Shell δ18O Values Research Articles

Shell sclerochronology and stable oxygen isotope ratios from the limpet Patella depressa Pennant, 1777: Implications for palaeoclimate reconstruction and archaeology in northern Spain

Stable oxygen isotope ratios of mollusc shells (δ18Oshell) offer the possibility to reconstruct coastal resource exploitation patterns and changes in the oceanographic conditions of direct relevance to past human populations. This method relies on the fact that shell carbonate is deposited by molluscs in equilibrium with their surrounding environment and actualistic investigation of modern specimens is needed to ensure that selected species can be used as accurate palaeoclimate indicators. The limpet Patella depressa Pennant, 1777 is one of the most common mollusc species found in Holocene archaeological assemblages along the Atlantic coast of Europe. However, this taxon has not, to date, been tested as a seawater palaeothermometer. Here, we explore the ability of P. depressa to be used as an environmental recorder in this littoral region, specifically in northern Iberia where we obtained live-collected specimens throughout the year. We undertook sclerochronological investigations combining observations of incremental shell growth patterns with δ18Oshell values. Carbonate samples were taken on (i) the ventral margins of shells collected alive year-round in order to test for isotopic equilibrium and (ii) along the axis of maximum shell growth of four modern specimens to decipher the shell growth pattern of this species. Isotopic data showed that calcium carbonate is precipitated in predictable isotopic disequilibrium with the ambient seawater (R2 = 0.95; p < 0.0001), reporting a consistent offset of +1.08‰. Some periods of growth cessation were observed in the shells when thermal tolerances were exceeded in winter and occasionally in summer. Nevertheless, estimated seawater temperatures from modern shell δ18Oshell values agreed closely with instrumentally measured temperatures (R2 = 0.88–0.93; p < 0.0001) and correctly reflected seasonal temperature patterns. As a result, multi-proxy analysis of P. depressa shells can provide a high-resolution palaeothermometer with significant implications for future palaeoclimate and archaeological studies along the Atlantic coast of Europe.

Calibration of shell δ18O from the common whelk Buccinum undatum highlights potential for palaeoenvironmental reconstruction

The common whelk, Buccinum undatum, is a commercially important gastropod found throughout the North Atlantic. One method of age and life history analysis for gastropod species is the use of oxygen isotope ratio (δ18O) measurements from their shells, which is a well-established technique for the reconstruction of historical seawater temperatures at the time of shell biomineralization. Palaeotemperature calibrations have been developed for different types of calcium carbonate as well as species-specific equations to produce the most accurate seawater temperature reconstructions. Here we investigate the four-layer internal structure of B. undatum shells and confirm an aragonite composition using Micro-Raman Spectroscopy (MRS). We then calibrate a species-specific palaeotemperature equation for this gastropod species. This was achieved through the isotopic analysis of shells from laboratory reared specimens of known provenance reared at specific seawater temperatures to produce the following:t(°C) = 14.96 (± 0.15) – 4.94 (± 0.22) x (δ18Oshell – δ18Owater)The calibrated equation differs significantly from previously published data derived from both aragonite and calcite. An offset of 1.04‰ (± 0.41‰) was discovered between observed δ18OShell values and those expected under equilibrium, suggesting a species-specific vital effect. The calibrated equation was used to reconstruct accurate, high resolution historical seawater temperatures from three sites across the U.K. (Shetland, the Menai Strait and Jersey). With this new accurate calibration, both modern and fossil B. undatum shells now have the potential to be employed as high-resolution archives of recent and historical seawater temperature.

Stress in the tropics? Impact of a latitudinal seawater δ18O gradient on Middle Jurassic temperature reconstructions at low latitudes

Abstract Previous stable oxygen isotope records of calcitic fossils (δ18Oshell) from Europe have been interpreted to reflect strong climatic perturbations throughout the Jurassic, but it is unknown whether they reflect global trends because data from other regions are sparse. Here, we use bivalve and brachiopod shells from western Asia and northern Africa to examine seawater temperatures at low latitudes as well as latitudinal temperature gradients in the Middle Jurassic. Our results include the first absolute temperature estimates for most of the study areas and this time interval. Furthermore, we acknowledge a latitudinal gradient in δ18O values of seawater (δ18Osea) that leads to more realistic temperature reconstructions of tropical water temperatures, which were underestimated by earlier methods. Following this approach, δ18Oshell values translate into average equatorial water temperatures during the Middle Jurassic several degrees higher than today (as high as 34–35 °C). Such high temperatures cause environmental stress, probably explaining previously documented low diversities of low-latitudinal ecosystems (e.g., coral reefs) during this time interval. A comparison with literature data raises doubts over whether previously published European temperature curves are truly representative of global patterns. Instead, they might reflect the complex paleogeography of European basins influenced by regional and/or short-term changes in δ18Osea values.

Oxygen isotope disequilibrium in the juvenile portion of oyster shells biases seawater temperature reconstructions

For decades, bivalve shells have constituted one of the most common supports for paleoclimatic archives based on stable isotope approaches. In this work, we conducted chemical marking and recapture techniques to study the fluctuation of δ18O values of oyster shells of the species Magallana gigas reared in natural environment in Normandy (France) for two years. The results were compared to the continuous monitoring of temperature and salinity and monthly records of seawater δ18O. Isotopic measurements were performed on the hinge area that regroups the whole life of the oysters. Here we demonstrate that oysters mineralized their shells with no significant growth breaks during the two-years experiment, even at temperatures below 6 °C. The results confirm that adult oysters (i.e. > 1 yr) mineralized their shells at equilibrium. However, juvenile specimens exhibit a strong isotopic disequilibrium, with a maximum shift of the δ18O values of 3‰ in winter, likely due to kinetic isotope effects. This corresponds to a reconstructed temperature up to 13 °C warmer than expected. This work indicates that although these oyster shells can be used as an accurate archive of (paleo)environmental conditions, the shell portion mineralized during the juvenile stage (i.e. < 1 yr) should be avoided for paleotemperature reconstructions. Given the wide use of bivalve shells as environmental archives, similar studies on others species are required.

Last Interglacial sea surface warming during the sea-level highstand in the Canary Islands: Implications for the Canary Current and the upwelling off African coast

The Canary Islands, east of the North Atlantic Ocean (27°N-29°N), are under the influence of the Canary Current, the descending branch of the North Atlantic Gyre, which is modulated by coastal upwelling off North-West Africa. They constitute strategic sites for palaeoclimatic reconstructions, especially for the Last Interglacial (LIG, 129 to 116 ky BP) estimated to be warmer than present. Seventy-four carbon and oxygen isotope bulk analyses and time series measurements were performed on 32 aragonitic mollusc shells from the LIG marine deposits on Lanzarote and Fuerteventura islands during a period of sea-level highstand that we estimated to occur between ≈125 and 119-116 ky BP. Our SST calculations, inferred from shell δ18O values using available isotopic fractionation equations, provide a seasonal SST amplitude ranging from 3.5 °C to 6.0 °C, in agreement with the modern seasonal amplitude, along with a mean SST comprised between 20.4 ± 1.3 °C and 22.2 ± 1.2 °C. With respect to the pre-industrial times, we deduce a positive SST anomaly in the range of +1.0 ± 1.4 °C to +2.8 ± 1.3 °C, consistent with the presence of the species Persististrombus latus, typical of warm SSTs. Although this finding does not match with the zonal negative anomaly of a reconstructed SST at low latitudes of the North Atlantic, it is nevertheless corroborated by other climate reconstructions in the northeastern tropical Atlantic region. We attribute this trend to an excess of summer insolation during the LIG which warmed the Canary Current, enhanced by a weakening of the North African upwelling. The entire North Atlantic Gyre was probably warmer during the LIG.

Annual shell growth patterns of three venerid bivalve mollusk species in the subtropical northwestern Pacific as revealed by sclerochronological and stable oxygen isotope analyses

Annual shell growth patterns of the three venerid bivalve species, Gafrarium pectinatum, Pitar citrinus, and Katelysia japonica were investigated based on the results of sclerochronological and stable oxygen isotope analyses of live-caught specimens from the intertidal zone of Iriomote Island, southern Ryukyu Archipelago. In the study area, these three species temporally stopped shell deposition, when sea surface temperature (SST) dropped to 23–26 °C, during the first three years. However, the shutdown temperature for shell growth increased slightly to higher than 26 °C after 6 years old for G. pectinatum combined with a shortening in the length of shell growing period. Seasonal changes in daily shell growth in these species were controlled mainly by SST and primary production. Shell δ18O-derived summer temperatures recorded in the annual increments were higher by 3–5 °C than the highest SST records of the habitat. This data mismatch might be caused by an abrupt decrease in seawater δ18O values during the summer and fall typhoon seasons because of the influx of fresh water into the study area from nearby rivers. This study suggests that in the study area the annual shell growth patterns and shell δ18O values in the three species examined were controlled by mutually related biological and environmental factors such as ontogenetic age and seasonal changes in SST, salinity and primary production.

High-resolution records of growth temperature and life history of two Nacella limpet species, Tierra del Fuego, Argentina

Stable isotope ratios in patelloid limpets of the genus Patella have been established as proxies for coastal environmental change at sub-monthly resolution along the eastern North Atlantic and Mediterranean Sea. Nacella deaurata (Gmelin 1791) and N. magellanica (Gmelin 1791) are common intertidal species of patelloid limpets inhabiting the coast of Tierra del Fuego, Argentina/Chile and are commonly found in Holocene archaeological deposits. Here, we examine oxygen and carbon isotope ratios (δ18Oshell and δ13Cshell, respectively) of modern specimens of N. deaurata and N. magellanica to test the hypotheses that: 1) they form their shells in isotopic equilibrium with ambient water; and 2) prominent growth lines form annually. Based on growth margin analysis of δ18Oshell values, we identified a positive offset of 1.3 ± 0.4‰ (N. deaurata) and 1.3 ± 0.3‰ (N. magellanica) from expected equilibrium, similar to other patelloid limpets. Because the offset is relatively consistent between observed and expected values, it can be taken into account to reliably reconstruct growth temperature. Seawater temperature estimated from oxygen isotope time series data falls within the observed range. Thus, N. deaurata and N. magellenica shells serve as reliable proxy archives of seasonal variation in coastal seawater temperature. Time series of δ13Cshell values do not vary seasonally in all shells; hence, the influence on its variation requires further study. The timing of prominent growth lines contextualized by the δ18Oshell time series form twice a year and therefore cannot be used to estimate lifespan. Future isotopic analysis of archaeological Nacella shells can potentially provide much needed information about Holocene climate change at sub-monthly resolution from high-latitude South American locations, and contribute to our understanding of human behavior and human-climate interactions.

OXYGEN ISOTOPE PROFILES OF UPPERMOST JURASSIC VERTEBRATE TEETH AND OYSTER SHELLS: A RECORD OF PALEOENVIRONMENTAL CHANGES AND ANIMAL HABITATS

ABSTRACTHigh resolution oxygen isotopes profiles of well-preserved uppermost Jurassic oyster shells and vertebrate (ichthyosaur and fish) teeth from Central Poland were obtained using a microdrill and an ion microprobe (SHRIMP IIe/MC). Internal variability of δ18O values of oyster shells (-1.7 to 1.3‰ VPDB) collected from the offshore interval of the studied section is greater than that determined previously from bulk oysters. Relatively high δ18O values of the studied oysters may be linked to a slight increase in the salinity of the mid-Polish basin. Low δ18O values of co-occurring ichthyosaur tooth enamel substantiate endothermy of these reptiles. An observed ontogenetic increase in the ichthyosaur enamel δ18O values (from ∼ 19.4 to ∼ 21.6‰ VSMOW) may be a result of long distance migrations although the effect of metabolic factors at rising body mass cannot be completely excluded. It is postulated that δ18O values of the latest portion of the ichthyosaur tooth enamel can be used for the calculation of ancient water oxygen isotope composition and verification of oxygen isotope temperatures. High δ18O values of the enameloid of Caturus lungfishes (23.6 to 26.5‰ VSMOW) collected from an upper part of the studied section, which was deposited in a restricted lagoonal environment after a marine regression, are probably a result of a high evaporation rate. The fishes could have breathed atmospheric air and withstood episodes of water hypoxia. Internal oxygen isotope records of the shells and teeth are promising proxies for studies of the paleoenvironment and activity of animals.

Clumped and stable isotopes of land snail shells on the Chinese Loess Plateau and their climatic implications

We report stable (δ13Cshell, δ18Oshell) and clumped isotope (Δ47) compositions of modern and last glacial fossil snail shell carbonates from the Luochuan and Weinan sections on the central and southern Chinese Loess Plateau (CLP). Our study reveals that the average Δ47 temperature (T47) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is ~10 °C higher than that of fossil snails from glacial time. Moreover, the average δ13Cshell of modern snails is more depleted than that of fossils. We argue that the δ13Cshell cannot record changes in plant communities (i.e., the C3/C4 ratio) on the CLP and may mainly indicate arid conditions with depleted values reflecting reduced aridity. Additionally, the reconstructed snail body water δ18O (δ18Owater) of modern snails is more enriched than δ18O in modern growing season precipitation and δ18Owater of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water 18O in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using δ18Oshell to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher δ18Oshell and δ18Owater values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

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

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

A global assessment of freshwater mollusk shell oxygen isotope signatures and their relation to precipitation and stream water

Records of δ18O in stream flow are critical for understanding and modeling hydrological, ecological, biogeochemical and atmospheric processes. However, the number of such records are extremely limited globally and the length of such time series are usually less than a decade. This situation severely handicaps their use in model testing and evaluation. Here we present a global assessment of freshwater mollusk (bivalves & gastropods) isotope data from 25 river basins that have stream water isotope values, water temperature data and shell material isotope signatures. Our data span a latitude range of 37.50°S to 52.06°N. We show that δ18O signatures in freshwater mollusks are able to explain 95% of the variance of stream water δ18O. We use shell δ18O values and water temperature data to reconstruct stream water δ18O signatures. With freshwater mussel life expectancy ranging from a few years up to 200 years, this translation of mollusk metabolic properties into long term stream water isotope records is a promising approach for substantially extending global stream water isotope records in time and space.

Sedimentary, geochemical and hydrological history of Lake Kinneret during the past 28,000 years

The sedimentary, geochemical and hydrological history of Lake Kinneret (Sea of Galilee) is reconstructed for the past ∼28 kyrs, based on three sedimentary cores drilled at the lake and a trench dug at the shore of the prehistorical Ohalo–II site. During the past 28 kyrs either laminated or massive fine-grained sediments were deposited in the lake comprising primary calcites and fine-grain detritus. Sr/Ca, Mg/Ca, 87Sr/86Sr ratios and δ18O values of live and fossil ostracod shells (Cyprideis torosa) and primary calcites together with XRD, grain-size, and carbonate content analyses indicate contribution of the following types of waters to the lake: (1) Jordan River; (2) Regional runoff; and (3) Ca-chloride brines (currently comprises the Tiberias Spa brine). During the last glacial period (∼28-24 ka) the lake rose to its highest stand of ∼170 m below sea level (bsl), expanding over the Kinnarot Basin and converging with the southern hypersaline Lake Lisan. At that time, waters were mainly supplied to the lake by the Jordan River and regional runoff with enhanced contribution of the Dead Sea Ca-chloride brine. Primary calcites were precipitated from the lake's solution forming sequences of laminated sediments on the lake's floor. At ∼24-22 ka (coinciding with Heinrich event H2 at the North Atlantic) the lake retreated below the modern level (of ∼214 m bsl), depositing mainly flood-related sediments at its margins. The lake slightly rose during the Younger Dryas and subsequently declined towards the modern level with decreasing contributions of the Jordan River waters, brines and regional runoff, reflecting a continuous aridification of the region during the Holocene.

Holocene cultural and climate shifts in NW Africa as inferred from stable isotopes of archeological land snail shells

Cultural transitions or even societal collapses have often been associated with long-term drought events. Linkages between humans and the environment are best documented in well-constrained archeological records that offer multi-centennial to multi-millennial cultural and paleoclimate data. Two Holocene Capsian sites, Kef Zoura D and Aïn Misteheyia, from NE Algeria document a marked change in subsistence strategies near 8200 cal. yr BP. Radiocarbon dated archeological shells (from 10,300 to 6700 cal. yr BP) of the terrestrial gastropod Helix melanostoma were studied to examine the role of climate change on cultural shifts. Oxygen (δ18O) and carbon (δ13C) stable isotopes were measured from whole shells and time-series profiles along shell ontogeny to assess average annual paleoclimate and degree of seasonality, respectively. Shell δ18O values illustrate that conditions were wetter between 10,300 and 9000 cal. yr BP, coinciding with the ‘African Humid Period’, whereas the environment turned drier at 8000–7600 cal. yr BP, immediately after the 8.2-ka climate event, feasibly comparable in magnitude to the drought episode initiated in 1968 in the Sahel and N Africa. A snail evaporative steady-state flux balance-mixing model suggests that snails at around 8000 years ago precipitated shells under notably lower relative humidity conditions than previous wetter scenarios of the earlier Holocene. The well-known 8.2 ka cold event of the Northern Hemisphere was detected as a drought event that seems to have lasted several centuries in Algeria. The δ13C values indicate that snails only consumed and assimilated C3 plants without noticeable shifts. This study points to multi-millennial humidity fluctuations in NE Algeria, which likely affected the economy and subsistence strategies of prehistoric human groups in the area.

Quantification of source contributions to the water budgets of the Ganga (Hooghly) River estuary, India

This study is based on comprehensive data on salinity and δ18O values of the Ganga (Hooghly) River estuary waters sampled during six seasons of contrasting water discharge over two years. In addition, the data for the groundwater collected from areas in proximity to the estuarine transect are also discussed. The δ18O values and salinity depict strong positive correlations (r2: 0.96–0.99) suggestive of efficient mixing in the estuary. The intercepts of the regression lines in the plots of δ18O vs. salinity suggest that the average δ18O values of freshwater were the lowest (−8.1 to −8.4‰) in the post-monsoon periods whereas the highest values (−6.2 to −6.6‰) were recorded in the pre-monsoon periods. The results of mass balance calculations carried out using the δ18O values of the meltwater and non-melt water (groundwater and rainwater) indicate that meltwater accounts for ~10–25% of the freshwater budget of the Hooghly River in the pre-monsoon and monsoon periods during two years of the study. In the plot of δ18O vs. salinity, several samples plot above the theoretical line of conservative mixing of river water and the seawater; indicating that there is internal source(s) of water within the estuary. The contributions of water from three sources (river water, seawater and groundwater) to the estuary were estimated based on the inverse modeling. The model results of higher fractions of seawater at Gangasagar in the monsoon period compared to the post-monsoon period are consistent with the more intense tidal incursion in the summer months that is driven by the prevalent wind circulation and the surface currents in the northern coastal Bay of Bengal. In the upper estuary, the groundwater constitutes ≤10% of the total water flux whereas, in the middle to the lower estuary, the groundwater fractions are typically 20–30%. The observations that the maximum contribution from the meteoric recharge to the Hooghly water flux at Gangasagar (5–16%) is lower than the estimates of the groundwater contributions and that the salinity of shallow groundwater is as high as 4.21‰; together indicate that there is a component of recirculated saline water in the submarine groundwater discharge to the Hooghly River estuary. Our results of salinity-δ18O covariation over several seasons have implications for the reconstruction of paleosalinity in the estuarine and coastal regions using δ18O values of carbonate shells.

Temperature and cyclone frequency in Kimmeridgian Greenhouse period (late Jurassic)

Abstract The relationship between palaeoclimate warming and cyclone frequency is still poorly understood, even though tempestites (i.e. storm deposits) may be abundant in Greenhouse period deposits. The Kimmeridgian marl-limestone alternations of the North Aquitaine Platform include abundant tempestites that formed in shallow to moderate-depth marine environments affected by westward-moving cyclones. High-resolution sedimentological and sequence stratigraphic interpretations, combined with biostratigraphy have allowed us to date these deposits. However, changes in cyclone frequency and temperature have never been determined and compared for this interval. In the present work, the relationship between temperature, cyclone frequency and their controlling factors are addressed for the first time. Oxygen isotope analyses were performed on brachiopod and oyster shells and on bulk rock samples from three sections close to La Rochelle, western France. To define a western European temperature tendency for the entire Kimmeridgian, these δ18Oshell values were supplemented by new oxygen isotope measurements from Kimmeridgian brachiopod shells of the Burgundy and Jura Platforms. These new δ18Oshell values were compared with a selection of published contemporaneous δ18Oshell values in order to specify the Kimmeridgian Greenhouse climatic conditions. The resulting overview shows that the average temperature of shallow seawater did not change through the Kimmeridgian while deep seawater warmed close to the early-late Kimmeridgian boundary (i.e. during the Cymodoce zone of the Sub-boreal realm or time equivalent zone). This global seawater thermal homogenization coincides with the most important increase in cyclone frequency recorded through the Kimmeridgian of western France. At the western European scale, a small decrease in temperature took place during the early Kimmeridgian, which is interpreted as a switch in oceanic circulation, on the basis of new mineralogical and elemental analyses performed on the Kimmeridgian of the North Aquitaine Platform. This switch in oceanic circulation may be due to widespread Kimmeridgian extensional tectonic activity, and to proto-North Atlantic seaway opening in particular. This kind of tectonically-induced regional divergence from the late Jurassic Greenhouse climate conditions probably diminished or even disappeared at the early-late Kimmeridgian boundary, favouring the global thermal homogenization of marine waters and the increasing frequency of cyclones observed in the present work.

Significant seasonal contrast in the Arabian Sea during deglaciation: Evidence from oxygen isotopic analyses of individual planktic foraminifera

The variation of stable isotopes between individual shells of planktic foraminifera of a given species and size may provide short-term seasonal insight into past sea surface temperature (SST) and salinity (SSS). In this context, oxygen isotope analyses of individual Globigerinoides sacculifer were carried out from the Ocean Drilling Program Site 723A in the western Arabian Sea to unravel seasonal changes over the last 22 kyr. δ18O values of single shells of G. sacculifer range from of 0.54–2.09‰ at various discrete depths in the core which spans the last 22 kyr. Maximum inter-shell δ18O variability and relatively higher standard deviation is noticed from 20 to 10 kyr (deglaciation), whereas from 10 kyr onwards (Holocene) inter-shell δ18O variability decreases. Based on statistical inferences, we interpret this result as greater seasonal SST and SSS contrasts during the deglaciation compared to the Holocene. These winter-to-summer contrasts during the deglaciation can likely be explained by a net reduction in summer upwelling linked to changes in wind forcing and net evaporation-minus-precipitation that might, along with local causes, be related to deglacial shifts in the intertropical convergence zone (ITCZ). We note that a common winter-dominated SST signal in the Arabian Sea and in Greenland was pervasive during the last glacial period, strengthening evidence of a North Atlantic link with Indian Ocean SST. Thus, the present study has wider implications in understanding whether the forcing mechanism of tropical monsoon climate lies in high latitudes or in the tropics.

Ligament, hinge, and shell cross-sections of the Atlantic surfclam (Spisula solidissima): Promising marine environmental archives in NE North America.

The Atlantic surfclam (Spisula solidissima) is a commercially important species in North American waters, undergoing biological and ecological shifts. These are attributed, in part, to environmental modifications in its habitat and driven by climate change. Investigation of shell growth patterns, trace elements, and isotopic compositions require an examination of growth lines and increments preserved in biogenic carbonates. However, growth pattern analysis of S. solidissima is challenging due to multiple disturbance lines caused by environmental stress, erosion in umbonal shell regions, and constraints related to sample size and preparation techniques. The present study proposes an alternative method for describing chronology. First, we analyzed growth patterns using growth lines within the shell and hinge. To validate the assumption of annual periodicity of growth line formation, we analyzed the oxygen isotope composition of the outer shell layer of two specimens (46°54'20"N; 56°18'58"W). Maximum δ18Oshell values occurred at the exact same location as internal growth lines in both specimens, confirming that they are formed annually and that growth ceases during winter. Next, we used growth increment width data to build a standardized growth index (SGI) time-series (25-year chronology) for each of the three parts of the shell. Highly significant correlations were found between the three SGI chronologies (p < 0.001; 0.55 < τ < 0.68) of all specimens. Thus, ligament growth lines provide a new method of determining ontogenetic age and growth rate in S. solidissima. In a biogeographic approach, the shell growth performance of S. solidissima in Saint-Pierre and Miquelon was compared to those in other populations along its distribution range in order to place this population in a temporal and regional context.

Food Water Contribution to the Oxygen Isotope Composition of Land Snail Body Water and Its Environmental Implication

AbstractThis is the first study focused on the oxygen isotope composition (δ18O) of laboratory‐cultivated land snails. Land snails were raised under various controlled environmental parameters, permitting an evaluation of the effectiveness of the flux balance model developed by Balakrishnan and Yapp (). The slope of body water δ18O values vs. provided water δ18O values, suggest that food water is an important source (19‐37%) of land snail body water. However, the flux balance model does not account for this water source. We suggest a revised framework to understand the sources and controlling factors of the land snail body fluid, which should be helpful to future studies on paleo‐environment reconstruction using land snail fossils. This investigation also indicates that snails living in non‐optimal conditions (e.g. low temperature) can adapt their biological behaviors to the environment, which will in turn affect the δ18O values of both body fluid and shells, giving rise to δ18O values which deviate from those of snails living at optimal temperatures.

Reevaluation of carbonate concentration and oxygen isotope records from Lake Qinghai, the northeastern Tibetan Plateau

Lake Qinghai is the largest lake on the Tibetan Plateau, and it is also one of the important sites for studying global environmental changes. Over the past 30 years, many studies have used oxygen isotope of authigenic carbonates from the lake as the proxy to infer past environmental and climate changes on the Plateau. However, debate on interpretations of isotopic data and their environmental implications still exist, largely due to the complex arid environment settings and multiple sources/species for carbonate minerals within the lake. In this study, we systematically analyze δ18O values in different-type carbonates collected [i.e. bulk carbonates, ostracode shells, Chara encrustations, and fine-grain (<63 μm) carbonate minerals] from modern lake sediments and surrounding soils, as well as the down-core δ18O values of bulk/fine-grain carbonates since the Last Glacial Maximum. Together with previously published δ18O records from ostracode shells, we try to re-evaluate the controlling factors of variations in lacustrine carbonate δ18O data and to infer environmental changes on the northeastern Tibetan Plateau since the Last Glacial Maximum. Our results show that the lake depth, or the size of the water body, is an important factor to influence the lake water and carbonate δ18O values. A shallow and small lake would be more easily influenced by precipitation δ18O which is characterized by negative values at Lake Qinghai region, while a deep and large lake would be better to reflect environmental changes such as the precipitation-evaporation balance. The “lake volume” effect might be an explanation for the negative carbonate δ18O values during the early Holocene, which was likely caused by an increased influence of negative δ18O values in precipitation and glacial melt water under a small and shallow water body. The δ18O values of ostracode shells and bulk carbonates show similar variations since both of them are dominated by lake water oxygen isotopic composition, but they still have distinct geochemical information. The isotopic differences between ostracode and bulk carbonates probably reflect the temperature differences between the surface and the bottom of lake water. In addition, the δ18O values of evaporative induced carbonates may correlate with carbonate contents, while those of Chara encrustations do not show any correlation with carbonate contents. Our results suggest that special caution would be necessary when using lacustrine δ18O values of authigenic carbonates to infer past hydrological and climate changes in an arid environment.

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.