Spatial and seasonal variations in the stable C isotope composition of dissolved inorganic carbon and in physico-chemical water parameters in the Plitvice Lakes system

Plitvice Lakes waters were collected at 14 sampling points, including springs, tributaries and lakes, for the period 2002–2007. The results of the physical and chemical conditions of calcite precipitation as well as the δ¹³C values of dissolved inorganic carbon (DIC) were used to study the processes influencing calcite precipitation. Significant differences between spring, lake and stream waters as well as changes in the downstream direction were observed. The correlation between δ¹³C_DIC values and physico-chemical conditions for calcite precipitation showed that calcite precipitates in lake waters which are oversaturated with respect to CaCO₃ (I _sat values 4–10) and with δ¹³C_DIC values between?11.5 and?8.5 ‰. In spring waters, the δ¹³C_DIC values were more negative, from?14 to?12 ‰, and I _sat values of 1–2 indicated that equilibrium conditions for calcite precipitation were not attained. The downstream increase in δ¹³C_DIC correlated with the increase in the δ¹³C values of calcite in the lake sediments, suggesting that the freshwater calcite was mainly of autochthonous origin and precipitated within the water column in isotopic equilibrium with DIC.     

Reliability of stable carbon and oxygen isotope compositions of pedogenic needle fibre calcite as environmental indicators: examples from Western Europe

Stable carbon and oxygen isotope analyses were conducted on pedogenic needle fibre calcite (NFC) from seven sites in areas with roughly similar temperate climates in Western Europe, including the Swiss Jura Mountains, eastern and southern France, northern Wales, and north-eastern Spain. The δ(13)C values (-12.5 to-6.8 ‰ Vienna Pee Dee Belemnite (VPDB)) record the predominant C(3) vegetation cover at the sites. A good correlation was found between mean monthly climatic parameters (air temperature, number of frost days, humidity, and precipitation) and δ(18)O values (-7.8 to-3.4‰ VPDB) of all the NFC. Similar seasonal variations of δ(18)O values for monthly NFC samples from the Swiss sites and those of mean monthly δ(18)O values of local precipitation and meteorological data point out precipitation and preferential growth/or recrystallisation of the pedogenic needle calcite during dry seasons. These covariations indicate the potential of stable isotope compositions of preserved NFC in fossil soil horizons as a promising tool for palaeoenvironmental reconstructions.     

Stable carbon isotopes in dissolved inorganic carbon: extraction and implications for quantifying the contributions from silicate and carbonate weathering in the Krishna River system during peak discharge

We present a comparative study of two offline methods, a newly developed method and an existing one, for the measurement of the stable carbon isotopic composition (δ¹³C) of dissolved inorganic carbon (DIC; δ¹³C_DIC) in natural waters. The measured δ¹³C_DIC values of different water samples, prepared from laboratory Na₂CO₃, ground and oceanic waters, and a laboratory carbonate isotope standard, are found to be accurate and reproducible to within 0.5 ‰\ (1σ). The extraction of CO₂ from water samples by these methods does not require pre-treatment or sample poisoning and can be applied to a variety of natural waters to address carbon cycling in the hydrosphere. In addition, we present a simple method (based on a two-end-member mixing model) to estimate the silicate-weathering contribution to DIC in a river system by using the concentration of DIC and its δ¹³C. This approach is tested with data from the Krishna River system as a case study, thereby quantifying the contribution of silicate and carbonate weathering to DIC, particularly during peak discharge.     

Stable water isotope patterns in a climate change hotspot: the isotope hydrology framework of Corsica (western Mediterranean)

The Mediterranean is regarded as a region of intense climate change. To better understand future climate change, this area has been the target of several palaeoclimate studies which also studied stable isotope proxies that are directly linked to the stable isotope composition of water, such as tree rings, tooth enamel or speleothems. For such work, it is also essential to establish an isotope hydrology framework of the region of interest. Surface waters from streams and lakes as well as groundwater from springs on the island of Corsica were sampled between 2003 and 2009 for their oxygen and hydrogen isotope compositions. Isotope values from lake waters were enriched in heavier isotopes and define a local evaporation line (LEL). On the other hand, stream and spring waters reflect the isotope composition of local precipitation in the catchment. The intersection of the LEL and the linear fit of the spring and stream waters reflect the mean isotope composition of the annual precipitation (δ_P) with values of?8.6(±0.2) ‰ for δ¹⁸O and?58(±2) ‰ for δ²H. This value is also a good indicator of the average isotope composition of the local groundwater in the island. Surface water samples reflect the altitude isotope effect with a value of?0.17(±0.02) ‰ per 100 m elevation for oxygen isotopes. At Vizzavona Pass in central Corsica, water samples from two catchments within a lateral distance of only a few hundred metres showed unexpected but systematic differences in their stable isotope composition. At this specific location, the direction of exposure seems to be an important factor. The differences were likely caused by isotopic enrichment during recharge in warm weather conditions in south-exposed valley flanks compared to the opposite, north-exposed valley flanks.     

Reliability of stable carbon and oxygen isotope compositions of pedogenic needle fibre calcite as environmental indicators: examples from Western Europe

Stable carbon and oxygen isotope analyses were conducted on pedogenic needle fibre calcite (NFC) from seven sites in areas with roughly similar temperate climates in Western Europe, including the Swiss Jura Mountains, eastern and southern France, northern Wales, and north-eastern Spain. The δ¹³C values (?12.5 to?6.8 ‰ Vienna Pee Dee Belemnite (VPDB)) record the predominant C₃ vegetation cover at the sites. A good correlation was found between mean monthly climatic parameters (air temperature, number of frost days, humidity, and precipitation) and δ¹⁸O values (?7.8 to?3.4‰ VPDB) of all the NFC. Similar seasonal variations of δ¹⁸O values for monthly NFC samples from the Swiss sites and those of mean monthly δ¹⁸O values of local precipitation and meteorological data point out precipitation and preferential growth/or recrystallisation of the pedogenic needle calcite during dry seasons. These covariations indicate the potential of stable isotope compositions of preserved NFC in fossil soil horizons as a promising tool for palaeoenvironmental reconstructions.     

Determination of (13)C/(12)C-Ratios in Rumen Produced Methane and CO(2) of Cows, Sheep and Camels

Abstract Naturally produced methane shows different δ(13)C-values with respect to its origin, e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane-given in literature-range between -80‰ and -50‰ and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gases via implanted fistulae we compared δ(PDB) (13)C-values of methane and CO(2) between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n = 47) we calculated δ(PDB) (13)C-medians resulting in small but not significant differences within and significant differences between the species for CO(2) and methane. The δ(PDB) (13)C-differences between methane and CO(2) were statistically equal within and also between the species. Therefore a linear regression of methane values on CO(2) is appropriate and leads to: δ(PDB) (13)C(methane)‰ = 1,57 * δ(PDB) (13)C(CO(2))‰-47‰ with a correlation coefficient of r = 0,87.     

Mineral- und heilwässer in sachsen - eine isotopenanalytische charakterisierung

Abstract The groundwaters studied and labelled as mineral water were "natural mineral waters" for bottled waters and "natural curative waters" for heal therapeutical applications. They were characterized either by a specific mineralization or their suitability for balneology. To reveal the actual hydrological situation isotope investigations using (2)H, (18)O, (3)H, (12)C and (14)C (DIC) and (34)S (sulphate) were included in a study describing samples of 24 mineral water deposits in Saxonia. The water was classified into 4 hydrochemical types of genesis. Due to different hydrogeological and hydrochemical situations widely scattered isotope ratios were measured. Most of the investigated mineral waters are containing at least parts of younger waters (with residence times less than about 40 years). Correlations between chemical composition and the tritium content could be observed within different springs from the areas Bad Brambach, Bad Elster and Burkhardswalde. Strong variations in δ(34)S were found in samples with low sulphate content, showing different sulphur sources, as well as microbiological reactions. On the other hand mineral waters from Bad Brambach and Bad Elster show nearly the same δ(34)S value of about 6‰ CDT despite beeing of a different chemical type. The δ(13)C values between -22 and -2.2y PDB are related to different sources of CO(2).     

C and O stable isotopic signatures of fast-growing dripstones on alkaline substrates: reflection of growth mechanism, carbonate sources and environmental conditions

Secondary carbonate precipitates (dripstones) formed on concrete surfaces in four different environments--Mediterranean and continental open-space and indoor environments (inside a building and in a karstic cave)--were studied. The fabric of dripstones depends upon water supply, pH of mother solution and carbonate-resulting precipitation rate. Very low δ(13)C (average-28.2‰) and δ(18)O (average-18.4‰) values showed a strong positive correlation, typical for carbonate precipitated by rapid dissolution of CO(2) in a highly alkaline solution and consequent disequilibrium precipitation of CaCO(3). The main source of carbon is atmospheric or biogenic CO(2) in the poorly ventilated karstic cave, which is reflected in even lower δ(13)C values. Statistical analysis of δ(13)C and δ(18)O values of the four groups of samples showed that the governing factor of isotope fractionation is not the temperature, but rather the precipitation rate.     

Measuring δ(13)C values of atmospheric acetaldehyde via sodium bisulfite adsorption and cysteamine derivatisation

δ(13)C values of gaseous acetaldehyde were measured by gas chromatograph-combustion-isotope ratio mass spectrometer (GC-C-IRMS) via sodium bisulfite (NaHSO(3)) adsorption and cysteamine derivatisation. Gaseous acetaldehyde was collected via NaHSO(3)-coated Sep-Pak(?) silica gel cartridge, then derivatised with cysteamine, and then the δ(13)C value of the acetaldehyde-cysteamine derivative was measured by GC-C-IRMS. Using two acetaldehydes with different δ(13)C values, derivatisation experiments were carried out to cover concentrations between 0.009×10(-3) and 1.96×10(-3)?mg·l(-1)) of atmospheric acetaldehyde, and then δ(13)C fractionation was evaluated in the derivatisation of acetaldehyde based on stoichiometric mass balance after measuring the δ(13)C values of acetaldehyde, cysteamine and the acetaldehyde-cysteamine derivative. δ(13)C measurements in the derivertisation process showed good reproducibility (<0.5?‰) for gaseous acetaldehyde. The differences between predicted and measured δ(13)C values were 0.04-0.31?‰ for acetaldehyde-cysteamine derivative, indicating that the derivatisation introduces no isotope fractionation for gaseous acetaldehyde, and obtained δ(13)C values of acetaldehyde in ambient air at the two sites were distinct (-34.00?‰ at an urban site versus-31.00?‰ at a forest site), implying potential application of the method to study atmospheric acetaldehyde.     

Spring response to precipitation events using δ18O and δ2H in the Tanour catchment,NW Jordan

The Tanour spring is one of the several karst springs located in the northern part of Jordan. Water samples from the Tanour spring and precipitation were collected in the area of Ajloun in NW Jordan for the analysis of stable oxygen and hydrogen isotopes to evaluate the spring response to precipitation events. Rainwater and snow samples were collected from different elevations during winters of 2013–2014 and 2014–2015. In addition, spring samples were collected between December 2014 and March 2015. δ¹⁸O values in rainwater vary from ?3.26 to ?17.34?‰ (average: ?7.84?±?3.23?‰), while δ²H values range between ?4.4 and ?110.4?‰ (average: ?35.7?±?25.0?‰). Deuterium excess ranges from 17.8 to 34.1?‰ (average: 27.1?±?4.0?‰). The Local Meteoric Water Line for the study area was calculated to be δ²H?=?7.66*δ¹⁸O?+?24.43 (R²?=?0.98). Pre-event spring discharge showed variation in δ¹⁸O (range ?6.29 to ?7.17?‰; average ?6.58?±?0.19?‰) and δ²H values (range ?28.8 to ?32.7?‰; average: ?30.5?±?1.0?‰). In contrast, δ¹⁸O and δ²H rapidly changed to more negative values during rainfall and snowmelt events and persisted for several days before returning to background values. Spring water temperature, spring discharge, and turbidity followed the trend in isotopic composition during and after the precipitation events. The rapid change in the isotopic composition, spring discharge, water temperature, and turbidity in response to recharge events is related to fast water travel times and low storage capacity in the conduit system of the karst aquifer. Based on the changes in the isotopic composition of spring water after the precipitation events, the water travel time in the aquifer is in the order of 5–11 days.     

Effect of preservation on the δ¹³C value of dissolved inorganic carbon in different types of water samples

Recent studies have shown that the stable carbon isotope compositions of dissolved inorganic carbon (δ(13)C(DIC)) of water samples preserved with HgCl(2) and CuSO(4) vary. Furthermore, mercury and cuprum compounds are toxic to the human or biological system and require proper waste disposal. To test the effect of preservation on the δ(13)C value of DIC in different types of water samples, a set of water samples with different DIC concentrations was preserved using different methods, including preserving with inhibitors (CuSO(4) or HgCl(2)), preserving under frozen conditions, filtering through a 0.4 μ m paper filter, and the DIC species precipitated in the form of solid BaCO(3). Our results show that δ(13)C(DIC) values of the samples preserved with CuSO(4) and HgCl(2) become more positive with increased storage time. The δ(13)C(DIC) of the water samples preserved under frozen conditions and the precipitated DIC as BaCO(3) are also more positive than original water samples. However, the δ(13)C values were relatively stable for up to 90 days in all water samples filtered through the 0.4 μ m paper filter and stored under cool conditions (0-4 °C). Therefore, we suggest that the better method for the storage of water samples is to filter the samples through a 0.4 μ m paper filter while out in the field and preserve them under cool conditions, thereby avoiding the use of preservatives.     

C and O stable isotopic signatures of fast-growing dripstones on alkaline substrates: reflection of growth mechanism,carbonate sources and environmental conditions

Secondary carbonate precipitates (dripstones) formed on concrete surfaces in four different environments – Mediterranean and continental open-space and indoor environments (inside a building and in a karstic cave) – were studied. The fabric of dripstones depends upon water supply, pH of mother solution and carbonate-resulting precipitation rate. Very low δ¹³C (average?28.2‰) and δ¹⁸O (average?18.4‰) values showed a strong positive correlation, typical for carbonate precipitated by rapid dissolution of CO₂ in a highly alkaline solution and consequent disequilibrium precipitation of CaCO₃. The main source of carbon is atmospheric or biogenic CO₂ in the poorly ventilated karstic cave, which is reflected in even lower δ¹³C values. Statistical analysis of δ¹³C and δ¹⁸O values of the four groups of samples showed that the governing factor of isotope fractionation is not the temperature, but rather the precipitation rate.     

Stable isotopic and geochemical variability within shallow groundwater beneath a hardwood hammock and surface water in an adjoining slough (Everglades National Park,Florida, USA)

Data from a 10-month monitoring study during 2007 in the Everglades ecosystem provide insight into the variation of δ¹⁸O, δD, and ion chemistry in surface water and shallow groundwater. Surface waters are sensitive to dilution from rainfall and input from external sources. Shallow groundwater, on the other hand, remains geochemically stable during the year. Surface water input from canals derived from draining agricultural areas to the north and east of the Everglades is evident in the ion data. δ¹⁸O and δD values in shallow groundwater remain near the mean of?2.4 and?12 ‰, respectively. ¹⁸O and D values are enriched in surface water compared with shallow groundwater and fluctuate in sync with those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement with the global meteoric water line; however, the local evaporation line (LEL) for surface water and shallow groundwater is δ D=5.6 δ¹⁸O+1.5, a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades is tropical cyclones or fronts. δ deuterium to δ¹⁸O excess (D_ex values) generally reveal two moisture sources for precipitation, a maritime source during the fall and winter (D _ex>10 ‰) and a continental-influenced source (D _ex<10 ‰) in the spring and summer.     

Mytilus galloprovincialis as a bioindicator of environmental conditions: the case of the eastern coast of the Adriatic Sea

The marine mussel Mytilus galloprovincialis lives attached to the surface of hard substrata, where its exposure and relative immobility allow it to record changes in ambient seawater. It is also found along the eastern coast of the Adriatic Sea. Oxygen and carbon isotopes were analysed for calcite and aragonite in separate shell layers, while major, minor and trace elements in the bulk shell were analysed to evaluate environmental conditions such as the temperature of carbonate deposition, freshwater influence and locations of anthropogenic pollution. We found that, on average, aragonite is enriched by 1.1‰ in (13)C and by 0.2‰ in (18)O compared with calcite. The calculated temperatures for M. galloprovincialis shell growth from the investigated area range from 13.4 to 20.9 °C for calcite and from 16.6 to 23.1 °C for aragonite. According to the δ(18)O and δ(13)C values of shell layers, we can separate the investigated area into three locations: those with more influence of freshwater, those with less influence of freshwater and those with marine environments. The highest concentrations of manganese, barium, boron, arsenic, nickel and chromium were observed in shells from Omis, Bacvice and Zablace (Central Adriatic) and Sv. Ivan (South Adriatic), where chemical and heavy industries are located and where sewage is known to be discharged into coastal areas. The highest concentrations of zinc, lead and copper were measured in samples from Pula, Rijeka and Gruz, where there are also ports in addition to industry.     

Stable carbon and nitrogen isotope signatures of root-holoparasitic Cynomorium songaricum and its hosts at the Tibetan plateau and the surrounding Gobi desert in China

We first measured the δ(13)C and δ(15)N values of root holoparasite Cynomorium songaricum and its hosts from 19 sites across four provinces in northwest China, in an attempt to investigate their nutritional relationship at the Tibetan plateau and the surrounding Gobi desert. Our study showed that the δ(13)C of C. songaricum closely mirrored the values of its hosts, Nitraria tangutorum and N. sibirica across all sampling sites. C. songaricum was significantly depleted in (13)C compared to host plants at the Tibetan plateau, showing an average parasite/host δ(13)C difference of-0.6?‰. In contrast, (15)N of C. songaricum was significantly enriched by+1.3?‰ compared to the hosts, implying that these holoparasites had other nitrogen resources. Although no difference in the δ(13)C and δ(15)N values between holoparasites and hosts was detected, the δ(13)C and δ(15)N values of holoparasites were significantly correlated with those of their hosts at the Gobi desert. The δ(13)C versus δ(15)N values were significantly but negatively correlated for the hosts; however, holoparasite/host variation in δ(13)C was not correlated with the variation in δ(15)N. The δ(13)C versus δ(15)N values were negatively correlated in C. songaricum, and this relationship tended to be magnified along the increasing elevations independent of the host plants. C. songaricum at the Tibetan plateau exhibited different δ(13)C and δ(15)N signatures compared with those at the Gobi desert. Furthermore, both δ(13)C and δ(15)N values of C. songaricum and its host plants in salt marshes at the Tibetan plateau were different from those in sand sites at the Tibetan plateau and the Gobi desert. Our results indicate that the isotopic difference depends on the different altitudes and habitats and is host-specific.     

Preface - Contributions to the 8th International Conference on Applications of Stable Isotope Techniques to Ecological Studies (ISOECOL), Brest,France, 20–24 August 2012

Abstract

The study is focused on the dolomite-limestone drinking water aquifers in the Bo? massif, as well as on the andesite-aquifer containing mineral water in the vicinity of Roga?ka Slatina. The catchment area is limited and both drinking and mineral waters are discharged from the same source. The increasing use of deeper aquifers means that natural springs and shallow wells have become sporadic. Consequently new techniques of investigating recharge and aquifer capacity are required which can augment classical hydrogeological methods.

Current research into the mineral and drinking water aquifers in the area of Roga?ka Slatina is based on measuring the isotopic composition of light elements, (H, C and O) as natural tracers. It can be concluded that all the groundwaters investigated are typically infiltrated meteoric water. The drinking waters are generally young and were infiltrated up to about ten years ago. The isotopic composition of oxygen is similar to recent precipitation (δ¹⁸O = -9.3 ± 1‰) and the drinking waters contain tritium. It was found that exploited mineral waters recharged aquifers during colder periods; they are only partly mixed with younger water as can be seen from the isotopic composition of oxygen and corrected ¹⁴C dating, which puts the mean ages at between around 100 and 8,000 years. With regard to the “nuclear period” (1960-64) with abnormally high tritium activities of precipitation, all the waters examined can be divided into at least three main infiltration groups depending on their measured tritium content: around 35 years old (> 80 T.U.), older (> 10 T.U.) and younger (10 to 60 T.U.). Detailed dating is possible following the above classification. Isotope exchange between rocks and water is negligible and therefore very deep circulation at the temperature conditions above 80°C does not occur. Dissolved inorganic carbon (DIC) in the drinking waters is the result of equilibrium reactions between carbonates and organically produced CO₂ (δ¹³C = - 14.5 ± ‰), while the high concentrations of DIC (δ¹³C = + 3 ±‰) and CO₂ observed in the mineral waters are generated by low-temperature decarbonatization processes and indicate the deep origin of CO₂, from where gas migrates into mineral water aquifers.

Correlation analyses between the parameters studied are performed. Useful conclusions concerning water circulation and the capacities of aquifer reservoirs are described which support the future optimal pumping of mineral and drinking water at the limited catchment area of Roga?ka Slatina and Bo?.     

13C and 15N Abundances in the Sediment Core (VER 92/1-St-10-GC2) from Northern Lake Baikal

Abstract

Carbon and nitrogen stable isotope compositions of organic matter, TOC/TN ratio, and manganese concentration in a sediment core that was collected in northern part of Lake Baikal (VER92ST10-GC2, water depth at 922 m, about 3 m long) were investigated to elucidate the origin of the sedimentary organic matter and its associated environmental factors.

The sediment core was composed of mainly two parts: turbidite sections and other sections. Constant δ¹³C and δ¹⁵N values of the turbidite sections were observed (- 26.8 ±0.2 ‰ for δ¹³C and 3.2 ± 0.1 ‰ for δ¹⁵N) throughout the core. The higher δ¹³C in turbidite sections (about - 27 ‰) than that of the other sections (- 31 to - 29 ‰) was clearly observed, and δ¹⁵N was different between turbidite sections (about 3‰) and other sections (3 to 5 ‰). δ¹³C of other sections was close to that of pelagic phytoplankton, indicating that sediment other than turbidite sections is composed of autochthonous components. The variation of stable isotopes in other sections may be possibly caused by the changes in either phytoplankton growth rate or contribution ratios of terrestrial to aquatic plants for δ¹³C. Either denitrification or fluctuation of δ¹⁵N in pelagic phytoplankton can be the cause of variable δ¹⁵N in other sections.     

Seasonal variation of oxygen-18 in precipitation and surface water of the Poyang Lake Basin,China

Based on the monthly δ¹⁸O value measured over a hydrology period in precipitation, runoff of five tributaries and the main lake of the Poyang Lake Basin, combined with hydrological and meteorological data, the characteristics of δ¹⁸O in precipitation (δ¹⁸O_PPT) and runoff (δ¹⁸O_SUR) are discussed. The δ¹⁸O_PPT and δ¹⁸O_SUR values range from?2.75 to?14.12 ‰ (annual mean value=?7.13 ‰ ) and from?2.30 to?8.56 ‰, respectively. The seasonal variation of δ¹⁸O_PPT is controlled by the air mass circulation in this region, which is dominated by the Asian summer monsoon and the Siberian High during winter. The correlation between the wet seasonal averages of δ¹⁸O_SUR in runoff of the rivers and δ¹⁸O_PPT of precipitation at the corresponding stations shows that in the Poyang Lake catchment area the river water consists of 23% direct runoff (precipitation) and 77% base flow (shallow groundwater). This high proportion of groundwater in the river runoff points to the prevalence of wetland conditions in the Poyang Lake catchment during rainy season. Considering the oxygen isotopic composition of the main body of Poyang Lake, no isotopic enrichment relative to river inflow was found during the rainy season with maximum expansion of the lake. Thus, evaporation causing isotopic enrichment is a minor component of the lake water balance in the rainy period. During dry season, a slight isotopic enrichment has been observed, which suggests a certain evaporative loss of lake water in that period.     

Determination of 13C/12C-Ratios in Rumen Produced Methane and CO2 of Cows,Sheep and Camels

Abstract

Naturally produced methane shows different δ¹³C-values with respect to its origin, e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane—given in literature—range between ?80‰ and -50‰ and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gases via implanted fistulae we compared δ_PDB ¹³C-values of methane and CO₂ between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n = 47) we calculated δ_PDB ¹³C-medians resulting in small but not significant differences within and significant differences between the species for CO₂ and methane. The δ_PDB ¹³C-differences between methane and CO₂ were statistically equal within and also between the species. Therefore a linear regression of methane values on CO₂ is appropriate and leads to: δ_PDB ¹³C(methane)‰ = 1,57 * δ_PDB ¹³C(CO₂)‰-47‰ with a correlation coefficient of r = 0,87.