Earth-Science Reviews 56Ž. 2001 1–159 www.elsevier.comrlocaterearscirev A critique of Phanerozoic climatic models involving changes in the CO2 content of the atmosphere A.J. Boucot a,), Jane Gray b,1 a Department of Zoology, Oregon State UniÕersity, CorÕallis, OR 97331, USA b Department of Biology, UniÕersity of Oregon, Eugene, OR 97403, USA Received 28 April 1998; accepted 19 April 2001 Abstract Critical consideration of varied Phanerozoic climatic models, and comparison of them against Phanerozoic global climatic gradients revealed by a compilation of Cambrian through Miocene climatically sensitive sedimentsŽ evaporites, coals, tillites, lateritic soils, bauxites, calcretes, etc.. suggests that the previously postulated climatic models do not satisfactorily account for the geological information. Nor do many climatic conclusions based on botanical data stand up very well when examined critically. Although this account does not deal directly with global biogeographic information, another powerful source of climatic information, we have tried to incorporate such data into our thinking wherever possible, particularly in the earlier Paleozoic. In view of the excellent correlation between CO2 present in Antarctic ice cores, going back some hundreds of thousands of years, and global climatic gradient, one wonders whether or not the commonly postulated Phanerozoic connection between atmospheric CO2 and global climatic gradient is more coincidence than cause and effect. Many models have been proposed that attempt to determine atmospheric composition and global temperature through geological time, particularly for the Phanerozoic or significant portions of it. Many models assume a positive correlation between atmospheric CO2 and surface temperature, thus viewing changes in atmospheric CO2 as playing the critical role in r regulating climate temperature, but none agree on the levels of atmospheric CO2 through time. Prior to the relatively recent interval of time in which atmospheric CO2 is directly measurable, a variety of biological and geological proxies have been r proposed to correlate with atmospheric CO22 level or with pCO temperature. Atmospheric models may be constructed for the Pre-Cenozoic but the difficulties of assessing variables in their construction are many and complex. None of the modelers have gathered enough biological and geological data to impart reliability to the model constructs. Most modelers focus almost exclusively on one or a few variables as proxy to measure atmospheric CO2 , nor consider the many other variables involved, nor agree on what these variables are or how to measure them. In this paper, it is the reliability of the present data bases used in these atmospheric models that we wish to consider. We focus most attention on the Berner models, such as GEOCARB I, II and BLAG, because of the basic role they attribute to tracheophytes in regulating atmospheric CO2 and our own interest in pre-tracheophytic land plants and the atmospheric composition of the pre-tracheophytic Paleozoic. We survey the presence of symbiotic mycorrhizae and question the assumption that all tracheophytes are obligately associated with them. ) Corresponding author. Fax: q1-541-737-0501. E-mail address: [email protected]Ž. A.J. Boucot . 1 Deceased. 0012-8252r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0012-8252Ž. 01 00066-6 2 A.J. Boucot, J. GrayrEarth-Science ReÕiews 56() 2001 1–159 Although pre-tracheophytic embryophytes, cyanobacteria, and possibly other organisms preceded tracheophytes on land by millions of years, Berner’s models do not consider a significant role for them in affecting pre-Devonian climatertempera- ture and atmospheric composition. In effect, Berner assumes that pre-tracheophytic life inhabited a world governed largely by abiotic physical and chemical reactions. We consider uncertainties raised by minimizing possible roles for pre- tracheophytic oxygenic and heterotrophic microorganisms analogous to those speculated to be unique to tracheophytes both with regard to an active role in biodeterioration of rock and soil mineral substrates and in the sequestration of organic carbon. Additionally, Berner does not consider marine productivity, which might have been high in the Precambrian and Early Paleozoic and possibly consequent organic carbon sequestration, even in the possible absence of terrestrial organisms, or even in the absence of a significant preserved biomass of terrestrial and marine organisms. The important roles played by cyanobacteria, for example, are briefly reviewed by Giller and Malmqvist in lakes and rivers as regards both planktonic and benthic taxa, and it is not safe to assume that these organisms were absent or of no potential significance in the pre-embryophytic, i.e. earlier Ordovician and well back into the Precambrian. Berner’s models have met with a large measure of consensus about CO2 balance during the Phanerozoic, about the role played by tracheophytes, and have been used to test or evaluate other data. After reviewing the biological and geological assumptions and estimates on which these Models are based, we conclude that they do not provide reliable information about atmospheric CO2 composition through Phanerozoic time, particularly in the Early Phanerozoic. We compare many atmospheric CO2 models, while considering the numerous proxies on which they are based and conclude that the competing models are inadequate for atmospheric CO2 estimation. Many possibilities not considered in present models must either be included or eliminated based on reliable evidence. We suggest that assessing Phanerozoic climatertemperature based on the available geologicalrclimatic proxies would appear to provide a more reliable method of estimating variations in CO222 , and hence atmospheric CO :O balance, than most proxy constructs on which atmospheric models are presently based, because of the critical role postulated for atmospheric CO2 in regulating Earth’s surface temperature. We present our own Phanerozoic climate estimate, based on r readily available geological climatic data, for comparison with postulated coeval atmospheric CO2 levels as a test of the postulated correlation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: global climatic gradient; atmospheric CO2 ; Phanerozoic; stomates; mycorrhizae; paleogeography; weathering Contents 1. Introduction ............................................................... 5 2. Phanerozoic climate and atmospheric composition ........................................... 7 2.1 Introduction ............................................................ 7 2.2 Carbon and CO2 sources and sinks ................................................ 9 2.2.1 Carbon and CO2 sources ................................................. 9 2.2.2 Carbon and CO2 sinks................................................... 10 2.3 Criticisms ............................................................. 10 2.4 An alternative ........................................................... 11 3. Atmospheric CO2 proxies ........................................................ 11 4. Proxies without independent atmospheric CO2 curves ......................................... 12 4.1 Stomates .............................................................. 12 4.1.1 Physical and biotic variables affecting stomatal densities and occurrences ....................... 13 4.1.2 Application of stomatal data to atmospheric CO2 estimates ............................... 15 4.2 Tracheophyte productivity and standing crop........................................... 17 4.3 Proxies with atmospheric CO2 curve models ........................................... 17 4.3.1 Berner models ....................................................... 17 a. Introduction ...................................................... 17 b. Atmospheric CO2 fluctuations in the Berner models ................................ 19 c. Berner’s data bases .................................................. 19 A.J. Boucot, J. GrayrEarth-Science ReÕiews 56() 2001 1–159 3 I. Estimates and assumptions........................................... 19 a. Sequestered organic carbonŽ. continental ................................. 19 b. Sequestered inorganic carbonŽ. marine .................................. 21 II. The modern autotrophic weathering basis of Berner’s 1984–2001 models.................. 21 a. 1991 Model ................................................ 21 b. 1992, 1994, 1995, 1997 models ...................................... 22 4.3.2 Comment and criticism of the Berner models ...................................... 26 I. Marine phytoplankton ............................................... 26 II. Pretracheophytic autotrophs ............................................ 26 III. Phytogeography and evolution ........................................... 27 IV. Climatically sensitive rocks ............................................ 27 V. Need to collect adequate geological data...................................... 27 VI. Is tracheophyte weathering evolutionarily progressive? .............................. 27 VII. Sequestered organic carbon ............................................ 29 VIII. Natural hydrocarbons ................................................ 30 IX. Deep, subsurface bacterial carbon ......................................... 30 X. Were Early and Middle Devonian floras confined to the lowlands? ....................... 30 5. Organic carbon ............................................................