Why the atmosphere became oxygenated: A proposal

The increase in the oxidation state of the atmosphere during Earth history is well documented, but not the reasons for this process. The course of atmospheric evolution has been dominated by the effects of photosynthesis and by the composition of volcanic gases. The H₂/H₂O ratio of volcanic gases has remained essentially constant during most of Earth history. It is here proposed that their CO₂/H₂O and SO₂/H₂O ratios have increased with time. A simple model for the degassing and the recycling of CO₂, SO₂, and H₂O can account for many of the major steps in the oxygenation of the atmosphere.     

Volatiles in pillows of the Mid-Ocean-Ridge-Basalt (MORB) and vitreous basaltic rims

Temperature-resolved analyses of volatiles from Mid-Ocean-Ridge-Basalt (MORB) and vitreous basaltic rims were carried out to investigate the total volatile contents of basaltic melts and the influence of magma contamination on the degassing behaviour of volcanic rocks.With respect to the sources of methane evolution from the MORB the investigations are taken into consideration, the hydrocarbon (HC) release especially from the melt.The current paper presents data for H₂O, CO₂, SO₂, He, H₂, HF, HCl, CO, N₂, O₂, and HC degassing profiles of samples from the MORB sampling cruise 02.10.1983-11.11.1983 with FS Sonne 28 during the GEMINO-1 project near the Carlsberg Ridge (CR) and the Mid-Indian-Ocean-Ridge (MIOR).It aims to estimate the magnitude and nature of source magma volatiles and contamination (crustal material, seawater, atmospheric gases).The degassing of H₂O, CO₂, HCs as well as sulphur and chlorine species, or O₂ from vitreous specimens shows characteristic differences associated with sample position with respect to the lava surface.From the water release by bubbling and diffusion above 700 °C it must be concluded that any assimilation of sea water in vitreous rim is very low. The water content in the vitreous rim is about 0.1-0.2 wt%. The low interaction of melt with sea water is supported by the missing of a significant release of chlorine species during the heat treatment of the sample up to 1450 °C.Mixed H₂O/CO₂ bubbles escape between 700 and 800 °C from the vitreous rim. The CO₂ release in the temperature range of 1060-1170 °C from the basalt and the vitreous rim is interpreted as an indication for the primary carbon-dioxide content in the melt.Above 1100 °C CO₂ and SO₂ are evolved by both diffusion and small bubbles. The quantities of CO₂ in the vitreous rim and the basalt are similar (between 0.05 and 0.15 wt%), whereas the quantities of SO₂ escaping both from the vitreous rim and the crystalline basalt are between 0.013 and 0.024 wt%.Simultaneous with the CO₂ release by bubbling, HC species, especially CH fragments, were observed. The fact that the temperature of release maxima are above 1050 °C in both the vitreous rim and in the basalt is an indication for a geogenetic origin of HCs, e.g. methane.A low temperature of release for methane, which is consistent with biogenetic HC, was observed from the gas-release profiles of the basalts only. The maxima of the low-temperature gas releases are between 80 and 200 °C with a high correlation between the fragments m/z 13 and m/z 15. This correlation is a significant indication for a methane release.The oxygen release profiles of vitreous and crystalline basalts give significant indications for oxygen fugacity below the (QMF) of basaltic magma.Secondary minerals, generated by alteration of basaltic rocks, can be characterized by gas release profiles (GRPs) due to their decomposition in the temperature range below 800 °C. Only in the basalt were there observed indications of alteration processes. Small traces of carbonates (<0.0001 wt%) were detected by the gas release during the decomposition.Processes of degassing at temperatures higher than 800 °C are correlated to volatiles in the melt and to fluid inclusions of the minerals. There are no obvious correlations in the degassing characteristics between H₂O, CO₂ and SO₂. The different maxima of the degassing velocity, especially of CO₂, and SO₂, are indications of the different bonding forces of the site occupancy of the volatiles in the melt and in the glass. A micelle model for bonding sites in the basaltic glass for dissolved volatiles is discussed.     

Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N₂/Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hemipelagic sediment on the down-going plate. Using a mantle endmember with δ¹⁵N = −5‰ and N₂/He = 100 and a subducted sediment component with δ¹⁵N = +7‰ and N₂/He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcán Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO₂/N_{2 exc.} ratios (N_{2 exc.} is the N₂ abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 × 10⁸ mol N/a from ³He flux, 6.9 × 10⁸ mol/a from SO₂ flux, and 2.1 × 10⁸ and 1.3 × 10⁹ mol/a from CO₂ fluxes calculated from ³He and SO₂, respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hemipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism.     

Tracing industrial sulfur contributions to atmospheric sulfate deposition in the Athabasca oil sands region,Alberta, Canada

Anthropogenic S emissions in the Athabasca oil sands region (AOSR) in Alberta, Canada, affect SO₄ deposition in close vicinity of industrial emitters. Between May 2008 and May 2009, SO₄-S deposition was monitored using open field bulk collectors at 15 sites and throughfall collectors at 14 sites at distances between 3 and 113 km from one of the major emission stacks in the AOSR. At forested plots >90 km from the operations, SO₄ deposition was ∼1.4 kg SO₄-S ha⁻¹ yr⁻¹ for bulk deposition and ∼3.3 kg SO₄-S ha⁻¹ yr⁻¹ for throughfall deposition. Throughfall SO₄ deposition rates in the AOSR exceeded bulk deposition rates at all sites by a factor of 2–3, indicating significant inputs of dry deposition especially under forest canopies. Both bulk and throughfall SO₄ deposition rates were elevated within 29 km distance of the industrial operations with deposition rates as high as 11.7 kg SO₄-S ha⁻¹ yr⁻¹ for bulk deposition and 39.2 kg SO₄-S ha⁻¹ yr⁻¹ for throughfall at industrial sites. Sulfur isotope ratio measurements of atmospheric SO₄ deposited in the AOSR revealed that at a few selected locations ³⁴S-depleted SO₄, likely derived from H₂S emissions from tailing ponds contributes to local atmospheric SO₄ deposition. In general, however, δ³⁴S values of SO₄ deposition at distant forested plots (>74 km) with low deposition rates were not isotopically different from δ³⁴S values at sites with high deposition rates in the AOSR and are, therefore, not suitable to determine industrial S contributions. However, O isotope ratios of atmospheric SO₄ in bulk and throughfall deposition in the AOSR showed a distinct trend of decreasing δ¹⁸O-SO₄ values with increasing SO₄ deposition rates allowing quantification of industrial contributions to atmospheric SO₄ deposition. Two-end-member mixing calculations revealed that open field bulk SO₄ deposition especially at industrial sites in close proximity (<29 km) to the operations is significantly (17–59%) affected by industrial S emissions and that throughfall generally contained 49–100% SO₄ of industrial origin. Hence, it is suggested that δ¹⁸O values of SO₄ may constitute a suitable tracer for quantifying industrial contributions to atmospheric SO₄ deposition in the AOSR.     

Composition and structure of an iron-bearing, layered double hydroxide (LDH) - Green rust sodium sulphate

Mixed-valent Fe(II),Fe(III)-layered hydroxide, known as green rust, was synthesized from slightly basic, sodium sulphate solutions in an oxygen-free glove box. Solution conditions were monitored with pH and Eh electrodes and optimized to ensure a pure sulphate green-rust phase. The solid was characterised using Mössbauer spectroscopy, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The composition of the solution from which the green rust precipitated was established by mass and absorption spectroscopy. The sulphate form of green rust is composed of brucite-like layers with Fe(II) and Fe(III) in an ordered distribution. The interlayers contain sulphate, water and sodium in an arrangement characteristic for the nikischerite group. The crystal structure is highly disordered by stacking faults. The composition, formula and crystallographic parameters are: NaFe(II)₆Fe(III)₃(SO₄)₂(OH)₁₈·12H₂O, space group P-3, a = 9.528(6) Å, c = 10.968(8) Å and Z = 1. Green rust sodium sulphate, GR_Na,SO4, crystallizes in thin, hexagonal plates. Particles range from less than 50 nm to 2 μm in diameter and are 40 nm thick or less. The material is redox active and reaction rates are fast. Extremely small particle size and high surface area contribute to rapid oxidation, transforming green rust to an Fe(III)-phase within minutes.     

Carbon dioxide degassing and thermal energy release in the Monte Amiata volcanic-geothermal area (Italy)

The quaternary volcanic complex of Mount Amiata is located in southern Tuscany (Italy) and represents the most recent manifestation of the Tuscan Magmatic Province. The region is characterised by a large thermal anomaly and by the presence of numerous CO₂-rich gas emissions and geothermal features, mainly located at the periphery of the volcanic complex. Two geothermal systems are located, at increasing depths, in the carbonate and metamorphic formations beneath the volcanic complex. The shallow volcanic aquifer is separated from the deep geothermal systems by a low permeability unit (Ligurian Unit). A measured CO₂ discharge through soils of 1.8 × 10⁹ mol a⁻¹ shows that large amounts of CO₂ move from the deep reservoir to the surface. A large range in δ¹³C_TDIC (−21.07 to +3.65) characterises the waters circulating in the aquifers of the region and the mass and isotopic balance of TDIC allows distinguishing a discharge of 0.3 × 10⁹ mol a⁻¹ of deeply sourced CO₂ in spring waters. The total natural CO₂ discharge (2.1 × 10⁹ mol a⁻¹) is slightly less than minimum CO₂ output estimated by an indirect method (2.8 × 10⁹ mol a⁻¹), but present-day release of 5.8 × 10⁹ mol a⁻¹ CO₂ from deep geothermal wells may have reduced natural CO₂ discharge. The heat transported by groundwater, computed considering the increase in temperature from the infiltration area to the discharge from springs, is of the same order of magnitude, or higher, than the regional conductive heat flow (>200 mW m⁻²) and reaches extremely high values (up to 2700 mW m⁻²) in the north-eastern part of the study area. Heat transfer occurs mainly by conductive heating in the volcanic aquifer and by uprising gas and vapor along fault zones and in those areas where low permeability cover is lacking. The comparison of CO₂ flux, heat flow and geological setting shows that near surface geology and hydrogeological setting play a central role in determining CO₂ degassing and heat transfer patterns.     

Insights into the Hadean Earth from experimental studies of zircon

Geologists investigate the evolution of the atmosphere, crust, and mantle through time by direct study of the rock record. However, the Hadean eon (>3.85 Ga) has been traditionally viewed as inaccessible due to the absence of preserved rocks. The discovery of >4.0 Ga detrital zircons from Western Australia in the 1980s — coupled with the development of new micro-analytical capabilities — made possible new avenues of early Earth research. The prevailing view that emerged is that the early Earth may have contained a stable hydrosphere, water-saturated or (near watersaturated) granitic magmas, and volcanic emanations dominated by neutral gas species (e.g., CO₂, H₂O, and SO₂). The Hadean Earth may have been capable of supporting life ～200 Ma after accretion and perhaps earlier. Many of these models are formulated — or have been subsequently supported — by laboratory experiments of zircon. Important petrological variables such as temperature, pressure, oxygen fugacity, and component activities (e.g., SiO₂/TiO₂-activities) can be controlled. These experiments are fundamental for extrapolation to ‘deep time’ because they provide a means to understand primary chemistry preserved in ancient zircons. This review paper specifically focuses on zircon experimental studies (oxygen isotope fractionations, Ti-thermometry, and redox sensitive element incorporation into zircon), which have influenced our view of the very early Earth.     

Diffuse gas emissions at the Ukinrek Maars,Alaska: Implications for magmatic degassing and volcanic monitoring

Diffuse CO₂ efflux near the Ukinrek Maars, two small volcanic craters that formed in 1977 in a remote part of the Alaska Peninsula, was investigated using accumulation chamber measurements. High CO₂ efflux, in many places exceeding 1000 g m⁻² d⁻¹, was found in conspicuous zones of plant damage or kill that cover 30,000–50,000 m² in area. Total diffuse CO₂ emission was estimated at 21–44 t d⁻¹. Gas vents 3-km away at The Gas Rocks produce 0.5 t d⁻¹ of CO₂ that probably derives from the Ukinrek Maars basalt based on similar δ¹³C values (∼−6‰), ³He/⁴He ratios (5.9–7.2 R_A), and CO₂/³He ratios (1–2 × 10⁹) in the two areas. A lower ³He/⁴He ratio (2.7 R_A) and much higher CO₂/³He ratio (9 × 10¹⁰) in gas from the nearest arc-front volcanic center (Mount Peulik/Ugashik) provide a useful comparison. The large diffuse CO₂ emission at Ukinrek has important implications for magmatic degassing, subsurface gas transport, and local toxicity hazards. Gas–water–rock interactions play a major role in the location, magnitude and chemistry of the emissions.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

Chemical monitoring of volcanic gas using remote FT-IR spectroscopy at several active volcanoes in Japan

Chemical compositions of volcanic gases of several Japanese active volcanoes have been monitored from distant safe places since the beginning of the 1990s using an FT-IR spectral radiometer. For absorption measurements, an infrared light source behind volcanic gas emissions is necessary in a volcanic environment. In the early observations, infrared radiation from hot lava domes (Unzen volcano) and hot ground heated by high-temperature fumaroles (Usu, Aso, and Satsuma-Iwojima volcanoes) were used as infrared light sources. However, these sources were not available in many cases. This remote FT-IR method became more commonly applied to chemical monitoring of volcanic gases emitted from the summit or slopes of active volcanoes using scattered solar infrared light as infrared light sources (Sakurajima, Miyakejima, and Asama volcanoes). To date, eight species have been measured using this method: SO₂, HCl, HF, CO, CO₂, COS, SiF₄, and H₂O. The observations indicate that volcanic gases for each volcano have different chemical composition on a SO₂–HCl–HF ternary diagram in spite of similar tectonic settings, suggesting that vapor/melt volume ratios during volcanic gas formation differ among volcanoes. During more than 15 years of monitoring, chemical changes in volcanic gases attributable to ascent of magma were observed only at Asama, where HCl/SO₂ and HF/HCl ratios in the eruptive period were higher than those in non-eruptive period because of scrubbing of more soluble components in surface hydrothermal systems in the non-eruptive stage or solubility-controlled fractionation processes. Results show that these parameters are the most prospective ones among the various parameters measured using the remote FT-IR method to monitor volcanic activities.     

Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group,southern margin of the North China Craton

The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO₂ contents ranging from 52.10 wt% to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88 wt%) and accordingly low Mg^# values of 11–53. The Nd isotopes are enriched and show a weak variation with ?_Nd(t) = −7.12 to −9.63. Zircon Hf isotopes are also enriched with ?_Hf(t) = −12.02 ± 0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO₂ contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000 km²) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean.     

Quantification of atmospheric oxygen levels during the Paleoproterozoic using paleosol compositions and iron oxidation kinetics

The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (P_O2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (M_R), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and Fe_R values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. Fe_R increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. Mn_R also increased gradually similar to Fe_R but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe²⁺ and Mn²⁺ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled P_O2 variations during the Paleoproterozoic by applying Fe²⁺ oxidation kinetics to the Fe²⁺ and Fe³⁺ concentrations in paleosols, which enabled us to derive an Fe²⁺ oxidation term referred to as ψ. Possible changes in temperature and P_CO2 during this time window and their effects on resulting models of P_O2 evolution have been also considered. We assumed four cases for the calculations of P_O2 variations between 2.5 and 2.0 Ga: no change in either temperature or P_CO2, long-term change in only P_CO2, long-term changes in both temperature and P_CO2, and short-term fluctuations of both temperature and P_CO2 during the possible, multiple global-scale glaciations. The calculations indicate that P_O2 increased gradually, linearly on the logarithmic scale, from <∼10⁻⁶ to >∼10⁻³ atm between 2.5 and 2.0 Ga. Our calculations show that the P_O2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life.     

The oxygen geochemical cycle: dynamics and stability

The first and possibly only major rise of atmospheric oxygen, from p_O2 ≤ 0.1% PAL (the present atmospheric level) to p_O2 ≥ 10% PAL, appears to have occurred sometime before 2 Ga ago, although the exact time of and the cause(s) for the rise have been hotly debated. Equally important questions on the atmospheric oxygen concern its stability, especially the mechanisms regulating the atmospheric p_O2 level and the causes and magnitude of p_O2 variations since the first major rise of atmospheric oxygen. Previous efforts to model the p_O2 variation during the Phanerozoic time have typically relied on secondary information, such as the carbon and sulfur isotopic records of sedimentary rocks, and on simple dynamics of the geochemical cycles of O, C, S, and P based on box-type models. As a result, many kinetic questions about the variation and stability of atmospheric oxygen could not have been answered. Here we quantitatively evaluate the dynamics and stability of atmospheric O₂ and CO₂, using recent experimental data, field observations, and a new model for the C-O coupled geochemical cycles. We examine the change with time in the fluxes of various compounds (O₂, CO₂, phosphate, organic C, carbonate C, C-bearing reduced volcanic gases, and C-free reduced volcanic gases) among the various reservoirs (atmosphere, soil, surface ocean, deep ocean, the lower crust and mantle, and upper crust) under a variety of scenarios. Our model does not assume steady-state fluxes for any of the reservoirs. Rather, the model incorporates the kinetic experimental data on oxidation of coal, a proxy for kerogen, the dynamics of soil formation and erosion, the kinetics of decomposition of organic matter in the Oceans by aerobic and anaerobic bacteria, the equilibrium ocean-atmosphere carbonate model, the observed relationships among the organic burial flux, dissolved O₂ content of deep ocean, and sedimentation rates, and the three-box model ocean. The important parameters that strongly influence the dynamics of atmospheric O₂, are found to be (a) the total area of soil formation on Earth; (b) the average soil depth; (c) the average rate of physical erosion of soils, which is linked to the average rate of accumulation of clastic sediments in the oceans; (d) the composition and flux of volcanic gas; and (e) the level of atmospheric CO₂. We develop kinetic equations linking these parameters to the production and consumption fluxes of atmospheric oxygen and also to stable p_O2 values. Considering the likely ranges of variations in these parameters in geologic history, we suggest that the atmospheric p_O2 level is likely to have stayed within a very narrow range of 0.6-2 PAL and that the entire ocean, except for local euxinic basins, is likely to have been basically oxygenated since the first major rise of atmospheric oxygen more than 2 Ga ago.     

River dissolved and solid loads in the Lesser Antilles: New insight into basalt weathering processes

We present here the first available estimations of chemical weathering and associated atmospheric CO₂ consumption rates as well as mechanical erosion rate for the Lesser Antilles. The chemical weathering (100–120 t/km²/year) and CO₂ consumption (1.1–1.4 × 10⁶ mol/km²/year) rates are calculated after subtraction of the atmospheric and hydrothermal inputs in the chemical composition of the river dissolved loads. These rates thus reflect only the low-temperature basalt weathering. Mechanical erosion rates (approx. 800–4000 t/km²/year) are estimated by a geochemical mass balance between the dissolved and solid loads and mean unaltered rock. The calculated chemical weathering rates and associated atmospheric CO₂ consumption rates are among the highest values worldwide but are still lower than those of other tropical volcanic islands and do not fit with the HCO₃⁻ concentration vs. 1/T correlation proposed by Dessert et al. (2001). The thick soils and explosive volcanism context of the Lesser Antilles are the two possible keys to this different weathering behaviour; the development of thick soils limits the chemical weathering and the presence of very porous pyroclastic flows allows an important water infiltration and thus subsurface weathering mechanisms, which are less effective for atmospheric CO₂ consumption.     

Relationship between soil CO<Subscript>2</Subscript> flux and volcanic tremor at Mt. Etna: Implications for magma dynamics

Large variations of the CO₂ flux through the soil were observed between November 2002 and January 2006 at Mt. Etna volcano. In many cases, the CO₂ flux was strongly influenced by changes in air temperature and atmospheric pressure. A new filtering method was then developed to remove the atmospheric influences on soil CO₂ flux and, at the same time, to highlight the variations strictly related to volcanic activity. Successively, the CO₂ corrected data were quantitatively compared with the spectral amplitude of the volcanic tremor by cross correlation function, cross-wavelet spectrum and wavelet coherence. These analyses suggested that the soil CO₂ flux variations preceded those of volcanic tremor by about 50 days. Given that volcanic tremor is linked to the shallow (a few kilometer) magma dynamics and soil CO₂ flux related to the deeper (~12 km b.s.l.) magma dynamics, the “delayed similarity” between the CO₂ flux and the volcanic tremor amplitude was used to assess the average speed in the magma uprising into the crust, as about 170–260 m per day. Finally, the large amount of CO₂ released before the onset of the 2004–2005 eruption indicated a deep ingression of new magma, which might have triggered such an eruption.     

Deccan volcanism linked to the Cretaceous-Tertiary boundary mass extinction: New evidence from ONGC wells in the Krishna-Godavari Basin

A scientific challenge is to assess the role of Deccan volcanism in the Cretaceous-Tertiary boundary (KTB) mass extinction. Here we report on the stratigraphy and biologic effects of Deccan volcanism in eleven deep wells from the Krishna-Godavari (K-G) Basin, Andhra Pradesh, India. In these wells, two phases of Deccan volcanism record the world’s largest and longest lava mega-flows interbedded in marine sediments in the K-G Basin about 1500 km from the main Deccan volcanic province. The main phase-2 eruptions (∼80% of total Deccan Traps) began in C29r and ended at or near the KTB, an interval that spans planktic foraminiferal zones CF1–CF2 and most of the nannofossil Micula prinsii zone, and is correlative with the rapid global warming and subsequent cooling near the end of the Maastrichtian. The mass extinction began in phase-2 preceding the first of four mega-flows. Planktic foraminifera suffered a 50% drop in species richness. Survivors suffered another 50% drop after the first mega-flow, leaving just 7 to 8 survivor species. No recovery occurred between the next three mega-flows and the mass extinction was complete with the last phase-2 mega-flow at the KTB. The mass extinction was likely the consequence of rapid and massive volcanic CO₂ and SO₂ gas emissions, leading to high continental weathering rates, global warming, cooling, acid rains, ocean acidification and a carbon crisis in the marine environment.     

CO2 solubility in Martian basalts and Martian atmospheric evolution

To understand possible volcanogenic fluxes of CO₂ to the Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the Adirondack-class Humphrey basalt at 1-2.5 GPa and 1400-1625 °C. Starting materials included both oxidized and reduced compositions, allowing a test of the effect of iron oxidation state on CO₂ solubility. CO₂ contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and Fe³⁺/Fe^T was measured by Mössbauer spectroscopy. The CO₂ contents of glasses show no dependence on Fe³⁺/Fe^T and range from 0.34 to 2.12 wt.%. For Humphrey basalt, analysis of glasses with gravimetrically-determined CO₂ contents allowed calibration of an integrated molar absorptivity of 81,500 ± 1500 L mol⁻¹ cm⁻² for the integrated area under the carbonate doublet at 1430 and 1520 cm⁻¹. The experimentally determined CO₂ solubilities allow calibration of the thermodynamic parameters governing dissolution of CO₂ vapor as carbonate in silicate melt, K_II, (Stolper and Holloway, 1988) as follows: , ΔV⁰ = 20.85 ± 0.91 cm³ mol⁻¹, and ΔH⁰ = −17.96 ± 10.2 kJ mol⁻¹. This relation, combined with the known thermodynamics of graphite oxidation, facilitates calculation of the CO₂ dissolved in magmas derived from graphite-saturated Martian basalt source regions as a function of P, T, and f_O2. For the source region for Humphrey, constrained by phase equilibria to be near 1350 °C and 1.2 GPa, the resulting CO₂ contents are 51 ppm at the iron-wüstite buffer (IW), and 510 ppm at one order of magnitude above IW (IW + 1). However, solubilities are expected to be greater for depolymerized partial melts similar to primitive shergottite Yamato 980459 (Y 980459). This, combined with hotter source temperatures (1540 °C and 1.2 GPa) could allow hot plume-like magmas similar to Y 980459 to dissolve 240 ppm CO₂ at IW and 0.24 wt.% of CO₂ at IW + 1. For expected magmatic fluxes over the last 4.5 Ga of Martian history, magmas similar to Humphrey would only produce 0.03 and 0.26 bars from sources at IW and IW + 1, respectively. On the other hand, more primitive magmas like Y 980459 could plausibly produce 0.12 and 1.2 bars at IW and IW + 1, respectively. Thus, if typical Martian volcanic activity was reduced and the melting conditions cool, then degassing of CO₂ to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water. However, if a significant fraction of Martian magmas derive from hot and primitive sources, as may have been true during the formation of Tharsis in the late Noachian, that are also slightly oxidized (IW + 1.2), then significant contribution of volcanogenic CO₂ to an early Martian greenhouse is plausible.     

The role of sulfate groups in controlling CaCO3 polymorphism

The nucleation and growth of CaCO₃ phases from aqueous solutions with SO₄²⁻:CO₃²⁻ ratios from 0 to 1.62 and a pH of ∼10.9 were studied experimentally in batch reactors at 25 °C. The mineralogy, morphology and composition of the precipitates were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and microanalyses. The solids recovered after short reaction times (5 min to 1 h) consisted of a mixture of calcite and vaterite, with a S content that linearly correlates with the SO₄²⁻:CO₃²⁻ ratio in the aqueous solution. The solvent-mediated transformation of vaterite to calcite subsequently occurred. After 24 h of equilibration, calcite was the only phase present in the precipitate for aqueous solutions with SO₄²⁻:CO₃²⁻ ? 1. For SO₄²⁻:CO₃²⁻ > 1, vaterite persisted as a major phase for a longer time (>250 h for SO₄²⁻:CO₃²⁻ = 1.62). To study the role of sulfate in stabilizing vaterite, we performed a molecular simulation of the substitution of sulfate for carbonate groups into the crystal structure of vaterite, aragonite and calcite. The results obtained show that the incorporation of small amounts (<3 mole%) of sulfate is energetically favorable in the vaterite structure, unfavorable in calcite and very unfavorable in aragonite. The computer modeling provided thermodynamic information, which, combined with kinetic arguments, allowed us to put forward a plausible explanation for the observed crystallization behavior.     

The role of metal sulfates in thermochemical sulfate reduction (TSR) of hydrocarbons: Insight from the yields and stable carbon isotopes of gas products

The mechanism of thermochemical sulfate reduction (TSR) was investigated by separately heating n-C₂₄ with three different sulfates (CaSO₄, Na₂SO₄, MgSO₄) in sealed gold tubes at 420 °C and measuring the stable carbon isotope values of hydrocarbon (C₁-C₅) and non-hydrocarbon (CO₂) products. Extensive TSR was observed with the MgSO₄ reactant as reflected by increasing concentrations of H₂S, ¹³C depleted CO₂ and relatively low concentrations of H₂ (compared to the control). H₂S yields were already very high at the first monitoring time (12 h) when the temperature had just reached 420 °C, suggesting that TSR had commenced well prior to this temperature. Only trace amounts of n-C₂₄ and secondary C₃-C₅ alkanes were detected at 12 h, reflecting the efficient TSR utilization of the reactant and lower molecular weight alkane products. Ethane levels were still relatively high at 12 h, but declined thereafter as it was subject to TSR in the absence of higher molecular weight alkanes which had already been utilized. Methane yields were consistently high throughout the 48 h MgSO₄ treatment. The temporal decrease in the concentrations of alkanes available for TSR may also contribute to the sharp enhancement of CO₂ after 36 h. Absence or dampening of the molecular and isotopic trends of MgSO₄ TSR was observed with Na₂SO₄ and CaSO₄ respectively, directly reflecting the levels of TSR reached using these sulfate treatments.For all treatments, the δ¹³C values of C_1-5n-alkanes showed an increase with both molecular weight and treatment time. MgSO₄ TSR led to a 5-10‰ increase in the δ¹³C values of the C₁-C₅ hydrocarbons and a 20‰ decrease in the δ¹³C value of CO₂. The significant ¹³C depletion of the CO₂ may be due to co-production of ¹³C enriched MgCO₃, although this remains unproven as the δ¹³C of MgCO₃ was not measured. The difference in the δ¹³C values of ethane and propane (Δδ¹³C_EP) increased in magnitude with the degree of TSR, and this trend could be used to help evaluate the occurrence and extent of TSR in subsurface gas reservoirs.     

Role of low solar luminosity in the history of the biosphere

It was shown that the history of the biosphere is closely related to processes caused by low solar luminosity. Solar radiation is insufficient to maintain the Earth’s surface temperature above the freezing point of water. Positive temperatures are kept owing to the presence of greenhouse gases in the atmosphere: CO₂, CH₄, and others. Certain stages in the development of the biosphere and climate are related to these effects. Methane was the main carbon-bearing gas in the primordial atmosphere. It compensated the low solar luminosity. Life originated under the reduced conditions of the early Earth. Methane-producing biota was formed. Methane remained to be the main greenhouse gas in the Archean. The release of molecular oxygen into the atmosphere 2.4 Ga ago resulted in the disruption of the established mechanism of the compensation of the low solar luminosity. Methane ceased to cause a significant greenhouse effect, and the content of carbon dioxide was insufficient to play this role. A global glaciation began and had lasted for approximately 200 million years. However, the increasing CO₂ content in the atmosphere reached eventually a level sufficient for the compensation for the low solar luminosity. The glaciation period came to an end. Simultaneously, a conflict arose between the role of CO₂ as a gas controlling the thermal regime of the planet and as an initial material for biota production. As long as the resource of biotic carbon was inferior to that of atmospheric CO₂, the uptake of atmospheric CO₂ related to sporadic increases in biologic production was insufficient for a significant change in the thermal regime. This was the reason for a long-term climate stabilization for 1.5 billion years. By 0.8 Ga, the resource of oceanic biota reached the level at which variations in the uptake of atmospheric CO₂ related to variations in the production of organic and carbonate carbon became comparable with the resource of atmospheric CO₂. Since then, an oscillatory equilibrium has been established between the intensity of biota development and climate-controlling CO₂ content in the atmosphere. Glaciation and warming periods have alternated. These changes were triggered by various geologic events: intensification or attenuation of volcanism; growth, breakup, or migration of continents; large-scale magmatism; etc. A new relation between atmospheric CO₂ and biotic carbon was established in response to the emergence of terrestrial biota and the appearance of massive buffers of organic carbon on land. The interrelation of the biosphere and climate changed.