Stable Carbon Isotope Signatures Research Articles

The influence of lipid content and taxonomic affiliation on methane and carbon dioxide production from phytoplankton biomass in lake sediment

AbstractThe greenhouse gases methane (CH4) and carbon dioxide (CO2) are end products of microbial anaerobic degradation of organic matter (OM) in lake sediments. Although previous research has shown that phytoplankton lipid content influences sediment methanogenesis, current understanding on how OM quality affects methanogenesis is still limited. Such information is needed to more accurately assess how lake greenhouse gas emissions may change in response to anthropogenic activities. We cultured 11 phytoplankton species from five classes and studied how taxonomic identity, C : N ratio, lipid content, and fatty acid composition of phytoplankton biomass affects the CH4 and net CO2 production in anaerobic lake sediments with an incubation experiment that lasted > 100 d. The carbon‐normalized potential CH4 (0.09–0.23 μmol mg C−1 d−1) and net CO2 (0.09–0.28 μmol mg C−1 d−1) production rates were not related to phytoplankton taxonomic affiliation (e.g., class, species), C : N ratio, or fatty acid composition of algal biomass. Methane or net CO2 production potentials did not increase with higher lipid content (10–30%); however, total fatty acid content had a weak correlation with CH4 production potential. In contrast to previous research, our results suggest that lipid content is of minor importance in determining methanogenesis rates from the biomass of multispecies phytoplankton communities settling on sediments. The decrease in CO2 concentration and the correlation between stable carbon isotope signatures of CH4 and molar ratio of CH4 and CO2 at the end of the experiment may indicate that importance of hydrogenotrophic methanogenesis, which uses CO2 when other substrates become limiting, increased during the long incubation.

Incorporation of local dissolved organic carbon into floodplain aquatic ecosystems

Environmental flow releases in lowland Australian rivers are currently timed to avoid high-carbon production on floodplains. Moreover, return flows (water draining from floodplains back into rivers) are avoided if there exists a risk of introducing deoxygenated “blackwater” into the main channel. This concern has restricted the range of possible watering scenarios being considered by environmental flow managers. We utilised a series of blackwater flows in the lower Murrumbidgee floodplain, Australia, in 2016 and 2017 to determine the origin and trophic contribution of blackwater dissolved organic carbon (DOC) in a floodplain wetland. We demonstrate a consistent difference in the isotope signature of blackwater DOC compared to both dissolved inorganic carbon (DIC) and river water DOC, explained by the greater contribution of floodplain vegetation (including the river red gum Eucalyptus camaldulensis) to blackwater DOC. Stable carbon isotope signatures suggest a contribution of blackwater to algal production, whereby microbial-mediated conversion of blackwater DOC into DIC may create opportunities for primary autotrophic productivity. This carbon signature was incorporated by the common yabby Cherax destructor. In the main river channel, C. destructor, the native gudgeon Hypseleotris spp. and the introduced European carp Cyprinus carpio may utilise the same basal carbon source. The use of small to moderate floodplain inundation with return flow to the river, properly monitored, would ameliorate the risk of hypoxia while providing the benefit of floodplain-derived DOC and associated increases to in-stream productivity.

Nannoplankton and 13C and 13O stable isotope stratigraphy record of the mid Cretaceous sequences, Zagros Basin (western Iran)

Deposition of black shales and dark argillaceous limestones across the Cenomanian to Turonian interval has been recorded in many parts of the world. In this regard, a stratigraphic section in the Iran's hydrocarbon field in the Kermanshah region in the Zagros Basin has been studied in detail to reconstruct palaeoceanographic conditions under a reliable biostratigraphic framework. Biostratigraphy based on calcareous nannofossils integrated with high resolution carbonate and oxygen stable isotope analyses were carried out on the Cenomanian-early Turonian interval of the upper part of Garau Formation. In this formation, 24 species of calcareous nannofossils were identified in the pelagic marine deposits. Our study allowed the definition of nannoplankton bio zones from the upper part of Eiffellithus turriseiffelii Zone to the lower part of Quadrum gartneri Zone in the mid Cretaceous. This research first recorded the upper part of the Garau Formation at site and it is consistent with the previous bio zones identified in the Tethyan domain in the region. The middle Cretaceous events of calcareous nannofossils proposed here correlate with the isotope global profile for the early Cenomanian-earliest Turonian interval and with those, reported for the southeastern France and south England reference curves, represented by the positive Cenomanian to Turonian δ13C excursion. Thus, the sequences of dark argillaceous limestone rich in organic matter in Kermanshah region can be considered as a continuation of the global trend in the Tethyan domain, where the Cenomanian-Turonian Oceanic Anoxic Event constrained to about 91.5 ± 8.6 millions of years, occurred in an anoxic pelagic sedimentary environment correlates with both, global and local sea-level rises, represented in this study by the characterization of the nannoflora assemblage reflected in a variety of widespread paleoenvironmental changes that coincides with significant shifts in the stable carbon and oxygen isotope signature.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Abstract Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto–Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb –11.6‰ and –15.7‰ (χ average –13.1‰; 1σ = 1.6) and δ13Ccarb –2.6‰ to –8.0‰ (χ average –5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Models estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

Physiological traits of income breeding strategy in the sea cucumber Apostichopus japonicus

The sea cucumber, Apostichopus japonicus, is an important marine aquaculture species in Asian countries. The small body size but high fecundity of A. japonicus suggests that using income resources to support gonad development might be an appropriate life history strategy, but the physiological mechanisms involved in this approach remain unclear. In this study, we conducted three experiments to assess the physiological traits that underlie income breeding strategy of the A. japonicus. As inferred by the carbon and nitrogen stable isotope signatures, the results showed that income breeding allows all adults, even individuals with poor body condition and small maternal size, to reproduce annually by flexibly adjusting their reproductive output. In addition, functionally, we found that the sea cucumbers allocate income resources to the demands of reproduction, growth and storage simultaneously during reproduction. However, dynamic changes of activities of Na+/K+-ATPase, hexokinase and pyruvate kinase suggested that income breeding places a metabolic burden on the breeding adults. Our results provided new insights into the physiological mechanisms underlying income breeding strategy in sea cucumbers.

Temperature affects carbon and nitrogen stable isotopic signatures of aquatic plants

Aquatic plants are vital components of shallow aquatic ecosystems, and they can substantially contribute to food webs. However, the large spatial and temporal variations of δ13C and δ15N signatures of aquatic plants have hindered the interpretation of their trophic interactions with organisms at higher trophic levels, and the effects of temperature on plant isotopic signatures remain to be fully elucidated. Herein, we cultured three common submerged macrophytes [Elodea nuttallii (Planch.) St. John, Vallisneria spiralis L., and Potamogeton lucens L.] at four temperatures (10, 15, 20 and 25 °C) for 16 weeks and analyzed their δ13C and δ15N signatures. Results showed that temperature altered the isotopic signatures of all three plant species. δ13C and δ15N varied by 16.06‰ and 11.68‰ in P. lucens at different temperatures, respectively. Plant δ15N significantly decreased with rising temperature in all three plant species and was correlated with plant growth, N content, and pore water dissolved inorganic N (DIN) concentrations. Conversely, δ13C responded non-linearly with temperature: a hump-shaped response of δ13C with temperature was observed for P. lucens. Plant δ13C was not correlated with any of the measured parameters. Temperature can alter plant metabolism and photosynthesis and the compositions and concentrations of C and N sources, thereby influencing plant δ13C and δ15N signatures, respectively. Temperature plays a key role in altering plant C and N isotopic signatures. Therefore, we recommend future studies to carefully consider the effects of temperature on plant stable isotopic signatures when interpreting the food contribution of aquatic plants in food webs and long-term environmental changes via historical isotopic signatures of plants exposed to different temperatures, particularly in light of changing climate conditions.

Coupling of stable carbon isotopic signature of methane and ebullitive fluxes in northern temperate lakes.

Stable isotopic analysis is a popular method to understand the mechanisms sustaining methane (CH4) emissions in various aquatic environments. Yet, the general lack of concurrent measurements of isotopes and fluxes impedes our ability to establish a connection between the variation in the rates of CH4 emission and isotopic signature. Here, we examine the magnitude of CH4 ebullition (bubbling) and stable carbon isotopic signature (δ13C-CH4) of bubble CH4 in four northern temperate lakes and evaluate the in-lake processes shaping their variability. The ebullitive CH4 flux and bubble δ13C-CH4 varied from 0.01 to 37.0mmolm-2d-1 and between -71.0‰ and -50.9‰, respectively. The high emission lakes in general and high fluxing shallow zones within each lake consistently showed enriched δ13C-CH4 signature. Subsequently, in addition to the temperature dependence (1.4±0.1eV), the rates of ebullition strongly correlated with the variability of δ13C-CH4 across our study lakes. Our results suggest that higher ebullitive emissions are sustained by acetoclastic methanogenesis, likely fueled by fresh organic matter inputs. Further, the annual whole-lake estimate of bubble isotopic flux alone showed depleted δ13C-CH4 values (-64.6±0.6‰ to -60.1±3.2‰), yet the signature of the total CH4 emission (ebullition+diffusion) was relatively enriched (-60.7‰ to -52.6‰) due to high methanotrophic activity in the water column. We show that δ13C-CH4 signature of bubbles can be linked to the magnitude of ebullition itself, yet we suggest there is a need to account for different emission pathways and their isotopic signature to allocate CH4 source signature to northern lakes.

The Role of Methane Transport From the Active Layer in Sustaining Methane Emissions and Food Chains in Subarctic Ponds

AbstractGroundwater discharge from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH4) into Arctic lakes and streams, but its contribution to CH4 emissions from thaw ponds and its influence on the trophic support and nutritional quality of pond food chains remains unexplored. We quantified the transport of CH4 from the active layer through groundwater discharge into thaw ponds in a subarctic catchment in northern Sweden, using radon (222Rn) as groundwater tracer. We analyzed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of groundwater‐mediated CH4 inputs on the aquatic food chains. Our results indicate that active layer groundwater discharge flows are nontrivial (range 6%–46% of pond volume per day) and the associated CH4 fluxes (median 339 mg C m−2day−1, interquartile range [IQR]: 179–419 mg C m−2 day−1) can sustain the diffusive CH4 emissions from most of the ponds (155 mg C m−2 day−1, IQR: 55–234 mg C m−2 day−1). Consumers in ponds receiving greater CH4 inputs from the active layer had lower stable carbon (C) isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Overall, this work links physical (CH4 transport from the active layer), biogeochemical (CH4 emission), and ecological (MOB‐consumer interaction) processes to provide direct evidence for the role of active layer groundwater discharge in CH4 cycling of subarctic thaw ponds.

Interaction between small-scale habitat properties and short-term temporal conditions on food web dynamics of a warm temperate intertidal rock pool ecosystem

The physical properties of habitats are crucial determinants of community structure with significant effects on food web dynamics. We examined the effects of rock pool physical properties (e.g. pool size, water depth) and short-term temporal (seasonal) change (e.g. temperature, nutrients) on food web structure at small spatial scales (c. 500 m) on the warm temperate coast of South Africa. Using stable carbon and nitrogen isotope signatures, we characterised food web structure with Layman metrics and quantified food web size, defined as the total area in isotopic space occupied by consumer species. The effects of rock pool physical properties and seasonal changes on species counts and food web structure were evaluated using a Bayesian generalised linear mixed model approach. Substratum type, water depth, pool size, phosphate concentrations, salinity and temperature all influenced overall food web dynamics. A winter reduction of sand cover led to a more heterogeneous substratum and a significant increase in species counts, while consumer niche similarity decreased with increasing pool size in summer. The range of δ15N values, or trophic height, was negatively and positively affected by salinity and phosphate levels, respectively. Overall, the effects of pool physical properties on food web structure were moderated by seasonality.

The fate of anaerobic syntrophy in anaerobic digestion facing propionate and acetate accumulation

How the acetate and propionate accumulation impact anaerobic syntrophy during methane formation is not well understood. To investigate such effect, continuous acetate (35 g/L), propionate (11.25 g/L) and bicarbonate (30 g/L) supplementation were used during mesophilic anaerobic digestion. The high throughput sequencing (16S rRNA and mcrA), Real-Time quantitative PCR, and stable carbon isotope fingerprinting were applied to investigate the structure and activity of microbial community members. The results demonstrated that the abundance of syntrophic acetate oxidizing bacteria exhibited a gradual decrease coupled with heavier stable carbon isotopic signature of methane (δ 13CH4) in the three reagents impacted reactors. The increased acetate and propionate concentrations exerted negative influence on biogas production but the relatively stable hydrogenotrophic methanogens together with syntrophic acetate/propionate oxidizing bacteria kept the stable methane formation facing acetate and propionate accumulation. The functional genes copy number of the hydrogenotrophic Methanocellaceae and Methanomicrobiaceae correlated significantly with δ 13CH4 (R2 > 0.74), but only the abundance of Methanocellaceae fitted well with δ 13CH4 (p < 0.05). The δ 13CH4 signatures can predict methanogenesis, as it directly reflects the main methanogenic pathway; yet, further investigation of isotope fractionation in acetate/propionate coupled with δ 13CH4 is needed. Collectively, these results provide deep insight into anaerobic syntrophy and reveal changes of synergistic relationships, both of which may contribute to the stability of biogas reactors.

Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model

Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmospheric pCO2. Here we extend and calibrate a temperature-dependent representation of marine carbon cycling in the cGENIE.muffin Earth system model, intended for both past and future climate applications. In this, we combine a temperature-dependent remineralisation scheme for sinking organic matter with a biological export production scheme that also includes a dependence on ambient seawater temperature. Via a parameter ensemble, we jointly calibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopic signature (δ13C) of dissolved inorganic carbon in the ocean with modern observations. We additionally explore the role of temperature in the creation and recycling of dissolved organic matter (DOM) and hence its impact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version shows a fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact of accounting for temperature-dependent remineralisation of POM is in driving higher rates of remineralisation in warmer waters, in turn driving a more rapid return of nutrients to the surface and thereby stimulating organic matter production. As a result, more POM is exported below 80 m but on average reaches shallower depths in middle- and low-latitude warmer waters compared to the standard model. Conversely, at higher latitudes, colder water temperature reduces the rate of nutrient resupply to the surface and POM reaches greater depth on average as a result of slower subsurface rates of remineralisation. Further adding temperature-dependent DOM processes changes this overall picture only a little, with a slight weakening of export production at higher latitudes. As an illustrative application of the new model configuration and calibration, we take the example of historical warming and briefly assess the implications for global carbon cycling of accounting for a more complete set of temperature-dependent processes in the ocean. We find that between the pre-industrial era (ca. 1700) and the present (year 2010), in response to a simulated air temperature increase of 0.9 ∘C and an associated projected mean ocean warming of 0.12 ∘C (0.6 ∘C in surface waters and 0.02 ∘C in deep waters), a reduction in particulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.

Ontogenetic diet shifts: an additional mechanism for successful invasion of a piranha species in a Neotropical floodplain.

Ontogenetic shifts in food preference reduces intraspecific competition as immature individuals eat different food types than adults. This diet plasticity could facilitate species' ability to successfully invade and establish itself in a new environment, even when co-occurring with phylogenetically close species. Here, ontogenetic diet shifts of a non-native piranha species (Serrasalmus marginatus) was tested by analyzing the relationship between its body length and the carbon and nitrogen stable isotope signatures. Carbon stable isotope was not correlated to fish length, but positive significant correlation between δ15N values and fish length was found for the non-native piranha. Also, immature and adult S. marginatus showed low isotopic niche overlap. The correlation between fish length and δ15N, and the low trophic overlap between immature and adult, indicate that the non-native S. marginatus had ontogenetic shifts in food preference, which may be viewed as an additional mechanism underlying its successful establishment in the upper Paraná River. Our findings indicate that ontogenetic shifts in food preference may be an invasive trait that facilitates the establishment of non-native fish species in tropical aquatic systems.

Carbon isotope seasonal characteristics of fine carbonaceous aerosol in Jinzhong City, Shanxi Province, China

Stable carbon isotope signatures were observed in organic carbon (OC) and elemental carbon (EC) fractions of fine carbonaceous aerosol (PM2.5) sampled in spring, summer, and winter (December 2013 to August 2014) in Jinzhong in Shanxi Province. The stable carbon isotope composition of EC (δ13CEC) varied from −25.93‰ to −22.20‰ during the sampling (mean value of −24.35 ± 1.11‰), with depletion in summer (−25.44 ± 0.32‰) and enrichment in winter (−23.04 ± 0.71‰). The contribution of vehicle exhaust was 65% in summer, and the contribution of fossil fuel combustion was 92% in winter based on carbon isotope mass balance model. The stable carbon isotope compositions of OC (δ13COC) over the whole sampling period ranged from −29.75‰ to −22.87‰ (mean value of −25.10 ± 1.47‰), with enrichment in winter (−24.12 ± 0.88‰) and depletion in spring (−26.40 ± 1.50‰). Compared with δ13CEC, the δ13COC in spring is depleted in 13C (Δ13COC-EC = δ13COC − δ13CEC = −1.55‰) because of the formation of secondary OC (SOC) from the photo-oxidation reactions. Summertime δ13COC (−24.52 ± 0.28‰) is more positive than δ13CEC because of the strong photochemical aging. The negative value of Δ13COC-EC in winter may be attributed to a complex set of reasons, including the unfavorable meteorological conditions (e.g., low temperature, low boundary layer height, and frequent temperature inversions), coal combustion, vehicle exhaust, gaseous fuel burning, and SOC formation from photo-oxidation reactions of volatile organic compounds (VOCs) caused by fossil fuel combustion. The seasonal variation of δ13CTC was consistent with that of δ13COC, and the annual average δ13CTC (−24.80 ± 1.12‰) implicated fossil fuels combustion. This study highlights that the isotope signatures of EC can be used for source apportionment, reveals the strong influence of atmospheric processing on the isotope signature of OC (e.g., SOC formation and photochemical aging), and suggests the potential application of isotope technology in air pollution research.

Effects of Tissue Preservation on Carbon and Nitrogen Stable Isotope Signatures in Syngnathid Fishes and Prey.

Simple SummaryStable isotope analysis (SIA) was used to assess the influence of various preservation methods (freezing, ethanol and formaldehyde) on syngnathid (seahorses and pipefishes) fins, seahorse newborns (seahorses), and prey (copepods and Artemia). The first available conversion models for syngnathids are provided, enabling their application to isotopic studies in the field and in the laboratory.Isotopic stable analysis (SIA) is a powerful tool in the assessment of different types of ecological and physiological studies. For that, different preservation methods for sampled materials are commonly used prior to isotopic analysis. The effects of various preservation methods (freezing, ethanol and formaldehyde) were analyzed for C:N, and δ13C and δ15N signals on a variety of tissues including dorsal fins (three seahorse and two pipefish species), seahorse newborns (three seahorses species), and prey (copepods and different stages of Artemia) commonly used to feed the fishes under rearing conditions. The aims of the study were: (i) to evaluate isotopic effects of chemical preservation methods across different types of organisms and tissues, using frozen samples as controls, and (ii) to construct the first conversion models available in syngnathid fishes. The chemical preservation in ethanol and, to a lesser extent, in formaldehyde significantly affected δ13C values, whereas the effects on δ15N signatures were negligible. Due to their low lipid content, the isotopic signals in fish fins was almost unaffected, supporting the suitability of dorsal fins as the most convenient material in isotopic studies on vulnerable species such as syngnathids. The regression equations provided resulted convenient for the successful conversion of δ13C between preservation treatments. Our results indicate that the normalization of δ15N signatures in preserved samples is unnecessary. The conversion models should be applicable in isotopic field studies, laboratory experiments, and specimens of historical collections.

Sedimentology and Stratigraphic Evolution of the Early Eocene Nammal Formation, Salt Range, Pakistan

The Early Eocene succession of the Salt Range from base to top comprises the Nammal Formation, Sakesar Limestone and Chor Gali Formation. The Nammal Formation of Ypresian age is well exposed throughout the Salt Range. Detailed sedimentological and palaeontological analyses of the Nammal Formation were carried out, based on six stratigraphically important measured sections in the Salt Range. Lithologically, the formation is predominantly composed of interbedded nodular limestone, marl and shale. Wackestone, packstone, wackestone to packstone, dolomitic limestone and grainstone facies dominate the Nammal Formation in a fine-grained bioclastic matrix with abundant grains of larger benthic foraminifera. The diagnostic larger benthic foraminifera are recorded, which includes Nummulites mamillatus, Assilina spinosa, Assilina subspinosa, Assilina granulosa, Assilina laminosa, Discocyclina dispansa, Alveolina dolioliformis, Alveolina pasticillata, Alveolina globula, Lockhartia tipperi, and Lockhartia conditi. Stable carbon and oxygen isotopic signatures of the Nammal Formation designate shallow marine depositional environment. During Eocene a carbonate sequence developed in the Salt Range, lower boundary of which is marked as SB-II at the base of the Nammal Formation overlying the Palaeocene Patala Formation. The Nammal Formation presents the retrogradational facies suggesting the transgressive system tracts. The Sakesar Limestone shows agradational to progradational pattern of facies, which developed in highstand system tracts. The Chor Gali Formation possesses shallowing-upward trend by forming progradational shift of facies, and represents the falling stage system tracts. Early Eocene carbonate sequence is terminated by a regional sub-aerial unconformity SB-I marked between marine carbonate sequence of the Chor Gali Formation and the overlying non-marine clastic Miocene Kamlial Formation. Overall, the Nammal Formation presents shallow water neritic carbonate deposits containing larger benthic foraminifers.

Stable Isotopes Reveal Variation in Consumption of Pacific Salmon by Brown Bears, Despite Ready Access in Small Streams

AbstractBrown bears Ursus arctos consume a wide range of organisms, including ungulates and plants, but Pacific salmon Oncorhynchus spp. are especially important to their diet where their ranges overlap. Although some brown bears minimize antagonistic encounters with other brown bears or infanticide by avoiding streams where salmon spawn, studies generally assume that brown bears with ready access to salmon feed heavily on them. To test this assumption, and the hypothesis that male brown bears would feed more heavily on salmon than females (owing to their sexual size dimorphism), we collected hair samples from brown bears by using barbed wire placed on six small tributaries of Lake Aleknagik, Alaska, USA, where adult Sockeye Salmon Oncorhynchus nerka are readily accessible and frequently consumed by brown bears. Analysis of DNA distinguished among the different brown bears leaving the hair samples, some of which were sampled multiple times within and among years. We assessed the contribution of salmon to the diet of individual brown bears by using carbon and nitrogen stable isotope signatures. The 77 samples analyzed from 31 different bears over 4 y showed isotopic ratios consistent with reliance on salmon, but the wide range of isotopic signatures included values suggesting variable, and in one case considerable, use of terrestrial resources. Stable isotope signatures did not differ between male and female brown bears, nor did they differ between two sides of the lake, despite marked differences in Sockeye Salmon density. We collected the hair samples when salmon were present, so there was some uncertainty regarding whether they reflected feeding during the current or previous season. Notwithstanding this caveat, the results are consistent with the hypothesis that salmon were sufficiently available to provide food for the brown bears and that the considerable isotopic variation among brown bears with access to salmon reflected their age, status, and behavior.

Linking the karst record to atmospheric, precipitation, and vegetation dynamics in Portugal

Cave deposits can be valuable sources of paleoclimate data, especially when atmospheric circulation patterns, precipitation variability, vegetation changes, and the chemical evolution of waters moving through the karst environment can be mechanistically linked to speleothem proxies. In particular, an evaluation of the factors that control the isotopic composition of precipitation and the evolution of rainwater during migration from the land surface to the cave are needed to robustly develop speleothems as hydroclimate-sensitive proxies. One area in which precipitation and atmospheric variability are closely linked is western Iberia, where rainfall is strongly influenced by the Azores High, part of the North Atlantic Oscillation (NAO) dipole. Therefore, in order to better characterize the factors that influence the isotopic composition of precipitation in Portugal and to evaluate the potential of using stalagmites from this region as hydroclimate (and NAO-sensitive) proxies, we investigated Global Network of Isotopes in Precipitation (GNIP) data from ten mainland Portugal sites spanning multiple decades. In addition, we use more than one hydrologic year of precipitation amount and isotope data from Buraca Gloriosa (BG), a cave in western Portugal, the site of on-going speleothem analyses, as well as six years of environmental monitoring from BG. We present an integrated analysis of rainfall and vegetation through the normalized difference vegetation index (NDVI) following extremely wet and dry winters. Summer vegetation density, related to the amount of precipitation in the preceding winter wet season, as well as prior calcite precipitation (PCP), would largely control the stable carbon isotopic signature (δ13C) in stalagmites at BG. Cool season recharge is likely the dominant factor for the oxygen isotopic composition (δ18O) of water percolating through the cave system, while amount effects exert the primary control on precipitation δ18O values. Based on HYSPLIT modeling, moisture sources overwhelmingly originate from the Atlantic Ocean as opposed to the Mediterranean or elsewhere; thus, variability in δ18O values in the precipitation (and, thus, by inference, those of the dripwater and stalagmites) are primarily reflecting changes in precipitation amount and not changing temperatures or source regions. Together these data constitute an important analysis of the controls of isotope proxies in Portuguese cave systems.

Multi-proxy approach to unravel methane emission history of an Arctic cold seep

Arctic Ocean sediments contain large amounts of methane in the form of free gas and gas hydrate. This highly dynamic methane reservoir is susceptible to be modified by bottom water warming. The warming may lead to gas hydrate destabilization releasing elevated methane fluxes to the seafloor and seawater. Reconstructing past methane dynamics can be achieved by using specific proxies left in the geological record. In this study, we apply a multi-proxy approach for paleo seepage reconstruction from sediment records at gas hydrate mounds (GHMs) in Storfjordrenna (south of the Svalbard archipelago). These shallow water (∼380 m water depth) systems are potentially vulnerable to global warming related temperature changes. 14C dating of foraminifera shells indicated an onset of deglaciation in the Storfjordrenna region at ∼20 kyr BP and allowed us to establish a stratigraphic context based on sediment Zr/Rb and Fe/Ca ratios. Several major (between 15 and 17 kyr BP) and minor methane venting phases were identified and interpreted to be related to gas hydrate instability triggered by isostatic adjustment right after the onset of the deglaciation. The detection of all major methane releases was only possible by combining data sets of stable carbon isotope compositions of foraminifera, mineralogy and δ13C values of authigenic carbonates, and abundance and stable carbon isotope signatures of lipid biomarkers. The most robust single proxy in this study was provided by the δ13C values of archaeal biomarkers. In contrast, the sediment Ba/Ti ratios recorded only the major events. Our results highlight the complexity and heterogeneity of methane dynamics in a small area of some hundred meters across.

Stable Nitrogen and Carbon Isotope Signatures (δ 15N and δ 13C Values in ‰) of Fresh and Preserved Button Mushrooms (Agaricus bisporus) from the German Market

Fresh and preserved button mushrooms (Agaricus bisporus) are among the most consumed varieties worldwide. The cultivation is highly optimized and largely based on the use of different compost materials from agricultural wastes as substrates. In this study, we determined the stable carbon (13C) and nitrogen (15N) isotope signatures of the fruiting bodies and used the measured δ values (‰) to obtain indirect information on the medium on which they were grown. The δ 13C values (‰) of the investigated fresh and processed button mushrooms from the German market were quite narrow (δ 13C values −24.8 ± 0.9 ‰, n = 72), which indicated C3 plant materials as carbon sources. Overall high δ 15N values (δ 15N values > 5.5 ‰) of the cultivated mushrooms enabled a differentiation to wild growing saprophytes. Furthermore, the broad range of δ 15N values (Δ=7 ‰) pointed toward a cultivation on compost materials with varying shares of farmyard manure. While only few samples suggested a cultivation on dominantly plant-based substrates, more than 50% of the investigated button mushrooms were related to high shares of animal manures in the compost mixture. Little variation of δ 15N values (‰) in related products collected on subsequent years indicated a high level of standardization of the individual cultivation processes over several years. Analysis of the carbon and nitrogen isotope signatures of cultivated button mushrooms could thus be valuable parameters used for long-term monitoring and controlling purposes.

Du Feu à l'Eau: Source and Flux of Dissolved Black Carbon From the Congo River

AbstractPyrogenic organic residues from wildfires and anthropogenic combustion are ubiquitous in the environment and susceptible to leaching from soils into rivers, where they are known as dissolved black carbon (DBC). Here we quantified and isotopically characterized DBC from the second largest river on Earth, the Congo, using 12 samples collected across three annual hydrographs from 2010 to 2012. We find that the Congo River exports an average of 803 ± 84 Gg‐C as DBC per year, comprising 7.5% of the river's average annual dissolved organic carbon (DOC) flux (10.7 ± 1.2 Tg‐C yr−1). Concentrations of DBC were strongly correlated with discharge and DOC concentration, indicating transport limitation for DBC flux from the Congo River Basin. Stable carbon isotopic signatures of DBC revealed a seasonal shift in pyrogenic source from forest dominant to an increasing contribution from savannah biomass, which derives from the North‐South bimodal hydrologic regime within the basin. Our results also indicate that black carbon produced within the Congo Basin is exported by the river on relatively short time scales and that total DBC export will increase with climate change predictions for the central African region.