Earth-Science Reviews 54Ž. 2001 349–392 www.elsevier.comrlocaterearscirev Phanerozoic atmospheric CO2 change: evaluating geochemical and paleobiological approaches Dana L. Royer a,), Robert A. Berner a,1, David J. Beerling b,2 a Kline Geology Laboratory, Yale UniÕersity, P.O. Box 208109, New HaÕen, CT 06520-8109, USA b Department of Animal and Plant Sciences, UniÕersity of Sheffield, Sheffield S10 2TN, UK Accepted 8 November 2000 Abstract The theory and use of geochemical modeling of the long-term carbon cycle and four paleo-PCO2 proxies are reviewed and discussed in order to discern the best applications for each method. Geochemical models provide PCO2 predictions for the entire Phanerozoic, but most existing models present 5–10 m.y. means, and so often do not resolve short-term excursions. Error estimates based on sensitivity analyses range from "75–200 ppmV for the Tertiary to as much as "3000 ppmV during the early Paleozoic. 13 The d C of pedogenic carbonates provide the best proxy-based PCO2 estimates for the pre-Tertiary, with error estimates ranging from "500–1000 ppmV. Pre-Devonian estimates should be treated cautiously. Error estimates for Tertiary reconstructions via this proxy are higher than other proxies and models Ž."400–500 ppmV , and should not be solely relied upon. We also show the importance of measuring the d13C of coexisting organic matter instead of inferring its value from marine carbonates. The d13C of the organic remains of phytoplankton from sediment cores provide high temporal resolutionŽ up to 1034 –10 " " year.Ž , high precision 25–100 ppmV for the Tertiary to 150–200 ppmV for the Cretaceous . PCO2 estimates that can be near continuous for most of the Tertiary. Its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short-term excursions. This method, however, must correct for changes in growth ; rate and oxygen level. At elevated PCO2 Ž.750–1250 ppmV , this proxy loses its sensitivity and is not useful. The stomatal density and stomatal index of land plants also provide high temporal resolution Ž-102 year. , high " precision Ž.10–40 ppmV for the Tertiary and possibly Cretaceous PCO2 estimates, and so also is ideal for discerning short-term excursions. Unfortunately, this proxy also loses sensitivity at some level of PCO2 above 350 ppmVŽ which, currently, is largely undetermined. Our analysis of the recently developed d11B technique shows that it currently is not yet well constrained. Most importantly, it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through ) Corresponding author. Fax: q1-203-432-3134. E-mail addresses: [email protected]Ž. D.L. Royer , [email protected] Ž R.A. Berner . , [email protected] Ž.D.J. Beerling . 1 Fax: q1-203-432-3134. 2 Fax: q44-0114-222-0002. 0012-8252r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0012-8252Ž. 00 00042-8 350 D.L. Royer et al.rEarth-Science ReÕiews 54() 2001 349–392 time. In addition, it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur. A fifth CO2 proxy, based on the redox chemistry of marine cerium, has several fundamental flaws and is not discussed in detail here. q 2001 Elsevier Science B.V. All rights reserved. Keywords: carbon dioxide; paleoatmosphere; paleoclimatology; indicators 1. Introduction mination of the composition of air trapped in glacial iceŽ. e.g., Friedli et al., 1986; Petit et al., 1999 . During the last decade, numerous methods for However, this method is only useful for the past 400 evaluating past concentration of atmospheric carbon ka because of the absence of ice older than this. r dioxideŽ. PCO2 have been developed and or re- Thus, other methods have been applied to the older fined. The most reliable method has been the deter- geologic record. Quantifying paleo-CO2 is vital for Nomenclature: Symbols commonly used in text ) Ca2PCO ) 3 Cs2soil COŽ. mol cm C37:2 diunsaturated long-chained alkenones ) 2 y1 Ds2diffusion coefficient for soil COŽ. cm s r piap ratio of internal:ambient CO 2 S CO2 contributed by biological respiration z depth in a soil profileŽ. cm z characteristic CO2 production depthŽ. cm adiffand ´ diff fractionation factor for diffusion during carbon fixationŽ. ‰ affand ´ kinetic fractionation factor for carbon fixationŽ. ‰ a PPand ´ combined fractionation for carbon fixationŽ. ‰ 13 dda C of the atmosphereŽ. ‰ 13 ddcc C of pedogenic carbonateŽ. ‰ 13 ddi2C of CO at the site of carbon fixationŽ. ‰ 13 ddocc C of marine carbonateŽ. ‰ 13 ddom C of organic matterŽ. ‰ 13 ddp C of photosynthateŽ. ‰ 13 dds2C of soil COŽ. ‰ 13 ddf C of soil-respired CO2 Ž. ‰ 11 ddÝB B of total dissolved BŽ. ‰ ´ free air porosity in soil ´a ambient atmospheric partial pressure of water vapor ´i intercellular partial pressure of water vapor ) y1 y3 fs2CO production rateŽ. mol s cm m growth rateŽ. dy1 r tortuosity D.L. Royer et al.rEarth-Science ReÕiews 54() 2001 349–392 351 understanding climate dynamics, most notably its basis is dominated by the exchange of carbon be- effect on global temperatureŽ e.g., Sloan and Rea, tween rocks and the surficial reservoir consisting of 1995; Kothavala et al., 1999. through the so-called the atmosphereqbiosphereqoceansqsoils, and the ‘greenhouse effect’. PCO2 affects many other as- processes involved in the long-term cycle are differ- pects of the biosphere including particularly the ent from those that are involved in the short-term physiology, productivity and distribution of terres- cycle, which considers only the exchange of carbon trial vegetation. This in turn influences our interpre- within the surficial reservoirŽ. see Berner, 1999 . tation of the plant fossil recordŽ. Beerling, 1998b and Because the amount of CO2 in the atmosphere is exerts an important impact on the feedback between exceedingly small compared to the carbon fluxes vegetation and climate by changing the exchange of into and out of it, it is very difficult to calculate energy and water vapor between the land surface and changes in PCO2 due to imbalances between these the overlying atmosphereŽ. e.g., Bounoua et al., 1999 . fluxes over millions of yearsŽ Berner and Caldeira, Quantifying the PCO2 of the ancient atmosphere, 1997. Calculations of PCO2 from imbalances via therefore, provides a firmer basis for assessing the time-dependent modeling have been madeŽ Berner et linkages between PCO2 and the biosphere that are al., 1983; Lasaga et al., 1985. , but they require urgently required for understanding the geologic past treatment of fluxes of additional elements such as Ž.Beerling, 2000 . Ca, Mg, and S, as well as fluxes of carbon between Here, we critically consider the underlying theory, the ocean and atmosphere as well as between rocks practice, and applicability of geochemical modeling and the surficial reservoir. Because of their complex- of the long-termŽ. multimillion year carbon cycle ity, these models will not be covered here. Instead, and four paleo-PCO2 proxies. Several of the proxies simpler models dealing only with carbon, principally were developed for and first applied to the Quater- the GEOCARB and similar modelsŽ Budyko et al., nary, but their application to the pre-Quaternary 1987; Berner, 1991, 1994; Caldeira and Kasting, record will be emphasized here. The four proxies are 1992; Franc¸ois and Walker, 1992; Kump and Arthur, the d13C of phytoplankton, the d13C of pedogenic 1997; Tajika, 1998; Berner and Kothavala, 2001; carbonatesŽ. including the goethite method , stomatal Wallman, 2001. , will be discussed. Several other density and stomatal index, and the d11 B of marine modeling approaches to the long-term carbon cycle calcium carbonate. In considering the applicability of have been made, but they do not actually calculate the various approaches to the geologic record, their CO2 concentrations and will not be discussed here associated temporal resolutions and precision of ŽWalker et al., 1981; Caldeira, 1992; Raymo and PCO2 estimates will be emphasized. Ruddiman, 1992; Godderis and Franc¸ois, 1995, 1996; Derry and France-Lanord, 1996; France-Lanord and Derry, 1997; Gibbs et al., 1997, 1999; Kump and 2. Geochemical modeling of the long-term carbon Arthur, 1999; McCauley and DePaolo, 1997; Raymo, cycle 1997; Franc¸ois and Godderis, 1998. The GEOCARB model considers the input of The level of atmospheric CO2 over geologic time CO2 to the atmosphere from the thermal breakdown can be estimated by constructing mass balance ex- of carbon in rocks resulting in volcanic, metamor- pressions for all the processes that bring about inputs phic, and diagenetic degassing. Additional CO2 in- and outputs of CO2 to and from the atmosphere. This put comes from the weathering of organic matter in is a daunting task at any timescale. The best one can rocks exposed on the continents. Carbon dioxide is do is to construct theoretical models and attempt to removed from the atmosphere by the weathering of devise rates and rate laws for processes involved in Ca–Mg silicate and carbonate minerals to form dis- the carbon cycle and how these rates may have solved bicarbonate in groundwater, followed by y changed over time. Since this review is concerned transport of the HCO3 by rivers to the oceans where y with pre-Quaternary PCO2 , only models that treat the HCO3 is removed as Ca–Mg carbonates in CO2 changes over millions of years will be dis- bottom sediments. Thus, carbon is transferred from cussed. The carbon cycle on a multimillion year the atmosphere to carbonate rocks. In some models 352 D.L. Royer et al.rEarth-Science ReÕiews 54() 2001 349–392 s Ž.e.g., Wallman, 2001 , direct weathering of Ca–Mg where: Fwc, F wg rate of release of carbon to the silicates to carbonates on the seafloor is also consid- atmosphereqbiosphereqoceansqsoils system via ered.