Palaeoceanography: methane release in the Early Jurassic period

Dramatic global warming, triggered by release of methane from clathrates, has been postulated to have occurred during the early Toarcian age in the Early Jurassic period. Kemp et al. claim that this methane was released at three points, as recorded by three sharp excursions of delta13C(org) of up to 3 per thousand magnitude. But they discount another explanation for the excursions: namely that some, perhaps all, of the rapid excursions could be a local signature of a euxinic basin caused by recycling of isotopically light carbon from the lower water column. This idea has been proposed previously (see ref. 3, for example) and is supported by the lack evidence for negative delta13C excursions in coeval belemnite rostra. Kemp et al. dismiss this alternative, claiming that each abrupt shift would have required the recycling of about double the amount of organic carbon that is currently present in the modern ocean; however, their measurements are not from an ocean but from a restricted, epicontinental seaway and so would not require whole-ocean mixing to achieve the excursions.     

Astronomical pacing of methane release in the Early Jurassic period

A pronounced negative carbon-isotope (delta13C) excursion of approximately 5-7 per thousand (refs 1-7) indicates the occurrence of a significant perturbation to the global carbon cycle during the Early Jurassic period (early Toarcian age, approximately 183 million years ago). The rapid release of 12C-enriched biogenic methane as a result of continental-shelf methane hydrate dissociation has been put forward as a possible explanation for this observation. Here we report high-resolution organic carbon-isotope data from well-preserved mudrocks in Yorkshire, UK, which demonstrate that the carbon-isotope excursion occurred in three abrupt stages, each showing a shift of -2 per thousand to -3 per thousand. Spectral analysis of these carbon-isotope measurements and of high-resolution carbonate abundance data reveals a regular cyclicity. We interpret these results as providing strong evidence that methane release proceeded in three rapid pulses and that these pulses were controlled by astronomically forced changes in climate, superimposed upon longer-term global warming. We also find that the first two pulses of methane release each coincided with the extinction of a large proportion of marine species.     

Increased soil emissions of potent greenhouse gases under increased atmospheric CO2

Increasing concentrations of atmospheric carbon dioxide (CO(2)) can affect biotic and abiotic conditions in soil, such as microbial activity and water content. In turn, these changes might be expected to alter the production and consumption of the important greenhouse gases nitrous oxide (N(2)O) and methane (CH(4)) (refs 2, 3). However, studies on fluxes of N(2)O and CH(4) from soil under increased atmospheric CO(2) have not been quantitatively synthesized. Here we show, using meta-analysis, that increased CO(2) (ranging from 463 to 780 parts per million by volume) stimulates both N(2)O emissions from upland soils and CH(4) emissions from rice paddies and natural wetlands. Because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrestrial carbon sink under increased atmospheric CO(2) concentrations. Our results therefore suggest that the capacity of land ecosystems to slow climate warming has been overestimated.     

Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources

Atmospheric methane (CH(4)) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium, after which levels increased once more. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors. Here we show that the late-twentieth-century changes in the CH(4) growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH(4) mixing and (13)C/(12)C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of (13)C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The (13)C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51?±?18%) of the decrease in Northern Hemisphere CH(4) emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application and reductions in water use.     

A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills

The recent discovery of diamond-graphite inclusions in the Earth's oldest zircon grains (formed up to 4,252 Myr ago) from the Jack Hills metasediments in Western Australia provides a unique opportunity to investigate Earth's earliest known carbon reservoir. Here we report ion microprobe analyses of the carbon isotope composition of these diamond-graphite inclusions. The observed delta(13)C(PDB) values (expressed using the PeeDee Belemnite standard) range between -5 per mil and -58 per mil with a median of -31 per mil. This extends beyond typical mantle values of around -6 per mil to values observed in metamorphic and some eclogitic diamonds that are interpreted to reflect deep subduction of low-delta(13)C(PDB) biogenic surface carbon. Low delta(13)C(PDB) values may also be produced by inorganic chemical reactions, and therefore are not unambiguous evidence for life on Earth as early as 4,250 Myr ago. Regardless, our results suggest that a low-delta(13)C(PDB) reservoir may have existed on the early Earth.     

Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era

Methanogenic microbes may be one of the most primitive organisms, although it is uncertain when methanogens first appeared on Earth. During the Archaean era (before 2.5 Gyr ago), methanogens may have been important in regulating climate, because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity during these times. Nevertheless, no direct geological evidence has hitherto been available in support of the existence of methanogens in the Archaean period, although circumstantial evidence is available in the form of approximately 2.8-Gyr-old carbon-isotope-depleted kerogen. Here we report crushing extraction and carbon isotope analysis of methane-bearing fluid inclusions in approximately 3.5-Gyr-old hydrothermal precipitates from Pilbara craton, Australia. Our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than -56 per thousand included within original precipitates. This provides the oldest evidence of methanogen (> 3.46 Gyr ago), pre-dating previous geochemical evidence by about 700 million years.     

Detection and classification of atmospheric methane oxidizing bacteria in soil

Well-drained non-agricultural soils mediate the oxidation of methane directly from the atmosphere, contributing 5 to 10% towards the global methane sink. Studies of methane oxidation kinetics in soil infer the activity of two methanotrophic populations: one that is only active at high methane concentrations (low affinity) and another that tolerates atmospheric levels of methane (high affinity). The activity of the latter has not been demonstrated by cultured laboratory strains of methanotrophs, leaving the microbiology of methane oxidation at atmospheric concentrations unclear. Here we describe a new pulse-chase experiment using long-term enrichment with 12CH4 followed by short-term exposure to 13CH4 to isotopically label methanotrophs in a soil from a temperate forest. Analysis of labelled phospholipid fatty acids (PLFAs) provided unambiguous evidence of methane assimilation at true atmospheric concentrations (1.8-3.6 p.p.m.v.). High proportions of 13C-labelled C18 fatty acids and the co-occurrence of a labelled, branched C17 fatty acid indicated that a new methanotroph, similar at the PLFA level to known type II methanotrophs, was the predominant soil micro-organism responsible for atmospheric methane oxidation.     

Extreme deuterium enrichment in stratospheric hydrogen and the global atmospheric budget of H2

Molecular hydrogen (H2) is the second most abundant trace gas in the atmosphere after methane (CH4). In the troposphere, the D/H ratio of H2 is enriched by 120 per thousand relative to the world's oceans. This cannot be explained by the sources of H2 for which the D/H ratio has been measured to date (for example, fossil fuels and biomass burning). But the isotopic composition of H2 from its single largest source--the photochemical oxidation of methane--has yet to be determined. Here we show that the D/H ratio of stratospheric H2 develops enrichments greater than 440 per thousand, the most extreme D/H enrichment observed in a terrestrial material. We estimate the D/H ratio of H2 produced from CH4 in the stratosphere, where production is isolated from the influences of non-photochemical sources and sinks, showing that the chain of reactions producing H2 from CH4 concentrates D in the product H2. This enrichment, which we estimate is similar on a global average in the troposphere, contributes substantially to the D/H ratio of tropospheric H2.     

Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air

Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH(4)) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C(2)H(6)) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20?years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14-16 teragrams per year (1?Tg = 10(12)?g) and dropped to 8-10?Tg yr(-1) by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane.     

D to H ratio and the origin and evolution of Titan's atmosphere

A value of 1.7 x 10(-3) has been reported for the ratio of CH3D to CH4 in the stratosphere of the saturnian moon Titan. A lower value of 6 x 10(-4) for this ratio in the deeper part of Titan's atmosphere was reported by de Bergh et al. For comparison we note that the CH3D to CH4 ratio on Saturn and Jupiter is 8.7 x 10(-5) and 6.7 x 10(-5), respectively. We estimate the uncertainties in all these observations and data reduction to be about a factor of 2. Despite these uncertainties it appears that Titan's atmosphere is enriched in deuterium by a factor of > or = 3 relative to Jupiter and Saturn. Potential causative factors examined here for this enrichment are condensation to form tropospheric methane clouds, fractionation occurring over a hypothetical CH4-C2H6 ocean and between the ocean and the clathrate crust beneath, fractionation which occurred during the formation of Titan and fractionation occurring as a result of the evolution of Titan's atmosphere. We conclude that the greater part of the observed fractionation is probably derived from the formation of Titan and the subsequent evolution of Titan's atmosphere driven by photochemistry.     

A conspicuous nickel protein in microbial mats that oxidize methane anaerobically

Anaerobic oxidation of methane (AOM) in marine sediments is an important microbial process in the global carbon cycle and in control of greenhouse gas emission. The responsible organisms supposedly reverse the reactions of methanogenesis, but cultures providing biochemical proof of this have not been isolated. Here we searched for AOM-associated cell components in microbial mats from anoxic methane seeps in the Black Sea. These mats catalyse AOM rather than carry out methanogenesis. We extracted a prominent nickel compound displaying the same absorption spectrum as the nickel cofactor F430 of methyl-coenzyme M reductase, the terminal enzyme of methanogenesis; however, the nickel compound exhibited a higher molecular mass than F430. The apparent variant of F(430) was part of an abundant protein that was purified from the mat and that consists of three different subunits. Determined amino-terminal amino acid sequences matched a gene locus cloned from the mat. Sequence analyses revealed similarities to methyl-coenzyme M reductase from methanogenic archaea. The abundance of the nickel protein (7% of extracted proteins) in the mat suggests an important role in AOM.     

Indicators of δ13C and δ18O of gas hydrate-associated sediments

The analyses of δ13C and δ18O of gas hydrate-associated sediments from two cores on Hydrate Ridge in Cascadia convergent margin offshore Oregon, eastern North Pacific show the values of d 13C from -29.81‰ to -48.28‰ (PDB) and d 18O from 2.56‰ to 4.28‰ (PDB), which could be plotted into a group called typical carbonate minerals influenced by the methane in cold venting. Moreover, the values of d 13C and d 18O show a consistent trend in both cores from top to bottom with increasing of d 13C and decreasing of d 18O. This trend could be explained as an effect caused by the anaerobic oxidation of methane (AOM) in depth and the oxygen fraction during the formation of gas hydrate in depth together. These characteristics of d 13C and d 18O indicate that the gas hydrate-associated sediments are significantly different from the normal marine carbonates, and they are deeply influenced by the formation and evolution of gas hydrate. So, the distinct characteristics of d 13C and d 18O of gas hydrate-associated sediments could be undoubtedly believed as one of parameters to determine the presence of gas hydrates in other unknown marine sediment cores.     

Triassic-Jurassic atmospheric CO2 spike.

I question the claim by Tanner et al. that atmospheric CO2 levels remained constant across the Triassic-Jurassic boundary on the grounds of problems with stratigraphic completeness and contamination with atmospheric methane. Because methanogenic CH4 has a light isotope composition and oxidizes readily to CO2, methane-clathrate dissociation and oxidation events cannot be detected by palaeobarometers that use the carbon-isotope composition of palaeosol carbonate.     

High density methane inclusions in Puguang Gasfield: Discovery and a T-P genetic study

Based on measurement of homogenization temperature of inclusions and Raman spectral analysis, high density methane inclusions were discovered in the Triassic reservoirs of Puguang Gasfield. The methane inclusions show a homogenization temperature Th = -117.5― -118.1℃, a corresponding density of 0.3455―0.3477 g/cm3, and a Raman scatter peak v1 shift varying between 2911―2910 cm-1, which signifies a very high density of methane inclusions. The salt water inclusions paragenetic with methane inclusions show a homogenization temperature Th=170―180℃. Based on the composition of methane inclusions as determined by Raman spectra, PVTsim software was used to simulate the trapping pressure for high density methane inclusions in geologic history, and the trapping pressure was found to be as high as 153―160 MPa. Even though Puguang Gasfield is currently a gas pool of normal pressure, and the fluid pressure for the gas pool ranges between 56―65 MPa. However, data from this study indicates that remarkable overpressure may be generated at the stage of mass production of gas cracked from oils in Cretaceous, as high density methane inclusions constitute key evidence for overpressure in gas pool in geologic history. Meanwhile, discovery of small amounts of H2S, CO2 or heavy hydrocarbon in part of the high density methane inclusions indicates that the geochemical environment for trapping of inclusions may be related to formation of H2S. Therefore, the observation results can help to explore the thermochemical sulfate reduction （TSR） conditions for oil cracking and H2S formation.     

Secondary biogenic coalbed gas in some coal fields of China

The secondary biogenic coalbed gas, a new genetic and energy source type of coalbed gas in China, has been found in Xinji, Liyazhuang and Enhong areas. The essential characteristics of this type of gas are: (i) the major component of the gas is methane, with C1/C1-5 value higher than 0.99, indicating that the gas is part of dry gas; (ii) theδ13C1 value is in the range of -61.7‰to -47.9‰, mostly lower than -55‰, which is much lower than the estimatedδ13C1 value of thermogenic methane according to the thermal evolution degree of the coal rocks (with R0 value from 0.87% to 1.43%), showing the characteristics of the secondary biogenic gas; (iii) theδ5D value of methane ranges from -244‰to -196‰; (iv)δ13C 2 value ranges from -26.7‰to -15.9‰andδ13C 3 value ranges from -10.8‰to -25.3‰, indicating that the heavier hydrocarbons have a thermogenic origin; (v) the content of CO2 is very low, andδ13CCO2 value changes greatly, reflecting a characteristic of secondary change; (vi)δ15N2 value ranges mainly from -1‰to +1‰, indicating N2 derived significantly from air. The negative linear correlation between the contents of N2 and CH4 reflects the activity of bacteria bearing surface water infiltrating into coal beds. The comprehensive tracing indices show that the coalbed gas in the studied areas is the mixed gas of primarily secondary biogenic gas and a part of remnant thermogenic gas. The uplift of coal beds and the development of faults in the studied areas create favorable conditions for the formation of the secondary biogenic gas.     

水稻品种以及施肥措施对稻田甲烷排放的影响

１９９４年４月至１０月，在北京稻田试验田研究了不同水稻品种对甲烷排放的影响以及施肥措施对降低甲烷排放和提高水稻产量的匹配关系。实验表明稻田甲烷排放在品种之间存在着明显差异，实验中品种间甲烷排放通量最大差异可达到五倍。以有机肥混合硫酸铵作底肥并配合不同时间和不同用量的硫酸铵作追肥与仅施混合底肥不施施硫酸铵作追肥的相比较可减少甲烷排放约５８％，水稻产量提高３１．７％。可明显地抑制水稻生长期的甲烷排放高     

Plant biomarkers in aerosols record isotopic discrimination of terrestrial photosynthesis

Carbon uptake by the oceans and by the terrestrial biosphere can be partitioned using changes in the (12)C/(13)C isotopic ratio (delta(13)C) of atmospheric carbon dioxide, because terrestrial photosynthesis strongly discriminates against (13)CO(2), whereas ocean uptake does not. This approach depends on accurate estimates of the carbon isotopic discrimination of terrestrial photosynthesis (Delta; ref. 5) at large regional scales, yet terrestrial ecosystem heterogeneity makes such estimates problematic. Here we show that ablated plant wax compounds in continental air masses can be used to estimate Delta over large spatial scales and at less than monthly temporal resolution. We measured plant waxes in continental air masses advected to Bermuda, which are mainly of North American origin, and used the wax isotopic composition to estimate Delta simply. Our estimates indicate a large (5 6 per thousand) seasonal variation in Delta of the temperate North American biosphere, with maximum discrimination occurring in late spring, coincident with the onset of production. We suggest that the observed seasonality arises from several factors, including seasonal shifts in the proportions of production by C(3) and C(4) plants, and environmentally controlled adjustments in the photosynthetic discrimination of C(3)-plant-dominated ecosystems.     

气相中二氧化铬与乙烯反应机理的理论研究

用密度泛函B3LYP/6-311++G(3df,3pd)//6-311G(2d,p)法研究了CrO2(+2A1/4A″)+C2H4生成P1[Cr(OCH2)(+2A"/4A")+CH2O]和P2[CrO(+2Σg/4Σg)+C2H4O]的气相反应,重点对影响反应机理和反应速率的势能面交叉现象进行了讨论,并运用Hammond假设和Yoshizawa等的内禀反应坐标单点垂直激发计算的方法大致确定了势能面交叉点(crossing point(CP))或势能面交叉缝(crossing seam)的位置。研究结果表明,CrO2+活化乙烯C-C键是一个[2+2]类型反应,整个反应经历了重排过程。     

体外产气法研究植物精油对水牛瘤胃发酵和甲烷生成的影响

为探讨植物精油对水牛瘤胃体外发酵的影响,本研究采用体外产气法初步研究了不同植物油对水牛瘤胃发酵过程中甲烷和二氧化碳生成及微生物蛋白和pH的影响。与对照组相比结果表明:1)在相同添加浓度下,桂皮油在发酵过程中基本抑制了甲烷生成(P0.05),也显著降低了二氧化碳产量(P0.05),微生物蛋白含量减少了15%;2)薄荷油在发酵12 h、24 h均显著抑制甲烷生成(P0.05),对二氧化碳产量稍有降低,但差异不显著;在发酵36 h,抑制甲烷效果不明显,二氧化碳反而显著降低(P0.05)。微生物蛋白含量则减少了12.8%;3)生姜油和丁香油的抑制甲烷效果不明显,只在发酵12 h,有部分降甲烷效果,之后反而促进了甲烷生成;4)5种精油中,只有辣椒油处理组发酵液pH显著降低,其它对pH值影响不大,但辣椒油无抑制甲烷效果。     

Chronology of the early Solar System from chondrule-bearing calcium-aluminium-rich inclusions

Chondrules and Ca-Al-rich inclusions (CAIs) are high-temperature components of meteorites that formed during transient heating events in the early Solar System. A major unresolved issue is the relative timing of CAI and chondrule formation. From the presence of chondrule fragments in an igneous CAI, it was concluded that some chondrules formed before CAIs (ref. 5). This conclusion is contrary to the presence of relict CAIs inside chondrules, as well as to the higher abundance of 26Al in CAIs; both observations indicate that CAIs pre-date chondrules by 1-3 million years (Myr). Here we report that relict chondrule material in the Allende meteorite, composed of olivine and low-calcium pyroxene, occurs in the outer portions of two CAIs and is 16O-poor (Delta17O approximately -1 per thousand to -5 per thousand). Spinel and diopside in the CAI cores are 16O-rich (Delta17O up to -20 per thousand), whereas diopside in their outer zones, as well as melilite and anorthite, are 16O-depleted (Delta17O = -8 per thousand to 2 per thousand). Both chondrule-bearing CAIs are 26Al-poor with initial 26Al/27Al ratios of (4.7 +/- 1.4) x 10(-6) and <1.2 x 10(-6). We conclude that these CAIs had chondrule material added to them during a re-melting episode approximately 2 Myr after formation of CAIs with the canonical 26Al/27Al ratio of 5 x 10(-5).