Variation In δ13C Values Research Articles

Pedogenic siderites fossilizing Ediacaran soil microorganisms on the Baltica paleocontinent

AbstractEdiacaran siderite-bearing sedimentary rocks from the western part of the East European craton exhibit features typical of redoximorphic paleosols, including pedogenic siderite (e.g., sphaerosiderite) with uniform and negative δ18O and highly variable δ13C values. The siderite precipitated in water-logged soils in a hot and humid climate, and represents a rare example of pedogenic siderite formed before the rise of vascular plants. Morphology of soil microorganisms was preserved in three dimensions due to the early siderite precipitation. These are mainly filamentous and tube-like threads, which might belong to cyanobacteria or fungi, and spherical structures resembling green algae. This microbial life fostered development of strongly reducing soils in tropical wetlands on the Baltica paleocontinent. The Ediacaran sediments of the western East European craton, traditionally regarded as marine, are reinterpreted as containing sections affected by freshwater conditions, documenting the presence of microbial life on the Ediacaran land.

Speleothem Records from the Eastern Part of Europe and Turkey—Discussion on Stable Oxygen and Carbon Isotopes

The region comprising of East Central Europe, South East Europe and Turkey contributed to the SISAL (Speleothem Isotopes Synthesis and AnaLysis) global database with stable carbon and oxygen isotope time-series from 18 speleothems from 14 caves. The currently available oldest record from the studied region is the ABA-2 flowstone record (Abaliget Cave; Hungary) reaching back to MIS 6. The temporal distribution of the compiled 18 records from the region points out a ~20 kyr-long period, centering around 100 ka BP, lacking speleothem stable isotope data. The regional subset of SISAL_v1 records displays a continuous coverage for the past ~90 kyr for both δ13C and δ18O, with a mean temporal resolution of ~12 yr for the Holocene, and >50 yr for the pre-Holocene period. The highest temporal resolution both for the Holocene and the pre-Holocene was achieved in the So-1 record (Sofular Cave; Turkey). The relationship between modern day precipitation δ18O (amount weighted annual and winter season mean values; 1961–2017) and climatological parameters was evaluated. The strong positive correlation found in East Central Europe reinforces the link between modern day precipitation δ18O, temperature and large-scale circulation (North Atlantic Oscillation) expected to be preserved in the speleothem δ18O record; while a negative relationship was documented between precipitation amount and oxygen isotope compositions in South East Europe. Variations of δ13C values are primarily interpreted as reflecting dry/wet periods across the region. Elevation gradients from three non-overlapping periods of the last ~5 kyr indicated elevation gradients around −0.26‰ per 100 m−1 for calcite δ18O.

Inter-individual variation in trophic history of Dosidicus gigas, as indicated by stable isotopes in eye lenses

Isotopic signatures have been extensively used to reconstruct marine organism trophic habitat over their lifetime. In this study, sequential stable isotopic values of δ13C and δ15N in eye lenses were measured to investigate the trophic history of Dosidicus gigas during ontogeny. A total of 281 segments from 33 lenses were examined with δ13C ranging from −20.17‰ to −16.11‰ and δ15N ranging from 5.15‰ to 18.74‰. Sequential increasing but variable δ13C and δ15N values in eye lenses indicated that the prey used in the model had a wide range of values, it became able to consume prey from higher trophic levels, perhaps to maximize feeding success. High inter-individual differences in the range and considerable inter-individual variation in the trend of the isotopic values of the sequential segments from the nuclear to the periphery were considered to be the result of the effects of food and the best feeding strategy. These inter-individual isotopic differences in the life history of D. gigas confirmed the high degree of plasticity of feeding for this squid and its high potential to adapt to environmental change. Inter-individual differences of δ15N values in the growth of D. gigas were more likely to be caused by differences in the baseline values of isotopes. According to the population discrimination model of δ15N value changes during ontogeny, the D. gigas off Peru fit the model of “generalist population-generalist individuals”.

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery.

Ocean warming and the increased prevalence of coral bleaching events threaten coral reefs. However, the biology of corals during and following bleaching events under field conditions is poorly understood. We examined bleaching and postbleaching recovery in Montipora capitata and Porites compressa corals that either bleached or did not bleach during a 2014 bleaching event at three reef locations in Kāne‘ohe Bay, O‘ahu, Hawai‘i. We measured changes in chlorophylls, tissue biomass, and nutritional plasticity using stable isotopes (δ 13C, δ 15N). Coral traits showed significant variation among periods, sites, bleaching conditions, and their interactions. Bleached colonies of both species had lower chlorophyll and total biomass, and while M. capitata chlorophyll and biomass recovered 3 months later, P. compressa chlorophyll recovery was location dependent and total biomass of previously bleached colonies remained low. Biomass energy reserves were not affected by bleaching, instead M. capitata proteins and P. compressa biomass energy and lipids declined over time and P. compressa lipids were site specific during bleaching recovery. Stable isotope analyses did not indicate increased heterotrophic nutrition in bleached colonies of either species, during or after thermal stress. Instead, mass balance calculations revealed that variations in δ 13C values reflect biomass compositional change (i.e., protein : lipid : carbohydrate ratios). Observed δ 15N values reflected spatiotemporal variability in nitrogen sources in both species and bleaching effects on symbiont nitrogen demand in P. compressa. These results highlight the dynamic responses of corals to natural bleaching and recovery and identify the need to consider the influence of biomass composition in the interpretation of isotopic values in corals.

Do drifts deposited adjacent to carbonate platforms record the signal of global carbon isotopic values?

AbstractThis study investigates the δ13C values of Middle Miocene–Modern drift deposits and periplatform sediments in the Maldives and compares these data with the global δ13C values derived from bulk oceanic sediments and foraminifera. This comparison reveals that while the δ13C values of the early Miocene periplatform sediments in the Maldives appear to track the global record of δ13C values, including increases associated with the Oligocene–Miocene boundary as well as the variations within the Monterey Event, the correlation with the Monterey Event may be coincidental. It is suggested that variations in δ13C values do not reflect changes in oceanic dissolved inorganic carbon, but instead pulses of sediment arising from platform progradation that contribute carbonates with elevated δ13C values derived from the adjacent shallow‐water atolls. This conclusion is supported both by correlations between the seismic sequence architecture and the δ13C values which document progradation of 13C‐rich platform sediments, and also by the continuation of the interval of 13C‐rich sediments past the end of the Monterey Event at 13 Ma within the drift.

Trophic consistency of supraspecific taxa in below‐ground invertebrate communities: Comparison across lineages and taxonomic ranks

AbstractAnimals that have similar morphological traits are expected to share similar ecological niches. This statement applies to individual animals within a species and thus species often serve as the functional units in ecological studies. Species are further grouped into higher‐ranked taxonomic units based on their morphological similarity and thus are also expected to be ecologically similar. On the other hand, theory predicts that strong competition between closely related species may result in differentiation of ecological niches. Due to high diversity and limited taxonomic expertise, soil food webs are often resolved using supraspecific taxa such as families, orders or even classes as functional units.Here, we tested the trophic differentiation and consistency of supraspecific taxa across major lineages of temperate forest soil invertebrates: Annelida, Chelicerata, Myriapoda, Crustacea and Hexapoda. Published data on stable isotope compositions of carbon and nitrogen were used to infer basal resources and trophic level, and explore the relationship between taxonomic and trophic dissimilarity of local populations.Genera and families had normal and unimodal distributions of isotopic niches, suggesting that supraspecific taxa are trophically consistent. The isotopic niche of local populations varied considerably resulting in large overlap of niches among species. Within the same genus, the effect of species identity on stable isotope composition of populations was not significant in 92% of cases. More than 50% of the variability in Δ15N values (trophic level) across lineages was explained by classes and orders, while the variability in Δ13C values (basal resources) was explained mostly by families and genera. The variability in stable isotope composition in Chelicerata and Hexapoda was explained by lower taxonomic ranks than in Myriapoda.We compiled a comprehensive list of mean Δ13C and Δ15N values of invertebrate taxa from temperate forest soils allowing to refine soil food web models when measurements of trophic niches of local populations are not feasible. Supraspecific taxa are meaningful as trophic nodes in food web studies, but the consistency varies among taxa and the choice of taxonomic resolution depends on the research question; generally, identification of taxa should be more detailed in more diverse taxonomic groups.Aplain language summaryis available for this article.

Isotopic perspectives on pastoralist mobility in the Late Bronze Age South Caucasus

Isotopic methods of studying the geographic and seasonal movements of herd animals provide new data pertinent to ongoing scholarly re-consideration of the relationships between pastoralist mobility and the development of new forms of political organization. Radiogenic strontium and mass-dependent oxygen and carbon isotope analysis on a large sample of herd animals (sheep, goats, and cattle) from Late Bronze Age (1500–1100 BC) sites in the Tsaghkahovit Plain, Armenia indicate that long-distance movements across geological zones were not common, but that animals were drinking from a range of water sources. Connecting this variation to movement is complicated by limited intra-tooth variation in δ13C values. Data from the study provide preliminary evidence for foddering. These results reveal a gap between the emphasis on mobility in theoretical models of pastoralism and political organization and the way that animal diets mediate isotopic proxies for movement. These interpretive difficulties emphasize the need to consider herds’ seasonal and geographic movement within a wider range of pastoralist practices. These findings also highlight the need for large sample sizes in isotopic investigations of pastoralist mobility, in order to accurately identify and evaluate the diversity in both geographic and seasonal movements within a single ancient pastoralist system.

Effects of early diagenesis on Mg isotopes in dolomite: The roles of Mn(IV)-reduction and recrystallization

Abstract Sedimentary dolomite plays an important role in global Mg cycling, and Mg isotopes in massive dolostones may be used to infer secular changes in seawater chemistry through geological history. However, sedimentary dolomite is generally regarded as a diagenetic product, and many details about the effects of early diagenesis on the Mg isotope composition of dolomite remain unclear. The mid-Cretaceous (Albian) Soreq and Givat Ye’arim formations near Jerusalem, Israel, contain exceptionally well-preserved massive dolostones, which provide an ideal opportunity to investigate the Mg isotope responses to early diagenesis. Dolomite samples from this section show large variations in δ13C values and Mn contents that are negatively correlated, reflecting degradation of organic matter and mineralization of organic carbon in the Mn(IV) reduction zone within soft sediment during dolomite formation. This is a rare example of a clear link between Mn(IV) reduction and dolomite precipitation based on geochemical evidence in the rock record. The dolomite samples also exhibit large variations in d(104) values and the degree of cation ordering. The latter is negatively correlated with Sr contents, implying that variable degrees of dolomite recrystallization occurred during diagenesis. δ26Mg values of >50 dolomite samples from this section vary from −2.28‰ to −1.78‰, but do not correlate with indicators of organic matter degradation (δ13C values and Mn contents) or dolomite recrystallization (e.g., Sr contents), suggesting that Mg isotopes behave conservatively after initial dolomite (or proto-dolomite) precipitation during the very early stages of diagenesis. We propose that the Mg isotope composition of dolomite formed due to Mn(IV) reduction is buffered by seawater due to the shallowness of the Mn(IV) reduction zone in platform sediments, which is different from the dolomitization associated with bacterial sulfate reduction or methanogenesis, where Mg supply can be diffusion-limited. Furthermore, Mg isotopes in dolomite are robust to resetting by recrystallization during burial. Magnesium isotope compositions of platform dolomite that show variable and negatively correlated δ13C values and Mn contents can, therefore, be considered robust archives for reconstructing paleo-seawater Mg isotope compositions. The variation in δ26Mg values of the dolostones in the Soreq and Givat Ye’arim formations is interpreted to reflect Rayleigh fractionation in response to dolomitization in a restricted water body. Therefore, the lowest δ26Mg value is considered to be the Mg isotope composition of dolomite that was in equilibrium with coeval seawater in the open ocean, and thus the δ26Mg value of Albian seawater was around −0.4‰.

New Bioarchaeological Evidence and Radiocarbon Dates from the Lambayeque/Sicán Culture Camelids from the El Brujo Complex (Northern Coast of Peru): Implications for Funerary and Herd Management Practices

ABSTRACT Pastoralism and camelid management have been essential to all aspects of pre-Hispanic Andean societies. Here, we present zooarchaeological and isotopic data on domestic camelid remains from Huaca Cao Viejo (El Brujo archaeological complex) on the northern coast of Peru, and dated to the Lambayeque/Sicán period—to characterise their biological age, diet, life history, possible geographic origin and ritual use. Domestic camelids, representing a wide range of biological ages and a high rate of polydactyly, were found as burial offerings in direct association with human funerary bundles (fardos). Direct AMS dates indicated that camelids were buried over a short period of time (AD 1022–1176) confirming the Lambayeque presence in the Chicama Valley during the first half of the Late Intermediate Period. Stable isotopic analyses were carried out on both bone collagen and hair keratin, including incremental analysis. A considerable variability in δ13C values at both the intra-individual and the intra-group level and a large contribution of C4 resources to diet are shown. This clearly supports local management and camelids originating from various herds. Zooarchaeological and isotopic evidences suggest diversity in herding practices and suggest the importance of the herds in fulfilling the transportation demands for trade in goods.

Carbon stable isotopic compositions of citric acid and malic acid in Japanese apricot liqueur decrease as the fruit ripens.

The carbon stable isotopic composition (δ13C) is often analyzed to quantify the addition of acidulants to Japanese apricot liqueur, but little is known about the variation in the δ13C values of the main organic acids arising from differences in the ripeness of Japanese apricots. We show that in Japanese apricot liqueur prepared using fruits at different stages of ripeness, the δ13C values of citric acid and malic acid ranged from -25.1‰ to -23.7‰ and from -22.3‰ to -19.7‰, respectively, and the δ13C values decreased as the fruit ripened. The average δ13C value of citric acid from liqueurs was 0.7‰ higher than that from fresh fruits, whereas the δ13C values of malic acid showed no isotope discrimination. The variation in δ13C values of the main organic acids in Japanese apricot liqueurs will help detect acidulant addition and control authenticity.

Environmental and taxonomic controls of carbon and oxygen stable isotope composition in <i>Sphagnum</i> across broad climatic and geographic ranges

Abstract. Rain-fed peatlands are dominated by peat mosses (Sphagnum sp.), which for their growth depend on nutrients, water and CO2 uptake from the atmosphere. As the isotopic composition of carbon (12,13C) and oxygen (16,18O) of these Sphagnum mosses are affected by environmental conditions, Sphagnum tissue accumulated in peat constitutes a potential long-term archive that can be used for climate reconstruction. However, there is inadequate understanding of how isotope values are influenced by environmental conditions, which restricts their current use as environmental and palaeoenvironmental indicators. Here we tested (i) to what extent C and O isotopic variation in living tissue of Sphagnum is species-specific and associated with local hydrological gradients, climatic gradients (evapotranspiration, temperature, precipitation) and elevation; (ii) whether the C isotopic signature can be a proxy for net primary productivity (NPP) of Sphagnum; and (iii) to what extent Sphagnum tissue δ18O tracks the δ18O isotope signature of precipitation. In total, we analysed 337 samples from 93 sites across North America and Eurasia using two important peat-forming Sphagnum species (S. magellanicum, S. fuscum) common to the Holarctic realm. There were differences in δ13C values between species. For S. magellanicum δ13C decreased with increasing height above the water table (HWT, R2=17 %) and was positively correlated to productivity (R2=7 %). Together these two variables explained 46 % of the between-site variation in δ13C values. For S. fuscum, productivity was the only significant predictor of δ13C but had low explanatory power (total R2=6 %). For δ18O values, approximately 90 % of the variation was found between sites. Globally modelled annual δ18O values in precipitation explained 69 % of the between-site variation in tissue δ18O. S. magellanicum showed lower δ18O enrichment than S. fuscum (−0.83 ‰ lower). Elevation and climatic variables were weak predictors of tissue δ18O values after controlling for δ18O values of the precipitation. To summarize, our study provides evidence for (a) good predictability of tissue δ18O values from modelled annual δ18O values in precipitation, and (b) the possibility of relating tissue δ13C values to HWT and NPP, but this appears to be species-dependent. These results suggest that isotope composition can be used on a large scale for climatic reconstructions but that such models should be species-specific.

A guano‐derived δ13C and δ15N record of climate since the Medieval Warm Period in north‐west Romania

ABSTRACTA 285‐cm core of bat guano was recovered from Măgurici Cave in north‐west Romania and analyzed for δ13C, δ15N and pollen. Guano deposition occurred from AD 881 until 1240 and from AD 1651 to 2013, allowing for the interpretation of summer variations in precipitation and temperature during the Medieval Warm Period (MWP) and the Little Ice Age (LIA). A 2‰ increase in δ13C, 1.5‰ decrease in δ15N, and the presence of Ulmus, Quercus and Carpinus betulus indicate a warm and dry MWP occurred in the region. The lack of deposition during the beginning of the LIA suggests a possible climate‐induced change in prey availability resulting in bats vacating the cave. Variation of δ13C values between −25 and −23‰ at AD 1650 (LIA) indicates similar drier conditions as at the end of MWP. However, a 2‰ decrease in δ13C values that occurred between AD 1790 and 1900 suggests climate was trending towards wetter conditions at the end of the LIA. From AD 1938 to 2013, δ13C values appear to be more influenced by temperature, indicating that this parameter had a more significant effect on carbon discrimination than water availability.

Compound-Specific Carbon Isotopes and Concentrations of Carbohydrates and Organic Acids as Indicators of Tree Decline in Mountain Pine

We investigated seasonal variations in δ13C values and concentrations of carbohydrates and organic acids in needles of declining and healthy mountain pine (Pinus mugo ssp. uncinata (DC.) Domin) trees from the Swiss National Park (SNP), using compound-specific isotopes analysis (CSIA). Our goal was to study the impact of climatic drivers on the individual compounds and understand the reasons of partial tree declines in relation to healthy mountain pine trees under seasonal weather patterns. We found that temperature is the main climatic driver determining the seasonal carbon dynamics at the needle level. Lower seasonal δ13C variability and lower concentration levels of sucrose in needles suggest less photosynthetic activity and sink carbon demand in declining compared to healthy mountain pine trees. Higher concentration levels of hexose (glucose and fructose) can play a reserve function for surviving mechanisms of mountain pine trees. Seasonal patterns of organic acid (malate and citrate) suggest an increasing investment in maintenance and repair mechanisms. The seasonal course of carbohydrates and organic acids can therefore be considered an indicator for a modified carbon metabolism within the leaves and possibly within the other tree tissues, partially explaining the decline of mountain pine trees.

Spatial and interspecific variation in the food sources of sympatric estuarine nereidid polychaetes: stable isotopic and enzymatic approaches

Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of sympatric nereidid polychaetes including Hediste diadroma, H. atoka, Tylorrhynchus osawai, and Simplisetia erythraeensis were examined at six stations along the Japanese coast to reveal the trophic niches of nereidid polychaetes in estuarine benthic food webs. The species studied exhibited a large degree of spatial variation in δ13C values (− 25.3 to − 13.7‰) among stations, suggesting site-based differences in nereidid dietary components. The low δ13C values of nereidids suggest that terrestrial plant materials are important components of their diets in river-affected estuarine habitats, which were also shown by a δ13C-based stable isotope mixing model (SIAR). δ13C values of H. diadroma and H. atoka were 0.3–3.1‰ lower than those of the sympatric T. osawai and S. erythraeensis, while no significant difference was found between Hediste species. A biochemical assay revealed cellulase activity in H. diadroma, H. atoka, and T. osawai (S. erythraeensis was not examined), while the apparent molecular size of cellulase differed between the two genera. These observations suggest that some nereidids can directly assimilate terrestrial plant matters using cellulases, although the dietary importance of terrestrial matter varies among genera. Species with the ability to digest cellulose may be better able to colonize river-dominated estuarine habitats, where refractory terrestrial detritus is a major carbon source.

Variability in δ13C values between individual Daphnia ephippia: Implications for palaeo-studies

The stable carbon isotope ratio (δ13C value) of Daphnia spp. resting egg shells (ephippia) provides information on past changes in Daphnia diet. Measurements are typically performed on samples of ≥20 ephippia, which obscures the range of values associated with individual ephippia. Using a recently developed laser ablation-based technique, we perform multiple δ13C analyses on individual ephippia, which show a high degree of reproducibility (standard deviations 0.1–0.5‰). We further measured δ13C values of 13 ephippia from surface sediments of three Swiss lakes. In the well-oxygenated lake with low methane concentrations, δ13C values are close to values typical for algae (−31.4‰) and the range in values is relatively small (5.8‰). This variability is likely driven by seasonal (or inter-annual) variability in algae δ13C values. In two seasonally anoxic lakes with higher methane concentrations, average values were lower (−41.4 and −43.9‰, respectively) and the ranges much larger (10.7 and 20.0‰). We attribute this variability to seasonal variation in incorporation of methane-derived carbon. In one lake we identify two statistically distinct isotopic populations, which may reflect separate production peaks. The potentially large within-sample variability should be considered when interpreting small-amplitude, short-lived isotope excursions based on samples consisting of few ephippia. We show that measurements on single ephippia can be performed using laser ablation, which allows for refined assessments of past Daphnia diet and carbon cycling in lake food webs. Furthermore, our study provides a basis for similar measurements on other chitinous remains (e.g. from chironomids, bryozoans).

Seasonal variation in natural abundance of δ13C and 15N in Salicornia brachiata Roxb. populations from a coastal area of India

ABSTRACTHigh and fluctuating salinity is characteristic for coastal salt marshes, which strongly affect the physiology of halophytes consequently resulting in changes in stable isotope distribution. The natural abundance of stable isotopes (δ13C and δ15N) of the halophyte plant Salicornia brachiata and physico-chemical characteristics of soils were analysed in order to investigate the relationship of stable isotope distribution in different populations in a growing period in the coastal area of Gujarat, India. Aboveground and belowground biomass of S. brachiata was collected from six different populations at five times (September 2014, November 2014, January 2015, March 2015 and May 2015). The δ13C values in aboveground (−30.8 to −23.6 ‰, average: −26.6 ± 0.4 ‰) and belowground biomass (−30.0 to −23.1 ‰, average: −26.3 ± 0.4 ‰) were similar. The δ13C values were positively correlated with soil salinity and Na concentration, and negatively correlated with soil mineral nitrogen. The δ15N values of aboveground (6.7–16.1 ‰, average: 9.6 ± 0.4 ‰) were comparatively higher than belowground biomass (5.4–13.2 ‰, average: 7.8 ± 0.3 ‰). The δ15N values were negatively correlated with soil available P. We conclude that the variation in δ13C values of S. brachiata was possibly caused by soil salinity (associated Na content) and N limitation which demonstrates the potential of δ13C as an indicator of stress in plants.

Correction to 'Starvation effects on nitrogen and carbon stable isotopes of animals: an insight from meta-analysis of fasting experiments'.

[This corrects the article DOI: 10.1098/rsos.170633.].

Starvation effects on nitrogen and carbon stable isotopes of animals: an insight from meta-analysis of fasting experiments.

Nitrogen and carbon stable isotopic compositions (δ15N and δ13C) of consumers have been used for physiological and food web studies. Previous studies have shown δ15N and δ13C values are affected by several biological and environmental factors during starvation, but the generality of the effect of starvation on δ15N and δ13C values has not yet been tested. Here, we performed a meta-analysis to evaluate the effects of starvation on δ15N and δ13C values of consumers, and the underlying factors that may explain the observed variation. The δ15N and δ13C values were calculated as the differences between the final δ15N and δ13C values of consumers (post-starvation) and the pre-starvation values on each experiment. Our meta-analysis showed a large variation in the δ15N and δ13C values of consumers (δ15N range: –0.82 to 4.30‰; mean: 0.47‰ and δ13C range: –1.92 to 2.62‰; mean: 0.01‰). The δ15N values of most consumers increased along the length of the starvation period and were influenced by nitrogen excretion and thermoregulation types, probably because differences in nitrogen metabolism and thermoregulation affect nitrogen processing and excretion rates. None of our predictor variables accounted for the variation in δ13C values, which showed both increases and decreases due to fasting. Our findings suggest that starvation results in changes in consumer δ15N values which are mainly explained by the length of the fasting period and by nitrogen and energy metabolism, but the underlying mechanisms of the starvation effects on δ13C values seem to be more complex than previously thought.

Dated eclogitic diamond growth zones reveal variable recycling of crustal carbon through time

Monocrystalline diamonds commonly record complex internal structures reflecting episodic growth linked to changing carbon-bearing fluids in the mantle. Using diamonds to trace the evolution of the deep carbon cycle therefore requires dating of individual diamond growth zones. To this end Rb–Sr and Sm–Nd isotope data are presented from individual eclogitic silicate inclusions from the Orapa and Letlhakane diamond mines, Botswana. δ13C values are reported from the host diamond growth zones. Heterogeneous 87Sr/86Sr ratios (0.7033–0.7097) suggest inclusion formation in multiple and distinct tectono-magmatic environments. Sm–Nd isochron ages were determined based on groups of inclusions with similar trace element chemistry, Sr isotope ratios, and nitrogen aggregation of the host diamond growth zone. Diamond growth events at 0.14±0.09, 0.25±0.04, 1.1±0.09, 1.70±0.34 and 2.33±0.02 Ga can be directly related to regional tectono-magmatic events. Individual diamonds record episodic growth with age differences of up to 2 Ga. Dated diamond zones have variable δ13C values (−5.0 to −33.6‰ vs PDB) and appear to imply changes in subducted material over time. The studied Botswanan diamonds are interpreted to have formed in different tectono-magmatic environments that involve mixing of carbon from three sources that represent: i) subducted biogenic sediments (lightest δ13C, low 87Sr/86Sr); ii) subducted carbonate-rich sediments (heavy δ13C, high 87Sr/86Sr) and iii) depleted upper mantle (heavy δ13C, low 87Sr/86Sr). We infer that older diamonds from these two localities are more likely to have light δ13C due to greater subduction of biogenic sediments that may be related to hotter and more reduced conditions in the Archaean before the Great Oxidation Event at 2.3 Ga. These findings imply a marked temporal change in the nature of subducted carbon beneath Botswana and warrant further study to establish if this is a global phenomenon.

Seasonality of cladoceran and bryozoan resting stage δ13C values and implications for their use as palaeolimnological indicators of lacustrine carbon cycle dynamics

The stable carbon isotope composition, expressed as δ13C values, of chitinous resting stages of planktivorous invertebrates can provide information on past changes in carbon cycling in lakes. For example, the δ13C values of cladoceran ephippia and bryozoan statoblasts have been used to estimate the past contribution of methane-derived carbon to lake food webs and variations in the δ13C value of planktonic algae. Limited information, however, is available concerning seasonal variations in δ13C values of these organisms and their resting stages. We measured the seasonal variation in δ13C values of Daphnia (Branchiopoda: Cladocera: Daphniidae) and their floating ephippia over a 2-year period in small, dimictic Lake Gerzensee, Switzerland. Floating ephippia of Ceriodaphnia (Branchiopoda: Cladocera: Daphniidae) and statoblasts of Plumatella (Phylactolaemata: Plumatellida: Plumatellidae) were analysed during parts of this period. Furthermore, δ13C values of remains from all three organism groups were analysed in a 62-cm-long sediment core. Throughout the year, Daphnia δ13C values tracked the δ13C values of particulate organic matter (POM), but were more negative than POM, indicating that Daphnia also utilize a relatively 13C-depleted carbon source. Daphnia ephippia δ13C values did not show any pronounced seasonal variation, suggesting that they are produced batch-wise in autumn and/or spring and float for several months. In contrast, δ13C values of Ceriodaphnia ephippia and Plumatella statoblasts followed variations in δ13CPOM values, Ceriodaphnia values being the most negative of the resting stages. Average cladoceran ephippia δ13C values in the flotsam agreed well with ephippia values from Gerzensee surface sediments. In contrast, average Plumatella statoblast δ13C values from the flotsam were 4‰ more negative than in the surface sediments. In the sediment core, δ13C values of the two cladocerans remained low (mean −39.0 and −41.9‰) throughout the record. In contrast, Plumatella had distinctly less negative δ13C values (mean −32.0‰). Our results indicate that in Gerzensee, Daphnia and Ceriodaphnia strongly relied on a 13C-depleted food source throughout the past 150 years, most likely methane-oxidising bacteria, whereas this food source was not a major contribution to the diet of bryozoans.