13C Depletion Research Articles

Enhanced production of secondary biogenic coalbed natural gas from a subbituminous coal treated by hydrogen peroxide and its geochemical and microbiological analyses

Unmineable coal accounts for over 90% of the world’s fossil fuel resources. Fortunately, many coal seams contain indigenous microorganisms capable of utilizing the coal as a carbon source to produce secondary biogenic coalbed natural gas. However, coal bioavailability has been shown to be a significant factor that limits the extent of bioconversion. In this study, we have analyzed the rate and yield of the biogas production and assessed the gas potential, carbon balance, stable carbon isotopes, microbial communities, and microbial pathways changes resulting from the hydrogen peroxide pretreatment with the Wyoming’s Powder River Basin subbituminous coal. The results showed that coal pretreated with hydrogen peroxide can significantly enhance the bioavailability of coal for enhancing the biogas production, with a peak yield of 552.6 μmol/g coal (437.1 Scf/ton coal) at the day 184. The stable carbon isotopic analysis indicated that the δ13C values of the methane and carbon dioxide were much less negative than the published field data. This suggested that the enrichment or depletion of the precursor 13C could contribute to the shift of the carbon isotopic composition in the subsequence processes. Therefore, the data should be used with cautions for interpreting genesis of the thermogenic/biogenic methane and the methanogenic pathways. The methanogenic pathways were also investigated with re-fed experiment and microbial community analysis. The results indicated that the hydrogenotrophic pathway was not active in the original inoculum became activated after the gas production. The microbial community analysis demonstrated that the obligate hydrogenotrophic methanobacterium was the most dominant methanogens in the microcosms after the gas production. This suggested that the chemical treatment of coal has impacts on the microbial structure during the subsequent methane production phase.

The geochemistry of modern calcareous barnacle shells and applications for palaeoenvironmental studies

Thoracican barnacles of the Superorder Thoracicalcarea Gale, 2016 are sessile calcifiers which are ubiquitous in the intertidal zone and present from very shallow to the deepest marine environments; they also live as epiplankton on animals and detritus. The geochemical composition of their shell calcite has been shown to yield information about environmental conditions, but comprehensive analyses of barnacle shell geochemistry are so far lacking.Here, a dataset is reported for Mg/Ca, Sr/Ca, Mn/Ca, Fe/Ca, as well as carbon and oxygen isotope ratios for 42 species from the Balaniformes, Verruciformes, Scalpelliformes and Lepadiformes. Barnacles predominantly form low-Mg-calcite with very high Sr/Ca ratios averaging 4.2 mmol/mol. The Mn/Ca and Fe/Ca ratios in shell plates are variable and can exceed >4 mmol/mol in barnacles that are attached to manmade structures or live close to (anthropogenic) sources of Mn and Fe. No strong phylogenetic control on the average element/Ca ratios is observed in barnacles. The Balaniformes show a ca. 40% enrichment of Mg in their scuta and terga as compared to other shell plates—a pattern which is not seen in other barnacles. The combination of low to medium Mg/Ca ratios and high Sr/Ca ratios is rare for marine biogenic calcite. Barnacles may thus become important for robustly reconstructing past seawater composition, if this signature is also present in fossil barnacle calcite and can be used alongside other fossil taxa with different Sr incorporation behaviour.Carbon and oxygen isotope data support the view that the oxygen isotope thermometer for barnacles is robust and that most barnacle species form their calcite near isotopic equilibrium with ambient water. The Lepadiformes, however, show a tendency for strong co-variation of δ13C with δ18O values and depletion in 13C and 18O which is attributed to isotopic disequilibrium during shell secretion.Strong systematic fluctuations in Mg/Ca ratios over length scales of ca. 5–15 µm are exhibited by the scalpelliform species Capitulum mitella, the only studied species which consistently forms high Mg calcite, and are tentatively linked to tidal control on the shell secretion pattern. Cathodoluminescence images for this species suggest that additionally a seasonal pattern of Mn distribution in its shell plates is recorded, pointing to a potential use for reconstruction of seasonal changes in terrestrial element supply.

Compositional fractionation of petroleum from reservoir to wellhead in the Niobrara shale oil play

Hydraulic fracturing is routinely used to stimulate unconventional reservoirs. In the Niobrara shale oil play there are several fracking targets, but the actual stimulated rock volume is not well constrained. This means that allocating produced oil to specific reservoirs, in essence oil-reservoir correlation, is not straightforward. The data presented here support that the chalk units are the principal production zones in the Niobrara shale oil play. Several lines of evidence, such as the presence of heavy-end n-alkanes (>n-C31), consistence in composition of gross fractions, the distribution of C27-28-29 diasteranes, as well as the pristane to phytane ratio, all suggest that oils were produced from the A- & B- chalks, particularly from the A-Chalk. During production, a selective partitioning of lighter hydrocarbons into the migrating fluids could nevertheless occur, in particular as regards the n-alkanes domain and their carbon isotopic compositions. Regarding the latter, a depletion of 13C in the produced oil's n-alkanes by more than three per mil relative to all of those core extracts was observed. This finding suggests that compound specific isotope analysis in both oil-reservoir and oil-source rock correlations cannot be taken at face value, i.e. by simple matching of values. Besides that, fractionation appears to result in the enrichment of rather aliphatic nitrogen-containing compounds in crude oils.

Depletion of 13C in residual ethane and propane during thermal decomposition in sedimentary basins

Explanations for the thermal decomposition of ethane and propane in sedimentary basins have long been conceptual without quantitative evidence. This work bridges the gap between 13C depletion and the conversion of ethane and propane, and confirms their thermal decomposition during the late catagenesis stage. Isotopic fractionation was rigorously investigated in a partly reversible reaction scheme for ethane and propane decompositions. Analytical solutions under isothermal conditions were derived. Non-monotonic isotopic fractionation with respect to conversion is demonstrated, which may be ignored if a steady-state approximation is applied. The results imply that depletion of 13C in ethane and propane as their molar fractions decrease (isotopic inverse with respect to thermal maturity) results from the reversible conversion from alkane to alkyl groups. Kinetic parameters were obtained for ethane and propane decomposition in sedimentary basins, and the chemical kinetic method applies to different isotopic fractionation systems. The results help to better understand the chemical form of deep carbon and the thermal conversion of gaseous hydrocarbons in the geochemical carbon cycle.

Influence of Hydrodynamic Processes on the Fate of Sedimentary Organic Matter on Continental Margins

AbstractUnderstanding the effects of hydrodynamic forcing on organic matter (OM) composition is important for assessment of organic carbon (OC) burial in marginal seas on regional and global scales. Here we examine the relationships between regional oceanographic conditions (bottom shear stress), and the physical characteristics (mineral surface area and grain size) and geochemical properties (OC content [OC%] and carbon isotope compositions [13C, 14C]) of a large suite of surface sediments from the Chinese marginal seas to assess the influence of hydrodynamic processes on the fate of OM on shallow continental shelves. Our results suggest that 14C content is primarily controlled by organo‐mineral interactions and hydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (and bottom shear stress). Hydrodynamic processes influence 14C content due to both OC aging during lateral transport and accompanying selective degradation of OM associated with sediment (re) mobilization, these effects being superimposed on the original 14C characteristics of carbon source. Our observations support the hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev‐marine‐120709‐142717) and Leithold et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sediment translocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin settings than on active margins. On a global scale, this may influence the extent to which continental margins act as net carbon sources and sinks. Our findings thus suggest that hydrodynamic processes are important in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas.

Methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the Mid-Okinawa Trough

Abstract Widely distributed cold seeps are crucial sources of carbon to the seawater, whereas anaerobic oxidation of methane (AOM) and precipitation of authigenic carbonate might change the compositions of the methane-rich fluids and thus reduce the outputs of dissolved carbon to seawater. In this work, we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the Mid-Okinawa Trough. For the northern research area, active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes (sites C01 and C10), and strong emissions of gas and fluids may occur in the central mud volcanoes. In a submarine canyon at the south, C23 site exhibits rarely methane seepage, while high rate transportation of methane from deep sediments and associated AOM are identified at site C25 where small vertical faults and gas-bearing layers are developed at the surrounding. AOM at site C25 is indicated by (1) quasi-linearly decrease of sulfate concentrations with depths, (2) intense increases of dissolved inorganic carbon (DIC) concentrations and (3) significantly negative δ13C values (vary from −7.2 to −26.3‰) of DIC. Organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of NH4+, as well as negligible sulfate consumption at three of the four sites. Our data reveal that the cold systems are one of the DIC sources (the fluxes are ca. 11.2 mmol/m2/yr) to the seawater in the Okinawa Trough, which can be tracked by high depletion of 13C (δ13CDIC ranges from −7.2 to −26.3‰ VPDB).

Large carbon isotope variability during methanogenesis under alkaline conditions

High carbon isotope values (δ13CCH4 > −40‰) have widely been used as evidence that methane in alkaline rock-hosted fluids was formed abiotically, particularly in serpentinizing systems. However, isotope fractionation during microbial methanogenesis is relatively understudied at high pH. We isolated a hydrogenotrophic Methanobacterium sp. from hyperalkaline subsurface fluids in the Samail ophiolite to assess how carbon and hydrogen isotope values of CH4 varied depending upon pH and carbonate mineral source (NaHCO3 or CaCO3). The hydrogen isotope fractionation αH20/CH4 (1.46–1.66) did not vary across pH. In contrast, the expressed carbon isotope fractionation, αCO2/CH4, ranged from 1.028 to 1.089. Carbon isotope fractionation increased with pH, reaching a maximum 13C depletion of −85‰. However, the 13C depletion significantly diminished at pH ≥ 9 for CaCO3-amended experiments, generating δ13CCH4 as high as −28‰. To evaluate the large variability in δ13CCH4, we developed a steady-state model to assess how the rates of carbonate dissolution, cellular uptake of CO2 and irreversible CH4 production can affect the net isotope fractionation during methanogenesis. Methanobacterium sp. can produce highly depleted δ13CCH4 in simulated alkaline serpentinizing fluids when dissolved inorganic carbon levels are high and methanogenesis rates are slow. However, small carbon isotope fractionation occurs when rates of carbonate dissolution are slower than cellular uptake, leading to relatively high δ13CCH4 values (>∼−35‰) that are traditionally interpreted to be purely “abiotic”. Thus, microbial CH4 can be produced in carbon-limited mafic and ultramafic rock-hosted environments on Earth and potentially other planetary bodies, but it may be difficult to isotopically identify biogenic methane when mineral carbonates are the dominant carbon source.

Petrographical and organic geochemical study of the lignite from the Smederevsko Pomoravlje field (Kostolac Basin, Serbia)

Three Upper Miocene (Pontian) lignite seams are present in the Smederevsko Pomoravlje field (Kostolac Basin, Serbia). The origin of their organic matter (OM), the characteristics of the depositional environment and certain utilisation properties have been evaluated based on petrographic data, bulk organic geochemical parameters, biomarker patterns and their isotope signatures. Moreover, results of isotopic analysis were used for the investigation of the influence of diagenetic aromatisation on δ13C signatures of biomarkers.The studied lignites are typical humic coals. The OM of lignites is derived from woody vegetation and herbaceous peat-forming plants, with a strong prevalence of the former. The peat-forming vegetation is dominated by decay resistant conifers, including gymnosperm families Cupressaceae, Taxodiaceae, and Pinaceae. Angiosperms occurred in lower amounts. Minor contribution of ferns, fungi and emergent aquatic macrophyta to the biomass is also evident. Chemoautotrophic- and heterotrophic bacteria played an import role during diagenesis. Diagenetic alterations, associated with change in the number of carbon atoms, influence δ13C ratios. Diagenetic aromatisation of di- and non-hopanoid triterpenoids is accompanied with 13C depletion, whereas aromatisation of hopanoids displays the opposite trend.Peatification proceeded in a fresh water environment under variable, anoxic to slightly oxic redox conditions. The lowermost coal seam III accumulated in a topogenous fresh water peat mire with open water areas, which changed occasionally into a wet forest swamp. This resulted in the deposition of mineral-rich coal. The characteristics of lignite in coal seam II are similar to those of coal seam III. This is supported also by generally similar δ13C values of individual biomarkers. Coal seam I is dominated by xylite-rich coal, formed under mesotrophic to ombrotrophic conditions. Rapid flooding of the bogs stopped peat growth in all three coal seams. The ratios of ring-A-degraded and non-degraded aromatic diterpenoids and non-hopanoid triterpenoids, proposed in this study, as well as degree of aromatisation of these biomarkers, reflect changes in the water table.Calorific values of the samples indicate that they meet basic requirements for utilisation in the thermal power plants. None of the lignite samples is suitable for coal briquetting, whereas, based on petrographic data, lignite from coal seam I possesses certain potential for fluidized bed gasification.

Deep K-band Observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, Nondetection of HC11N, and a Search for New Organic Molecules

Abstract The 100 m Robert C. Byrd Green Bank Telescope K-band (KFPA) receiver was used to perform a high-sensitivity search for rotational emission lines from complex organic molecules in the cold interstellar medium toward TMC-1 (cyanopolyyne peak), focussing on the identification of new carbon-chain-bearing species as well as molecules of possible prebiotic relevance. We report a detection of the carbon-chain oxide species HC7O and derive a column density of cm−2. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C7H−, and/or reaction of C6H2 + with CO (followed by dissociative electron recombination). Upper limits are given for the related HC6O, C6O, and C7O molecules. In addition, we obtained the first detections of emission from individual 13C isotopologues of HC7N, and derive abundance ratios HC7N/HCCC13CCCCN = 110 ± 16 and HC7N/HCCCC13CCCN = 96 ± 11, indicative of significant 13C depletion in this species relative to the local interstellar elemental 12C/13C ratio of 60–70. The observed spectral region covered two transitions of HC11N, but emission from this species was not detected, and the corresponding column density upper limit is (at 95% confidence). This is significantly lower than the value of previously claimed by Bell et al. and confirms the recent nondetection of HC11N in TMC-1 by Loomis et al. Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline, and pyrimidine.

Changes in CO2 Emission Sources in Mexico City Metropolitan Area Deduced from Radiocarbon Concentrations in Tree Rings

AbstractWe present radiocarbon (14C) in tree rings from Mexico City and a reconstruction of fossil CO2 concentrations for the last five decades, as part of a research program to understand the 14C dynamics in this complex urban area. Background values were established by 14C concentrations in tree rings from a nearby clean area. Agreement between background and NH-zone 2 values indicate Taxodium mucronatum is a good biomonitor for annual atmospheric 14C variations. Values for the urban tree rings were significantly lower than background values, indicating a 14C depletion from fossil CO2 emissions. There is an increasing trend of fossil CO2 between 1960 and 1990, in agreement with the population growth and the increasing demand for fossil fuels in Mexico City. Between 1990 and 2000, there is an apparent decrease in fossil CO2 concentration, increasing again after 2000. The decrease in 2000, despite being of the same magnitude as the overall uncertainty, may reflect environmental policies that improved the energy efficiency and reduced CO2 emissions in the area. The increase in fossil CO2 concentration between 2000 and 2010 may be attributable to the significant growth of motor vehicle usage in Mexico City, which made transportation the main energy-demanding and -emitting sector.

Insights into the role of methionine synthase in the universal 13C depletion in O- and N-methyl groups of natural products

Many O-methyl and N-methyl groups in natural products are depleted in 13C relative to the rest of the molecule. These methyl groups are derived from the C-1 tetrahydrofolate pool via l-methionine, the principle donor of methyl units. Depletion could occur at a number of steps in the pathway. We have tested the hypothesis that methionine biosynthesis is implicated in this depletion by using a combined experimental and theoretical approach. By using isotope ratio monitoring 13C NMR spectrometry to measure the position-specific distribution of 13C within l-methionine of natural origin, it is shown that the S-methyl group is depleted in 13C by ∼20‰ relative to the other positions in the molecule. In parallel, we have conducted a basic theoretical analysis of the reaction pathway of methionine synthase to assess whether the enzyme cobalamin-independent l-methionine synthase (EC 2.1.1.14)—that catalyzes the synthesis of l-methionine from 5-methyl-tetrahydrofolate and homocysteine—plays a role in causing this depletion. Calculation predicts a strong normal 13C kinetic isotope effect (1.087) associated with this enzyme. Hence, depletion in 13C in the S-methyl of l-methionine during biosynthesis can be identified as an important factor contributing to the general depletion seen in many O-methyl and N-methyl groups of natural products.

Carbon isotope analysis and a solid state 13C NMR study of Mongolian lignite: Changes in stable carbon isotopic composition during diagenesis

The results of a combined carbon isotope analysis and NMR study of a set of lignite samples from Mongolia show that a correlation exists between δ13C value and the ratios of oxygenated carbon (O-aliphatic, carboxyl/carbonyl and O-aromatic carbons) over protonated and carbon substituted aliphatic and aromatic carbons. The results indicate that the δ13C values of the lignite are controlled by two mechanisms: (i) Depletion in 13C as a result of loss of isotopically enriched O-bonded carbons and (ii) enrichment in 13C caused by a loss of isotopically depleted CH4 from aliphatic and aromatic carbons. At the rank of lignite, coal is enriched in 13C because the amount of isotopically enriched CO2 and CO, released from coal as a result of change in chemical structure, is less than that of isotopically depleted CH4.

Exploiting mycorrhizas in broad daylight: Partial mycoheterotrophy is a common nutritional strategy in meadow orchids

Abstract Partial mycoheterotrophy (PMH) is a nutritional mode in which plants utilize organic matter, i.e. carbon, both from photosynthesis and a fungal source. The latter reverses the direction of plant‐to‐fungus carbon flow as usually assumed in mycorrhizal mutualisms. Based on significant enrichment in the heavy isotope 13C, a growing number of PMH orchid species have been identified. These PMH orchids are mostly associated with fungi simultaneously forming ectomycorrhizas with forest trees. In contrast, the much more common orchids that associate with rhizoctonia fungi, which are decomposers, have stable isotope profiles most often characterized by high 15N enrichment and high nitrogen concentrations but either an insignificant 13C enrichment or depletion relative to autotrophic plants. Using hydrogen stable isotope abundances recent investigations showed PMH in rhizoctonia‐associated orchids growing under light‐limited conditions. Hydrogen isotope abundances can be used as substitute for carbon isotope abundances in cases where autotrophic and heterotrophic carbon sources are insufficiently distinctive to indicate PMH. To determine whether rhizoctonia‐associated orchids growing in habitats with high irradiance feature PMH as a nutritional mode, we sampled 13 orchid species growing in montane meadows, four forest orchid species and 34 autotrophic reference species. We analysed δ2H, δ13C, δ15N and δ18O and determined nitrogen concentrations. Orchid mycorrhizal fungi were identified by DNA sequencing. As expected, we found high enrichments in 2H, 13C, 15N and nitrogen concentrations in the ectomycorrhiza‐associated forest orchids, and the rhizoctonia‐associated Neottia cordata from a forest site was identified as PMH. Most orchids inhabiting sunny meadows lacked 13C enrichment or were even significantly depleted in 13C relative to autotrophic references. However, we infer PMH for the majority of these meadow orchids due to both significant 2H and 15N enrichment and high nitrogen concentrations. Pseudorchis albida was the sole autotrophic orchid in this study as it exhibited neither enrichment in any isotope nor a distinctive leaf nitrogen concentration. Synthesis. Our findings demonstrate that partial mycoheterotrophy is a trophic continuum between the extreme endpoints of autotrophy and full mycoheterotrophy, ranging from marginal to pronounced. In rhizoctonia‐associated orchids, partial mycoheterotrophy plays a far greater role than previously assumed, even in full light conditions.

Foliar water use efficiency of Platycladus orientalis sapling under different soil water contents

The determination of plant foliar water use efficiency will be of great value to improve our understanding about mechanism of plant water consumption and provide important basis of regional forest ecosystem management and maintenance, thus, laboratory controlled experiments were carried out to obtain Platycladus orientalis sapling foliar water use efficiency under five different soil water contents, including instantaneous water use efficiency (WUEgs) derived from gas exchange and short-term water use efficiency (WUEcp) caculated using carbon isotope model. The results showed that, controlled by stomatal conductance (gs), foliar net photosynthesis rate (Pn) and transpiration rate (Tr) increased as soil water content increased, which both reached maximum va-lues at soil water content of 70%-80% field capacity (FC), while WUEgs reached a maximum of 7.26 mmol·m-2·s-1 at the lowest soil water content (35%-45% FC). Both δ13C of water-soluble leaf and twig phloem material achieved maximum values at the lowest soil water content (35%-45% FC). Besides, δ13C values of leaf water-soluble compounds were significantly greater than that of phloem exudates, indicating that there was depletion in 13C in twig phloem compared with leaf water-soluble compounds and no obvious fractionation in the process of water-soluble material transportation from leaf to twig. Foliar WUEcp also reached a maximum of 7.26 mmol·m-2·s-1 at the lowest soil water content (35%-45% FC). There was some difference between foliar WUEgs and WUEcp under the same condition, and the average difference was 0.52 mmol·m-2·s-1. The WUEgs had great space-time variability, by contrast, WUEcp was more representative. It was concluded that P. orientalis sapling adapted to drought condition by increasing water use efficiency and decreasing physiological activity.

Exploring potential pine litter biodegradability as a natural tool for low-carbon forestry

Litter decomposition has a remarkable role in the terrestrial C cycle not only for being an important C source of CO2 effluxes but also for leading soil organic matter (OM) formation with remarkable influences on sustainability of forest ecosystems, either broadleaf or coniferous. Thus, in order to evaluate the decomposability of litter from humid-temperate coniferous forests, two of the most widespread types of pine forests in the NW of Spain were selected (Pinus radiata D. Don, Pinus pinaster Ait.). In addition to litter characterization and solid state 13C NMR chemical structural identification, potential biodegradability and isotopic dynamics during the decomposition of litter from these two types of pine plantations were determined by using long-term controlled laboratory incubations (≈1year) under optimal environmental conditions for microbial growth (28°C and 80% moisture content). For both pine species, CO2 fluxes released during the experimental decomposition process fitted a common litter biodegradability model that considers two OM pools (labile and recalcitrant). The labile C pool represented 7.9% or 8.6% of the litter total C content in radiata or maritime pine forests, respectively, with the recalcitrant fraction showing averaged values close to 92% of the litter C. This recalcitrant pool had instantaneous mineralization rates between 80 and 170 times lower than the corresponding labile fraction. Radiata pine litter presented significantly higher instantaneous mineralization rates of the labile C pool compared with maritime litter (possibly related with their interspecific compositional differences, mean C/N ratio of 48 for radiata and 70 for maritime pine litter), which resulted in lower residence times for the former than the latter with an average half-life of 18days for radiata litter and of 34days for maritime litter. Multilinear regression modelling by combining the C mineralization indices and the biochemical characteristics of the litter as predictor variables of its potential biodegradability pointed to the labile C pool as the main driver of the whole litter decay process, since this parameter alone explained more than the 60% of the total variance. For both species, a brief 13C depletion was detected at the beginning of litter incubation and a negative significant correlation between litter 13C contents and early C fluxes was observed. Despite the already mentioned differences between radiata and maritime pine litter layers, the overall general findings on potential biodegradability suggest a similar short-to-medium term behaviour of the soil organic inputs for both types of forest ecosystems under the envisaged global warming challenge.

Experimental and Theoretical Evidence for Surface-Induced Carbon and Nitrogen Fractionation during Diamond Crystallization at High Temperatures and High Pressures

Isotopic and trace element variations within single diamond crystals are widely known from both natural stones and synthetic crystals. A number of processes can produce variations in carbon isotope composition and nitrogen abundance in the course of diamond crystallization. Here, we present evidence of carbon and nitrogen fractionation related to the growing surfaces of a diamond. We document that difference in the carbon isotope composition between cubic and octahedral growth sectors is solvent-dependent and varies from 0.7‰ in a carbonate system to 0.4‰ in a metal-carbon system. Ab initio calculations suggest up to 4‰ instantaneous 13C depletion of cubic faces in comparison to octahedral faces when grown simultaneously. Cubic growth sectors always have lower nitrogen abundance in comparison to octahedral sectors within synthetic diamond crystals in both carbonate and metal-carbon systems. The stability of any particular growth faces of a diamond crystal depends upon the degree of carbon association in the solution. Octahedron is the dominant form in a high-associated solution while the cube is the dominant form in a low-associated solution. Fine-scale data from natural crystals potentially can provide information on the form of carbon, which was present in the growth media.

Geochemistry of the brachiopod Hemithiris psittacea from the Canadian Arctic: Implications for high latitude palaeoclimate studies

Fossil shell material from high palaeo-latitudes is increasingly studied for environmental forcings specific to these latitudes and as sensitive archives of climate change. However, more detailed studies of modern analogues are necessary for putting findings into global and high latitude perspectives.Here we present δ13C and δ18O values, as well as Mg, Sr, Na, Mn and Fe contents for the rhynchonellide brachiopod Hemithiris psittacea from Friday Harbor, U.S.A. and twenty-five sites from Canada ranging in latitude from 48° to 71°N. Median Mn/Ca ratios are below 0.1mmol/mol and Fe/Ca ratios are below 0.3mmol/mol, with no strong correlation between ratios and no apparent geographical relationship. Mg/Ca and Sr/Ca ratios vary within relatively narrow limits and yield median ratios of 3.7mmol/mol and 1.13mmol/mol. Na/Ca ratios vary widely amongst the sites with median values ranging from 9 to 20mmol/mol. Relatively uniform median δ13C values of +1.8±0.6‰ (2 sd, n=25) are observed for the Canadian sites, whereas at Friday Harbor the signature of 13C depletion in local DIC is imprinted in the shell calcite. Median δ18O values of −0.5 to +2.7‰ are more variable than carbon isotope signatures and point to precipitation of shell material in seawater diluted by variable amounts of freshwater and affected by seasonal sea ice.Our results imply that robust elemental patterns and carbon isotope signatures of high latitude brachiopod fossils can be expected for most palaeogeographic situations. In contrast, δ18O signatures of shell material from high latitude marginal seas, particularly during glacial periods, would yield unreasonably high seawater temperature estimates unless the oxygen isotopic composition of the ambient water is accurately accounted for in palaeotemperature calculations.

Mixotrophy in Pyroleae (Ericaceae) from Estonian boreal forests does not vary with light or tissue age.

In temperate forests, some green plants, namely pyroloids (Pyroleae, Ericaceae) and some orchids, independently evolved a mode of nutrition mixing photosynthates and carbon gained from their mycorrhizal fungi (mixotrophy). Fungal carbon is more enriched in 13C than photosynthates, allowing estimation of the proportion of carbon acquired heterotrophically from fungi in plant biomass. Based on 13C enrichment, mixotrophic orchids have previously been shown to increase shoot autotrophy level over the growth season and with environmental light availability. But little is known about the plasticity of use of photosynthetic versus fungal carbon in pyroloids. Plasticity of mixotrophy with leaf age or light level (estimated from canopy openness) was investigated in pyroloids from three Estonian boreal forests. Bulk leaf 13C enrichment of five pyroloid species was compared with that of control autotrophic plants along temporal series (over one growth season) and environmental light gradients (n=405 samples). Mixotrophic 13C enrichment was detected at studied sites for Pyrola chlorantha and Orthilia secunda (except at one site for the latter), but not for Chimaphila umbellata, Pyrola rotundifolia and Moneses uniflora. Enrichment with 13C did not vary over the growth season or between leaves from current and previous years. Finally, although one co-occurring mixotrophic orchid showed 13C depletion with increasing light availability, as expected for mixotrophs, all pyroloids responded identically to autotrophic control plants along light gradients. A phylogenetic trend previously observed is further supported: mixotrophy is rarely supported by 13C enrichment in the Chimaphila + Moneses clade, whereas it is frequent in the Pyrola + Orthilia clade. Moreover, pyroloid mixotrophy does not respond plastically to ageing or to light level. This contrasts with the usual view of a convergent evolution with orchids, and casts doubt on the way pyroloids use the carbon gained from their mycorrhizal fungi, especially to replace photosynthetic carbon.

Soil Aggregate Dynamics Influenced by Biochar Addition using the 13C Natural Abundance Method

Understanding the mechanisms of biochar C sequestration within soil aggregates requires us to investigate the role of biochar in the soil aggregation process. We aimed to compare aggregate dynamics in soils amended with organic matter or with biochar and to quantify the amount of biochar and organic matter included in different sizes of aggregate. In free‐draining incubation jars, soils collected from grassland and rice (Oryza sativa L.) paddy were amended with fresh corn stalks (Zea mays L) and leaves (CORN_RAW) and with biochar made from corn stalks and leaves (CORN_CHAR). Temporal change in the aggregates were investigated. Soils were fractionated into macroaggregates (250–1000 μm), microaggregates (53–250 μm), and organo‐mineral complexes (<53 μm). The added materials were measured using the difference in the relative depletion of 13C (δ13C) values between the soil and the amendments. Although the macroaggregates in the CORN_RAW treatment increased throughout the incubation period, those in the CORN_CHAR treatment only increased during the initial 60 d. This indicates that macroaggregates in biochar‐amended soil might not be stable enough. However, microaggregates in the CORN_CHAR treatment showed an increase of >200% throughout the incubation period, probably as a result of the breakdown of macroaggregates and formation through chemical adsorptive interactions between biochar surface and soil particles. The proportion of biochar‐derived C within aggregates was greatly increased by 204% on average from the initial to later periods, indicating that biochar C is preferentially enriched because of its chemical recalcitrance and physical occlusion within the aggregates. The overall results imply that biochar amendment could initiate macroaggregation; however, the temporal change pattern was different. Biochar C could be both chemically and physically protected from decomposition during long‐term sequestration.

Carbon isotope biogeochemistry of acetate in sub-seafloor sediments in the Sea of Okhotsk near Sakhalin Island, Russia

Biogeochemical processes involving acetate in sub-seafl oor sediments were investigated by examining the stable carbon isotopic relationships of acetate and other relevant carbon-bearing materials (i.e., total organic carbon (TOC) and ΣCO2) in fi ve piston cores retrieved from the Sea of Okhotsk off Sakhalin Island, Russia. The cores were classifi ed into two types on the basis of SO42– content: (I) those with sulfate-reducing sediments in which the depletion of pore-water SO42– with increasing depth was slight, and (II) those with methanogenic sediments in which SO42– concentrations were less than 2 mM more than 3 m below the seafl oor. Acetate was detected in all cores. The acetate content in methanogenic sediments (2.6–23.0 μM) was relatively higher than in the sulfate-reducing sediment (2.8–8.8 μM). In the sulfate-reducing sediments, the depth profi les of δ13Cacetate approximately parallel those of δ13CTOC, with the δ13Cacetate values (–39‰ to –33‰) depleted by about 12‰ relative to δ13CTOC (–24.5‰ to –22.3‰). These approximately parallel depth profi les suggest that the principal acetate production process in the sulfate-reducing sediments is fermentation of dissolved organic compounds. The fermentation products, however, tend to be similar or slightly enriched in 13C compared to their substrates. Therefore, the 13C depletion of acetate relative to TOC in the sulfate reduction zone suggests that some portion of the acetate was synthesized by acetogenesis in which the synthesized acetate is depleted in 13C compared with its precursor. Given the large contribution of land-derived organic matter in the studied sediments, organoautotrophic acetogenesis using the lignin-derived syringate monomer, which originates from land plants, is likely. In the methanogenic sediments, the δ13Cacetate values in sediments throughout the cores (–39‰ to –25‰) were depleted compared to δ13CTOC (–25.5‰ to –21.4‰). This suggests some acetogenic contribution to the total acetate production. The depth profi les of δ13Cacetate in methanogenic sediments did not parallel those of either δ13CTOC or δ13CΣCO2, probably because of the mixed isotopic effect from some production and consumption processes; namely fermentation, acetogenesis, and acetoclastic methanogenesis. * Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan ** Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan *** Institute of Low Temperature Science, Hokkaido University, N19W8 Kita-ku, Sapporo, Hokkaido 060-0819, Japan **** Present address: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 4648601, Japan a Corresponding author. Tel: +81358414250. Fax: +81358418791. e-mail: ijiri@eps.s.u-tokyo.ac.jp Present address: Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan