cretaceous pco2 estimation from stomatal frequency analysis of gymnosperm leaves of patagonia argentina abstract stomatal frequency of fossil conifers and ginkgoals cuticles were considered as potential indicators of the p co 2 values in the patagonian palaeoatmosphere during the cretaceous samples are from continental deposits of early upper aptian age anfiteatro de tico and punta del barco formations and upper albian lower cenomanian age kachaike formation both outcropping in andean and extra andean portions of santa cruz province argentina it can be inferred that the co 2 atmospheric content was similar during both stages or was slightly higher toward the late albian early cenomanian the co 2 content estimated from the conifers is between 700 and 1400 ppmv or lower from the mid aptian ginkgoals this co 2 range is consistent with those predicted worldwide by previous studies stomatal indices biogeochemical models and carbon isotopes and implies a co 2 content 2 3 4 0 times higher than present the high atmospheric co 2 content during the middle aptian and late albian early cenomanian was probably linked to a warming process at high latitudes during these times this is consistent with the floristic development recorded during both stages in patagonia

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

new constraints on atmospheric co2 concentration for the phanerozoic earth s atmospheric co2 concentration ca for the phanerozoic eon is estimated from proxies and geochemical carbon cycle models most estimates come with large sometimes unbounded uncertainty here we calculate tightly constrained estimates of ca using a universal equation for leaf gas exchange with key variables obtained directly from the carbon isotope composition and stomatal anatomy of fossil leaves our new estimates validated against ice cores and direct measurements of ca are less than 1000 ppm for most of the phanerozoic from the devonian to the present coincident with the appearance and global proliferation of forests uncertainties obtained from monte carlo simulations are typically less than for ca estimates from other approaches these results provide critical new empirical support for the emerging view that large 2000 3000 ppm long term swings in ca do not characterize the post devonian and that earth s long term climate sensitivity to ca is greater than originally thought key points a novel co2 proxy calculates past atmospheric co2 with improved certainty co2 is unlikely to have exceeded 1000 ppm for extended periods post devonian earth s long term climate sensitivity to co2 is greater than originally thought

marked decline in atmospheric carbon dioxide concentrations during the paleogene 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 from the middle eocene to the late oligocene approximately 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

high sea surface temperatures during the early cretaceous epoch the early cretaceous epoch about 145 100 million years ago is generally thought of as a greenhouse period with high atmospheric co2 concentrations1 and high global mean temperatures2 but evidence for episodes of cooler conditions and even transient glaciations has been proposed3 4 5 6 7 8 9 here we present sea surface temperature records spanning the period from 142 to 128 million years ago berriasian barremian ages from low and mid latitudes reconstructed using the tex86 palaeotemperature proxy during this period we find sea surface temperatures exceeding 32 c at 15 20 n and averaging 26 c at 53 s these temperatures substantially exceed modern temperatures at equivalent latitudes and are incompatible with the notion of consistently cooler conditions in the earliest cretaceous moreover we find little variability in the sea surface temperature records even during the valanginian carbon isotope excursion 138 135 million years ago which was thought to be associated with marked temperature fluctuations5 we conclude that the earliest cretaceous was characterized by a warm stable climate with a lower meridional temperature gradient than today

coccolith chemistry reveals secular variations in the global ocean carbon cycle the mismatch between the 100 and 400 k y components of pleistocene climate and the relative power of those terms from the eccentricity of the earth s orbit remains a challenge to the milankovitch hypothesis coccolithophores have the potential to respond to parameters of orbital forcing other than insolation and as a critical component of the ocean carbon cycle can act to modify the climate response the first direct comparison of coccolith fraction sr ca alkenone abundance and automated coccolithophore counts shows that cf sr ca is largely driven by changing production of bloom species with unusually high sr ca ratios the periods of high cf sr ca and high bloom production mark periods of high global coccolithophore production which correlate inversely with the low amplitude 100 and higher amplitude 400 k y eccentricity orbital frequency 400 k y cycles of coccolithophore bloom production correspond to periods of enhanced carbonate accumulation in some parts of the ocean deep ocean dissolution in others positive shifts in global ocean 13c and acmes of gephyrocapsa caribbeanica and emiliania huxleyi the link between production of coccolithophore blooms and eccentricity may be due to orbital control of silica leakage from the southern ocean to the orbitally defined inverse correlation between insolation and growing season length and the asymptotic growth response to these parameters or to changes in nutrient input from weathering during the pleistocene the eccentricity induced coccolithophore acmes have no apparent influence on atmospheric carbon dioxide pco2 due to the shift towards small bloom coccolithophores or to coupling with increased diatom productivity or the ballast effect of the calcium carbonate rain such that pleistocene climate has no significant variance at the largest amplitude eccentricity forcing of 400 k y coccolithophores and their influence on the carbon cycle may act as a filter between the incident orbital forcing and resultant climate

alkenone and boron based pliocene pco2 records abstract 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 11b 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

co2 forced climate thresholds during the phanerozoic abstract 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

fish tooth δ18o revising late cretaceous meridional upper ocean water temperature gradients the oxygen isotope composition of fossil fi sh teeth a paleo upper ocean temperature proxy exceptionally resistant to diagenetic alteration provides new insight on the evolution of the low to middlelatitude thermal gradient between the middle cretaceous climatic optimum and the cooler latest cretaceous period the new middle cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera 18o recovered from deep sea drilling project and ocean drilling program drilling sites although the isotopic temperatures derived from 18o of fish teeth are uniformly higher by 3 4 c in contrast our new latest cretaceous thermal gradient is markedly steeper than those previously published for this period fish tooth 18o data demonstrate that low to middle latitude thermal gradients of the middle cretaceous climatic optimum and of the cooler latest cretaceous are similar to the modern one despite a cooling of 7 c between the two periods our new results imply that no drastic changes in meridional heat transport are required to explain the late cretaceous climate based on climate models such a cooling without any change in the low to middle latitude thermal gradient supports an atmospheric co2 decrease as the primary driver of the climatic evolution recorded during the late cretaceous

a pedogenic goethite record of soil co2 variations as a response to soil moisture content abstract the terrestrial carbon cycle and the role of atmospheric co 2 concentrations in controlling global temperatures can be inferred from the study of ancient soils paleosols soil formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric p co 2 from ancient terrestrial records in the case of goethite optimum sampling strategies for reconstructing p co 2 focus on the portion of the soil profile that displays steep gradients in both soil co 2 concentration and 13 c values of soil co 2 such that a keeling plot can be developed for a given soil and atmospheric p co 2 can be calculated from it we report data from a carboniferous paleosol that depart from the expected linear trends the results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil co 2 over a range of timescales and can record these variations in the carbon isotope composition and mole fraction of fe co 3 oh in solid solution with goethite we explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired co 2 or in the residence time of carbon in soils the implications of this result are overestimation of paleoatmospheric p co 2 from pedogenic goethite

was the antarctic glaciation delayed by a high degassing rate during the early cenozoic abstract the cenozoic is a period of major climatic changes marked by the formation of the antarctic ice sheet at the eocene oligocene e o boundary the opening of the southern ocean seaways and the decrease in atmospheric co2 are two processes generally evoked to explain this e o cooling the debate is still ongoing but modeling studies tend to demonstrate that the decrease in atmospheric co2 is the main driver of the cooling however uncertainties persist on what drove the decrease in atmospheric co2 during the cenozoic in this study we investigate the impact of continental drift lithology distribution and volcanic degassing rates on the atmospheric carbon dioxide concentration over the cenozoic within a coupled climate carbon model geoclim in the model the continental drift results in driving low atmospheric co2 levels during the eocene and the oligocene the dispersed configuration and the location of a large continental area north africa northern south america within the inter tropical convergence zone itcz promote co2 consumption by weathering forcing co2 to remain low icehouse conditions are also promoted by the drifting of india and the weathering of the deccan basalts in the itcz during the eocene and by the weathering of the ethiopian traps during the oligocene to prevent the building up of the antarctic ice sheet at the eocene the model needs enhanced solid earth degassing flux by 50 so that atmospheric co2 levels stay above the glacial threshold 750 ppm we find that the decrease in atmospheric co2 from the eocene to the oligocene is probably due to a reduction in the source of volcanic co2 rather than an increase in silicate weathering the model results furthermore suggest that during the miocene period the northward drifting of both the african plate and india including the deccan traps might have decreased the continental surface exposed to chemical weathering therefore generating higher co2 values finally the uplift of the tibetan plateau from the miocene to the present day induces in the model an increase in silicate weathering through the intensification of the south eastern asian monsoon causing atmospheric co2 to come back to a pre industrial value

obscuring of long eccentricity cyclicity in pleistocene oceanic carbon isotope records abstract long eccentricity 400 kyr cycles in carbon isotope records from the pacific and atlantic oceans and the mediterranean sea of the past 5 0 ma are compared all records show maximum 13c values 13cmax at eccentricity minima during the pliocene but this relationship obscured in the pleistocene after 1 6 ma in particular for the open ocean deep water 13c records since a clear anti phase relationship was set up between oceanic 18o and 13c in the 100 kyr band from this time we attribute the obscured 400 kyr signal to a major change in the oceanic carbon reservoir probably associated with restructure of the southern ocean a similar change occurred in the miocene at 13 9 ma when the 400 kyr cyclicity in 13c records flattened out together with a drastic cooling and antarctic ice sheet expansion a remarkable exception is the mediterranean surface water 13c record which remained paced by the long term eccentricity cycle throughout the pliocene and pleistocene suggesting a low latitude climatic origin of the 400 kyr signal that is independent of glacial interglacial forcing since the earth is currently passing through an eccentricity minimum it is crucial to understand the nature of the 13cmax events

atmospheric carbon dioxide linked with mesozoic and early cenozoic climate change carbon isotopes of fossil plants and model simulations suggest that atmospheric carbon dioxide levels were variable during the period 200 to 60 million years ago the large decreases in the partial pressure of co2 coincide with glaciations providing evidence against climate co2 decoupling during the mesozoic

potential origins of 400 500 kyr periodicities in the ocean carbon cycle a box model approach 1 cenozoic proxy records of both the isotopic composition of dissolved inorganic carbon in the oceans 13cdic and deep ocean carbonate preservation show significant periodicities in the range 400 500 kyr sensitivity analysis of the ocean carbon cycle to potential variability on this timescale in patterns of global oceanic primary productivity and or continental weathering fluxes is performed using a 7 box ocean atmosphere model the data constraints imposed by plio pleistocene proxy records of 13cdic carbonate preservation and atmospheric pco2 variability allow different scenarios to be evaluated forcing with the global oceanic primary production ratio of inorganic and organic carbon leads to the relative phases of response most consistent with the proxy data however only when changes also occur in total marine primary production can the observed relative amplitudes of 13cdic and pco2 variability also be reproduced this scenario is consistent with oscillations between a highly productive coccolithophore rich global ocean and a less productive diatom rich one such oscillations have been proposed to originate from the orbital eccentricity cycle leading to changes in seasonality and thus silica utilization in the southern ocean however the period of response in model 13cdic is always close to that of the forcing function used and thus a significant discrepancy remains between the orbital eccentricity cycle period of 413 kyr and the observed 500 kyr 13cdic periodicity seen in the pleistocene

a synthesis of approaches for modelling coupled thermal hydraulic mechanical chemical processes in a single novaculite fracture experiment the geological formation immediately surrounding a nuclear waste disposal facility has the potential to undergo a complex set of physical and chemical processes starting from construction and conti

evolution of tropical watersheds and continental hydrology during the late cretaceous greenhouse impact on marine carbon burial and possible implications for the future abstract regional climate modelling for the late cretaceous greenhouse and high resolution marine stratigraphic records from both sides of the low latitude atlantic show that tropical south american and african hydrology and watersheds had a strong effect on freshwater transfer into the equatorial atlantic and subsequently the marine carbon record this conclusion is derived from new detailed geochemical records from demerara rise off suriname drilled at ocean drilling program odp site 1261 combined with frequency analyses and climate simulations providing evidence for mainly eccentricity driven changes in carbon burial in the western tropical atlantic shorter orbital frequencies in particular precession clearly dominating black shale cycles off tropical africa odp site 959 are far less dominant at demerara rise despite comparable time resolution of the geochemical records we suggest that these different frequency patterns in carbon burial were related to the regional evolution of cretaceous watersheds and hydrology in tropical south america and africa river discharge deduced from simulations indicates higher and less variable discharge from south america compared to western africa at that time this runoff pattern would have supported more permanent anoxic conditions off south america compared to africa at least indirectly and caused the lack of strong higher frequency geochemical cycles in the western sector of the equatorial atlantic furthermore climate simulations show a general switch of primary runoff from either side of the cretaceous equatorial atlantic every half precession cycle i e every 10 kyr similarities between the developments of cretaceous and holocene hydrology in the tropical atlantic area imply that orbital scale evolution of watersheds is a robust feature through time that is independent from the mean global climate state based upon the comparison of cretaceous and holocene trends in hydrology we infer that future hydrology in the study region may develop in a comparable direction to the one observed in the cretaceous if true this suggests that the modern amazon rain forest could shrink over the next millennium due to a 30 loss of moisture while the congo rain forest in africa is likely to expand in response to a 14 gain in moisture

a 100 myr history of the carbon cycle based on the 400 kyr cycle in marine δ13c benthic records documenting the past coevolution of earth temperatures and of the carbon cycle is of paramount importance for our understanding of climate dynamics atmospheric co2 is well constrained over the last million years through direct measurements in air bubbles from antarctic ice cores for older times many different and sometimes conflicting proxies have been suggested here we provide a new methodology to constrain the carbon cycle in the past based on marine benthic 13c records marine 13c data are recording a persistent 400 kyr cycle with an amplitude primarily linked to the total amount of carbon in the ocean or dissolved inorganic carbon dic by extracting this amplitude from published records we obtain a new strong constraint on the 100 myr history of earth s carbon cycle the obtained cenozoic evolution of dic is in surprisingly in a good agreement with existing reconstructions of pco2 suggesting that pco2 is mostly driven by dic changes over this period in contrast we find no strong decreasing trend in dic between the cretaceous and the cenozoic suggesting that cretaceous atmospheric pco2 levels were limited and high temperatures at this time should be explained by other mechanisms alternatively high cretaceous atmospheric pco2 could occur as a consequence of changes in oceanic chemistry but not carbon content

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

atmospheric carbon dioxide concentration across the mid pleistocene transition 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 approximately 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