Stable isotope of some selected Egyptian pectinids and their paleoenvironmental implications

Eight pectinid shells were collected and subjected to quantitative study using δ¹⁸O and δ¹³C isotopic analysis in order to study the paleoenvironment which prevailed during their calcification. The scalerochronological variations in δ¹⁸O and δ¹³C values, among these shells are also discussed. The Early Miocene pectinid shells display highly depleted δ¹⁸O and δ¹³C signature as a result of paleo-meteoric water with heavy rainfall that was produced by Tropical Cyclones when the Mediterranean Sea was open. The Early Pliocene pectinid shells reveal depleted δ¹⁸O values, related to the influx of fresh water influenced by monsoonal activity following the formation of the Tibetan Plateau. Their enrichment in the δ¹³C isotopic excursion is referred to high productivity of the Indian Ocean, which was the main source of the Red Sea water. The Pleistocene pectinid shell shows highly depleted δ¹⁸O and δ¹³C signature with obvious diagenetic shell structure, indicating that a wetter humid climate prevailed during the Early–Middle Pleistocene and long sub-arial exposure of the shell. The Recent Mediterranean pectinid shell displays slight enrichment in δ¹⁸O and δ¹³C values referring to deeper inhabitation of this species with a low temperature and high salinity environment. The scalerochronological variations in both δ¹⁸O and δ¹³C values, along these shells is referred to seasonal variations or kinetic effects.     

Stable isotopic insights into paleoclimatic conditions and alluvial depositional processes in the Kaiparowits Formation (Campanian,south-central Utah,U.S.A.)

The Kaiparowits Formation contains an exceptionally rich history of tectonic, climatic, and biologic conditions within the Western Interior of North America during the Campanian. Here we reconstruct aspects of the southern Cordilleran foreland basin's paleohydrology using δ¹⁸O and δ¹³C values determined from unionoid bivalve shells and pedogenic carbonate nodules derived from a suite of lithofacies associations. Unionoid shells derived from fluvial deposits display average water δ¹⁸O estimates of −13.7‰ ± 2.1 (1σ) (VSMOW) and shell δ¹³C values of −4.0‰ ± 1.5 (VPDB), whereas pedogenic carbonate nodules display average values of −6.0‰ ± 0.5 and −8.7‰ ± 0.8, respectively. Unionoid shells derived from pond deposits fall in between the two other environments with average values of −9.5‰ ± 1.8 and −5.7‰ ± 2.1, in δ¹⁸O and δ¹³C values respectively. Water δ¹⁸O estimates are interpreted to represent high altitude runoff within river systems, low elevation precipitation within the basin onto floodplain soils, and varying degrees of mixing between these two components within floodplain ponds. δ¹³C values track the isotopic composition of dissolved inorganic carbon within river, soil, and pond waters with high values likely reflecting greater contribution from chemically weathered marine carbonates exposed in the hinterland and lower values reflecting greater contributions from the in situ degradation of plant matter. Up-section there is a shift to lower δ¹⁸O values and higher δ¹³C values in fluvially-derived unionoid shells that post-dates an incursion of the Western Interior Seaway, but coincides with a shift in sediment provenance, an increase in basin sedimentation rates, and a change to a more anastomosed-style channel morphology within the basin foredeep depocentre. By combining the isotopic patterns with previously published sedimentologic, climate model, and paleofloral records we find: 1) additional evidence for humid, wet, and potentially monsoonal conditions within the region, 2) support for a tectonic uplift event, potentially related to Laramide deformation, and 3) greater aggradation and overbank flooding within the alluvial system in response to the uplift event.     

Small-scaled environmental changes: indications from stable isotopes of gastropods (Early Miocene, Korneuburg Basin, Austria)

The Korneuburg Basin, with mainly upper Lower Miocene (Karpatian) sediment filling, is divided by the Mollmannsdorf–Obergänserndorf Swell into two sub-basins characterised by different environmental settings. Paleoecological data indicate a marine northern part and a mainly estuarine southern part. Nevertheless, short-termed marine ingressions from the north allowed marine faunas (ostracods, molluscs, and echinoids) to temporarily settle the southern part of the basin. The carbon and oxygen isotopic composition of gastropod shells from these different environmental settings were investigated. Highest δ¹⁸O and δ¹³C values are found in Turritella shells from the northern part of the basin, and in Turritella shells from layers interpreted as a marine ingression in the south. Generally, components of the mudflat fauna (Tympanotonos cinctus, Granulolabium bicinctum, Terebralia bidendata, and Ocenebra crassilabiata) have slightly lower isotope values. Considerable freshwater influx in the southern part is documented by abundant freshwater genera such as Melanopsis, which show low carbon and oxygen isotope values. Data of identical taxa, especially Turritella and Granulolabium, reflect a trend from higher isotope values at the marine northern part to slightly lower values in the mainly estuarine southern part of the basin. Differences in δ¹⁸O between the marine and the estuarine assemblages are interpreted to be caused by changes in salinity and isotopic composition of ambient water rather than by temperature. Paleotemperature estimates derived from oxygen isotope data are in good agreement with existing paleoclimatic proxies for the Korneuburg Basin. Hence, an annual range of the sea-surface temperature from 13 to 26°C can be predicted within that protected basin.     

The oxygen and carbon isotope composition of Carboniferous fossil components: sea-water effects

The aragonitic molluscs and lime-mud of the Pennsylvanian Buckhorn asphalt (Deese Group) of southern Oklahoma precipitated calcium carbonate in oxygen and carbon isotopic equilibrium with ambient sea-water. In addition, δ¹⁸O values indicate that the pelecypods precipitated their shells during the warmer months of the year. The coiled nautiloids probably precipitated their shells in the warm surface water and throughout the year. For the orthocone nautiloids, the δ¹⁸O values suggest that they precipitated their shells in deeper/cooler water. The low-Mg calcite brachiopods of the Mississippian Lake Valley Formation of New Mexico precipitated shells in oxygen and carbon isotopic equilibrium with ambient sea-water. The δ¹⁸O and δ¹³C values of the Buckhorn and Lake Valley faunas, in conjunction with other published results, suggest that Carboniferous sea-water was, on a average, depleted in δ¹⁸O by 1·5 ± 2‰, PDB, relative to Recent sea-water. However, the δ¹³C value of +2.6 ± 2‰, PDB, for average Carboniferous sea-water is similar to that of Recent ocean water. Early diagenetic alteration of metastable carbonates probably occurs in a meteoric-sea-water mixing zone. In this zone the oxygen and carbon isotopic compositions of these components are increased by about 2-4‰, PDB over their marine composition.     

Deciphering late Quaternary land snail shell δO and δC from Franchthi Cave (Argolid,Greece)

This paper investigates the stable isotopic composition from late Pleistocene–Holocene (~ 13 to ~ 10.5 cal ka BP) shells of the land snail Helix figulina, from Franchthi Cave (Greece). It explores the palaeoclimatic and palaeoenvironmental implications of the isotope palaeoecology of archaeological shells at the time of human occupation of the cave. Modern shells from around the cave were also analysed and their isotopic signatures compared with those of the archaeological shells. The carbon isotope composition of modern shells depicts the consumption of C₃ vegetation. Shell oxygen isotopic values are consistent with other Mediterranean snail shells from coastal areas. Combining empirical linear regression and an evaporative model, the δ¹⁸Os suggest that modern snails in the study area are active during periods of higher relative humidity and lower rainfall δ¹⁸O, probably at night. Late glacial and early Holocene δ¹⁸Os show lower values compared to modern ones. Early Holocene δ¹⁸Os values likely track enhanced moisture and isotopic changes in the precipitation source. By contrast, lower late glacial δ¹⁸O could reflect lower temperatures and δ¹⁸Op, compared to the present day. Shell carbon isotope values indicate the presence of C₃ vegetation as main source of carbon to late glacial and early Holocene snails.     

Isotopic variability in the aragonite shells of freshwater gastropods living in springs with nearly constant temperature and isotopic composition

We conducted a year-long, intensive monitoring program of live aquatic gastropods (Helisoma duryi, Melanoides tuberculata, Physa virgata, Pyrgulopsis sp., and Tyronia sp.) and their host springs in the Ash Meadows National Wildlife Refuge of southern Nevada. Our purpose was to constrain the degree of natural variation in the isotopic values of shell aragonite for gastropods living in near-constant conditions. Inter- and intraspecies variations, as well as within-shell variations, of δ¹⁸O and δ¹³C values for all taxa were larger than predicted based on variations in environmental conditions alone. This result suggests that different organisms growing in identical or nearly identical environmental conditions may not produce shells with equilibrium isotopic compositions and that these offsets from equilibrium may differ by small, but statistically significant amounts. For the gill-breathing, fully aquatic gastropods M. tuberculata, Pyrgulopsis sp., and Tyronia sp., the deviation of measured isotopic values compared to predicted values based on average environmental conditions were consistent with differences between taxa in the seasonal timing of shell growth. Measured values for the lung-breathing gastropods H. duryi and P. virgata were higher for δ¹⁸O and lower for δ¹³C than predicted at isotopic equilibrium, even when accounting for seasonality effects. We suggest that explaining the differences between the shell isotopic composition of lung- and gill-breathing snails requires a combination of both behavioral and physiologic factors. Our results illustrate the potential complexities of interpreting stable isotopic data from fossil gastropod shells even when environmental conditions are nearly constant, and place limitations on the paleoenvironmental deductions that can be made from the isotopic measurements on fossil gastropods.     

Isotopic evidence for the incorporation of methane-derived carbon into foraminifera from modern methane seeps, Hydrate Ridge, Northeast Pacific

The presence of modern methane seeps at Hydrate Ridge, offshore Oregon, provide an opportunity to study the influence of methane seeps on the ecology and geochemistry of living foraminifera. A series of cores were collected from the southern summit of Hydrate Ridge in 2002. Samples were preserved and stained to determine the δ¹³C composition of three species of live (stained) and dead benthic foraminifera: Uvigerina peregrina, Cibicidoides mckannai, and Globobulimina auriculata. Specimens were examined under light and Scanning Electron Microscopy (SEM) and exhibit no evidence of diagenesis or authigenic carbonate precipitation. Individual living foraminifera from seep sites recorded δ¹³C values from −0.4‰ to −21.2‰, indicating the isotopic influence of high methane concentrations. Average δ¹³C values (calculated from single specimens) range from −1.28 to −5.64‰ at seep sites, and −0.81 to −0.85‰ at a control (off seep) site.Two distinct seep environments, distinguished by the presence of microbial mats or clam fields, were studied to determine environmental influences on δ¹³C values. Individual foraminifera from microbial mat sites exhibited more depleted δ¹³C values than those from clam field sites. We interpret these differences as an effect of food source and/or symbiotic microbes on foraminiferal carbon isotopic values, acting to magnify the negative δ¹³C values recorded via the DIC pool. No statistical difference was found between δ¹³C values of live vs. dead specimens. This suggests that authigenic carbonate precipitation did not play a dominant role in the observed isotopic compositions. However, a few dead specimens with extremely negative δ¹³C composition (<-12‰) do indicate potential evidence for an authigenic influence on the recorded δ¹³C composition.     

Oxygen and carbon isotope measurements of land snail shell carbonate

δ¹³C and δ¹⁸O analyses have been performed on the aragonite shells of a variety of modern land snails from a number of different geographic and climatic locales. The δ¹⁸O values of the waters assumed to be in isotopic equilibrium with the shell carbonate were calculated. These calculated δ¹⁸O values are more positive than the δ¹⁸O values of the average meteoric waters in the locales in which the snails lived. The ¹⁸O enrichment appears to be linearly related to the reciprocal of the local relative humidity, which is consistent with the notion that these ambient waters have undergone isotopic steady-state evaporation. Measurements of the δ¹⁸O values of ancient land snail shells from the excavation of Sudden Shelter (42SV6) at Ivie Creek, Utah, suggest that the climate at this site was probably warmer and/or drier around 7100–7800 BP than at present.     

Isotopic partitioning between scallop shell calcite and seawater: effect of shell growth rate

The relationship between molluscan shell growth rate and skeletal δ¹⁸O and δ¹³C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ¹⁸O and δ¹³C. At low shell growth rates, shell δ¹⁸O were more positive (of the order +0.4‰) and δ¹³C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ¹⁸O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ¹³C were consistent with incorporation of isotopically depleted respiratory ¹³C (i.e., a metabolic effect). A trend toward more depleted shell δ¹⁸O and δ¹³C values occurred at higher shell growth rates, with negative δ¹⁸O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ¹⁸O and δ¹³C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.     

Sea-rain-lake relation in the Last Glacial East Mediterranean revealed by δO-δC in Lake Lisan aragonites

We investigated the Sea-Rain-Lake relation during the Last Glacial-Holocene in the East Mediterranean region by comparing the δ¹⁸O and δ¹³C records of authigenic aragonite deposited in Lake Lisan, the Dead Sea, Mediterranean foraminifera, and speleothems. The Lisan Formation data display long- and short-term variations of δ¹⁸O, representing steady-state conditions of the lake (e.g., 5.6‰ ± 0.5‰ and 4.5‰ ± 1‰ in the Upper and Lower Members of the Lisan Formation, respectively), and short-term excursions reflecting large floods and droughts. The long-term (steady-state) δ¹⁸O values of the Lisan aragonites show similarity to the corresponding time-equivalent records of the Eastern Mediterranean foraminifera and Judea Mountain speleothems: The Last Glacial deposits are in all of them 2‰-3‰ heavier than the Holocene ones. We interpret this similarity as reflecting the significance of the source effect on the long-term behavior of isotopic reservoirs: Speleothem δ¹⁸O is strongly influenced by the marine reservoir that contributes its vapor to rain formation; the lake δ¹⁸O is dominated by the composition of the inflowing water. Short-term variations in the isotopic composition of rainfall are dominated by the amount effect and the temperature and those of the Lake’s upper water mass by the lake’s water balance.δ¹³C values are more variable than δ¹⁸O in the same Lisan sequences (e.g., δ¹³C in the Lower Member is 1.0‰ ± 1.7‰, whereas δ¹⁸O is 4.6‰ ± 0.7‰) and are 1‰ to 1.5‰ higher in the Upper Member than in the Lower and Middle Members of the Lisan Formation. These variations reflect significant increase in primary productivity of the lake and algal bloom activity. It appears that the hypersaline-saline lakes were not as “dead” as the Dead Sea is and that algal activity had an important impact upon the geochemistry of Lake Lisan.The δ¹⁸O data combined with independent geochemical and limnologic information (e.g., level fluctuations) indicate that Lisan time was characterized by high precipitation-high lake stands-high atmospheric humidity, whereas the Holocene Dead Sea shows the opposite behavior. This paleoclimatic reconstruction is consistent with independent evidence for significantly wetter conditions in the East Mediterranean region during the Last Glacial period.     

Environmental significance of C/C and O/O ratios of modern land-snail shells from the southern great plains of North America

¹³C/¹²C and ¹⁸O/¹⁶O ratios of aragonite shells of modern land snails from the southern Great Plains of North America were measured for samples from twelve localities in a narrow east-west corridor that extended from the Flint Hills in North Central Oklahoma to the foothills of the Sangre de Cristo Mountains in Northern New Mexico, USA. Across the study area, shell δ¹⁸O values (PDB scale) ranged from −4.1‰ to 1.2‰, while δ¹³C values ranged from −13.2‰ to 0.0‰. δ¹⁸O values of the shell aragonite were predicted with a published, steady state, evaporative flux balance model. The predicted values differed (with one exception) by less than 1‰ from locality averages of measured δ¹⁸O values. This similarity suggests that relative humidity at the time of snail activity is an important control on the δ¹⁸O values of the aragonite and emphasizes the seasonal nature of the climatic information preserved in the shells. Correlated δ¹³C values of coexisting Vallonia and Gastrocopta suggest similar feeding habits and imply that these genera can provide information on variations in southern Great Plains plant ecology. Although there is considerable scatter, multispecies, transect average δ¹³C values of the modern aragonite shells are related to variations in the type of photosynthesis (i.e., C₃, C₄) in the local plant communities. The results of this study emphasize the desirability of obtaining isotope ratios representing averages of many shells in a locale to reduce possible biases associated with local variations among individuals, species, etc., and thus better represent the “neighborhood” scale temporal and/or spatial environmental variations of interest in studies of modern and ancient systems.     

Stable carbon and oxygen isotopic variations in modern Rabdotus land snail shells in the southern Great Plains, USA, and their relation to environment

Variations of stable isotopic ratios of carbon (¹³C/¹²C) and oxygen (¹⁸O/¹⁶O) were investigated in modern shells of two species of Rabdotus land snails (R. dealbatus and R. alternatus) in the southern Great Plains. Geographic variation in relation to climate and vegetation, microgeographic variation, variability among individuals, and detailed records of seasonal variations within individual shells were studied. Stable carbon isotopic ratios in shell carbonate are primarily a function of the isotopic composition of the diet of the snails, as represented by the isotopic composition of shell organic matter. This in turn reflects the presence or absence of CAM (Crassulacean Acid Metabolism) or C₄ plants. Vegetation density may have a small effect on the carbon isotope ratios. Microgeographic variation (samples within 25 to 300 m) is greater than that seen across different climatic regions and points to very local control of isotopic variations, predominantly related to vegetation. Seasonal variations, as assessed through serial analysis of individual shells (up to 35 samples per shell), may provide a means for distinguishing between isotopic influences of perennial CAM vs. annual C₄ plants. Carbon isotopic variations in time-series of shells from a site provide a means of reconstructing temporal changes in environment and climate.Oxygen isotopic values of shell carbonate are uniform across the region and also show no significant microgeographic variation. The oxygen isotopic composition appears to be mainly a function of the rainwater isotopic composition, with no direct influence of rainfall amount or evaporative effects. The δ¹⁸O values are only 2‰ enriched relative to estimated equilibrium with rainwater. Variability is low (SD of 0.8‰ among sites), so the isotopic composition of fossil Rabdotus shells can provide a precise record of changes in the isotopic composition of rain over time.     

Early‐middle Holocene land snail shell stable isotope record from Grotta di Latronico 3 (southern Italy)

This paper compares stable isotope (δ¹⁸O and δ¹³C) records of early–middle Holocene land snail shells from the archaeological deposits of Grotta di Latronico 3 (LTR3; southern Italy) with modern shell isotopic data. No substantial interspecific variability was observed in shell δ¹⁸O (δ¹⁸Os) of modern specimens (Pomatias elegans, Cornu aspersum, Eobania vermiculata, Helix ligata and Marmorana fuscolabiata). In contrast, interspecific shell δ¹³C (δ¹³Cs) variability was significant, probably due to different feeding behaviour among species. The δ¹⁸Os values of living land snails suggest that species hibernate for a long period during colder months, so that the signal of ¹⁸O‐depleted winter rainfall in their δ¹⁸Os is lost. This suggests that δ¹⁸Os and δ¹³Cs values of Pomatias elegans from this archaeological succession provide valuable clues for seasonal (spring–autumn) climatic conditions during the early–middle Holocene. The δ¹⁸Os values of fossil specimens are significantly lower than in modern shells and in agreement with other palaeoclimatic records, suggesting a substantial increase of precipitation and/or persistent changes in air mass source trajectories over this region between ca. 8.8 cal ka BP and 6.2–6.7 ka ago. The δ¹³Cs trend suggests a transition from a slightly ¹³C‐enriched to a ¹³C‐depleted diet between early and middle Holocene compared to present conditions. We postulate that this δ¹³Cs trend might reflect changes in the C3 vegetation community, potentially combined with other environmental factors such as regional moisture increase and the progressive decrease of atmospheric CO₂ concentration. Copyright © 2010 John Wiley & Sons, Ltd.     

The reef builder gastropod Dendropomapetreaum - A proxy of short and long term climatic events in the Eastern Mediterranean

High-resolution δ¹⁸O and δ¹³C records obtained from seven cores were drilled from ledges of the reef builder gastropod Dendropomapetreaum and used to reconstruct variations in the Levantine basin sea surface temperature, hydrology and productivity during the past 500 years. The δ¹⁸O of the aragonite shell of living D. petreaum indicate that skeletal deposition occurs under isotopic equilibrium and faithfully record the temperature and surface water δ¹⁸O during summer and autumn. The mean down core δ¹⁸O record clearly captures global and local climatic events, such as the Little Ice Age (LIA) and the recent warming of surface waters in the Eastern Mediterranean. Comparison to the Western Mediterranean vermetid δ¹⁸O record reveals changes in the freshwater/evaporation budgets of the two basins during cold and warm periods. The Eastern basin had lower surface temperatures and excess evaporation during the LIA and experienced a relatively larger warming and/or a decrease in freshwater/evaporation during the past 70 years. The D. petraeum δ¹³C is strongly related to δ¹³C of dissolved inorganic carbon and to the primary productivity of the surface water. The mean down core δ¹³C record exhibits enrichment during the LIA maximum and a strong depletion trend during the last century. The LIA δ¹³C enrichment is attributed to an increase in primary production and high nutrient levels which resulted from increased vertical mixing and upwelling. The last century δ¹³C depletion is mostly related to the increased anthropogenic emissions of ¹³C depleted carbon dioxide and to a certain decrease in primary production. The data indicate that D. petraeum isotopic signatures are unique proxies for last 500 years high-resolution reconstruction of paleo-oceanographic environments in the Mediterranean and potentially in the sub-tropical Atlantic regions.     

Carbon isotopic fractionation in live benthic foraminifera—comparison with inorganic precipitate studies

Carbon and oxygen isotopic analyses have been performed on live-stained aragonitic and calcitic benthic foraminifera and dissolved inorganic carbon (DIC) from the Southern California Borderland to examine carbon isotopic fractionation in foraminifera. Temperature, salinity and pH data have also been collected to permit accurate determination of the δ¹³C of bicarbonate ion and thus aragonite-HCO₃ and calcite-HCO^?₃ isotopic enrichment factors (?_ar-b and ?_cl-b, respectively). Only species which precipitate in ¹⁸O equilibrium have been considered.?_ar-b values based on Hoeglundina elegans range from 1.9%. at 2.7°C to 1.1%. at 9.5°C. Only the lower temperature values agree with a tentative carbon isotope equilibrium equation for aragonite based on the data of Rubinson and Clayton (1969) and Emrich et al. (1970). The temperature dependence of ?_ar-b is considerably greater than the equilibrium equation would predict and may be due to a vital effect.The calcitic foraminifera Cassidulina tortuosa, Cassidulina braziliensis, and Cassidulina limbata, Bank and Terrace dwellers, have similar δ¹³C values and yield an average ?_cl-b value of ?0.2 ± .1%. between 8° and 10°C. Calcitic Uvigerina curticosta, Uvigerina peregrina, and megalospheric B. argentea, Slope and Basin dwellers, are ?0.7 ± .1%. enriched relative to ambient bicarbonate for 3 to 9°C. No temperature dependence for ?_cl-b was observed for the species in either habitat. The ?_cl-b values for Cassidulina species are close (± 0.3%.) to the values given by the tentative equilibrium curve for calcite, while Uvigerina and Bolivina species give values 0.2–0.8%. less. The ?_cl-b difference between the Cassidulina species and the Uvigerina and Bolivina species is attributed to the incorporation of ¹³C-depleted pore water DIC by the latter group rather than to taxonomic or temperature differences.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Carbon and Oxygen Isotopic Characteristics of REE—Fluorocarbonate Minerals and Their Genetic Implications,Bayan Obo Deposit,Inner Mongloia,China

REE－fluorocarbonates as major REE minerals in the Bayan Obo deposit,the largest REE deposit in the world,were analyzed for their stable isotopic compositions,The δ^13 C and δ^18 O values of huanghoite,cebaite and bastnaesite from late-stage veins vary in the ranges of 7.8--4.0‰ and 6.7-9.4‰,respectively,These data are relatively similar to those of bastnaesites from banded ores:δ^13C-5.6--5.2‰ andδ^18O3.6-5.5‰.The REE fluorocarbonates from both late-staege veins and banded ores are characterized by lower δ^13 C and δ^18O values,especially the δ^18O values of bastnaesites from banded ores.Compared with them,the disseminated bastnaesits the dolomite-type ores possess rather highδ^13 C and δ^18O values,i.e.,-2.1-0.4‰ and 8.6-12.9‰ respectively.The high values are typical of the sedimentary host dolomite rocks as well as of the dolomite-type-ores.The carbon and oxygen isotopic characteristics of REE fluorocarbonate minerals provide new evidence for the hypothesis on the origin of Bayan Obo deposit-epigenetic hydrothermal metasomatism.     

Cenomanian events in the deep western Basque Basin: the Leioa section

Analysis of the microfaunas (foraminifera, ostracods) and the stable isotope values (δ¹³C, δ¹⁸O) of the Leioa section, as representative of the deep Basque Basin, has allowed us to propose a detailed palaeoenvironmental reconstruction of this region during the Cenomanian, as well as to register global chronostratigraphic reference levels to facilitate interregional correlations. During the Cenomanian, part of the basin, the Plentzia Trough, was occupied by intermediate water masses, as deduced by the relative percentages of planktonic (Rotalipora) and benthonic foraminifera. A noticeable change is observed at the middle-late Cenomanian transition: the replacement of the dominance of keeled (rota-liporids) by incipiently-keeled (dicarinellids, praeglobotruncanids) planktonic foraminifera, indicative of the new influence of the upper intermediate waters. The temporary effect of shallow waters is deduced in one interval of the latest early Cenomanian and two more of the middle Cenomanian, as indicated by the dominance of globular planktonic foraminifera (hedbergellids). These water masses were moderately to strongly hypoxic (<4 to <2 ml/l of disolved oxygen) after the ostracod platycopid signal and benthonic foraminiferal hypoxic indicators. The dysaerobia may have been particulary strong (almost anoxia?) during part of the middle Cenomanian. Micronutrient availability was also restricted during several intervals of the middle Cenomanian, as indicated by the sudden decrease in the species diversity of the calcitic benthonics during the period when increased trends of the δ¹³C isotopic signals are observed. The combination of both hypoxia and nutrient depletion produced drastic changes in the microfaunal assemblages, with emigrations and local extinctions, showing benthonic perturbations from the time indicated by the base of theRotalipora reicheliZone onwards. From the beginning until the end of the middle Cenomanian, eleven of these perturbations are recorded as regional bioevents, using as bioevent-markers, intervals where microfauna was absent (including benthic-free intervals, B-FI; benthonic calcitic-free intervals, BC-FI; and ostracod-free intervals, O-FI). These changes led to the renewal of the microfaunas; benthonic foraminifera renewed their specific stocks during the early to early middle Cenomanian, with planktonic foraminifera and ostracods undergoing renewal at the end of the middle Cenomanian. Isotope values of δ¹⁸O and δ¹³C are consistent with the palaeoenvironmental changes detected by the microfaunas; their maximum and minimum shifts coincide with the bioevents. The double-peaked positive shift of δ¹³C for the mid-Cenomanian of northwest Europe (Jenkynset al., 1994; Paulet al., 1994a) has been recognized in this series of the Basque Basin. The palaeoenvironmental perturbations deduced in the Cenomanian of the Leioa section are attributed essentially to palaeoceanographic changes, where intermediate water masses profoundly influenced the planktonic and benthonic ecosystems. The influence of other local causes, such as volcanic activity at that time, or tectonics between the Iberian and European plates, are more difficult to prove. Several of the bioevents defined in the middle Cenomanian of this basin could probably be global in nature, and thus may be useful for establishing interregional correlations.     

Paleoclimatic implications of the spatial patterns of modern and LGM European land-snail shell δO

The oxygen isotopic composition of land-snail shells may provide insight into the source region and trajectory of precipitation. Last glacial maximum (LGM) gastropod shells were sampled from loess from Belgium to Serbia and modern land-snail shells both record δ¹⁸O values between 0‰ and − 5‰. There are significant differences in mean fossil shell δ¹⁸O between sites but not among genera at a single location. Therefore, we group δ¹⁸O values from different genera together to map the spatial distribution of δ¹⁸O in shell carbonate. Shell δ¹⁸O values reflect the spatial variation in the isotopic composition of precipitation and incorporate the snails' preferential sampling of precipitation during the warm season. Modern shell δ¹⁸O decreases in Europe along a N-S gradient from the North Sea inland toward the Alps. Modern observed data of isotopes in precipitation (GNIP) demonstrate a similar trend for low-altitude sites. LGM shell δ¹⁸O data show a different gradient with δ¹⁸O declining toward the ENE, implying a mid-Atlantic source due to increased sea ice and a possible southern displacement of the westerly jet stream. Balkan LGM samples show the influence of a Mediterranean source, with δ¹⁸O values decreasing northward.     

The impact of African aridity on the isotopic signature of Atlantic deep waters across the Middle Pleistocene Transition

A high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m-long marine core located off the Mauritanian margin spanning the last ~ 1.2 Ma, documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1.0–0.9 Ma, within the so-called Mid-Pleistocene Transition (MPT) time period. The increase in the oxygen isotopic composition of deep waters, as seen through the benthic foraminifera δ¹⁸O values, is consistent with the growth of larger ice sheets known to have occurred during this transition. Deep-water mass δ¹³C changes, also estimated from benthic foraminifera, show a strong depletion for the same time interval. This drastic change in δ¹³C values is concomitant with a worldwide 0.3‰ decrease observed in the major deep oceanic waters for the MPT time period. The phase relationship between aeolian terrigeneous signal increase and this δ¹³C decrease in our record, as well as in other paleorecords, supports the hypothesis of a global aridification amongst others processes to explain the deep-water masses isotopic signature changes during the MPT. In any case, the isotopic shifts imply major changes in the end-member δ¹⁸O and δ¹³C values of deep waters.