Geochemical studies of pore fluid in surface sediment on the Daini Atsumi Knoll

We collected sediment samples and pore water samples from the surface sediment on the Daini Atsumi Knoll, and analyzed the sediments for CH₄, C₂H₆, and δ¹³C_CH4, and the pore fluids for CH₄, C₂H₆, δ¹³C_CH4, Cl⁻, SO₄²⁻, δ¹⁸O_H2O, and δD_H2O, respectively. A comparison of the measured concentration and isotopic composition of methane in pore water samples with those in sediment samples revealed that methane was present in the sediment samples at a higher concentration and was isotopically heavier than those in the pore water samples. It suggests that the effect of the release of a sorbed gas bound to organic particles when heated prior to analysis of hydrocarbons was larger than that of the degassing process. A large amount of a sorbed gas would be a significant source of natural gas. Two striking features are the chemical and isotopic composition of the pore water samples taken from the different sites around the Daini Atsumi Knoll. In the KL09, KL10, and KP07 samples, Cl⁻ concentrations in the pore water samples showed depletion to a minimum of 460 mmol/kg, correspond to  17% dilution of seawater, however the latter was not enriched in CH₄. The isotopic compositions of pore water samples suggested the low-Cl⁻ fluids in the pore water were not derived from dissociation of methane hydrate, but were derived from input of meteoric water. In contrast, in the KP05 samples from the north flank of the Daini Atsumi Knoll, pore water were characterized by CH₄ enrichment more than 370 μmol/kg, but not depleted in Cl⁻ concentrations. The observed methane concentration in the KP05 samples is not sufficient for methane hydrate to form in situ, indicating that the existence of methane hydrate in the surface sediment is negligible, as supported by Cl⁻ concentration. Based on the stable carbon isotope ratio of methane in the pore fluid from the KP05 site (δ¹³C_CH4 < − 50‰PDB), methane is thought to be of microbial origin. The pore waters in the surface sediments in the north flank of the Daini Atsumi Knoll were not directly influenced by upward fluid bearing methane of thermogenic origin from a deeper part of the sedimentary layer. However, extremely high methane concentration in the north flank site as compared with the concentration of pore water taken from the normal seafloor suggests that the north flank site is not the normal seafloor. We hypothesize that upward migration of chemically-reduced fluids from a deeper zone of the sedimentary layer reduces chemically-oxidized solutes in the surface sediment. As a consequence methane production replaced sulfate reduction as the microbial metabolism in the reduced environment of the surface sediment.     

Volatile C1C8 organic compounds in sediments from the Peru upwelling region

Volatile C₁C₈ organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C₄C₇ alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C₁, C₂, C₃, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C₄C₆ alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C₁C₇ alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).     

Light hydrocarbons geochemistry of surface sediments from Pranhita-Godavari Basin,Andhra Pradesh

A geochemical study of surface sediments from Pranhita-Godavari Basin, Andhra Pradesh, India was carried out using light hydrocarbon compounds to assess the hydrocarbon potential of the basin. Suite of 80 soil samples were collected from the depth of 2.5 m and analyzed for adsorbed light gaseous hydrocarbons namely methane (CH₄), ethane (C₂H₆) and propane (C₃H₈) in Gas chromatograph. Compound specific Carbon isotope ratios for CH₄ and C₂H₆ were also determined using GC-C IRMS (Gas Chromatograph Combustion Isotope Mass Spectrometer). The presence of moderate to low concentrations of methane (C_CH4 C_{CH_4 } : 1 to 138 ppb), ethane (H₄{H_4 }: 1 to 35 ppb) and propane (C_{C3 H8} C_{C_3 H_8 } : 1 to 20 ppb) was measured in the soil samples. Carbon isotopic composition of d¹³ C_CH4 \delta ^{13} C_{CH_4 } ranges between −27.9 to −47.1 ‰ and d¹³ C_{C2 H6} \delta ^{13} C_{C_2 H_6 } ranged between −36.9 to −37.2 ‰ (V-PDB) indicating that these gases are of thermogenic origin. Study of soil samples suggests the area has good potential for hydrocarbon.     

Carbon-Isotope Characteristics of CO2 and CH4 in Geothermal Springs from the Central Andes

Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ¹³C_CO2 and, of these, 10 were selected for δ¹³C_CH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994).

The present results show a wide range of δ¹³C_CO2 (-14.9 to -0.6‰) and a surprisingly heavy δ¹³C_CH4 (?20.9 to ?12.3‰). The difference between δ¹³C_CO2 and δ¹³C_CH4 (δ¹³C_CO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ¹³C_CO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the Eastern Cordillera. The widest ranges occur in the Eastern Cordillera (?15.0 to ?4.8‰) and the Altiplano (?20 to ?6‰). The δ¹³C_CO2 results for geothermal samples from the Volcanic Arc range between ?8.0‰ (Surire) and ?0.6‰ (Abra de Nappa), whereas δ¹³C_CO2 measured in gases collected from geothermal springs in the Precordillera range from ?10 to ?5‰.

The relationships between 3He/4He, δ¹³C_CO2 , and δ¹³C_CH4 are used to distinguish between crustal and mantle origins. The wide (21‰) range in the is interpreted to reflect contributions from different CO₂ sources that include organic and inorganic crustal and mantle carbon. Assuming isotopic equilibrium between coexisting methane and carbon dioxide, Δ¹³C_CO2-CH4 suggests very high equilibrium temperatures, in excess of 530°C, for some geothermal systems that also are characterized by a high (up to 63%) mantle-derived helium component.

δ¹³C_CH4 results suggest that methane has not formed by bacteriogenic processes or by thermal decomposition of organic matter, but rather abiogenically through the high-temperature reaction between H₂ and CO₂. The δ¹³C_CH4 results for the samples from the Volcanic Arc and from two CO₂-rich geothermal springs in the Altiplano (Coipasa-2 and Belen de Andamarca) are similar to those reported from hydrothermal fluids emitted from the East Pacific Rise (Welhan, 1988) and White Island, New Zealand (Hulston and McCabe, 1962), suggesting a mantle-derived carbon component in the methane.     

Stable isotope geochemistry of coals, humic kerogens and related natural gases

Isotope systematics are well defined for conventional sapropelic, Type I/II kerogens and their associated bacterial and thermogenic natural-gas products. These geochemical tools are used to estimate source type, maturity and depositional environment, and as a correlation technique. In many cases the natural gas signatures in near-surface samples and drill cuttings can be used to classify or predict a deeper lying source rock or reservoir.Corresponding interpretative schemes for coals, Type III kerogens and their associated hydrocarbons are progressing quickly. The shift in attention to humic sources is driven primarily by depletion of conventional oil and gas resources and the economic and societal requirements of coal and coal-bed methane.Carbon, hydrogen and nitrogen stable isotope variations can be large between different coals and humic kerogens. These differences can often be recognized in their bulk δ¹³C_org, δD_org and δ¹⁵N_org values. Isotope signatures of coals can be diagnostic of several factors, including deposit age, type, geographic location, maturity and generation history. However, these characteristic isotopic variations are substantially better defined by the C-, H- and N-isotope ratios of the separate maceral groups, such as vitrinite, exinite and inertinite. This new application of stable isotopes, at the maceral and compound levels, have great potential to improve the interpretative precision over conventional whole coal or bulk techniques.Hydrocarbon gases, including coal gases, derived from coals and humic kerogens can be distinguished from Type I/II sources, based on their molecular rations, i.e., C₁/(C₂ + C₃) and by comparing their stable isotope compositions, especially δ¹³C_CH4 and δD_CH4. The δ¹³C_C2H6 can also be valuable, but ethane is generally present in small amount (<1 vol. %) and requires     

A model of the diagenetic evolution of coaly sedimentary organic matter

Elemental composition was used to calculate the amounts of compounds produced during the diagenetic evolution of a coal series from the Mahakam delta (Kalimantan, Indonesia). These calculations were based on the following hypotheses: organic nitrogen does not take part in reactions and remains unchanged in the residual organic matter, the only compounds produced are water, carbon dioxide and hydrocarbons.This approach shows that carbon loss during diagenesis is mainly as CO₂, and hydrogen loss is mainly as H₂O. Hydrocarbon production is negligible, in accordance with absence of bacterial methane accumulations in the Mahakam delta.The δ¹³C of coals in the sequence becomes about 2 per mil more positive over the diagenetic depth range of coal evolution. Accounting for the coal δ¹³C change in terms of CO₂ loss requires that the CO₂ given off have δ¹³C of about ?40%.. Such negative CO₂ has not been observed in natural systems, except when CH₄ is undergoing oxidation. Several plausible causes for this effect are discussed.     

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Stable isotope ratios of oxygen and carbon were determined for CO₂ in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO₂ concentrations detected previously by McCarthy et al. (1986). Values of δ¹³C in soil gas CO₂ from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ¹³C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C₃ (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C₃ and C₄ (Hatch-Slack) plants, together with atmospheric CO₂ are the likely sources of carbon in soil gas CO₂. Values of δ¹⁸O in soil-gas CO₂ range from 32 to 38‰ (SMOW). These δ¹⁸O values are intermediate between that calculated for CO₂ gas in isotopic equilibrium with local groundwaters and that for atmospheric CO₂. The δ¹⁸O data indicate that atmospheric CO₂ has been incorporated by mixing or diffusion. Any CO₂ generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO₂ taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ¹³C value of soil-gas CO₂ was observed, contrary to what would be expected if the anomalous CO₂ were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C₃ plant material were responsible for most CO₂ generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO₂ anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO₂ concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.     

Preliminary analysis of the first digital recordings obtained in the Portuguese seismographic network — Attenuation studies

During experiments with digital stations in the period 1985–1987, twenty five earthquakes with magnitudes m _b in the range 2.9 to 4.8 and epicentres located within the area 36°–42.3° (N) and 4.5°–13.6° (W) were recorded at Montemor (MOE) and Montachique (MTH). The three-component recordings were obtained by Geotech S13 instruments with 1 second period. A preliminary analysis of the recordings consisted in the determination of amplitudes and spectral contents of P and S waves, and led to the following observations: (1) The attenuation of waves is expressed by the equation V = exp(C ₂).R ^C ₁. exp(C ₃.M), where V stands for acceleration, velocity or displacement; M-magnitude; R-focal distance; C ₁, C ₂ and C ₃ are constants to be obtained by least square fitting. The application of this equation led to C ₁ of the order 1.7 for displacement, 1.8 for velocity and 2.0 for acceleration, with an average mean square error 0.8. (2) The ratios L/T (longitudinal/transversal amplitudes), for velocity and displacement, showed a tendency to reduce with increasing focal distance, being 2 for short distances (<50 km) and 1 for long distances (400 km). (3) The ratios S/P (S-wave/P-wave amplitudes), although with a large dispersion, showed a slight tendency for increasing with focal distance. (4) The predominant frequencies also showed a slight tendency to decrease with increasing focal distance and with magnitude. (5) The dependence of C ₁ with frequency (3 to 12 Hz) is well behaved from 0.95 to 1.75 (for the velocity trace).     

Arsenic as an indicator element for gold exploration in the region of the Xiangxi Au–Sb–W deposit, NW Hunan, PR China

The Xiangxi Au–Sb–W deposit, the largest of its type in northwestern Hunan, China, is a sulfide-dominated ore body hosted by low grade metamorphic red slates of the Neoproterozoic Madiyi Formation. Three stages of mineralization, quartz–scheelite, quartz–gold–pyrite, quartz–gold–stibnite, and one metal-barren stage of veining, quartz–calcite, are recognized. Arsenopyrite occurs only as a minor mineral phase in the second stage. Analyses for 21 trace elements show that the enrichment factors of As in the metal deposit (EC [=element concentration of sample/average content of an element in the upper crust]: 190; 43 samples) in ore veins and in the Guanzhuang and Yuershan reference sections (3.7 km and 2.7 km away from the Xiangxi mine, EC: 3.5; 96 samples) are much smaller than those of Sb (52855 [in ore veins], 117 [in the sections]), W (5665, 7.5) and Au (2727, 5.3). The background concentrations of Au and As in the two sections were 1.4 ppb and 1.4 ppm, respectively. Arsenic (with an anomaly coefficient [AC = number of anomalous samples/total number of samples] of 76%) forms a larger geochemical halo than W (AC: 8%) and Au (AC: 32%). Gold and As in the deposit were transported mainly as metal complexes such as Au(HS)⁻₂, H_nAs₃S⁻⁽³⁻ⁿ⁾₆ (n=1, 2 or 3) and HAsS⁰₂. Au(HS)⁻₂ is rapidly precipitated by a geochemical oxidation barrier — the red slates of the Madiyi Formation. As–S complexes in the stratigraphic horizon can be transformed into As–O complexes (e.g., H₃AsO⁰₃) under oxidizing conditions, and are continuously transported. Therefore, they can be widely distributed in the red slate units, thus forming extensive geochemical haloes, so that As can be used as an indicator element for Au exploration in the Xiangxi region.     

Carbon isotope thermometry in marbles of the Adirondack Mountains, New York

Abstract Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670° C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry. Data for amphibolite and granulite grade marbles show a 12%oo range in δ¹³C_cal and δ¹³C_gr. A strong correlation between carbon isotopic composition and the abundance of graphite (C_gr/C_rock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ¹³C _vs. C_gr/C_rock and δ¹³C_cal vs. δ¹³C_gr could not have been preserved if significant amounts of CO₂-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ¹³C vs. C_gr/C_rock indicates a biogenic origin for Adirondack graphites, even though low δ¹³C values are not preserved in marble. Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher δ¹³C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located >2km from anorthosite show a decrease of 0.5-0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(_cal-gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ¹³C(_cal-gr) = 3.56 ± 10⁶T^-2 (K).     

Silicate melt inclusions in the Qiushuwan granitoids,northern Qinling belt,China: Implications for the formation of a porphyry Cu–Mo deposit as a reduced magmatic system

Melt inclusions (MIs) in quartz from granitoids in the northern Qinling belt were studied using microthermometry and laser Raman spectroscopy. The total homogenization of melt inclusions occurs in a mean range between 1050 and 1100 °C. Laser Raman experiments reveal H₂O, C₂H₆, C₄H₆ and CH₄ as the dominant volatile compounds. Our results provide insights into the temperatures of magma crystallization and the dominantly reducing environment during the early magmatic stage. Based on ore mineralogy, and on the volatile species content in the MIs, we evidence firstly that the Qiushuwan porphyry Cu–Mo deposit in the Qinling–Dabie–Sulu orogenic belt was derived from a reduced magmatic system, emplaced at relatively deep domains more than 10 km deep, and secondly, that the magmas that are responsible for the generation of Qiushuwan were either derived from an inherently reduced source, or reduced during ascent and emplacement. The mechanism might have involved the assimilation of sedimentary material with minimal crustal interaction. The parental magmas likely underwent reduction essentially by loss of all of their SO₂ by degassing, as evidenced by the low S content in melt inclusions. These reduced materials provided adequate sulfur source for the formation of the porphyry Cu–Mo deposits with obvious zonation, which plays a key role in the mineralization; finally, we conclude that the reduced environment and the relatively deep domain of magma emplacement probably limited the extent of mineralization, generating only a relatively small Cu–Mo deposit in Qiushuwan, located within the northern Qinling accretionary belt.     

Lipid Biomarkers and Their Stable Carbon Isotopes inAncient Seep Carbonates from SW Taiwan,China

Four massive brecciated, chimney-like, and slender pipe network carbonate samples(JA-4, JA-5, JX-8 and BG-12) were collected from southwestern Taiwan, which were suggested to have formed as a result of anaerobic oxidization of methane(AOM). Considering that the environmental conditions of the carbonates precipitation and the sources of carbon and organic matter need to be further declared, molecular fossils and compound-specific carbon isotopic investigations of the carbonates were conducted in this study. According to lipid biomarkers of 2,6,10,15,19-pentamethyleicosane(PMI) and squalane diagnostic to methanotrophic archaea, as well as the extremely low δ¹³C values(as low as -1¹³.4‰) detected in samples JA-4, JA-5 and JX-8, these carbonates were revealed to be a result of AOM. Based on the varied δ¹³C values of characteristic archaea biomarkers in specific samples, biogenic methane was proposed to be responsible for the formation of samples JA-4 and JA-5, whereas a mixed carbon source of ¹³C-depleted methane and ¹³C-enriched residual CO_2 from methanogenesis was suggested for the carbonate of JX-8 due to the co-occurrence of a highly positive δ¹³ C_carb value(+8‰) and a moderate ¹³C depletion of PMI. The low content of AOM-related biomarkers and the absence of indicators for ANME-2 suggested that these carbonates were formed in weak seep settings. By comparison, no typical lipid biomarkers for methanotrophic archaea was detected in carbonate BG-12. The short-chain and long-chain n-alkanes accounted for 30% and 45% of all hydrocarbons, respectively, with a CPI value of 1.2, suggesting that the n-alkanes were derived from both marine organisms and terrestrial inputs. A low thermal maturity could be revealed by the incomplete equilibrium value of the C³¹αβ 22S/(22S+22R) ratio(0.5), and the carbonate BG-12 was probably deposited in a suboxic condition indicated by a value of Pr/Ph ratio(2.5).     

Origin of light hydrocarbon gases from the Matsukawa geothermal area in Japan

Light hydrocarbon and isotope compositions of methane were analyzed in well steam samples from the Matsukawa vapour-dominated type geothermal system. Alkanes (C₁-C₄) and alkene (C₂) were detected in all samples. Light hydrocarbon contents of CO₂-type steam are slightly higher than those of CO₂-H₂S-type steam. The isotope composition of methane and the relationship between methane/ethane ratio and δ¹³C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at f_o2 defined by the fayalite-magnetite-quartz buffer (FMQ).     

THE EXTENT OF EXPLORATION OF A MINERAL DEPOSIT PREPARATORY TO ITS COMMERCIAL DEVELOPMENT

Certain problems of successive conduction of prospecting-exploration work as well as a detailed exploration of various deposits are considered in the paper and it is stated that the correct solution of these problems essentially increases the efficiency of geological exploratory work. The solution of the above problems may be considerably facilitated if the commercial requirements as to the amount and accuracy of estimated ore reserves are quite definite. These requirements however have many shortcomings, e.g., such important factors as the deposit scale and the reserves of category C₂ are not taken into account. A new version of such commercial requirements is suggested which takes into account the scale of the deposits and the prospective reserves (cat. C₂) along with different (A₂, B and C₁) category reserve ratios. — Auth. English Summ.     

Lipid Distribution in Marine Sediments from the Northern South China Sea and Association with Gas Hydrate

The distributions of lipids in surface and subsurface sediments from the northern South China Sea were determined. The n-alkanes were in bimodal distribution that is characterized by a centre at n-C16 –n-C20 with maximum at C18(or C19) and n-C27 –n-C31 as well as at C29(or C31). The short-chain alkanes suffered from significant losses due to their slow deposition in the water column, and their presence with a slight even carbon predominance in shallow seafloor sediments was ascribed mainly to the direct input from the benthos. The long-chain alkanes with odd predominance indicate transportion of terrigenous organic matter. Immature hopanoid biomarkers reflect the intense microbial activity for bacteria–derived organic matter and the gradual increase of maturity with burial depth. Abundant n-fatty acid methyl esters(n-FAMEs) that are in distributions coincident with fatty acids were detected in all samples. We proposed that the observed FAMEs originated from the methyl esterification of fatty acids; methanol production by methanotrophs and methanogenic archaea related to the anaerobic oxidation of methane, and sulfate reduction provided an O–methyl donor for methylation of fatty acids. The CH4 released from hydrate dissociation at oxygen isotope stage II of Cores ZD3 and ZS5, which had been confirmed by the occurrence of negative δ13C excursion and spherical pyrite aggregates, could have accelerated the above process and thus maximized the relative content of FAMEs at ZD3-2(400–420 cm depth) and ZS5-2(241–291 cm depth).     

Geochemical characteristics of methane from sediments of the underwater high Posolskaya Bank (Lake Baikal)

The component and carbon isotope compositions were studied in the hydrocabon gases from sediments of the underwater high Posolskaya Bank (Lake Baikal). It was established that sediments of this Baikal area contain methane of microbial (C₁/C₂ >16000; δ¹³C 70 ± 3‰) and thermocatalytic (C₁/C₂ <100; δ¹³C–46 ± 3‰) origin. Some samples represent a gas mixture of thermocatalytic and microbial origin. This gas is characterized by δ¹³C of methane varying from–60 to–70‰ and contains a significant amount of ethane. The main homolog of methane in the thermocatalytic and mixed gas is ethane. Owing to biodegradation, propane and butanes are present in trace amounts.     

Extracts of impact breccia samples from Sudbury, Gardnos, and Ries impact craters and the effects of aggregation on C60 detection

Fullerenes have been detected in carbonaceous chondrite meteorites and in breccia samples from meteorite impact craters, but questions have been raised about contradictory results from similar samples and the sensitivities and accuracies of different analytical methods. We analyzed samples from three impact craters and detected C₆₀ in samples from several locations; we also observed differences in the detection capabilities of various analytical techniques used in the search for fullerenes. The presence of C₆₀ in rocks from the Onaping Formation of the Sudbury impact crater was confirmed. Low levels of C₆₀ were also detected for the first time in samples from the Gardnos (Norway) and Ries (Germany) impact structures.We detected C₆₀ in these samples using surface-enhanced laser desorption/ ionization (SELDI), but the related technique of microprobe laser-desorption, laser-ionization mass spectrometry (μL²MS) did not observe C₆₀ above detection limits. We attribute the absence of μL²MS signal to aggregate formation caused by phthalic acid esters, which appear to easily contaminate samples either during storage or demineralization in plastic containers. The μL²MS technique is incapable of detecting aggregated C₆₀, but aggregation does not suppress detection in SELDI. Phthalate-induced aggregation did, however, enhance SELDI detection of C₆₀ in some cases, and we suggest that this enhancement may help explain previously reported differences in C₆₀ detection from natural samples between laser desorption mass spectrometry (LDMS, a technique analogous to SELDI that has detected fullerenes in meteorite and impact breccia samples) and high-pressure liquid chromatography (HPLC). This work highlights the effects of phthalates and other indigenous compounds or contaminants on certain mass spectrometric techniques and lends support to the idea that several complementary analytical methods should be employed to investigate complex natural samples.     

Variscan and post-Variscan lead–zinc mineralization, Rhenish Massif, Germany: evidence for sulfide precipitation via thermochemical sulfate reduction

A mechanical decrepitation device coupled with a gas chromatograph has been used to characterize the molecular composition of gaseous and liquid hydrocarbons contained in minerals. Application of this technique allows the identification of low-molecular-weight n-alkanes and some aromatic hydrocarbons in sulfides and gangue minerals from epigenetic Variscan and post-Variscan lead–zinc deposits in the Rhenish Massif, Germany. Based on the analysis of 200 samples, Variscan and post-Variscan mineralization can be distinguished by the composition of associated hydrocarbons. Variscan sulfides and gangue minerals contain high abundancies of methane. In contrast, n-alkanes in the C₂–C₉ range and aromatic hydrocarbons (benzene, toluene) are dominant in post-Variscan mineralization. The absence of high-molecular-weight hydrocarbons in ore minerals suggests highly mature gas associated with hydrothermal activity, during which hydrothermal fluids caused an increase in thermal maturation of organic matter and the generation of low-molecular-weight hydrocarbons in the adjacent organic-rich rocks. The hydrocarbon compositions contained in fluid inclusions of Variscan and post-Variscan minerals are probably governed by the maturation level of the potential source rocks. In Variscan time tectonic brines (T > 175 °C) generated predominantly methane, whereas basement brines (T < 175 °C) expelled higher-molecular-weight hydrocarbons (wet gases, condensates, aromatic hydrocarbons) from adjacent rocks during the Mesozoic event. The specific role of hydrocarbons in sulfide precipitation via thermochemical sulfate reduction is indicated by geochemical characteristics of organic matter associated with the Plombières Pb–Zn deposit, in eastern Belgium. Intense alteration phenomena were observed in near-ore kerogens, compared with unaltered kerogens far from the ore body, as well as by a very high maturity (5.40% R_o), a systematic depletion in ¹²C towards the vein-type mineralization, high atomic S/C ratios (0.49), and by low atomic H/C ratios (0.29). The data suggest that hydrothermal solutions caused a drastic increase in the thermal maturation of organic matter within the adjacent wall rock. Increased thermal maturation resulted in increased δ¹³C-values of organic carbon due to the preferential release of ¹²C. The change in the organic matter to a H-depleted and S-enriched bulk composition in association with sulfide ores strongly suggests that thermochemical sulfate reduction was responsible for organic degradation. Thus, thermochemical sulfate reduction probably triggered base metal sulfide precipitation in Variscan and post-Variscan ore deposits of the Rhenish Massif. Finally, based on data from this study and previous investigations, new genetic models are presented for both Variscan and post-Variscan mineralization in the Rhenish Massif. Received: 15 September 1999 / Accepted: 2 December 1999     

An 8-year record of gas geochemistry and isotopic composition of methane during baseline sampling at a groundwater observation well in Alberta (Canada)

Variability in baseline groundwater methane concentrations and isotopic compositions was assessed while comparing free and dissolved gas sampling approaches for a groundwater monitoring well in Alberta (Canada) over an 8-year period. Methane concentrations in dissolved gas samples (n?=?12) were on average 4,380?±?2,452 μg/L, yielding a coefficient of variation (CV) >50 %. Methane concentrations in free gas samples (n?=?12) were on average 228,756?±?62,498 ppm by volume, yielding a CV of 27 %. Quantification of combined sampling, sample handling and analytical uncertainties was assessed via triplicate sampling (CV of 19 % and 12 % for free gas and dissolved gas methane concentrations, respectively). Free and dissolved gas samples yielded comparable methane concentration patterns and there was evidence that sampling operations and pumping rates had a marked influence on the obtained methane concentrations in free gas. δ¹³C_CH4 and δ²H_CH4 values of methane were essentially constant (?78.6?±?1.3 and ?300?±?3?‰, respectively) throughout the observation period, suggesting that methane was derived from the same biogenic source irrespective of methane concentration variations. The isotopic composition of methane constitutes a robust and highly valuable baseline parameter and increasing δ¹³C_CH4 and δ²H_CH4 values during repeat sampling may indicate influx of thermogenic methane. Careful sampling and analytical procedures with identical and repeatable approaches are required in baseline-monitoring programs to generate methane concentration and isotope data for groundwater that can be reliably compared to repeat measurements once potential impact from oil and gas development, for example, may occur.     

Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting

We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for organic matter present in ancient methane vent systems (cold seeps). The limestone contains high concentrations of the tail-to-tail linked, acyclic C₂₀ isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C₂₅ homolog 2,6,10,15,19-pentamethylicosane (PME), and a distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. The chemical structures of these biomarkers indicate a common origin from archaea. Their extremely ¹³C-depleted isotope compositions (δ¹³C ≈ −108 to −115.6‰ PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. We postulate that a second major cluster of biomarkers showing heavier isotope values (δ¹³C ≈ −88‰) is derived from sulfate-reducing bacteria (SRB). The observed biomarkers sustain the idea that methanogenic bacteria, in a syntrophic community with SRB, are responsible for the anaerobic oxidation of methane in marine sediments. Marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined, two-membered bacterial consortium: (1) archaea that perform reversed methanogenesis by oxidizing methane and producing CO₂ and H₂; and (2) SRB that consume the resulting H₂. Furthermore, the respective organic molecules are, unlike other compounds, tightly bound to the crystalline carbonate phase. The Marmorito carbonates can thus be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates.