Molecular and carbon isotopic analysis of individual biological markers: evidence for sources of organic matter and paleoenvironmental conditions in the Upper Ordovician Maquoketa Group, Illinois Basin, U.S.A.

Variations in the carbon isotopic composition (δ¹³C) of pristane, phytane, n-heptadecane (n-C₁₇), C₂₉ ααα 20R sterane, and aryl isoprenoids provide evidence for a diverse community of algal and bacterial organisms in organic matter of the Upper Ordovician Maquoketa Group of the Illinois Basin. Carbon isotopic compositions of pristane and phytane from the Maquoketa are positively covariant (r = 0.964), suggesting that these compounds were derived from a common source inferred to be primary producers (algae) from the oxygenated photic zone. A variation of 3‰ in δ¹³C values (−31 to −34‰) for pristane and phytane indicates that primary producers utilized variable sources of inorganic carbon. Average isotopic compositions of n-C₁₇ (−32‰) and C₂₉ ααα 20R sterane (−31‰) are enriched in ¹³C relative to pristane and phytane (−33‰) suggesting that these compounds were derived from a subordinate group of primary producers, most likely eukaryotic algae. In addition, a substantial enrichment of ¹³C in aryl isoprenoids (−14 to −18‰) and the identification of tetramethylbenzene in pyrolytic products of Maquoketa kerogen indicate a contribution from photosynthetic green sulfur bacteria to the organic matter. The presence of anaerobic, photosynthetic green sulfur bacteria in organic matter of the Maquoketa indicates that anoxic conditions extended into the photic zone.The δ¹³C of n-alkanes and the identification of an unusual suite of straight-chain n-alkylarenes in the m/z 133 fragmentograms of Ordovician rocks rich in Gloeocapsomorpha prisca (G. prisca) indicate that G. prisca did not contribute to the organic matter of the Maquoketa Group.     

Sources and transport of carbon and nitrogen in the River Sava watershed, a major tributary of the River Danube

Carbon and nitrogen dynamics were examined throughout the River Sava watershed, a major tributary of the River Danube, in 2005 and 2006. The River Sava exported 2.1 × 10¹¹ mol C/yr as dissolved inorganic carbon (DIC), and emitted 2.5 × 10¹⁰ mol C/yr as CO₂ to the atmosphere. Stable carbon isotope ratios indicate that up to 42% of DIC originated from carbonate weathering and 23% from degradation of organic matter. Loads of dissolved and particulate organic carbon increased with discharge and export rates were calculated to be 2.1 × 10¹⁰ mol C/yr and up to 4.1 × 10⁹ mol C/yr, respectively. Isotopic compositions (δ¹³C and δ¹⁵N) and C/N ratios indicated that soil organic matter was the dominant source of particulate organic matter for 59% of the samples. Eighteen percent of the samples were dominated by plankton, 12% by periodic inputs of fresh terrestrial plant detritus with C/N > 15, and about 11% of the samples were dominated by the contribution of aquatic vascular plants. Nitrate inputs were controlled by land use in the River Sava watershed. δ¹⁵N_NO3 values <6‰ were found in predominantly forested watersheds, while values >6‰ typically represented watersheds with a higher percentage of agricultural and/or urban land use. Elevated δ¹⁵N_NO3 values (up to +25.5‰) at some sites were probably due to the combined effects of low-flow and inputs from sewage and/or animal waste.     

Origins and accumulation of organic matter in expanded Albian to Santonian black shale sequences on the Demerara Rise, South American margin

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ¹³C_org values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C₃ land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ¹³C_org values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.     

Geochemistry and mineralogy of shallow alluvial aquifers in Daudkandi upazila in the Meghna flood plain, Bangladesh

The shallow alluvial aquifers of the delta plains and flood plains of Bangladesh, comprises about 70% of total land area are mostly affected by elevated concentrations of arsenic (As) in groundwater exposing a population of more than 35 million to As toxicity. Geochemical studies of shallow alluvial aquifer in the Meghna flood plain show that the uppermost yellowish grey sediment is low in As (1.03 mg/kg) compared to the lower dark grey to black sediment (5.24 mg/kg) rich in mica and organic matter. Sequential extraction data show that solid phase As bound to poorly crystalline and amorphous metal (Fe, Mn, Al)-oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level (3.05 mg/kg) in the mica rich layers. Amount of As bound to sulphides and organic matter also peaks in the dark grey to black sediment. Vertical distributions of major elements determined by X-ray fluorescence (XRF) show that iron (Fe₂O₃), aluminum (Al₂O₃) and manganese (MnO) follow the general trend of distribution of As in the sediments. Concentrations of As, Mn, Fe, HCO₃ ⁻, SO₄ ²⁻ and NO₃ ⁻ in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry. Mineralogical analysis by X-ray diffraction (XRD) and scanning electron microscopy (SEM) fitted with energy dispersive X-ray spectrometer (EDS) revealed dominance of crystalline iron oxides and hydroxides like magnetite, hematite and goethite in the oxidised yellowish grey sediment. Amorphous Fe-oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of Fe(III)-oxyhydroxides which in turn act as sink for As. Presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

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.     

12,000-Year, preliminary results of the stable nitrogen and carbon isotope record from the Empakai Crater lake sediments, Northern Tanzania

The stable isotope compositions of organic carbon and nitrogen, the contents of organic carbon and nitrogen and C/N ratios for two cores recovered from the Empakai Crater at water depths of 11 and 20 m are used to document climatic changes in northern Tanzania. Eight ¹⁴C AMS dates determined on total organic matter (OM) indicate that the sedimentation rate in this lake is about 30 cm/ka for the late Pleistocene to early Holocene period. There are differences in the δ¹³C values of organic carbon between the two cores, which may be a result of differences in location from the present shoreline and of different water depths. In the deeper-water core the δ¹³C values show a general downcore decrease to the base of the core with a sharp change to lower values of about 4‰ at a depth of 100 cm (8.7 ka). The general trend of downcore decrease in ¹³C values can be attributed either to a systematic decrease in the relative proportion of C₄ type of OM, owing to an increase in precipitation and change in vegetation cover from grassland to forest, or to utilization of isotopically enriched carbon during photosynthesis. The δ¹⁵N values show a general downcore increase with again a sharp change of about 5‰ to lower values at about 8.7 ka. A sharp change of about 5‰ and 4‰ to more depleted values at a depth of 100 cm of both ¹⁵N and ¹³C, respectively, suggests either hiatus or abrupt change in climatic condition from wetter conditions to drier conditions. There is enhanced preservation of OM in the lake as depicted by high mean values of organic carbon and nitrogen at both sites.     

Isotopic and molecular composition of coal-bed gas in the Amasra region (Zonguldak basin—western Black Sea)

Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios (δ¹³C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C₅, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ¹³C₁: −51.1 to −48.3‰, δ¹³C₂: −37.9 to −25.3‰, δ¹³C₃: −26.0 to −19.2 ‰, respectively. The δD₁ values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling.     

Relations of petrographical and geochemical parameters in the middle Miocene Lavanttal lignite (Austria)

Samples from two lignite seams (Lower Seam, Upper Seam) of the Lavanttal basin (Austria) and additional xylite were investigated for variations in maceral composition, petrography-based facies indicators, bulk geochemical parameters, and molecular composition of hydrocarbons. Both seams originated in a topogenous mire and evolved within a transgressive setting. The final drowning of the mire is indicated by sapropelic shales. Whereas the sapropelic shale overlying the Lower Seam was deposited in a freshwater lake, the sapropelic shale above the Upper Seam represents a brackish lake.Numerous relationships are found between petrography-based facies indicators and the geochemical composition of organic matter. The contents of macerals of the liptinite group are positively correlated with soluble organic matter (SOM) yields and hydrogen index (HI). Consistent with maceral composition and high HI values, enhanced proportions of short-chain n-alkanes, which are predominantly found in algae and microorganisms, are obtained from samples of the sapropelic shales. The final drowning of the mire is reflected by decreasing pristane/phytane ratios, due to the rise in (ground)water table and the establishment of anaerobic conditions, as well as by decreasing ratios of diasterenes/sterenes, indicating increasing pH values in the mire. The degree of gelification of plant tissue (gelification index) is governed by the microbial activity in the mire, as indicated by the hopanes concentration. The differences in floral assemblage during the formation of the Lavanttal lignite seams are reflected by major differences in tissue preservation. Preservation of plant tissue (TPI) in the Lavanttal lignite is obviously controlled by the presence/absence of decay-resistant gymnosperms in the peat-forming vegetation, and additionally influenced by the relative contribution of wood to coal formation. The results provide evidence that valuable information for coal facies characterization could be obtained by petrography-based and geochemical facies indicators. An influence of the floral assemblage (gymnosperms/angiosperms ratio) and of the contribution of algal biomass on carbon isotopic composition of the organic matter (δ¹³C = − 24.2 to − 28.6‰) is proposed. Carbon cycling during biogeochemical decomposition of plant tissue by bacteria is suggested to affect the δ¹³C values of the coal. The chemotaxonomical classification of the xylites as gymnosperm remnants, based on the molecular composition of terpenoid biomarkers, is corroborated by the carbon isotopic composition of the xylites (mean δ¹³C = − 24.1‰) and the extracted cellulose (mean δ¹³C = − 20.2‰). The higher isotopic difference of about 3.9‰ between cellulose and total organic carbon of the xylites, compared to the difference between cellulose and wood found in modern trees, is explained by the smaller effect of decomposition on δ¹³C of cellulose.     

Origin of methane in high-arsenic groundwater of Taiwan – Evidence from stable isotope analyses and radiocarbon dating

Groundwaters in the confined aquifers of the Chianan and Ilan coastal plains of Taiwan are rich in dissolved methane (CH₄). Serious endemic “blackfoot disease”, which occurred in the Chianan plain, especially during AD1950-1970, has been demonstrated to have arisen from drinking highly reducing groundwater with abnormal arsenic and humic substance levels. In order to explore the origin of CH₄ and its hydrological implications, stable carbon isotope ratios (δ¹³C) and radiocarbon (¹⁴C) ages of exsolved CH₄, dissolved inorganic carbon (DIC), and sedimentary biogenic sediments from a total of 34 newly completed water wells at 16 sites were determined. The main results obtained are as follows: (1) The δ¹³C_CH4 (−65‰ to −75‰) values indicate that, except for one thermogenic sample (δ¹³C_CH4=38.2‰) from the Ilan plain, all CH₄ samples analyzed were produced via microbially mediated CO₂ reduction. Many δ¹³C_DIC values are considerably greater than −10‰ and even up to 10‰ due to Rayleigh enrichment during CO₂ reduction. (2) Almost all the ¹⁴C ages of CH₄ samples from the shallow aquifer (I) (<60 m depth) are greater than the ¹⁴C ages of coexisting DIC and sediments, suggesting the presence of CH₄ from underlying aquifers. (3) The ¹⁴C ages of coexisting CH₄, DIC and sediments from aquifer (II) of the Chianan plain are essentially equal, reflecting in-situ generation of CH₄ and DIC from decomposition of sedimentary organic matter and sluggishness of the groundwater flow. On the other hand, both CH₄ and DIC from each individual well of the relatively deep aquifers (III) and (IV) in the Chianan plain are remarkably younger than the deposition of their coexisting sediments, indicating that current groundwaters entered these two aquifers much later than the deposition of aquifer sediments. (4) Each CH₄ sample collected from the Ilan plain is older than coexisting DIC, which in turn is distinctly older than the deposition of respective aquifer sediments, demonstrating the presence of much older CO₂ and CH₄ from underlying strata.     

The carbon isotopic response of algae, (cyano)bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367)

The stable carbon isotopic compositions of free and sulfur (S)-bound biomarkers derived from algae, (cyano)bacteria, archaea and higher plants and total organic carbon (TOC) during the first phase of the late Cenomanian/Turonian oceanic anoxic event (OAE) were measured in black shales deposited in the southern proto-Atlantic Ocean in the Cape Verde basin (DSDP Site 367) to determine the response of these organisms to this major perturbation of the global carbon cycle resulting from widespread burial of marine organic matter. The average positive isotope excursions of TOC and biomarkers varied from 5.1‰ to 8.3‰. The δ¹³C values were cross correlated to infer potential common sources of biomarkers. This revealed common sources for C₃₁ and C₃₂ hopanes but no 1:1 relationship for pristane and phytane. The correlation of δ¹³C_TOC with the δ¹³C value of sulfur (S)-bound phytane is the strongest. This is because S-bound phytane is derived from phytol that originates from all marine primary producers (algae and cyanobacteria) and thus represents a weighted average of their carbon isotopic compositions. The δ¹³C values of S-bound phytane and C₃₅ hopane were also used to estimate pCO₂ levels. Before the OAE burial event, pCO₂ levels are estimated to be ca. 1300 ppmv using both biomarkers and the independent maximum Rubisco fractionation factors. At times of maximum organic carbon burial rates during the OAE, reconstructed pCO₂ levels are estimated to be ca. 700 ppmv. However, compared to other C/T OAE sections the positive isotope excursion of S-bound phytane is also affected by an increased production during the OAE. When we compensate for this, we arrive at pCO₂ levels around 1000 ppmv, a reduction of ca. 25%. This indicates that burial of organic matter can have a large effect on atmospheric CO₂ levels.     

Petrology and isotopic geochemistry of dawsonite-bearing sandstones in Hailaer basin, northeastern China

The CO₂ gas reservoir sandstones in the Hailaer Basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of CO₂. The origins of dawsonite and CO₂ in these sandstones were studied by petrographic and isotopic analysis. According to the paragenetic sequence of the sandstones, dawsonite grew later than CO₂ charging at 110–85 Ma. The dawsonite δ¹⁸O value is 7.4‰ (SMOW), and the calculated δ¹⁸O values of the water present during dawsonite growth are from −11.4‰ to −9.2‰ (SMOW). This, combined with the NaHCO₃-dominated water linked to dawsonite growth, suggests meteoric water being responsible for dawsonite growth. The δ¹³C values of gas phase CO₂ and the ratios of ³He/⁴He of the associated He suggest a mantle magmatic origin of CO₂-rich natural gas in Hailaer basin. Dawsonite δ¹³C values are −5.3‰ to −1.5‰ (average −3.4‰), and the calculated δ¹³C values of CO₂ gas in isotopic equilibrium with dawsonite are −11.4‰ to −7.3‰. These C isotopic values are ambiguous for the dawsonite C source. From the geological context, the timing of events, together with formation water conditions for dawsonite growth, dawsonite possibly grew in meteoric-derived water, atmospherically-derived CO₂ maybe, or at least the dominant, C source for dawsonite. It seems that there are few relationships between dawsonite and the modern gas phase of CO₂ in the Hailaer basin.     

A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).     

Carbonate precipitation in artificial soils as a sink for atmospheric carbon dioxide

Turnover of C in soils is the dominant flux in the global C cycle and is responsible for transporting 20 times the quantity of anthropogenic emissions each year. This paper investigates the potential for soils to be modified with Ca-rich materials (e.g. demolition waste or basic slag) to capture some of the transferred C as geologically stable CaCO₃. To test this principal, artificial soil known to contain Ca-rich minerals (Ca silicates and portlandite) was analysed from two sites across NE England, UK. The results demonstrate an average C content of 30 ± 15.3 Kg C m⁻² stored as CaCO₃, which is three times the expected organic C content and that it has accumulated at a rate of 25 ± 12.8 t C ha⁻¹ a⁻¹ since 1996. Isotopic analysis of the carbonates gave values between −6.4‰ and −27.5‰ for δ¹³C and −3.92‰ and −20.89‰ for δ¹⁸O, respectively (against V-PDB), which suggests that a combination of carbonate formation mechanisms are operating including the hydroxylation of gaseous CO₂ in solution, and the sequestration of degraded organic C with minor remobilisation/precipitation of lithogenic carbonates. This study implies that construction/development sites may be designed with a C capture function to sequester atmospheric C into the soil matrix with a maximum global potential of 290 Mt C a⁻¹.     

Methane derived organic matter and carbonates

¹³C depleted materials of three types are encountered in the south-eastern Mediterranean Coastal Plain of Israel and Northern Sinai. Sub-surface coarse carbonates associated with elemental sulfur have gd¹³C values of −50‰. Organic matter (humic substances and lipid soluble) in the Be'eri sulfur quarries have δ¹³C values of −80 to −90‰. Dissolved bicarbonate in groundwaters show δ¹³C values of −13 to −17‰.It is suggested that all these values can be explained by biogenic methane leaking from sub-surface reservoir which is oxidized under different depositional and environmental conditions.     

Stable carbon-isotopic compositions of lipids isolated from the ammonia-oxidizing chemoautotroph Nitrosomonas europaea

For the ammonia-oxidizing bacterium Nitrosomonas europaea, grown autotrophically using semicontinuous culturing, average biomass was depleted in ¹³C relative to CO₂ dissolved in the medium by ca. 20‰ and the total-lipid extract was depleted in ¹³C relative to biomass by 3.7‰. The n-alkyl lipids (weighted average of fatty acids) and isoprenoid lipids (weighted average of hopanoids) were both depleted in ¹³C relative to biomass by about 9‰. The large depletion in the isoprenoid lipids seems to indicate that isotopic fractionations associated with the biosynthesis of methylerythritol phosphate (MEP) affected at least two carbon positions in each isoprene unit. Among the fatty acids, trans-9-hexadecenoic acid was most depleted (13.0‰ relative to biomass), followed by cis-9- hexadecenoic acid (9.6‰) and hexadecanoic acid (6.9‰). Isotopic relationships between the three acids suggest that significant isotope effects were associated with the desaturation and cis to trans isomerization of fatty acids. Given these observations, hopanoids produced by ammonia-oxidizing bacteria growing in natural waters are likely to be depleted in ¹³C by 26–30‰ relative to dissolved CO₂. Since CO₂ at aquatic oxyclines is often depleted in ¹³C, the range of δ values expected for hopanoids is ca. −34‰ to −55‰. The δ values of geohopanoids observed in numerous studies and attributed to unspecified chemoautotrophs fall within this range.     

Long-term biogeochemical cycling in agroecosystems inferred from C, C and N

We investigated C and N cycling in long-term agroecological experiments initiated over 50 years ago at a cool, semi-arid site on the North American Great Plains. We used isotopes at natural abundance to trace C and N exchange between soils and plants in contrasting cropping systems. Both ¹³C and ¹⁴C indicated that the soil organic matter was isotopically distinct from current plant inputs, suggesting that recently added plant C was cycling independently of much of the soil C pool. For tracing recent C flows, bomb-¹⁴C was more sensitive than ¹³C, and increased more in high – than in low – yielding systems. Analysis of ¹⁵N in plant tissues, as an index of ¹⁵N in actively cycling soil N, suggested that biological and industrial N fixation both tended to decrease plant ¹⁵N, whereas livestock manure addition increased ¹⁵N abundance. Collectively, the data suggest that soil organic matter is kinetically heterogeneous, so that a majority of soil C and N inputs and outputs exchange with only the small pool of soil organic matter that is actively cycling. Consequently, recently photosynthesized C and deposited N may not readily enter the old, stable fractions of soil organic matter. Practices to retain CO₂ from the atmosphere and prevent leakage of reactive N to non-agricultural systems should therefore focus on management of this active pool.     

Holocene hydrological reconstructions from stable isotopes and paleolimnology, Cordillera Real, Bolivia

Multiproxy analyses of sediment cores from Lago Taypi Chaka Kkota (LTCK) Cordillera Real, Bolivia, provide a record of drier conditions following late Pleistocene deglaciation culminating in pronounced aridity between 6.2 and 2.3 ka B.P. Today LTCK is a glacial-fed lake that is relatively insensitive to changes in P–E because it is largely buffered from dry season draw-down through the year-round supply of glacial meltwater. This was not the case during the middle to late Holocene when glaciers were absent from the watershed. Lake-water δ¹⁸O values inferred from δ¹⁸O analysis of sediment cellulose range from −12.9 to −5.3‰ and average −8.7‰ between 6.2 and 2.3 ka B.P. Modern lake-water δ¹⁸O from LTCK averages −14.8‰ which is compatible with the δ¹⁸O_lw value of −14.3‰ for the surface sediment cellulose. Analyses of δ¹⁸O from modern surface waters in 23 lakes that span the range from glacial-fed to closed basin vary from −16.6 to −2.5‰. This approximates the magnitude of the down-core shift in δ¹⁸O_lw values in LTCK during the middle to late Holocene from −12.9 to −5.3‰. Strong paleohydrologic change during the middle Holocene is also evident in diatom assemblages that consist of shallow-water, non-glacial periphytic taxa and bulk organic δ¹³C and δ¹⁵N that show increases likely resulting from degradation of lacustrine organic matter periodically exposed to subaerial conditions. Neoglaciation began after 2.3 ka B.P. as indicated by changes in the composition of the sediments, lower δ¹⁸O values, and a return to diatom assemblages characteristic of the glacial sediments that formed during the Late Pleistocene. Collectively, these data indicate that the past 2.3 ka B.P. have been the wettest interval during the Holocene. Millennial-scale shifts in the paleohydrologic record of LTCK during the early to middle Holocene conform to other regional paleoclimatic time-series, including Lake Titicaca and Nevado Sajama, and may be driven by insolation and resultant changes in atmospheric circulation and moisture supply. In contrast, an apparent 1200-year lag in the onset of wetter conditions at LTCK (2.3 ka B.P.) compared to Lake Titicaca (3.5 ka B.P.) provides evidence for variable sub-regional hydrologic response to climate change during the middle to late Holocene.     

Stable isotope evidence for multiple fluid regimes during carbonate cementation of the Upper Tertiary Hazeva Formation, Dead Sea Graben, southern Israel

Stable carbon- and oxygen-isotope compositions of calcite and dolomite cements have been used to understand porewater evolution in the Upper Tertiary Hazeva Formation within the Dead Sea Graben, southern Israel. Sandstone samples were obtained from four boreholes in three tectonic blocks of the graben over depths of 253–6448 m, a variation that largely reflects differential subsidence of individual fault-bounded blocks. Early carbonate cements dominate diagenesis. Calcite occurs at <1600 m, but was replaced by dolomite at greater depths. Dolomite at 1600–2700 m is Fe-poor (<0.8 mol% FeCO₃), and at 4700–6200 m, Fe-rich (0.5–7.2 mol% FeCO₃). Magnesite, anhydrite and halite are the final diagenetic phases. Calcite has positively correlated δ¹⁸O (+21‰ to +25‰) and δ¹³C (−6‰ to −2‰) values that generally decrease with depth. Dolomite has a wider variation in δ¹⁸O (+18‰ to +30‰) and δ¹³C (−8‰ to −1‰) values, which also generally are lower with increasing depth. However, the δ¹³C and δ¹⁸O values of dolomite from the uppermost 400 m of the Hazeva Formation in the Sedom Deep-1 borehole are anomalous in spanning the entire range of stable carbon and oxygen isotopic compositions over this relatively small interval.The decreasing dolomite δ¹³C values likely indicate an increased contribution of carbon from organic sources with increasing depth. Except for the uppermost 400 m, Hazeva Formation dolomite in the Sedom Deep-1 borehole has stable carbon-isotope compositions that imply initial dolomitization at much shallower levels, prior to the preferential subsidence of this tectonic block. The oxygen isotopic compositions of the calcite cement are best explained by equilibration at present burial temperatures (≤55 °C) with porewater of meteoric origin. Its δ¹⁸O values increased from −5‰ at the shallowest depths to 0‰ at 1600 m. The dolomite oxygen isotopic compositions also reflect equilibration at present burial temperatures with porewaters ranging from 0‰ at 1600 m to +7‰ at 3600 m (100 °C). In the deepest fault block (Sedom Deep-1 borehole), however, increasingly Fe-rich dolomite has (re)equilibrated with porewater whose δ¹⁸O values decreased from +9‰ at 4750 m (120 °C) to +1‰ to +2‰ by 6200 m (150 °C).Much of the dolomite likely formed at relatively shallow depths from saline brines derived from precursors to the Dead Sea. These infiltrated the Hazeva Formation, mixing with and largely displacing meteoric water, and dolomitizing calcite. Rock–water ratios tended to be high during these processes. However, the upper 400 m of the Hazeva Formation in the deepest fault block were likely deposited during its rapid tectonic subsidence, and largely escaped the initial style of dolomitization pervasive elsewhere in the study area. These sediments were also capped by evaporites. This relatively thin interval likely became a preferential conduit for brines that escaped underlying and overlying strata, including the Fe-rich, lower ¹⁸O fluids (evolved seawater?) present in the deepest part of the graben. These rocks present the most promising target for the passage and accumulation of hydrocarbons in the study area.     

A review of the geochemistry of methane in natural gas hydrate

The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO₂ from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ¹³C_PDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ¹³C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd