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5.18 Soils and Global Change in the Carbon Cycle Over Geological Time

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5.18 Soils and Global Change in the Carbon Cycle Over Geological Time 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
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