Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 121 (2013) 240–262 www.elsevier.com/locate/gca Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic Ralf Tappert a,b,⇑, Ryan C. McKellar a,c, Alexander P. Wolfe a, Michelle C. Tappert a, Jaime Ortega-Blanco c,d, Karlis Muehlenbachs a a Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada b Institute of Mineralogy and Petrography, University of Innsbruck, Innsbruck, Austria c Division of Entomology (Paleoentomology), Natural History Museum, University of Kansas, Lawrence, KS, USA d Departament d’Estratigrafia, Paleontologia i Geocie`ncies Marines, Facultat de Geologia, Universitat de Barcelona, Barcelona, Spain Received 28 September 2012; accepted in revised form 10 July 2013; Available online 24 July 2013 Abstract Estimating the partial pressure of atmospheric oxygen (pO2) in the geological past has been challenging because of the lack of 13 reliable proxies. Here we develop a technique to estimate paleo-pO2 using the stable carbon isotope composition (d C) of plant resins—including amber, copal, and resinite—from a wide range of localities and ages (Triassic to modern). Plant resins are par- ticularly suitable as proxies because their highly cross-linked terpenoid structures allow the preservation of pristine d13Csignatures over geological timescales. The distribution of d13C values of modern resins (n = 126) indicates that (a) resin-producing plant fam- ilies generally have a similar fractionation behavior during resin biosynthesis, and (b) the fractionation observed in resins is similar to that of bulk plant matter. Resins exhibit a natural variability in d13C of around 8& (d13Crange:À31& to À23&,mean: À27&), which is caused by local environmental and ecological factors (e.g., water availability, water composition, light exposure, temperature, nutrient availability). To minimize the effects of local conditions and to determine long-term changes in the d13Cof 13 13 resin 13 resins, we used mean d C values (d Cmean) for each geological resin deposit. Fossil resins (n = 412) are generally enriched in C 13 resin compared to their modern counterparts, with shifts in d Cmean of up to 6&. These isotopic shifts follow distinctive trends through time, which are unrelated to post-depositional processes including polymerization and diagenesis. The most enriched fossil resin 13 resin samples, with a d Cmean between À22& and À21&, formed during the Triassic, the mid-Cretaceous, and the early Eocene. Exper- 13 imental evidence and theoretical considerations suggest that neither change in pCO2 nor in the d C of atmospheric CO2 can 13 resin 13 13 account for the observed shifts in d Cmean. The fractionation of C in resin-producing plants (D C), instead, is primarily influ- 13 resin enced by atmospheric pO2, with more fractionation occurring at higher pO2. The enriched d Cmean values suggest that atmospheric pO2 during most of the Mesozoic and Cenozoic was considerably lower (pO2 = 10–20%) than today (pO2 = 21%). In addition, a 13 resin 18 correlation between the d Cmean and the marine d O record implies that pO2, pCO2, and global temperatures were inversely linked, which suggests that intervals of low pO2 were generally accompanied by high pCO2 and elevated global temperatures. Inter- vals with the lowest inferred pO2, including the mid-Cretaceous and the early Eocene, were preceded by large-scale volcanism dur- ing the emplacement of large igneous provinces (LIPs). This suggests that the influx of mantle-derived volcanic CO2 triggered an initial phase of warming, which led to an increase in oxidative weathering, thereby further increasing greenhouse forcing. This pro- cess resulted in the rapid decline of atmospheric pO2 during the mid-Cretaceous and the early Eocene greenhouse periods. After the cessation in LIP volcanism and the decrease in oxidative weathering rates, atmospheric pO2 levels continuously increased over tens of millions of years, whereas CO2 levels and temperatures continuously declined. These findings suggest that atmospheric pO2 had a considerable impact on the evolution of the climate on Earth, and that the d13C of fossil resins can be used as a novel tool to assess the changes of atmospheric compositions since the emergence of resin-producing plants in the Paleozoic. Ó 2013 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Institute of Mineralogy and Petrography, University of Innsbruck, Innsbruck, Austria. Tel.: +43 512 492 5519. E-mail address: [email protected] (R. Tappert). 0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2013.07.011 R. Tappert et al. / Geochimica et Cosmochimica Acta 121 (2013) 240–262 241 1. INTRODUCTION Epstein, 1960; Brugnoli and Farquhar, 2000). In addition, some plant groups use distinctive metabolic Our knowledge of the composition of the atmosphere pathways that fundamentally affect their isotopic through time and of its impact on climate change and composition. For example, C4 and CAM plants are the evolution of life is based on evidence preserved in generally more enriched in 13C compared to C3 the fossil record. For the Pleistocene and Holocene, a di- plants (O’Leary, 1988; Farquhar et al., 1989). There- rect and continuous record of the partial pressures and fore, if stable isotopes are used in chemostratigraphic isotopic compositions of atmospheric gases is preserved studies, it is crucial to determine the organic compo- in the form of air vesicles entrapped in polar ice sheets sition of the material under investigation and its (e.g., Petit et al., 1999). With increasing age, the record botanical source. of atmospheric compositions becomes less directly accessi- (2) Environmental and ecological factors can play an ble, and inferences about the composition of the atmo- important role in the isotopic fractionation of plants, sphere in the geological past rely on indirect measures and individual isotope values of fossil or modern or proxies. The quality of such information depends di- plant material may reflect local instead of large-scale rectly on the ability of proxies to reliably capture the com- environmental conditions. position of ancient atmospheres. (3) Plant constituents can undergo substantial composi- Attempts have been made to reconstruct atmospheric tional changes during and after burial and diagenesis. pCO2 and pO2 using mass balance calculations and flux These changes have the potential to alter the isotopic modeling (Berner, 1999; Berner and Kothavala, 2001; Ber- composition of the fossil plant material, to the extent ner et al., 2007). Other approaches to estimate the paleo- of preventing a meaningful paleoenvironmental pCO2 have utilized the density of stomata on fossil leaf cuti- interpretation. cles (Retallack, 2001, 2002; Beerling and Royer, 2002) and the carbon stable isotopic composition of pedogenic car- To determine environmentally mediated changes in the bonates (Cerling and Hay, 1986; Ekart et al., 1999; Bowen d13C of fossil plant organic matter through time, we ana- and Beerling, 2004). One of the richest sources of informa- lyzed modern and fossil resins from a wide range of locali- tion on ancient atmospheric compositions comes from the ties and ages. Compared to other terrestrial plant marine sedimentary record, in particular from the stable constituents, resins have chemical properties that make isotope composition (d13C, d18O, d11B) of biogenic carbon- them particularly suitable as proxies of environmental ates (e.g., Clarke and Jenkyns, 1999; Pearson and Palmer, changes over geological time. Resins can polymerize into 2000; Royer et al., 2001; Zachos et al., 2001). This informa- highly cross-linked hydrocarbons that have considerable tion has been used to infer changes in the isotopic compo- preservation potential in the geological record (Fig. 1). This sition of atmospheric CO2 through time and to derive means that a reliable terrestrial stable isotope history can paleo-temperature estimates. More recently, the d13Cof potentially be reconstructed well into the Paleozoic, consid- marine phytoplankton biomarkers have been used to esti- ering that the earliest known fossil resins are from the Car- mate the pCO2 of Cenozoic atmospheres (Pagani et al., boniferous (White, 1914; Bray and Anderson, 2009). 2005). Additional constraints on the composition of ancient Unlike many other fossil plant organic constituents, atmospheres were obtained from the marine record using resins can often be assigned to a specific source plant fam- the biochemical cycles of sulfur (Berner, 2006; Halevy ily, and only a few plant families produce significant et al., 2012). amounts of preservable resins, all of which have C3 For terrestrial environments, attempts to estimate paleo- metabolism. Furthermore, resins are chemically conserva- 13 pCO2 have frequently exploited the d C of fossil organic tive, which means that the composition of the resins did matter, primarily plant organic matter (Strauss and Pe- not change significantly as plants evolved. These factors ters-Kottig, 2003; Jahren et al., 2008). A range of plant con- limit not only the chemical variability of fossil resins, stituents have been targeted for d13C analysis, including but also any potential family-specific biases, which could wood, foliage, and cellulose extracts, as well as the coals influence the isotopic composition of fossil resins in to which they contribute