CO₂ and temperature decoupling at the million-year scale during the Cretaceous Greenhouse

Abstract

CO₂ is considered the main greenhouse gas involved in the current global warming and the primary driver of temperature throughout Earth’s history. However, the soundness of this relationship across time scales and during different climate states of the Earth remains uncertain. Here we explore how CO₂ and temperature are related in the framework of a Greenhouse climate state of the Earth. We reconstruct the long-term evolution of atmospheric CO₂ concentration (pCO₂) throughout the Cretaceous from the carbon isotope compositions of the fossil conifer Frenelopsis. We show that pCO₂ was in the range of ca. 150–650 ppm during the Barremian–Santonian interval, far less than what is usually considered for the mid Cretaceous. Comparison with available temperature records suggest that although CO₂ may have been a main driver of temperature and primary production at kyr or smaller scales, it was a long-term consequence of the climate-biological system, being decoupled or even showing inverse trends with temperature, at Myr scales. Our analysis indicates that the relationship between CO₂ and temperature is time scale-dependent at least during Greenhouse climate states of the Earth and that primary productivity is a key factor to consider in both past and future analyses of the climate system.

Introduction

Greenhouse gas emissions resulting from human activity are considered to be an important driver of the current global warming. Accurate understanding of this cause-effect relationship is essential to propose realistic scenarios of the future evolution of Earth’s climate, but the human monitoring history of these variables may be very tenuous to properly take into account the relative importance of their large time-scale dynamics into the climate system. Geological records offer an opportunity to address this time-scale issue as they provide track of the long-term dynamics of atmospheric greenhouse gas contents and surface temperature throughout Earth’s history. An analysis of this relationship throughout the whole Phanerozoic pointed out atmospheric pCO₂ as the main multi-scale driver of global warming¹. However, recent improvement and refining of methods to reconstruct pCO₂ evidence that this conclusion was based on outdated imprecise estimates^{2, 3}. Additionally, some factors involved in long-term climate dynamics such as O₂ have been put forward to challenge this paradigm⁴. This raises questions about the extent to which we are correctly interpreting the relative influence of atmospheric pCO₂ forcing on climate⁵.

The analysis of the relationship between pCO₂ and temperature for the Phanerozoic as a whole may not be pertinent because it comprised shifts between drastically different climate states of the Earth in which the relative impact and the dynamics of atmospheric CO₂ may have strongly differed. Of these, Greenhouse is considered to be the default planetary climate state of the Earth, prevailing in more than 70% of the Phanerozoic⁶. The Cretaceous is one of the longest and most studied Greenhouse periods of Earth’s history, with an extensive background of pCO₂ ^7–10 and temperature^11–13 reconstructions. This makes the Cretaceous an exceptional ‘laboratory’ of the past to explore the relationship between the two factors. Nevertheless, pCO₂ reconstructions made so far for the Cretaceous defined temporal trends that are not consistent with those described for temperature^11–13 or the carbon cycle dynamics evidenced by carbon isotopes^14–16. This raises questions about how accurate are pCO₂ reconstructions made so far and how significant was the relative contribution of pCO₂ to temperature evolution during the Cretaceous.

Carbon isotopes are relevant tools for describing past carbon cycle and CO₂ dynamics^14–20. Carbon isotope compositions of terrestrial plants are particularly interesting as they are strongly linked to atmospheric CO₂, which is the main source of carbon assimilated by plants via photosynthesis. The recent construction of models relating carbon isotope fractionation by plants and pCO₂ ^19–21 coupled with the good preservation of the pristine carbon isotope signal within plant fossil materials^22–25, make carbon isotope compositions of plant fossils valuable proxies to analyze the evolution of atmospheric pCO₂ during past periods.

Herein the long-term evolution of atmospheric pCO₂ during the Cretaceous is reconstructed based on the carbon isotope compositions of leaves of the fossil conifer Frenelopsis (δ¹³C_leaf). pCO₂ is estimated from twelve stages spanning the Barremian–Santonian interval (129.4–83.6 Ma) during which major disturbance events of the carbon cycle related to CO₂ release to the atmosphere^{14, 15, 18} and major variations in temperature^11–13 described for the Cretaceous took place. We explore the correspondence between estimated pCO₂ trends and those previously published for temperature to analyze the relative influence of pCO₂ forcing on climate during the Cretaceous.

Results and Discussion

Carbon isotope compositions of the fossil conifer Frenelopsis

δ¹³C_leaf values from the twelve studied stages range from −27.7 to −22.1‰ in average (Fig. 1; Table 1). In most of these stages, δ¹³C_leaf values show similar standard deviations (ca. 0.9‰), thus reflecting a similar range of local environmental variability. Santonian stages are the only exceptions, which may reflect higher climatic instability related to the development of Oceanic Anoxic Event (OAE) 3. Carbon isotope compositions of Frenelopsis in the Barremian, upper Albian and lower Santonian stages are notably ¹³C-depleted compared to others, even though it is not significant in the latter case (Fig. 1). These lower δ¹³C_leaf values are consistent with decreases in the carbon isotope composition of atmospheric CO₂ (δ¹³C_CO2) recently described by Barral et al.¹⁶ (Fig. 1). Overall, there is a good agreement between δ¹³C_leaf and δ¹³C_CO2 curves, though trends in the former are generally magnified. This magnification is more pronounced during the Cenomanian-Turonian interval, coinciding with a critical increase in global temperatures¹¹, which may have slightly reduced carbon isotope fractionation by plants (Δ¹³C_leaf) by reducing the discrimination effect of RuBisCO against ¹³C^{26, 27}.

Figure 1.

Comparison of the carbon isotope compositions of leaves of the fossil conifer Frenelopsis (δ¹³C_leaf) obtained for each studied episode with the evolution of the carbon isotope composition of atmospheric CO₂ (δ¹³C_CO2) throughout the Cretaceous estimated by Barral et al.¹⁶. Envelopes and bar errors correspond to ±1σ.

Table 1.

δ¹³C_leaf, δ¹³C_CO2, Δ¹³C_leaf and pCO₂ values for the twelve studied Cretaceous stages. μ and σ correspond to mean and standard deviation, respectively.

Locality	Age	δ13Cleaf (‰)			δ13CCO2 (‰)		Δ13Cleaf (‰)		pCO2 (ppm)
		μ	σ	n	μ	σ	μ	σ	μ	σ
Hautrage	upper lower Barremian–lower upper Barremian	−27.7	0.9	30	−6.1	0.5	22.3	1.1	476.5	135.2
Uña	upper Barremian	−27.6	0.9	30	−5.7	0.7	22.5	1.1	502.2	155.1
Mas de la Parreta	uppermost Barremian–lower Aptian	−23.0	0.9	30	−5.8	0.7	17.6	1.1	191.6	37.6
El Soplao	lower Albian	−23.1	0.9	30	−5.0	0.3	18.6	1.0	225.6	39.4
San Just	lower–middle Albian	−24.9	0.8	30	−5.8	0.5	19.6	1.0	272.3	53.8
Reillo	upper Albian	−26.8	1.0	30	−5.8	0.3	21.6	1.1	408.9	99.0
Archingeay (A1sl–A)	uppermost Albian	−24.7	1.0	26	−5.7	0.4	19.5	1.1	265.8	53.3
Archingeay (A2sm1–2)	lowermost Cenomanian	−24.4	0.9	30	−5.9	0.4	18.9	1.0	240.9	43.9
La Buzinie	uppermost lower–lowermost middle Cenomanian	−23.1	0.9	30	−5.9	0.5	17.6	1.0	190.5	34.5
Infiesto	upper Turonian–Coniacian	−22.1	0.8	30	−5.1	0.4	17.4	0.9	185.0	30.1
Piolenc	lower Santonian	−26.2	1.6	17	−5.2	0.6	21.5	1.7	432.5	220.3
Belcodème	upper Santonian	−23.4	0.6	30	−4.8	0.5	19.1	0.8	246.0	34.5

Atmospheric CO₂ concentration during the Cretaceous

Δ¹³C_leaf values inferred from δ¹³C_leaf and concomitant δ¹³C_CO2 values (Table 1) were used to estimate pCO₂ values. pCO₂ estimates are in the range of ca. 150–650 ppm for the Barremian–Santonian interval, and the resolution is better than 55 ppm (i.e. 1σ, standard deviation) for most of the stages (Fig. 2; Table 1). This range of values is consistent with the conclusions by Franks et al.²⁰ from a method based on universal equations of leaf gas exchange using stomatal anatomy and carbon isotope ratios of fossil leaves suggesting pCO₂ values below 1000 ppm during most of the Phanerozoic, although their conclusions remain controversial^28–30. Our pCO₂ estimates average ca. 300 ppm for the studied interval, which is in accordance with the most recent revision of GEOCARBSULF model³ and the range of values obtained in most recent works dealing with proxy-based reconstructions^7–10 (Fig. 2). All these reconstructions suggest that atmospheric CO₂ concentrations were comparable to those of present-day atmosphere, which is a conclusion in deep contrast with the classical view that the Cretaceous period was characterized by atmospheric CO₂ concentrations considerably higher than today³¹. This is also consistent with several other approaches. Boron isotope compositions (δ¹¹B) of marine biogenic carbonates are closely related to pCO₂ and pH^32–34. Higher atmospheric concentrations would imply more CO₂ dissolution into the ocean, causing a lowering of seawater pH and δ¹¹B of marine biogenic carbonates^{33, 34}. However, δ¹¹B of Cretaceous and present-day brachiopods as well as ophiolitic serpentinites are close to each other³⁵, thus suggesting that seawater pH and atmospheric pCO₂ were of similar magnitude. Analysis of reef abundance and diversity throughout the Phanerozoic also agrees with our results, indicating that during the Cretaceous no significant long-term rise in ocean acidification, reflecting a long-term rise in pCO₂, took place³⁶. Only discrete short-term events of ocean acidification have been described during the Cretaceous based on marine calcifiers, associated with major Cretaceous OAEs and Hothouse episodes^{37, 38}.

Figure 2.

Comparison of the δ¹³C_leaf-based pCO₂ estimates with most recent proxy-based and model-based pCO₂ reconstructions for the Cretaceous^{3, 7–10}. Envelopes and bar errors represent ±1σ. GCSM: GEOCARBSULF Model reconstructions; PB: Paleosol Barometer reconstructions; SI: Stomatal Index reconstructions.

Was pCO₂ a long-term driver of temperature during the Cretaceous?

Several events of volcanic CO₂ release into the atmosphere linked to the emplacement of large igneous provinces have been inferred from pronounced negative shifts of both carbon and strontium isotope compositions of marine carbonates^{14, 38} associated with the main Cretaceous OAEs^{14, 18, 37}. These CO₂ release events are also evident regarding the evolution of δ¹³C_CO2 throughout the Cretaceous¹⁶ (Fig. 1) and are most likely responsible for temperature increases at the kyr scale^{12, 13}. The biggest of these CO₂ release events in terms of the magnitude of carbon isotope excursions was related to OAE1b and lasted ca. 25 kyr^{14, 39}. This event was responsible for a fast warming of the atmosphere of ca. 0.3 °C and followed by cooler climatic conditions during a period of ca. 45 kyr³⁹. Greater kyr-scale increases in temperature of ca. 6–7 °C have been described associated with the CO₂ release events related to OAE1a^{12, 13} and OAE2¹¹ (Fig. 3). However, our results indicate that drastic Myr-scale pCO₂ drawdowns took place during the warmest time intervals described for the Cretaceous^11–13: a drawdown of ca. 310 ppm (from ca. 500 to ca. 190 ppm) during the upper Barremian-lower Aptian interval (ca. 7.5 Myr), and ca. 225 ppm (from ca. 410 ppm to ca. 190 ppm) during the upper Albian-lower Cenomanian interval (ca. 5 Myr) (Fig. 3). In fact, at the Myr-scale, trends in pCO₂ are even inverse compared to temperature during significant time intervals such as the upper Barremian–lower Aptian (considering the higher resolution temperature curves based on TEX₈₆ proxy^{12, 13}) and the upper Albian–Santonian in which pCO₂ minima are coeval with thermal maxima (Fig. 3). If pCO₂ was rather low and usually decoupled from temperature at the Myr-scale, it may not have been the main long-term driver of global warming during the Cretaceous.

Figure 3.

Reconstructed pCO₂ compared with available sea surface temperature (SST) records^11–13 for the Cretaceous. Vertical grey bars represent major Oceanic Anoxic Events (OAE) occurring during the Cretaceous¹⁸. Envelopes represent ±1σ.

Time scale dependency of the relationship between pCO₂ and temperature

Episodes of enhanced primary production have been associated with the kyr-scale CO₂ release events that took place during major OAEs and the warmest intervals of the Cretaceous^40–44. Barclay et al.⁷ showed that a 20% increase in pCO₂ during OAE2 triggered extensive primary productivity leading to a global increase in rates of organic carbon burial, which eventually resulted in a pCO₂ decrease down to 26% in ca. 100 kyr. This indicates that, as a consequence of a critical CO₂ release event to the atmosphere, primary productivity can account for pCO₂ drawdowns of at least up to the 10⁵ yr scale. The time resolution of our record is not high enough to fully discuss pCO₂ trends on the kyr scale; however, when the time uncertainty of our estimates allows clear perusal, pCO₂ levels are consistently low well after the events of CO₂ release to the atmosphere described in the literature^{18, 45} (Fig. 3). This indicates that the aforementioned primary productivity effect may exceed the kyr scales resulting in a negative balance or a low-level stasis of pCO₂ up to the Myr scales. If primary production were responsible for a long-term decrease in atmospheric pCO₂ during the Cretaceous, we should find a co-varying increase in atmospheric pO₂ as a product of long-term enhanced photosynthesis. This is in accordance with the long-term trends obtained by the recent revision of the GEOCARBSULF model, which describes both a drawdown of pCO₂ and an increase in pO₂ levels from the earliest stages of the Cretaceous up to the mid Cretaceous³. Poulsen et al.⁴ emphasized the importance of O₂ in long-term climate dynamics as under low pO₂ conditions shortwave scattering by clouds and air molecules is reduced; this implies a significant increase in surface shortwave forcing, leading to increase of atmospheric water vapour and consequent enhanced greenhouse forcing which increases global surface temperatures. Thus, their simulations predict that temperature increases under low pO₂ and high pCO₂ conditions. However, during the Cretaceous temperature was maintained high during the late Albian–Santonian interval coinciding with relatively high pO₂ ³ and low pCO₂ conditions^{3, 7} (Fig. 3). This observation reflects that both pO₂ and pCO₂ parameters may not be enough to explain long-term evolution of temperature.

In addition to the conclusions of previous work on atmospheric CO₂ dynamics at kyr time scales⁷ our results indicate that the relationship between CO₂ and temperature was probably scale-dependent during the Cretaceous. In the framework of a Greenhouse climate state functioning, CO₂ appears to have been a main driver of global warming and primary production at timescales up to the kyr⁷. However, our results indicate that it was decoupled from temperature and most likely a long-term consequence of the climate-biological system at Myr scales. Direct extrapolation of these conclusions to time intervals corresponding to different climate states of the Earth is not pertinent because C cycle dynamics, and thus the evolution of pCO₂, might be strongly modulated by different global scale functioning of climate factors. For instance, temperature strongly affects carbon sequestration by living organisms^{26, 27}, and global oceanic and atmospheric circulation patterns are involved in heat transport⁴⁶ and distribution of climate zones⁴⁷ that strongly influence primary productivity. Different dynamics and relative effects of those factors within different climate states of the Earth⁶ may result in different functioning of atmospheric CO₂ dynamics. However, nowadays anthropogenic CO₂ release into the atmosphere also constitutes a main driver of climate change at a human time scale. At larger scales, primary productivity appears to have been an important driver of atmospheric CO₂ concentration during both interglacial⁴⁸ and glacial intervals⁴⁹. Further work on the Myr-scale interrelationships between primary productivity, CO₂ and temperature within these different climate state intervals of the Earth is needed to explore how universal are the conclusions obtained herein. Primary production constitutes a major factor involved in climate dynamics and that regulates the balance between pO₂ and pCO₂. Thus, productivity must be taken into consideration in the evaluation of the relative effect of O₂ and CO₂ on climate dynamics. Present-days anthropogenic CO₂ release event can be compared in magnitude with those of the Cretaceous, reflected in a ca. −1.8‰ shift in δ¹³C_CO2 since the industrial revolution⁵⁰ against the ca. −2‰ shift associated with OAE1b¹⁶ (Fig. 1). However, its critically shorter time of development makes it a more intense disturbance event, and so its forthcoming consequences are difficult to predict. Evaluating the response time of primary production to cushion CO₂ forcing will be critical to allow the prediction of climate evolution and its impact in the near future of life on Earth.

Methods

Twelve stages corresponding to eleven sites from the Cretaceous of western Europe were used for pCO₂ reconstruction (see Supplementary Fig. S1). The sites represent similar terrestrial environments in which the fossil conifer genus Frenelopsis was present, and they are distributed over a narrow palaeolatitude band (30–40° N) of the same climate zone throughout the time range analyzed⁵¹ (see Supplementary Table S1). These stages and sites were selected to reduce as much as possible environmental biases on δ¹³C_leaf.

Frenelopsis was chosen as a pCO₂ proxy because it was frequent and abundant in Cretaceous ecosystems^52–55. Frenelopsis is a valuable material for performing stable carbon isotope analysis due to its resistance to decay and diagenesis. It provides accurate δ¹³C_leaf values, which are comparable to those of living plant leaves^{25, 56, 57}.

Frenelopsis leaves were treated with HCl 32% following the protocol recommended by Barral et al.²⁵. Thirty Frenelopsis leaves per locality were separately reduced to powder using an agate mortar. Three replicates of ca. 100 μg per leaf were randomly selected and weighted using a precision balance Sartorius ME36S and loaded into tin capsules. Carbon isotope analyses were performed using a continuous flow IRMS configuration, being He the carrier gas, using a EuroEA3028™–HT elemental analyzer working in combustion mode and interfaced with an IP60 isotope ratio mass spectrometer. Carbon isotope ratios were calibrated against the international standard IAEA-C4 (δ¹³C = −23.96‰) and casein (δ¹³C = −22.67‰) as an internal reference. Based on repeated analysis of standards, analytical reproducibility was better than ±0.2‰.

pCO₂ values were estimated for each stage using the equation relating pCO₂ to δ¹³C_leaf described by Schubert and Jahren^{19, 21} (r = 0.94):

$${\mathrm{\Delta }}^{13}{\mathrm{C}}_{\mathrm{leaf}}=[(28.26)(0.22)(p{\mathrm{CO}}_2+23.9)]/[28.26+(0.22)(p{\mathrm{CO}}_2+23.9)]$$

Δ¹³C_leaf values were estimated from δ¹³C_leaf and concomitant δ¹³C_CO2 values by using the equation proposed by Farquhar et al.⁵⁸:

$${\mathrm{\Delta }}^{13}{\mathrm{C}}_{\mathrm{leaf}}={\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{Co2}}-{\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{leaf}}/1+({\mathrm{\delta }}^{13}{\mathrm{C}}_{\mathrm{co2}}/1000)$$

δ¹³C_CO2 values were taken from the δ¹³C_carb-based δ¹³C_CO2 estimates provided by Barral et al.¹⁶. We avoid discrimination bias due to different physiological functioning related to taxonomic specificity by using always the same taxon, and thus δ¹³C_leaf faithfully represents environmental variability. In order to avoid misrepresented pCO₂ estimates due to subtle uncorrelations between the beds from which δ¹³C_CO2 and δ¹³C_leaf were obtained, we accounted for δ¹³C_CO2 variability in the uncertainty timespan of the Frenelopsis localities by averaging and estimating cumulated error for each stage. This was made by randomly drawing 10,000-repetition Gaussian distributions for each single case within the interval of uncertainty of a given stage, defined by the pre-determined means and standard deviations of each case, and calculating the mean and the standard deviations of the 10,000 randomly generated sets. This procedure was also implemented to include the associated errors to each δ¹³C_leaf, δ¹³C_CO2 and Δ¹³C_leaf values into the aforementioned equations to account for all sources of error in the resolution of pCO₂ estimates.

Author Contributions

A.B. conceived the original idea for this study and led the writing of the manuscript under the advising of B.G. and C.L. Plant fossil materials were provided by B.G. and V.D.-G. δ¹³C_leaf measurements were made by A.B., F.F. and C.L. All the authors contributed to data interpretation and manuscript editing.

Competing Interests

The authors declare that they have no competing interests.