Present Atmospheric Level Research Articles

Controls on Permo-Carboniferous precipitation over tropical Pangaea: A GCM sensitivity study

A series of Late Paleozoic climate model simulations was developed using the GENESIS atmospheric general circulation model to investigate the independent roles of Gondwanan deglaciation, atmospheric CO 2 rise, and regional tectonism on the long-term (10 6 yr) evolution of continental precipitation over tropical Pangaea. The model results indicate that either deglaciation or a CO 2 increase from 1× to 8× present-atmospheric levels could have caused substantial drying and warming at low latitudes, a result that is consistent with Late Paleozoic proxy records. Yet, the atmospheric processes that led to drying differ between these cases. The deglaciation of Gondwana reduces low-latitude temperature gradients in the Southern Hemisphere, decreasing by as much as 60% the meridional overturning in the Southern Hemisphere winter cell of the Hadley circulation. The decline in overturning reduces convective precipitation over equatorial Pangaea. In contrast, high levels of CO 2 have little effect on the large-scale meridional overturning circulation but increase continental temperatures, leading to high evaporation rates and reduced available soil moisture, the source of convective precipitation on land. In comparison to deglaciation and pCO 2, regional uplift/erosion of the Central Pangaean Mountains (CPMs) has a secondary effect on tropical precipitation in GENESIS and cannot explain the long-term aridification and warming of western Pangaea. We suggest that the deglaciation of Gondwana and a concomitant rise in atmospheric pCO 2 could explain much of the Late Paleozoic climate change over low-latitude Pangaea.

Isotopic evidence for Mesoarchaean anoxia and changing atmospheric sulphur chemistry

The evolution of the Earth's atmosphere is marked by a transition from an early atmosphere with very low oxygen content to one with an oxygen content within a few per cent of the present atmospheric level. Placing time constraints on this transition is of interest because it identifies the time when oxidative weathering became efficient, when ocean chemistry was transformed by delivery of oxygen and sulphate, and when a large part of Earth's ecology changed from anaerobic to aerobic. The observation of non-mass-dependent sulphur isotope ratios in sedimentary rocks more than approximately 2.45 billion years (2.45 Gyr) old and the disappearance of this signal in younger sediments is taken as one of the strongest lines of evidence for the transition from an anoxic to an oxic atmosphere around 2.45 Gyr ago. Detailed examination of the sulphur isotope record before 2.45 Gyr ago also reveals early and late periods of large amplitude non-mass-dependent signals bracketing an intervening period when the signal was attenuated. Until recently, this record has been too sparse to allow interpretation, but collection of new data has prompted some workers to argue that the Mesoarchaean interval (3.2-2.8 Gyr ago) lacks a non-mass-dependent signal, and records the effects of earlier and possibly permanent oxygenation of the Earth's atmosphere. Here we focus on the Mesoarchaean interval, and demonstrate preservation of a non-mass-dependent signal that differs from that of preceding and following periods in the Archaean. Our findings point to the persistence of an anoxic early atmosphere, and identify variability within the isotope record that suggests changes in pre-2.45-Gyr-ago atmospheric pathways for non-mass-dependent chemistry and in the ultraviolet transparency of an evolving early atmosphere.

Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (∼2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of Δ33S varies from −0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero Δ33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative Δ33S value, however, Superior-type BIFs which are far away from the volcanic center have positive Δ33S value. The δ 34S varies in a large range from −22.0‰ to +11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10−2–10−3 of the present atmospheric level (PAL).

Coping with cyclic oxygen availability: evolutionary aspects

Both the gradual rise in atmospheric oxygen over the Proterozoic Eon as well as episodic fluctuations in oxygen over several million-year time spans during the Phanerozoic Era, have arguably exerted strong selective forces on cellular and organismic respiratory specialization and evolution. The rise in atmospheric oxygen, some 2 billion years after the origin of life, dramatically altered cell biology and set the stage for the appearance of multicelluar life forms in the Vendian (Ediacaran) Period of the Neoproterozoic Era. Over much of the Paleozoic, the level of oxygen in the atmosphere was near the present atmospheric level (21%). In the Late Paleozoic, however, there were extended times during which the level of atmospheric oxygen was either markedly lower or markedly higher than 21%. That these Paleozoic shifts in atmospheric oxygen affected the biota is suggested by the correlations between: (1) Reduced oxygen and the occurrences of extinctions, a lowered biodiversity and shifts in phyletic succession, and (2) During hyperoxia, the corresponding occurrence of phenomena such as arthropod gigantism, the origin of insect flight, and the evolution of vertebrate terrestriality. Basic similarities in features of adaptation to hyopoxia, manifest in living organisms at levels ranging from genetic and cellular to physiological and behavioral, suggest the common and early origin of a suite of adaptive mechanisms responsive to fluctuations in ambient oxygen. Comparative integrative approaches addressing the molecular bases of phenotypic adjustments to cyclic oxygen fluctuation provide broad insight into the incremental steps leading to the early evolution of homeostatic respiratory mechanisms and to the specialization of organismic respiratory function.

A two‐dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O3 and near‐surface ultraviolet radiation history

We use the Cambridge two‐dimensional (2‐D) chemistry‐radiation transport model to investigate the implications for column O3 and near‐surface ultraviolet radiation (UV), of variations in atmospheric O2 content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1‐D model investigations showing that global annual mean O3 column increases monotonically with atmospheric O2. Sensitivity studies indicate that changes in temperature and N2O exert a minor influence on O3 relative to O2. We reconstructed Earth's O3 history by interpolating the modeled relationship between O3 and O2 onto two Phanerozoic O2 histories. Our results indicate that the largest variation in Phanerozoic column O3 occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O2 to ∼1.5 times the present atmospheric level (PAL) and subsequent fall to ∼0.5 PAL. The O3 response to this O2 decline shows latitudinal differences, thinning most at high latitudes (30–40 Dobson units (1 DU = 0.001 atm cm) at 66°N) and least at low latitudes (5–10 DU at 9°N) where a “self‐healing” effect is evident. This O3 depletion coincides with significant increases in the near‐surface biologically active UV radiation at high latitudes, +28% as weighted by the Thimijan spectral weighting function. O3 and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N2O production as the climate switched from an icehouse to a greenhouse mode. On the basis of a summary of field and laboratory experimental evidence, we suggest that these UV radiation increases may have exerted subtle rather than catastrophic effects on ecosystem processes.

Mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria

Filamentous and shrub-like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca. 1200 Ma Society Cliffs Formation, Baffin and Bylot Islands, Arctic Canada, are 400 m.y. older than previously reported examples. In vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms (CCMs) and is a direct ecophysiological link to atmospheric CO 2 concentration. CCMs are induced in present-day cyanobacteria under experimental conditions when pCO 2 is below ∼0.36% (∼10 times present atmospheric level, PAL). Society Cliffs calcified cyanobacteria consequently imply pCO 2 levels of 2 of 7–10 PAL in the late Mesoproterozoic. Combined, petro-graphic, experimental, and modeling results therefore suggest that Mesoproterozoic pCO 2 concentrations were not substantially different from Phanerozoic values and were significantly less than previous estimates of up to 200 PAL. Assuming 10% lower solar luminosity in the late Mesoproterozoic, pCO 2 levels of 10 PAL or less require the presence of additional greenhouse gases for maintenance of an ice-free Earth. At 10 PAL pCO 2 , methane concentrations of 100–200 ppm would have been sufficient to sustain warm Earth surface conditions. The low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of Earth9s Proterozoic biosphere.

Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition

ABSTRACTPhotosynthetic uptake of inorganic carbon can raise the pH adjacent to cyanobacterial cells, promoting CaCO3 precipitation. This effect is enhanced by CO2 concentrating mechanisms that actively transport into cells for carbon fixation. CO2 concentrating mechanisms presumably developed in response to atmospheric decrease in CO2 and increase in O2 over geological timescales. In present‐day cyanobacteria, CO2 concentrating mechanisms are induced when the atmospheric partial pressure of CO2 (pCO2) falls below ∼0.4%. Reduction in pCO2 during the Proterozoic may have had two successive effects on cyanobacterial calcification. First, fall in pCO2 below ∼1% (33 times present atmospheric level, PAL) resulted in lower dissolved inorganic carbon (DIC) concentrations that reduced pH buffering sufficiently for isolated CaCO3 crystals to begin to nucleate adjacent to cyanobacterial cells. As a result, blooms of planktic cyanobacteria induced precipitated ‘whitings’ of carbonate mud in the water column whose sedimentary accumulation began to dominate carbonate platforms ∼1400–1300 Ma. Second, fall in pCO2 below ∼0.4% (10 PAL) induced CO2‐concentrating mechanisms that further increased pH rise adjacent to cells and promoted in vivo cyanobacterial sheath calcification. Crossing of this second threshold is indicated in the fossil record by the appearance of Girvanella 750–700 Ma. Coeval acquisition of CO2 concentrating mechanisms by planktic cyanobacteria further stimulated whiting production. These inferences, that pCO2 fell below ∼1%∼1400–1300 Ma and below ∼0.4% 750–700 Ma, are consistent with empirical and modelled palaeo‐atmosphere estimates. Development of CO2 concentrating mechanisms was probably temporarily slowed by global cooling ∼700–570 Ma that favoured diffusive entry of CO2 into cells. Lower levels of temperature and DIC at this time would have reduced seawater carbonate saturation state, also hindering cyanobacterial calcification. It is suggested that as Earth emerged from ‘Snowball’ glaciations in the late Neoproterozoic, global warming and O2 rise reactivated the development of CO2 concentrating mechanisms. At the same time, rising levels of temperature, calcium ions and DIC increased seawater carbonate saturation state, stimulating widespread cyanobacterial in vivo sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics.

Bistability of atmospheric oxygen and the Great Oxidation

The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability. The largest chemical transition was the 'Great Oxidation', approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10(-5) of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved, but the causes of this delay and of the Great Oxidation itself remain uncertain. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10(-5) PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 x 10(-3) PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10(-5) PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis and for the evolution of multicellular life.

Potential of ozone formation by the smog mechanism to shield the surface of the early Earth from UV radiation

We propose that the photochemical smog mechanism produced substantial ozone (O3) in the troposphere during the Proterozoic period, which contributed to ultraviolet (UV) radiation shielding, and hence favoured the establishment of life. The smog mechanism is well established and is associated with pollution hazes that sometimes cover modern cities. The mechanism proceeds via the oxidation of volatile organic compounds such as methane (CH4) in the presence of UV radiation and nitrogen oxides (NOx). It would have been particularly favoured during the Proterozoic period given the high levels of CH4 (up to 1000 ppm) recently suggested. Proterozoic UV levels on the surface of the Earth were generally higher compared with today, which would also have favoured the mechanism. On the other hand, Proterozoic O2 required in the final step of the smog mechanism to form O3 was less abundant compared with present times. Furthermore, results are sensitive to Proterozoic NOx concentrations, which are challenging to predict, since they depend on uncertain quantities such as NOx source emissions and OH concentrations. We review NOx sources during the Proterozoic period and apply a photochemical box model having methane oxidation with NOx, HOx and Ox chemistry to estimate the O3 production from the smog mechanism. Runs suggest the smog mechanism during the Proterozoic period can produce approximately double the present-day ozone columns for NOx levels of 1.53×10−9 by volume mixing ratio, which was attainable according to our NOx source analysis, with 1% of the present atmospheric levels of O2. Clearly, forming ozone in the troposphere is a trade-off for survivability – on the one hand, harmful UV radiation is blocked, but on the other hand ozone is a respiratory irratant, which becomes fatal at concentrations exceeding about 1 ppmv.

Can biomass burning produce a globally significant carbon-isotope excursion in the sedimentary record?

Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios ( δ 13C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO 2), and thereby provide a source of low 13C CO 2 when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO 2 from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ 13C excursion in the atmosphere (∼ 2.4‰), vegetation (∼ 2.4‰) and the surface ocean (∼ 1.2‰), but not for the deep ocean (∼ 0.9‰). Release of CO 2 initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ 13C excursions in the rock record, even during times of elevated pCO 2.

Evolution of the oceanic sulfur cycle at the end of the Paleoproterozoic

Here, we present new measurements of 32S, 33S, 34S, and 36S in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late Paleoproterozoic marine basin. We target the transition in ocean chemistry from the deposition of Paleoproterozoic iron formations (Gunflint Formation, Biwabik Formation, Trommald Formation, and Mahnomen iron formations) to the inferred sulfidic ocean conditions recorded by overlying shale (Rove Formation). The data suggest that certain features of the global sulfur cycle, such as a control by sulfate reducing prokaryotes, and low (mM) concentrations of oceanic sulfate, were maintained across this transition. This suggests that the transition was associated with changes in the structure of the basin-scale sulfur cycle during deposition of these sediments. Sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore-water sulfate that was supplied through steady-state exchange with an overlying oceanic sulfate reservoir. The sulfide data for the euxinic Rove Formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a Rayleigh-like process. We suggest that the prevalence of large and variable heavy isotope enrichments observed in Rove Formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column, either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink. The inclusion of 36S measurements (in addition to 32S, 33S, and 34S) illustrates the mass-dependent character of these sedimentary environments, ruling out contributions from the weathering of Archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen (>10 −5 present atmospheric levels of O 2).

Magnitude and origin of the anthropogenic CO2 increase and 13C Suess effect in the Nordic seas since 1981

This study evaluates the anthropogenic changes of CO2 (ΔCant) and δ13C (Δδ13Cant) in the Nordic seas, the northern limb of the Atlantic Meridional Overturning Circulation, that took place between 1981 and 2002/2003. The changes have been determined by comparing data obtained during the Transient Tracers in the Ocean, North Atlantic Study (TTO‐NAS) with data obtained during the Nordic seas surveys of R/V Knorr in 2002 and R/V G.O. Sars in 2003 using an extended multilinear regression approach. The estimated Δδ13Cant and ΔCant and their relationship to each other and to water mass distribution suggest that the Polar Water entering the Nordic seas from the north is undersaturated with respect to the present atmospheric anthropogenic CO2 levels and promotes a local uptake of Cant within the Nordic seas. In contrast, the Atlantic Water entering from the south appears equilibrated. It carries with it anthropogenic carbon which will be sequestered at depth as the water overturns. This preequilibration leaves no room for further uptake of Cant in the parts of the Nordic seas dominated by Atlantic Water. The upper ocean pCO2 in these regions appears to have increased at a greater rate than the atmospheric pCO2 over the last 2 decades; this is reconcilable with a large lateral advective supply of Cant.

Early evolution of atmospheric oxygen from multiple-sulfur and carbon isotope records of the 2.9 Ga Mozaan Group of the Pongola Supergroup, Southern Africa

Sulfur isotope mass-independent fractionation (S-MIF) is a unique geologic record of Archean atmospheric chemistry that provides important constraints on the evolution of the early Earth’s atmosphere and its impact on early life. In this contribution, we report multiple-sulfur (33S/32S, 34S/32S, and 36S/32S) isotope ratios of sulfide minerals and carbon (13C/12C) isotope ratios of organic carbon for shale in the ~2.96 to ~2.84 Ga Mozaan Group of the Pongola Supergroup, Southern Africa. The δ13C of organic carbon shows two populations: one with δ13C of ~ −26 ‰ and another with δ13C of −32 ‰. The Δ33S values from nine samples ranges from −0.49 to +0.36 ‰, which is considerably smaller than what was measured for the sulfide and sulfate minerals from other Archean intervals but outside the range of Δ33S values measured for post-2.0 Ga sulfide and sulfate minerals. Moreover, some samples from the Mozaan Group yield Δ36S/Δ33S ratios that are different from Phanerozoic sulfides, suggesting sulfide sulfur from the Mozaan Group carries mass-independent isotope fractionation originated from atmospheric photochemistry. The relatively small Δ33S values for the Mozaan Group may suggest that the atmosphere became slightly oxidized at ~2.9 Ga with oxygen level above 10−5 but below 10−2 times present atmospheric level. This intermediate oxygen level would allow production of S-MIF in atmospheric chemistry but prohibit preservation of large S-MIF signatures in surface deposits. Our hypothesis implies the evolution of oxygenic photosynthesis as early as ~2.9 Ga. Such an ephemeral oxidation event could have triggered the Mozaan-Witwatersrand glaciation by destabilizing an existing methane-rich Archean atmosphere.

Precambrian paleosols and atmospheric CO 2 levels

Precambrian atmospheric pCO 2 levels were previously estimated using a simple thermodynamic model based on the mineral assemblage in paleosols. In addition to theoretical flaws with that model, recalculation of pCO 2 levels using more recent thermodynamic data gives significantly lower values. A new model based on the mass balance of weathering in paleosols gives consistent results from three separate ∼2.2 Ga old paleosols. The calculated pCO 2 value of 23 ÷ 3 × 3 times present atmospheric levels is insufficient to overcome the “faint young Sun” paradox, suggesting that another greenhouse gas such as CH 4 was present at elevated levels relative to the present atmosphere to account for the discrepancy. Applying this new approach to other Precambrian paleosols a Proterozoic pCO 2 curve is generated, which indicates consistently high pCO 2 from 2.5 to 1.8 Ga ago, and a substantial drop in atmospheric pCO 2 at some time between 1.8 and 1.1 Ga ago.

A mid-Caradocian (453 Ma) drawdown in atmospheric pCO 2 without ice sheet development?

Previous workers have recognized a positive δ 13C excursion during the mid-Caradocian in Laurentian and Baltic platform carbonates. This study documents the mid-Caradocian Guttenberg excursion (GICE) in the Kullsberg Limestone in central Sweden. Significantly, we observed increases in both δ 13C and δ 18O unlike most previous studies of GICE which only observed increased δ 13C marine values. Brachiopods from the lower Kullsberg Limestone reflect pre-excursion conditions having more negative δ 13C (0–1‰) and δ 18O (− 4‰ to − 6‰) values compared with least-altered marine cement from the upper Kullsberg Limestone, which have more positive δ 13C (2.1–3.6‰) and δ 18O (− 1.8‰ to − 3.6‰) values. Least-altered marine cements from the Kullsberg Limestone include: columnar calcite, first-generation equant calcite, and translucent fibrous calcite (from earliest to latest in timing of formation). Kullsberg seawater during the δ 13C excursion event likely decreased in temperature by up to 15 °C with an approximate ≈1‰ increase in δ 18O H 2O attributable to increased salinity during the mid-Caradocian. The results of this study are consistent with multiple independent approaches to characterize the composition of the Ordovician atmosphere/hydrosphere systems. Specifically, results of this study are consistent with formation of abiotic calcite at moderately low paleotemperatures during the GICE excursion of 10–17 °C and Ordovician atmospheric pCO 2 values of < 10 present atmospheric level.

COPSE: A new model of biogeochemical cycling over Phanerozoic time

We present a new model of biogeochemical cycling over Phanerozoic time. This work couples a feedback-based model of atmospheric O2 and ocean nutrients (Lenton and Watson, 2000a, 2000b) with a geochemical carbon cycle model (Berner, 1991, 1994), a simple sulfur cycle, and additional components. The resulting COPSE model (Carbon-Oxygen-Phosphorus-Sulfur-Evolution) represents the co-evolution of biotic and abiotic components of the Earth system, in that it couples interactive and evolving terrestrial and marine biota to geochemical and tectonic processes. The model is forced with geological and evolutionary forcings and time-dependent solar insolation. The baseline model succeeds in giving simultaneous predictions of atmospheric O2, CO2, global temperature, ocean composition, δ13C and δ34S that are in reasonable agreement with available data and suggested constraints. The behavior of the coupled model is qualitatively different to single cycle models. While atmospheric pCO2 (CO2 partial pressure) predictions are mostly determined by the model forcings and the response of silicate weathering rate to pCO2 and temperature, multiple negative feedback processes and coupling of the C, O, P and S cycles are necessary for regulating pO2 while allowing δ13C changes of sufficient amplitude to match the record. The results support a pO2 dependency of oxidative weathering of reduced carbon and sulfur, which raises early Paleozoic pO2 above the estimated requirement of Cambrian fauna and prevents unrealistically large δ34S variation. They do not support a strong anoxia dependency of the C:P burial ratio of marine organic matter (Van Cappellen and Ingall, 1994, 1996) because this dependency raises early Paleozoic δ13C and organic carbon burial rates too high. The dependency of terrestrial primary productivity on pO2 also contributes to oxygen regulation. An intermediate strength oxygen fire feedback on terrestrial biomass, which gives a pO2 upper limit of ∼1.6PAL (present atmospheric level) or 30 volume percent, provides the best combined pO2 and δ13C predictions. Sulfur cycle coupling contributes critically to lowering the Permo-Carboniferous pCO2 and temperature minimum. The results support an inverse dependency of pyrite sulfur burial on pO2 (for example, Berner and Canfield, 1989), which contributes to the shuttling of oxygen back and forth between carbonate carbon and gypsum sulfur. A pO2 dependency of photosynthetic carbon isotope fractionation (Berner and others, 2000; Beerling and others, 2002) is important for producing sufficient magnitude of δ13C variation. However, our results do not support an oxygen dependency of sulfur isotope fractionation in pyrite formation (Berner and others, 2000) because it generates unrealistically small variations in δ34S. In the Early Paleozoic, COPSE predicts pO2=0.2–0.6PAL and pCO2>10PAL, with high oceanic [PO43-] and low [SO4=]. Land plant evolution caused a ‘phase change’ in the Earth system by increasing weathering rates and shifting some organic burial to land. This change resulted in a major drop in pCO2 to 3 to 4PAL and a rise in pO2 to ∼1.5PAL in the Permo-Carboniferous, with temperatures below present, ocean variables nearer present concentrations, and PO4:NO3 regulated closer to Redfield ratio. A second O2 peak of similar or slightly greater magnitude appears in the mid-Cretaceous, before a descent towards PAL. Mesozoic CO2 is in the range 3 to 7PAL, descending toward PAL in the Cretaceous and Cenozoic.

Organic- and carbonate-rich soil formation ∼2.6 billion years ago at Schagen, East Transvaal district, South Africa

A ∼17-m paleosol sequence at Schagen, South Africa, which developed on a serpentinized dunite intrusion in a granite-gneiss terrain ∼2.6 Ga ago, is characterized by an alternating succession of thick (∼1–3 m) carbonate-rich (dolomite and calcite) zones and silicate-rich (serpentines, talc, and quartz) zones; the upper ∼8 m section is especially rich in organic C (up to ∼1.4 wt.%). Petrologic and geochemical data suggest the upper ∼8 m section is composed of at least three soil profiles that developed on: (i) silicate-rich rock fragments (and minerals) that were transported from local sources (serpentinite and granite) by fluvial and/or eolian processes; and (ii) dolomite and calcite zones that formed by locally discharged groundwater. The Mg and Fe in the paleosol sequence were largely supplied from local sources (mostly serpentinite), but the Ca, Sr, and HCO 3 − were supplied by groundwater that originated from a surrounding granite-gneiss terrain. In the uppermost soil profile, the (Fe is retained, the Fe 3+/Fe 2+ ratio increases, and ferri-stilpnomelane is abundant. These data suggest the atmospheric pO 2 was much greater than ∼10 −3.7 atm (>0.1% present atmospheric level [PAL]). The carbonaceous matter in the soils is intimately associated with clays (talc, chlorite, and ferri-stilpnomelane) and occurs mostly as seams (20 μm to 1 mm thick) that parallel the soil horizons. These occurrences, crystallographic structures, H/C ratios, and δ 13C org values (−17.4 to −14.4‰ PDB) suggest that the carbonaceous matter is a remnant of in situ microbial mats, originally ∼1 to ∼20 mm thick. The microbial mats developed: (a) mostly on soil surfaces during the formation of silicate-rich soils, and (b) at the bottom of an evaporating, anoxic, alkaline pond during the precipitation of the Fe-rich dolomite. These δ 13C org values are difficult to be explained by a current popular idea of a methane- and organic haze-rich Archean atmosphere (Pavlov et al., 2001); these values, however, can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment.

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

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.

Exogenous carbon acquisition of photosynthesis in Porphyra haitanensis (Bangiales, Rhodophyta) under emersed state*

Abstract The photosynthetic performances of Porphyra haitanensis thalli were investigated in order to understand its mechanisms for exogenous carbon acquisition during emersion at low tide. The emersed photosynthesis was studied by altering the pH value in the water film on the thalli surface, treating them with carbonic anhydarase inhibitors (acetazolamide and 6-ethoxyzolamide), adjusting the CO2 concentrations in the air. and comparing the theoretical maximum CO2 supply rates within the adherent water film with the observed photosynthetic CO2 uptake rates. It was found that the principal exogenous inorganic carbon source for the photosynthesis of P. haitanensis during emersion was atmospheric CO2. The driving force of CO2 flux across the waterfilm was the CO2 concentration gradient within it. Carbonic anhydrase accelerated both extracellular and intracellular CO2 transport. The emersed photosynthesis of P. haitanensis was limited by the present atmospheric CO2 level, and would be enhanced by atmospheric CO2 rise that would trigger global warming.

Dating the rise of atmospheric oxygen.

Several lines of geological and geochemical evidence indicate that the level of atmospheric oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels by 2.22 Gyr ago. Here we present evidence that the rise of atmospheric oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in organic-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic composition of sulphur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atmospheric oxygen was present at significant levels (that is, greater than 10(-5) times that of the present atmospheric level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of haematitic pisolites and oölites in the upper Timeball Hill Formation indicate that atmospheric oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Palaeoproterozoic glacial events.