Variability of Stable Isotope Fingerprints of the Serpulid Ficopomatus enigmaticus Within a Permanently Stratified Estuary: Implications for (Palaeo)environmental Interpretations

This paper examines how the mixing of freshwater and seawater, and related mixing of freshwater and marine particulate organic matter (POM) in the permanently stratified estuary of the River Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and tubes of the serpulid Ficopomatus enigmaticus. The carbon stable isotope composition (δ¹³C values) of the river-borne POM is retained over long distances, causing a depletion in ¹³C of POM in brackish waters. A trophic depletion in ¹³C was recorded in serpulid soft tissues. The serpulid carbonate tubes were depleted in ¹³C even at locations with salinity close to that of the sea and were subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making carbonate depleted in ¹³C by several per mil compared with DIC. These effects, though large in the freshwater zone, fade towards the sea. The carbonate δ¹⁸O values of tubes reflect the δ¹⁸O values of the water. The temperature-related differences in δ¹⁸O values of tubes from different sites are masked by source-related differences in the δ¹⁸O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, in serpulide tubes, the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in ¹³C in freshwater and brackish environments.     

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.     

Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes

The oxygen (δ¹⁸O, δ¹⁷O) and carbon (δ¹³C, FMOD¹⁴C-the fraction of modern ¹⁴C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ¹³C and FMOD¹⁴C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO₂ that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.     

Flux balance models for the oxygen and carbon isotope compositions of land snail shells

A simple flux balance model with a diffusive, evaporative boundary layer indicates that the time constant (characteristic time) for approach to oxygen isotope steady state in the body fluid of land snails is ∼19 min or less. These comparatively short times support an assumption that the snail’s aragonitic shell is commonly precipitated from a body fluid that is at, or near, isotopic steady state. The model indicates that the steady-state δ¹⁸O value of snail shell carbonate depends upon the temperature, relative humidity, δ¹⁸O of the input liquid water, and δ¹⁸O of ambient water vapor. Model shell δ¹⁸O values were calculated for the warm, wet months corresponding to times of snail activity at some European sites. Linear regression of these predicted values against published, measured values yielded the expression: δ¹⁸O_calc = 0.93(±0.13) δ¹⁸O_meas −0.9(±0.2), with r² = 0.65. As indicated by the value of r², there is scatter in the relationship, but the slope and intercept are close to one and zero, respectively, which lends credence to the model. Therefore, temporal or spatial changes recorded in the δ¹⁸O values of land snail shells appear to be selectively seasonal—commonly the warm, wet months—and include the effects of relative humidity.For carbon, the time constant for approach to isotopic steady state in the bicarbonate dissolved in the body fluid of land snails is predicted to be ∼16 min or less. New and published δ¹³C measurements of aragonite shell and associated organic matter exhibit an overall correlation, but with considerable scatter. As noted by previous workers, ¹³C-rich dietary “limestone” may account for some of the scatter. Additional scatter, according to the model presented herein, could arise from changes in the proportion of total oxidized carbon that is expelled by the snail as bicarbonate dissolved in body fluid (i.e., effects of relative changes in metabolic rates). These results affirm the need for caution in the interpretation of δ¹³C values of land snail aragonite shells solely in terms of dietary proportions of C₃ and C₄ plants.     

Oxygen and carbon stable isotopes of modern land snail shells as environmental indicators from a low-latitude oceanic island

Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a low-latitude oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10-2160 m a.s.l.) in Tenerife Island (∼28°N) of the Canary Archipelago.Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ¹⁸O value of the shell (δ¹⁸O_shell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship observed between temperature and the δ¹⁸O value of rain (δ¹⁸O_rain). Calculated δ¹⁸O values of the snail body water (δ¹⁸O_body) derived from observed temperatures and δ¹⁸O_shell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.)59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ¹⁸O values for local rain water. The calculated δ¹⁸O_body values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the “altitude effect” observed for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ¹⁸O_shell values indicate lower temperatures, lower δ¹⁸O_rain values and possibly, higher rainfall totals. A positive correlation between the mean δ¹³C values of shells (δ¹³C_shell) and plants by site suggests that shells potentially record information about the surrounding vegetation. The δ¹³C_shell values varied between −15.7 and −0.6‰ (VPDB), indicating that snails consumed C₃ and C₄/CAM plants, where more negative δ¹³C_shell values probably reflects the preferential consumption of C₃ plants which are favored under wetter conditions. Individuals with more positive δ¹³C_shell values consumed a larger percentage of C₄ plants (other potential factors such as carbonate ingestion or atmospheric CO₂ contribution were unlikely) that were more common at lower elevations of the hotter and drier south facing transect. The relatively wide range of shell isotope values within a single site requires the analysis of numerous shells for meaningful paleoclimatic studies. Although small differences were observed in isotope composition among snail species collected at a single sampling site, they were not significant, suggesting that isotope signatures extracted from multi-taxa snail data sets may be used to infer environmental conditions over a broad range of habitats.     

C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia)

Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ¹³C) and oxygen (δ¹⁸O) in the carbonate and organic carbon (δ¹³C_org) were determined and compared with δ¹⁸O of water and δ¹³C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC₀]?1) vs. (δ¹³C‐DIC × ([DIC]/[DIC₀])) ( Sayles & Curry, 1988 ). The δ¹³C value of added DIC was ?13·6‰, corresponding to the dissolution of CO₂ with δ¹³C between ?19·5 and ?23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ¹³C and decreasing δ¹⁸O. The lack of covariation between δ¹³C and δ¹⁸O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ¹³C and δ¹⁸O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.     

Lake carbonate-δ18O records from the Yukon Territory,Canada: Little Ice Age moisture variability and patterns

A 1000-yr history of climate change in the central Yukon Territory, Canada, is inferred from sediment composition and isotope geochemistry from small, groundwater fed, Seven Mile Lake. Recent observations of lake-water δ¹⁸O, lake level, river discharge, and climate variations, suggest that changes in regional effective moisture (precipitation minus evaporation) are reflected by the lake’s hydrologic balance. The observations indicate that the lake is currently ¹⁸O-enriched by summer evaporation and that during years of increased precipitation, when groundwater inflow rates to the lake increase, lake-water δ¹⁸O values decrease. Past lake-water δ¹⁸O values are inferred from oxygen isotope ratios of fine-grained sedimentary endogenic carbonate. Variations in carbonate δ¹⁸O, supplemented by those in carbonate and organic δ¹³C, C/N ratios, and organic carbon, carbonate and biogenic silica accumulation rates, document changes in effective moisture at decadal time scales during the early Little Ice Age period to present. Results indicate that between ～AD 1000 and 1600, effective moisture was higher than today. A shift to more arid climate conditions occurred after ～AD 1650. The 19th and 20th centuries have been the driest of the past millennium. Temporal variations correspond with inferred shifts in summer evaporation from Marcella Lake δ¹⁸O, a similarly small, stratified, alkaline lake located ～250 km to the southwest, suggesting that the combined reconstructions accurately document the regional paleoclimate of the east-central interior. Comparison with regional glacial activity suggests differing regional moisture patterns during early and late Little Ice Age advances.     

Oxygen isotope compositions and magmatic epidote from two contrasting metaluminous granitoids,NE Brazil

Oxygen isotope compositions of mineral separates were determined from two metaluminous granitoids (Emas and São Rafael plutons) from northeastern Brazil. The I-type Emas pluton has high δ¹⁸O (WR) values (11.5–11.8‰), whereas the São Rafael pluton has low δ¹⁸O (WR) values (7.5–8.1‰), but Sr and Nd are characteristics of S-type granitoids. Measured mineral–mineral fractionations suggest continuous sub-solidus inter-mineral isotope exchange among all minerals except zircon. There is a large and consistent quartz–epidote fractionation that gives apparent temperatures that are much lower than anticipated closure temperatures for epidote. Oxygen isotope fractionation between natural zircon and magmatic epidote is opposite to that predicted from theoretical determinations, as δ¹⁸O (epidote) <δ¹⁸O (zircon). An empirical calibration based on these results would suggests a closure T for oxygen in epidote of ~500 °C and Δ(qtz–epi) ~5.19 at 500 °C.     

环境因素对蜗牛Achatina fulica壳体碳酸盐δ13C 组成影响的实验研究

陆生蜗牛化石稳定同位素组成是一种良好的古环境信息载体,常被用于古环境古气候的重建。由于缺少古环境因子 对蜗牛壳体稳定同位素组成机理性的结论,因此越来越多的研究侧重于对现代蜗牛壳体的环境效应进行探讨。实验室蜗牛 饲养实验则可以通过控制环境因素来确定其对壳体碳酸盐稳定同位素组成的影响程度,从而得到较为确切的结果。该文在 前人的研究基础上,利用Achatina fulica 进行实验室饲养实验。结果表明,在相同的温、湿度下,同种食物喂养的蜗牛壳体 有非常稳定的分馏值,不同食物类型的结果有一定的差异,莴苣叶、玉米粉和饲料喂养的蜗牛壳体相对于食物的分馏值分 别为16.70‰±0.2‰,10.57‰±0.2‰和10.65‰±0.2‰;在20~30℃实验条件下,壳体δ13C并不受环境温度的影响,主要受食 物的影响,并得到两者之间的回归方程为δ13Cs=0.6665δ13Cv+6.2302（n=26）;无机碳酸盐对Achatina fulica 壳体文石δ13C值影 响很少;根据端元组分分析方法的统计结果显示,食物是Achatina fulica 壳体碳同位素组成的主要影响因素,贡献值约为 80％±5％,除此以外,大气CO2的贡献值约为20％±5％。     

An alternative model for positive shifts in shallow-marine carbonate δ13C and δ18O

Abstract Positive shifts in global seawater δ¹³C_DIC are related to changes in the ratio of organic relative to inorganic carbon burial in oceanic basins, whereas factors such as climatic cooling and the accumulation of polar ice are known to cause positive shifts in δ¹⁸O. Here, an alternative model is proposed for the formation of local positive isotope shifts in shallow-marine settings. The model involves geochemically altered platform-top water masses and the effects of early meteoric diagenesis on carbonate isotopic composition. Both mechanisms are active on modern (sub)tropical carbonate platforms and result in low carbonate δ¹³C and δ¹⁸O relative to typical oceanic values. During high-amplitude transgressive events, the impact of isotopically light meteoric fluids on the carbonate geochemistry is much reduced, and ¹³C-depleted platform-top water mixes with open oceanic water masses having higher isotope values. Both factors are recorded as a transient increase in carbonate ¹³C and ¹⁸O relative to low background values. These processes must be taken into consideration when interpreting the geochemical record of ancient epeiric seas.     

The geochemistry of carbonate diagenesis: The past,present and future

Stable carbon and oxygen isotopes (δ¹⁸O and δ¹³C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ¹⁸O values of carbonate rocks are dependent both upon the temperature and the δ¹⁸O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ¹⁸O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well‐known tendency for δ¹⁸O values of carbonate rocks to decrease with increasing age. Unlike the δ¹⁸O, δ¹³C values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time. However, many studies have not sufficiently emphasized the effects of diagenesis and other post‐depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored. Finally, many additional proxies are poised to contribute to the interpretation of carbonate diagenesis. Although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods, care should be taken to understand both old and new proxies before applying them to the ancient record.     

Isotope geochemistry of caliche developed on basalt

Enormous variations in oxygen and carbon isotopes occur in caliche developed on < 3 Ma basalts in 3 volcanic fields in Arizona, significantly extending the range of δ¹⁸O and δ¹³C observed in terrestrial caliche. Within each volcanic field, δ¹⁸O is broadly co-variant with δ¹³C and increases as δ¹³C increases. The most ¹⁸O and ¹³C enriched samples are for subaerial calcite developed on pinnacles, knobs, and flow lobes that protrude above tephra and soil. The most ¹⁸O and ¹³C depleted samples are for pedogenic carbonate developed in soil atmospheres. The pedogenic caliche has δ¹⁸O fixed by normal precipitation in local meteoric waters at ambient temperatures and has low δ¹³C characteristic of microbial soil CO₂. Subaerial caliche has formed from ¹⁸O-rich evapoconcentrated meteoric waters that dried out on surfaces after local rains. The associated ¹³C enrichment is due either to removal of ¹²C by photosynthesizers in the evaporating drops or to kinetic isotope effects associated with evaporation. Caliche on basalt lava flows thus initially forms with the isotopic signature of evaporation and is subsequently over-layered during burial by calcite carrying the isotopic signature of the soil environment. The large change in carbon isotope composition in subsequent soil calcite defines an isotopic biosignature that should have developed in martian examples if Mars had a “warm, wet” early period and photosynthesizing microbes were present in the early soils. The approach can be similarly applied to terrestrial Precambrian paleocaliche in the search for the earliest record of life on land. Large variations reported for δ¹⁸O of carbonate in Martian meteorite ALH84001 do not necessarily require high temperatures, playa lakes, or flood runoff if the carbonate is an example of altered martian caliche.     

Isotopic investigations of carbonate growth on concrete structures

Stable C and O isotope ratios were measured in carbonate minerals, growing under concrete structures from two locations in the United States. These locations were under a bridge in Michigan and under an overpass in New York. The δ¹³C of the carbonate samples ranged from −21.6 to −31.4‰ (with respect to V-PDB) and clearly indicated precipitation under non-equilibrium conditions. Indeed, the values in some cases were more negative than could be accounted for by existing models that invoke 4 stages of kinetic fractionation. There have been suggestions that microbial activity involving C from gasoline and other fossil fuel sources might be responsible for the relatively low C isotope ratios measured in these carbonates. To explore this possibility, ¹⁴C measurements were made in some of the samples. All samples measured for ¹⁴C contained bomb C. The range of ¹⁴C concentrations suggested a non-uniform growth rate, although possible fossil fuel-derived carbon in the system needs future investigation. The δ¹⁸O values of the carbonates analyzed from Michigan range from 12.5 to 15.7‰ (with respect to V-SMOW), with a mean value of 13.7‰. The δ¹⁸O values of the NY samples range from 11.8 to 15.2‰, with a mean value of 13.1‰. The nearly identical mean values at both locations favors incorporation of O from atmospheric CO₂ in carbonate precipitation. Additionally, the ²¹⁰Pb radiometric technique was also attempted to explore the applicability of this technique in dating concrete derived carbonates as well as recent carbonates forming in a wide variety of environments. The results gave ages between 64 and 3.8 a and are consistent when compared with the date the bridge was constructed.     

Stable isotopes of pedogenic carbonates from the Somma–Vesuvius area,southern Italy,over the past 18 kyr: palaeoclimatic implications

Stable isotopes were measured in the carbonate and organic matter of palaeosols in the Somma–Vesuvius area, southern Italy in order to test whether they are suitable proxy records for climatic and ecological changes in this area during the past 18000 yr. The ages of the soils span from ca. 18 to ca. 3 kyr BP. Surprisingly, the Last Glacial to Holocene climate transition was not accompanied by significant change in δ¹⁸O of pedogenic carbonate. This could be explained by changes in evaporation rate and in isotope fractionation between water and precipitated carbonate with temperature, which counterbalanced the expected change in isotope composition of meteoric water. Because of the rise in temperature and humidity and the progressive increase in tree cover during the Holocene, the Holocene soil carbonates closely reflect the isotopic composition of meteoric water. A cooling of about 2°C after the Avellino eruption (3.8 ka) accounts for a sudden decrease of about 1‰ in δ¹⁸O of pedogenic carbonate recorded after this eruption. The δ¹³C values of organic matter and pedogenic carbonate covary, indicating an effective isotope equilibrium between the organic matter, as the source of CO₂, and the pedogenic carbonate. Carbon isotopes suggest prevailing C₃ vegetation and negligible mixing with volcanogenic or atmospheric CO₂. Copyright © 2000 John Wiley & Sons, Ltd.     

An explanation of the effect of seawater carbonate concentration on foraminiferal oxygen isotopes

Stable oxygen isotope ratios of foraminiferal calcite are widely used in paleoceanography to provide a chronology of temperature changes during ocean history. It was recently demonstrated that the stable oxygen isotope ratios in planktonic foraminifera are affected by changes of the seawater chemistry carbonate system: the δ¹⁸O of the foraminiferal calcite decreases with increasing CO₃²⁻ concentration or pH. This paper provides a simple explanation for seawater chemistry dependent stable oxygen isotope variations in the planktonic foraminifera Orbulina universa which is derived from oxygen isotope partitioning during inorganic precipitation. The oxygen isotope fractionation between water and the dissolved carbonate species S = [H₂CO₃] + [HCO₃⁻] + [CO₃²⁻] decreases with increasing pH. Provided that calcium carbonate is formed from a mixture of the carbonate species in proportion to their relative contribution to S, the oxygen isotopic composition of CaCO₃ also decreases with increasing pH. The slope of shell δ¹⁸O vs. [CO₃²⁻] of Orbulina universa observed in culture experiments is −0.0022‰ (μmol kg⁻¹)⁻¹ (Spero et al., 1997), whereas the slope derived from inorganic precipitation is −0.0024‰ (μmol kg⁻¹)⁻. The theory also provides an explanation of the nonequilibrium fractionation effects in synthetic carbonates described by Kim and O’Neil (1997) which can be understood in terms of equilibrium fractionation at different pH. The results presented here emphasize that the oxygen isotope fractionation between calcium carbonate and water does not only depend on the temperature but also on the pH of the solution from which it is formed.     

Unusual isotopic phenomena in Welsh quarry water and carbonate crusts

Modern terrestrial speleothem-like calcareous deposits in streams draining a disused lime quarry on Black Mountain, South Wales have anomalously negative δ¹⁸O and δ¹³C compositions compared with other similar European deposits. Black Mountain water chemistry is unusual only in its locally very high pH (> 11.5) and carbonate ion concentrations. The high pH is caused by dissolution of lime spoil, resulting in high OH^– concentrations. This high alkalinity causes uptake of atmospheric CO₂ and strong fractionation of both carbon and oxygen stable isotopes, resulting in calcite precipitates with unusually negative isotopic compositions. Since shifts in δ¹⁸O of < 1° are highly significant for Holocene palaeoclimate reconstructions, depletions caused by hyperalkaline waters must be avoided. While extreme lime spoil contamination should be obvious, less heavily affected sites will record smaller fractionation effects and might escape detection. Even small depletions from low-level contamination will have large effects on palaeotemperatures based on carbonate crust δ¹⁸O values.     

Constraints on the application of carbon isotope thermometry in high- to ultrahigh-temperature metamorphic terranes

Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ¹³C and δ¹⁸O values in calcite and heterogeneous single crystal δ¹³C values (? 1 to ? 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ¹³C and δ¹⁸O values in marbles by infiltration of low δ¹³C–δ¹⁸O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO₂‐rich fluid, and that the infiltration had occurred under high temperature and low f_O2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ¹³C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.     

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian platform

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian Platform (eastern Ladoga Klint, Lynna River, and Babino quarry sections) are considered. In the studied section interval, average δ¹³C and δ¹⁸O values are 0 ± 0.5 and ?5 ± 0.5‰ (V-PDB), respectively. Two closely-spaced negative carbon isotope excursions with the amplitude of 2‰ are established near the Lower-Middle Ordovician boundary (between the Floian and Dapingian stages). The lower part of the Darriwilian Stage is marked by the gradual decrease in δ¹³C values to 1‰. Excursions of δ¹³C do not correlate with δ¹⁸O variations and can be considered as primary. The carbon isotope event defined at the Lower-Middle Ordovician boundary is traceable at the interregional level and represents a promising stratigraphic reference level. It may likely be explained by decrease in the relative rate of organic matter burial due to sea level fall and expansion of well-aerated shallow-water basins with a low primary production of phytoplankton.     

CO clumping in speleothems: Observations from natural caves and precipitation experiments

The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water δ¹⁸O. Interpreting speleothem δ¹⁸O records in terms of absolute paleotemperatures and δ¹⁸O values of paleo-precipitation requires quantitative separation of the effects of these two parameters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of Δ₄₇ (a geochemical variable reflecting the statistical overabundance of ¹³C¹⁸O bonds in CO₂ evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Application of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of modern speleothems. The only modern stalagmite reported so far in the literature yielded a lower Δ₄₇ value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation.Here we report Δ₄₇ values measured in natural speleothems from various cave settings, in carbonate produced by cave precipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower Δ₄₇ and heavier δ¹⁸O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of Δ₄₇ disequilibrium and that of δ¹⁸O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium δ¹⁸O and Δ₄₇ values, probably inherited from prior degassing within the cave system.In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evidence that the amount of disequilibrium affecting Δ₄₇ in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of Δ₄₇ in inorganic carbonates.     

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.