Papers on atmospheric CO2 from proxies

This is a list of papers on atmospheric carbon dioxide contents determined from proxies. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

UPDATE (June 3, 2012): Hönisch et al. (2009) added. Thank to Nick for pointing it out.
UPDATE (April 18, 2012): Breecker et al. (2010) added. Thanks to Barry for pointing it out.

Atmospheric CO2 over the Past 66 Million Years from Marine Archives – Rae et al. (2021). “Throughout Earth’s history, CO₂ is thought to have exerted a fundamental control on environmental change. Here we review and revise CO₂ reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO₂ and climate throughout the Cenozoic, with peak CO₂ levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO₂ drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO₂ record highlights the truly geological scale of anthropogenic CO₂ change: Current CO₂ levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO₂ levels of the Miocene or Eocene in the coming century.” James W.B. Rae, Yi Ge Zhang, Xiaoqing Liu, Gavin L. Foster, Heather M. Stoll, Ross D.M. Whiteford. Annual Review of Earth and Planetary Sciences 46:609-641. https://doi.org/10.1146/annurev-earth-082420-063026.

Atmospheric CO2 during the Mid-Piacenzian Warm Period and the M2 glaciation – de la Vega et al. (2020). “The Piacenzian stage of the Pliocene (2.6 to 3.6 Ma) is the most recent past interval of sustained global warmth with mean global temperatures markedly higher (by ~2–3 °C) than today. Quantifying CO₂ levels during the mid-Piacenzian Warm Period (mPWP) provides a means, therefore, to deepen our understanding of Earth System behaviour in a warm climate state. Here we present a new high-resolution record of atmospheric CO₂ using the δ¹¹B-pH proxy from 3.35 to 3.15 million years ago (Ma) at a temporal resolution of 1 sample per 3–6 thousand years (kyrs). Our study interval covers both the coolest marine isotope stage of the mPWP, M2 (~3.3 Ma) and the transition into its warmest phase including interglacial KM5c (centered on ~3.205 Ma) which has a similar orbital configuration to present. We find that CO₂ ranged from 394−9+34 ppm to 330−21+14 ppm: with CO₂ during the KM5c interglacial being 391−28+30 ppm (at 95% confidence). Our findings corroborate the idea that changes in atmospheric CO₂ levels played a distinct role in climate variability during the mPWP. They also facilitate ongoing data-model comparisons and suggest that, at present rates of human emissions, there will be more CO₂ in Earth’s atmosphere by 2025 than at any time in at least the last 3.3 million years.” Elwyn de la Vega, Thomas B. Chalk, Paul A. Wilson, Ratna Priya Bysani, Gavin L. Foster. Scientific Reports 10:11002. https://doi.org/10.1038/s41598-020-67154-8.

Atmospheric paleo-CO₂ estimates based on Taxodium distichum (Cupressaceae) fossils from the Miocene and Pliocene of Eastern North America – Stults et al. (2011) “Neogene atmospheric paleo-CO₂ estimates based on fossils of the extant cupressaceous conifer species Taxodium distichum from the Brandywine Formation of Maryland and the Citronelle Formation of southern Alabama are presented. These are important as the first such estimates from eastern North American paleofloras, and provide new evidence from a time for which the role of CO₂ in climate change is controversial. Comparisons of the stomatal density (SD) of the fossil leaf cuticles to a calibration curve constructed from modern leaves of the same species collected over the last century of anthropogenic CO₂ increase produces Miocene and Pliocene atmospheric paleo-CO₂ mean estimates of 360 and 351 ppmv, respectively. Although the temporal resolution of the fossil sites is low, these results are in agreement with multiple independent proxies that indicate near modern CO₂ levels during this interval, and demonstrate the utility of T. distichum leaves as instruments for stomatal frequency analysis.” Debra Z. Stults, Friederike Wagner-Cremer and Brian J. Axsmith, Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2011.06.017.

Transient Middle Eocene Atmospheric CO₂ and Temperature Variations – Bijl et al. (2010) “The long-term warmth of the Eocene (~56 to 34 million years ago) is commonly associated with elevated partial pressure of atmospheric carbon dioxide (pCO₂). However, a direct relationship between the two has not been established for short-term climate perturbations. We reconstructed changes in both pCO₂ and temperature over an episode of transient global warming called the Middle Eocene Climatic Optimum (MECO; ~40 million years ago). Organic molecular paleothermometry indicates a warming of southwest Pacific sea surface temperatures (SSTs) by 3° to 6°C. Reconstructions of pCO₂ indicate a concomitant increase by a factor of 2 to 3. The marked consistency between SST and pCO₂ trends during the MECO suggests that elevated pCO₂ played a major role in global warming during the MECO.” Peter K. Bijl, Alexander J. P. Houben, Stefan Schouten, Steven M. Bohaty, Appy Sluijs, Gert-Jan Reichart, Jaap S. Sinninghe Damsté and Henk Brinkhuis, Science 5 November 2010: Vol. 330 no. 6005 pp. 819-821, DOI: 10.1126/science.1193654. [Full text]

Alkenone and boron-based Pliocene pCO₂ records – Seki et al. (2010) “The Pliocene period is the most recent time when the Earth was globally significantly ( 3 °C) warmer than today. However, the existing pCO₂ data for the Pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo-pCO₂. This disagreement, coupled with the general low temporal resolution of the published records, does not allow a robust assessment of the role of declining pCO₂ in the intensification of the Northern Hemisphere Glaciation (INHG) and a direct comparison to other proxy records are lacking. For the first time, we use a combination of foraminiferal (δ¹¹B) and organic biomarker (alkenone-derived carbon isotopes) proxies to determine the concentration of atmospheric CO₂ over the past 5 Ma. Both proxy records show that during the warm Pliocene pCO₂ was between 330 and 400 ppm, i.e. similar to today. The decrease to values similar to pre-industrial times (275–285 ppm) occurred between 3.2 Ma and 2.8 Ma — coincident with the INHG and affirming the link between global climate, the cryosphere and pCO₂.” Osamu Seki, Gavin L. Foster, Daniela N. Schmidt, Andreas Mackensen, Kimitaka Kawamura and Richard D. Pancost, Earth and Planetary Science Letters, Volume 292, Issues 1-2, 15 March 2010, Pages 201-211, doi:10.1016/j.epsl.2010.01.037. [Full text]

A benthic δ¹³C-based proxy for atmospheric pCO₂ over the last 1.5 Myr – Lisiecki (2010) “A high-resolution marine proxy for atmospheric pCO₂ is needed to clarify the phase lag between pCO2 and marine climate proxies and to provide a record of orbital-scale pCO₂ variations before the oldest ice core measurement at 800 ka. Benthic δ¹³C data should record deep ocean carbon storage and, thus, atmospheric pCO₂. This study finds that a modified δ¹³C gradient between the deep Pacific and intermediate North Atlantic (Δδ¹³C_P-^NA/2) correlates well with pCO₂. Δδ¹³C_P-^NA/2 reproduces characteristic differences between pCO₂ and ice volume during Late Pleistocene glaciations and indicates that pCO₂ usually leads terminations by 0.2–3.7 kyr but lags by 3–10 kyr during two “failed” terminations at 535 and 745 ka. Δδ¹³C_P-^NA/2 gradually transitions from 41- to 100-kyr cyclicity from 1.3–0.7 Ma but has no secular trend in mean or amplitude since 1.5 Ma. The minimum pCO₂ of the last 1.5 Myr is estimated to be 155 ppm at ∼920 ka.” Lisiecki, L. E. (2010), Geophys. Res. Lett., 37, L21708, doi:10.1029/2010GL045109. [Full text]

Atmospheric CO₂ concentrations during ancient greenhouse climates were similar to those predicted for A.D. 2100 – Breecker et al. (2010) “Quantifying atmospheric CO₂ concentrations ([CO₂]_atm) during Earth’s ancient greenhouse episodes is essential for accurately predicting the response of future climate to elevated CO₂ levels. Empirical estimates of [CO₂]_atm during Paleozoic and Mesozoic greenhouse climates are based primarily on the carbon isotope composition of calcium carbonate in fossil soils. We report that greenhouse [CO₂]_atm have been significantly overestimated because previously assumed soil CO₂ concentrations during carbonate formation are too high. More accurate [CO₂]_atm, resulting from better constraints on soil CO₂, indicate that large (1,000s of ppmV) fluctuations in [CO₂]_atm did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A.D. 2100..” D. O. Breecker, Z. D. Sharp, and L. D. McFadden, PNAS January 12, 2010 vol. 107 no. 2 576-580, doi: 10.1073/pnas.0902323106. [Full text]

Coupling of CO₂ and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years – Tripati et al. (2009) “The carbon dioxide (CO₂) content of the atmosphere has varied cyclically between ~180 and ~280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, the partial pressure of CO₂ (pCO₂) is much less certain, and the relation between pCO₂ and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO₂ during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were ~3° to 6°C warmer and sea level was 25 to 40 meters higher than at present, pCO₂ appears to have been similar to modern levels. Decreases in pCO₂ were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).” Aradhna K. Tripati, Christopher D. Roberts and Robert A. Eagle, Science 4 December 2009: Vol. 326 no. 5958 pp. 1394-1397, DOI: 10.1126/science.1178296. [Full text]

Atmospheric Carbon Dioxide Concentration Across the Mid-Pleistocene Transition – Hönisch et al. (2009) “The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons. Here we present a 2.1-million-year record of sea surface partial pressure of CO₂ (Pco₂), based on boron isotopes in planktic foraminifer shells, which suggests that the atmospheric partial pressure of CO₂ (pco₂) was relatively stable before the mid-Pleistocene climate transition. Glacial Pco₂ was ~31 microatmospheres higher before the transition (more than 1 million years ago), but interglacial Pco₂ was similar to that of late Pleistocene interglacial cycles (<450,000 years ago). These estimates are consistent with a close linkage between atmospheric CO₂ concentration and global climate, but the lack of a gradual decrease in interglacial Pco₂ does not support the suggestion that a long-term drawdown of atmospheric CO₂ was the main cause of the climate transition.” Bärbel Hönisch, N. Gary Hemming, David Archer, Mark Siddall and Jerry F. McManus, Science 19 June 2009: Vol. 324 no. 5934 pp. 1551-1554, DOI: 10.1126/science.1171477.

CO₂-forced climate thresholds during the Phanerozoic – Royer (2006) “The correspondence between atmospheric CO₂ concentrations and globally averaged surface temperatures in the recent past suggests that this coupling may be of great antiquity. Here, I compare 490 published proxy records of CO₂ spanning the Ordovician to Neogene with records of global cool events to evaluate the strength of CO₂-temperature coupling over the Phanerozoic (last 542 my). For periods with sufficient CO₂ coverage, all cool events are associated with CO₂ levels below 1000 ppm. A CO₂ threshold of below 500 ppm is suggested for the initiation of widespread, continental glaciations, although this threshold was likely higher during the Paleozoic due to a lower solar luminosity at that time. Also, based on data from the Jurassic and Cretaceous, a CO₂ threshold of below 1000 ppm is proposed for the initiation of cool non-glacial conditions. A pervasive, tight correlation between CO₂ and temperature is found both at coarse (10 my timescales) and fine resolutions up to the temporal limits of the data set (million-year timescales), indicating that CO₂, operating in combination with many other factors such as solar luminosity and paleogeography, has imparted strong control over global temperatures for much of the Phanerozoic.” Dana L. Royer, Geochimica et Cosmochimica Acta, Volume 70, Issue 23, 1 December 2006, Pages 5665-5675, doi:10.1016/j.gca.2005.11.031. [Full text]

Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals – McElwain et al. (2005) “The marine sedimentary record exhibits evidence for episodes of enhanced organic carbon burial known as ‘oceanic anoxic events’ (OAEs). They are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Causal mechanisms for the enhancement of organic carbon burial during OAEs are still debated, but it is thought that such events should draw down significant quantities of atmospheric carbon dioxide. In the case of the Toarcian OAE (183 million years ago), a short-lived negative carbon-isotope excursion in oceanic and terrestrial reservoirs has been interpreted to indicate raised atmospheric carbon dioxide caused by oxidation of methane catastrophically released from either marine gas hydrates or magma-intruded organic-rich rocks. Here we test these two leading hypotheses for a negative carbon isotopic excursion marking the initiation of the Toarcian OAE using a high-resolution atmospheric carbon dioxide record obtained from fossil leaf stomatal frequency. We find that coincident with the negative carbon-isotope excursion carbon dioxide is first drawn down by 350 +/- 100 p.p.m.v. and then abruptly elevated by 1,200 +/- 400 p.p.m.v, and infer a global cooling and greenhouse warming of 2.5 +/- 0.1 °C and 6.5 +/- 1 °C, respectively. The pattern and magnitude of carbon dioxide change are difficult to reconcile with catastrophic input of isotopically light methane from hydrates5 as the cause of the negative isotopic signal. Our carbon dioxide record better supports a magma-intrusion hypothesis, and suggests that injection of isotopically light carbon from the release of thermogenic methane occurred owing to the intrusion of Gondwana coals by Toarcian-aged Karoo-Ferrar dolerites.” Jennifer C. McElwain, Jessica Wade-Murphy & Stephen P. Hesselbo, Nature 435, 479-482 (26 May 2005), doi:10.1038/nature03618

Mid-Cretaceous pCO₂ based on stomata of the extinct conifer Pseudofrenelopsis (Cheirolepidiaceae) – Haworth et al. (2005) “Stomatal characteristics of an extinct Cretaceous conifer, Pseudofrenelopsis parceramosa (Fontaine) Watson, are used to reconstruct atmospheric carbon dioxide (pCO₂) over a time previously inferred to exhibit major fluctuations in this greenhouse gas. Samples are from nonmarine to marine strata of the Wealden and Lower Greensand Groups of England and the Potomac Group of the eastern United States, of Hauterivian to Albian age (136–100 Ma). Atmospheric pCO₂ is estimated from the ratios between stomatal indices of fossil cuticles and those from four modern analogs (nearest living equivalent plants). Using this approach, and two calibration methods to explore ranges, results show relatively low and only slightly varying pCO₂ over the Hauterivian–Albian interval: a low of ∼560–960 ppm in the early Barremian and a high of ∼620–1200 ppm in the Albian. Data from the Barremian Wealden Group yield pCO₂ values indistinguishable from a soil-carbonate–based estimate from the same beds. The new pCO₂ estimates are compatible with sedimentological and oxygen-isotope evidence for relatively cool mid-Cretaceous climates.” Matthew Haworth, Stephen P. Hesselbo, Jennifer C. McElwain, Stuart A. Robinson and James W. Brunt, Geology, v. 33 no. 9 p. 749-752, doi: 10.1130/G21736.1.

Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene – Pagani et al. (2005) “The relation between the partial pressure of atmospheric carbon dioxide (pCO2) and Paleogene climate is poorly resolved. We used stable carbon isotopic values of di-unsaturated alkenones extracted from deep sea cores to reconstruct pCO2 fromthe middle Eocene to the late Oligocene (∼45 to 25 million years ago). Our results demonstrate that pCO2 ranged between 1000 to 1500 parts per million by volume in the middle to late Eocene, then decreased in several steps during the Oligocene, and reached modern levels by the latest Oligocene. The fall in pCO2 likely allowed for a critical expansion of ice sheets on Antarctica and promoted conditions that forced the onset of terrestrial C4 photosynthesis.” Mark Pagani, James C. Zachos, Katherine H. Freeman, Brett Tipple and Stephen Bohaty, Science 22 July 2005: Vol. 309 no. 5734 pp. 600-603, DOI: 10.1126/science.1110063. [Full text]

Atmospheric CO₂ During the Late Paleozoic and Mesozoic: Estimates from Indian Soils – Ghosh et al. (2005) No abstract. Prosenjit Ghosh, S.K. Bhattacharya and Parthasarathi Ghosh, A History of Atmospheric CO2 and Its Effects on Plants, Animals, and Ecosystems, Ecological Studies, 2005, Volume 177, Part 1., 8-34, DOI: 10.1007/0-387-27048-5_2. [Full text]

Fossil bryophytes as recorders of ancient CO₂ levels: Experimental evidence and a Cretaceous case study – Fletcher et al. (2005) “Biological and geochemical CO₂ proxies provide critical constraints on understanding the role of atmospheric CO₂ in driving climate change during Earth history. As no single existing CO₂ proxy is without its limitations, there is a clear need for new approaches to reconstructing past CO₂ concentrations. Here we develop a new pre-Quaternary CO₂ proxy based on the stable carbon isotope composition (δ¹³C) of astomatous land plants. In a series of CO₂-controlled laboratory experiments, we show that the carbon isotope discrimination (Δ¹³C) of a range of bryophyte (liverwort and moss) species increases with atmospheric CO₂ across the range 375 to 6000 ppm. Separate experiments establish that variations in growth temperature, water content and substrate type have minor impacts on the Δ13C of liverworts but not mosses, indicating the greater potential of liverworts to faithfully record past variations in CO₂. A mechanistic model for calculating past CO₂ concentrations from bryophyte Δ13C (White et al., 1994) is extended and calibrated using our experimental results. The potential for fossil liverworts to record past CO₂ changes is investigated by analyzing the δ¹³C of specimens collected from Alexander Island, Antarctica dating to the “greenhouse” world of the mid-Cretaceous. Our analysis and isotopic model yield mid-Cretaceous CO₂ concentrations of 1000–1400 ppm, in general agreement with independent proxy data and long-term carbon cycle models. The exceptionally long evolutionary history of bryophytes offers the possibility of reconstructing CO₂ concentrations back to the mid-Ordovician, pre-dating all currently used quantitative CO₂ proxies.” Fletcher, B. J., D. J. Beerling, S. J. Brentnall, and D. L. Royer (2005), Global Biogeochem. Cycles, 19, GB3012, doi:10.1029/2005GB002495. [Full text]

Fe(CO₃)OH in goethite from a mid-latitude North American Oxisol: estimate of atmospheric CO₂ concentration in the Early Eocene “climatic optimum” – Yapp (2004) “Measured mole fractions (X) and δ13C values of the Fe(CO₃)OH component in pedogenic goethite from a mid-latitude Oxisol of Early Eocene age (≈52 Ma B.P.) range from 0.0014 to 0.0064 and −20.1 to −15.4‰, respectively. These values of X imply that concentrations of CO₂ gas in the paleosol were ≈7400 to ≈34,000 ppm. δ13C and 1/X are correlated and define a linear, soil-CO₂ diffusive mixing line with a positive slope. Such positive slopes are characteristic of mixing of two isotopically distinct CO₂ endmembers (atmospheric CO₂ and CO₂ from oxidation of soil organic matter). From the intercept of the mixing line, it is calculated that the δ 13C value of organic matter in the ancient soil was ≈−28.0‰. The magnitude of the slope implies an Early Eocene atmospheric CO₂ concentration of ≈2700 ppm. A simple model for forest soils suggests that a “canopy effect” may cause atmospheric CO₂ concentrations deduced from pedogenic minerals to underestimate the actual concentrations of atmospheric CO₂. If a significant forest canopy were present at the time of formation of pedogenic goethite in the Ione Fm, the concentration of 2700 ppm calculated for atmospheric CO₂ could be slightly low, but the underestimate is expected to be < ≈300 ppm (i.e., less than the analytical uncertainty). The relatively high concentration of 2700 ppm inferred for atmospheric CO₂ at ≈52 Ma B.P. would have been coincident with the Early Eocene climatic optimum. This result seems to support the case for an important role for variations of atmospheric CO₂ in the modification of global paleoclimate.” Crayton J. Yapp, Geochimica et Cosmochimica Acta, Volume 68, Issue 5, 1 March 2004, Pages 935-947, doi:10.1016/j.gca.2003.09.002.

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO₂ concentrations – Tabor et al. (2004) “Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO₂ components—atmospheric CO₂ and CO₂ from in situ oxidation of organic matter. The δ¹³C values measured for the Fe(CO₃)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (270 Ma BP) paleosol (SAP). These goethites contain the most ¹³C-rich Fe(CO3)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO₂ components. δ¹³C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco₂ value of about 1 × PAL, but the scatter in the measured δ¹³C values of organic matter permits a calculated maximum Pco₂ of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO₃)OH in MCGoeth and SAP correspond to soil CO₂ concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO₂ concentrations are similar to modern soils in warm, wet environments. The average δ¹³C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ¹³C value for all profiles of −5.4‰. Thus, the value of Δ¹³C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco₂ and organic matter δ¹³C values were the same for both paleosol types. Furthermore, the atmospheric Pco₂ calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco₂ (permitted by one standard deviation of the organic matter δ¹³C value) of 5 × PAL. If, however, measured average δ¹³C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of ¹³C-depleted, labile organic compounds, calculated Permian atmospheric Pco2 using these ¹³C-enriched organic values would underestimate the actual atmospheric Pco₂ using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO₂ concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO₂. The Fe(CO₃)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in ¹³C relative to Fe(CO₃)OH in goethites from soils in which there are mixtures of two isotopic CO₂ components. Field-relationships and the δ¹³C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO₂ components at the time of goethite crystallization. The three components were probably atmospheric CO₂, CO₂ from in situ oxidation of organic matter and CO₂ from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO₂ components, as recorded by Fe(CO₃)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco₂.” Neil J. Tabor, Crayton J. Yapp and Isabel P. Montañez, Geochimica et Cosmochimica Acta, Volume 68, Issue 7, 1 April 2004, Pages 1503-1517, doi:10.1016/S0016-7037(03)00497-6.

Terrestrial Evidence for Two Greenhouse Events in the Latest Cretaceous – Nordt (2003) “We present a terrestrial record of stable carbon and oxygen isotopes from paleosol carbonate for climate interpretations between ca. 71.0 and 63.6 Ma. Isotopic ratios point to covarying and elevated atmospheric CO₂ pressures and temperatures between ca. 70.0 and 69.0 Ma and ca. 65.5 and 65.0 Ma. These two greenhouse episodes were characterized by atmospheric CO₂ levels between 1000 and 1400 ppmV (V = volume) and by mean annual temperatures in west Texas between 21 and 23 {degrees}C (~35{degrees}N paleo-latitude). Atmospheric CO₂ and temperature relations indicate that a doubling of pCO₂ was accompanied by an ~0.6 {degrees}C increase in temperature. A temperature gradient of ~0.4 {degrees}C per degree of latitude is proposed for North America across the Cretaceous-Tertiary boundary when comparing temperature proxies from west Texas with paleobotanical work in North Dakota. Our data demonstrate strong coupling between terrestrial climates and ocean temperatures that were possibly forced by Deccan trap volcanic degassing, leading to dramatic global climate changes.” Lee Nordt, Stacy Atchley, and Steve Dworkin, GSA Today, v. 13(12), p. 4-9. [Full text]

Leaf stomatal frequency in the Australian tropical rainforest tree Neolitsea dealbata (Lauraceae) as a proxy measure of atmospheric pCO₂ – Greenwood et al. (2003) “A putative relationship has been demonstrated for European and North American woody dicots and gymnosperms between leaf stomatal frequency and historical levels of atmospheric CO₂. However, hitherto no such study has been presented for Australian tropical broadleaved evergreen trees. In this study, variation of stomatal index (SI) along environmental gradients is examined for the broadleaved evergreen tropical rainforest tree Neolitsea dealbata (Lauraceae). Historical herbarium samples from natural populations in northeastern and southeastern Queensland were analysed. Leaf SI for Neolitsea dealbata is shown to be insensitive to mean annual rainfall or seasonal totals, or to temperature variables, indicating the climatic factors that influence the water budget of the plants were not a factor controlling SI. Contrary to the pattern most commonly observed in temperate areas, SI decreased with elevation for collections within a single year for 3 out of 4 years surveyed, a pattern consistent with environmental factors other than pCO₂ acting as the control over SI. However, an overall decrease in SI was recorded for samples collected in Queensland (30°–17°S) over the period 1899–1988, corresponding to a Southern Hemisphere increase in pCO₂ from 295 to 350 ppm. Restricting the analysis to sites within an altitudinal band of 640–1120 m demonstrated a significant relationship between SI and pCO₂ (r2=0.8942, F11=84.5, P<0.001). This data set was used to estimate atmospheric pCO₂ from fossil leaf cuticle for the closely related genus Litsea, giving pCO₂ values comparable to those estimated using Ginkgo from Northern Hemisphere fossil sites.” David R. Greenwood, Mark J. Scarr and David C. Christophel, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 196, Issues 3-4, 1 August 2003, Pages 375-393, doi:10.1016/S0031-0182(03)00465-6. [Full text]

Atmospheric pCO₂ since 60 Ma from records of seawater pH, calcium, and primary carbonate mineralogy – Demicco et al. (2003) “A 60 m.y. record of atmospheric pCO₂ has been refined from knowledge of (1) secular changes in the major ion composition of seawater (particularly Ca and Mg) and (2) oscillations in the mineralogy of primary oceanic carbonate sediments. Both factors have had a significant impact on the chemistry of the ocean carbonate buffer system. Calculated atmospheric pCO₂ oscillated between values of 100–300 ppm and to maxima of 1200–2500 ppm from 60 to 40 Ma and varied between 100 and 300 ppm from 25 Ma to the present. The refined pCO₂ values are significantly lower than previous estimates made from seawater pH data where total dissolved inorganic carbon was assumed constant and more in line with modeling and stomatal index estimations of atmospheric pCO₂ for the Tertiary.” Robert V. Demicco, Tim K. Lowenstein and Lawrence A. Hardie, Geology, v. 31 no. 9 p. 793-796, doi: 10.1130/G19727.1. [Full text]

Atmospheric pCO₂ and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England) – Robinson et al. (2002) “Nodular soil carbonates (calcretes) are present in overbank facies of Lower Cretaceous, non-marine Wealden Beds (Wessex Formation) of southern England. Field evidence suggests that these calcretes formed mostly under semi-arid Mediterranean-type climatic conditions. Typical calcrete fabrics, identified petrographically, include floating detrital grains, corroded grain margins and circumgranular cracks defining peds. Localized alteration of primary micrites is mainly associated with large cracks where early non-ferroan diagenetic cementation and neomorphism was focused. Diagenetic ferroan calcites occur as void fills and yield relatively light carbon-isotope and oxygen-isotope compositions (δ¹³C= –15.0‰; δ¹⁸O= –6.3‰) compared to well-preserved micrite (δ¹³C= –10.2‰; δ¹⁸O= –4.0‰). Precise definition of δ¹³C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO₂ (pCO₂) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO₂ was low during the mid-Early Cretaceous before rising to a previously defined mid-Cretaceous high. Data from calcretes in the Weald Clay highlight the need for selection of appropriate material and careful evaluation before pCO₂ calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO₂ into the soil-zone was either reduced or prevented.” Stuart A. Robinson, Julian E. Andrews, Stephen P. Hesselbo, Jonathan D. Radley, Paul F. Dennis, Ian C. Harding & Perce Allen, Journal of the Geological Society; 2002; v. 159; issue.2; p. 215-224; DOI: 10.1144/0016-764901-015. [Full text]

Paleosol barometer indicates extreme fluctuations in atmospheric CO₂ across the Cretaceous-Tertiary boundary – Nordt et al. (2002) “We present an atmospheric pCO₂ (p is partial pressure) curve showing extreme fluctuations for the interval between ca. 77 and 63 Ma in southern Alberta, Canada, using a paleosol barometer. Paleosol carbonate nodules (micrite) were collected from 40 Bk horizons among 6 stratigraphic sections for stable carbon isotope analysis. Based on results from the study area, declining atmospheric pCO₂ from 1200 ppmV (V is volume) in the Campanian to 780 ppmV in the Maastrichtian correlates with Late Cretaceous climate cooling and falling sea level as documented in global records. The remarkable rise in atmospheric pCO₂ near 65.5 Ma (1440 ppmV) correlates with volcanic activity associated with the Deccan Traps, rising sea level, and warmer global climates. The decline in atmospheric pCO₂ (760 ppmV) at the Cretaceous-Tertiary boundary and subsequent sharp rise into the Danian (1000 ppmV) occurred during static terrestrial temperatures and sea level. This work provides compelling evidence that atmospheric pCO₂ curves modeled for the Phanerozoic do not offer the resolution needed to understand environmental conditions during catastrophic events in Earth’s history.” Lee Nordt, Stacy Atchley and S.I. Dworkin, Geology, v. 30 no. 8 p. 703-706, doi: 10.1130/0091-7613(2002).

Fossil plants as indicators of the Phanerozoic global carbon cycle – Beerling & Royer (2002) “Developments in plant physiology since the 1980s have led to the realization that fossil plants archive both the isotopic composition of atmospheric CO₂ and its concentration, both critical integrators of carbon cycle processes through geologic time. These two carbon cycle signals can be read by analyzing the stable carbon isotope composition (δ¹³C) of fossilized terrestrial organic matter and by determining the stomatal characters of well-preserved fossil leaves, respectively. We critically evaluate the use of fossil plants in this way at abrupt climatic boundaries associated with mass extinctions and during times of extreme global warmth. Particular emphasis is placed on evaluating the potential to extract a quantitative estimate of the δ¹³C of atmospheric CO₂ because of the key role it plays in understanding the carbon cycle. We critically discuss the use of stomatal index and stomatal ratios for reconstructing atmospheric CO₂ levels, especially the need for adequate replication, and present a newly derived CO₂ record for the Mesozoic that supports levels calculated from geochemical modeling of the long-term carbon cycle. Several suggestions for future research using stable carbon isotope analyses of fossil terrestrial organic matter and stomatal measurements are highlighted.” D.J. Beerling and D.L. Royer, Annual Review of Earth and Planetary Sciences, Vol. 30: 527-556 (Volume publication date May 2002), DOI: 10.1146/annurev.earth.30.091201.141413. [Full text]

An atmospheric pCO₂ reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils – Beerling et al. (2002) “The end-Cretaceous mass extinctions, 65 million years ago, profoundly influenced the course of biotic evolution. These extinctions coincided with a major extraterrestrial impact event and massive volcanism in India. Determining the relative importance of each event as a driver of environmental and biotic change across the Cretaceous-Tertiary boundary (KTB) crucially depends on constraining the mass of CO2 injected into the atmospheric carbon reservoir. Using the inverse relationship between atmospheric CO₂ and the stomatal index of land plant leaves, we reconstruct Late Cretaceous-Early Tertiary atmospheric CO₂ concentration (pCO₂) levels with special emphasis on providing a pCO₂ estimate directly above the KTB. Our record shows stable Late Cretaceous/Early Tertiary background pCO₂ levels of 350–500 ppm by volume, but with a marked increase to at least 2,300 ppm by volume within 10,000 years of the KTB. Numerical simulations with a global biogeochemical carbon cycle model indicate that CO₂ outgassing during the eruption of the Deccan Trap basalts fails to fully account for the inferred pCO₂ increase. Instead, we calculate that the postboundary pCO₂ rise is most consistent with the instantaneous transfer of ≈4,600 Gt C from the lithic to the atmospheric reservoir by a large extraterrestrial bolide impact. A resultant climatic forcing of +12 W m⁻² would have been sufficient to warm the Earth’s surface by ≈7.5°C, in the absence of counter forcing by sulfate aerosols. This finding reinforces previous evidence for major climatic warming after the KTB impact and implies that severe and abrupt global warming during the earliest Paleocene was an important factor in biotic extinction at the KTB.” D. J. Beerling, B. H. Lomax, D. L. Royer, G. R. Upchurch, Jr., and L. R. Kump, PNAS June 11, 2002 vol. 99 no. 12 7836-7840, doi: 10.1073/pnas.122573099. [Full text]

Low atmospheric CO₂ levels during the Permo- Carboniferous glaciation inferred from fossil lycopsids – Beerling (2002) “Earth history was punctuated during the Permo-Carboniferous [300–250 million years (Myr) ago] by the longest and most severe glaciation of the entire Phanerozoic Eon. But significant uncertainty surrounds the concentration of CO₂ in the atmosphere through this time interval and therefore its role in the evolution of this major prePleistocene glaciation. Here, I derive 24 Late Paleozoic CO₂ estimates from the fossil cuticle record of arborsecent lycopsids of the equatorial Carboniferous and Permian swamp communities. Quantitative calibration of Late Carboniferous (330–300 Myr ago) and Permian (270–260 Myr ago) lycopsid stomatal indices yield average atmospheric CO₂ concentrations of 344 ppm and 313 ppm, respectively. The reconstructions show a high degree of self-consistency and a degree of precision an order of magnitude greater than other approaches. Low CO₂ levels during the Permo-Carboniferous glaciation are in agreement with glaciological evidence for the presence of continental ice and coupled models of climate and ice-sheet growth on Pangea. Moreover, the Permian data indicate atmospheric CO₂ levels were low 260 Myr ago, by which time continental deglaciation was already underway. Positive biotic feedbacks on climate, and geotectonic events, therefore are implicated as mechanisms underlying deglaciation.” D. J. Beerling, PNAS October 1, 2002 vol. 99 no. 20 12567-12571, doi: 10.1073/pnas.202304999 . [Full text]

Stability of atmospheric CO₂ levels across the Triassic/Jurassic boundary – Tanner et al. (2001) “The Triassic/Jurassic boundary, 208 million years ago, is associated with widespread extinctions in both the marine and terrestrial biota. The cause of these extinctions has been widely attributed to the eruption of flood basalts of the Central Atlantic Magmatic Province. This volcanic event is thought to have released significant amounts of CO₂ into the atmosphere, which could have led to catastrophic greenhouse warming, but the evidence for CO₂-induced extinction remains equivocal. Here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations, spanning a 20-Myr period across the Triassic/Jurassic boundary. Using a standard diffusion model, we interpret these isotopic data to represent a rise in atmospheric CO₂ concentrations of about 250 p.p.m. across the boundary, as compared with previous estimates of a 2,000–4,000 p.p.m. increase. The relative stability of atmospheric CO₂ across this boundary suggests that environmental degradation and extinctions during the Early Jurassic were not caused by volcanic outgassing of CO₂. Other volcanic effects—such as the release of atmospheric aerosols or tectonically driven sea-level change—may have been responsible for this event.” Lawrence H. Tanner, John F. Hubert, Brian P. Coffey & Dennis P. McInerney, Nature 411, 675-677 (7 June 2001) | doi:10.1038/35079548.

A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles – Retallack (2001) “To understand better the link between atmospheric CO₂ concentrations and climate over geological time, records of past CO₂ are reconstructed from geochemical proxies. Although these records have provided us with a broad picture of CO₂ variation throughout the Phanerozoic eon (the past 544 Myr), inconsistencies and gaps remain that still need to be resolved. Here I present a continuous 300-Myr record of stomatal abundance from fossil leaves of four genera of plants that are closely related to the present-day Ginkgo tree. Using the known relationship between leaf stomatal abundance and growing season CO₂ concentrations I reconstruct past atmospheric CO₂ concentrations. For the past 300 Myr, only two intervals of low CO₂ (<1,000 p.p.m.v.) are inferred, both of which coincide with known ice ages in Neogene (1–8 Myr) and early Permian (275–290 Myr) times. But for most of the Mesozoic era (65–250 Myr), CO₂ levels were high (1,000–2,000 p.p.m.v.), with transient excursions to even higher CO₂ (>2,000 p.p.m.v.) concentrations. These results are consistent with some reconstructions of past CO₂ (refs 1, 2) and palaeotemperature records, but suggest that CO₂ reconstructions based on carbon isotope proxies may be compromised by episodic outbursts of isotopically light methane. These results support the role of water vapour, methane and CO₂ in greenhouse climate warming over the past 300 Myr.” Gregory J. Retallack, Nature 411, 287-290 (17 May 2001) | doi:10.1038/35077041. [Full text]

CO₂ levels in the Late Palaeozoic and Mesozoic atmosphere from soil carbonate and organic matter, Satpura basin, Central India – Ghosh et al. (2001) “A number of calcic palaeosols have been identified within the fluvial deposits of the Motur (Permian), the Denwa (Triassic), the Bagra (Jurassic) and the Lameta (Cretaceous) Formations of the Satpura sedimentary succession, Central India. These palaeosols show accumulation of pedogenic carbonates in rhizocretions and glaebules. The carbon isotopic compositions of these carbonates and the coexisting soil organic matters are used to determine the isotopic composition and the partial pressure of atmospheric CO₂ using the CO₂ palaeobarometer developed by Cerling [Am. J. Sci., 291 (1991) 377]. It is seen that the atmospheric CO₂ level increased by a factor of 8 from the Permian to the Jurassic and declined again during the Cretaceous. The nature of the changes agrees with the result of the CO₂ evolution model of Berner (GEOCARB II) but the magnitude of the CO₂ increase in the Middle Jurassic and the Late Cretaceous was higher than the predicted value. Degassing of Earth’s interior due to rapid break-up of the Gondwana landmass during the Triassic and Jurassic period could have caused the rapid CO₂ increase.” Prosenjit Ghosh, P Ghosh and S.K Bhattacharya, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 170, Issues 3-4, 15 June 2001, Pages 219-236, doi:10.1016/S0031-0182(01)00237-1. [Full text]

Paleobotanical Evidence for Near Present-Day Levels of Atmospheric CO₂ During Part of the Tertiary – Royer et al. (2001) “Understanding the link between the greenhouse gas carbon dioxide (CO₂) and Earth’s temperature underpins much of paleoclimatology and our predictions of future global warming. Here, we use the inverse relationship between leaf stomatal indices and the partial pressure of CO₂ in modern Ginkgo bilobaand Metasequoia glyptostroboides to develop a CO₂ reconstruction based on fossil Ginkgo andMetasequoia cuticles for the middle Paleocene to early Eocene and middle Miocene. Our reconstruction indicates that CO₂ remained between 300 and 450 parts per million by volume for these intervals with the exception of a single high estimate near the Paleocene/Eocene boundary. These results suggest that factors in addition to CO₂ are required to explain these past intervals of global warmth.” Dana L. Royer, Scott L. Wing, David J. Beerling, David W. Jolley, Paul L. Koch, Leo J. Hickey and Robert A. Berner, Science 22 June 2001: Vol. 292 no. 5525 pp. 2310-2313, DOI: 10.1126/science.292.5525.2310. [Full text]

Atmospheric carbon dioxide concentrations over the past 60 million years – Pearson & Palmer (2000) “Knowledge of the evolution of atmospheric carbon dioxide concentrations throughout the Earth’s history is important for a reconstruction of the links between climate and radiative forcing of the Earth’s surface temperatures. Although atmospheric carbon dioxide concentrations in the early Cenozoic era (about 60 Myr ago) are widely believed to have been higher than at present, there is disagreement regarding the exact carbon dioxide levels, the timing of the decline and the mechanisms that are most important for the control of CO₂ concentrations over geological timescales. Here we use the boron-isotope ratios of ancient planktonic foraminifer shells to estimate the pH of surface-layer sea water throughout the past 60 million years, which can be used to reconstruct atmospheric CO2 concentrations. We estimate CO₂ concentrations of more than 2,000 p.p.m. for the late Palaeocene and earliest Eocene periods (from about 60 to 52 Myr ago), and find an erratic decline between 55 and 40 Myr ago that may have been caused by reduced CO₂ outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial. Since the early Miocene (about 24 Myr ago), atmospheric CO₂ concentrations appear to have remained below 500 p.p.m. and were more stable than before, although transient intervals of CO₂ reduction may have occurred during periods of rapid cooling approximately 15 and 3 Myr ago.” Paul N. Pearson & Martin R. Palmer, Nature 406, 695-699 (17 August 2000), doi:10.1038/35021000. [Full text]

Fossil Plants and Global Warming at the Triassic-Jurassic Boundary – McElwain et al. (1999) “The Triassic-Jurassic boundary marks a major faunal mass extinction, but records of accompanying environmental changes are limited. Paleobotanical evidence indicates a fourfold increase in atmospheric carbon dioxide concentration and suggests an associated 3° to 4°C “greenhouse” warming across the boundary. These environmental conditions are calculated to have raised leaf temperatures above a highly conserved lethal limit, perhaps contributing to the >95 percent species-level turnover of Triassic-Jurassic megaflora.” J. C. McElwain, D. J. Beerling and F. I. Woodward, Science 27 August 1999: Vol. 285 no. 5432 pp. 1386-1390, DOI: 10.1126/science.285.5432.1386.

Stable isotopic composition of pedogenic carbonates of the Early Cretaceous Shimonoseki Subgroup, western Honshu, Japan – Lee & Hisada (1999) “Abundant pedogenic carbonate nodules are present in the Shimonoseki Subgroup in the Kanmon Basin of Kyushu and Honshu, Japan. The oxygen isotope compositions of these pedogenic carbonates range from −20.1 to −22.8‰ (PDB), which is 7‰ lower than those of the septarian crystallaria in the carbonate nodules. The meteoric water composition estimated from the oxygen isotope composition of the soil water in equilibrium with the carbonates is much depleted compared with the estimate from other sources. This suggests that the oxygen isotope compositions of the Shimonoseki pedogenic carbonates were modified during diagenesis. The carbon isotopic compositions of the Shimonoseki carbonates range from −5.4 to −7.0‰ (PDB), with an average of −6.7‰ (PDB), suggesting carbonate formation in soils dominated by C₃ type of vegetation. An analysis of these values using Cerling’s model (Cerling, T.E., Wright, V.P., Vanstone, S.D., 1992. Further comments on using carbon isotopes in paleosols to estimate the CO₂ content of the palaeo-atmosphere. J. Geol. Soc. London 148 (1991), 945–947; 149, 673–676) indicates that the partial pressure of CO₂ in the Early Cretaceous Shimonoseki atmosphere was about 1700–3200 ppmV.” Yong Il Lee and Ken-ichiro Hisada, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 153, Issues 1-4, 15 September 1999, Pages 127-138, doi:10.1016/S0031-0182(99)00069-3.

Late Miocene Atmospheric CO₂ Concentrations and the Expansion of C₄ Grasses – Pagani et al. (1999) “The global expansion of C₄ grasslands in the late Miocene has been attributed to a large-scale decrease in atmospheric carbon dioxide (CO₂) concentrations. This triggering mechanism is controversial, in part because of a lack of direct evidence for change in the partial pressure of CO₂(pCO₂) and because other factors are also important determinants in controlling plant-type distributions. Alkenone-based pCO₂ estimates for the late Miocene indicate that pCO₂ increased from 14 to 9 million years ago and stabilized at preindustrial values by 9 million years ago. The estimates presented here provide no evidence for major changes in pCO₂ during the late Miocene. Thus, C₄ plant expansion was likely driven by additional factors, possibly a tectonically related episode of enhanced low-latitude aridity or changes in seasonal precipitation patterns on a global scale (or both).” Mark Pagani, Katherine H. Freeman and Michael A. Arthur, Science 6 August 1999: Vol. 285 no. 5429 pp. 876-879, DOI: 10.1126/science.285.5429.876.

Miocene evolution of atmospheric carbon dioxide – Pagani et al. (1999) “Changes in pCO₂ or ocean circulation are generally invoked to explain warm early Miocene climates and a rapid East Antarctic ice sheet (EAIS) expansion in the middle Miocene. This study reconstructs late Oligocene to late Miocene pCO2 from εp values based on carbon isotopic analyses of diunsaturated alkenones and planktonic foraminifera from Deep Sea Drilling Project sites 588 and 608 and Ocean Drilling Program site 730. Our results indicate that highest pCO₂ occurred during the latest Oligocene (∼350 ppmv) but decreased rapidly at ∼25 Ma. The early and middle Miocene was characterized by low pCO₂ (260–190 ppmv). Lower intervals of pCO₂ correspond to inferred organic carbon burial events and glacial episodes with the lowest concentrations occurring during the middle Miocene. There is no evidence for either high pCO₂ during the late early Miocene climatic optimum or a sharp pCO₂ decrease associated with EAIS growth. Paradoxically, pCO₂ increased following EAIS growth and obtained preindustrial levels by ∼10 Ma. Although we emphasize an oceanographic control on Miocene climate, low pCO₂ could have primed the climate system to respond sensitively to changes in heat and vapor transport” Pagani, M., M. A. Arthur, and K. H. Freeman (1999), Paleoceanography, 14(3), 273–292, doi:10.1029/1999PA900006. [Full text]

Stable isotopic composition of calcic paleosols of the Early Cretaceous Hasandong Formation, southeastern Korea – Lee (1999) “Abundant pedogenic carbonate nodules are present in the Cretaceous non-marine Hasandong Formation, southeastern Korea. The oxygen isotope compositions of these pedogenic carbonates range from −14.2 to −18.0‰ (PDB). The meteoric water composition estimated from the oxygen isotope composition of the soil-water in equilibrium with the carbonates is much depleted compared with the previous estimate. This suggests that the oxygen isotope compositions of the Hasandong pedogenic carbonates were modified during diagenesis. The carbon isotopic compositions of the Hasandong carbonates range from −2.4 to −9.3‰ (PDB) with an average of −5.6‰ (PDB) suggesting carbonate formation in soils dominated by C₃ type of vegetation. The estimated average composition of the vegetation is about 6‰ (PDB) enriched compared with the present-day C₃ vegetation. This is probably due to atmospheric influence contributing about 35% of the total CO₂ in the soil. An analysis of this contribution using the model of Cerling (1991) indicates that the partial pressure of CO₂ in the Early Cretaceous Hasandong atmosphere was about 2300 ppmV.” Yong Il Lee, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 150, Issues 1-2, 15 June 1999, Pages 123-133, doi:10.1016/S0031-0182(99)00010-3. [Full text]

Do fossil plants signal palaeoatmospheric carbon dioxide concentration in the geological past? – McElwain (1998) “Fossil, subfossil, and herbarium leaves have been shown to provide a morphological signal of the atmospheric carbon dioxide environment in which they developed by means of their stomatal density and index. An inverse relationship between stomatal density/index and atmospheric carbon dioxide concentration has been documented for all the studies to date concerning fossil and subfossil material. Furthermore, this relationship has been demonstrated experimentally by growing plants under elevated and reducedcarbon dioxide concentrations. To date, the mechanism that controls the stomatal density response to atmospheric carbon dioxide concentration remains unknown. However, stomatal parameters of fossil plants have been successfully used as a proxy indicator of palaeo–carbon dioxide levels. This paper presents new estimates of palaeo–atmospheric carbon dioxide concentrations for the Middle Eocene (Lutetian), based on the stomatal ratios of fossil Lauraceae species from Bournemouth in England. Estimates of atmospheric carbon dioxide concentrations derived from stomatal data from plants of the Early Devonian, Late Carboniferous, Early Permian and Middle Jurassic ages are reviewed in the light of new data. Semi–quantitative palaeo–carbon dioxide estimates based on the stomatal ratio (a ratio of the stomatal index of a fossil plant to that of a selected nearest living equivalent) have in the past relied on the use of a Carboniferous standard. The application of a new standard based on the present–day carbon dioxide level is reported here for comparison. The resultant ranges of palaeo–carbon dioxide estimates made from standardized fossil stomatal ratio data are in good agreement with both carbon isotopic data from terrestrial and marine sources and long–term carbon cycle modelling estimates for all the time periods studied. These data indicate elevated atmospheric carbon dioxide concentrations during the Early Devonian, Middle Jurassic and Middle Eocene, and reduced concentrations during the Late Carboniferous and Early Permian. Such data are important in demonstrating the long–term responses of plants to changing carbon dioxide concentrations and in contributing to the database needed for general circulation model climatic analogues.” Phil. Trans. R. Soc. Lond. B 29 January 1998 vol. 353 no. 1365 83-96, doi: 10.1098/rstb.1998.0193. [Full text]

Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO₂ pressure – Yapp & Poths (1996) “Abundance and carbon isotope data from an Fe(CO₃)OH component in apparent solid solution in oolitic goethites have been used to infer ancient atmospheric CO₂ pressures. A test of the validity of these estimates might be comparisons of the carbon isotope compositions of Fe(CO₃)OH in oolitic goethites with time-equivalent pedogenic calcites. Temporal trends of the oolitic goethite and pedogenic calcite δ13C values are generally similar, but time-equivalent samples from each of these two groups are not common in the existing data. To facilitate discussion of the concept, comparisons were made of available goethite and calcite samples even though ages of the compared samples in each pair were not identical. In four out of the five comparisons, Fe(CO₃)OH abundance and δ13C data were combined with pedogenic calcite δ13C data to calculate physically reasonable soil CO₂ concentrations for the ancient calcitic soids. This suggests that the compared oolitic goethite and pedogenic calcite systems were responding to the same global scale phenomenon (i.e., atmospheric CO₂). Atmospheric PCO₂ as determined from the goethites in these four “well-behaved” cases ranged from values indistinguishable from modern (within analytical uncertainty) to values up to approximately 16 time modern (modern atmospheric PCO₂ was taken to be 10−3.5 atm). One interpretation of the fifth, “anomalous”, comparison is that atmospheric CO₂ levels increased from about 3 times modern to about 18 times modern from the Triassic into the Early Jurassic. This inferred value for the PCO2 of the Early Jurassic atmosphere is not uniquely constrained by the existing data and needs to be substantiated. However, even considerably lower Early Jurassic atmospheric PCO₂ values of 6 to 9 times modern (i.e., 1/3 to 1/2 of the estimated value of 18 times modern) would still indicate significant differences between the global carbon cycles then and now. These results highlight the need for more research on the behavior of the atmosphere during and after the Triassic-Jurassic transition.” Crayton J. Yapp and Harald Poths, Earth and Planetary Science Letters, Volume 137, Issues 1-4, January 1996, Pages 71-82, doi:10.1016/0012-821X(95)00213-V.

Middle to Late Paleozoic Atmospheric CO2 Levels from Soil Carbonate and Organic Matter – Mora et al. (1996) “The stable carbon isotope compositions of ancient soil carbonate and coexisting soil organic matter indicate that atmospheric CO₂ levels decreased by a factor of 10 during the middle to late Paleozoic era. Proxy measurements of CO₂ were made by application of a soil carbonate CO₂ paleobarometer to a suite of paleosols that share key physical and chemical characteristics. The estimates agree with theoretical models that imply that a decrease in Paleozoic atmospheric CO₂ levels was associated with afforestation of the land surface by terrestrial plants and with global climate change leading to the extensive Permo-Carboniferous glaciation.” Claudia I. Mora, Steven G. Driese and Lee Ann Colarusso, Science 23 February 1996: Vol. 271 no. 5252 pp. 1105-1107, DOI: 10.1126/science.271.5252.1105.

A 400 million year carbon isotope record of pedogenic carbonate; implications for paleoatomospheric carbon dioxide – Ekart et al. (1995) “A 400 record of atmospheric carbon dioxide levels has been estimated by applying a CO₂ paleobarometer to a database of 758 analyses of paleosol (fossil soil) carbonates. This database is a compilation of new data and previously published values from the literature. Many new analyses of Mesozoic paleosols are reported, an era poorly represented in the literature. Results indicate that large fluctuations in atmospheric carbon dioxide levels have occurred over the study interval, ranging from the current level up to ten times the current level. Declining pCO₂ levels through the middle Paleozoic culminate in low levels in the Early Permian. An abrupt increase in pCO{sub 2} in the Early Permian is followed by a decrease prior to the Permo-Triassic boundary. Carbon dioxide levels increase through the Triassic to approx. 3,000 ppmV, a level maintained through the Jurassic period. Levels lowered through the Cretaceous, dropping to less than 1,000 ppmV prior to the Cretaceous-Tertiary boundary. Relatively low levels persisted throughout the Cenozoic, with some evidence of higher levels in the Eocene and Oligocene.” Douglas D. Ekart, Thure E. Cerling, Isabel P. Montanez, and Neil J. Tabor, American Journal of Science, Vol. 299, December 1999, P.805-827; doi:10.2475/ajs.299.10.805. [Full text]

Concentration of carbon dioxide in the Late Cretaceous atmosphere – Andrews et al. (1995) “Stable carbon isotope data from Late Cretaceous (Maastrichtian) palaeosols in India are used to estimate the concentration of carbon dioxide in the Late Cretaceous atmosphere. We show that the Maastrichtian atmosphere is unlikely to have contained more than about 1300 ppm by volume of CO₂.This value agrees with an independently modeled value of CO₂ in the Late Cretaceous atmosphere. A low concentration of the greenhouse gas carbon dioxide in the Maastrichtian atmosphere (relative to concentrations in the earlier Cretaceous) is consistent with palaeotemperature information from terrestrial plant and marine fossils, which suggest that the global climate cooled toward the end of the Cretaceous Period.” Andrews, J.E., Tandon, S.K., and Dennis, P.F. 1995, Journal of the Geological Society, London, v. 152, p. 1-3, DOI: 10.1144/gsjgs.152.1.0001.

Stomatal Density and Index of Fossil Plants Track Atmospheric Carbon Dioxide in the Palaeozoic – McElwain & Chaloner (1995) “It has been demonstrated that the leaves of a range of forest tree species have responded to the rising concentration of atmospheric CO₂ over the last 200 years by a decrease in both stomatal density and stomatal index. This response has also been demonstrated experimentally by growing plants under elevated CO₂ concentrations. Investigation of Quaternary fossil leaves has shown a corresponding stomatal response to changing CO₂ concentrations through a glacial-interglacial cycle, as revealed by ice core data. Tertiary leaves show a similar pattern of stomatal density change, using palynological evidence of palaeo-temperature as a proxy measure of CO₂ concentration. The present work extends this approach into the Palaeozoic fossil plant record. The stomatal density and index of Early Devonian, Carboniferous and Early Permian plants has been investigated, to test for any relationship that they may show with the changes in atmospheric CO₂ concentration, derived from physical evidence, over that period. Observed changes in the stomatal data give support to the suggestion from physical evidence, that atmospheric CO₂ concentrations fell from an Early Devonian high of 10-12 times its present value, to one comparable to that of the present day by the end of the Carboniferous. These results suggest that stomatal density of fossil leaves has potential value for assessing changes in atmospheric CO₂ concentration through geological time.” Jennifer C. McElwain and William G. Chaloner, Ann Bot (1995) 76 (4): 389-395, doi: 10.1006/anbo.1995.1112. [Full text]

Palaeoclimate and palaeovegetation in central India during the Upper Cretaceous based on stable isotope composition of the palaeosol carbonates – Ghosh et al. (1995) “The oxygen isotope compositions of the pedogenic carbonates formed on the sediments of the Lameta Formation of Central India during the Upper Cretaceous range from −6.7 to −8.9‰. Estimates of the oxygen isotope composition of the soil-water in equilibrium with the carbonates suggest average meteoric water composition of −8‰. This value is considerably lighter compared to the modern precipitation in Central India (−3‰). The lighter oxygen isotope composition can be explained in terms of cumulative effects of highly seasonal (monsoon-like) climatic regime in a rain shadow zone and a more pronounced “continental effect” due to a bigger size of Cretaceous India. The carbon isotopic compositions of the Lameta carbonates range from −7.1 to −10.7 with an average of −9.1‰ suggesting pedogenesis in soils dominated by C₃ type of vegetation. The estimated average composition of the vegetation is about 3‰ enriched compared to the modern day C₃ vegetation. This is probably due to atmospheric influence contributing about 15% of the total CO₂ in the soil. An analysis of this contribution using the model of Cerling (1991) indicates that the partial pressure of CO₂ in the Late Cretaceous atmosphere was 800 – 12,000 ppmV” P. Ghosh, S.K.Bhattacharya and R.A. Jani, Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 114, Issues 2-4, April 1995, Pages 285-296, doi:10.1016/0031-0182(94)00082-J.

New atmospheric pCO₂ estimates from palesols during the late Paleocene/early Eocene global warming interval – Sinha & Stott (1994) “The late Paleocene to early Eocene was one of the warmest intervals in Earth’s history. Superimposed on this long-term warming was an abrupt short-term extreme warm event at or near the Paleocene/Eocene boundary and centered in the higher latitudes. This short-term climate warming was associated with a major benthic foraminiferal extinction and a dramatic 3–4% drop in the ocean’s carbon isotopic composition. It has been suggested that the late paleocene/early Eocene global warming was caused by an enhanced greenhouse effect associated with higher levels of atmospheric CO₂ relative to present levels. We present carbon isotopic data from the co-existing paleosols organic matter and carbonates from a terrestrial sequence in the Paris Basin, France that contradict the notion that an increase in atmospheric CO₂ level was the cause of extreme warming for this time interval. Atmospheric pCO₂ estimates for the Late Paleocene/early Eocene estimated from the terrestrial carbon isotopic record spanning the Paleocene/Eocene transition, are indistinguishable from each other and were generally between 300 and 700 ppm.” Ashish Sinha and Lowell D. Stott, Global and Planetary Change, Volume 9, Issues 3-4, December 1994, Pages 297-307, doi:10.1016/0921-8181(94)00010-7.

Paleoatmospheric Signatures in Neogene Fossil Leaves – Van Der Burgh et al. (1993) “An increase in the atmospheric carbon dioxide (CO₂) concentration results in a decrease in the number of leaf stomata. This relation is known both from historical observations of vegetation over the past 200 years and from experimental manipulations of microenvironments. Evidence from stomatal frequencies of fossil Quercus petraea leaves indicates that this relation can be applied as a bioindicator for changes in paleoatmospheric CO₂ concentrations during the last 10 million years. The data suggest that late Neogene CO₂ concentrations fluctuated between about 280 and 370 parts per million by volume.” Johan Van Der Burgh, Henk Visscher, David L. Dilcher and Wolfram M. Kürschner, Science 18 June 1993: Vol. 260 no. 5115 pp. 1788-1790, DOI: 10.1126/science.260.5115.1788.

Fractionation of carbon isotopes by phytoplankton and estimates of ancient CO₂ levels – Freeman & Hayes (1992) “Reports of the ¹³C content of marine particulate organic carbon are compiled and on the basis of GEOSECS data and temperatures, concentrations, and isotopic compositions of dissolved CO₂ in the waters in which the related phytoplankton grew are estimated. In this way, the fractionation of carbon isotopes during photosynthetic fixation of CO₂ is found to be significantly correlated with concentrations of dissolved CO₂. Because ancient carbon isotopic fractionations have been determined from analyses of sedimentary porphyrins [Popp et al., 1989], the relationship between isotopic fractionation and concentrations of dissolved CO2 developed here can be employed to estimate concentrations of CO₂ dissolved in ancient oceans and, in turn, partial pressures of CO2 in ancient atmospheres. The calculations take into account the temperature dependence of chemical and isotopic equilibria in the dissolved-inorganic-carbon system and of air-sea equilibria. Paleoenvironmental temperatures for each sample are estimated from re-constructions of paleogeography, latitudinal temper- ature gradients, and secular changes in low-latitude sea surface temperature. It is estimated that atmospheric partial pressures of CO₂ were over 1000 μatm 160–100 Ma ago, then declined to values near 300 μatm during the next 100 Ma. Analysis of a high-resolution record of carbon isotopic fractionation at the Cenomanian-Turonian boundary suggests that the partial pressure of CO₂ in the atmosphere was drawn down from values near 840 μatm to values near 700 μatm during the anoxic event.” Freeman, K. H., and J. M. Hayes (1992), Global Biogeochem. Cycles, 6(2), 185–198, doi:10.1029/92GB00190.

Correlation between isotope records in marine and continental carbon reservoirs near the Palaeocene/Eocene boundary – Koch et al. (1992) “CHANGES in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land. The carbon isotope compositions of marine carbonate sediments from the late Palaeocene vary considerably, exhibiting a sudden decrease close to the Palaeocene/Eocene boundary which coincides with deep-sea benthic extinctions1 and with changes in ocean circulation. Here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel. In using palaeosol carbonates to reconstruct the CO₂ content of the ancient atmosphere2, isotope shifts of this sort will have to be taken into account. The sharp decrease in ¹³C/¹²C ratios in the late Palaeocene provides a datum for precise correlation of marine and continental records, and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals.” Paul L. Koch, James C. Zachos & Philip D. Gingerich, Nature 358, 319 – 322 (23 July 1992); doi:10.1038/358319a0. [Full text]

Higher temperatures and lower oceanic pCO₂: A climate enigma at the end of the Paleocene Epoch – Stott (1992) “One of the largest and most abrupt climatic warming events documented in the geologic record occurred at the end of the Paleocene epoch. Oceanic deep waters warmed to 10°C, and high-latitude surface waters warmed from ∼10°C to ∼20°C within several thousand years. This coincided with weakened atmospheric circulation and the extinction of ∼50% of deep-sea benthic foraminiferal species. It has been suggested that this warm excursion was forced by higher atmospheric pCO₂ and greenhouse effects caused by a pulse of hydrothermal activity and/or volcanism. Stable isotopic evidence is presented from two widely separated locations that suggest this warming was associated with a drop in oceanic pCO₂ rather than an increase. Oceanic pCO₂ change across this event was estimated using a model of ¹³C fractionation in photosynthate organic carbon versus [CO_2aq], with solubility constants for CO₂ and stable isotopic paleotemperature estimates. To derive a well-preserved record for surface ocean δ¹³C change the organic carbon bound within the calcite lattice of well-preserved planktonic foraminifera was extracted for isotopic analysis. With allowance for uncertainty in the isotopic differences between phytoplankton and foraminiferal organic matter, the initial results indicate a drop in surface ocean pCO₂ at high and low latitudes from 600–700 parts per million (ppm) to ∼200 ppm. Lower pCO₂ persisted for at least 10,000 years. The duration of the pCO₂ excursion was long enough for the ocean and atmosphere to have reached a new steady state condition. There is no evidence of increased organic carbon burial in the deep sea during this period. Two alternative explanations are presented to account for such a rapid drop in oceanic pCO₂. One involves reduced upwelling induced by diminished wind stress as atmospheric circulation weakened in response to climate warming. This would have reduced the rate of metabolic CO₂ recycling into the surface ocean. It will be necessary to obtain data from regions outside potential upwelling zones in order to evaluate this. The second involves a readjustment of carbonate equilibria in the ocean to higher [CO₃=] in the surface ocean, particularly at high latitudes where surface waters warmed to approximately 20°C. Such a shift in carbonate equilibria would have lowered the ocean’s capacity to take in CO₂. If the initial results presented here do accurately reflect a change in the global ocean [CO_2aq], the Paleocene/Eocene boundary event may provide clues about the ocean’s physical and biological response to rapid, large-scale perturbations in atmospheric pCO₂ and to global warming.” Stott, L. D. (1992), Paleoceanography, 7(4), 395–404, doi:10.1029/92PA01183.

Use of carbon isotopes in paleosols as an indicator of the P(CO₂) of the paleoatmosphere – Cerling (1992) “Measurements of carbon isotopic composition of coexisting paleosol organic matter and carbonate results in improved estimates of the paleo-P(CO₂) content of the paleoatmosphere. Measurements from Tertiary paleosols indicate that the levels of CO₂ in the early Eocene, middle and late Miocene, and Pliocene were less than 1000 ppmV. Early Cretaceous levels of P(CO₂) were significantly higher, of the order of 1500 to 3000 ppmV.” Cerling, T. E. (1992), Global Biogeochem. Cycles, 6(3), 307–314, doi:10.1029/92GB01102.

Carbon dioxide in the atmosphere; evidence from Cenozoic and Mesozoic Paleosols – Cerling (1991) “A diffusion-reaction model for the isotopic composition of soil CO₂ and soil carbonate is evaluated. It shows which variables are important under different conditions and shows that under certain circumstances the carbon isotopic composition of soil carbonate can be used to estimate P(CO₂) of the atmosphere from late Paleozoic to the present. The isotopic composition of soil carbonate produced under uniform conditions is essentially constant below about 20 cm in most soils. Today, the isotopic composition of soil organic matter, which is mostly determined by the fraction of C₄ biomass present, is the most important factor in determining the carbon isotopic composition of soil carbonate. However, prior to the advent of C₄ plants making up a significant fraction of the biomass, probably in the Tertiary, the concentration of CO₂ in the atmosphere can be estimated because of mixing of atmospheric CO₂ with CO₂ produced in the soil. The diffusion-reaction model also suggests that before the advent of higher land plants most soil carbonate should have {delta}¹³C values between {minus}6.5 and {minus}8.5 permil suggesting that P(CO₂) was significantly higher than today, probably on the order of 1,500 to 3,000 ppmV.” Thure E. Cerling, American Journal of Science, Vol. 291, April 1991, P.377-400; doi:10.2475/ajs.291.4.377. [Full text]

Lacustrine carbonates and pedogenesis: sedimentology and origin of palustrine deposits from the Early Cretaceous Rupelo Formation, W Cameros Basin, N Spain – Platt (1989) “The Berriasian Rupelo Formation of the W Cameros Basin consists of a 2–200 m thickness of marginal and open lacustrine carbonate and associated deposits. Open lacustrine facies contain a non-marine biota with abundant charophytes (both stems and gyrogonites), ostracods, gastropods and rare vertebrates. Carbonate production was mainly biogenic. The associated marginal lacustrine (‘palustrine’) facies show strong indications of subaerial exposure and exhibit a wide variety of pedogenic fabrics. Silicified evaporites found near to the top of the sequence reflect a short hypersaline phase in the lake history. The succession was laid down in a low gradient, shallow lake complex characterized by wide fluctuations of the shoreline. Carbon and oxygen stable isotope analyses from the carbonates show non-marine values with ranges of δ¹³ from − 7 to − 11‰and δ¹⁸ from − 3 to − 7.5‰. Differences in the isotopic composition of open lacustrine carbonates are consistent with sedimentary evidence of variation in organic productivity within the lake. Analyses from the entire sample suite plot on a linear trend; isotopic compositions become lighter with increasing evidence of pedogenic modification. This suggests progressive vadose zone diagenesis and influence of meteoric waters rich in soil-derived CO₂. The stable isotope data thus support evidence from petrography and facies relations that ‘palustrine’limestones form through pedogenic modification of lake carbonates.” Nigel H. Platt, Sedimentology, Volume 36, Issue 4, pages 665–684, August 1989.

Isotopic imprint of climate and hydrogeochemistry on terrestrial strata of the Triassic-Jurassic Hartford and Fundy rift basins – Suchecki et al. (1988) “Late Triassic to Early Jurassic terrestrial sequences in the Hartford and Fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite. The isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine, playa, and fluvial environments. Dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the Hartford basin have variable isotopic compositions (delta ¹³ C = -5.8 to + 1.8 per thousand PDB; delta ¹⁸ O = -7.2 to +0.7 per thousand PDB). Within any single symmetrical cycle of playa red mudstone–lacustrine gray, black, gray mudstone–playa red mudstone, there is a systematic change to relatively enriched ¹³ C compositions in dolomite in the grayish black and black mudstones in the center of the cycle. These carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where ¹³ C-depleted carbon was concentrated in organic matter that was then buried. Calcites from lacustrine, micritic, and biomicritic limestones of the Scots Bay Formation of the Fundy basin have stable isotopic compositions (delta ¹³ C = -4.6 to -2.2 per thousand PDB; delta ¹⁸ O = -6.1 to -3.0 per thousand PDB) that become more enriched in ¹⁸ O and ¹³ C upward in shallowing depositional sequences. These isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels, followed by progressively lower inflow, resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface water. The lake waters were well oxygenated at all times. In the Hartford basin, caliche calcites in fluvial mudstones and sandstones have isotopic compositions (delta ¹³ C = -7.3 to -3.8 per thousand PDB; delta ¹⁸ O = -8.0 to – 5.6 per thousand PDB) that reflect paleosol processes during climatic conditions that varied from warm and dry in Late Triassic time to relatively cooler and probably wetter in the Early Jurassic. Isotopic compositions of caliche calcites in redbeds in the Fundy basin indicate a parallel climate change from Late Triassic to Early Jurassic time, but also that the climate was relatively hotter and probably drier over the entire interval, as compared to the Hartford basin.” Robert K. Suchecki, John F. Hubert, and Carol C. Birney de Wet, Journal of Sedimentary Research; September 1988; v. 58; no. 5; p. 801-811; DOI: 10.1306/212F8E6D-2B24-11D7-8648000102C1865D.

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Molecular proxies for paleoclimatology – Eglinton & Eglinton (2008) “We summarize the applications of molecular proxies in paleoclimatology. Marine molecular records especially are proving to be of value but certain environmentally persistent compounds can also be measured in lake sediments, loess deposits and ice cores. The fundamentals of this approach are the molecular parameters, the compound abundances and carbon, hydrogen, nitrogen and oxygen isotopic contents which can be derived by the analysis of sediment extracts. These afford proxy measures which can be interpreted in terms of the conditions which control climate and also reflect its operation. We discuss two types of proxy; those of terrigenous and those of aquatic origin, and exemplify their application in the study of marine sediments through the medium of ten case studies based in the Atlantic, Mediterranean and Pacific Oceans, and in Antarctica. The studies are mainly for periods in the present, the Holocene and particularly the last glacial/interglacial, but they also include one study from the Cretaceous. The terrigenous proxies, which are measures of continental vegetation, are based on higher plant leaf wax compounds, i.e. long-chain (circa C₃₀) hydrocarbons, alcohols and acids. They register the relative contributions of C₃ vs. C₄ type plants to the vegetation in the source areas. The two marine proxies are measures of sea surface temperatures (SST). The longer established one, (U₃₇K′) is based on the relative abundances of C₃₇ alkenones photosynthesized by unicellular algae, members of the Haptophyta. The newest proxy (TEX₈₆) is based on C₈₆ glycerol tetraethers (GDGTs) synthesized in the water column by some of the archaeal microbiota, the Crenarchaeota.” Timothy I. Eglinton and Geoffrey Eglinton, Earth and Planetary Science Letters, Volume 275, Issues 1-2, 30 October 2008, Pages 1-16, doi:10.1016/j.epsl.2008.07.012. [Full text]

Geocarb III: A Revised Model of Atmospheric CO₂ over Phanerozoic Time – Berner & Kothavala (2001) “Revision of the GEOCARB model (Berner, 1991, 1994) for paleolevels of atmospheric CO₂, has been made with emphasis on factors affecting CO₂ uptake by continental weathering. This includes: (1) new GCM (general circulation model) results for the dependence of global mean surface temperature and runoff on CO₂, for both glaciated and non-glaciated periods, coupled with new results for the temperature response to changes in solar radiation; (2) demonstration that values for the weathering-uplift factor fR(t) based on Sr isotopes as was done in GEOCARB II are in general agreement with independent values calculated from the abundance of terrigenous sediments as a measure of global physical erosion rate over Phanerozoic time; (3) more accurate estimates of the timing and the quantitative effects on Ca-Mg silicate weathering of the rise of large vascular plants on the continents during the Devonian; (4) inclusion of the effects of changes in paleogeography alone (constant CO₂ and solar radiation) on global mean land surface temperature as it affects the rate of weathering; (5) consideration of the effects of volcanic weathering, both in subduction zones and on the seafloor; (6) use of new data on the 13C values for Phanerozoic limestones and organic matter; (7) consideration of the relative weather- ing enhancement by gymnosperms versus angiosperms; (8) revision of paleo land area based on more recent data and use of this data, along with GCM-based paleo-runoff results, to calculate global water discharge from the continents over time. Results show a similar overall pattern to those for GEOCARB II: very high CO₂ values during the early Paleozoic, a large drop during the Devonian and Carbonifer- ous, high values during the early Mesozoic, and a gradual decrease from about 170 Ma to low values during the Cenozoic. However, the new results exhibit considerably higher CO₂ values during the Mesozoic, and their downward trend with time agrees with the independent estimates of Ekart and others (1999). Sensitivity analysis shows that results for paleo-CO₂ are especially sensitive to: the effects of CO₂ fertilization and temperature on the acceleration of plant-mediated chemical weathering; the quantitative effects of plants on mineral dissolution rate for constant temperature and CO₂; the relative roles of angiosperms and gymnosperms in accelerating rock weather- ing; and the response of paleo-temperature to the global climate model used. This emphasizes the need for further study of the role of plants in chemical weathering and the application of GCMs to study of paleo-CO2 and the long term carbon cycle.” Robert A. Berner and Zavareth Kothavala, American Journal of Science, Vol. 301, February 2001, P.182-204; doi:10.2475/ajs.301.2.182. [Full text]