Australia: The Land Where Time Began

A biography of the Australian continent 

Mesozoic and Early Cainozoic Climate Linked to atmospheric Carbon Dioxide

In the Quaternary period the relationship between carbon dioxide (CO₂) and climate has been investigated extensively, but the role of CO₂ during changes of temperature during the remainder of the history of the Earth is less clear (Veizer et al., 2000). It has been particularly difficult to interpret the range of geological evidence for cool periods during the high CO₂ Mesozoic ‘green house world’ (Royer et al., 2004; Royer et al., 2006) of high concentrations of CO₂ that has been indicated by models (Berner, 2004) and fossil soils (Ekart, Cerling, Montañez & Tabor, 1999). In this paper Fletcher et al., present high resolution records of Mesozoic and early Cainozoic atmospheric CO₂ from a combination of carbon-isotope analyses of nonvascular plant (bryophyte) fossils and theoretical modelling (Fletcher et al., 2005; Fletcher et al., 2006). Atmospheric CO₂ concentrations are indicated by these records to have risen from ~420 ppmv in the Triassic (about 200 Ma) to a peak in the middle Cretaceous (about 100 Ma). Then atmospheric concentrations of CO₂ declined to ~680 ppmv by 60 Ma. These variations are shown by time-series comparison to coincide with large climate shifts in the Mesozoic (Dromart et al., 2003; Wilson et al., 2002; Shouten et al., 2003), which contrasts with earlier suggestions of cimate-CO₂ decoupling during this interval (Veizer, Godderis & François, 2000). On several occasions these reconstructed concentrations of atmospheric CO₂ drop below the threshold for the initiation of glaciations (DeConto & Pollard, 2003) and therefore help to explain the occurrence of cold intervals in a ‘greenhouse world’ (Royer,2006).

For a number of reasons the fundamental role of CO₂ as a driver of past warm climates in the Mesozoic and early Cainozoic has remained obscure. Widely disparate estimates of CO₂ for this critical phase in the history of the Earth (Royer, 2006) have been yielded by different proxies, such as stomatal indices of fossil leaves and palaeosols suggest, in general, that CO₂ levels were thousands of ppmv higher than fossil leaves (Retallack, 2001). Furthermore, it is not clear whether mismatches between oxygen-isotope (δ¹⁸O) that are based on palaeoclimate records and ancient estimates (Veizer, Godderis & François, 2000) of CO₂, from either proxies or numerical carbon-cycle models (Royer et al., 2004; Royer, 2006; Berner, 2004), reflect genuine decoupling between CO₂ and climate or are an artefact that is associated with difficulties of reconstructing accurately these critical features of the history of the Earth (Kump, 2002). However, evidence for brief cool pulses, that are typically <1 Myr, that punctate climates that are generally warm in the Jurassic and Cretaceous is challenging the prevailing high CO₂ Mesozoic ‘greenhouse world’ paradigm (Royer, 2006) that is indicated by palaeosols (Ekart, 1999) and geochemical carbon cycle models (Berner, 2004),

In this paper Fletcher et al., report a series of CO₂ estimates for the ancient atmosphere as far back as >200 Ma, by use of a new method that was based on the strong dependency of carbon-isotope fractionation (Δ¹³C) in terrestrial bryophytes on atmospheric CO₂ during photosynthetic carbon uptake (Fletcher et al., 2005; Fletcher et al., 2006). This approach is the terrestrial analogue of the marine phytoplankton CO₂ proxy (Freeman & Hayes, 1992), used successfully to reconstruct glacial-interglacial CO₂ changes from peat cores that dated to the Pleistocene (White et al., 1994). In terms of function, Δ¹³C approaches the maximum value (30‰) expressed by the primary carboxylating enzyme carboxylase/oxygenase (RuBisCo), and the concentration of CO₂ at the site of fixation (C_i) approaches the external concentration of CO₂ (C_a) (Fletcher et al., 2005; Fletcher et al., 2006). Because Ci is set by the combined influence of net photosynthetic CO₂ uptake (A) and resistance to the inward diffusion of CO₂ (r), Δ¹³C is proportional to C_i/C_a, where C_a – C_i = A x r. r is not subject to stomatal control in bryophytes, as it is in the leaves of vascular plants, and Δ¹³C varies with C_a and the photosynthetic demand for CO₂ by RuBisCo (Fletcher et al., 2005; Fletcher et al., 2006).

In this study ancient concentrations of CO₂ were estimated on the basis of 93 δ¹³C measurements on 61 liverwort gametophyte compression fossils. The samples were from 12 localities on 5 continents and spanned 150 Myr. Conversion of δ¹³C measurements to palaeoatmospheric concentrations of CO₂ is achieved after calculating Δ¹³C to account for variations in the δ¹³C of atmospheric CO₂, and inverting a mechanistic mathematical model that has been well validated (BRYOCARB) describing the CO₂ dependency of Δ¹³C in bryophytes. Fossil Δ¹³C values that have been calculated lie within the range that is sensitive to variations in CO₂, with the exception of those for the (Ypresian) specimens from the Palaeocene. The balance between the photosynthetic CO₂ draw-down and the supply of diffusional CO₂ from the atmosphere to the tissues is determined by the Bryocarb models Δ¹³C as a function of C_a/C_i. Photosynthesis is represented with established biochemical theory for assimilation of CO₂ and accounts explicitly for the interactive effects of irradiance, O₂ and temperature (Fletcher et al., 2006) (see Supplementary Information for derivation of values). The supply of atmospheric CO₂ to the site of photosynthesis is modelled by analogy with Ohm’s law (Fletcher et al., 2006). In liverworts with pores, where r is regulated by varying density of pores and morphology, BRYOCARB _p calibrates Δ¹³C change. However, some groups of liverworts do not have pores and so cannot regulate in this manner. In this study a second version of the model, BRYOCARB_NP, to calibrate Δ¹³C changes in liverworts that have no pores, with a prescribed resistance and a lower maximum rate of carboxylation, that is RuBisCo limited, (V_cmax; see supplementary Information) (Fletcher et al., 2005). For each CO₂ estimate uncertainties are characterised by deriving probability density functions (PDFs) by the use of large (25,000) ensemble Monte Carlo simulations to integrate uncertainties into inferred and measured input variables for both versions of BRYOCARB).

The new reconstruction of atmospheric CO₂, that spans approximately ⅓ of the Phanerozoic eon, which covers the past 450 Myr, in this study exhibits trends that are coherent throughout the Mesozoic and early Cainozoic. These trends are primarily a reflection of the systematic shifts that are driven by CO₂ of up to 5‰ of fossil bryophyte Δ¹³C; a secondary role is played by other environmental inputs in the determination of the pattern. Estimates of CO₂ that are derived from fossil bryophyte Δ¹³C calibrated by using either BRYOCARB or BRYOCARB_NP both rise from comparatively low concentrations in the Triassic and Early Jurassic to a peak of ~1,139 ppmv in the Middle Cretaceous, following which they decline toward 680 ppmv in the early Cainozoic. Using BRYOCARB_NP calibrated Δ¹³C leads to CO₂ estimates that are approximately 60 ppmv higher than those using BRYOCARB_P, as the increased resistance to inwards diffusion of CO₂ meant that a higher C_a is required to produce C_i/C_a inferred from fossil Δ¹³C. A singular high CO₂ estimate during the Ypresian (48.6-55.8 Ma 2,300-4,700 ppmv) is the only exception to a coherent trend which might be a reflection of the reduced sensitivity of the bryophyte proxy at high CO₂ and should therefore be regarded as less certain than the others. According to Fletcher et al. they have omitted this from their subsequent time-series analyses until it is understood better by investigation of further fossil material, though they recognise that it coincides with the peak of warmth during the Cainozoic.

In this study the records of Bryophyte CO₂ were used to evaluate the role of this greenhouse gas in the evolution of warm climates in the Mesozoic and early Cainozoic by calculating the temperatures that were forced by CO₂ for comparison with independent palaeoclimate records. In this study cross-correlations were computed between the changes that resulted in mean global surface temperature (ΔT) and tropical ocean surface temperature series that were adjusted for pH that were based on the δ¹⁸O of marine calcium carbonate fossils (Veizer, Godderis & François, 2000; Ridgwell, 2005), after accounting for uncertainties in ΔT that were due to imperfect knowledge of vital effects (Veizer, Godderis & Françoise, 2000), alteration that occurred after deposition (Wilson, Norris & Cooper, 2002) and the δ¹⁸O of ocean water (Roche, Donnadieu, Pucéat & Paillard, 2006). The major pattern that was revealed by the pH-adjusted δ¹⁸O record, that is supported by other evidence, that shows cooler Middle Jurassic (167-160 Ma) (Dromart et al., 2003) and warmer climates in the Cretaceous (Wilson, Norris & Cooper, 2002; Shouten et al., 2003), which suggests that it is a useful metric for evaluating the greenhouse forcing by calculated CO₂ trends. Agreement between calculated changes in ΔT was quantified by the use of a       hierarchical curve reconstruction technique that accommodated the irregular spacing of CO₂/ΔT determinations and uncertainties in their estimation and dating. The same approach was also used in the evaluation of CO₂ as a driver of climate change using an extensive  compilation of estimates of CO₂ that were derived from studies of fossil leaves and palaeosols (Royer, 2006).

It was indicated by the results that bryophyte and stomatal histories reproduce the average climate of the Mesozoic, which yields average global temperatures within 0.7^oC of the temperature that had been estimated in the pH-adjusted δ¹⁸O climate record. Calculated temperatures with palaeosol CO₂ histories were, in contrast, 2.0-2.5^oC higher than the marine record. It was revealed by cross-correlations that only ΔT patterns that were calculated from CO₂ histories of fossil bryophytes were correlated positively with ΔT from the marine δ¹⁸O record (medium coefficient = 0.2-0.5). Positive correlations with independent records of climate were not produced by the stomatal or palaeosol proxies. It was suggested by Fletcher et al. that the capacity for the bryophyte proxy to describe overall temperatures as well as the magnitude of climate change by evidence of forcing by CO₂ of major climate shifts, though this suggestion requires the assessment of reliability of each of the 3 proxies that were used to reconstruct ancient concentrations of atmospheric CO₂.

Therefore, they benchmarked proxy performance against predictions of numerical carbon cycle models in order to identify those that are most consistent with variations that are well known in the long term sources and sinks for CO₂. Comparisons were made with a series of 3 histories of CO₂ that were simulated by using a coupled carbon-oxygen-sulphur cycle geochemical model (Berner, 2006) that incorporated different representations of the weathering of basalt in order to describe the range of field observations (Dessert et al., 2003; Taylor, 2000). Treatment of all other source and sink terms remained unchanged in each of the following 3 simulations:

1.    Standard weathering (Berner, 2006) (GEO_SW), ,

2.    Moderate weathering that was linked to variations in the production and exposure of volcanic rocks (Berner, 2006) assuming values that are unusually low for basalt-seawater (BSW) reaction rate (GEO_VW) and,

3.    As 2 but with enhanced weathering of basalts (Dessert et al., 2003) and very low rates of (BSW), or with moderate weathering and reasonable BSW rates (GEO_VW).

Proxy performance was defined with summary statistics for absolute differences in proxy-model CO₂ values that were integrated over the Mesozoic and early Cainozoic, and correlation coefficients between proxy-model CO₂ time series.

Fossil bryophytes are indicated by these comparisons to provide a record of fluctuations that is more coherent in the atmospheric CO₂ content of the Earth over the Mesozoic and early Cainozoic than either of the other 2 existing proxies that cover the same interval. Fossil bryophytes and the CO₂ concentrations that have been estimated by stomatal proxy are lower than the concentrations in all of the 3 model simulations though this underestimation was reduced progressively from ~-750 ppmv to only ~-90 ppmv with increasing emphasis being placed on the weathering of volcanic silicates. Contrasting with this, the CO₂ values were overestimated considerably by the palaeosol proxy, increasing from ~+590 ppmv to ~+1,370 ppmv across the same series of simulations. The case for variations in the production and exposure of basalts as playing a prominent role in the regulation of CO₂ levels on a timescale of multimillion years (Dessert et al., 2003; Berner, 2006), is strengthened by the closest agreement between 2 out of 3 proxies with GEOEVW. It is indicated, however, by cross-correlation analyses that the bryophyte proxy is the only one that was both a good fit and exhibited a positive correlation in the trends in CO₂ simulated with GEO_EV and GEO_EVW. It is suggested by this that the bryophyte proxy has accuracy and captures some patterns of the histories of CO₂ simulated by 2 versions of the geochemical model. The other 2 proxies do not fulfil both criteria simultaneously. The stomatal CO₂ proxy record correlated best with GEOSW CO₂ predictions, though this simulation gave the largest CO₂ difference. The palaeosol proxy CO₂ record did not correlate strongly with any of the model simulations, and had concentrations of CO₂ that differed from model results.

It was concluded by Fletcher et al. that forcing by CO₂ played an important role in Mesozoic and early Cainozoic climate change, and that earlier claims for a decoupling in the CO₂-climate change relationship at this critical phase in the history of the Earth (Veizer, Godderis & François, 2000) are premature. Changes in the CO₂ greenhouse effect that are driven by geochemical controls on the long-term climate cycle (Berner, 2004; Berner, 2006), thereby contributing an explanation for climates of the Jurassic and Cretaceous without the need to invoke the influence of cosmic rays on cloud cover and planetary albedo (Wallmann, 2004). Furthermore, reconstructed concentrations of CO₂ in the study (500-1,300 ppmv) coincide with the threshold for the initiation of glaciations as determined by global climate modelling calibrated for mid-Cainozoic conditions (DeConto & Pollard, 2003) (560-1,120 ppmv). CO₂ histories that are derived from palaeosols (Ekart et al., 1999) and earlier geochemical carbon cycle models (Berner, 2004) exceed the threshold by several thousand ppmv, though the exact range is likely to be dependent on the particular climate model. Therefore, the new CO₂ reconstructions in this study offer a resolution for the high CO₂ ‘greenhouse world’ paradox by explaining better the apparent susceptibility of the Earth system to experience brief discrete cool events during the Mesozoic (Royer, 2006).

Fletcher, B., et al. (2007). "Atmospheric carbon dioxide linked Mesozoic and Early Cenozoic climatic change." Nature Geoscience 1: 43-48.