Isotope Composition Of Shells Research Articles

Controls on the Barium and Strontium Isotopic Records of Water Chemistry Preserved in Freshwater Bivalve Shells.

Biogenic carbonates, including bivalve shells, record past environmental conditions, but their interpretation requires understanding environmental and biological factors that affect trace metal uptake. We examined stable barium (δ138Ba) and radiogenic strontium (87Sr/86Sr) isotope ratios in the aragonite shells of four native freshwater mussel species and two invasive species in five streams and assessed the effects of species identity, growth rate, and river water chemistry on shell isotopic composition. Shells were robust proxies for Sr, accurately reflecting 87Sr/86Sr ratios of river water, regardless of species or growth rate. In contrast, shell δ138Ba values, apart from invasive Corbicula fluminea, departed widely from those of river water and varied according to species and growth rate. Apparent fractionation between river water and the shell (Δ138Bashell-water) reached -0.86‰, the greatest offset observed for carbonate minerals. The shell deposited during slow growth periods was more enriched in lighter Ba isotopes than the rapidly deposited shell; thus, this phenomenon cannot be explained by aragonite precipitation kinetics. Instead, biological ion transport processes linked to growth rate may be largely responsible for Ba isotope variation. Our results provide information necessary to interpret water chemistry records preserved in shells and provide insights into biomineralization processes and bivalve biochemistry.

Stadial‐Interstadial Temperature and Aridity Variations in East Central Europe Preceding the Last Glacial Maximum

AbstractLast glacial North Atlantic climate is characterized by abrupt, centennial‐millennial scale climate oscillations, called Dansgaard‐Oeschger (D‐O) events. Understanding the cause and propagation of these D‐O events into Eurasia is hampered by the scarcity of quantitative paleotemperature estimates from continental archives with precise, independent age models. Here, we present land snail shell carbonate clumped isotope‐based active season paleotemperature estimates and δ13C/δ18O‐based aridity reconstructions from Greenland stadial/interstadials (GS/GI) between 31 and 26.5 ka from the 14C‐dated Dunaszekcső loess section (Hungary). This reconstruction is complemented with a new 230Th‐dated flowstone stable isotope record covering 30‐26 ka. Our snail shell clumped isotope (Δ47) data indicate growing season temperatures (GSTs) of 16°C–18°C and 7°C–14°C for the investigated interstadials and stadials, respectively. Stable carbon and oxygen stable isotope compositions of shells and flowstone calcite reveal milder interstadials with drier summers and more available moisture over the winter season, and colder stadials with annually/seasonally (winter) drier conditions, promoting increased loess/dust deposition. We propose that large‐scale ocean‐atmospheric variability, characterized by NAO phases, may have imparted a major control on transmitting abrupt North Atlantic climate event signals into continental Europe during the last glacial.

Geochemistry of modern shells of the gastropod Radix in the Tibetan Plateau and its implications for palaeoenvironmental reconstruction

The widely distributed shells of the gastropod Radix are a potential high resolution archive of environmental information. However, previous studies of Radix have used the shells of unidentified species, and the implications of inter- and intra-specific differences in the geochemical attributes of the shells for paleoenvironmental reconstruction are unclear. Here we report the results of a modern process study and species level classification of Radix based on measurements of the trace element and stable isotope composition of the shells, together with an investigation of the occurrence of living Radix in lakes, wetlands and rivers in the Tibetan Plateau. The results indicate that vital effects on the trace element and stable isotope composition of the modern aragonite shells of the four most common species (R. auricularia, R. cucunorica, R. lagotis and R. acuminata Lamark) are not big, especially for the two dominant species (R. auricularia and R. cucunorica). Intraspecies differences in both the major elemental ratios and the stable isotopic values can be ignored in the larger shells of R. auricularia (length >14 mm). For different species of Radix of similar sizes, the values of Sr/Cashell (＜0.0003) and Mg/Cashell (＜0.0002) are similar in all four common species. By evaluating the importance of vital effects on quantitative paleohydrochemical constructions, we demonstrate that Sr/Cashell of Radix is a valuable tool for reconstructing the Sr/Ca ratio and conductivity of the paleo-lake water. However, the Mg/Cashell ratios of Radix have no clear environmental implications, possibly because of significant vital effects. Intraspecies differences of <0.4‰ and interspecies difference of <0.8‰ for δ18Oshell are observed amongst the four species; in addition, the intraspecies (<0.6‰) and interspecies variations of δ13Cshell of Radix (<1.7‰) are larger than those of δ18Oshell. δ13Cshell of the identified Radix mainly reflects the δ13CDIC of the ambient water and is probably limited by DIC, which is of inorganic origin and is seldom limited by DIC derived from the decomposition of organic matter. By evaluating the effects of different transfer functions of living Radix on paleohydrochemical reconstructions (Sr/Capalaeo-water, ECpalaeo-water, δ18Opalaeo-water and δ13Cpalaeo-DIC), we show that the results obtained from transfer functions using R. auricularia and R. cucunorica are very similar, and we recommend the use of transfer functions based on living Radix which are of the same species as the fossil shells. Finally, we show that δ18Oshell of both identified and unidentified Radix can be used to determine lake hydrological status (open or closed).

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.

Raman spectroscopy as a tool for magnesium estimation in Mg‐calcite

Despite their strong Raman scattering and importance in several applications in the geological and biological sciences, Mg‐calcites have not been thoroughly investigated by Raman spectroscopy. In this study, we investigated whether Raman spectra of carbonates are sensitive to the structural and chemical changes occurring when Mg2+ substitutes Ca2+ in the calcite lattice. Different carbonate samples with variable Mg content (from 0 to 20 mol% MgCO3) of biological and inorganic origin have been first beamed under a Raman spectrometer and then analysed by scanning electron microscopy‐energy dispersive spectrometer and electron microprobe analysis‐wavelength dispersive spectrometer microprobes to determine their chemistry. The biological samples (foraminifers, algae, barnacles and echinoderms) were collected from areas situated at different latitudes and water temperature and saturation, factors affecting the chemical and isotopic composition of shells in marine organisms. The Raman band positions are directly linked to the amount of magnesium present in the calcite lattice, and all peaks of Mg‐calcite spectra show a consistent and linear increase in Raman shifts according to their Mg content, which is a consequence of the decreased inter‐atomic distances following the substitution of Ca2+ with the smaller Mg2+ ion.This study demonstrates that Raman spectroscopy provides an innovative work perspective in marine biology, provenance studies and oil exploration when estimates of mineralogical and chemical changes are the focus of the study. Raman spectroscopy is even more attractive – for the mentioned fields of research – than many other methods because of the non‐destructive nature and its very short analytical time. Copyright © 2017 John Wiley &amp; Sons, Ltd.

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.

The ormer (Haliotis tuberculata): A new, promising paleoclimatic tool

Abstract This study aimed to investigate the environmental controls on the oxygen isotope composition of shells of the European abalone, Haliotis tuberculata . Seasonal δ 18 O profiles from the outer prismatic layer of four abalone shells, collected live in northwest Brittany (France) in 2002 and 2012, were compared to local temperatures and salinities. According to the findings herein, δ 18 O variations in abalone shells corresponded to seasonal variations, and thus, shell composition represented a reliable tool for aging and growth studies. Seawater temperatures estimated from the abalone collected in 2012 reflected the in situ measured temperatures, but the reconstructed temperatures from shells of the three specimens collected in 2002 deviated from measured temperatures by 2.5 °C. This overestimation of temperatures corresponded to a “kinetic effect” related to very high annual abalone growth rates; thus, it could be corrected by applying + 0.53‰ to the δ 18 O shell . This methodology was then applied to a fossil (6000 cal yr BP) collected in the Bay of Biscay. Given the worldwide distribution of both live and fossilized abalones, the results of the present study showed that this genus represents a promising paleoclimatic tool.

Exceptional High Contribution of Metabolic Effects to Stable Carbon Isotope Composition of Experimental Cultured Freshwater Bivalve Shell

The stable isotope compositions of bivalve shells have been widely used to reconstruct palaeoclimate, but the metabolic effect obscured the climatic and environmental significance of the carbon isotope composition. In order to assess the contribution of metabolic C to the shell and ascertain whether its variations have any environmental or physiological significance, Corbicula fluminea (Müller, 1774) was bred in the laboratory last for one year; two shell samples (shell A and B) were selected at the end of the experiment to determine the carbon isotope composition of the shell increment that deposited during the experiment. The results indicated that ڄ13CA ranged from 4.76 to 2.09 and ڄ13CB from 8.49 to 2.89 . ڄ13C of shell A and B are both more negative than the predicted equilibrium value, indicated that the shells had used the metabolic carbon during the shell forming process. According to computation, the proportion of shell A was 32.0% to 52.0% (mean value 38.7%); and shell B was 42.7% to 92.1% (mean value 70.3%). Such exceptional high proportion of metabolic carbon to cultured C. fluminea shells quite different from the field C. fluminea shells and other previous studies results, and may suggest that the improper bred food that indicate by the slowly growth rate of species caused the bivalve abnormal biological behaviors, which induce complicated and exceptional metabolic effect. Therefore, it can be used as indicator of deviant food change events during bivalve growth.

Stable isotope compositions of recent Dreissena polymorpha (Pallas) shells: paleoenvironmental implications

Stable carbon and oxygen isotope studies are among the major proxies in investigations of recent and ancient freshwater environments. Mollusc shells are one the most frequently studied carbonates. In the present paper, stable carbon and oxygen isotope compositions of Dreissena polymorpha (Pallas) shells, regarded as the most aggressive freshwater invader worldwide, is compared with the stable isotope composition of ambient water. Macrophytes with modern D. polymorpha shells attached were sampled twice, in June and August 2011, from four transects established within the littoral of Lake Lednica (western Poland). The macrophytes were sampled between 0.5 and 7 m of depth at each site from the restricted area of the lake bottom. In order to avoid the influence of ontogeny on the results obtained the stable isotope compositions of shells of equal or nearly equal sizes within one population were compared. A significant spread was observed in the stable isotope signatures in the D. polymorpha shells, particularly in the δ13C values derived from one population. The spread in δ13C and δ18O values was observed in both juvenile and adult shells; however, it increased with age. It is suggested that stable isotope investigations of D. polymorpha shells should not be performed on single shells, as the isotope values will not be representative of the coeval individuals within population. While the shells of D. polymorpha were close to oxygen isotope equilibrium with the ambient water, they were characterised by a 1.5–2 ‰ depletion in 13C relative to δ13CDIC. Both the spread in δ13C values in the shells and the 13C depletion observed in the shells are interpreted as resulting from a strong metabolic influence on shell composition. Because the offset observed between dissolved inorganic carbon (DIC) and shells is relatively constant, the stable carbon isotope composition of D. polymorpha shells may reflect environmental conditions and thus may be used as a palaeolimnological proxy.

The Sr isotope composition of the world ocean, marginal and inland seas: Implications for the Sr isotope stratigraphy

We studied the Sr isotope composition of shells of modern shallow-water mollusks and coral fragments. Twenty five of the studied samples were collected in beach zones of open oceans and marginal seas; twelve and eight additional samples are from saline and freshened inland seas respectively. The 87Sr/86Sr ratio in samples from the Atlantic, Indian, and Pacific oceans and their marginal seas corresponds on average to 0.709202 ± 0.000003 and coincides with the average ratio in the standard USGS EN-1 sample. The average 87Sr/86Sr ratio in inner parts of evaporite subbasins of the Mediterranean and Red seas is identical to that of the oceanic water. In shells of shallow-water mollusks from the Black Sea and Sea of Azov, where the degree of seawater dilution by riverine runoff is as high as 50 to 70%, the 87Sr/86Sr ratio is lower than that in the oceans by only a value of 0.00002 on average. As oceanic waters penetrated into these freshwater basins no earlier than in the Holocene, we conclude that the Sr isotopic equilibration with the oceanic water is realized very rapidly in the epicontinental seas even under conditions of restricted water exchange with the World Ocean. The established uniformity of the Sr isotope composition in all geographic types of currently existing sea basins open to the World Ocean proves the efficiency of the Sr isotope stratigraphy in correlation of contemporaneous chemogenic sediments.

Positive precipitation–evaporation budget from AD 460 to 1090 in the Saloum Delta (Senegal) indicated by mollusk oxygen isotopes

There is a critical need to document the long-term variability of the West African Monsoon (WAM) in the Sahel region. We present here a multidecadal proxy record of the past hydrology from AD 460 to 1090 in the Saloum Delta, Senegal. The Saloum Delta is a hypersaline estuary where the salinity and the water isotopic composition are highly sensitive to rainfall variations. The past hydrology was studied using the oxygen isotopic ratio of Anadara senilis fossil shells, since mollusk shell isotopic composition (δ18O) in this environment is primarily determined by the precipitation–evaporation budget. Successive samples of shells were taken along the stratigraphy of the massive Dioron Boumak fossil shell middens for new insights into the past WAM multi-decadal to centennial variability. The averaged δ18O value of fossil shells was more negative by 1.4‰ compared to modern shells' isotopic signature. This result indicates substantially fresher mean conditions in the Saloum Delta, that was likely not hypersaline as it is today. The precipitation–evaporation budget was thus more positive in response to a more intense and/or longer monsoon season during the studied period. Our record suggests that strong multidecadal droughts as observed in the Sahel in the late 20th century did likely not occur in Senegal during this ~600-yr time period.

Specific pathways for the incorporation of dissolved barium and molybdenum into the bivalve shell: An isotopic tracer approach in the juvenile Great Scallop (Pecten maximus)

Dissolved barium and molybdenum incorporation in the calcite shell was investigated in the Great Scallop Pecten maximus. Sixty six individuals were exposed for 16 days to two successive dissolved Ba and Mo concentrations accurately differentiated by two different isotopic enrichments (97Mo, 95Mo; 135Ba, 137Ba). Soft tissue and shell isotopic composition were determined respectively by quantitative ICP-MS (Inductively Coupled Plasma Mass Spectrometer) and laser ablation – ICP-MS. Results from Ba enrichment indicate the direct incorporation of dissolved Ba into the shell in proportion to the levels in the water in which they grew with a 6–8 day delay. The low spike contributions and the low partition coefficient (DMo = 0.0049 ± 0.0013), show that neither the soft tissue nor the shell were significantly sensitive to Mo enrichment. These results eliminate direct Mo shell enrichment by the dissolved phase, and favour a trophic uptake that will be investigated using the successive isotopic enrichment approach developed in this study.

An isotope mass balance model for the correlation of freshwater bivalve shell (Unio pictorum) carbonate δ18O to climatic conditions and water δ18O in Lake Balaton (Hungary)

The oxygen isotope composition of bivalve shells (δ18Oshell) can potentially record environmental variability of shallow lakes and therefore it has been extensively used as a proxy in the reconstruction of past climate and environmental conditions. As δ18Oshell reflects - besides the water temperature - the oxygen isotope composition of lake water (δ18OL), it is required to interpret the quality and impact of parameters influencing the δ18OL. Using the isotope mass balance model, I tested the hypothesis that Balaton lake water δ18O variability can be described as a result of the combined effects of three main climatic parameters such as river runoff, precipitation and evaporation. I calculated δ18OL time series for the period 1999-2008 for the whole water body at Siófok (eastern part of Lake Balaton, Hungary) based on measured precipitation, inflow and evaporation amount and measured inflow, precipitation δ18O and calculated vapour δ18O data. The comparison of the modelled δ18OL time series to measured surface δ18OL data revealed that δ18O of Balaton water is sensitive for variation of climatic parameters. This variability is most striking at the surface, while according to the results of the model, the whole water body itself is less sensitive. Monthly differences suggest that generally during summer the whole water body is mixed up, while moderate isotope stratification (0.3-0.7‰ difference between surface and whole water body) can be assumed in early spring and autumn. Predictions of shell δ18O values were made using the measured surface water δ18O data and the modelled δ18O values for the whole water body. High-resolution sampling was conducted on two Unio pictorum shells covering the period of 2001-2008, and both predictions were compared to measured shell δ18O records. The results showed that the prediction for the whole water body gives a better fit to the measured shell δ18O, suggesting that the whole water body better describes the isotope variability of shell carbonate. As a practical application of the isotope mass balance model was used to present the effect of precipitation and evaporation on δ18Oshell. The relationship between intra-shell δ18O-variability and precipitation amount (precipitation/evaporation ratio) were determined, which allows the quantified prediction the impact of meteorological parameters affecting the oxygen isotope composition of shell carbonate.

Integrated Archaeozoological Research of Shell Middens: New Insights Into Hunter-Gatherer-Fisher Coastal Exploitation in Tierra Del Fuego

ABSTRACT Shells of Nacella magellanica are abundant in archaeological shell middens distributed along the Beagle Channel. In this study we analyzed the oxygen isotopic composition of shells of this species to obtain information on its seasonal exploitation patterns by hunter-gatherer-fisher groups in this region during the nineteenth century. Oxygen isotopic data from shells recovered from Lanashuaia and from some occupations of Túnel VII (both contact sites) indicate that N. magellanica was gathered in different seasons. Our data agree with fauna-derived seasonal reconstructions, and collectively these data provide more detailed economic scenarios. Comprehensive results confirm that in the nineteenth century, Yamana people continuously occupied the coast of the Beagle Channel by means of short-lived campsites.

Radiocarbon age anomalies of land snail shells in the Chinese Loess Plateau

In the Chinese Loess Plateau, land snail shells are often the only material available for dating in paleoenvironmental and archaeological research. However, the geochronological suitability of land snail shells is limited because of poor knowledge about their deposition dynamics, particularly with regards to the incorporation of inorganic carbonate and the resulting age anomalies. To evaluate the factors controlling these age anomalies, radiocarbon and stable carbon analyses were carried out on surface soils, as well as the shells and organic bodies of different modern snail species from different ecological habitats. The results showed that all specimens were depleted in 14C, indicating the influence of inorganic, radiocarbon-free carbonate on the 14C-activity of the snail shells. The apparent 14C-deficiencies and the resultant age anomalies of both the Cathaica and the Bradybaena snail shells were within close ranges across the Chinese Loess Plateau, indicating that the shells of these species could, after corrections for radiocarbon anomalies, provide reliable age estimates. The apparent 14C-deficiencies were closely associated with the ecological habitats of the snails. The shells of the ground-dwelling Bradybaena had the smallest age offsets (533 ± 150 a), followed grass-dwelling Cathaica (1107 ± 138 a) and Cathaica living on trees (1550 ± 345 a). These results suggest that the availability of calcium in the respective ecological habitats is an important factor in explaining the apparent 14C-deficiencies. The influence of carbonate on the stable carbon isotope composition of shells is overwhelmed by the organic diets of snails, making δ 13C unsuitable for identifying and correcting shell age anomalies. The radiocarbon activities of surface soils ( A calc) increase with weathering intensity. Thus, a significant uncertainty could be caused by assuming that A calc is zero when estimating the proportions of different carbon sources in shells, as has been the case in most previous studies.

C and O isotope compositions of modern fresh-water mollusc shells and river waters from the Himalaya and Ganga plain

The aim of this paper is to unfold the relationship between the O and C isotope compositions of modern fresh-water mollusc shells and water in order to refine the basis of interpretation for paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases. The O isotopic compositions of river waters in the Himalaya and Ganga plain have a large range, from − 18‰ in the north of the high range up to − 8‰ to − 4‰ in the Ganga plain. δ 18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for the Main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between − 15‰ and − 5‰. Average δ 18O Ara values are consistent with precipitation at equilibrium with host waters at a temperature range of 20–25 °C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along the Main Himalayan rivers have δ 18O values uniformly distributed within − 11‰ and − 6‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of δ 18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in 18O due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0‰ to − 10‰ reflecting input of soil derived alkalinity and plant productivity in the river. δ 13C of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell δ 13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year.

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 δ 18O and δ 13C 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 δ 18O and lower for δ 13C 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.

Stable isotope fractionation between mollusc shells and marine waters from Martinique Island

Forty-nine aragonitic and calcitic shells from 14 species of marine tropical molluscs (Bivalvia, Gastropoda, Polyplacophora) and ambient waters from Martinique have been analyzed for their carbon and oxygen isotope compositions. Mineralogy of shells was systematically determined by Raman spectroscopy that reveals composite shell structures and early processes of diagenetic alteration. In mangrove, brackish waters result from the mixing between 89±1% of seawater and 11±1% of freshwater, a hydrological budget quantified by both oxygen isotope and salinity mass balance calculations. Mollusc shells from the mangrove environment ( S=31‰; δ 18O=0.5‰) are characterized by mean δ 13C values (−1.2‰) lower than those (+2.6‰) living in the open sea ( S=35‰; δ 18O=1‰). These low carbon isotope compositions result from the oxidation of organic matter into bicarbonate ions used in the building of mollusc shells. The oxygen isotope compositions of the studied mollusc species are mainly controlled by the temperature and composition of seawater whereas the role of the so-called “vital effects” is negligible. Contrasting with carbon isotopes, variability in the δ 18O values among and within species of mollusc shells is very low (1 σ=0.15) for a given littoral environment. Using ambient temperatures of seawater (28–30 °C), oxygen isotope fractionations between all studied living species and environmental waters match those extrapolated from the fractionation equation established for molluscs by Grossman and Ku [Chem. Geol., Isot. Geosci. Sect. 59 (1986) 59] in the range 3–20 °C. By analyzing calcite and aragonite layers from the same shell or by comparing shells from different species living in the same environment, there is no evidence that oxygen isotope fractionation between aragonite and water differs from that between calcite and water. On the basis of these results, we conclude that the oxygen isotope compositions of shells from most fossil mollusc species are suitable to estimate past seawater temperatures at any paleolatitude.

Stable isotope composition of Littoridina australis from the coast of Buenos Aires province, Argentina, during Holocene climatic fluctuations

Stable isotope (carbon and oxygen) analyses were performed on Littoridina australis shells collected from molluscan concentrations within Holocene littoral deposits along the Bonaerensian coastal area of Argentina (south-western Atlantic). Isotope data allow us to define two very different areas: the Samborombon Bay, where isotope composition of shells was mainly governed by mixing between marine and freshwater, and the Mar Chiquita lagoon, where the original brackish environment was dominated by evaporation of water that originated high isotope shell values. In both areas some isotope profiles show short and quite large oscillations in δ 18O. Their origin may be tentatively explained as due to the changes in moisture regime that control freshwater supply. The results suggest that these deposits can represent natural archives potentially useful for palaeoclimate reconstruction.

Deep-dwelling planktonic foraminifera of the northeastern Pacific Ocean reveal environmental control of oxygen and carbon isotopic disequilibria

We assess the utility of four species of living planktonic foraminifera as tracers of thermocline and intermediate water masses in the northern Pacific Ocean, based on their water-column distribution and shell isotopic composition. Assuming oxygen isotopic equilibria with the water column, we infer apparent calcification depths. This allows an estimate of apparent carbon isotopic disequilibria. We then relax the assumption of oxygen isotopic equilibrium to examine habitat effects on kinetic disequilibrium processes. In the California Current, left-coiling Neogloboquadrina pachyderma and Globigerina calida calcify in the upper thermocline. Globorotalia scitula resides within the Shallow Salinity Minumum (potential density σ θ = 25.1–26.7), while Globoquadrina hexagona is associated with the deeper, North Pacific Intermediate Water (σ θ = 26.7–26.9). Apparent carbon isotopic disequilibria corrected for oxygen isotopic disequilibria, range from 1.0–1.9‰ in these asymbiotic species. The carbon isotopic disequilibrium can be modeled as a single exponential function of temperature or a logarithmic function of potential food supply. We infer that carbon isotopic disequilibrium increases with metabolic rate, related to temperature and/or food supply. Kinetic processes of oxygen and carbon isotopic disequilibria yields reasonable depth habitats if the slope of the oxygen:carbon isotopic shift is about 0.35, consistent with culture data. Our finding of a link between environment, metabolism, and isotopic disequilibrium observed in oceanic settings suggests the potential to better reconstruct the structure and biological processes of the upper water column from geologic data.