Isotopic biosignatures in carbonate‐rich,cyanobacteria‐dominated microbial mats of the Cariboo Plateau,B.C.

Photosynthetic activity in carbonate‐rich benthic microbial mats located in saline, alkaline lakes on the Cariboo Plateau, B.C. resulted in pCO₂ below equilibrium and δ¹³C_DIC values up to +6.0‰ above predicted carbon dioxide (CO₂) equilibrium values, representing a biosignature of photosynthesis. Mat‐associated δ¹³C_carb values ranged from ~4 to 8‰ within any individual lake, with observations of both enrichments (up to 3.8‰) and depletions (up to 11.6‰) relative to the concurrent dissolved inorganic carbon (DIC). Seasonal and annual variations in δ¹³C values reflected the balance between photosynthetic ¹³C‐enrichment and heterotrophic inputs of ¹³C‐depleted DIC. Mat microelectrode profiles identified oxic zones where δ¹³C_carb was within 0.2‰ of surface DIC overlying anoxic zones associated with sulphate reduction where δ¹³C_carb was depleted by up to 5‰ relative to surface DIC reflecting inputs of ¹³C‐depleted DIC. δ¹³C values of sulphate reducing bacteria biomarker phospholipid fatty acids (PLFA) were depleted relative to the bulk organic matter by ~4‰, consistent with heterotrophic synthesis, while the majority of PLFA had larger offsets consistent with autotrophy. Mean δ¹³C_org values ranged from ?18.7 ± 0.1 to ?25.3 ± 1.0‰ with mean Δ¹³C_inorg‐org values ranging from 21.1 to 24.2‰, consistent with non‐CO₂‐limited photosynthesis, suggesting that Precambrian δ¹³C_org values of ~?26‰ do not necessitate higher atmospheric CO₂ concentrations. Rather, it is likely that the high DIC and carbonate content of these systems provide a non‐limiting carbon source allowing for expression of large photosynthetic offsets, in contrast to the smaller offsets observed in saline, organic‐rich and hot spring microbial mats.     

Environmental niches and metabolic diversity in Neoarchean lakes

The diversification of macro‐organisms over the last 500 million years often coincided with the development of new environmental niches. Microbial diversification over the last 4 billion years likely followed similar patterns. However, linkages between environmental settings and microbial ecology have so far not been described from the ancient rock record. In this study, we investigated carbon, nitrogen, and molybdenum isotopes, and iron speciation in five non‐marine stratigraphic units of the Neoarchean Fortescue Group, Western Australia, that are similar in age (2.78–2.72 Ga) but differ in their hydro‐geologic setting. Our data suggest that the felsic‐dominated and hydrologically open lakes of the Bellary and Hardey formations were probably dominated by methanogenesis (δ¹³C_org = ?38.7 ± 4.2‰) and biologic N₂ fixation (δ¹⁵N_bulk =?0.6 ± 1.0‰), whereas the Mt. Roe, Tumbiana and Kylena Formations, with more mafic siliciclastic sediments, preserve evidence of methanotrophy (δ¹³C_org as low as ?57.4‰, δ¹³C_carb as low as ?9.2‰) and NH₃ loss under alkaline conditions. Evidence of oxygenic photosynthesis is recorded only in the closed evaporitic Tumbiana lakes marked by abundant stromatolites, limited evidence of Fe and S cycling, fractionated Mo isotopes (δ^98/95Mo = +0.4 ± 0.4‰), and the widest range in δ¹³C_org (?57‰ to ?15‰), suggesting oxidative processes and multiple carbon fixation pathways. Methanotrophy in the three mafic settings was probably coupled to a combination of oxidants, including O₂ and SO₄^2‐. Overall, our results may indicate that early microbial evolution on the Precambrian Earth was in part influenced by geological parameters. We speculate that expanding habitats, such as those linked to continental growth, may have been an important factor in the evolution of life.     

Cambrian high-resolution biostratigraphy and carbon isotope chemostratigraphy in Scania, Sweden: first record of the SPICE and DICE excursions in Scandinavia

A core drilling (Andrarum‐3), from the classical locality at Andrarum, Scania, southernmost Sweden, penetrated a 28.90‐m‐thick Cambrian succession. The core comprises dark grey to black, finely laminated mudstones and shales with early concretionary carbonate lenses (stinkstones or orsten) and a few primary carbonate beds. The middle Cambrian (provisional Series 3) part of the core comprises 17.35 m, whereas the Furongian Series (upper Cambrian) part covers the remaining 11.55 m. Nineteen trilobite and two phosphatocopine genera are present in the middle Cambrian, whereas the less diverse Furongian interval yielded four trilobite and three phosphatocopine genera. Other, less frequent, faunal elements include conodonts (s. l.), brachiopods, sponge spicules, bradoriids, and coprolites. Trilobites and phosphatocopines were used to subdivide the core into seven biozones ranging from the Ptychagnostus atavus Zone to the Parabolina spinulosa Zone (P. spinulosa Subzone). Carbon isotopic analyses (δ¹³C_org) through the core show two important excursions, the negative DrumIan Carbon isotope Excursion (DICE) in the Pt. atavus Zone, and the Steptoean Positive Carbon Isotope Excursion (SPICE) beginning near the first appearance of Glyptagnostus reticulatus and extending upward into the Olenus and Agnostus (Homagnostus) obesus Zone. The DICE displays a peak value, in the samples at hand, of –30.45‰δ¹³C_org in the lower part of the P. atavus Zone. The δ¹³C_org values increase through the overlying L. laevigata and A. pisiformis zones and display peak values of c. –28.00‰δ¹³C_org in the lowermost Furongian Olenus wahlenbergi and O. attenuatus subzones. Thereafter the values decrease significantly through the O. scanicus Subzone. Both isotopic excursions have been documented from several palaeocontinents, but never before from Baltica. Moreover, for the first time these excursions are recorded from organic matter in an alum shale setting. The recorded shift of +1.50–2.00‰δ¹³C_org is approximately half the magnitude of the SPICE documented from other regions. This discrepancy may be related to temporal variations in the type, origin, or diagenesis of the organic fraction analysed.     

Carbon isotopic composition of lipid biomarkers from an endoevaporitic gypsum crust microbial mat reveals cycling of mineralized organic carbon

Microbial mats that inhabit gypsum deposits in ponds at Guerrero Negro, Baja California Sur, Mexico, developed distinct pigmented horizons that provided an opportunity to examine the fixation and flow of carbon through a trophic structure and, in conjunction with previous phylogenetic analyses, to assess the diagenetic fates of molecular δ¹³C biosignatures. The δ¹³C values of individual biomarker lipids, total carbon, and total organic carbon (TOC) were determined for each of the following horizons: tan‐orange (TO) at the surface, green (G), purple (P), and olive‐black (OB) at the bottom. δ¹³C of individual fatty acids from intact polar lipids (IPFA) in TO were similar to δ¹³C of dissolved inorganic carbon (DIC) in the overlying water column, indicating limited discrimination by cyanobacteria during CO₂ fixation. δ¹³C_TOC of the underlying G was 3‰ greater than that of TO. The most δ¹³C‐depleted acetogenic lipids in the upper horizons were the cyanobacterial biomarkers C₁₇ n‐alkanes and polyunsaturated fatty acids. Bishomohopanol was 4 to 7‰ enriched, relative to alkanes and intact polar fatty acids (IPFA), respectively. Acyclic C₂₀ isoprenoids were depleted by 14‰ relative to bishomohopanol. Significantly, ?[δ¹³C_TOC ? δ¹³C_∑IPFA] increased from 6.9‰ in TO to 14.7‰ in OB. This major trend might indicate that ¹³C‐enriched residual organic matter accumulated at depth. The permanently anoxic P horizon was dominated by anoxygenic phototrophs and sulfate‐reducing bacteria. P hosted an active sulfur‐dependent microbial community. IPFA and bishomohopanol were ¹³C‐depleted relative to upper crust by 7 and 4‰, respectively, and C₂₀ isoprenoids were somewhat ¹³C‐enriched. Synthesis of alkanes in P was evidenced only by ¹³C‐depleted n‐octadecane and 8‐methylhexadecane. In OB, the marked increase of total inorganic carbon δ¹³C (δ¹³C_TIC) of >6‰ perhaps indicated terminal mineralization. This δ¹³C_TIC increase is consistent with degradation of the osmolyte glycine betaine by methylotrophic methanogens and loss of ¹³C‐depleted methane from the mat.     

Effects of charring on the carbon isotopic composition of grass (Poaceae) epidermis

Charred modern grass epidermis preserves the carbon isotopic composition of the parent plant photosynthetic pathway. Fifty-nine modern grasses and sedges were collected in lowland western Uganda. All charred epidermal samples from C₄ grasses or sedges preserve a carbon isotopic value within the range typical for C₄ plants (−17 to −10‰), and charred epidermal fragments from C₃ plants have carbon isotopic values between −30 and −26‰. The process of charring results in a slightly enriched carbon isotopic signature (−11.9‰ mean charred value as compared to −12.8‰ mean unaltered grass tissue value). δ¹³C measurements of replicate samples from the same plant vary within 1–2‰, yet all values for the same plant stay within the expected values for the photosynthetic pathway of the plant. δ¹³C measurements on >180-μm charred grass epidermal fragments extracted from surface sediment samples from three lakes on the lowland western Ugandan landscape confirm the predominant lowland C₄ grass input (δ¹³C=−16 to −19‰). These results demonstrate the utility of using carbon isotopic analysis of charred grass epidermis to reconstruct C₃ vs. C₄ grassland assemblages on the landscape. Furthermore, such downcore δ¹³C profiles can be used to highlight key zones of C₃ vs. C₄ grass change for which taxonomic analysis of fossil grass epidermis could provide more detailed information regarding grassland community composition.     

Spatial variability in photosynthetic and heterotrophic activity drives localized δ13Corg fluctuations and carbonate precipitation in hypersaline microbial mats

Modern laminated photosynthetic microbial mats are ideal environments to study how microbial activity creates and modifies carbon and sulfur isotopic signatures prior to lithification. Laminated microbial mats from a hypersaline lagoon (Guerrero Negro, Baja California, Mexico) maintained in a flume in a greenhouse at NASA Ames Research Center were sampled for δ¹³C of organic material and carbonate to assess the impact of carbon fixation (e.g., photosynthesis) and decomposition (e.g., bacterial respiration) on δ¹³C signatures. In the photic zone, the δ¹³C_org signature records a complex relationship between the activities of cyanobacteria under variable conditions of CO₂ limitation with a significant contribution from green sulfur bacteria using the reductive TCA cycle for carbon fixation. Carbonate is present in some layers of the mat, associated with high concentrations of bacteriochlorophyll e (characteristic of green sulfur bacteria) and exhibits δ¹³C signatures similar to DIC in the overlying water column (?2.0‰), with small but variable decreases consistent with localized heterotrophic activity from sulfate‐reducing bacteria (SRB). Model results indicate respiration rates in the upper 12 mm of the mat alter in situ pH and concentrations to create both phototrophic CO₂ limitation and carbonate supersaturation, leading to local precipitation of carbonate minerals. The measured activity of SRB with depth suggests they variably contribute to decomposition in the mat dependent on organic substrate concentrations. Millimeter‐scale variability in the δ¹³C_org signature beneath the photic zone in the mat is a result of shifting dominance between cyanobacteria and green sulfur bacteria with the aggregate signature overprinted by heterotrophic reworking by SRB and methanogens. These observations highlight the impact of sedimentary microbial processes on δ¹³C_org signatures; these processes need to be considered when attempting to relate observed isotopic signatures in ancient sedimentary strata to conditions in the overlying water column at the time of deposition and associated inferences about carbon cycling.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Lower–middle Frasnian organic carbon isotope record of conodonts in East European Platform

The early–middle Frasnian boundary interval of the northern part of the East European Platform (northwest of Russia) corresponding to the transitans-punctata isotope event is revealed by biostratigraphically constrained conodont carbon stable isotope data (δ¹³C_con). The dynamics of δ¹³C_con demonstrate a three-fold pattern with positive peaks at the onset of the main phase of the transitans-punctata isotope event (upper part of Montagne Noire 4 conodont Zone, MN4; up to -22.5‰ VPDB), and during the late part of the event (lower and middle parts of MN6 Zone; up to -24.0‰ VPDB and -22.1‰ VPDB). The stratigraphic level near the MN5 and MN6 boundary is marked by a prominent negative excursion in δ¹³C_con (down to -31.8‰ VPDB) that resembles the negative excursion in the terminal phase of the transitans-punctata isotope event worldwide. The δ¹³C_con variations in different taxa are generally consistent but demonstrate some differences in values and amplitudes. It is assumed that variations in the carbon isotopic compositions of conodonts were mainly controlled by changes in the isotope composition of the planktons as the main food source for conodonts.     

Isotopic discrimination and kinetic parameters of RubisCO from the marine bloom‐forming diatom,Skeletonema costatum

The cosmopolitan, bloom‐forming diatom, Skeletonema costatum, is a prominent primary producer in coastal oceans, fixing CO₂ with ribulose 1,5‐bisphosphate carboxylase/oxygenase (RubisCO) that is phylogenetically distinct from terrestrial plant RubisCO. RubisCOs are subdivided into groups based on sequence similarity of their large subunits (IA–ID, II, and III). ID is present in several major oceanic primary producers, including diatoms such as S. costatum, coccolithophores, and some dinoflagellates, and differs substantially in amino acid sequence from the well‐studied IB enzymes present in most cyanobacteria and in green algae and plants. Despite this sequence divergence, and differences in isotopic discrimination apparent in other RubisCO enzymes, stable carbon isotope compositions of diatoms and other marine phytoplankton are generally interpreted assuming enzymatic isotopic discrimination similar to spinach RubisCO (IB). To interpret phytoplankton δ¹³C values, S. costatum RubisCO was characterized via sequence analysis, and measurement of its K_CO2 and V_max, and degree of isotopic discrimination. The sequence of this enzyme placed it among other diatom ID RubisCOs. Michaelis‐Menten parameters were similar to other ID enzymes (K_CO2 = 48.9 ± 2.8 μm ; V_max = 165.1 ± 6.3 nmol min^?1 mg^?1). However, isotopic discrimination (ε = [¹²k/¹³k ? 1] × 1000) was low (18.5‰; 17.0–19.9, 95% CI) when compared to IA and IB RubisCOs (22–29‰), though not as low as ID from coccolithophore, Emiliania huxleyi (11.1‰). Variability in ε‐values among RubisCOs from primary producers is likely reflected in δ¹³C values of oceanic biomass. Currently, δ¹³C variability is ascribed to physical or chemical factors (e.g. illumination, nutrient availability) and physiological responses to these factors (e.g. carbon‐concentrating mechanisms). Estimating the importance of these factors from δ¹³C measurements requires an accurate ε‐value, and a mass‐balance model using the ε‐value for S. costatum RubisCO is presented. Clearly, appropriate ε‐values must be included in interpreting δ¹³C values of environmental samples.     

Uncovering the spatial heterogeneity of Ediacaran carbon cycling

Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to ?12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ¹³C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ¹³C_carb and δ¹³C_org) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ¹³C_org are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ³⁴S of carbonate‐associated sulfate (CAS) and increased ⁸⁷Sr/⁸⁶Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ¹³C_org records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O₂ and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ¹³C presented in this study.     

Nanoscale petrographic and geochemical insights on the origin of the Palaeoproterozoic stromatolitic phosphorites from Aravalli Supergroup,India

Stromatolites composed of apatite occur in post‐Lomagundi–Jatuli successions (late Palaeoproterozoic) and suggest the emergence of novel types of biomineralization at that time. The microscopic and nanoscopic petrology of organic matter in stromatolitic phosphorites might provide insights into the suite of diagenetic processes that formed these types of stromatolites. Correlated geochemical micro‐analyses of the organic matter could also yield molecular, elemental and isotopic compositions and thus insights into the role of specific micro‐organisms among these communities. Here, we report on the occurrence of nanoscopic disseminated organic matter in the Palaeoproterozoic stromatolitic phosphorite from the Aravalli Supergroup of north‐west India. Organic petrography by micro‐Raman and Transmission Electron Microscopy demonstrates syngeneity of the organic matter. Total organic carbon contents of these stromatolitic phosphorite columns are between 0.05 and 3.0 wt% and have a large range of δ¹³C_org values with an average of ?18.5‰ (1σ = 4.5‰). δ¹⁵N values of decarbonated rock powders are between ?1.2 and +2.7‰. These isotopic compositions point to the important role of biological N₂‐fixation and CO₂‐fixation by the pentose phosphate pathway consistent with a population of cyanobacteria. Microscopic spheroidal grains of apatite (MSGA) occur in association with calcite microspar in microbial mats from stromatolite columns and with chert in the core of diagenetic apatite rosettes. Organic matter extracted from the stromatolitic phosphorites contains a range of molecular functional group (e.g. carboxylic acid, alcohol, and aliphatic hydrocarbons) as well as nitrile and nitro groups as determined from C‐ and N‐XANES spectra. The presence of organic nitrogen was independently confirmed by a CN^? peak detected by ToF‐SIMS. Nanoscale petrography and geochemistry allow for a refinement of the formation model for the accretion and phototrophic growth of stromatolites. The original microbial biomass is inferred to have been dominated by cyanobacteria, which might be an important contributor of organic matter in shallow‐marine phosphorites.     

Stable isotope enrichment in laboratory ant colonies: effects of colony age,metamorphosis, diet,and fat storage

Ecologists use stable isotopes to infer diets and trophic levels of animals in food webs, yet some assumptions underlying these inferences have not been thoroughly tested. We used laboratory‐reared colonies of Solenopsis invicta Buren (Hymenoptera: Formicidae: Solenopsidini) to test the effects of metamorphosis, diet, and lipid storage on carbon and nitrogen stable isotope ratios. Effects of metamorphosis were examined in ant colonies maintained on a control diet of domestic crickets and sucrose solution. Effects of a diet shift were evaluated by adding a tuna supplement to select colonies. Effects of lipid content on stable isotopes were tested by treating worker ants with polar and non‐polar solvents. δ¹³C and δ¹⁵N values of larvae, pupae, and workers were measured by mass spectrometry on whole‐animal preparations. We found a significant effect of colony age on δ¹³C, but not δ¹⁵N; larvae, pupae, and workers collected at 75 days were slightly depleted in ¹³C relative to collections at 15 days (Δδ¹³C = ?0.27‰). Metamorphosis had a significant effect on δ¹⁵N, but not δ¹³C; tissues of each successive developmental stage were increasingly enriched in ¹⁵N (pupae, +0.5‰; workers, +1.4‰). Availability of tuna resulted in further shifts of about +0.6‰ in isotope ratios for all developmental stages. Removing fat with organic solvents had no effect on δ¹³C, but treatment with a non‐polar solvent resulted in enriched δ¹⁵N values of +0.37‰. Identifying regular patterns of isotopic enrichment as described here should improve the utility of stable isotopes in diet studies of insects. Our study suggests that researchers using ¹⁵N enrichment to assess trophic levels of an organism at different sites need to take care not to standardize with immature insect herbivores or predators at one site and mature ones at another. Similar problems may also exist when standardizing with holometabolous insects at one site and spiders or hemimetabolous insects at another site.     

Bacterial abundance and composition in marine sediments beneath the Ross Ice Shelf,Antarctica

Marine sediments of the Ross Sea, Antarctica, harbor microbial communities that play a significant role in the decomposition, mineralization, and recycling of organic carbon (OC). In this study, the cell densities within a 153‐cm sediment core from the Ross Sea were estimated based on microbial phospholipid fatty acid (PLFA) concentrations and acridine orange direct cell counts. The resulting densities were as high as 1.7 × 10⁷ cells mL^?1 in the top ten centimeters of sediments. These densities are lower than those calculated for most near‐shore sites but consistent with deep‐sea locations with comparable sedimentation rates. The δ¹³C measurements of PLFAs and sedimentary and dissolved carbon sources, in combination with ribosomal RNA (SSU rRNA) gene pyrosequencing, were used to infer microbial metabolic pathways. The δ¹³C values of dissolved inorganic carbon (DIC) in porewaters ranged downcore from ?2.5‰ to ?3.7‰, while δ¹³C values for the corresponding sedimentary particulate OC (POC) varied from ?26.2‰ to ?23.1‰. The δ¹³C values of PLFAs ranged between ?29‰ and ?35‰ throughout the sediment core, consistent with a microbial community dominated by heterotrophs. The SSU rRNA gene pyrosequencing revealed that members of this microbial community were dominated by β‐, δ‐, and γ‐Proteobacteria, Actinobacteria, Chloroflexi and Bacteroidetes. Among the sequenced organisms, many appear to be related to known heterotrophs that utilize OC sources such as amino acids, oligosaccharides, and lactose, consistent with our interpretation from δ¹³C_PLFA analysis. Integrating phospholipids analyses with porewater chemistry, δ¹³C_DIC and δ¹³C_POC values and SSU rRNA gene sequences provides a more comprehensive understanding of microbial communities and carbon cycling in marine sediments, including those of this unique ice shelf environment.     

Spatial variation in flux, δ<Superscript>13</Superscript>C and δ<Superscript>2</Superscript>H of methane in a small Arctic lake with fringing wetland in western Greenland

Small lakes in northern latitudes represent a significant source of CH₄ to the atmosphere that is predicted to increase with warming in the Arctic. Yet, whole-lake CH₄ budgets are lacking as are measurements of δ¹³C-CH₄ and δ²H-CH₄. In this study, we quantify spatial variability of diffusive and ebullitive fluxes of CH₄ and corresponding δ¹³C-CH₄ and δ²H-CH₄ in a small, Arctic lake system with fringing wetland in southwestern Greenland during summer. Net CH₄ flux was highly variable, ranging from an average flux of 7 mg CH₄ m^?2 d^?1 in the deep-water zone to 154 mg CH₄ m^?2 d^?1 along the lake margin. Diffusive flux accounted for ~8.5 % of mean net CH₄ flux, with plant-mediated and ebullitive flux accounting for the balance of the total net flux. Methane content of emitted ebullition was low (mean ± SD 10 ± 17 %) compared to previous studies from boreal lakes and wetlands. Isotopic composition of net CH₄ emissions varied widely throughout the system, with δ¹³C-CH₄ ranging from ?66.2 to ?55.5 ‰, and δ²H-CH₄ ranging from ?345 to ?258 ‰. Carbon isotope composition of CH₄ in ebullitive flux showed wider variation compared to net flux, ranging from ?69.2 to ?49.2 ‰. Dissolved CH₄ concentrations were highest in the sediment and decreased up the water column. Higher concentrations of CH₄ in the hypoxic deep water coincided with decreasing dissolved O₂ concentrations, while methanotrophic oxidation dominated in the epilimnion based upon decreasing concentrations and increasing values of δ¹³C-CH₄ and δ²H-CH₄. The most depleted ¹³C- and ²H-isotopic values were observed in profundal bottom waters and in subsurface profundal sediments. Based upon paired δ¹³C and δ²H observations of CH₄, acetate fermentation was likely the dominant production pathway throughout the system. However, isotopic ratios of CH₄ in deeper sediments were consistent with mixing/transition between CH₄ production pathways, indicating a higher contribution of the CO₂ reduction pathway. The large spatial variability in fluxes of CH₄ and in isotopic composition of CH₄ throughout a single lake system indicates that the underlying mechanisms controlling CH₄ cycling (production, consumption and transport) are spatially heterogeneous. Net flux along the lake margin dominated whole-lake flux, suggesting the nearshore littoral area dominates CH₄ emissions in these systems. Future studies of whole-lake CH₄ budgets should consider this significant spatial heterogeneity.     

Microbiological and biogeochemical properties of the Caspian Sea sediments and water column

The work presents the results of investigation of microbial and biogeochemical processes at the water-sediment interface in the samples of three Caspian Sea profiles obtained during the 39th cruise of RV “Rift” in May–June 2012. The decrease in suspended C_org content from the surface to the bottom resulted from the activity of aerobic heterotrophic microorganisms. Autotrophic methanogenesis occurred in anoxic water of deep-sea depressions, where methane concentrations were up to 2.2–3.75 μL CH₄ L^?1, which was an order of magnitude higher than in the aerobic water column (0.04–0.32 μL CH₄ L^?1). Methanogenesis was accompanied by a considerable decrease in δ¹³C of suspended C_org (?26 to ?30‰). The numbers of microbial cells in the water column varied from 40 to 3200 × 10³ cells mL^?1. The results of microbiological and biogeochemical investigation demonstrated that, in spite of the absence of connection with the ocean and other specific features, the Caspian Sea has the characteristics of a typical marine basin.     

Trophic structure and major trophic links in conventional versus organic farming systems as indicated by carbon stable isotope ratios of fatty acids

Using bulk tissue and fatty acid ¹³C analysis we investigated major trophic pathways from soil microorganisms to microbial consumers to predators in conventional versus organic farming systems planted for the first time with maize. Organic farming led to an increase in microbial biomass in particular that of fungi as indicated by phospholipid fatty acids (PLFAs). Microbial PLFAs reflected the conversion from C₃ to C₄ plants by a shift in δ¹³C of 2‰, whereas the isotopic signal in fatty acids (FAs) of Collembola was much more pronounced. In the euedaphic Protaphorura fimata the δ¹³C values in maize fields exceeded that in C₃ (soybean) fields by up to 10‰, indicating a close relationship between diet and vegetation cover. In the epedaphic Orchesella villosaδ¹³C values shifted by 4‰, suggesting a wider food spectrum including carbon of former C₃ crop residues. Differences in δ¹³C of corresponding FAs in consumers and resources were assessed to assign food web links. P. fimata was suggested as root and fungal feeder in soybean fields, fungal feeder in conventional and leaf consumer in organically managed maize fields. O. villosa likely fed on root and bacteria under soybean, and bacteria and fungi under maize. Comparison of δ¹³C values in FAs of the cursorial spider Pardosaagrestis and O. villosa implied the latter as important prey species in soybean fields. In contrast, the web‐building spider Mangora acalypha showed no predator–prey relationship with Collembola. The determination of δ¹³C values in trophic biomarker FAs allowed detailed insight into the structure of the decomposer food web and identified diet‐shifts in both consumers at the base of the food web and in top predators in organic versus conventional agricultural systems. The results indicate changes in major trophic links and therefore carbon flux through the food web by conversion of conventional into organic farming systems.     

Acid treatment effects on the stable isotopic signatures of fossils

Abstract: Prior to geochemical analyses, fossil bones and teeth are often extracted from any surrounding lithified sediments using chemical techniques such as immersion in acid. As stable isotope analysis becomes more commonplace in palaeoecological investigations, it is important to consider what effects these chemical preparation techniques may have on any subsequent isotopic data and to constrain these effects as quantitatively as possible. This study aims to elucidate these effects, as it is vital that variability in a data set should not be introduced as a result of protocols used during sample preparation; in addition, it defines the most effective and viable method of carbonate removal for processing bulk fossil samples without causing alteration of their stable isotopic signatures. Various strengths of two weak acids commonly used during palaeontological preparation were tested to evaluate their effects on the δ¹⁵N and δ¹³C_org isotopic signatures of the vertebrae of a large Eocene fossil fish. Changes in the isotopic values occurred over time regardless of which acid was used, each causing a variable response in both δ¹⁵N and δ¹³C_org isotopic values. Without careful monitoring of the acidification process in a controlled environment, any resulting data could therefore confound interpretation. Based on these experiments, it is recommended that 2 m acetic acid be used for the pretreatment of fossils prior to the acquisition of N and C isotope data where carbonate removal is necessary.     

Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of δ¹³C values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (δ¹³C_p). The δ¹³C_p for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be ?23.9 ± 0.4 ‰ as compared to a centric diatom (Porosira glacialis, δ¹³C_p = ?20.0 ± 1.6 ‰) and a marine haptophyte (Emiliana huxleyi, δ¹³C_p = ?22.7 ± 0.5 ‰) at a growth rate (µ) of 0.1 divisions per day (d^?1). δ¹³C_p values increased by ~2.5 ‰ when µ increased from 0.1 to a maximum growth rate of 1.4 d^?1. We compared our estimates of δ¹³C_p values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.     

The Pignola‐Abriola section (southern Apennines,Italy): a new GSSP candidate for the base of the Rhaetian Stage

The base of the Rhaetian stage (Norian/Rhaetian boundary, NRB) is still awaiting formal designation by the International Commission on Stratigraphy. At present, only the 4.30‐m‐thick Steinbergkogel section (Austria) has been proposed as GSSP (Global Stratotype Section and Point) candidate for the base of the Rhaetian. Here we present data from the 63‐m‐thick Pignola‐Abriola section (Southern Apennines, Italy) that we consider an alternative candidate for the Rhaetian GSSP. The Pignola‐Abriola basinal section, represented by hemipelagic–pelagic carbonate successions belonging to the Lagonegro Basin, matches all the requirements for a GSSP: 1, it is well exposed with minimal structural deformation; 2, it is rich in age diagnostic fossils (e.g. conodonts and radiolarians); 3, it yields a geochemical record suitable for correlation (e.g. δ¹³C_org/carb); and 4, it has a robust magnetostratigraphy and is correlated with the Newark APTS for age approximation of the NRB and additional Rhaetian bioevents. In the Pignola‐Abriola section, we opt to place the NRB at the 44.4 metre level, coincident with a prominent negative shift of ca. 6‰ of the δ¹³C_org. This level is located 50 cm below the FAD of conodont Misikella posthernsteini s.s within the radiolarian Proparvicingula moniliformis Zone. Both the negative δ¹³C_org shift and the FAD of Misikella posthernsteini occur within Pignola‐Abriola magnetozone MPA‐5r, at ~205.7 Ma, according to magnetostratigraphical correlation to the Newark APTS. We also illustrate the coeval Mt. Volturino stratigraphical section deposited below the calcite compensation depth (CCD) within the same Lagonegro Basin and characterized by a detailed radiolarian biostratigraphy and strong δ¹³C_org negative shift around the NRB.     

Carbonate facies‐specific stable isotope data record climate,hydrology, and microbial communities in Great Salt Lake,UT

Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ¹⁸O_water, δ¹³C_DIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ¹⁸O_carb, δ¹³C_carb, ?₄₇), as well as carbon isotopic compositions of bulk organic matter (δ¹³C_org) and dissolved inorganic carbon (DIC; δ¹³C_DIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ?₄₇ isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(?₄₇)), δ¹⁸O_carb, and calculated δ¹⁸O_water in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation.