,1986; Truswell, MiddleMiocene(16 Ma;Schwarz,1997). These 1987; Crane etal.,1995). The keyevolutionary eventsarenotableasshort-lived ( , 0.5Myr) innovation offloweringplants wasanabbreviated spikelike warmingsinbothstableisotopic records life cycle, inwhich pollination,fertilization,and from the ocean(Veizer etal.,2000; Zachos etal., germination followed oneanotherinquick 2001) andstomatalindexstudies(Retallack, succession (WingandBoucher,1998). 2001b). The latest Paleocenewarm spikeis Early angiospermswerelargely confined to associated withsuch profoundcarbon isotopic weakly developed soils (Entisols) ofdisturbed lighteningthatitcanonly reasonably be attributed coastalandstreamsidehabitats,which they to the methanefrom isotopically lightmethane colonizedandweathered morerapidly clathratesfrom the oceanfloor or permafrost thanassociated conifers andcycadlikeplants (Koch, 1998). Short-term physicalforcingsare (Retallack andDilcher,1981).Angiosperm leaves thus alsorecordedinthe paleosol recordof wereless coriaceous andless well defended with paleoclimate. Figure12 Geochemicaldatafor the Siestapaleosol ofPlioceneage (3 Ma)from the MeyerDesert Formation atOliverBluffs,centralTransantarctic Mountains (reproduced by permission ofGeologicalSociety ofLondon from Geol. Soc.London J ., 2001, 158,925–935). RecordofPast SoilandGlobalChange 597 5.18.3.11 NeogeneIcehousePaleosols andEast Africahaverevealed abroad schedule oforiginandpaleoclimatic expansion of PeriglacialpaleosolsappearduringLate bunch andthensodgrasslands(Figure14). Miocenetime(8Ma)inAntarctica (Sugden etal., Theincreased organic carbon content,high 1995;Retallack etal.,2001),wheresoildevelop- internalsurface area,elevated albedo,andgreater ment issoslow thatsomesurface soils maybe of waterretention capacity ofgrasslandscompared comparableantiquity (Campbell andClaridge, withwoodlandsofcomparableclimatic regions 1987).Antarctic soilformation isnot only wouldhavebeenapotent force for globalcooling promoted bygroundice deformation,but includes asgrasslandsemergedto occupy almost aquarter the effects ofsalt accumulation andeolianmass ofthe current landarea (Bestland, 2000; addition inanextremely dry continentalfrigid Retallack, 2001d). Mountainupliftandocean climate(Figure12). currentsplayed aroleinNeogeneclimate TheLateMioceneisbest known for the change aswell (Raymo andRuddiman,1992; Messiniansalinity crisis,whenthe Mediterranean Ramstein etal.,1997),but thereremainproblems Sea becameadesert (Krijgsman etal.,1999). withthe timingandmagnitude ofcarbon Itwasalso asignificant timefor geographic and sequestration bythesephysicalmechanisms climatic expansion ofgrasslandbiomesandtheir (Retallack, 2001d). characteristic soils:Mollisols ofSoilSurveyStaff (1999) or Chernozems ofFAO (1988). Evidence for thistransformation intropicalregions comes from the dramatic change to aless depleted (less 5.18.3.12 PleistoceneGlacialandInterglacial negative)carbon isotopic composition ( d 13C)of Paleosols pedogenic carbonateandorganic matter,andof Overthe past million years large ice caps have the apatiteoffossilmammaliantoothenamel grown to engulfthe present-daylocation of attributed to the tropicalexpansion ofC4 grasses Chicagowithinmorethanakilometerofice (Cerling etal.,1997; MacFadden,2000). Thereis duringglacialmaxima, thenretreated to the alsoevidence from adaptations to grazingin current ice caps ofGreenlandandAlaska during fossilmammals (Janis etal.,2002),from traces interglacialtimesatMilankovitch scalefrequen- ofgrasslandinvertebratessuch asdungbeetles ciesof100 ka.Therehavealsobeenless extreme (Genise etal.,2000),andfromincreased paleoclimatic oscillations on the otherMilanko- abundance ofsilicabodies(phytoliths) andpollen vitch frequenciesof42ka and23 ka (Hays etal., characteristic ofgrasses(Stro¨mberg, 2002). 1976; Petit etal.,1999). InIllinois,interglacials Paleosolsalso demonstrateLateMiocene aredefined bypaleosolssuch asthe Sangamon expansion ofgrasslandscapableofformingsod paleosol,which iscomparablewithmodern forest ofthe sort thatisunrolled to createlawns and soils underoak–hickory forest.The42ka and golfcourses. Thedensegrowthoffine( , 2mm 23 ka interstadials aredefined bypaleosols such as diameter) adventitious roots,togetherwiththe the Farmdalepaleosol,which iscomparablewith slimeofabundant earthworms,createacharac- modern borealforest paleosols underspruce forest teristic soilstructureconsistingoffinecrumb peds,which canbe preserved inpaleosols (Figure13). Grasslandsoils arealso unusually rich inorganic matter,intimately admixed with clay,oftenwithasmuch as10wt.% Cdown to a meterormore, although thisorganic matterisnot always preserved inpaleosols. Thesoft,low- density upperhorizons ofgrasslandsoils arealso 2 2 rich inmineralnutrients (Ca þ ,Mg þ ,Na þ ,Kþ ), andtheirsubsurface horizons commonly include nodulesofsoilcarbonate(usually micritic low magnesium calcite). Ithaslongbeenknown that such pedogenic nodulesform atshallow depths withinsoilprofilesindry climatesanddeeper withinthe profileinmorehumid climates(Jenny, 1941; Retallack, 1994c). Observations ofdepthto carbonatehorizon togetherwithroot tracesand Figure13 Tall grassland(Mollisol) paleosol with crumbpedsofgrasslandpaleosolscanbe used to thick, darkbrown crumb-textured surface overadeep constrainthe paleoclimatic range ofgrasslands (79 cm) whitenodularcalcic horizon,overathinner (Retallack, 1997d, 2001d). short grasslandpaleosols withcarbonatenodulesata Observations on hundredsofpaleosolsinthe depthof39 cm,inthe LateMiocene(7 Ma)AshHollow NorthAmericanGreatPlains,Oregon,Pakistan, Formation,13 kmnorthofEllis,Kansas. 598 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime

Figure14 Ascenariofor climatic andgeographic expansion ofgrasslandsandtheirsoils inthe GreatPlains ofNorth America (from Palaios 1997d , 12,pp. 380–390. reproduced bypermission ofSociety for Sedimentary Geology.).

(Follmer etal.,1979).Ice, till,loess,and inferred from carbon isotopic valuestypical periglacialsoils (Gelisols) alternated withforest for CAM saltbushinthe sagebrushpaleosols, soils (Alfisols or Inceptisols)through these andfor C 3 grassesinthe grasslandpaleosols, paleoclimatic fluctuations. aswell asfrom the characteristic phytolithsof Oscillations betweendifferent ecosystems can theseplants(Blinnikov etal.,2002). be inferred from many paleosol sequences,even Comparablealternationsofecosystems with beyondthe icemargin(Figure15). Inthe paleoclimatic fluctuation areseeninmany Palouseloess ofWashington,for example, Quaternary sequencesofpaleosols(Paepeand grasslandsoils (Mollisols)withcrumbpeds vanOverloop,1990;Feng etal.,1994;Wang andearthworm castingsduringinterglacials and etal.,1998). Differencesinprimary production interstadials alternatewithsagebrushsoils (Ari- andcarbon sequestration ofthesealternating disols) withcicada burrows andshallow carbon- ecosystemtypesonaglobalbasismayhave atehorizons duringglacials andinterstadial played aroleinthe relativeabundance of minima(Busacca, 1989; O’GeenandBusacca, greenhousegasesduringglacial–interglacial 2001). Vegetation ofthe paleosols canbe paleoclimatic cycles. Soils andGlobalCarbon CycleChanges 599

Figure15 Alternatingpaleoenvironments ofdesert CAM shrublandsandC3 grasslandsrepresented byasequence ofQuaternary paleosols (0–100 ka)inthe Palouseloess nearKahlotus,Washington,USA;showing(lefttoright), fieldsection withthermoluminescence datesandpaleosol identification (Moll. isMollisol,Arid.isAridisol),paleosol position anddevelopment (represented byblack boxes),paleosol calcareousness (scalebased on acid reaction), Munsell hue(measured dry infield),sand-silt-clayproportion, d 13Cofpedogenic carbonate, d 18 Oofopalphytoliths, abundance ofearthworm pellets,andabundance ofcicada burrows (source Retallack, 2001c).

Ice corerecordsshow aslittleas180ppmv CO2 5.18.4SOILS AND GLOBAL CARBON duringglacialperiodsand280ppmvduring CYCLE CHANGES interglacials,inastrongly asymmetric pattern of gradualdrawdowns followed bysteeprisesknown Overgeologicaltimetherehavebeendramatic changesinsoil,life, andairthatarewell asterminations (Figure2(b)).EvenhigherCO 2 represented inthe fossilrecordofsoils. Paleosols levels duringinterglacials areprevented byhigh areanunderexploited recordofpast environments plant productivity offorests inhumid, previously inland.Thisreviewhasemphasized mainlythe glaciated terrains andofgrasslandsinarid evidence from paleosols for changesincarbon rangelands. Thisslow weatheringandbiomass cyclingandgreenhousegases(CO2 ,CH 4 ,H2 O) building, togetherwithnutrient leakage to the inthe atmosphereovergeologicaltimescales. oceanandcarbon burialthere, coulddrawdown Itisunremarkablethatpaleosols wouldchange, greenhousegasesandbringon cooling.Asice particularly athigh latitudes,asglobalclimates expandsandgrasslandsareconverted to deserts, warmed or cooled withchangingatmospheric the carbon sequestration capacity ofsoils and loadsofgreenhousegases. Itisnotablethat ecosystems isdiminished.Large herdsof sequencesofpaleosols can,undercertaincircum- mammals or populationsofhumans could stances,behigh-resolution recordsofsuch disturbtheseimpoverished soils into dustbowl paleoclimatic change (Figure15). conditionsandthe massivecarbon oxidation Parallels betweenbiologicalactivity within eventsofaglacialtermination. Such long-term soils andgreenhousegascomposition havebeen biologicaltrends,metered bysteadily declining emphasized inthisreviewasfertilegroundfor andthenabruptly renewed soilnutrients,could futureresearch (Figure16). Olsen(1993) has amplifyotherdrivers ofclimate, which include suggested thatsoilproducers such asplants cool large ice caps,oceancurrents,mountain the planet,but soilconsumers such asanimals building, andorbitalconfiguration (Mullerand warm it. Thisidea, which the author hasdubbed MacDonald, 2000). the Proserpinaprincipleafterthe ancient Roman 600 Soils andGlobalChange inthe Carbon CycleoverGeologicalTime

Figure16 The Proserpinaprinciplerelatesvariation inatmospheric CO2 concentration withcoevalevolutionary andecologicalevents on avariety oftimescales. Carbon sequesteringevolutionary innovations andecological transitions (closed symbols) alternatewithcarbon oxidizingevolutionary innovations andecologicaltransitions (opensymbols). The CO2 curveisacompositeofthoseshown inFigure2andbyKasting(1992). goddess ofspring(Retallack, 2000b),isundeni- eventsalso affectlifeandthe carbon cycle, ablefor the annualspringfall andautumn riseof producingtransient greenhouseevents (Retallack, atmospheric CO2 withnorthern hemisphereleaf 2001b). Oceancurrents andmountainbuilding sproutingandshedding(Figure2(a)).This alsoarelikely to playaroleincarbon sequestra- explanation isespecially demonstrated bythe tion (Raymo andRuddiman,1992;Ramstein etal., muted andout-of-phaseannualfluctuation ofCO 2 1997).Soils andtheirecosystems playan inthe southern hemisphere(Mooney etal.,1987), important roleinthe carbon cycletoday,and wherethereisless fertileland, moreevergreen the historyofthatrolenowdecipherable plants,anddifferent seasons.The question from paleosols appears ripefor modelingand addressed hereiswhetherthe Proserpinaprinciple otherquantitativecomparisons withotherlikely operatesongeologically significant timescales, controls on globalpaleoclimatechange. andso far,such asimpleidea doesnot conflict withthe historyoflife andpaleoclimateoutlined here. ACKNOWLEDGMENTS Onevolutionary timescales,itisthe biochemi- NathanSheldon,HopeJahren,andTimWhite calevolution oflignin,pyrethrin,caffeine, and havebeensoundingboardsfor the ideaspresented othersubstancesthatdeterherbivory,digestion, here.IalsothankJ.I.DreverandH.D.Holland anddecay,which affectratesofcarbon burialin for helpful reviews. sediments asthe principallong-term control on atmospheric CO2 levels. Theroleoftreesand theirsoils inLatePaleozoic carbon seques- REFERENCES tration,cooling, andglaciation iswidely accepted (Berner,1997; AlgeoandScheckler,1998;see AlgeoT.J.andSchecklerS.E.(1998)Terrestrial–marine teleconnections inthe Devonian:linksbetweenthe evolution Chapter5.06). The roleofhumans inglobal oflandplants,weatheringprocessesandanoxic events. Roy. warmingisalso becomingwell known (Vitousek Soc.London Phil. Trans. B353 ,113–130. etal.,1997b). Accordingto the Proserpina AllenJ.R.L.(1974)GeomorphologyofSiluro-Devonian principle, wemaynot havebeenthe only alluvialplains. Nature 249 ,644–645. organismstohavehad significant effects on Amundson R.,Stern L.,BaisdenT.,andWangY.(1998)The isotopic composition ofsoilandrespired CO2 . Geoderma 82 , climate.Thereremainmany otherinstancesof 83–114. globalchange less clearlyrelated to changesin BakkerR.T.(1985) The Dinosaur Heresies .WilliamMorrow, life andsoils,inpart becausethe numerous NewYork. paleosolsofappropriateage havenot yetbeen BarleyM.E.,PickardA.L.,andSylvesterP.J.(1997) Emplacement ofalarge igneous province asapossiblecause studied indetail. Asteroid impacts,volcanic ofbanded iron formation 2.45 billion years ago. Nature 385, eruptions,andmethaneclathratedissociation 55–58. References 601

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