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GEOCARB III: A REVISED MODEL OF ATMOSPHERIC CO2 …

GEOCARB III: A REVISED MODEL OF ATMOSPHERIC CO2 OVERPHANEROZOIC TIMEROBERT A. BERNER and ZAVARETH KOTHAVALAD epartment of Geology and Geophysics, Yale University,New Haven, Connecticut of the GEOCARB MODEL (Berner, 1991, 1994) for paleolevelsof ATMOSPHERIC CO2, has been made with emphasis on factors affecting CO2uptake bycontinental weathering. This includes: (1) new GCM (general circulation MODEL )results for the dependence of global mean surface temperature and runoff on CO2,for both glaciated and non-glaciated periods, coupled with new results for thetemperature response to changes in solar radiation; (2) demonstration that values forthe weathering-uplift factor fR(t) based on Sr isotopes as was done in GEOCARB II arein general agreement with independent values calculated from the abundance ofterrigenous sediments as a measure of global physical erosion rate over Phanerozoictime; (3) more accurate estimates of the timing and the quantitative effects on Ca-Mgsilicate weathering of the rise of large vascular plants on the continents during theDevonian.

to include findings in the earth, biological, and climatological sciences that have occurred over the past seven years. Most of the findings are related to chemical weathering on the continents. The long-term carbon cycle.—On a multimillion year time scale the major process affecting atmospheric CO

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Transcription of GEOCARB III: A REVISED MODEL OF ATMOSPHERIC CO2 …

1 GEOCARB III: A REVISED MODEL OF ATMOSPHERIC CO2 OVERPHANEROZOIC TIMEROBERT A. BERNER and ZAVARETH KOTHAVALAD epartment of Geology and Geophysics, Yale University,New Haven, Connecticut of the GEOCARB MODEL (Berner, 1991, 1994) for paleolevelsof ATMOSPHERIC CO2, has been made with emphasis on factors affecting CO2uptake bycontinental weathering. This includes: (1) new GCM (general circulation MODEL )results for the dependence of global mean surface temperature and runoff on CO2,for both glaciated and non-glaciated periods, coupled with new results for thetemperature response to changes in solar radiation; (2) demonstration that values forthe weathering-uplift factor fR(t) based on Sr isotopes as was done in GEOCARB II arein general agreement with independent values calculated from the abundance ofterrigenous sediments as a measure of global physical erosion rate over Phanerozoictime; (3) more accurate estimates of the timing and the quantitative effects on Ca-Mgsilicate weathering of the rise of large vascular plants on the continents during theDevonian.

2 (4) inclusion of the effects of changes in paleogeography alone (constantCO2and solar radiation) on global mean land surface temperature as it affects the rateof weathering; (5) consideration of the effects of volcanic weathering, both insubduction zones and on the seafloor; (6) use of new data on thed13C values forPhanerozoic limestones and organic matter; (7) consideration of the relative weather-ing enhancement by gymnosperms versus angiosperms; (8) revision of paleo land areabased on more recent data and use of this data, along with GCM-based paleo-runoffresults, to calculate global water discharge from the continents over show a similar overall pattern to those for GEOCARB II: very high CO2values during the early Paleozoic, a large drop during the Devonian and Carbonifer-ous, high values during the early Mesozoic, and a gradual decrease from about 170 Mato low values during the Cenozoic.

3 However, the new results exhibit considerablyhigher CO2values during the Mesozoic, and their downward trend with time agreeswith the independent estimates of Ekart and others (1999). Sensitivity analysis showsthat results for paleo-CO2are especially sensitive to: the effects of CO2fertilizationand temperature on the acceleration of plant-mediated chemical weathering; thequantitative effects of plants on mineral dissolution rate for constant temperature andCO2; the relative roles of angiosperms and gymnosperms in accelerating rock weather-ing; and the response of paleo-temperature to the global climate MODEL used. Thisemphasizes the need for further study of the role of plants in chemical weathering andthe application of GCMs to study of paleo-CO2and the long term carbon 1991 a new MODEL for the evolution of the carbon cycle and of ATMOSPHERIC CO2over Phanerozoic time was presented based on inputs of geological, geochemical,biological, and climatological data (Berner, 1991).

4 This MODEL was later REVISED in 1994and given the name GEOCARB , whereupon the REVISED MODEL was labelled asGEOCARB II (Berner, 1994). The purpose of the present paper is to amend the modelto include findings in the earth, biological, and climatological sciences that haveoccurred over the past seven years. Most of the findings are related to chemicalweathering on the long-term carbon cycle. On a multimillion year time scale the major processaffecting ATMOSPHERIC CO2is exchange between the atmosphere and carbon stored inrocks. This long-term, or geochemical carbon cycle is distinguished from the morefamiliar short-term cycle that involves the transfer of carbon between the oceans,atmosphere, biosphere, and soils (see Berner, 1999 for a comparison of the two cycles).In the long-term cycle loss of CO2from the atmosphere is accomplished by photosyn-[American Journal of Science, , February,2001, 204]182thesis and burial of organic matter in sediments and by the reaction of atmosphericCO2with Ca and Mg silicates during continental weathering to form, ultimately, Caand Mg carbonates on the ocean floor (after transport of the weathering-derived Ca,Mg, and carbon to the sea by rivers).

5 Release of CO2to the atmosphere in thelong-term carbon cycle takes place via the oxidative weathering of old organic matterand by the thermal breakdown of buried carbonates and organic matter (via diagen-esis, metamorphism and volcanism) resulting in degassing to the earth above description can be represented by succinct overall chemical reactions (Ebelmen, 1845; Urey, 1952; Holland, 1978; Berner, 1991) are:CO21 CaSiO37 CaCO31 SiO2(i)CO21 MgSiO37 MgCO31 SiO2(ii)CH2O1O27CO21H2O(iii)The arrows in reactions (i) and (ii) refer to Ca-Mg silicate weathering plus sedimenta-tion of marine carbonates when reading from left-to-right. These two weatheringreactions summarize many intermediate steps including photosynthetic fixation ofCO2, root/mycorrhizal respiration, organic litter decomposition in soils, the reactionof carbonic and organic acids with primary silicate minerals, the conversion of CO2toHCO32in soil and ground water, the flow of riverine HCO32to the sea, and theprecipitation of oceanic HCO32as Ca-Mg carbonates in bottom sediments.

6 Reactions(i) and (ii) reading from right-to-left represent thermal decomposion of carbonates atdepth resulting in degassing of CO2to the surface. The double arrow in reaction (iii)refers to weathering (or thermal decomposition plus ATMOSPHERIC oxidation of re-duced gases) when reading from left to right and burial of organic matter (the net ofglobal photosynthesis over respiration) when reading from right to modeling of the long term carbon cycle consists of equations express-ing carbon and carbon isotope mass balance along with formulations for rates ofweathering and degassing and how these rates have changed over time. Details of thederivation of these equations can be found in GEOCARB I and II (Berner, 1991, 1994),and they are simply presented here in appendix 1. (For discussion of models similar toGEOCARB; see Kump and Arthur, 1997; Tajika, 1998, Gibbs and others, 1999; andWallmann, 2001).

7 Any modification of the equations from GEOCARB II to III arediscussed in the present should be emphasized that GEOCARB modeling has only a long time resolu-tion. Data are input into the MODEL at 10 my intervals with linear interpolationbetween. In the case of rock abundance data, averages for up to 30 my time slices aresometimes used. Thus, shorter term phenomena occurring over a few million years orless are generally missed in this type of GEOCARB II paper ended with suggestions for future research that includeda need for better input data on (1) the quantitative effects of plants on weathering; (2)the quantitative effect of changes in relief, as it affects physical erosion and silicateweathering, including independent checks on the use of Sr isotopes as a proxy forcontinental relief and erosion; (3) changes in continental size and position as theyaffect weathering by way of changes in runoff and land temperature; (4) the applica-tion of GCM modeling, based on past and not just present geography, to the deductionof the effects of changes in ATMOSPHERIC CO2on global temperature and runoff.

8 Theseproblems are now addressed in the present to the modelingApplication of new GCM results to fB(T, CO2). The weathering feedback parameterfB(T, CO2) reflects the effects of changes in CO2and global temperature on the rate Berner and Z. Kothavala183weathering of Ca-Mg silicates. Factors considered that affect temperature are theevolution of the sun, the ATMOSPHERIC greenhouse effect (relating temperature toCO2), and changes in paleogeography. In addition, the direct effect of CO2onweathering in the presence and absence of vascular plants is included in this parame-ter. Appropriate expressions (see Berner, 1994, for further discussion) are:fB~T, CO2!5f~T!f~CO2!(1)f~T!5exp$ACT@ RCO22Ws~t/570!1 GEOG~t!(3)f~CO2!5~RCO2! pre-vascular plant weathering(4)f~CO2!5@2 RCO2/~11 RCO2!#FERTfor weathering affected by vascular plants (5)where.

9 T5global mean temperaturet5timeRCO25the ratio of mass of CO2at time t to that at present (t50)G5coefficient derived from GCM modeling that expresses the response ofglobal mean temperature to change in ATMOSPHERIC CO2due to theatmospheric greenhouse effectWs5factor expressing the effect on global mean temperature of the increase insolar radiation over geological timeGEOG(t)5the effect of changes in paleogeography on temperatureACT5E/RT25coefficient expressing the effect of mineral dissolution activationenergy E on weathering rate (R5gas constant)RUN5coefficient expressing the effect of temperature on global river runoffFERT5exponent reflecting the proportion of plants globally that are fertilized byincreasing CO2and that accelerate mineral weatheringSimilar expressions are derived for the weathering feedback parameter fBB(T, CO2) forcarbonates but with different activation energies and a different formulation for runoff(Berner, 1994).

10 Changes from GEOCARB II are the addition of the expression for theeffect of changes in paleogeography on temperature GEOG(t) to the expression forf(T) and the allowance of variation of the CO2fertilization factor FERT which waspreviously assumed to be equal to (equivalent to 35 percent of plants globallyresponding to CO2fertilization).Values of GEOG(t), the greenhouse and solar response factorsGand Ws, and theriver runoff factor RUN can be obtained from the application of general circulationmodels (GCM) to paleo-environments. In GEOCARB II the results of Marshall andothers (1994) and Manabe and Bryan (1985) for the present Earth were used to obtainG, Ws and RUN for all times. Recently GCM work, covering a large range of CO2values,has shown a lower sensitivity of temperature and runoff to CO2and solar forcing(Kothavala, Oglesby, and Saltzman, 1999, 2000).


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