Atmospheric CO2 Partial Pressure Research Articles

Differential protein expression using proteomics from a crustacean brine shrimp (Artemia sinica) under CO2-driven seawater acidification

Gradually increasing atmospheric CO2 partial pressure (pCO2) has caused an imbalance in carbonate chemistry and resulted in decreased seawater pH in marine ecosystems, termed seawater acidification. Anthropogenic seawater acidification is postulated to affect the physiology of many marine calcifying organisms. To understand the possible effects of seawater acidification on the proteomic responses of a marine crustacean brine shrimp (Artemia sinica) three groups of cysts were hatched and further raised in seawater at different pH levels (8.2 as control and 7.8 and 7.6 as acidification stress levels according to the predicted levels at the end of this century and next century, respectively) for 1, 7 and 14 days followed by examination of the protein expression changes via two-dimensional gel electrophoresis. Searches of protein databases revealed that 67 differential protein spots were altered due to lower pH level (7.6 and 7.8) stress in comparison to control groups (pH 8.2) by mass spectrometry. Generally, these differentially expressed proteins included the following: 1) metabolic process-related proteins involved in glycolysis and glucogenesis, nucleotide/amino acid/fatty acid metabolism, protein biosynthesis, DNA replication and apoptosis; 2) stress response-related proteins, such as peroxiredoxin, thioredoxin peroxidase, 70-kDa heat shock protein, Na/K ATPase, and ubiquinol-cytochrome c reductase; 3) immune defence-related proteins, such as prophenoloxidase and ferritin; 4) cytoskeletal-related proteins, such as myosin light chain, TCP1 subunit 2, tropomyosin and tubulin alpha chain; and 5) signal transduction-related proteins, such as phospholipase C-like protein, 14-3-3 zeta, translationally controlled tumour protein and RNA binding motif protein. Taken together, these data support the idea that CO2-driven seawater acidification may affect protein expression in the crustacean A. sinica and possibly also in other species that feed on brine shrimp in the ecosystem, particularly marine food webs.

Yield responses of wild C3 and C4 crop progenitors to subambient CO2 : a test for the role of CO2 limitation in the origin of agriculture.

Limitation of plant productivity by the low partial pressure of atmospheric CO2 (Ca ) experienced during the last glacial period is hypothesized to have been an important constraint on the origins of agriculture. In support of this hypothesis, previous work has shown that glacial Ca limits vegetative growth in the wild progenitors of both C3 and C4 founder crops. Here, we present data showing that glacial Ca also reduces grain yield in both crop types. We grew four wild progenitors of C3 (einkorn wheat and barley) and C4 crops (foxtail and broomcorn millets) at glacial and postglacial Ca , measuring grain yield and the morphological and physiological components contributing to these yield changes. The C3 species showed a significant increase in unthreshed grain yield of ~50% with the glacial to postglacial increase in Ca , which matched the stimulation of photosynthesis, suggesting that increases in photosynthesis are directly translated into yield at subambient levels of Ca . Increased yield was controlled by a higher rate of tillering, leading to a larger number of tillers bearing fertile spikes, and increases in seed number and size. The C4 species showed smaller, but significant, increases in grain yield of 10-15%, arising from larger seed numbers and sizes. Photosynthesis was enhanced by Ca in only one C4 species and the effect diminished during development, suggesting that an indirect mechanism mediated by plant water relations could also be playing a role in the yield increase. Interestingly, the C4 species at glacial Ca showed some evidence that photosynthetic capacity was upregulated to enhance carbon capture. Development under glacial Ca also impacted negatively on the subsequent germination and viability of seeds. These results suggest that the grain production of both C3 and C4 crop progenitors was limited by the atmospheric conditions of the last glacial period, with important implications for the origins of agriculture.

Reduced plant water status under sub-ambient pCO2 limits plant productivity in the wild progenitors of C3 and C4 cereals.

The reduction of plant productivity by low atmospheric CO2 partial pressure (pCO2) during the last glacial period is proposed as a limiting factor for the establishment of agriculture. Supporting this hypothesis, previous work has shown that glacial pCO2 limits biomass in the wild progenitors of C3 and C4 founder crops, in part due to the direct effects of glacial pCO2 on photosynthesis. Here, we investigate the indirect role of pCO2 mediated via water status, hypothesizing that faster soil water depletion at glacial (18 Pa) compared to post-glacial (27 Pa) pCO2, due to greater stomatal conductance, feeds back to limit photosynthesis during drying cycles. We grew four wild progenitors of C3 and C4 crops at glacial and post-glacial pCO2 and investigated physiological changes in gas exchange, canopy transpiration, soil water content and water potential between regular watering events. Growth parameters including leaf area were measured. Initial transpiration rates were higher at glacial pCO2 due to greater stomatal conductance. However, stomatal conductance declined more rapidly over the soil drying cycle in glacial pCO2 and was associated with decreased intercellular pCO2 and lower photosynthesis. Soil water content was similar between pCO2 levels as larger leaf areas at post-glacial pCO2 offset the slower depletion of water. Instead the feedback could be linked to reduced plant water status. Particularly in the C4 plants, soil-leaf water potential gradients were greater at 18 Pa compared with 27 Pa pCO2, suggesting an increased ratio of leaf evaporative demand to supply. Reduced plant water status appeared to cause a negative feedback on stomatal aperture in plants at glacial pCO2, thereby reducing photosynthesis. The effects were stronger in C4 species, providing a mechanism for reduced biomass at 18 Pa. These results have added significance when set against the drier climate of the glacial period.

The occurrence of <italic>Quercus heqingensis</italic> n. sp. and its application to palaeo-CO<sub>2</sub> estimates

Many leaf fossils belonging to Quercus sect. Heterobalanus were collected from the Upper Pliocene Sanying Formation in Heqing County, Yunnan Province, China. Leaves are oblong, elliptic or obovate with entire margin or spinous serrations, 3.1–8.7 cm in length and 1.8–5.7 cm in width. Leaf apex is round, mucronate or blunt round, while leaf base is heart-shaped, slightly heart-shaped or blunt round, and occasionally asymmetric. The petiole is slightly sturdy, straight or a bit curved, with 1.8–4.9 mm in length. The main vein of these specimens is slightly zigzagging, with a sturdy base attenuating to the apex. Secondary veins are pinnate, 7–12 pairs, alternate and parallel with the stretching angles from 40° to 73°, bifurcating near 1/3 to 1/2 of the leaf margin. Tertiary veins are, relatively thin, 5–7 bands in 1 cm2, and perpendicular to secondary veins. Quaternary, blind and marginal veins are not clear. The upper epidermis has both unicellular and multicellular trichomes without stomata, and most of the epidermal cells are nearly quadrangular. The lower epidermis has only unicellular trichomes and cyclocytic stomatal apparatuses, and the epidermal cells are nearly quadrangular or pentagonal. The stomatal apparatuses are 20–33 m m in length, 17–27 m m in width , with 2–6 m m wide in apertures. They spread casually with the stomatal density of 503±98 mm - 2 and the stomatal index of 8.90%±0.32%. The guard cells show kidney-shaped, with 5–7 subsidiary cells. These features are similar to Q. rehderiana, Q. guyavifolia , Q. preguyavifolia and some other extant or fossil species of Q . sect. Heterobalanus , but they are different in terms of density and type of trichomes. Both Q . guyavifolia and Q . preguyavifolia have more trichomes in total, and multicellular trichomes in lower epidermis which spread in veins and between veins. The fossil leaves of Q . sect. Heterobalanus in Heqing and Q . rehderiana have less trichomes, and the former has no multicellular trichome in lower epidermis, while the latter’s multicellular trichome are just found in veins. Hence, it was designated as a new species— Quercus heqingensis n. sp. The estimates of palaeoatmospheric-CO2 using the positive relationships between stomatal frequency and atmospheric CO2 partial pressure in its Nearest Living Relatives (NLRs)— Q . guyavifolia show that, the palaeo-CO2 levels in the late Pliocene reconstructed by Q. heqingensis were 263.42±24.86 ppm (equation with extant field collections) and 234.25±22.49 ppm (equation with historical herbarium samples). These results are slightly lower than most previous estimates of the same age. Our results are in agreement with other estimates when palaeoaltitude is considered. This indicates that it is feasible to apply the stomatal index of Q . heqingensis to the positive relationship between stomatal index of Q . guyavifolia and atmospheric p CO2 to reconstruct palaeo-CO2 levels. Meanwhile, the results further support lower palaeo-CO2 level in the late Pliocene than present. This study provides credible evidence for future palaeo-CO2 estimates using NLRs as proxies.

Use of carbon isotopic composition of alkenone as a CO2 proxy in the East Sea/Japan Sea

The carbon isotopic composition of alkenones (δ13C37:2) can be used as a pCO2 proxy, although the applicability of this method is still controversial. In this study, the present oceanic pCO2 was reconstructed by analysing the δ13C37:2 of sinking particles collected by sediment traps in the East Sea/Japan Sea. The reconstructed pCO2 was compared with observed pCO2 to examine the applicability of this method. We applied the same method to marine sediments of core 05PC-21 to reconstruct the past pCO2 of the East Sea for the last 190 cal ka BP. The seasonal variation patterns of pCO2 which was reconstructed from the δ13C37:2 of sinking particles in the water column corresponded with those of observed oceanic pCO2. The pCO2 was relatively low in spring, and increased in summer. In consideration of the factors that can affect the seasonal variation of the pCO2, sea surface temperature seems to be a main factor. However, from January to March, there would be an influence of a deepening of the mixed layer depth and the vertical mixing of seawater. In April, the pCO2 decreased by a phytoplankton bloom. During the last 190 cal ka BP, oceanic pCO2 values and its variation pattern which were reconstructed by analysing the δ13C37:2 of marine sediment were similar to those of atmospheric pCO2 that were reconstructed from ice cores at high latitudes. These suggest that the carbon isotopic composition of alkenones (δ13C37:2) can be used as a proxy of long-term change in the partial pressure of atmospheric CO2.

An Early Pleistocene atmospheric CO2 record based on pedogenic carbonate from the Chinese loess deposits

The tight coupling between temperature and atmospheric CO2 is shown by ice core records for the past 0.8 million years (Myr). However, the modern atmospheric partial pressure of CO2 (pCO2) has exceeded previous interglacial pCO2 levels over the past 0.8 Myr, suggesting that the earlier part of the Pleistocene and Pliocene might be a better analog of today's radiative forcing of CO2. The early Pleistocene experienced a constant cooling characterized by the intensification of northern hemisphere glaciations. Existing pCO2 records developed from marine organic matter and inorganic precipitates, however, disagree with the trends and absolute values of CO2 over this time interval. Here we present quantitative interglacial pCO2 estimates from ∼2.6–0.9 Myr using the stable carbon isotopic compositions of pedogenic carbonates collected from the Chinese Loess Plateau (CLP). Our pCO2 records provide the first documentation of pCO2 from continental sedimentary deposits over the early Pleistocene. The successive decrease of our pCO2 records is broadly consistent with the increase in deep-sea δO18 and the overall decline of sea surface temperature (SST) at this time, but in contrast with the increasing peak interglacial pCO2 recorded in ice cores for the last 0.8 Myr.

Stable carbon isotope fractionation of organic cyst-forming dinoflagellates: Evaluating the potential for a CO2 proxy

Over the past decades, significant progress has been made regarding the quantification and mechanistic understanding of stable carbon isotope fractionation (13C fractionation) in photosynthetic unicellular organisms in response to changes in the partial pressure of atmospheric CO2 (pCO2). However, hardly any data is available for organic cyst-forming dinoflagellates while this is an ecologically important group with a unique fossil record. We performed dilute batch experiments with four harmful dinoflagellate species known for their ability to form organic cysts: Alexandrium tamarense, Scrippsiella trochoidea, Gonyaulax spinifera and Protoceratium reticulatum. Cells were grown at a range of dissolved CO2 concentrations characterizing past, modern and projected future values (∼5–50μmolL−1), representing atmospheric pCO2 of 180, 380, 800 and 1200μatm. In all tested species, 13C fractionation depends on CO2 with a slope of up to 0.17‰ (μmolL)−1. Even more consistent correlations were found between 13C fractionation and the combined effects of particulate organic carbon quota (POC quota; pg C cell−1) and CO2. Carbon isotope fractionation as well as its response to CO2 is species-specific. These results may be interpreted as a first step towards a proxy for past pCO2 based on carbon isotope ratios of fossil organic dinoflagellate cysts. However, additional culture experiments focusing on environmental variables other than pCO2, physiological underpinning of the recorded response, testing for possible offsets in 13C values between cells and cysts, as well as field calibration studies are required to establish a reliable proxy.

Estimates of atmospheric CO2 in the Neoarchean–Paleoproterozoic from paleosols

Atmospheric CO2 levels reflect Earth’s surface temperatures directly, and have been discussed especially in association with the faint young Sun and Snowball Earth in the Precambrian. In addition, atmospheric O2 levels in the Precambrian have been estimated from paleosols, fossil weathering profiles, based on the estimates of atmospheric CO2 levels. Nevertheless, atmospheric CO2 levels in the Neoarchean and the Paleoproterozoic have remained as a debatable topic. In order to precisely estimate atmospheric CO2 levels in the Neoarchean–Paleoproterozoic, we developed a new method that calculates CO2 levels from the chemical compositions of paleosols. The new method (i) calculates the cation concentrations in porewaters at the time of weathering from those of paleosols, (ii) describes the relationships between partial pressure of atmospheric CO2 (PCO2), pH and cation concentrations based on the charge balance between the cations and anions including carbonate species in porewaters, and (iii) finally calculates PCO2 levels at a given temperature constraining pH by thermodynamics of weathering secondary-minerals. By applying the new method to modern weathering profiles, we obtained a good agreement between the calculated and observed PCO2 levels. The weathering rate deduced from the new method was proportional to PCO2 with fractional dependence of 0.18 and the apparent activation energy of weathering was 40–55kJmol−1, which is consistent with the laboratory and field results. The application to modern weathering and the formulated characteristics of weathering strongly indicate that the new method is valid and robust.The new method was then applied to eight paleosols formed in the Neoarchean–Paleoproterozoic. We made constraints on the local temperatures, at which the paleosols were formed, mainly by the temperature and solute-concentration relationships in the literature, because they should have been different between the paleosols and from the average global surface temperatures. Under the constrained local temperatures, the PCO2 levels were calculated to be 85–510times the present day atmospheric level (PAL) at ∼2.77Ga, 78–2500PAL at ∼2.75Ga, 160–490PAL at ∼2.46Ga, 30–190PAL at ∼2.15Ga, 20–620PAL at ∼2.08Ga and 23–210PAL at ∼1.85Ga. The estimated PCO2 levels are higher than those to maintain the average global surface temperature of the Earth above the freezing point of water only by CO2 itself. The newly estimated PCO2 levels probably imply that atmospheric CO2 decreased gradually in long term in the Neoarchean–Paleoproterozoic and that the glaciations at ∼2.9 and ∼2.4–2.2Ga were differently triggered.

Effect of Variable pCO2 on Ca2+ Removal and Potential Calcification of Cyanobacterial Biofilms —An Experimental Microsensor Study

Two different cyanobacterial biofilms from German karstwater creeks were investigated with respect to their photosynthetic effect on Ca2+ removal and potential CaCO3 precipitation in artificial creek waters of different CO2 partial pressures at a given, constant calcite supersaturation. CO2 partial pressures were adjusted to 350 ppmV, 2200 ppmV and 8700 ppmV respectively, covering the range of Phanerozoic atmospheric CO2 partial pressures inferred from palaeosoils, stomatal indices and model calculations. Microsensor measurements of calcium, pH and oxygen revealed differences in the potential to precipitate CaCO3 between the two model organisms Tychonema-relative strain SAG 2388 and Synechococcus sp. strain SAG 2387. Whereas a strong removal of Ca2+ from the solution was measured at Tychonema-relative biofilm, the Synechococcus sp. biofilm exercised a much lower Ca2+ removal during photosynthesis. Photosynthesis was enhanced in both organisms with increasing CO2 and HCO3−, as indicated by enhanced O2 production, but only for the motile filamentous taxon Tychonema-relative a concomitantly increasing calcium removal was measured. However, model calculations indicate that this short-term Ca2+ binding in the Tychonema-relative is due to complexation to exopolymers or oscillin, with no immediate CaCO3 precipitation. In contrast, Ca2+ and pH measurements at Synechococcus sp. biofilm could be consistent with immediate CaCO3 precipitation at the cells. In both biofilms, pH gradients increase with increasing pCO2 from 350 to 2200 ppmV due to enhanced photosynthesis, but decrease at a pCO2 of 8700 ppmV despite of further enhanced photosynthesis. This observation, regardless whether CO2 or HCO3− is used by the cyanobacteria, is in accordance with hydrochemical modeling demonstrating an increased DIC buffering at high pCO2 conditions. These results indicate that the potential of cyanobacteria to form spatially defined calcification pattern via pH gradients at their cell envelopes ('calcified cyanobacteria') increases at elevated pCO2, while at high pCO2 conditions Ca2+ binding and lowered pH microgradients lead to spatially diffuse calcification without defined cell envelope precipitates.

A new positive relationship between pCO2 and stomatal frequency in Quercus guyavifolia (Fagaceae): a potential proxy for palaeo-CO2 levels.

The inverse relationship between atmospheric CO2 partial pressure (pCO2) and stomatal frequency in many species of plants has been widely used to estimate palaeoatmospheric CO2 (palaeo-CO2) levels; however, the results obtained have been quite variable. This study attempts to find a potential new proxy for palaeo-CO2 levels by analysing stomatal frequency in Quercus guyavifolia (Q. guajavifolia, Fagaceae), an extant dominant species of sclerophyllous forests in the Himalayas with abundant fossil relatives. Stomatal frequency was analysed for extant samples of Q. guyavifolia collected from17 field sites at altitudes ranging between 2493 and 4497 m. Herbarium specimens collected between 1926 and 2011 were also examined. Correlations of pCO2-stomatal frequency were determined using samples from both sources, and these were then applied to Q. preguyavaefolia fossils in order to estimate palaeo-CO2 concentrations for two late-Pliocene floras in south-western China. In contrast to the negative correlations detected for most other species that have been studied, a positive correlation between pCO2 and stomatal frequency was determined in Q. guyavifolia sampled from both extant field collections and historical herbarium specimens. Palaeo-CO2 concentrations were estimated to be approx. 180-240 ppm in the late Pliocene, which is consistent with most other previous estimates. A new positive relationship between pCO2 and stomatal frequency in Q. guyavifolia is presented, which can be applied to the fossils closely related to this species that are widely distributed in the late-Cenozoic strata in order to estimate palaeo-CO2 concentrations. The results show that it is valid to use a positive relationship to estimate palaeo-CO2 concentrations, and the study adds to the variety of stomatal density/index relationships that available for estimating pCO2. The physiological mechanisms underlying this positive response are unclear, however, and require further research.

Nickel adsorption on chalk and calcite

Nickel uptake from solution by two types of chalk and calcite was investigated in batch sorption studies. The goal was to understand the difference in sorption behavior between synthetic and biogenic calcite. Experiments at atmospheric partial pressure of CO2, in solutions equilibrated with calcite and chalk and pH ranging from 7.7 to 8.8, explored the influence of initial concentration and the amount and type of sorbent on Ni uptake. Adsorption increases with increased surface area and pH. A surface complexation model describes the data well. Stability constants for the Ni surface complex are log KNi=−1.12 on calcite and log KNi=−0.43 and −0.50 on the two chalk samples. The study confirms that synthetic calcite and chalk both take up nickel, but Ni binds more strongly on the biogenic calcite than on inorganically precipitated, synthetic powder, because of the presence of trace amounts of polysaccharides and clay nanoparticles on the chalk surface.

Proteomic and metabolomic responses of Pacific oyster Crassostrea gigas to elevated pCO2 exposure.

The gradually increased atmospheric CO2 partial pressure (pCO2) has thrown the carbonate chemistry off balance and resulted in decreased seawater pH in marine ecosystem, termed ocean acidification (OA). Anthropogenic OA is postulated to affect the physiology of many marine calcifying organisms. However, the susceptibility and metabolic pathways of change in most calcifying animals are still far from being understood. To our knowledge, few studies have focused on the responses induced by pCO2 at both protein and metabolite levels. The pacific oyster C. gigas, widely distributed throughout most of the world's oceans, is a model organism for marine environmental science. In the present study, an integrated metabolomic and proteomic approach was used to elucidate the effects of ocean acidification on Pacific oyster C. gigas, hopefully shedding light on the physiological responses of marine mollusk to the OA stress.

Elevation-dependent responses of tree mast seeding to climate change over 45 years.

We use seed count data from a New Zealand mono-specific mountain beech forest to test for decadal trends in seed production along an elevation gradient in relation to changes in climate. Seedfall was collected (1965 to 2009) from seed trays located on transect lines at fixed elevations along an elevation gradient (1020 to 1370 m). We counted the number of seeds in the catch of each tray, for each year, and determined the number of viable seeds. Climate variables were obtained from a nearby (<2 km) climate station (914-m elevation). Variables were the sum or mean of daily measurements, using periods within each year known to correlate with subsequent interannual variation in seed production. To determine trends in mean seed production, at each elevation, and climate variables, we used generalized least squares (GLS) regression. We demonstrate a trend of increasing total and viable seed production, particularly at higher elevations, which emerged from marked interannual variation. Significant changes in four seasonal climate variables had GLS regression coefficients consistent with predictions of increased seed production. These variables subsumed the effect of year in GLS regressions with a greater influence on seed production with increasing elevation. Regression models enforce a view that the sequence of climate variables was additive in their influence on seed production throughout a reproductive cycle spanning more than 2 years and including three summers. Models with the most support always included summer precipitation as the earliest variable in the sequence followed by summer maximum daily temperatures. We interpret this as reflecting precipitation driven increases in soil nutrient availability enhancing seed production at higher elevations rather than the direct effects of climate, stand development or rising atmospheric CO2 partial pressures. Greater sensitivity of tree seeding at higher elevations to changes in climate reveals how ecosystem responses to climate change will be spatially variable.

Effect of Atmospheric Carbon Dioxide and Strontium Content on Surface Segregation in Strontium Doped Lanthanum Cobalt Iron Oxide Thin Films

This study investigates surface segregation behavior and phase formation in LaxSr1-xCo0.2Fe0.8O3-δ (LSCF), a commonly used cathode material for solid oxide fuel cells (SOFCs). (100)-oriented LSCF thin films with varying ‘x’ values were deposited on (110)-oriented NdGaO3 (NGO) substrates by pulsed laser deposition (PLD). Compositional changes on the surface of LSCF thin films were measured using total reflection x-ray fluorescence (TXRF) technique in real time at 800°C under different CO2 partial pressures. Ex-situ electronic structure measurements were carried on the post-annealed samples using hard x-ray photoelectron spectroscopy (HAXPES). The thin-films were also characterized in cross-section by high-resolution transmission electron microscopy (HRTEM), and composition profiles in the LSCF matrix adjacent to the surface segregated phases were analyzed using energy-dispersed x-ray spectroscopy (EDS). The thermodynamics and kinetics of the segregation and phase formation phenomena as a function of atmospheric CO2 partial pressure and Sr content is discussed in this presentation.

The Potential for Biologically Catalyzed Anaerobic Methane Oxidation on Ancient Mars

This study examines the potential for the biologically mediated anaerobic oxidation of methane (AOM) coupled to sulfate reduction on ancient Mars. Seven distinct fluids representative of putative martian groundwater were used to calculate Gibbs energy values in the presence of dissolved methane under a range of atmospheric CO2 partial pressures. In all scenarios, AOM is exergonic, ranging from -31 to -135 kJ/mol CH4. A reaction transport model was constructed to examine how environmentally relevant parameters such as advection velocity, reactant concentrations, and biomass production rate affect the spatial and temporal dependences of AOM reaction rates. Two geologically supported models for ancient martian AOM are presented: a sulfate-rich groundwater with methane produced from serpentinization by-products, and acid-sulfate fluids with methane from basalt alteration. The simulations presented in this study indicate that AOM could have been a feasible metabolism on ancient Mars, and fossil or isotopic evidence of this metabolic pathway may persist beneath the surface and in surface exposures of eroded ancient terrains.

Sulphide oxidation and carbonate dissolution as a source of CO2 over geological timescales

The observed stability of Earth's climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering, but reconstructions of sea-floor spreading do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth's atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses have been put forward to explain the 'Cenozoic isotope-weathering paradox', and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes, atmospheric CO2 partial pressure and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.

Differential effects of ocean acidification on carbon acquisition in two bloom-forming dinoflagellate species.

Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) with very low CO2 affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO2 partial pressure (pCO2) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom-forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO2 (i.e. 180–1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane-inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO2 range, but we observed a number of species-specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO2-dependent contribution of HCO3− uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO2-independent carbonic anhydrase activity. Comparing both species, a general trade-off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO2.

Vital effects in coccolith calcite: Cenozoic climate‐pCO2 drove the diversity of carbon acquisition strategies in coccolithophores?

Coccoliths, calcite plates produced by the marine phytoplankton coccolithophores, have previously shown a large array of carbon and oxygen stable isotope fractionations (termed “vital effects”), correlated to cell size and hypothesized to reflect the varying importance of active carbon acquisition strategies. Culture studies show a reduced range of vital effects between large and small coccolithophores under high CO2, consistent with previous observations of a smaller range of interspecific vital effects in Paleocene coccoliths. We present new fossil data examining coccolithophore vital effects over three key Cenozoic intervals reflecting changing climate and atmospheric partial pressure of CO2 (pCO2). Oxygen and carbon stable isotopes of size‐separated coccolith fractions dominated by different species from well preserved Paleocene‐Eocene thermal maximum (PETM, ∼56 Ma) samples show reduced interspecific differences within the greenhouse boundary conditions of the PETM. Conversely, isotope data from the Plio‐Pleistocene transition (PPT; 3.5–2 Ma) and the last glacial maximum (LGM; ∼22 ka) show persistent vital effects of ∼2‰. PPT and LGM data show a clear positive trend between coccolith (cell) size and isotopic enrichment in coccolith carbonate, as seen in laboratory cultures. On geological timescales, the degree of expression of vital effects in coccoliths appears to be insensitive topCO2 changes over the range ∼350 ppm (Pliocene) to ∼180 ppm (LGM). The modern array of coccolith vital effects arose after the PETM but before the late Pliocene and may reflect the operation of more diverse carbon acquisition strategies in coccolithophores in response to decreasing Cenozoic pCO2.

Evolution of carbon cycle over the past 100 million years

It is generally accepted that progressive cooling of global climate since the Late Cretaceous results from decreasing partial pressure of atmospheric CO2 (pCO2). However, details on how and why the carbon cycle evolved and how it would affect pCO2 have not been fully resolved. While the long-term decline of pCO2 might be caused by the decrease of volcanic degassing through the negative feedback between pCO2 and silicate weathering, seafloor spreading, the major control of CO2 degassing, seems to have remained relatively constant. Alternative explanation, known as ‘uplift driven climate change’ hypothesis, proposes that tectonic uplift may have enhanced the sink of atmospheric CO2 by silicate weathering, and thus produced the decline of pCO2. However, increasing weathering sink of CO2 could deplete atmosphere all of its CO2 within several million years while holding volcanic outgassing constant. In this work, major fluxes of long-term carbon cycle are calculated based on a reverse model constrained by marine C, Sr and Os isotopic records and the spreading rate of sea floor. Weathering of island basalt and continental silicate rocks are separated in the new model. The results indicate a long-term decline of island basalt weathering in consistent with the global cooling trend over the past 100 million years. Dramatic changes of the CO2 fluxes associated continental silicate weathering, reverse weathering, volcanic degassing and the growth of organic carbon reservoir have been observed. Disturbance of atmospheric CO2 cycle by these fluxes seems to be maintained by the concomitant adjustments of island basalt weathering that were sensitive to the pCO2 controlled environment factors such as temperature and runoff. The negative feedbacks between pCO2 and weathering of island basalt might have played a significant role in stabilizing the long-term carbon cycle.

Reproducing early Martian atmospheric carbon dioxide partial pressure by modeling the formation of Mg‐Fe‐Ca carbonate identified in the Comanche rock outcrops on Mars

The well defined composition of the Comanche rock's carbonate (Magnesite0.62Siderite0.25Calcite0.11Rhodochrosite0.02) and its host rock's composition, dominated by Mg‐rich olivine, enable us to reproduce the atmospheric CO2partial pressure that may have triggered the formation of these carbonates. Hydrogeochemical one‐dimensional transport modeling reveals that similar aqueous rock alteration conditions (including CO2partial pressure) may have led to the formation of Mg‐Fe‐Ca carbonate identified in the Comanche rock outcrops (Gusev Crater) and also in the ultramafic rocks exposed in the Nili Fossae region. Hydrogeochemical conditions enabling the formation of Mg‐rich solid solution carbonate result from equilibrium species distributions involving (1) ultramafic rocks (ca. 32 wt% olivine; Fo0.72Fa0.28), (2) pure water, and (3) CO2partial pressures of ca. 0.5 to 2.0 bar at water‐to‐rock ratios of ca. 500 molH2O mol−1rock and ca. 5°C (278 K). Our modeled carbonate composition (Magnesite0.64Siderite0.28Calcite0.08) matches the measured composition of carbonates preserved in the Comanche rocks. Considerably different carbonate compositions are achieved at (1) higher temperature (85°C), (2) water‐to‐rock ratios considerably higher and lower than 500 mol mol−1 and (3) CO2partial pressures differing from 1.0 bar in the model set up. The Comanche rocks, hosting the carbonate, may have been subjected to long‐lasting (&gt;104 to 105 years) aqueous alteration processes triggered by atmospheric CO2partial pressures of ca. 1.0 bar at low temperature. Their outcrop may represent a fragment of the upper layers of an altered olivine‐rich rock column, which is characterized by newly formed Mg‐Fe‐Ca solid solution carbonate, and phyllosilicate‐rich alteration assemblages within deeper (unexposed) units.