https://doi.org/10.1130/G47681.1 Manuscript received 30 October 2019 Revised manuscript received 31 March 2020 Manuscript accepted 12 April 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 29 May 2020 A 23 m.y. record of low atmospheric CO2 Ying Cui1, Brian A. Schubert2* and A. Hope Jahren3 1 Department of Earth and Environmental Studies, Montclair State University, Montclair, New Jersey 07043, USA 2 School of Geosciences, University of Louisiana at Lafayette, Lafayette, Louisiana 70504, USA 3 Centre for Earth Evolution and Dynamics, University of Oslo, N-0315 Oslo, Norway ABSTRACT universally shared mechanism of photorespi- Current atmospheric CO2 concentration is known to be higher than it has been during the past ration. Because CO2 is well mixed in Earth’s ∼800 k.y. of Earth history, based on direct measurement of CO2 within ice cores. A comparison atmosphere, and diminished photorespiration to the more ancient past is complicated by a deficit of CO2 proxies that may be applied across with increasing CO2 is fundamental to the bio- very long spans of geologic time. Here, we present a new CO2 record across the past 23 m.y. of chemistry of photosynthesis, this mechanism 13 Earth history based on the δ C value of terrestrial C3 plant remains, using a method applicable is recorded globally (Keeling et al., 2017). Our to the entire ∼400 m.y. history of C3 photosynthesis on land. Across the past 23 m.y., CO2 likely previous growth chamber experiments, in com- ranged between ∼230 ppmv and 350 ppmv (68% confidence interval:∼ 170–540 ppm). CO2 was bination with meta-analyses, established that 13 found to be highest during the early and middle Miocene and likely below present-day levels the effect of CO2 on δ Cp value is consistent th during the middle Pliocene (84 percentile: ∼400 ppmv). These data suggest present-day CO2 across a wide range of species and environ- (412 ppmv) exceeds the highest levels that Earth experienced at least since the Miocene, further ments (Schubert and Jahren, 2012, 2018). Re- highlighting the present-day disruption of long-established CO2 trends within Earth’s atmosphere. cent works have also shown that the influence 13 of CO2 on δ Cp value is not affected by water INTRODUCTION and Cerling, 1992; Ekart et al., 1999; Retallack, availability (Lomax et al., 2019) or atmospheric Knowledge of atmospheric CO2 concentra- 2014; Da et al., 2015, 2019); alkenones derived O2 levels (Porter et al., 2017), and it is recorded tion is vital for understanding Earth’s climate from marine phytoplankton (Seki et al., 2010; within multiple organic substrates (e.g., cellu- system because it imparts a controlling effect Badger et al., 2013a, 2013b; Zhang et al., 2013); lose and collagen [Hare et al., 2018], hair [Zhao on global temperatures across recent (Hegerl and the pH of ocean water as derived from bo- et al., 2019], and n-alkanes [Wu et al., 2017]) et al., 2006) and geologic (Foster et al., 2017) ron isotopes (Bartoli et al., 2011; Foster et al., and inorganic substrates (e.g., speleothems time scales. Proxies (Breecker et al., 2010) and 2012; Greenop et al., 2014; Martinez-Boti et al., [Breecker, 2017] and cave air [Bergel et al., models (Royer et al., 2014) indicate that CO2 has 2015; Stap et al., 2016). Each of these proxies 2017]). Consequently, researchers now correct 13 varied widely during the geologic past. Direct provides robust results for specific time periods δ C values for changes in CO2 across a myriad measurement of CO2 has been performed at the (Foster et al., 2017; Hollis et al., 2019); however, of fossil (e.g., ungulate teeth [Luyt et al., 2019; Mauna Loa Observatory (Hawaii, USA) for the a CO2 proxy for use across the entire history of Sealy et al., 2019], soil-respired carbon [Caves past 60+ yr, and historical CO2 has been sampled vascular land plants (i.e., the past ∼400 m.y.) Rugenstein and Page Chamberlain, 2018], soil continuously from ice-core bubbles recording is lacking. carbonate [Basu et al., 2019], pyrogenic car- the past 800 k.y. (Petit et al., 1999; Lüthi et al., Here, we present a method for calculating bon and n-alkanes [Zhou et al., 2017], and pol- 2008), allowing for trends in CO2 during the lat- CO2 that is based on a ubiquitous substrate, is len [Bell et al., 2019]), and modern (e.g., fungi ter portion of the Quaternary to be evaluated in sensitive across a wide range of CO2, and is root- [Hobbie et al., 2017] and leaves [Tibby et al., detail. Direct observations of atmospheric green- ed in a fully understood mechanism of response 2016]) substrates, and recent experiments have 13 house gases are also now available from discon- to changing CO2. We illustrate its efficacy by shown that the δ Cp value can produce accurate tinuous ice up to 2 m.y. old from East Antarctica presenting a novel, high-resolution record of estimates of paleo-CO2 concentration (Porter (Higgins et al., 2015; Yan et al., 2019). CO2 for the Neogene through the Quaternary et al., 2019). For time periods older than the Pleistocene, (i.e., the past 23 m.y.), a period that lacks a con- many CO2 proxies have been applied, including tinuous record of CO2 from any single proxy. METHODS 13 the proportion of epidermal cells that are stoma- Our approach is centered upon the δ C val- We reconstructed CO2 across the past 23 m.y. 13 13 tal pores (Kürschner et al., 1996, 2008; Beerling ue of C3 vascular land plants (hereafter δ Cp), using a compilation of 700 δ C measurements et al., 2009; Grein et al., 2013; Wang et al., 2015; which is available from terrestrial sediments for gathered from 12 previously published studies Reichgelt et al., 2016); the stable carbon isotope most of the Phanerozoic (Nordt et al., 2016). of terrestrial organic matter (TOM; n = 441) composition of paleosol carbonate (Breecker Our calculations of CO2 assumed that global and plant lipids (n = 259) that spanned at least changes in atmospheric composition affect the 1 m.y. of the Neogene (Table S1 in the Supple- 1 *E-mail: [email protected] plant tissues of all terrestrial C3 plants via the mental Material ). We chose these substrates 1 Supplemental Material. Description of the inputs used to calculate atmospheric CO2 concentration, uncertainty in CO2(t), Figure S1, and Tables S1 and S2. Please visit https://doi .org/10.1130/GEOL.S.12307451 to access the supplemental material, and contact [email protected] with any questions. CITATION: Cui, Y., Schubert, B.A., and Jahren, A.H., 2020, A 23 m.y. record of low atmospheric CO2: Geology, v. 48, p. 888–892, https://doi.org/10.1130/G47681.1 888 www.gsapubs.org | Volume 48 | Number 9 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/9/888/5137362/888.pdf by guest on 30 September 2021 because both TOM and plant lipid 13C values (i.e., 13C ) between two points in time, time 13 13 δ δ anomaly CO2()tt=−[(δδCCpp()=0 {[ ()t − have been shown to respond similarly to changes t = 0 (for which CO2 is known) and time t (for 13 13 2 ()δδCCatma(tt)(−+tm =0) ] ε}) ×[A in CO2 (Schubert and Jahren, 2012; Wu et al., which CO2 is not known): 2 2017; Chapman et al., 2019); these substrates ++AB()CO20()tt==2ABCB+ CC()O20() 13C ABCO C also represent an integrated signal with multiple δ anomaly =+()()( 2()t ) 22 2 13 ++BC]( ABCO2(t 0)()] /[(δ Cp t 0) photosynthetic inputs, which has been shown to = = / AB++()CO2()t C 13 13 13 improve the accuracy of the proxy (Porter et al., ( ) −−{[δδCCpa()tt()tm()−+δεCatm()t=0 ] }}) 13 2 22 2019). For studies that reported δ Cp data for ABCO C (3) − ()(()( 20()t = + ) ×−[.AB −−BC()OB20()t= CA]]+ B multiple n-alkanes (e.g., n-C27, n-C29, n-C31), we AB++CO C , (1) selected only one record, or the weighted mean / ()( 20()t = ) values (if reported), thus avoiding redundancy in Descriptions of the inputs are provided in the 13 our compiled data set. The δ Cp data set used where A, B, and C are curve-fitting parameters Supplemental Material. 13 for input exhibited a large range in δ Cp values (A = 28.26, B = 0.22, C = 23.9; Schubert and (∼8‰), sampled from a wide range of environ- Jahren, 2012, 2015; Cui and Schubert, 2016). RESULTS 13 ments; plant lipids generally exhibited lower When calculating δ Canomaly, it is necessary to Figure 1 shows a continuous record of CO2 13 13 13 δ Cp values than TOM of the same age, as is correct for (1) changes in the δ C value of at- across the past 23 m.y. based on changes in δ Cp 13 13 commonly observed (e.g., Chikaraishi and Nara- mospheric CO2 between time t [δ Catm(t)] and value (i.e., δ Canomaly). We calculated that the 13 oka, 2003). We limited our literature compilation time t = 0 [δ Catm(t = 0)], and (2) any biosynthetic median CO2 value was lower than that of today 13 to records with δ Cp values of TOM ≤ −22.0‰ fractionation when comparing across different across the entirety of the past 23 m.y., and it 13 and plant lipids ≤ −27.0‰, thus avoiding δ Cp plant tissues (e.g., TOM and lipids). Therefore, likely never fell below levels experienced during 13 13 values that reflected C4 ecosystems (O’Leary, δ Canomaly represents the change in δ Cp value, Pleistocene glacial advances (∼170 ppm; Petit 13 13 1988).