Shell δ13C Values Research Articles

Stable carbon and oxygen isotopic composition of modern land snails along a precipitation gradient in the mid-latitude East Asian monsoon region of China

The stable oxygen and carbon isotope composition (δ18Oshell and δ13Cshell) of land snail shells preserved in Chinese eolian loess deposits have the potential to reconstruct past changes in the East Asian Summer Monsoon (EASM) since the late Oligocene. To assess the potential of δ18Oshell and δ13Cshell for the semi-quantitative reconstruction of past variations of the EASM, we studied modern snails and the corresponding vegetation composition of three distinct ecosystems (woodland, grassland and mountains), spanning a large rainfall gradient (178–1403 mm) in the mid-latitude East Asian monsoon region. The results show that different snail species in the three ecosystems have significantly different values of δ18Oshell and δ13Cshell. The snails in woodland from the eastern plain area have significantly lower δ18Oshell and δ13Cshell values than those of snails in grassland from the Chinese Loess Plateau, and the snails from the Qinba Mountains region have values intermediate between the two. Systematic variations of the δ13C of snail shells, body tissues and local plants are evident along the rainfall gradient, indicating that δ13Cshell is mainly influenced by the δ13C of ingested plants, which is controlled by the EASM. We confirm that δ13Cshell is a reliable precipitation proxy and we obtained a new empirical equation (δ13Cshell = −0.0030(±0.0003) MAP − 8.80(±0.32), r = −0.76, n = 74, p < 0.001) for the relationship between δ13Cshell and mean annual precipitation, which is based on the results of the present study, together with those of Bao et al. (2018). We infer that δ18Oshell is influenced by precipitation δ18O, and that here is a significant relationship between δ18Oshell and mean annual precipitation (δ18Oshell = −0.0028(±0.0008) MAP − 1.32(±0.58), r = −0.65, p < 0.05, n = 21), possibly because of the rainfall amount effect in the study region. In addition, temperature and evaporation associated with relative humidity play non-negligible roles in determining δ18Oshell, and their effects should be considered before using the δ18O of fossil snail shells to reconstruct precipitation δ18O and/or rainfall amount.

The isotopic differences and implications for Pacific razor clams along the Washington coast

The Pacific razor clam fishery in Washington State has been co-managed by the coastal Indian Tribes and the state, but little is known about the growth and population structure of the clams due to difficulties of tagging and monitoring. Here we report the results of a pilot study using stable isotope ratios (δ13C and δ18O) of razor clam shells collected in two groups (juvenile vs. adult) and from two sites (Kalaloch Beach and Roosevelt Beach) where distinct biological differences in clam growth and survival rates were observed. The δ13C values of razor clam shells ranged from -2.9 to -0.3‰, whereas δ18O values of the same samples ranged from -2.2 to +1.4‰. Between the two sites there were significant differences in δ13C values especially for juvenile clams. The δ18O profiles from two representative shells demonstrated similar patterns of rapid growth as juveniles and seasonal patterns throughout the life span. Profiles of δ13C were sinusoidal but did not show seasonality and signatures of ocean acidification. We concluded that stable isotope analysis of razor clam shells is a potential new tool in shellfish research and management.

Light variability and mixotrophy: Responses of testate amoeba communities and shell δ13C values to a peatland shading experiment

Paleoecological records suggest that growing season length and/or cloudiness may affect peatland carbon accumulation and testate amoeba-based environmental reconstructions, highlighting a need to understand how light intensity affects microbial communities. We shaded plots on two peatlands for two years to examine effects on testate amoeba communities, the relative abundance of mixotrophic and heterotrophic testate amoebae, transfer-function performance, and δ13C values of two species of mixotrophic testate amoebae. Surprisingly, relative abundance of mixotrophic species increased in shade, although compositional changes did not affect transfer-function performance. Shading did not affect δ13C values of Hyalosphenia papilio and Heleopera sphagni, which ranged from −23.5 to −19.6‰ and −23.2 to −19.2‰, respectively. These δ13C values were higher than those of potential food sources and lower than literature-derived values for Chlorella, the zoochlorellae inhabiting mixotrophic testate amoebae. δ13C values thus suggest that these mixotrophic species obtain some carbon from Chlorella, although coupled dietary and isotope studies are needed to quantify this contribution. More research is needed to assess impacts of light variability on peatland microbial communities; however, carbon sources are recorded by δ13C values of testate amoebae, indicating potential for studies of carbon cycling and how mixotrophy varies temporally and spatially.

Stable and clumped isotope sclerochronologies of mussels from the Brazos River, Texas (USA): Environmental and ecologic proxy

Oxygen isotope sclerochronology with mollusk shells can provide vital environmental and ecologic information, but is complicated in regulated, subtropical rivers by irregular river water δ18O and consequently irregular shell δ18O patterns. We performed multi-proxy analyses (δ18O, δ13C, clumped isotopes) from serially-sampled freshwater mussels in two specimens each of Amblema plicata and Cyrtonaias tampicoensis from the Brazos River, Texas to age specimens and test whether they accurately record environmental conditions such as river discharge and water source. Oxygen isotopic measurements are similar to predicted aragonite δ18O values based on temperature and water δ18O, but the record is complex and irregular. To better resolve the chronologies, we performed clumped isotope analyses on select shell intervals. Clumped isotope temperatures (T(Δ47)) ranged from 19° to 36 °C. Summer T(Δ47) values were 3–5 °C higher than measured temperatures, suggesting an offset in paleothermometer calibration. Chronologies based on Δ47 and δ18O reveal specimen ages of 3–4 years and winter growth cessation (or dramatic slowing) in both C. tampicoensis and A. plicata, highlighting thermal limitations on growth and potential biases in environmental reconstructions. Compared with modern shells, historical shells collected before intensive damming showed lower δ18O values yet similar clumped temperatures, suggesting enhanced evaporation in modern river water due to impoundment and/or the 2011–12 drought.Carbon isotope trends were similar between conspecific shells, but differed between species, with substantially higher average values for A. plicata shells (−9‰) than C. tampicoensis shells (−12‰). The correlation between shell δ13C values and the release of 13C-enriched waters from upstream reservoirs demonstrates that δ13C can be used as a proxy for dam release in regulated rivers.

Redox conditions and authigenic mineralization related to cold seeps in central Guaymas Basin, Gulf of California

Highlights • In cold seeps of Guaymas Basin, aragonite, barite and pyrite precipitated from modified seawater. • Aragonite is highly depleted in 13C suggesting formation via anaerobic oxidation of methane. • Barite formed through mixing of reducing, Ba-rich seep fluids with a 34S-rich sulfate pool. • Pyrite framboids formed under anoxic-sulfidic water via microbial sulfate reduction. Abstract Authigenic carbonate crusts, surface muds and bivalve shell fragments have been recovered from inactive and active recently discovered cold seep sites in central Guaymas Basin. In this study, for first time, redox conditions and fluid sources involved in mineral precipitation were investigated by analyzing the mineralogy and textures of surface samples, along with skeletal contents, and C, O and S isotopes variations. The δ13C values of aragonitic bivalve shells and non-skeletal carbonate from some surface muds (1‰ to −3.7‰ V-PDB) suggest that carbonate precipitated from ambient dissolved inorganic carbon, whereas fibrous aragonite cement and non-skeletal carbonate from other sites are highly depleted in 13C (down to −47.6‰ V-PDB), suggesting formation via anaerobic oxidation of methane, characteristic of methane seepage environments. δ18O in most of the carbonates varies from +1.4‰ to +3.2‰ V-PDB, indicating that they formed from slightly modified seawater. Some non-skeletal carbonate grains from surface muds have lower δ18O values (−12.5‰ to −8.2‰ V-PDB) reflecting the influence of 18O-depleted pore water. Size distribution of pyrite framboids (mean value: 3.1 μm) scattered within diatomaceous sinter suggests formation from anoxic-sulfidic bottom waters. δ34S in pyrite is of −0.3‰ V-CDT compared to +46.6‰ V-CDT in barite, thus implying a fluid sulfate−sulfide fractionation of 21.3‰ that argues in favor of microbial sulfate reduction as the processes that mediated pyrite framboid formation, in a semi-closed system. Barite formation occurred through the mixing of reducing and Ba-rich seep fluids with a 34S-enriched sulfate pool that resulted from microbial sulfate reduction in a semi-closed system. The chemical composition of aragonite cement, barite and pyrite suggest mineral precipitation from modified seawater. Taken together, our data suggest that mineralization at the studied seep sites is controlled by the mixing of seawater with minor amounts of hydrothermal fluids, and oxygen-depleted conditions favoring anaerobic microbial processes.

Inter- and intra-specific variability in δ13C and δ18O values of freshwater gastropod shells from Lake Lednica, western Poland

AbstractThis study focuses on the inter- and intra-specific variability in δ13C and δ18O values of shells and opercula of gastropods sampled live from the littoral zone of Lake Lednica, western Poland. The δ13C and δ18O values were measured in individual opercula of Bithynia tentaculata and in shells of Bithynia tentaculata, Gyraulus albus, Gyraulus crista, Lymnaea sp., Physa fontinalis, Radix auricularia, Theodoxus fluviatilis and Valvata cristata. The gastropods selected for the study are among the species most commonly found in European Quaternary lacustrine sediments. The carbon isotope composition of the gastropod shells was species-specific and the same order of species from the most to the least13C-depleted was observed at all sites sampled. Differences in shell δ13C values between species were similar at all sampling sites, thus the factors influencing shell isotopic composition were interpreted as species-specific. The δ18O values of shells were similar in all the species investigated. Significant intra-specific variability in shell δ13C and δ18O values was observed not only within the populations of Lake Lednica, which can be explained by heterogeneity of δ13C DIC, δ18O water and water temperature between the sites where macrophytes with snails attached were sampled, but also between individuals sampled from restricted areas of the lake’s bottom. The latter points to the importance of factors related to the ontogeny of individual gastropods.

Paleoceanographic conditions at approximately 20 and 70 ka recorded in Kikaithyris hanzawai (Brachiopoda) shells

The δ13C and δ18O values of fossil brachiopod shells have been widely used as paleoenvironmental proxies. In this study, we investigated intrashell and intraspecific variations in the isotopic and minor element concentrations of well-preserved shells of the brachiopod Kikaithyris hanzawai (Yabe) from the last glacial period (∼20ka [Last Glacial Maximum; LGM] and ∼70ka [Marine Isotope Stage 4; MIS4]), collected in the Central Ryukyus, and used these data to estimate the paleoceanographic conditions (seawater temperature, concentration of dissolved inorganic carbon [DIC], and δ13C value of DIC [δ13CDIC]). The δ13C and δ18O profiles along the maximum growth axis, obtained from the inner shell surface, show three distinct intervals, corresponding to changes in shell morphology. These results suggest that the bulk isotopic compositions of brachiopods with complex shell morphologies are unsuitable for paleoenvironmental reconstructions. Nevertheless, there exists a specific shell portion with relatively small intrashell and intraspecific variations. The past seawater temperatures derived from the δ18O values of this portion are consistent with the alkenone- and planktic foraminiferal Mg/Ca-based past seawater temperatures reported in previous studies. The past δ13CDIC values estimated from the δ13C values of the specific shell portion are within the range of the past δ13CDIC values calculated from known atmospheric and oceanographic parameters. The past DIC concentrations reconstructed from the brachiopod-based δ13CDIC values are lower than the present concentrations in the East China Sea, which can be explained by low partial pressure of CO2 during the last glacial period. These results indicate that the δ13C and δ18O values obtained from K. hanzawai shells are potential paleoenvironmental indicators. The intrashell and intraspecific variations in the K. hanzawai shells are different for each minor element. Some anomalously high Mn and Fe concentrations in the shells are probably caused by metabolic factor(s), not by meteoric diagenesis. This suggests that the minor element concentrations are useful but not perfect for distinguishing diagenetically altered and unaltered portions of the shells of K. hanzawai in the studied succession.

Middle Pleistocene (MIS 14) environmental conditions in the central Mediterranean derived from terrestrial molluscs and carbonate stable isotopes from Sulmona Basin (Italy)

A paleosol from the Middle Pleistocene lacustrine-fluvial succession of Sulmona Basin, central Italy, was analysed for the land snail shell content, and the stable isotope composition of the shells and associated pedogenic carbonates. The paleosol – known as Fiorata Paleosol – is covered by a thick tephra layer dated to ca. 527ka allowing the pedogenetic horizons to be correlated to the marine isotope stage (MIS) 14-early MIS 13 interval. The terrestrial mollusc assemblage contained few individuals and was characterized by a low number of species which predominantly indicate open and dry habitats, thus suggesting that Fiorata Paleosol likely developed during glacial conditions of the MIS 14. The δ13C values of pedogenic carbonates and terrestrial shells indicate prevailing C3-type vegetation, probably marked by some degree of water stress. Calculation of the δ18O precipitation values, derived from pedogenic carbonates and shell δ18O values, indicate that the average temperature was 3–5°C lower than present day. This study highlights how paleosols, despite offering only snapshots of past climate and environments, provide valuable complementary information to paleoclimatic data obtained in the adjacent lacustrine intervals, specifically for the Sulmona successions.

δ18O and δ13C in fossil shells of Radix sp. from the sediment succession of a dammed palaeo‐lake in the Yarlung Tsangpo valley, Tibet, China

A series of confirmed and suspected dammed palaeo‐lake sedimentary successions is scattered within the middle Yarlung Tsangpo valley in Tibet. However, the chronology, the genesis of the dam and its location, the water level of the dammed lake, the process of dam failure and the spatiotemporal relationships between the sedimentary successions remain controversial. Here, we focus on one sedimentary succession of the suspected dammed palaeo‐lake at Xigazê. We measured the grain‐size distribution, magnetic susceptibility, organic and inorganic carbon content, and δ13Corg and δ15Ntotal ratios of the sediments. In addition, we measured the δ18Oshell and δ13Cshell values of modern and fossil Radix sp. shells, and the δ18Owater and δ13CDIC values of the ambient water with different hydrological regimes. The results indicate that the δ18Oshell values of modern Radix sp. and the δ18Owater of the ambient water body significantly depend on its hydrological status. In addition, a strong positive relationship was observed between δ18Oshell values of modern Radix sp. shells and the δ18Owater of the ambient water on the Tibetan Plateau. According to this correlation, the δ18Owater values of the palaeo‐water body are reconstructed using the δ18Oshell values of Radix sp. fossil shells in the Xigazê section. Further, based on the δ18Oshell values of fossil Radix sp., the reconstructed δ18Owater of the palaeo‐water body and the specific habitats of Radix sp., we infer that the sedimentary succession in the Xigazê broad valley was mainly formed within the backwater terminal zone of a dammed palaeo‐lake and that the elevation of the water level of the lake was approximately 3811 m a.s.l. AMS 14C dating indicates that the deposits of the dammed palaeo‐lake were formed at about 33–22 cal. ka BP. Finally, the presence of Radix sp. fossil shells within the Xigazê section suggests that Radix sp. survived the late Last Glacial Period on the Tibetan Plateau.

Conus Shell δ13C values as proxies for δ13CDIC in tropical waters

We use time-series δ18O and δ13C data from seawater and live-collected Conus shells from Panama's Pacific coast to test the fidelity of the gastropod's δ13C values as a proxy for the δ13C of marine dissolved inorganic carbon (DIC), and the potential of δ18O-δ13C correlations in shell profiles for resolving relative magnitudes of seasonal upwelling and freshening. Water samples were collected from March 2011 to August 2012 from Naos Island Marine Laboratory, and Conus specimens were collected from nearby Veracruz Beach in July 2013. In general, patterns corresponded with seasonal changes in rainfall and upwelling on the Pacific coast of Panama. During the long rainy season, the upwelling signal is absent and seawater salinity, δ18O, and δ13CDIC all decline. During the dry season, the upwelling signal increases and runoff declines increasing salinity, δ18O, and δ13CDIC values. Shell δ13C values strongly correlate with measured δ13CDIC values, but are lower than expected equilibrium for aragonite by approximately +2‰ reflecting the incorporation of light metabolic C. The co-dependences of δ18O and δ13C provide reliable indicators of upwelling (negative correlation) and freshening (positive correlation) for nearshore environments, allowing for the study of historical climate change and upwelling based on beach-collected museum specimens.

Shell sclerochronology and stable isotopes of the bivalve Anomalocardia flexuosa (Linnaeus, 1767) from southern Brazil: Implications for environmental and archaeological studies

This study presents the first stable isotopic and sclerochronological calibration of the bivalve Anomalocardia flexuosa (Linnaeus, 1767) in relation to environmental variables in a subtropical coastal area of southern Brazil. We investigate incremental shell growth patterns and δ18O and δ13C values of modern specimens collected alive from the Laguna Lagoonal System (LLS). Shells of Anomalocardia flexuosa are also one of the main biological components of pre-Columbian archaeological shell mounds and middens distributed along the Brazilian coastline. We therefore selected archaeological specimens from a local late Holocene shell mound (Cabeçuda) to compare their stable carbon and oxygen isotope values with those of modern specimens. Shell growth increments, δ18O and δ13C values respond to a complex of environmental conditions, involving, for example, the effects of temperature and salinity. The isotopic information extracted from archaeological specimens from Cabeçuda shell midden in the LLS indirectly indicates that environmental conditions during the late Holocene were different from present day. In particular, intra-shell δ18O and δ13C values of archaeological shells reveal a stronger marine influence at 3ka cal BP, which is in contrast to the seasonal freshwater/seawater balance that currently prevails at the LLS.

Calibration of the stable isotope composition and body size of the arid-dwelling land snail Sphincterochila candidissima, a climatic archive abundant in Mediterranean archaeological deposits

Land snails recovered from archeological deposits may be used to deduce climatic conditions during prehistoric occupation because their aragonitic skeletons are usually well-preserved and document valuable climatic information in the form of isotope codes. Since the snail Sphincterochila candidissima is common in archeological sites along the western Mediterranean but has been minimally investigated, the present work examines the relationship between the oxygen (δ18O) and carbon (δ13C) stable isotopes of modern specimens and relevant meteorological data. Individuals of Sphincterochila were regularly live-collected throughout 1 year (from September 2013 to October 2014) in Tarragona, NE Spain, and the δ18O and δ13C values were measured at the shell lip or aperture, which depicts the last growth episode closest to specimens’ collection date. Shell margin δ18O values varied from +3.1‰ in September to −0.8‰ in April. Average shell margin δ18O values per collection date positively correlated with monthly averaged rainwater δ18O, and negatively correlated with monthly averaged maximum relative humidity, while other variables (i.e. temperature and precipitation amount) did not explain monthly–submonthly isotopic variations in the shell lip. Shell margin δ13C values ranged from −11‰ in September to −4.8‰ in March, pointing to detectable variations in snails’ diet between seasons. Interestingly, snail body size positively correlated with shell margin δ18O and δ13C values, suggesting that larger snail size is reached under drier conditions. This work shows that S. candidissima from the western Mediterranean is a valid paleoprecipitation δ18O and/or paleohumidity proxy in the region at the monthly–submonthly scale. Although shell margin δ18O recorded fall and spring climate conditions only, limiting therefore their use to estimate the season of harvest, snails that grow year-round (e.g. at subtropical–tropical regions) may be used usefully for such analyses.

Assessing elemental ratios as a paleotemperature proxy in the calcite shells of patelloid limpets

Archaeological shell and fish middens are rich sources of paleoenvironmental proxy data. Patelloid limpet shells are common constituents in archaeological middens found along European, African, and South American coastlines. Paleotemperature reconstructions using oxygen isotope ratios of limpet shells depend on the ability to constrain the oxygen isotope ratio of seawater; therefore, alternative proxies are necessary for coastal localities where this is not possible. The study evaluates whether Mg/Ca, Sr/Ca, Li/Ca, Li/Mg, and Sr/Li ratios are reliable proxies of sea surface temperature (SST) in the calcite layer of shells of the patelloid limpets, Patella vulgata and Nacella deaurata. We compare Mg/Ca, Sr/Ca, Li/Ca, Li/Mg, and Sr/Li ratios to the seasonal variations in contemporaneous δ18Oshell values, which primarily record seasonal changes in SST. Elemental ratios (Mg/Ca, Sr/Ca, Li/Ca, Li/Mg, and Sr/Li) show no significant correlations with reconstructed SST in P. vulgata and N. deaurata shells, nor do they show sinusoidal cycles expected from a SST proxy. In addition, shell δ13C values show no significant ontogenetic trends, suggesting that these limpets exhibit little change in metabolic carbon incorporation into the shell with increasing ontogenetic age. Although shell growth rate exhibits a logarithmic decrease with age based on calculated linear extension rates, growth rate does not correlate with elemental profiles in these limpets. Overall, elemental ratios are not reliable recorders of paleotemperature in patelloid limpets.

Relationship between the shell geochemistry of the modern aquatic gastropod Radix and water chemistry of lakes of the Tibetan Plateau

Fossil shells of the aquatic mollusk Radix are common in the exposed sediments of lake terraces on the Tibetan Plateau. However, the living environment of Radix, and the geochemical characteristics of its shells, is unclear. Here, we report the results of an investigation of the occurrence of modern Radix in lakes of the southeastern and central Tibetan Plateau, as well as measurements of various geochemical characteristics of the shells. The results indicate that the nutritional status of the lake waters is the main limiting factor for the survival of Radix in these lakes. The Sr/Ca ratio of the Radix shells is significantly positively correlated with both the Sr/Ca ratio and the conductivity of the lake water. Initially, Kd Sr decreases rapidly with low values of Sr/Cawater; however, in the case of Sr/Cawater values above 0.0076, Kd Sr exhibits only a small range of variation. The δ 13Cshell values are controlled by the δ 13C of lake water dissolved inorganic carbon (DIC). In addition, the contribution of DIC of organic origin to the Radix shells increases when the lake water is deficient in DIC of inorganic origin. The δ 18O values of the Radix shells provide useful information about the isotopic composition of the ambient waters.

Influence of the upwelling events on the δ13C and δ18O of the benthic bivalve shells of the South Western Continental Margin of India

In this study, the influence of the spatial and temporal variability of upwelling intensity and the associated biological productivity observed during different phases of summer monsoon along the southwestern continental margin of India (SWCMI) on the δ13C and δ18O of the inorganic biogenic carbonate shells was investigated. Multispecies benthic bivalve shells (1–5 mm) separated from ten surface sediment samples of SWCMI (off 12°N, 10°N and 9°N) collected during the onset (OSM) and peak (PSM) phase of the summer monsoon of 2009 were analysed for δ13C and δ18O. Sea surface temperature along the study region indicates prominent upwelling in PSM than in OSM. A comparison of analytical and predicted values for δ18O in the bivalve shells confirmed their in situ origin during both the sampling periods. During PSM, the δ13C values in the benthic bivalve shells were more depleted in 13C than during OSM which recorded lower values of δ13C in dissolved inorganic carbon of bottom waters expected in the study region in PSM due to the upwelled waters, high surface productivity and the associated high degradation of the organic matter in the subsurface and bottom waters. However, this depletion of δ13C was not observed in benthic bivalve shells obtained from 10°N, since it is influenced by high export fluxes of carbon from the Cochin estuary since early monsoon months.

Discrepancies between the stable isotope compositions of water, macrophyte carbonates and organics, and mollusc shells in the littoral zone of a charophyte-dominated lake (Lake Lednica, Poland)

The aim of this study was to better understand the relations between carbon and oxygen stable isotope values of ambient water, mollusc shells, macrophytes and their carbonate encrustations, commonly used in palaeolimnological studies. Water, molluscs and macrophytes were sampled from the littoral zone in Lake Lednica, NW Poland. The influence of carbon species assimilated during photosynthesis and the net intensity of photosynthesis resulting from the size of charophyte species and the density of their stands were postulated to be the most important factors causing the species-specific δ13C values of charophyte thalli and encrustations. It was suggested that photosynthetic activity of charophytes affected not only the δ13C values of charophyte encrustations but also mollusc shells by changing δ13C values of DIC within charophyte stands. In addition, incorporation of metabolic carbon into the shell was proposed as the main cause of both the 13C depletion of mollusc shells relative to δ13C values of DIC and the species-specific δ13C values of shells. Mollusc shells were precipitated at the isotope equilibrium or close to the equilibrium with δ18O values of lake water. Charophyte encrustations were found to be 18O depleted due to the kinetic isotope effects during intense photosynthesis and thus fast precipitation of the calcite.

Quantitative analysis of intraspecific variations in the carbon and oxygen isotope compositions of the modern cool-temperate brachiopod Terebratulina crossei

This study unravels intraspecific variations in the carbon isotope (δ13C) and oxygen isotope (δ18O) compositions of shells of the modern cool-temperate brachiopod Terebratulina crossei collected at a water depth of 70m in Otsuchi Bay, northeastern Honshu, Japan. Brachiopod shells have been used as proxies of the δ13C values of dissolved inorganic carbon (DIC) (δ13CDIC) and seawater temperature/δ18O (δ18OSW) values to reconstruct the evolution of Phanerozoic oceans. To identify more reliable shell portions as the proxies, we conducted a rigorous time-series comparison of δ13C and δ18O values between the brachiopod shells and calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite) (δ13CEC and δ18OEC values, respectively). Samples were collected from the outer and inner surfaces of the secondary shell layer along the maximum growth axis (ontogenetic-series and inner-series samples, respectively). The ontogenetic-series δ13C values, which showed regular annual and irregular non-annual cycles, partly fell in but were mostly less than the range of the δ13CEC values. The δ13C cycles were often associated with one or two minor negative peaks. The peaks were likely resulted from an increased incorporation of respiration-derived 12C due to elevated metabolic activity during spawning. The ontogenetic-series δ18O values showed distinct seasonal variations and were mostly within the range of δ18OEC values. The amplitude of the δ18O profiles was relatively large during the younger fast-growth stage, and decreased during the senescent slow-growth stage. The inner-series δ13C and δ18O values of individual shells varied within narrow ranges. The inner-series δ13C values were close to the minimum δ13CEC values. The inner-series δ18O values were in the upper range of the δ18OEC values. Kinetic isotope fractionation effects were evident, but its degree varied among different shells. We identified the shell portions reliably recording past ocean environments. The best estimates of annual average δ13CDIC values were obtained from ontogenetic-series δ13C values near the anterior shell edge and from inner-series δ13C values; those of the annual average seawater temperature were obtained from ontogenetic-series δ18O values from the entire transect and from the fast-growth stage.

Within-sample variability of δ13C and δ18O values of freshwater gastropod shells and the optimum number of shells to measure per sediment layer in the Paddenluch palaeolacustrine sequence, Germany

This study aimed to: (1) investigate intra-sample variability in the stable isotope composition (δ13C and δ18O values) measured in several individual mollusc shells; and (2) determine the optimum number of individual mollusc shells required to obtain stable isotope values representative of the mean climate signal for a sediment sample. Seven hundred and six gastropod shells were analysed, representing eight species. These eight species were most abundant and consistently present in the sediment sequence of the Paddenluch outcrop, Brandenburg, northeast Germany. Significant differences between minimum and maximum δ13C and δ18O values of the shells were recorded within all sediment samples and for all analysed species. Within the sediment samples, the greatest mean variability in both δ13C and δ18O values was found for Hippeutis complanatus, 5.84 and 5.23 ‰, respectively. The greatest intra-specific variability in C and O isotope values within individual sediment samples, however, was 10.2 and 6.8 ‰, respectively. Significant differences between minimum and maximum δ13C and δ18O values of individual mollusc shells within the same sediment sample are probably the result of population isotope variability, variations in species life span, thickness of the sediment sample, sediment accumulation rate, and environmental fluctuations during the time span when the sediment was deposited. Shells were randomly selected to determine the optimum number of shells necessary to obtain stable isotope values that are representative of the mean climate signal, from a 1-cm sample of sediment. For each layer of the Paddenluch sediment sequence, 15 or more shells (N ≥ 15) was the required sample size to provide representative isotope values and yield a reliable mean isotope value. Sample sizes with fewer shells displayed more variable mean δ13C and δ18O values. Stable isotope composition of the analyzed molluscs was species-specific, with differences in mean δ13C and δ18O values of 2.5 and 3 ‰, respectively. This illustrates the need to use mono-specific samples throughout a sediment sequence to generate reliable data.

Coupling experimental and field-based approaches to decipher carbon sources in the shell of the great scallop, Pecten maximus (L.)

This research investigated how the carbon isotopic composition of food source (δ13Cfood) and dissolved inorganic carbon (δ13CDIC) influences the carbon isotopic composition of Pecten maximus shells (δ13Cshell) under both experimental and natural conditions. The objectives are to better understand the relationship between P. maximus and its environment, and to specifically distinguish conditions under which calcification is influenced by respired CO2 derived from food sources versus conditions in which calcification uses inorganic carbon from seawater. Laboratory experiment investigated carbon incorporation into shell carbonates by maintaining scallops under conditions where the stable carbon isotopic composition of food sources was considerably depleted (−54‰), relative to values observed in the natural environment (−21‰). Laboratory experiment ran for 78days under three temperature conditions, 15°C, 21°C and 25°C. A survey of the environmental parameters and stable carbon isotopic composition into shell carbonate of natural population of P. maximus was also realized during the same year in the Bay of Brest, France. Data collected from both laboratory experiment and the natural environment confirmed that both δ13CDIC and δ13Cfood influence δ13Cshell values and that organic carbon incorporation (CM) averages about 10% (4.3–6.8% under experimental conditions and 1.9–16.6% in the natural environment). The shift in stable carbon isotopic composition from the uptake of depleted food sources under experimental conditions realized a marked divergence in the predicted equilibrium between calcium carbonate and ambient bicarbonate, relative to the natural environment. This offset was 1.7±0.6‰ for scallops in their natural environment and 2.5±0.5 and 3.2±0.9‰ for scallops under experimental conditions at water temperatures of 15°C and 21°C, respectively. The offset of 3‰ for scallops subjected to laboratory experiment could not be explained in light of growth rate but may be related to food supply and/or temperature. Food source and temperature effects may also explain the annual variation observed in CM values measured from scallops in their natural environment. CM estimation from the natural population of P. maximus varied seasonally from around 2% at the end of winter, to 12% in summer. The seasonal variation resembles variability in the carbon isotopic composition of the food sources throughout the year with an exception at the end of winter.

Differences in stable isotope compositions of freshwater snails from surface sediments of two Polish shallow lakes

The study reports and discusses the differences in δ13C and δ18O values of shells between several species of freshwater snails. Shells were derived from sediment samples collected from depths of 0.5, 1, 2 and 3 m along transects in two shallow eutrophic lakes located in mid-western Poland. Mean δ13C values of the shells ranged between −7.5 and −3.8‰ in Lake Jarosławieckie and between −8.1 and −5.2‰ in Lake Rosnowskie Duże, whereas mean δ18O values ranged between −2.2 and −0.2‰ and between −2.2 and 0.4‰ respectively in the studied lakes. A similar order of species in terms of shell isotope values, from least to most 13C and 18O-depleted was observed in both lakes and seems to indicate constancy of the factors controlling the stable isotope compositions of snail shells. We postulate that the nearly 4‰ difference in the mean carbon stable isotope values between the species was primarily controlled by the amount of metabolic carbon incorporated into the shells and the δ13C values of the snail food. Different growth cessation temperatures and microhabitats of the species studied result in temporally and spatially varied DIC δ13C values, water δ18O values and water temperature of shell precipitation, and may thus differentiate the δ13C and δ18O values of shells. The range of δ13C and δ18O values of individual shells from a sediment sample (mean 2.35 and 2.15‰, respectively) is interpreted as reflecting an intraspecific variability of isotope compositions in shells from a population and changes of the ambient conditions during the accumulation of the sediment layer. The species-specificity and intraspecific variability in C and O isotopic compositions of shells allow concluding that in palaeolimnological studies, stable isotope analyses should be performed on a set of mono-specific shells representing mean isotope compositions of the species for the interval studied rather than single shells or multispecific bulk shell material.