δ13C Measurements Research Articles

HUMAN DIET DURING THE STONE AGE AND EARLY METAL PERIOD (7000–1 CAL BC) IN LITHUANIA: AN UPDATE

ABSTRACTIn this study we present new carbon (δ13C) and nitrogen (δ15N) stable isotope data of human (n=13) and animal (n=40) bone and/or dentine collagen samples, alongside accelerator mass spectrometry radiocarbon (AMS 14C) dates of human remains (n=16). The studied material was sampled from Lithuanian sites dating from the Late Mesolithic to the pre-Roman Iron Age. For the first time, we present δ13C and δ15N data from Lithuanian freshwater fish as well as AMS 14C, δ13C, and δ15N measurements of human remains from six disturbed graves at the Donkalnis cemetery and from two pre-Roman Iron Age graves. According to the new results, human diet derived protein from the Late Mesolithic to Subneolithic (ca. 7000–2900 cal BC) was primarily based on freshwater fish. While previous macrobotanical and stable isotope studies has suggested that C4 plants, i.e., millet, became more widely used from the Late Bronze Age (1100–500 cal BC), our data suggests that millet consumption may have decreased during the pre-Roman Iron Age (500–1 cal BC) in the southeastern Baltic.

Breastfeeding and Weaning in Roman Thessaloniki. An Investigation of Infant Diet based on Incremental Analysis of Human Dentine

ABSTRACT In the present study, we report isotope (δ15N, δ13C) measurements on dentine increments from first permanent molars, to investigate breastfeeding practices in Thessaloniki, a provincial capital of the Roman Empire in northern Greece (168 BC–324 AD). For this purpose, we reconstructed diets from birth to seven yoa and estimated weaning ages for 20 individuals. We compared the individuals in relation to their biological sex and investigated diet during three distinct phases i.e. breastfeeding, weaning and fully weaned. The results of our study show a general trend in breastfeeding duration (mean weaning age: two yoa) for seventeen individuals and no breastfeeding for three male individuals. Moreover, males (n = 10) were fully weaned approximately six months earlier than females (n = 10). The weaning diet consisted mainly of terrestrial sources (C3 and C4 plants, animals/animal-byproducts) or small fish. Our observations are in accordance with ancient texts (e.g. Gynaecology of Soranus) and other isotopic studies on Roman period datasets highlighting the pivotal role of Thessaloniki as a cultural hub of the Empire.

Sources and Cycling of Particulate Organic Matter in Baffin Bay: A Multi-Isotope δ13C, δ15N, and Δ14C Approach

The Canadian Arctic is warming three times faster than the rest of the planet. The impact of climate change on the Arctic carbon cycle, and in particular for Baffin Bay, remains poorly constrained. Sinking particulate organic matter (POMsink) is a key component of the biological carbon pump and provides a direct linkage between surface productivity and the preservation of carbon in marine sediments. While POMsink provides a rapid POM shunt to the deep ocean (days) the majority of marine POM is suspended (POMsusp) persists for years in the water column. Stable carbon (δ13C), nitrogen (δ15N) and radiocarbon (Δ14C) measurements are powerful tools for evaluating sources and cycling of POM. In this study, we measure depth-integrated (0-400m) POMsusp stable carbon (δ13C), nitrogen (δ15N) and radiocarbon (Δ14C) values at 11 stations in Baffin Bay. We use POM C:Na ratios to evaluate its diagenetic state. POMsusp δ13C and δ15N values ranged from −21.6‰ to −29.5‰, and +3.84‰ to +7.21‰, respectively. POMsusp Δ14C values ranged from −41.8‰ to +76.8‰. Together, our results suggest Baffin Bay POMsusp has multiple carbon and nitrogen sources. POMsusp δ13C and Δ14C are strongly correlated to surface salinity, indicating surface water has a strong influence on POMsusp isotopic composition and 14C-age. POMsusp from coastal Greenland stations have similar Δ14C values to surface dissolved inorganic carbon (DIC) and POM concentrations, consistent with primary production as the predominant POMsusp source in Eastern Baffin Bay. Positive POMsusp Δ14C values in central Baffin Bay suggest an accumulation of atmospheric “bomb” 14C in the sub-polar gyre. POMsusp exiting Davis Strait via the Baffin Island Current was determined to have a significant degraded, resuspended sediment component. Finally, POMsusp δ15N values highlight two distinct N sources in Baffin Bay: Pacific and Arctic nutrients at Northern gateway stations vs. the influx of Atlantic nutrients via Davis Strait along coastal Greenland. Overall, these first Baffin Bay POMsusp Δ14C values provide useful baseline data for rapidly changing Arctic ecosystem.

Delineating the Drivers and Functionality of Methanogenic Niches within an Arid Landfill.

ABSTRACTMicrobial communities mediate the transformation of organic matter within landfills into methane (CH4). Yet their ecological role in CH4 production is rarely evaluated. To characterize the microbiome associated with this biotransformation, the overall community and methanogenic Archaea were surveyed in an arid landfill using leachate collected from distinctly aged landfill cells (i.e., younger, intermediate, and older). We hypothesized that distinct methanogenic niches exist within an arid landfill, driven by geochemical gradients that developed under extended and age-dependent waste biodegradation stages. Using 16S rRNA and mcrA gene amplicon sequencing, we identified putative methanogenic niches as follows. The order Methanomicrobiales was the most abundant order in leachate from younger cells, where leachate temperature and propionate concentrations were measured at 41.8°C ± 1.7°C and 57.1 ± 10.7 mg L−1. In intermediate-aged cells, the family Methanocellaceae was identified as a putative specialist family under intermediate-temperature and -total dissolved solid (TDS) conditions, wherein samples had a higher alpha diversity index and near CH4 concentrations. In older-aged cells, accumulating metals and TDS supported Methanocorpusculaceae, “Candidatus Bathyarchaeota,” and “Candidatus Verstraetearchaeota” operational taxonomic units (OTUs). Consistent with the mcrA data, we assayed methanogenic activity across the age gradient through stable isotopic measurements of δ13C of CH4 and δ13C of CO2. The majority (80%) of the samples’ carbon fractionation was consistent with hydrogenotrophic methanogenesis. Together, we report age-dependent geochemical gradients detected through leachate in an arid landfill seemingly influencing CH4 production, niche partitioning, and methanogenic activity.IMPORTANCE Microbiome analysis is becoming common in select municipal and service ecosystems, including wastewater treatment and anaerobic digestion, but its potential as a microbial-status-informative tool to promote or mitigate CH4 production has not yet been evaluated in landfills. Methanogenesis mediated by Archaea is highly active in solid-waste microbiomes but is commonly neglected in studies employing next-generation sequencing techniques. Identifying methanogenic niches within a landfill offers detail into operations that positively or negatively impact the commercial production of methane known as biomethanation. We provide evidence that the geochemistry of leachate and its microbiome can be a variable accounting for ecosystem-level (coarse) variation of CH4 production, where we demonstrate through independent assessments of leachate and gas collection that the functional variability of an arid landfill is linked to the composition of methanogenic Archaea.

Tufa associated with karst features in a fracture-system fed by meteoric water, Araripe basin, NE Brazil

Karstic tufa from the Brazilian semi-arid zone offers a key opportunity to study the depositional evolution, unveiling the past climate history with examples generated outside of the phreatic zone. Morpho-petrological identification, in addition to a structural survey, petrographic analysis, X-ray diffractometry (XRD), scanning electronic microscopy (SEM), and measurements of δ18O and δ13C, provided elements to explain the occurrence of tufa associated with a fracture system. Three sedimentary facies were identified, one allochthonous (detrital) and two autochthonous (in situ). Field recognition and structural analysis showed that tufa occurs only in the deformed portion of the Crato Formation, at the Triunfo Fault splay sector. Facies formed in an open fracture-controlled hydrological system during rainfall periods. The infiltration of meteoric water into the limestones of the Crato Formation facilitated the dissolution of the parent rock through time, with the percolation controlled by open fractures in tilted blocks. Breccia tufa developed in the early stage of the karstification by carbonate clast forming from the limestone, previously collapsed from the walls and later cemented by percolation of solutions enriched in calcium carbonate within fractures. The two autochthonous facies developed in later stages through continuous percolation of water, rock dissolution, bryophyte-microbial growth, soil development, and calcium carbonate precipitation in progressively more humid conditions. Thin section analyses and SEM imaging revealed calcite crystals closely related with organic structures. The association between tufa and microbial filamentous organisms (likely cyanobacteria) indicates an active biogenic role for the tufa origin. Calcite stable-isotopes analyses indicated tufa as a product of meteoric water as supported by δ18O isotope. Changes of the carbon ratios through time indicated influence of soil development and probably C3–C4 photosynthetic activity. An integrated structural, sedimentological, petrological, and geochemical approach provided elements to define these tufa deposits as formed in a fracture-system fed by meteoric water, as result of the post-Mesozoic denudation process that affected the Araripe Basin.

Aquatic resource consumption at the Odense leprosarium: Advancing the limits of palaeodiet reconstruction with amino acid δ13C measurements

Dietary patterns of leprosy patients from the medieval leprosarium at Odense (Denmark) were investigated via carbon isotope ratio analysis of individual amino acids (δ13CAA) in bone collagen. The aim of this research was to explore in detail the consumption of aquatic resources (freshwater and marine) by these individuals; especially during the last few years of life, which were likely spent in the leprosy hospital. The analysis was conducted on ribs (n = 30) and long bones (n = 10) from 30 individuals, as well as on 9 local and contemporary faunal samples. A comparison of the Odense δ13CAA results with published δ13CAA values from the Baltic Sea region indicate that the diet was primarily terrestrial, but 12 of the 30 individuals (40%) consumed a mixed terrestrial-aquatic diet based on Δ13CVal-Phe values. Further, comparison of the rib-long bone pairs from 10 individuals found statistically significant differences in the Δ13CVal-Phe and Δ13CGly-Phe results, but not the δ13CLys results. This intra-individual dietary difference is also supported by MLA-PCA, and tentatively suggests these leprosy patients increased their consumption of marine protein over the last few years of life, indicating perhaps an institutional dietary program in the leprosarium. This work represents one of the largest archaeological δ13CAA studies in Europe and the first to compare bones with different turnover rates. Compound specific isotope analysis of δ13CAA has great potential to reveal subtle intra-individual dietary shifts that may remain undetected by bulk isotopic analysis.

Verification of the accuracy of the recent 50 years of tree growth and long‐term change in intrinsic water‐use efficiency using xylem Δ14C and δ13C in trees in an aseasonal tropical rainforest

Abstract Growth analysis based on tree‐ring chronology is difficult in trees in aseasonal tropical rain forests, because annual growth rings may be unclear or completely absent. Fortunately, tree growth history recorded in xylem tissue is capable of providing valuable information on the responses of trees and forests to past and present environmental changes, including global warming. We have developed a new technique for aseasonal tropical forest trees which derives their growth rates from xylem Δ14C, and verified its accuracy. We also determined, from xylem δ13C, the intrinsic water‐use efficiency (iWUE) in the past 50 years. We analysed changes in xylem Δ14C and δ13C in 23 canopy trees of 12 species in 6 families growing in Pasoh Forest Reserve, Malaysia; each stem diameter at breast height (DBH) was recorded 14 times from 1969 to 2011. We found a significant positive relationship between the growth rates determined by 14C dating and the past DBH data. On the other hand, leaf‐internal CO2 (Ci) content did not change with increasing atmospheric CO2 (Ca). Thus, the iWUE increased significantly over the last 50 years in all the families and species tested. This study showed that the simultaneous measurements of xylem Δ14C and δ13C could reveal a long‐term change in tree growth and iWUE during the past 50 years with high accuracy in various species and/or individuals in aseasonal tropical rainforests exhibiting high species diversity.

Simultaneous Sensitive Determination of δ13C, δ18O, and δ17O in Human Breath CO2 Based on ICL Direct Absorption Spectroscopy.

Previous research revealed that isotopes 13C and 18O of exhaled CO2 have the potential link with Helicobacter pylori; however, the 17O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous δ13C, δ18O, and δ17O analysis of human breath CO2 based on mid-infrared laser direct absorption spectroscopy with an interband cascade laser (ICL) at 4.33 μm. There was a gas cell with a small volume of less than 5 mL, and the pressure in the gas cell was precisely controlled with a standard deviation of 0.0035 Torr. Moreover, real-time breath sampling and batch operation were achieved in gas inlets. The theoretical drifts for δ13C, δ18O, and δ17O measurement caused by temperature were minimized to 0.017‰, 0.024‰, and 0.021‰, respectively, thanks to the precise temperature control with a standard deviation of 0.0013 °C. After absolute temperature correction, the error between the system responded δ-value and the reference is less than 0.3‰. According to Allan variance analysis, the system precisions for δ13C, δ18O, and δ17O were 0.12‰, 0.18‰, and 0.47‰, respectively, at 1 s integration time, which were close to the real-time measurement errors of six repeated exhalations.

Spatial distribution of fossil fuel derived CO2 over India using radiocarbon measurements in crop plants

Examining the contribution of fossil fuel CO2 to the total CO2 changes in the atmosphere is of primary concern due to its alarming levels of fossil fuel emissions over the globe, specifically developing countries. Atmospheric radiocarbon represents an important observational constraint and utilized to trace fossil fuel derived CO2 (CO2ff) in the atmosphere. For the first time, we have presented a detailed analysis on the spatial distribution of fossil fuel derived CO2 (CO2ff) over India using radiocarbon (Δ14C) measurements during three-year period. Analysis shows that the Δ14C values are varying between 29.33‰ to -34.06‰ across India in the year 2017, where highest value belongs to a location from Gujarat while lowest value belongs to a location from Chhattisgarh. Based on the Δ14C patterns, spatial distributions of CO2ff mole fractions have been determined over India and the calculated values of CO2ff mole fractions are varying between 4.85 ppm to 26.59 ppm across India. It is also noticed that the highest CO2ff mole fraction is observed as 26.59 ppm from a site in Chhattisgarh. CO2ff mole fraction values from four high altitude sites are found to be varied between 4.85 ppm to 14.87 ppm. Effect of sampling different crop plants from the same growing season and different crop plant organs (grains, leaves, stems) on the Δ14C and CO2ff have been studied. Annual and intra seasonal variations in the Δ14C and CO2ff mole fractions have also been analyzed from a rural location (Dholpur, Rajasthan).

An individual-based dataset of carbon and nitrogen isotopic data of Callinectessapidus in invaded Mediterranean waters.

BackgroundThe characterisation of functional traits of non-indigenous and invasive species is crucial to assess their impact within invaded habitats. Successful biological invasions are often facilitated by the generalist diet of the invaders which can modify their trophic position and adapt to new ecosystems determining changes in their structure and functioning. Invasive crustaceans are an illustrative example of such mechanisms since their trophic habits can determine important ecological impacts on aquatic food webs. The Atlantic blue crab Callinectessapidus is currently established and considered invasive in the Mediterranean Sea where it has been recorded for the first time between 1947 and 1949. In the last decade, the blue crab colonised most of the eastern and central Mediterranean Sea and the Black Sea and it is currently widening its distribution towards the western region of the basin.New informationStable isotope analysis is increasingly used to investigate the trophic habits of invasive marine species. Here, we collated individual measures of the blue crab δ13C and δ15N values and of its potential invertebrate prey into a geo-referenced dataset. The dataset includes 360 records with 236 isotopic values of the blue crab and 224 isotopic data of potential prey collected from five countries and 12 locations between 2014 and 2019. This dataset allows the estimation of the trophic position of the blue crab within a variety of invaded ecosystems, as well as advanced quantitative comparisons of the main features of its isotopic niche.

MARINE ORGANIC CARBON AND RADIOCARBON—PRESENT AND FUTURE CHALLENGES

ABSTRACTWe discuss present and developing techniques for studying radiocarbon in marine organic carbon (C). Bulk DOC (dissolved organic C) Δ14C measurements reveal information about the cycling time and sources of DOC in the ocean, yet they are time consuming and need to be streamlined. To further elucidate the cycling of DOC, various fractions have been separated from bulk DOC, through solid phase extraction of DOC, and ultrafiltration of high and low molecular weight DOC. Research using 14C of DOC and particulate organic C separated into organic fractions revealed that the acid insoluble fraction is similar in 14C signature to that of the lipid fraction. Plans for utilizing this methodology are described. Studies using compound specific radiocarbon analyses to study the origin of biomarkers in the marine environment are reviewed and plans for the future are outlined. Development of ramped pyrolysis oxidation methods are discussed and scientific questions addressed. A modified elemental analysis (EA) combustion reactor is described that allows high particulate organic C sample throughput by direct coupling with the MIniCArbonDAtingSystem.

Zeolite reduces losses and minimizes fractionation of various flavor compounds during EA-IRMS analysis

Among the methods for confirming the natural origin of flavor compounds, δ13C measurements using IRMS technique are becoming increasingly important. EA-IRMS measures can suffer from errors caused by the volatility of the analyzed compounds and the consequent isotope fractionation. This work suggests a method involving the adsorption of volatile molecules in zeolite before the EA-IRMS analysis, minimizing the possible error due to the volatility of the structures analyzed. The stabilizing effect on any fractionation was proven during the analysis of various molecules which cover a wide range of volatility. The work demonstrates that, by operating with the advised method, the repeatability of EA-IRMS data is very high not only for samples prepared before analysis, but also for samples prepared in a time span of up to 6 h. The proposed method facilitates serial preparation and prevents erroneous results due to the natural volatility of the molecules under evaluation.

Sugarcane yield loss due to water and nitrogen deficiencies evaluated by carbon isotopic discrimination method

Water and nitrogen (N) availability are determining factors for crop development and production. Assessments of sugarcane yield loss based on these factors may become more accurate by using the stable carbon (C) isotope technique. The aim of this work was to evaluate the effect of N and water on sugarcane yield, isolating the yield losses (YL) caused by N or by water limitations, and correlating them with the fractionation of C isotopes (Δ13C). The research was carried out in field conditions in the municipality of Jaú, Brazil. A statistical design in randomized block was used, considering the 2 ´ 2 ´ 2 factorial arrangement, composed of two cycles [first (2008/09) and second (2009/10)], two N rates (without and with N) and two water supplies (rainfed and irrigated). These variables were evaluated: biometric features (plant height, stalk diameter and tillering), yield elements [stalk yield (STY), sucrose yield (SUY), and stalk dry matter (SDM)], technological quality parameters [fiber content, sucrose juice content (SJC), and total sucrose recovery (TSR)], stalk N concentration (SNC) and accumulation (SNA), and Δ13C. The effect of irrigation associated with N fertilization on STY, SUY, and SDM, resulted in average increases of 53.7, 9.0, and 18.6 Mg ha-1, respectively. The average YL for STY in two crop seasons were 40.5%, 35.2%, and 48.2% due to limitations of N or water or N+water, respectively. The SNC was not affected by the water supply, but it was increased by N fertilization. The highest value of Δ13C was obtained for the treatment without N fertilization in rainfed conditions, and the least Δ13C was found for the one with N supply and irrigation. The Δ13C measurement was effective in identifying N and water deficiencies, presenting potential to be used as an indicator of N and water limitations for sugarcane yield.
 
 Keywords: Loss index of stalk biomass; Saccharum ssp.; Water deficit; Water‒N interaction; Water supply

Benchmarking source specific isotopic ratios of levoglucosan to better constrain the contribution of domestic heating to the air pollution

We report source specific isotope ratios of levoglucosan, the specific biomass burning tracer, in aerosol particles from the combustion of selected woods used for domestic heating in Europe, of coals containing cellulose (lignites) as well as of corn, a C4 plant. Here, we combine compound specific δ13C measurements of levoglucosan with total carbon δ13C of parent materials, to assess isotopic fractionations due to biosynthetic pathways or combustion processes. Levoglucosan formed during the combustion of cellulose from coals shows with δ13C of −21.1‰ and −18.6‰ a moderate enrichment in the heavier isotope compared to the C3 plant samples. Contrarily, observed levoglucosan isotope ratios of −25.0 to −21.5‰ for C3 plant samples are significantly lower than for the C4 plant sample (−12.4‰), as expected from the stronger 13C discrimination during the carbon fixation process by C3 compared to C4 plants. Overall, the C4 plant sample shows a 13C enrichment in all bulk measurements, on average by 12.2‰, 14.2‰ and 14.2‰ for total carbon (TC) in aerosol particle, whole plant/coal material and cellulose samples, respectively. Further, δ13C measurements of levoglucosan and TC for biomass burning aerosol particles, bulk plant/coal and cellulose in C3 plant samples agree well with the published observations. The combined levoglucosan/TC isotopic analyses can be used to differentiate among C3/coal/C4 origin of the smoke emissions from the cellulose-containing-fuel combustion. Noticeably, there is a consistent δ13C offset between C3 plant material and levoglucosan, which allows deriving emission levoglucosan isotope ratios when the combusted plant types are known.

Exploring the paleoceanographic changes registered by planktonic foraminifera across the Cenomanian-Turonian boundary interval and Oceanic Anoxic Event 2 at southern high latitudes in the Mentelle Basin (SE Indian Ocean)

Planktonic foraminiferal population dynamics and benthic foraminiferal and radiolaria distributions combined with δ13C and δ18O measurements of both bulk carbonate and foraminifera provide clues concerning the paleoceanographic changes across the Cenomanian-Turonian boundary interval and the Oceanic Anoxic Event 2 (OAE 2) at southern high latitudes. Samples analyzed are from Integrated Ocean Discovery Program (IODP) Expedition 369 Site U1516 in the Mentelle Basin (eastern flank of the Naturaliste Plateau, Indian Ocean, SW Australia). Site U1516 was located at 60°–62°S paleolatitude during the mid-Cretaceous, and it is the first high latitude locality in the Southern Hemisphere where planktonic foraminifera are consistently recorded across the OAE 2 interval and its associated positive δ13C excursion. The sedimentary record at Site U1516 consists of a sequence of alternating black, dark greenish gray, and light greenish gray claystone in the Cenomanian that grade to white and light gray calcareous chalk interbedded with chert in the Turonian. The correlation between the δ13C and δ18O profiles at Site U1516 and the European reference section at Eastbourne (England) coupled with the integrated calcareous plankton biostratigraphy and stable isotopic data at Site U1516, indicate that a complete record of OAE 2 at Site U1516 was recovered. Below and in the lower part of OAE 2, the planktonic foraminiferal assemblages are dominated by small-sized (125–38 μm) opportunistic species of Microhedbergella and radiolaria indicating a dominantly eutrophic regime. Above the onset of OAE 2, a trough in the δ13C profile (Plenus Carbon Isotope Event: P-CIE) coinciding with a δ18O increase may correspond to the Plenus Cold Event as observed at low latitudes, although no evidence of cooling is registered in the microfossil assemblages. At Site U1516, the middle part of OAE 2 at the initiation of the plateau phase of the δ13C profile is masked by absence of carbonate, by the highest TOC values, and high biogenic silica (dominance of radiolaria) indicating this interval corresponded to a time of highly stressed eutrophic conditions with possible shoaling of the Carbonate Compensation Depth (CCD). Above this interval, bulk isotopic results yield lower δ13C values, and the CaCO3 increases are associated with the presence of even smaller-sized Microhedbergella showing cyclic fluctuations in absolute abundances with benthic foraminifera indicating dominantly eutrophic conditions likely affected by upwelling of nutrient-rich and δ13C-depleted intermediate water masses. Toward the top of OAE 2 and across the Cenomanian-Turonian boundary interval, the planktonic foraminiferal assemblages show changes in composition (e.g., Microhedbergella is replaced by Muricohedbergella), species occupying relatively deep ecological niches appear and an overall increase in diversity is observed. These features coupled with the foraminiferal species-specific δ13C and δ18O patterns reveal that Site U1516 occupied a paleoceanographic setting still affected by eutrophy likely related to enhanced input of nutrients but with episodes of stability with ecological/thermal separation in the surface waters. This interval also records the highest sea surface water paleotemperatures values estimated as 20°–23°C based on δ18O values of foraminiferal test and assuming seawater δ18O values of −1‰V-SMOW. Mesotrophic to oligotrophic conditions persisted after the OAE 2 and throughout the Turonian as evidenced by a diverse planktonic foraminiferal assemblage with different species occupying separate ecological niches in the mixed layer and thermocline.

Paired analysis of tree ring width and carbon isotopes indicates when controls on tropical tree growth change from light to water limitations.

Light and water availability are likely to vary over the lifespan of closed-canopy forest trees, with understory trees experiencing greater limitations to growth by light and canopy trees greater limitation due to drought. As drought and shade have opposing effects on isotope discrimination (Δ13C), paired measurement of ring width and Δ13C can potentially be used to differentiate between water and light limitations on tree growth. We tested this approach for Cedrela trees from three tropical forests in Bolivia and Mexico that differ in rainfall and canopy structure. Using lifetime ring width and Δ13C data for trees of up to and over 200 years old, we assessed how controls on tree growth changed from understory to the canopy. Growth and Δ13C are mostly anti-correlated in the understory, but this anti-correlation disappeared or weakened when trees reached the canopy, especially at the wettest site. This indicates that understory growth variation is controlled by photosynthetic carbon assimilation due to variation in light levels. Once trees reached the canopy, inter-annual variation in growth and Δ13C at one of the dry sites showed positive correlations, indicating that inter-annual variation in growth is driven by variation in water stress affecting stomatal conductance. Paired analysis of ring widths and carbon isotopes provides significant insight in what environmental factors control growth over a tree’s life; strong light limitations for understory trees in closed-canopy moist forests switched to drought stress for (sub)canopy trees in dry forests. We show that combined isotope and ring width measurements can significantly improve our insights in tree functioning and be used to disentangle limitations due to shade from those due to drought.

Sources and sinks of methane in sea ice

We report on methane (CH4) stable isotope (δ13C and δ2H) measurements from landfast sea ice collected near Barrow (Utqiagvik, Alaska) and Cape Evans (Antarctica) over the winter-to-spring transition. These measurements provide novel insights into pathways of CH4 production and consumption in sea ice. We found substantial differences between the two sites. Sea ice overlying the shallow shelf of Barrow was supersaturated in CH4 with a clear microbial origin, most likely from methanogenesis in the sediments. We estimated that in situ CH4 oxidation consumed a substantial fraction of the CH4 being supplied to the sea ice, partly explaining the large range of isotopic values observed (δ13C between –68.5 and –48.5 ‰ and δ2H between –246 and –104 ‰). Sea ice at Cape Evans was also supersaturated in CH4 but with surprisingly high δ13C values (between –46.9 and –13.0 ‰), whereas δ2H values (between –313 and –113 ‰) were in the range of those observed at Barrow. These are the first measurements of CH4 isotopic composition in Antarctic sea ice. Our data set suggests a potential combination of a hydrothermal source, in the vicinity of the Mount Erebus, with aerobic CH4 formation in sea ice, although the metabolic pathway for the latter still needs to be elucidated. Our observations show that sea ice needs to be considered as an active biogeochemical interface, contributing to CH4 production and consumption, which disputes the standing paradigm that sea ice is an inert barrier passively accumulating CH4 at the ocean-atmosphere boundary.

Leaf 13C data constrain the uncertainty of the carbon dynamics of temperate forest ecosystems

AbstractStable carbon isotope discrimination occurred in plant biophysical and biogeochemical processes can help understand plant physiology and soil biogeochemistry with respect to carbon cycling. Here, we incorporated the photosynthetic carbon isotope discrimination into a process‐based land surface model (iTEM) to test if stable carbon isotope composition (δ13C) can impose additional constraint on model parameters. Sequential data assimilation was implemented at six eddy covariance flux tower sites using carbon flux observations including gross primary productivity (GPP) and net ecosystem exchange (NEE) with and without considering foliar δ13C (δ13Cf) measurement constraints, respectively. Our model‐data fusion showed that δ13Cf can provide useful constraint on photosynthetic (Vcmax25, the maximum rate of carboxylation at 25°C) and stomatal (g1, the slope of stomatal function) parameters as well as the posterior carbon fluxes. When including δ13Cf measurement, g1 spatially varies among the six sites and is significantly correlated with annual precipitation. We incorporated the statistical relationship between g1 and annual precipitation into iTEM, which is then used to quantify the regional carbon dynamic in temperate forest ecosystems of the Northern Hemisphere. Compared with the simulation only conditioned on carbon flux observations, regional carbon flux estimations performed slightly better against the FLUXCOM products and the uncertainties of modeled carbon fluxes were reduced by 27%. Our study demonstrated that δ13Cf data constrains carbon flux uncertainties across space.

Isolating Detrital and Diagenetic Signals in Magnetic Susceptibility Records From Methane‐Bearing Marine Sediments

AbstractVolume‐dependent magnetic susceptibility (κ) is commonly used for paleoenvironmental reconstructions in both terrestrial and marine sedimentary environments where it reflects a mixed signal between primary deposition and secondary diagenesis. In the marine environment, κ is strongly influenced by the abundance of ferrimagnetic minerals regulated by sediment transport processes. Post‐depositional alteration by H2S, however, can dissolve titanomagnetite, releasing reactive Fe that promotes pyritization and subsequently decreases κ. Here, we provide a new approach for isolating the detrital signal in κ and identifying intervals of diagenetic alteration of κ driven by organoclastic sulfate reduction (OSR) and the anaerobic oxidation of methane (AOM) in methane‐bearing marine sediments offshore India. Using the correlation of a heavy mineral proxy from X‐ray fluorescence data (Zr/Rb) and κ in unaltered sediments, we predict the primary detrital κ signal and identify intervals of decreased κ, which correspond to increased total sulfur content. Our approach is a rapid, high‐resolution method that can identify overprinted κ resulting from pyritization of titanomagnetite due to H2S production in marine sediments. In addition, total organic carbon, total sulfur, and authigenic carbonate δ13C measurements indicate that both OSR and AOM can drive the observed κ loss, but AOM drives the greatest decreases in κ. Overall, our approach can enhance paleoenvironmental reconstructions and provide insight into paleo‐positions of the sulfate‐methane transition zone, past enhancements of OSR or paleo‐methane seepage, and the role of detrital iron oxide minerals on the marine sediment sulfur sink, with consequences influencing the development of chemosynthetic biological communities at methane seeps.

Advances in carbon isotope analysis of trapped methane and volatile hydrocarbons in crystalline rock cores.

The isotopic composition of hydrocarbons trapped in rocks on the microscale (fluid inclusions, mineral grain boundaries, microfractures) can provide powerful information on geological and biological processes but are an analytical challenge due to low concentrations. We present a new approach for the extraction and carbon isotopic analysis of methane (CH4 ) and hydrocarbons in trapped volatiles in crystalline rocks. An off-line crusher with cryogenic trapping and a custom-made silica glass U-trap were attached to an external injector port on a continuous flow gas chromatograph/combustion/isotope ratio mass spectrometer to demonstrate the accuracy, reproducibility, and sensitivity of δ13 C measurements for CH4 . The method can isotopically characterize CH4 in crushed rock samples with concentrations as low as 3.5 × 10-9 mol/g of rock, and both sample and isotopic standards are analyzed with an accuracy and reproducibility of ±0.5‰. High H2 O/CH4 ratios of 98 to 500 have no effect on measured δ13 CCH4 values. The method is successfully applied to natural samples from the north range of Sudbury Basin, Ontario, Canada. The δ13 C isotopic signatures of CH4 trapped microscopically in rock from the north range overlap significantly with that of CH4 contained in larger scale flowing fracture fluids from the same part of the Sudbury Basin, indicating a potential genetic link. A novel method for δ13 CCH4 analysis was developed for the extraction of nanomole quantities of CH4 trapped microscopically in rocks. The technique has an accuracy and reproducibility comparable to that of on-line crushing techniques but importantly provides the capability of crushing larger rock quantities (up to 100 g). The benefit is improved detection levels for trace hydrocarbon species. Such a capability will be important for future extension of such crushing techniques for measurement of 2 H/1 H for CH4 , clumped isotopologues of CH4 and other trapped volatiles species, such as C2 H6 , C3 H8 , C4 H10 , CO2 and N2 .