Combined 14C and δ13C Monitoring of in Situ Biodegradation of Petroleum Hydrocarbons

The ecosystem carbon balance of high latitude and high altitude ecosystems is particularly sensitive to climate change, where increasing temperatures generally lead to a rise of the ecosystem carbon storage, but also increasing carbon turnover times. In this study, we investigated the carbon dynamics of the 150-year long Damma Glacier forefield chronosequence, Switzerland. Specifically, we performed radiocarbon analysis of a range of organic matter fractions, sampled in 2007, 2017 and 2022 from soils developing on areas having been exposed for 20-150 years due to the retreat of the glacier. To characterize the age spectrum of material making up the bulk soil carbon, we isolated a range of different fractions, from supposedly 'stable' carbon pools (fine mineral-bound, and peroxide-resistant carbon), microbially ‘labile’ respired CO2, dissolved soil organic carbon (DOC), hydrophobic leaf wax-derived alkanes, and microbial-derived fatty acids. Comparison of our results with the penetration of the radiocarbon bomb spike and the increase of soil and ecosystem carbon over the both the chronosequence (space-for-time) and over the sampling period (time-for-time) allowed us to make the following inferences: (i) A small but persistent contribution of ancient carbon is present in the forefield area exposed by the glacier, which is particularly visible in the hydrophobic leaf wax 14C data. From this we conclude that this old carbon pool is at least in part a remnant of ancient soil carbon from a previous warm and glacier-free period, potentially adding to contributions of fossil-fuel derived black carbon deposition. (ii) There is a significant portion of soil carbon with a decadal-scale carbon turnover rate, and (iii) mineral-bound carbon clearly has a slower turnover time. (iv) Microbial lipids, soil CO2 and DOC 14C content reflect different carbon sources: in younger soils, relatively low 14C contents indicate a higher relative contribution of ancient carbon decomposition, while in older soils this signal is swamped by decomposition of freshly photosynthesized organic matter.

Biodegradation of CO2, CH4 and volatile organic compounds (VOCs) in soil gas from the Vicano–Cimino hydrothermal system (central Italy)

A compact laser spectrometry instrument was developed for high precision measurements of isotope ratio of CO2 by tunable diode laser absorption spectroscopy in the mid-infrared at 2.7 μm. The experimental spectrum of carbon isotopologues in the gas phase near 3641 cm−1 is very suitable for real-time analysis of these isotopologues. Simultaneous measurements of the mixing ratio and the corresponding δ13C values of CO2 in the atmosphere were performed. The achieved standard deviation (1σ) of δ13C was 1.8‰. The Allan analysis of the time series of the mixing ratio of CO2 shows a measurement precision of 0.2‰ for δ13C with an optimum integration time of about 130 s. The spectrometer is capable of real-time measurements of stable carbon isotope ratios of CO2 under ambient conditions.

The effect of Europa and Enceladus analog seawater composition on isotopic measurements of volatile CO2

ABSTRACTWhile simultaneous radiocarbon and δ13C measurements have been available for organic materials (by accelerator mass spectrometry, AMS, and isotope ratio mass spectrometry, IRMS, respectively), this has not been possible for carbonates until now. Using an existing interface for gas ion source AMS measurements, we developed a prototype for a universal gas interface that allows simultaneous measurement of both carbon isotope ratios from potentially any source of CO2. First results obtained from reference materials (IAEA-C6, OxaII, PhA, IAEA-C1, IAEA-C2, ETH-4) show that for both organic as well as carbonate samples, the precision of radiocarbon measurements in the coupled mode is comparable to routine standalone AMS measurements. For IRMS δ13C measurements, the performance for different materials shows more variation with precisions ranging from 0.07‰ to 0.47‰. However, both organic materials and carbonates can achieve precisions as good as 0.13‰. Once fully automated, the method shows potential for filling the gap of simultaneous carbon isotope measurements for non-organic materials.

Global efforts target a soil organic carbon (SOC) enhancement rate of 2.4‰ y-1 in the upper 30cm of agricultural soils to address declining soil productivity associated with declining SOC stocks. We explored a unique chronosequence of homogeneous soils formed after mining in Germany, which serve conventional agriculture and exhibit a large margin for SOC storage, but limited SOC accrual, to study SOC protection mechanisms and accrual constraints. We hypothesized that limited SOC accrual is associated to insufficient nitrogen rather than to minerals saturation. Soil samples (0-30 and 30-60cm) were collected across the chronosequence (0-56years) and compared to an original non-mined soil (OS) managed similarly. The mean residence time (MRT) of SOC and its protection mechanisms were studied using soil incubation, organic matter density fractionation, and δ13C measurements. After 56years, total and mineral-associated SOC (MAOC) stocks remained 18% and 28% lower than in the OS at 0-30cm, with estimated replenishment times of 93 and 129years, respectively. Mineral-associated nitrogen (MAN) stocks stagnated along recultivation time below OS level. Together with significant linear correlation of MAOC with total SOC and MAN stocks, these results indicate that nitrogen rather than saturation of minerals limits SOC accrual. In fact, the MAOC stock deficit to saturation was estimated at 316.4Mgha-1. At 30-60cm, SOC and nitrogen stocks were restored within 30years, due to comparatively lower initial losses. The initial MRT of SOC at 0-30 and 30-60cm (15.3 and 27.9years) declined before finally becoming comparable to OS (11.7 and 7.7years). This reflected new carbon entering the soil that initially contained predominantly MAOC (78-82%), followed by its stabilization as MAOC. Due to their susceptibility to nitrogen losses, degraded soils require tailored nitrogen management to restore SOC stocks and comply with European laws requiring agricultural SOC accrual.

Afforestation has been considered to be the cost-effective way to sequestrate carbon (C) dioxide from the atmosphere in the soils, while concurrent responses of soil inorganic C (SIC) and soil organic C (SOC), and their turnover are still not well-defined. During the C cycle, inorganic C is enriched in heavy isotopes (13C), while organic C, due to photosynthetic fractionation, is enriched in light isotopes (12C). This leads to distinct C isotope fractionation in terrestrial ecosystems. In this study, 72 paired soils (0–20 cm) from poplar shelterbelts and adjacent farmland sites were collected in six regions (Zhaozhou, Fuyu, Dumeng, Zhaodong, Lanling, and Mingshui) of northeastern China. Five soil fractions of dissolved organic C (DOC), particulate organic matter (POM), sand and stable aggregates (S + A), silt and clay (S + C), and resistant SOC (rSOC) and bulk soils were used in C content assay and the natural δ13C determination. The results showed that, compared with SOC, poplar shelterbelts resulted in SIC accrual in the soils across all six sites; however, only half of the six sites showed SOC accrual, indicating an ambiguous effect of afforestation on SOC. The natural δ13C method could identify the SOC turnover owing to the C isotopic discrimination. The δ13C–SOC-derived turnover ratio was 23%. When SIC was included in the δ13C measurement, bulk soils and four soil fractions (S + C, S + A, rSOC, DOC) showed a 2%–10% lower turnover percentage than the δ13C–SOC-derived turnover ratios. The SIC inclusion resulted in the dependency of δ13C–TC (TC = SIC + SOC) values on SOC (negative, R2: 0.21–0.44) and SIC content (positive, R2: 0.39–0.63). By contrast, when SIC was excluded, the δ13C–SOC values were independent of them (R2 < 0.18). Redundancy ordination analysis manifested that more SOC in the soils, together with more POM and farming uses would be accompanied with the lower δ13C values. Moreover, forest characteristics (e.g., age and density) and farmland backgrounds (e.g., crop history and distance between forest and farmland) could explain differences in δ13C-related features. Our results highlighted that SIC in natural δ13C determination underestimated the C turnover ratio in general. However, SIC storage should be included in the soil C sequestration evaluation owing to a general SIC accrual pattern across regions when compared with those of SOC.

Usage of thermal energy contained in abandoned, flooded, coal mines has the potential to contribute to low carbon heating or cooling supply and assist in meeting net-zero carbon emission targets. However, hazardous ground gases, such as CH 4 and CO 2 , can be found naturally in superficial deposits, coal bearing strata and abandoned mines. Determining the presence, magnitude, and origin of subsurface gases, and how their geochemical fingerprints evolve within the shallow subsurface is vital to developing an understanding of how to manage the risk posed by ground gases in geoenergy technology development. Here, we present the first CH 4 and CO 2 concentration-depth profiles and stable isotope (δ 13 C CH4 , δ 13 C CO2 , and δD CH4 ) profiles obtained from UK mine workings, through analysis of headspace gas samples degassed from cores and chippings collected during construction of the Glasgow Observatory. These are used to investigate the variability of gas fingerprints with depth within unmined Carboniferous coal measures and Glasgow coal mine workings. Stable isotope compositions of CH 4 (δ 13 C CH4 = −73.4‰ to −14.3‰; δ 13 C CO2 = −29‰ to −6.1‰; δD CH4 = −277‰ to −88‰) provide evidence of a biogenic source, with carbonate reduction being the primary pathway of CH 4 production. Gas samples collected at depths of 63–79 m exhibit enrichments in 13C CH4 and 2 H, indicating the oxidative consumption of CH 4 . This correlates with their proximity to the Glasgow Ell mine workings, which will have increased exposure to O 2 from the atmosphere as a result of mining activities. CO 2 gas is more abundant than CH 4 throughout the succession in all three boreholes, exhibiting high δ 13 C CO2 values relative to the CH 4 present. Gases from unmined bedrock exhibit the highest δ 13 C CO2 values, with samples from near-surface superficial deposits having the lowest δ 13 C CO2 values. δ 13 C CO2 values become progressively lower at shallower depths (above 90 m), which can be explained by the increasing influence of shallow groundwaters containing a mixture of dissolved marine carbonate minerals (∼0‰) and soil gas CO 2 (−26‰) as depth decreases. Our findings provide an insight into the variability of mine derived gases within 200 m of the surface, providing an important ‘time-zero’ record of the site, which is required in the design of monitoring approaches.

Soil gas monitoring at the Illinois Basin – Decatur Project carbon sequestration site

The carbonate hydrochemistry of groundwater from the Hvinningdal aquifer (Denmark) was studied by radiocarbon (accelerator mass spectrometry (AMS)) and δ13C measurements as a preliminary step towards 14C groundwater dating. The 14C concentrations varied between 30 and 100 percent modern carbon (pMC) in apparent contradiction with tritium (3H) data, which in most cases indicate a post-bomb date. The dilution of 14C can be explained as being due to the combined effect of dissolution of old soil carbonate and oxidation of old organic carbon. The last effect proved to be essential. To calculate this correction the dissolved oxygen concentration was used together with the δ13C values. The combined corrections bring the 14C concentrations up to post-bomb levels in better agreement with the 3H data.

Continuous measurements of stable carbon isotopes in CO2 with a near-IR laser absorption spectrometer

We describe a comprehensive study of carbon isotopes in several karst springs and their environs in a contemporary karst environment in the region of the Cracow-Wieluñ Upland and Western Tatra Mountains, Southern Poland. We collected samples of water, plants and carbonate deposited on aquatic plants, and obtained13C values and14C concentrations. We also investigated a group of the youngest calcium carbonates from caves where deposition is still being observed or ceased no more than a few hundred years ago. The determination of a14C dilution factor (q) in these carbonates allows us to determine the “true” radiocarbon ages of old speleothems from caves in the area under investigation and enables the use of old speleothems as suitable material for extending the14C calibration time scale, the “Absolute” age having been determined by U/Th or amino acid racemization (AAR) dating methods. Measurements of δ13C and14C concentrations were made on dissolved inorganic carbon (DIC) extracted from water samples. Calculated values of q range from 0.55 to 0.68 and δ13C values range from −10% to −13% versus VPDB with mean values equal to 0.65 and −12%, respectively. Results indicate that the dissolution process of limestone bedrock is a closed system with the dominating contributor being biogenic carbon dioxide.Isotopic composition of carbon in contemporary plants collected at the karstic springs at 3 localities is highly diverse, with different species distinctly varying in both q and δ13C values. Extremely light values of13C (under −40%), observed in Algae andHyloconium splendens, are correlated with14C concentrations that are much lower than 100 pMC. Small systematic changes of isotopic composition were found in plants of the same species collected along streams at various distances from the spring. The youngest calcium carbonates from different caves show a relatively high scatter of both δ13C values and14C concentration. The lower reservoir effect for14C is observed in samples with higher value of δ13C, indicating equilibrium conditions in the sedimentation of carbonate. Pazdur et al. (1995b) presented14C dating results and paleoclimatic interpretation of 17014C analyses of 89 speleothems from 41 caves obtained through 1994. Investigations continued until early 1997, during which time a speleothem, JWi2, was dated by14C, U/Th and AAR dating methods, and its stable isotope composition (δ13C and δ18O) analyzed in detail (reported here). Carbon isotope analyses indicate very large differences among results obtained by U/Th, AAR, and14C dating methods.

The 222Rn and CO2 soil gas distribution at Lembang Fault Zone, West Java - Indonesia

SummaryWe propose that diets of consumers in a food web have various ages, where age is defined as the time elapsed since carbon (C) in the diet was fixed from atmospheric CO2by primary producers. To examine the diet ages for primary consumers in a detrital food web, we measured the radiocarbon (14C) content of termites collected in Thailand in 1998 and 2004. Diet ages were estimated by comparing the14C content of samples with records of atmospheric14CO2, which doubled in the early 1960s as a result of nuclear weapons tests and decreased after the nuclear test ban treaty. For comparison, we measured the14C content of bees as primary consumers in a grazing web at the same study site. Stable carbon and nitrogen (N) isotope ratios were also analysed.The14C contents of the same species of termites decreased during the sampling interval, indicating that they used organic matter produced after the peak in atmospheric14CO2. The diet ages were estimated to be 12–18, 7–13 and 5–9 years for the wood‐feeder (Microcerotermes crassus), the soil‐feeders (Dicuspiditermes makhamensisandTermes comis) and the fungus‐grower (Macrotermes carbonarius), respectively. One colony of soil‐feeder (T. comis), which nested in a fallen tree trunk, had exceptionally low14C content, and its diet age was estimated to be around 50 years. The two bee species had lower14C contents compared with the termites, and their diet ages were estimated to be 0 (Apis florea) and 2–4 years (Trigonasp.).Stable C and N isotope ratios of termites showed similar patterns as previously reported, and no clear difference was observed between 1998 and 2004. Although the bees and the fungus‐growing termite had similar stable C and N isotope ratios, their diet ages differed.Our study suggests that radiocarbon can be used to estimate the diet ages of consumers in terrestrial food webs. Diet age should provide new insight into the trophic positions of organisms in grazing and detrital food webs and the interactions between these two webs.

Bioenergy crops are expected to provide biomass to replace fossil resources and reduce greenhouse gas emissions. In this context, changes in soil organic carbon (SOC) stocks are of primary importance. The aim of this study was to measure changes in SOC stocks in bioenergy cropping systems comparing perennial (Miscanthus × giganteus and switchgrass), semi‐perennial (fescue and alfalfa), and annual (sorghum and triticale) crops, all established after arable crops. The soil was sampled at the start of the experiment and 5 or 6 years later. SOC stocks were calculated at equivalent soil mass, and δ13C measurements were used to calculate changes in new and old SOC stocks. Crop residues found in soil at the time of SOC measurements represented 3.5–7.2 t C ha−1 under perennial crops vs. 0.1–0.6 t C ha−1 for the other crops. During the 5‐year period, SOC concentrations under perennial crops increased in the surface layer (0–5 cm) and slightly declined in the lower layers. Changes in δ13C showed that C inputs were mainly located in the 0–18 cm layer. In contrast, SOC concentrations increased over time under semi‐perennial crops throughout the old ploughed layer (ca. 0–33 cm). SOC stocks in the old ploughed layer increased significantly over time under semi‐perennials with a mean increase of 0.93 ± 0.28 t C ha−1 yr−1, whereas no change occurred under perennial or annual crops. New SOC accumulation was higher for semi‐perennial than for perennial crops (1.50 vs. 0.58 t C ha−1 yr−1, respectively), indicating that the SOC change was due to a variation in C input rather than a change in mineralization rate. Nitrogen fertilization rate had no significant effect on SOC stocks. This study highlights the interest of comparing SOC changes over time for various cropping systems.