Thermal Degradation Of Organic Matter Research Articles

Taphonomic experiments on caridean shrimp elucidate the timing and influence of organic matter on diagenetic pyritization and sulfurization

Sulfur is an element particularly valuable for preserving organisms in the fossil record. On one hand, sulfurization (i.e. incorporation of sulfur into organic matter) forms macromolecules more likely to be preserved over geological timescales than their low-weight molecular precursors. On the other hand, pyrite (FeS2) is one of the main minerals replicating organisms and contributes to the exceptional preservation of soft tissues. However, the incomplete understanding of the processes, controls, timing and relation (competition, succession …) of these reactions may conceal critical biases within the fossil record, used to reconstruct past biodiversity and environment. In particular, organic matter (OM) is critical in these fossilization modes, but the roles of its composition and abundance remain to be better assessed, in particular for pyritization. Therefore, laboratory experiments were conducted on crustaceans' carcasses in mesocosms containing natural water and sediment. Tanks were treated with variable conditions to test the relative roles on preservation potential of oxic perturbation, thermal degradation of OM, and internal soft tissues, or lack thereof. Various bio-physico-chemical parameters and sulfur speciation were then monitored. The precipitation of a macroscopic amorphous iron monosulfide was observed after 100 days, both in the sediment and within carcasses; sulfurization of carcasses was detected within tens of days. A periodic aeration of the sediment where the carcasses were decayed impeded neither pyritization nor sulfurization while thermal degradation of OM did. Further, absence of internal soft tissues led solely to sulfurization, which suggests (i) that labile OM may be the factor driving the balance between pyritization and sulfurization and (ii) that internal soft tissues may promote pyritization through an organomineralization process, with sulfur-bearing organic ligands acting as nucleation sites. These results invite further experiments to test additional factors, in order to help refine environmental and taxonomical controls, and further investigation of fossil material, to evaluate the significance of sulfurization in the fossil record by searching sulfurized organisms.

Accumulation Conditions and Pattern of Tight Oil in the Lower Submember of the Fourth Member of the Shahejie Formation in the Damintun Sag, Bohai Bay Basin

To determine the accumulation conditions and pattern of tight oil in oil shales in the Damintun Sag, Bohai Bay Basin, this study investigated the basic geological conditions of the source rocks and reservoirs in the sag using methods such as organic carbon analysis, whole-rock XRD analysis, and field emission scanning electron microscopy. The results show that: (1) The high-quality source rocks in the lower submember of the fourth member of the Shahejie Formation (E2S42) in the Damintun Sag have high organic matter abundance, favorable organic matter types, high hydrocarbon expulsion efficiency, and high fluidity. Therefore, they provide sufficient oil sources for tight oil accumulation.; (2) During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which can dissolve carbonate minerals. In this way, secondary pores are formed.; (3) The special microscopic pore structure that connects fractures to pores is the key to the enrichment of tight oil is a key factor for the high oil saturation of pores in oil shales; (4) The breakthrough pressure (up to 100 MPa) and specific surface area of dolomitic mudstones in the E2S42 submember are significantly higher than those in other horizons. As a result, the dolomitic mudstones can effectively seal the underlying tight reservoirs; (5) Compared with the tight oil in tight sandstones, the tight oil in the oil shales in the study area has significantly superior geological conditions for reservoir formation, such as the favorable arrangement of hydrocarbon expulsion channels, low filling resistance, and the presence of reservoir spaces.

A maturation scale for molecular simulation of kerogen thermal degradation

Correlating molecular dynamics (MD) simulations of thermal degradation of organic matter with natural processes in ancient sediments is vital for revealing the mechanisms of hydrocarbon generation at molecular and atomic levels. This study establishes a practical maturation scale for molecular modeling of the thermal degradation of kerogen. ReaxFF reactive MD simulations of the thermal evolution of typical kerogen types (Type I, II, and III) were carried out at temperatures ranging from 1200 K to 2600 K. The results demonstrate that the H/C and O/C atomic ratios of kerogen molecules decrease as simulation temperatures increase and that the evolutionary paths of the three kerogen types fit well with those shown in the van Krevelen diagram. The corrected activation energies of pyrolysis for these three typical kerogen types were found to be 61.1 kcal/mol, 53.5 kcal/mol and 52.8 kcal/mol, respectively, consistent with those obtained from both laboratory simulations and basin modeling. By comparison with the relationships between measured vitrinite reflectance values and the H/C atomic ratios of kerogen samples reported in the literature, a quantitative relationship was established between the vitrinite reflectance (%Ro) values of kerogen and simulation temperatures (T). The cross-plots of T vs %Ro for kerogen types II and III show that vitrinite reflectance exponentially increases with increasing simulation temperature. Two logarithmic correlation equations were constructed to match geological maturation levels to the corresponding levels in MD simulations of thermal degradation of types II and III kerogen. These maturation scales are generally in line with the simulation temperatures set for thermal degradation in kerogen pyrolysis in previous studies. This work has promising practical implications for research on the thermal evolution behavior of ancient sedimentary organic matter using ReaxFF MD simulation.

Submarine metalliferous carbonate mounds in the Cambrian of the Baltoscandian Basin induced by vent networks and water column stratification

Two massive precipitation events of polymetallic ore deposits, encrusted by a mixture of authigenic carbonates, are documented from the Cambrian of the semi-enclosed Baltoscandian Basin. δ34S (‒9.33 to ‒2.08‰) and δ33S (‒4.75 to ‒1.06‰) values from the basal sulphide breccias, sourced from contemporaneous Pb–Zn–Fe-bearing vein stockworks, reflect sulphide derived from both microbial and abiotic sulphate reduction. Submarine metalliferous deposits were triggered by non-buoyant hydrothermal plumes: plumes of buoyant fluid were trapped by water column stratification because their buoyancy with respect to the environment reversed, fluids became heavier than their surroundings and gravitational forces brought them to a halt, spreading out laterally from originating vents and resulting in the lateral dispersion of effluents and sulphide particle settling. Subsequently, polymetallic exhalites were sealed by carbonate crusts displaying three generations of ikaite-to-aragonite palisade crystals, now recrystallized to calcite and subsidiary vaterite. Th of fluid inclusions in early calcite crystals, ranging from 65 to 78 ºC, provide minimum entrapment temperatures for carbonate precipitation and early recrystallization. δ13Ccarb (‒1.1 to + 1.6‰) and δ18Ocarb (‒7.6 to ‒6.5‰) values are higher than those preserved in contemporaneous glendonite concretions (‒8.5 to ‒4.7‰ and ‒12.4 to ‒9.1‰, respectively) embedded in kerogenous shales, the latter related to thermal degradation of organic matter. Hydrothermal discharges graded from highly reduced, acidic, metalliferous, and hot (~ 150 ºC) to slightly alkaline, calcium-rich and warm (< 100 ºC), controlling the precipitation of authigenic carbonates.

Insights into in-situ sulfur retention by co-combustion of dyeing sludge and wood sawdust

Dyeing sludge (DS) incineration is a promising strategy to achieve waste reduction and waste-to-energy conversion efficiency; however, the poor-combustion performance and high-emission level of the gaseous sulfur of DS limit its applications. This study attempts to introduce wood sawdust (WS) to improve combustion performance and inhibit the sulfur emission during DS incineration. The combustion characteristics and release behavior of sulfur gas products of DS-WS mixtures were investigated using thermogravimetry and mass spectrometry (TG-MS), as well as the influence of WS on sulfur fate of DS and the in-situ retention mechanism to sulfur were revealed by ash analysis. Results indicated that WS addition restrained the devolatilization of light volatiles and facilitated the degradation of heavy volatiles and fixed carbon. WS addition improved the combustibility, burnout performance and combustion stability of DS. Organic sulfur species occupied most sulfur-containing compounds in DS, the thermal degradation of organic matter was responsible for emitting SO2, CH3SH, H2S, COS. SO2 contributed > 40% of sulfurous gas emissions. WS addition (30%) significantly reduced the normalized sulfur content from DS, and the in-situ retention ratio to sulfur reached 16.72%. The in-situ retention mechanism to sulfur was explained as potassium iron sulfate and calcium hydrogen sulfate jointly retained the release of sulfur. Moreover, Ca-based sulfate was more stable, indicating that the newly formed mineral contributed to preventing the re-emission of sulfur from incineration residue.

A novel isotopic approach to distinguish primary microbial and thermogenic gases in shallow subsurface environments

The stable carbon isotopic compositions of methane and ethane have been used for several decades to characterize petroleum gas in the subsurface. However, difficulties have frequently remained in determining the exact origin of natural gas samples derived from microbial activity, thermal degradation of organic matter (e.g. thermogenic gas), and mixtures of both components. To further refine this approach, we represent the carbon isotope difference between methane and ethane, or Δ13CC1–C2, plotted versus the individual δ13C values of methane and ethane as a new graphical tool for a refined assessment of natural gas origins and their mixtures. In addition, a new mathematical notation using the δ13C values of methane and ethane is also proposed for discrimination of the origin of natural gas samples. This parameter has been termed isotope factor O(C1/C2) and is defined as the ratio [(δ13CC1*1000)/(δ13CC2)3]. When this factor is represented against the δ13C values of methane and ethane, an improved distinction between primary microbial and thermogenic gases is achieved. Our study does not propose mechanisms for microbial generation of ethane, but the data suggest that the isotopic composition of ethane should be taken into account for the characterization of the origin of natural gas samples from shallow subsurface environments. For primary microbial and thermogenic gases with similar δ13C values of methane, the primary microbial gas will be characterized by lower O(C1/C2) values (<1.5) compared to the thermogenic gas (>2), and will have a Δ13CC1–C2 value 10–20‰ more positive than that of thermogenic gas. Gas mixtures have intermediate O(C1/C2) and Δ13CC1–C2 values and are graphically identifiable using the here presented classification tools. The effectiveness of the new graphical tools is demonstrated using natural gas samples from Canadian groundwater wells, low-permeability petroleum reservoirs (including coalbed methane plays) in Canada and the USA, Quaternary primary microbial gas accumulations in China, and secondary microbial gases from Northern Africa and China.

Physicochemical, mineralogy, and thermo-kinetic characterisation of newly discovered Nigerian coals under pyrolysis and combustion conditions

In this study, the physicochemical, microstructural, mineralogical, thermal, and kinetic properties of three newly discovered coals from Akunza (AKZ), Ome (OME), and Shiga (SHG) in Nigeria were examined for potential energy recovery. Physicochemical analysis revealed high combustible but low levels of polluting elements. The higher heating values ranged from 18.65 MJ/kg (AKZ) to 26.59 MJ/kg (SHG). Microstructure and mineralogical analyses revealed particles with a rough texture, surface, and glassy lustre, which could be ascribed to metals, quartz, and kaolinite minerals. The major elements (C, O, Si, and Al), along with minor elements (Ca, Cu, Fe, K, Mg, S, and Ti) detected are associated with clays, salts, or the porphyrin constituents of coal. Thermal analysis showed mass loss (ML) ranges from 30.51% to 87.57% and residual mass (RM) from 12.44% to 69.49% under combustion (oxidative) and pyrolysis (non-oxidative) TGA conditions due to thermal degradation of organic matter and macerals (vitrinite, inertinite and liptinite). Kinetic analysis revealed the coals are highly reactive under the oxidative and non-oxidative conditions based on the Coats–Redfern Model. The activation energy (Ea) ranged from 23.81 to 89.56 kJ/mol, whereas the pre-exponential factor (ko) was from 6.77 × 10–4/min to 1.72 × 103/min under pyrolysis and combustion conditions. In conclusion, the coals are practical feedstocks for either energy recovery or industrial applications.

Two-stage mineral dissolution and precipitation related to organic matter degradation: Insights from in situ C–O isotopes of zoned carbonate cements

Mineral dissolution and precipitation, particularly of carbonate minerals, are ubiquitous and significant diagenetic processes in sedimentary rocks. However, the impacts of organic matter degradation on these processes in single systems remain unclear because the timing and fluid sources during progressive burial are poorly constrained. To address this issue, in situ C–O isotope analyses were conducted for zoned dolomite–ankerite/calcite cements from the Permian Lucaogou Formation in the Junggar Basin, China, using secondary ion mass spectrometry (SIMS) combined with mineralogical and elemental analyses. Carbonate minerals can be divided into early-stage and late-stage precipitations based on distinct variations in δ18O (−18.3 to −0.7‰ V-PDB), δ13C (−3.6 to +20.8‰ V-PDB), and Fe content (Fe# ranging from 0.002 to 0.373). The chemo-isotopically zoned carbonate cements record the thermal and chemical conditions of pore fluids during the burial process. The difference in δ18O values between the early and late precipitation (i.e., Δ18O [early–late]) of 11.9‰, and the difference in δ13C values (i.e., Δ13C [early–late]) of 11.7‰, reveal significant changes in precipitation temperatures and the availability of carbon sources, respectively. Early-stage precipitation occurred in association with microbial methanogenesis at lower temperatures (~25 °C) during shallow burial (~300 m), whereas late-stage precipitation was related to thermally-induced decarboxylation at higher temperatures (~90 °C or ~110 °C) during deep burial (~2500 m or ~3200 m). These two stages of dissolved pore-filling precipitation correspond to two major periods of dissolution (i.e., eogenetic and mesogenetic dissolution). Diagenetic fluids, mainly CO2 and/or organic acids, were derived from the biological and thermal degradation of organic matter, respectively. These results constrain the isotopic responses of carbonates to organic matter degradation with increasing burial depth and provide significant insights into carbonate diagenesis in organic-rich sedimentary sequences.

Focused fluid flow and methane venting along the Queen Charlotte fault, offshore Alaska (USA) and British Columbia (Canada)

AbstractFluid seepage along obliquely deforming plate boundaries can be an important indicator of crustal permeability and influence on fault-zone mechanics and hydrocarbon migration. The ∼850-km-long Queen Charlotte fault (QCF) is the dominant structure along the right-lateral transform boundary that separates the Pacific and North American tectonic plates offshore southeastern Alaska (USA) and western British Columbia (Canada). Indications for fluid seepage along the QCF margin include gas bubbles originating from the seafloor and imaged in the water column, chemosynthetic communities, precipitates of authigenic carbonates, mud volcanoes, and changes in the acoustic character of seismic reflection data. Cold seeps sampled in this study preferentially occur along the crests of ridgelines associated with uplift and folding and between submarine canyons that incise the continental slope strata. With carbonate stable carbon isotope (δ13C) values ranging from −46‰ to −3‰, there is evidence of both microbial and thermal degradation of organic matter of continental-margin sediments along the QCF. Both active and dormant venting on ridge crests indicate that the development of anticlines is a key feature along the QCF that facilitates both trapping and focused fluid flow. Geochemical analyses of methane-derived authigenic carbonates are evidence of fluid seepage along the QCF since the Last Glacial Maximum. These cold seeps sustain vibrant chemosynthetic communities such as clams and bacterial mats, providing further evidence of venting of reduced chemical fluids such as methane and sulfide along the QCF.

A NEW SUBSEAFLOOR REPLACEMENT MODEL FOR THE MACMILLAN PASS CLASTIC-DOMINANT Zn-Pb ± Ba DEPOSITS (YUKON, CANADA)

Abstract Sedimentary exhalative (SEDEX) deposits are a subset of sediment-hosted massive sulfide deposits and provide our dominant resource of Zn. In the SEDEX model, base metals (Zn, Pb, Fe) are hydrothermally vented into sulfidic (euxinic) seawater and deposited coevally with the organic-rich mudstone host rock, resulting in laterally extensive layered mineralization. In the Selwyn Basin (Canada) at Macmillan Pass, two deposits (Tom, Jason) are well preserved in a succession of Upper Devonian mudstones and are considered type-characteristic examples of the SEDEX deposit model. As with a number of SEDEX deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization. Early work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column. Recently, however, studies have both proposed an alternative diagenetic model for barite formation and provided more precise constraints on the chemistry of the hydrothermal fluid that entered the vent complexes. Here, we present a new model for Macmillan Pass in which sulfide mineralization occurred entirely within the subsurface. The introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (&amp;lt;1-km depth) resulted in the thermal degradation of organic matter and generated CO2; this promoted barite dissolution, which both provided a source of sulfate for thermochemical sulfate reduction and increased the porosity and permeability within the system. Importantly, there was clear potential for the development of positive feedbacks and self-organization between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. In contrast to the SEDEX model, alteration footprints will be controlled by the mass transfer involved in (barite) replacement reactions rather than hydrothermal venting, and exploration criteria at a district scale should strongly favor highly productive continental margins.

Geochemical characteristics of marine and terrestrial shale gas in China

Although the annual production of shale gas in China was 13 × 108 m3 in 2014, a systematic study on geochemical and isotopic characteristics of these unconventional gases has not been well addressed. In the present study, almost all shale gas samples available in China, including marine shale gas from the Wufeng-Longmaxi Formation in the Sichuan Basin (O3w–S1l) and terrestrial shale gas from Chang 7 Member (T3y7) in the Ordos Basin, were collected and analyzed for their geochemical and isotopic compositions. The shale gas from the Wufeng-Longmaxi Shale is dry gas with an average methane content of 98.38% and records a highest content of CH4 in the world (99.59%), which is consistent with the very high thermal maturity levels of the gas shales that have equivalent vitrinite reflectance (EqVRo) values between 2.4 and 3.6%. The δ13C1 values are correspondingly heavy and record a heaviest δ13C1 values (−26.7‰) for the shale gases found in the world as well. The average values of δ13C1, δ13C2 and δ13C3 for the Wufeng-Longmaxi shale gas are −31.3‰, −35.6‰, and −47.2‰, respectively, displaying a complete carbon isotopic reversal (i.e., δ13C1 > δ13C2 > δ13C3). δ2HCH4 and δ2HC2H6 has an average value of −148‰ and −173‰, respectively, also yielding a hydrogen isotopic reversal (i.e., δ2HCH4 > δ2HC2H6). The Chang 7 shale has an average TOC content of 13.81% with vitrinite reflectance (VRo) values between 0.7 and 1.2%. The Chang 7 shale gas is wet gas with an average methane content of 84.90% and is rich in heavy gaseous hydrocarbons (C2–C5). The respective values of δ13C1, δ13C2 and δ13C3 are −48.7‰, −36.4‰ and −31.3‰, displaying a positive carbon isotopic distribution pattern (i.e., δ13C1 < δ13C2 < δ13C3). The average δ2HCH4, δ2HC2H6 and δ2HC3H8 values are −256‰, −244‰ and −188‰, respectively, and are characterized by a positive distribution pattern (i.e., δ2HCH4 < δ2HC2H6 < δ2HC3H8). The differences in gas wetness and carbon and hydrogen isotopic distribution patterns between the shale gases from the Wufeng-Longmaxi and Chang 7 shale are attributed to variations in thermal maturity levels. CO2 is present in low content in both the Wufeng-Longmaxi and Chang 7 shale gases, mostly less than 1%. δ13C values for the CO2 in the Wufeng-Longmaxi Formation are between 8.9 and −9.2‰, indicating an inorganic origin from the cracking of carbonate mineral in the shales under high temperatures. In contrast, δ13C values of the CO2 in the Chang 7 shale gas range from −8.2 to −22.7‰, indicating an organic origin from the thermal degradation of organic matter. Helium in both the Wufeng-Longmaxi and Chang 7 shale gases is dominantly of curst origin in terms of their R/Ra ratios that vary from 0.01 to 0.08. Positive carbon isotopic distribution pattern is typical of primitive thermogenic gas. However, it can be converted into complete or partial carbon isotopic reversal patterns due to secondary alteration. The causes that yield carbon isotopic reversal include (1) mixing of gases with the same source but different thermal maturity levels; (2) secondary cracking of oil or wet gas; (3) formation water-involved reactions; (4) gas diffusion; and (5) carbon isotope exchange between alkane gases at high temperature. Among them, carbon isotopic exchange between alkane gases at high temperature is a key factor. Nine plots have been drawn based on the shale gases from China, USA and Canada. Among them, the plot of δ13C2 versus wetness demonstrates a “lying-S” shape with two inflection points on the gas wetness axis. The wetness value of 1.4% represents a critical point from pyrolytic gas (primary cracking gas) to cracking gas (secondary cracking gas) and whereas the wetness value of 6% marks the end of oil generation. On the diagram of wetness versus δ13C, shale gases with wetness values greater than 1.6% are characterized by positive carbon isotopic distribution pattern, whereas a complete or partial carbon isotopic reversals are observed for shale gases with wetness values less than 1.6%.

Origin and in situ concentrations of hydrocarbons in the Kumano forearc basin from drilling mud gas monitoring during IODP NanTroSEIZE Exp. 319

NanTroSEIZE Exp. 319 of the Integrated Ocean Drilling Program (IODP) was the first cruise in the history of scientific ocean drilling with drilling mud circulation through a riser. Drilling mud was pumped through the drill string and returned to the drill ship through the riser pipe during drilling of hole C0009A from 703 to 1604mbsf (meter below sea floor) and hole enlargement from 703 to 1569mbsf. During riser drilling, gas from returning drilling mud was continuously extracted, sampled and analyzed in real time to reveal information on the gas composition and gas concentrations at depth.Hydrocarbons were the only formation-derived gases identified in drilling mud and reached up to 14vol.% of methane and 48ppmv of ethane. The chemical and isotopic compositions of hydrocarbons exhibit a microbial origin. Hydrocarbons released from drilling mud and cuttings correlate with visible allochthonous material (wood, lignite) in drilling cuttings. At greater depth, addition of small but increasing amounts of hydrocarbons probably from low-temperature thermal degradation of organic matter is indicated. The methane content is also tightly correlated with several intervals of low Poisson’s ratio from Vp/Vs observed in sonic velocity logs, suggesting that the gas is situated in the pore space of the rock as free gas. The gas concentrations in the formation, determined from drilling mud gas monitoring, reaching up to 24Lgas/Lsediment for methane in hole C0009A, in line with gas concentrations from interpreted downhole sonic logs.

Thermal degradation of organic matter in the interlayer clay–organic complex: A TG-FTIR study on a montmorillonite/12-aminolauric acid system

The storage of natural organic matter within the interlayer space of layered silicate is an important type of clay–organic association in sediment. However, the role of the interlayer space of clay minerals in the thermal degradation of organics and the generation of hydrocarbons has not been well understood. In this study, an interlayer clay–organic complex was synthesized using montmorillonite (Mt) and 12-aminolauric acid (ALA). An Mt–ALA complex in which Mt and ALA were simply mixed was also prepared for comparison. Thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR) was applied to monitor the thermal events and the corresponding products during the thermal degradation of the Mt–ALA complexes. In the absence of Mt, ALA decomposed at 467°C via the cleavage of CC bonds, producing aliphatic hydrocarbon, N-containing compounds, and carboxylic acid. The decomposition temperatures of organic matter in the mixed Mt–ALA complex and the interlayer Mt–ALA complex decreased to 402 and 342°C, respectively. The most characteristic products of the interlayer Mt–ALA complex were NH3 and saturated hydrocarbons. The Brønsted acid sites in the interlayer space of Mt, arising from the dissociated interlayer water, initiated the deamination of ALA via the Hoffmann elimination pathway and significantly promoted the cracking of hydrocarbons via a carbonation mechanism. Lewis acid sites had little effect on the thermal degradation of ALA. This work indicated that the interlayer space of clay minerals provided the storage space for organic matter. Moreover, the active sites within the interlayer space strongly promoted the thermal degradation of organics.

Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (δ13C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO2 (δ13C–CO2 −2.0‰ to −3.8‰) and He isotope ratios (R/RA 0.12–0.27) were used to calculate the fraction of CO2 originating from mantle degassing vs. sedimentary sources. The results show that CO2 predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO2 in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO2; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH4 (δ13C–CH4 −6.1‰ to −22.7‰; δD–CH4 −9‰ to −129‰) show that CH4 does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO2 (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.

メタン循環に関わる海底下生命圏

Methane is one of the major end products from photosynthetic organic matter. Based on stable carbon and hydrogen isotopic compositions of methane, biological pathways mainly consisting of carbon dioxide reduction coupled to molecular hydrogen oxidation and acetate fermentation and abiological pathways such as thermal degradation of organic matter have been systematically classified. However, recent advances in subseafloor biosphere research have unveiled the complexity of processes involved in the transformation, migration and fate of methane. Particularly, it has been recognized that marine sediments with high flux of methane harbor novel lineages of microorganisms, the physiological traits of which are largely unknown due to their resistance to cultivation. In this review article, microbiological investigations that shed light on DNA-based microbial community structures and metabolic diversity and activity related to methane production and consumption in organic-rich marine sediments are briefly summarized.

CO 2 degassing and trapping during hydrothermal cycles related to Gondwana rifting in eastern Australia

Intensive carbonate and clay mineral authigenesis took place throughout the Late Permian Bowen–Gunnedah–Sydney basin system in eastern Australia. We conducted isotopic and trace element analyses of carbonate and clay minerals from clastic sedimentary rocks of the Gunnedah Basin and the Denison Trough in the Bowen Basin. Rb–Sr isochron age data of the illitic clays are consistent with episodic hydrothermal fluid flow events that occurred in association with Gondwana rifting accompanied by alkaline magmatism at ∼85 Ma and ∼95 Ma. Stable isotope data of carbonate and clay minerals from the Gunnedah Basin are indicative of meteoric waters from a high-latitude environment as the main fluid source, whereas trace element, Sr and Nd isotope data highlight mixing of meteoric fluids with magmatic and/or crustal components, with a possible input from marine carbonates for some samples. Trace metals, oxygen and strontium isotopes of dawsonites from the Denison Trough are interpreted to have been mobilised by fluids that interacted with evolved clastic sedimentary and marine carbonate end members. According to the carbon isotope data, CO 2 for calcite and ankerite precipitation was sourced mainly from thermal degradation of organic matter and magmatism, whereas the CO 2 used for dawsonite formation is inferred to have been derived from magmatic and marine sources. In the low permeability environments (particularly in coal seams), the increasing accumulation and oversaturation of CO 2 particularly promote the precipitation of dawsonite.

Thermal analysis of organic matter in cryogenic soils (Central Siberian Plateau)

The thermal degradation of organic matter was studied in cryogenic soils with methods of thermal analysis: differential scanning calorimetry and thermogravimetry (DSC and TG, respectively). The DSC curves of most of the samples within the temperature range from 221–247°C to 600°C were characterized by the presence of one wide exothermic peak (at 311–373°C) with a shoulder (or without it) on the descending branch at a temperature of about 400°C. This was connected mostly with the destruction of thermolabile compounds (oligo- and polysaccharides) and with the oxidation of low-aromatic complexes of plant residues and humus substances. Two exothermic peaks at 337–373°C and 448–492°C were found for some samples from the organic horizons. The high-temperature peaks were caused by the thermal destruction of lignin. The fraction of the thermolabile organic matter of the soil (237–261…331–377°C) reached 59–73% in the organic and 52–59% in the organomineral and mineral horizons.

Chemical and isotope compositions of drilling mud gas from the San Andreas Fault Observatory at Depth (SAFOD) boreholes: Implications on gas migration and the permeability structure of the San Andreas Fault

In this contribution we present results from two individual gas monitoring experiments which were conducted during the drilling of the SAFOD (San Andreas Fault Observatory at Depth) boreholes. Gas from circulating drilling mud was monitored during the drilling the SAFOD III side tracks and was later analyzed for δ 13C (CH 4, C 2H 6 and C 3H 8), H/D (CH 4) and noble gas isotopes. Furthermore, gas accumulations induced by drill pipe retrieval (“trip gas”) from the SAFOD MH and the SAFOD III boreholes were also investigated. The data are interpreted in the context of gas migration processes and the permeability structure of the San Andreas Fault (SAF) around two actively deforming zones at 3194 m and 3301 m borehole depth. Helium isotope ratios of 0.86 R a at 3203 m and between 0.51 and 0.88 R a at 3262 m (R a is the atmospheric 3He/ 4He ratio) indicate an improved flow of mantle volatiles between both fault strands. Much lower values were observed at 3147 m (0.26 R a) and 3312 m (0.22 R a). Hydrocarbon concentrations coincide with the occurrence of shale at ~ 3150–3200 m and below ~ 3310 m depth. The molecular and isotope composition of hydrocarbons and their spatial distributions imply hydrocarbon generation by thermal degradation of organic matter followed by extensive diffusion loss. Carbon isotope data furthermore suggest a thermal maturity of the source rock of approx. 1.4%R 0. The concentration of trip gas is generally low in the interval 3100 m–3450 m but exhibits high spatial variability. At 3128 m and 3223 m depth, the trip gas concentrations are as low as in the granite section of the SAFOD Main Hole. Considerable variations of R a values, trip gas concentrations, and the molecular composition of hydrocarbons when penetrating the active fault strands let us conclude that the permeability of the fault transverse to the fault direction is limited and that the active fault has not been breached over many earthquake cycles such that little or no fluid exchange took place. Diffusion is the dominant mechanism controlling hydrocarbon migration through the fault strands. The elevated R a values between both fault strands may reflect either episodic or continuous flow of mantle-derived fluids, suggestive of some limited permeability parallel to the fault direction.

Kinetics of organic matter degradation in the Murchison meteorite for the evaluation of parent-body temperature history

Abstract– To evaluate kinetic parameters for thermal degradation of organic matter, in situ heating experiments of insoluble organic matter (IOM) and bulk of Murchison (CM2) meteorite were conducted under Fourier transform infrared micro-spectroscopy combined with a heating stage. Decreases of aliphatic C–H band area under Ar flow were well fitted with Ginstling-Brounshtein three-dimensional diffusion model, and the rate constants for decreases of aliphatic C–H were determined. Activation energies Ea and frequency factors A obtained from these rate constants at different temperatures using the Arrhenius equation were Ea = 109 ± 3 kJ mol−1 and A = 8.7 × 104 s−1 for IOM, and Ea = 61 ± 6 kJ mol−1 and A = 3.8 s−1 for bulk, respectively. Activation energy values of aliphatic C–H decrease are larger for IOM than bulk. Hence, the mineral assemblage of the Murchison meteorite might have catalytic effects for the organic matter degradation. Using obtained kinetic expressions, the time scale for metamorphism can be estimated for a given temperature with aliphatic C–H band area, or the temperature of metamorphism can be estimated for a given time scale. For example, using the obtained kinetic parameters of IOM, aliphatic C–H is lost approximately within 200 years at 100 °C and 100 Myr at 0 °C. Assuming alteration period of 7.5 Myr, alteration temperatures could be calculated to be <15 ± 12 °C. Aliphatic C–H decrease profiles in a parent body can be estimated using time–temperature history model. The kinetic expression obtained by the infrared spectral band of aliphatic C–H could be used as an alternative method to evaluate thermal processes of organic matter in carbonaceous chondrites.

Excess methane in continental hydrothermal emissions is abiogenic

Thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100. We present hydrocarbon distribution data of continental hydrothermal gases, whose methane has been independently identified to derive from the abiogenic reduction of CO2. We find that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases. A similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis. These findings strongly suggest that abiogenic methane production in continental-hydrothermal systems is a more widespread process than previously assumed. The maximum contribution of such emissions to the modern atmospheric CH4 budget is estimated at ~1%.