Carbon isotope effects in the open-system Fischer-Tropsch synthesis

Carbon isotopic composition was measured for products of the Fischer-Tropsch synthesis: catalytic reaction between CO and H₂ to produce CO, CO₂, light hydrocarbons C1-C4 and “oil” fraction. Hydrogen isotopes were also measured in the oil fraction and the produced water. Experimental runs were conducted in the flow-through reactor at 260-310 °C and 30 bar using the synthesis gas composed of 5N₂ + 3H₂ + 2CO, on Fe-catalyst mixed with ZSM-5 synthetic zeolite. In the two of seven runs a Fe + Co-catalyst was used that gives a lower yield of unsaturated hydrocarbons in reaction products. The isotopic effects depended on the conversion of the carbon monoxide. Under steady-state conditions (CO conversion more than 90%) a strong kinetic fractionation was observed between CO and CO₂ (∼−10‰) and CO and hydrocarbons (∼+38‰). At low conversion a clear “inverse” isotopic trend of the depletion in ¹³C of longer hydrocarbon chains was observed. On average, Δ₁₂ = δ¹³C(CH₄) − δ¹³C(C₂H₆) correlates well with the CO conversion: the C₂H₆ is ∼6‰ isotopically lighter than CH₄ at low conversion and ∼2‰ heavier at steady-state regime. Under steady-state conditions there almost no difference was observed in the isotopic composition of methane and ethane and higher hydrocarbons. The chemical composition of light hydrocarbons in the products of flow-through, dynamic FTS is different from that found in the static FTS-type experiments with Fe-catalyst, but isotopic effects are similar. Our results suggest that the isotopic distribution of carbon found in so-called “abiogenic” hydrocarbons from some natural gases (δ¹³C1 > δ¹³C2 > δ¹³C3  >?) is somewhat similar to that at low conversion of CO, but do not resemble the distribution characteristic for the high conversion products, at least, on Fe-catalyst. Other processes (a simple mixing of two or more endmembers) or other P-T conditions of the carbon reduction could be responsible for the “inverse” isotopic trend found in meteorites and some natural gases.     

(U–Th)/He dating of fluorite: application to the La Azul fluorspar deposit in the Taxco mining district, Mexico

The (U–Th)/He dating method was applied to fluorite of the La Azul fluorspar deposit, Taxco mining district, Mexico. Ages of ten U-rich (4–94 g/g) samples range from 30 to 33 Ma (mean 32±2 Ma, 1). This age range is interpreted as the time of primary fluorite precipitation and is close to the K/Ar age of sericite from a small fluorspar deposit (Los Tréboles) in the same area, and the K/Ar age of the nearby volcanic succession, which is thought to be the main source of fluorine in both deposits. This age is also in concordance with the very few published ages of epithermal deposits in southern Mexico. The dating of fluorite by other methods, particularly for young samples, is a difficult task. We believe that the (U–Th)/He method, which has been applied before to thermochronological studies of apatite, zircon, titanite and hematite, can be used as a tool for direct dating of fluorite with microgram per gram levels of uranium and thorium.Editorial handling: B. Lehmann     

Energy Balance in the Corona over the 22-Year Solar Cycle

Wheatland and Litvinenko (2001) presented a model for dynamical energy balance in the flaring solar corona which predicts a time lag between flare occurrence and the supply of energy to the corona (`driving'). They also suggested that an observed net lag between flare numbers and sunspot numbers over cycles 21 and 22 might provide support for the model. Temmer, Veronig, and Hanslmeier (2003) examined data for five individual solar cycles (19–23) and confirmed a lag between flare and sunspot numbers for odd solar cycles, but found no lag for even cycles. Following the suggestion of Temmer, Veronig, and Hanslmeier, the energy balance model is here extended to incorporate 22-year driving consistent with the phenomenological Gnevyshev—Ohl rule. The model is found to exhibit a greater lag for the smaller (even) cycles, in contradiction with the findings of Temmer, Veronig, and Hanslmeier. A modification to the model is investigated in which the flaring rate is proportional to the free energy and to the driving rate for small driving rates, but is proportional only to the free energy for large driving rates. The modified model can in principle account for the observations.     

Researches on the stability of porphyra-334 solution and its influence factors

1　Introduction　Ultravioletradiationisahighlyactivecomponentofsolarspectrum .Ultravioletradiationhasdetrimentaleffectsonlivingthingsexposedtosunlight ,includingthedestructionofDNA ,proteinsandothermolecules(Harm ,1980 ) ,inhibitionofphotosynthesisandgrow…     

The impact of commercial and recreational harvesting for Paracentrotus lividus on shallow rocky reef sea urchin communities in North-western Sardinia, Italy

The fishery for the edible sea urchin Paracentrotus lividus is of great importance to many European regions, although in some of them this species has shown a wide scale decline in recent years. This paper reports on direct (on P. lividus) and indirect (on the co-occurring but not fished echinoid, Arbacia lixula) effects of sea urchin harvesting in shallow rocky reefs in North-western Sardinia (Italy, central-western Mediterranean Sea), where this activity is intensively practised. Density, size (test diameter), and biomass of both species were assessed in autumn 2004 at two locations impacted by P. lividus fishery, and at two controls within an adjacent Marine Protected Area. Density of P. lividus and A. lixula was significantly greater at the controls than at the exploited locations. The average test diameter of P. lividus was also significantly larger at the controls, where large-sized specimens (i.e. >5 cm) were notably more abundant. The average size of A. lixula did not significantly differ between the impacted locations and the controls. The average biomass of P. lividus was significantly greater at the controls than at the exploited locations, whereas no differences were detected for A. lixula. These results reveal the existence of a heavy fishing impact on P. lividus in North-western Sardinia and the need for regulation of its harvesting to prevent severe direct effects on its populations. Since no indirect effects on A. lixula have been detected, it could be hypothesized that this species did not benefit from P. lividus fishery due to an only moderate competition for habitat and resources between these two echinoids. Implications for management of edible sea urchin fishery are also discussed.     

Geothermal evolution of the Astrakhan Arch region of the Pricaspian basin

The Astrakhan Arch region contains one of the largest sub-salt structures of the Pricaspian basin, where perspectives for hydrocarbon generation and accumulation in the Devonian to Carboniferous deposits are considered to be high. The paper addresses the problem of structural and geothermal evolution of the region deformed by salt movements. Initially, we developed a model of the regional structural evolution along a geological profile using the volume-balancing and back-stripping methods and geological constraints on the sedimentation, erosion, and paleo-water depths. Then we developed geothermal models (along the study profile) associated with the regional structural evolution. The models were constrained by the temperatures measured in four deep boreholes along the profile. We show that the present temperatures and heat flux are influenced by the presence of salt diapirs. Since the Early Carboniferous and till Middle Permian times, the temperatures predicted by the models vary significantly due to the regional transgression and the presence of seawater. The temperature of Devonian–Carboniferous carbonates increases since the Late Permian (time of post-salt deposition) and attains its maximum values in the SW-part of the profile. If the model assumptions concerning the constant vertical and zero lateral heat fluxes are valid, we can conclude that hydrocarbons are most likely to be generated in the SW-part of the region for the post-Early Permian time.     

Siberian gas venting and the end-Permian environmental crisis

The end of the Permian period is marked by global warming and the biggest known mass extinction on Earth. The crisis is commonly attributed to the formation of the Siberian Traps Large Igneous Province although the causal mechanisms remain disputed. We show that heating of Tunguska Basin sediments by the ascending magma played a key role in triggering the crisis. Our conclusions are based on extensive field work in Siberia in 2004 and 2006. Heating of organic-rich shale and petroleum bearing evaporites around sill intrusions led to greenhouse gas and halocarbon generation in sufficient volumes to cause global warming and atmospheric ozone depletion. Basin scale gas production potential estimates show that metamorphism of organic matter and petroleum could have generated > 100,000 Gt CO₂. The gases were released to the end-Permian atmosphere partly through spectacular pipe structures with kilometre-sized craters. Dating of a sill intrusion by the U–Pb method shows that the gas release occurred at 252.0 ± 0.4 million years ago, overlapping in time with the end-Permian global warming and mass extinction. Heating experiments to 275 °C on petroleum-bearing rock salt from Siberia suggests that methyl chloride and methyl bromide were significant components of the erupted gases. The results indicate that global warming and ozone depletion were the two main drivers for the end-Permian environmental crisis. We demonstrate that the composition of the heated sedimentary rocks below the flood basalts is the most important factor in controlling whether a Large Igneous Provinces causes an environmental crisis or not. We propose that a similar mechanism could have been responsible for the Triassic-Jurassic (~ 200 Ma) global warming and mass extinction, based on the presence of thick sill intrusions in the evaporite deposits of the Amazon Basin in Brazil.     

Al–Mg and Pb–Pb systematics of Allende CAIs: Canonical solar initial Al/Al ratio reinstated

The precise knowledge of the initial ²⁶Al/²⁷Al ratio [(²⁶Al/²⁷Al)₀] is crucial if we are to use the very first solid objects formed in our Solar System, calcium–aluminum-rich inclusions (CAIs) as the “time zero” age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of ²⁷Al/²⁴Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined ²⁶Al–²⁶Mg fossil isochron with an (²⁶Al/²⁷Al)₀ of (5.23 ± 0.13) × 10^− 5. Internal mineral isochrons obtained for three of these CAIs (A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with (²⁶Al/²⁷Al)₀ = (4.96 ± 0.25) × 10^− 5, anchored to our precisely determined absolute ²⁰⁷Pb–²⁰⁶Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the “canonical” (²⁶Al/²⁷Al)₀ of 5 × 10^− 5 for the early Solar System. The uncertainty in (²⁶Al/²⁷Al)₀ corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the ²⁶Al–²⁶Mg and U–Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the “supra-canonical” ²⁶Al/²⁷Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical ²⁶Al/²⁷Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS.The radiogenic ²⁰⁸Pb/²⁰⁶Pb ratio obtained as a by-product from the Pb–Pb age dating is used to estimate time-integrated ²³²Th/²³⁸U ratio (κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.