18O Enrichment Research Articles

Simultaneous Visualization of Enzymatic Activity in the Cytoplasm and at Polyphosphate Inclusions in Beggiatoa sp. Strain 35Flor Incubated with 18O-Labeled Water.

Here we report on a new nanoscale secondary ion mass spectrometry (nanoSIMS) approach based on enzyme-mediated oxygen isotope exchange, which combines the visualization of general metabolic activity in the cytoplasm with insights into the activity of enzymes related to polyphosphate (polyP) inclusions. The polyP-accumulating strain of the large sulfur bacterium Beggiatoa was used as a model organism. Beggiatoa cultures were grown under oxic and anoxic conditions when exposed to either low- or high-sulfide conditions, which are known to influence polyP metabolism in this strain. Subsequent incubation with 18O-labeled water led to high 18O enrichments above the natural background in the cytoplasm and polyP granules derived from enzymatically mediated oxygen isotope exchange. The relative importance of polyP under the different sulfide regimes became evident by an apparent continued metabolic activity at polyP inclusions under stressfully high sulfide concentrations, in contrast to a decreased general metabolic activity in the cytoplasm. This finding confirms the role of polyP as a critical component in bacterial stress response and maintenance of a survival metabolism.IMPORTANCE Microbial organisms exert a large influence on the environment as they directly affect the turnover of essential elements. This is particularly true for polyphosphate-accumulating large sulfur bacteria, which can either accumulate phosphate as polyphosphate or degrade it and release phosphate into the environment, depending on environmental conditions. This study presents a new approach to simultaneously visualize general metabolic activity and enzymatic activity at polyphosphate granules by incubation with 18O-labeled water as the only stable isotope tracer. For this purpose, the well-studied Beggiatoa sp. strain 35Flor was used as a model organism and was exposed to different stress regimes. General metabolic activity was strongly impaired during high-stress regimes. In contrast, intense intracellular polyP cycling was not restricted to favorable or stressful conditions, highlighting the importance of polyP for general cell physiology, especially during hostile conditions. The nanoSIMS approach adds a new tool to study microorganisms involved in phosphorus cycling in the environment together with the identification of general metabolic activity.

Recycling of 18O enriched water used in 18F cyclotron production

A simple, effective, easy-handling and reliable method for recycling of [18O]H2O from fluorine-18 production, was developed based on co-precipitation, co-crystallization and distillation. Preliminary experiments with normal H2O were used to determine the optimal purification conditions, which were tested for purification of used [18O]H2O from a scientific and commercial 18F-production facility. The obtained recycled [18O]H2O had comparable quality to commercially available 18O enriched water. The loss of weight for enriched water was about only 7% and losses of 18O enrichment about 1.3% ± 0.2%. The method ensures elimination of radioisotopes (99.7% ± 0.1%), organic compounds (99.5% ± 0.1%) and trace metals (99.8% ± 0.1%) with special attention on radiation safety with dosimetry control checks and in total 1000 times mass reduction of the initial radioactive waste.

Breath water-based doubly labelled water method for the noninvasive determination of CO2 production and energy expenditure in mice

ABSTRACTWe explored a novel doubly labelled water (DLW) method based on breath water (BW-DLW) in mice to determine whole body CO2 production and energy expenditure noninvasively. The BW-DLW method was compared to the DLW based on blood plasma. Mice (n = 11, 43.5 ± 4.6 g body mass (BM)) were administered orally a single bolus of doubly labelled water (1.2 g H218O kg BM−1 and 0.4 g 2H2O kg BM−1, 99 atom% (AP) 18O or 2H). To sample breath water, the mice were placed into a respiration vessel. The exhaled water vapour was condensed in a cold-trap. The isotope enrichments of breath water were compared with plasma samples. The 2H/1H and 18O/16O isotope ratios were measured by means of isotope ratio mass spectrometry. The CO2 production (RCO2) was calculated from the 2H and 18O enrichments in breath water and plasma over 5 days. The isotope enrichments of breath water vs. plasma were correlated (R2 = 0.89 for 2H and 0.95 for 18O) linearly. The RCO2 determined based on breath water and plasma was not different (113.2 ± 12.7 vs. 111.4 ± 11.0 mmol d–1), respectively. In conclusion, the novel BW-DLW method is appropriate to obtain reliable estimates of RCO2 avoiding blood sampling.

Dual QM and MM Approach for Computing Equilibrium Isotope Fractionation Factor of Organic Species in Solution.

A dual QM and MM approach for computing equilibrium isotope effects has been described. In the first partition, the potential energy surface is represented by a combined quantum mechanical and molecular mechanical (QM/MM) method, in which a solute molecule is treated quantum mechanically, and the remaining solvent molecules are approximated classically by molecular mechanics. In the second QM/MM partition, differential nuclear quantum effects responsible for the isotope effect are determined by a statistical mechanical double-averaging formalism, in which the nuclear centroid distribution is sampled classically by Newtonian molecular dynamics and the quantum mechanical spread of quantized particles about the centroid positions is treated using the path integral (PI) method. These partitions allow the potential energy surface to be properly represented such that the solute part is free of nuclear quantum effects for nuclear quantum mechanical simulations, and the double-averaging approach has the advantage of sampling efficiency for solvent configuration and for path integral convergence. Importantly, computational precision is achieved through free energy perturbation (FEP) theory to alchemically mutate one isotope into another. The PI-FEP approach is applied to model systems for the 18O enrichment found in cellulose of trees to determine the isotope enrichment factor of carbonyl compounds in water. The present method may be useful as a general tool for studying isotope fractionation in biological and geochemical systems.

Production of stable isotopes of light elements: past, present and future

Taking into account the world trends in the production and use of stable isotopes of light chemical elements, the main directions of their application and the scale of modern production are considered. Particular attention is paid to stable isotopes, mainly used in clinical medicine and various biological studies. At examples for isotopes of 13C, 18O and 15N are analyzed the change in the production volume of isotope products by years, the technologies used for isotope enrichment are considered and manufacturers in different countries of the world are indicated. Particular attention is paid to the appearance of relatively new industrial processes for the enrichment of 13C and 18O. The dominant role of physicochemical separation methods in the production of light isotopes is noted. In addition, from the technology development point of view, technical proposals for the improvement of physicochemical separation methods aimed at enriching 15N and separation of boron isotopes are considered.

Abrupt climate change at ~2800 yr BP evidenced by a stalagmite record from peninsular India

This study presents an analysis of the Indian summer monsoon (ISM) rainfall variations for a 1460-year period (1720–3180, Before Present BP: 1950 AD), based on a long record of stable isotopic variations (δ18O) with high temporal resolution (~annual) obtained from a U-Th dated stalagmite from the Kadapa cave in peninsular India. This stalagmite proxy record captures variations associated with wet and dry monsoons on decadal to centennial time-scales, together with a general declining trend in the ISM during the 1460-year period. It is noted that the declining trend of the ISM follows the northern hemispheric summer insolation, which is known to influence the location and strength of the Inter tropical convergence zone (ICTZ). The stalagmite record also indicates an abrupt climate change, characterized by the decline of ISM around 2800 yr BP, as manifested in the enrichment of18O values. Furthermore, the enriched18O values around 2800 yr BP are corroborated by changes in the stalagmite growth rate, its trace elemental ratios (Sr/Ca, Ba/Ca and U/Ca) and crystallographic structure. In addition, the decline of ISM around 2800 yr BP coincides with a sudden rise in the atmospheric Δ14C, indicative of reduced solar activity. This period around 2800 yr BP is widely reported as the cold European climate associated with ice debris events in the North Atlantic (also known as the Iron Age Cold Epoch), which were reportedly forced by low solar activity. Syntheses of other available stalagmite records from the Indian region, during the common time-frame, show coherent variations with the Kadapa stalagmite and also the Dongge cave stalagmite (southern China), pointing to synchronous variations of the Indian and the East Asian monsoon systems.

APPLICATION OF 18O AND 13C STABLE ISOTOPES COMPOSITION OF THE CARBONATE ROCKS OF THE JERIBE FORMATION EASTERN IRAQ; AN APPROACH TO DEFINE THE PALEO TEMPERATURE AND PALEO DEPTH

Stable isotopes 18O/16O and 13C/12C in the carbonate rocks of the Jeribe Formation are examined here to define the depositional characters in the basin includes paleo temperatures and paleo depth. The Jeribe Formation comprises a transgressive unit belongs to the Latest Eocene-Recent Megasequence (Ap11). Complete sections of the formation were studied in the core of Himreen structure in eastern part of Iraq. The Himreen structure lies in the outer platform margin of unstable shelf of the Arabian plate. The stable isotope (18O/16O and 13C/12C) of Jeribe Formation provides the first records of paleotemperature for the Neogene (middle Miocene Transgression) of the Southern Teythes, which indicate cooling during deposition. Microfacies study imply that the skeletal grains are composed of green and red algae, stromatolite and verities of benthic foraminifera. The non-skeletal grains are pellets and intraclasts. Analysis of microfacies suggests lagoon, reef, shoal and open marine environments. The depletion of both 18O and 13 C indicated shallowing at the contact with overlying and underlying formations in the shoal and lagoonal facies respectively. While in the middle part in the coral reef facies reveals elevation of the sea surface temperature. These suggest that the enrichment of both 18O and 13 C at the cool water temperature associated with upwelling and arise of sea level represented by reef and open marine facies.

Coupling isotopic and piezometric data to infer groundwater recharge mechanisms in arid areas: example of Samail Catchment, Oman

Hydrochemistry and well hydrographs are coupled to assess groundwater recharge in the regional catchment of Samail, Oman. The complex geology comprises three aquifers: limestones of the Hajar Supergroup (HSG) at the highlands of North Oman Mountains (NOM); fractured/weathered ophiolites; and Quaternary alluvium. Groundwater flows south–north from the NOM to the coast. Samples from groundwater wells and springs (38) were analyzed for isotopes and major ions. Corrected 14C dating reveals modern groundwater across the entire catchment, while 87Sr/86Sr (0.70810–0.70895) shows greater homogeneity. Groundwater in the upper catchment is depleted in 2H and 18O, indicating a high-altitude recharge source (NOM), and becomes enriched downstream, with a slope indicating an evaporation effect. The hydrographs of nested piezometers located in the upper, middle and lower catchment show different recharge responses between deep and shallower depths. Head difference in response to recharge is observed upstream, suggesting a lateral recharge mechanism, contrary to vertical recharge downstream reflected in identical recharge responses. The homogeneous 87Sr/86Sr ratio, head changes, downstream enrichment of 2H and 18O, and the presence of modern groundwater throughout the catchment suggest that groundwater recharge takes place across the entire catchment and that the three aquifers are hydraulically connected. The recharge estimated using the chloride mass balance method is in the range of 0–43% of the mean annual rainfall.

Surrounding species diversity improves subtropical seedlings' carbon dynamics.

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two‐ and four‐species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant–soil–atmosphere continuum. We measured gas‐exchange variables (C assimilation, transpiration and above‐ and belowground respiration) and δ13C in leaf biomass, phloem, soil microbial biomass, leaf‐ and soil‐respired CO 2 as well as δ18O in leaf and xylem water. The 13C signal in phloem and respired CO 2 in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf‐water 18O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water‐use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth‐related C sinks in monoculture. These ecophysiological responses of L. glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.

Clay Mineral Suites in Submarine Mud Volcanoes in the Kumano Forearc Basin, Nankai Trough: Constraints on the Origin of Mud Volcano Sediments

Clay mineralogy is an important characteristic of mud volcano sediments. This study determined the clay mineral compositions of sediment from two submarine mud volcanoes in the Kumano forearc basin, Nankai Trough, by X-ray diffraction analysis. Similar compositions dominated by smectite in the two mud volcanoes indicate that the mud volcanoes in the basin are rooted in the same source sequence. These clay mineral compositions differed from those in Pleistocene basin sediment, suggesting that the mud volcano sediment originated beneath the Pleistocene sediment. The illite content in the illite–smectite mixed layer averaged 32% in the mud volcano sediment, which implies that the sediment experienced temperatures above 60 °C that promoted the smectite-to-illite transformation. However, porewater extracted from the mud volcano sediment had Cl‒ concentrations roughly half that of seawater and proportional enrichment in 18O and depletion in D, indicating that dehydration reactions of clay minerals had previously occurred in a deeply buried sedimentary layer. The smectite and illite contents (<60%) in the clay-size fraction rule out in situ smectite dewatering as the cause of the dilution of Cl‒ in porewater. Thus, fluids derived from clay dewatering must have originated from deeper than the source of the mud volcano sediment.

Oxygen isotope enrichment in rice (Oryza sativa L.) grain organic matter captures signature of relative humidity

The oxygen isotopic composition (δ18O) of plant organic matter (OM) is primarily governed by the δ18O of source water (δ18OSW) and climatic factor of relative humidity (RH). Among the cereals, the growth of rice plants is critically dependent on the water availability in the growth-environment. In the present study, we investigated the sensitivity of δ18O in the bulk organic matter of rice grains to RH of their growth-environment. Our experimental setup consisted of both glasshouse and field experiments, where eight genotypes were grown at RH levels ranging from 67% to 87%. The δ18O measured in bulk grain OM and source water was used to calculate the net oxygen isotopic enrichment (Δ18OOM). Regression analysis of Δ18OOM with RH demonstrated a significant relationship (r2 = 0.96; p < 0.0001), thereby implying that the isotopic signature of evaporative conditions gets recorded in the rice grain OM. In addition, our study involved a separate experiment that monitored the degree of oxygen isotope enrichment in water samples extracted from different parts of the rice plant. For this purpose, we sampled four of the above eight genotypes along with three other rice genotypes that were grown in both open cultivation fields and glasshouse. Water present in the culms, leaves, and grains were extracted quantitatively. Isotopic analyses revealed progressive 18O enrichment of the water in the culms and leaves and intermediate enrichment values of that in the grains. Based on the isotope data, we validated mechanistic models for prediction of δ18O of the leaf water and that of the plant carbohydrates. The model predictions were in close agreement with the experimental observations. The study provides insights into the rice plant’s oxygen isotope systematics that build the foundation for future applications of the stable isotope technique to study the interactions between rice and environment.

Stability of Zircon and Its Isotopic Ratios in High-Temperature Fluids: Long-Term (4 months) Isotope Exchange Experiment at 850°C and 50 MPa

Stability of zircon in hydrothermal fluids and vanishingly slow rates of diffusion identify zircon as a reliable recorder of its formation conditions in recent and ancient rocks. Debate, however, persists on how rapidly oxygen and key trace elements (e.g., Li, B, Pb) diffuse when zircon is exposed to hot aqueous fluids. Here, we report results of a nano- to micrometer-scale investigation of isotopic exchange using natural zircon from Mesa Falls Tuff (Yellowstone) treated with quartz-saturated, isotopically (18O, D, 7Li, and 11B) labeled water with a nominal δ18O value of +450‰ over 4 months at 850°C and 50 MPa. Frontside (crystal rim inwards) δ18O depth profiling of zircon by magnetic sector SIMS shows initially high but decreasing 18O/16O over a ~130 nm non-Fickian profile, with a decay length comparable to the signal from surficial Au coating deposited onto zircon. In contrast, backside (crystal interior outwards) depth profiling on a 2-3 µm thick wafer cut and thinned from treated zircon by focused ion beam (FIB) milling lacks any significant increase in 18O/16O during penetration of the original surface layer. Near-surface time-of-flight (TOF-SIMS) frontside profiles of uncoated zircon from 4-month and 1-day-long experiments as well as untreated zircons display similar enrichments of 18O over a distance of ~20 nm. All frontside 18O profiles are here interpreted as transient surface signals from nm-thick surface enrichment or contamination unrelated to diffusion. Likewise, frontside depth profiling of H, Li, and B isotopes are similar for long- and short-duration experiments. Additionally, surface U-Pb dating of zircon from the 4-month experiment returned U-Pb ages by depth profiling with ~1 µm penetration that were identical to untreated samples. Frontside and backside depth-profiling thus demonstrate that diffusive 18O enrichment in the presence of H2O is much slower than predicted from experiments in Watson and Cherniak (1997). Instead, intracrystalline exchange of oxygen between fluid and zircon in wet experimental conditions with excess silica occurred over length-scales equivalent to those predicted for dry diffusion. Oxygen diffusion coefficients even under wet conditions and elevated temperatures (850 °C) are <1-3×10-23 m2/sec, underscoring a virtual lack of oxygen diffusion and an outstanding survivability of zircons

Tropical Indo-Pacific hydroclimate response to North Atlantic forcing during the last deglaciation as recorded by a speleothem from Sumatra, Indonesia

Abrupt changes in Atlantic Meridional Overturning Circulation are known to have affected the strength of the Indian and Asian Monsoons during glacial and deglacial climate states. However, there is still much uncertainty around the hydroclimate response of the Indo-Pacific Warm Pool (IPWP) region to abrupt climate changes in the North Atlantic. Many studies suggest a mean southward shift in the intertropical convergence zone (ITCZ) in the IPWP region during phases of reduced Atlantic meridional overturning, however, existing proxies have seasonal biases and conflicting responses, making it difficult to determine the true extent of North Atlantic forcing in this climatically important region.Here we present a precisely-dated, high-resolution record of eastern Indian Ocean hydroclimate variability spanning the last 16 ky (thousand years) from δ18O measurements in an aragonite–calcite speleothem from central Sumatra. This represents the western-most speleothem record from the IPWP region. Precipitation arrives year-round at this site, with the majority sourced from the local tropical eastern Indian Ocean and two additional long-range seasonal sources associated with the boreal and austral summer monsoons. The Sumatran speleothem demonstrates a clear deglacial structure that includes 18O enrichment during the Younger Dryas and 18O depletion during the Bølling–Allerød, similar to the pattern seen in speleothems of the Asian and Indian monsoon realms. The speleothem δ18O changes at this site are best explained by changes in rainfall amount and changes in the contributions from different moisture pathways. Reduced rainfall in Sumatra during the Younger Dryas is most likely driven by reductions in moisture transport along the northern or southern monsoon transport pathways to Sumatra. Considered with other regional proxies, the record from Sumatra suggests the response of the IPWP to North Atlantic freshwater forcing is not solely driven by southward shifts of the ITCZ, but also a reduction in moisture transport along both monsoon pathways.

Enrichment of 18O in the mantle sources of the Antarctic portion of the Karoo large igneous province

Karoo continental flood basalt (CFB) province is known for its highly variable trace element and isotopic composition, often attributed to the involvement of continental lithospheric sources. Here, we report oxygen isotopic compositions measured with secondary ion mass spectrometry for hand-picked olivine phenocrysts from ~ 190 to 180 Ma CFBs and intrusive rocks from Vestfjella, western Dronning Maud Land, that form an Antarctic extension of the Karoo province. The Vestfjella lavas exhibit heterogeneous trace element and radiogenic isotope compositions (e.g., εNd from − 16 to + 2 at 180 Ma) and the involvement of continental lithospheric mantle and/or crust in their petrogenesis has previously been suggested. Importantly, our sample set also includes rare primitive dikes that have been derived from depleted asthenospheric mantle sources (εNd up to + 8 at 180 Ma). The majority of the oxygen isotopic compositions of the olivines from these dike rocks (δ18O = 4.4–5.2‰; Fo = 78–92 mol%) are also compatible with such sources. The olivine phenocrysts in the lavas, however, are characterized by notably higher δ18O (6.2–7.5‰; Fo = 70–88 mol%); and one of the dike samples gives intermediate compositions (5.2‒6.1‰, Fo = 83–87 mol%) between the other dikes and the CFBs. The oxygen isotopic compositions do not correlate with radiogenic isotope compositions susceptible to crustal assimilation (Sr, Nd, and Pb) or with geochemical indicators of pyroxene-rich mantle sources. Instead, δ18O correlates positively with enrichments in large-ion lithophile elements (especially K) and 187Os. We suggest that the oxygen isotopic compositions of the Vestfjella CFB olivines primarily record large-scale subduction-related metasomatism of the sub-Gondwanan mantle (base of the lithosphere or deeper) prior to Karoo magmatism. The overall influence of such sources to Karoo magmatism is not known, but, in addition to continental lithosphere, they may be responsible for some of the geochemical heterogeneity observed in the CFBs.

Fog Spatial Distributions over the Central Namib Desert - An Isotope Approach

Fog is a characteristic feature of the Namib Desert and is essential to life in this fog dependent system. It is often acknowledged that advective fog from the ocean is the dominant fog type over the Namib Desert fog-zone but recent evidence suggests that other fog types occur in this area. Knowledge of the existence and spatial distribution of different fog types will enhance the mechanistic understanding of fog formation and potential changes in this region, but such knowledge is limited in literature. In this study, we investigated fog spatial variations within the Namib Desert fog-zone by applying stable isotope (δ18O and δ2H) techniques to differentiate various fog types and identify their source waters. Isotope based results showed that at least three types of fog (advective, radiation and mixed) occurred in this region and what appears as a single fog event may include all three types. Results suggest that radiation fog was the dominant fog type during our study period. The results also suggest that advective fog (with Atlantic Ocean origins) either dissipated 30–50 km inland and the residual humidity combined with locally derived moisture to form mixed fog or advective fog incorporated local moisture along its trajectory inland resulting in mixed fog. Fog in the Namib Desert was consistently depleted in 18O and 2H compared to rainfall and this was attributed to sub-cloud evaporation of the rainfall as well as different sources of fog and rainfall. Sub-cloud evaporation led to enrichment of 18O and 2H in rainfall beyond that of the first stage condensate, fog. Advective fog is often considered the architect of the fog-zone in the Namib Desert, but our results demonstrated multiple dominant fog types during the study period, suggesting knowledge of both fog frequency and fog type is needed to better predict climate change impacts on the fog-zone.

A Giant Underwater, Encrusted Stalactite from the Blue Hole, Lighthouse Reef, Belize, Revisited: a Complex History of Biologically Induced Carbonate Accretion Under Changing Meteoric and Marine Conditions

A late Pleistocene to Holocene submerged and encrusted speleothem exhibits a complex history including a meteoric phase and two marine phases. A combined study using petrography, mineralogy, and inorganic and organic geochemistry, as well as geochronology has shown that phototrophic and heterotrophic biological activity impacted carbonate precipitation during all phases of carbonate accretion. The stalactite formed ca. 30 m below modern sea level at a marginal overhang in the Blue Hole of Lighthouse Reef Atoll. Unlike purely meteoric speleothems, the Belize example consists of a meteoric core, a marine aragonite crust, and a serpulid-micrite-rich outer crust as a result of postglacial flooding of the karst cave. The core of the stalactite has a tufaceous texture, containing algal or microbial remains, and consists entirely of low-magnesium calcite, formed 19.55-10.68 kyr BP. The texture suggests that the stalactite formed at the cave entrance, and, hence, the former cave ceiling had apparently collapsed earlier during the Pleistocene. Oxygen (δ18O) and carbon (δ13C) isotopes across the core suggest a trend towards drier conditions and reduced soil and plant cover after the last glacial maximum. The marine aragonite crust consists of stacked botryoids in which individual crystals up to 700 lm have dark terminations enriched in high-magnesium calcite. This crust accreted from 10.82 to 9.95 kyr BP in warm shallow water during the early Holocene thermal optimum. Carbonate accretion rates were considerable and averaged 125 μm/yr. The crust has a dense, laminated texture on one side and a porous, shrubby texture on the other. The presence of n-C16:1ω5, n-C17:1ω6, and 10Me-C16 fatty acids in the laminated crust suggests that sulfate-reducing bacteria contributed to aragonite formation in an environment that was less open than the formation environment of the porous crust, where these biomarkers are lacking (n-C16:1ω5,n-C17:1ω6) or are less abundant (10Me-C16). Enrichment of 34S and 18O in carbonate-associated sulfate (CAS) relative to seawater sulfate also suggests sulfate reduction during carbonate formation. The greater contribution of heterotrophic processes to aragonite precipitation in the laminated crust is also reflected in δ13C values as low as -1.3%, whereas no such depletion is observed in the aragonite of the porous crust (δ13C values as low as 0.0%). A pronounced isotopic variability and excursions to positive δ13C values as high as +3.5%0 in the inner half of the laminated crust indicate an episodic, local impact of photosynthesis on aragonite precipitation, whereas the lack of such excursions in the porous crust (δ13C values as high as +1.5%0) is again best explained by a more open environment of formation. After a ca. 5 kyr hiatus, from 4.39 kyr BP, a biogenic crust of abundant serpulids and finely crystalline, microbial and detrital carbonate, consisting of high-magnesium calcite and aragonite, accreted on the outer surface of the stalactite. Outermost crust accretion was probably influenced by the inundation of the Lighthouse Reef lagoon that started to shed abundant fine-grained carbonate sediment into the Blue Hole. The stalactite broke off the cave ceiling either before or after the formation of the outermost crust, likely due to seismic movements along the nearby plate boundary. The study demonstrates that like speleothems from the terrestrial realm, submerged stalactites may have had complex histories with great potential as paleoenvironmental archives.

Application of Bayesian analysis to the doubly labelled water method for total energy expenditure in humans.

RationaleThe doubly labelled water (DLW) method is the reference method for the estimation of free‐living total energy expenditure (TEE). In this method, where both 2H and 18O are employed, different approaches have been adopted to deal with the non‐conformity observed regarding the distribution space for the labels being non‐coincident with total body water. However, the method adopted can have a significant effect on the estimated TEE.MethodsWe proposed a Bayesian reasoning approach to modify an assumed prior distribution for the space ratio using experimental data to derive the TEE. A Bayesian hierarchical approach was also investigated. The dataset was obtained from 59 adults (37 women) who underwent a DLW experiment during which the 2H and 18O enrichments were measured using isotope ratio mass spectrometry (IRMS).ResultsTEE was estimated at 9925 (9106‐11236) [median and interquartile range], 9646 (9167–10540), and 9,638 (9220–10340) kJ·day−1 for women and at 13961 (12851–15347), 13353 (12651–15088) and 13211 (12653–14238) kJ·day−1 for men, using normalized non‐Bayesian, independent Bayesian and hierarchical Bayesian approaches, respectively. A comparison of hierarchical Bayesian with normalized non‐Bayesian methods indicated a marked difference in behaviour between genders. The median difference was −287 kJ·day−1 for women, and −750 kJ·day−1 for men. In men there is an appreciable compression of the TEE distribution obtained from the hierarchical model compared with the normalized non‐Bayesian methods (range of TEE 11234–15431 kJ·day−1 vs 10786–18221 kJ·day−1). An analogous, yet smaller, compression is seen in women (7081–12287 kJ·day−1 vs 6989–13775 kJ·day−1).ConclusionsThe Bayesian analysis is an appealing method to estimate TEE during DLW experiments. The principal advantages over those obtained using the classical least‐squares method is the generation of potentially more useful estimates of TEE, and improved handling of outliers and missing data scenarios, particularly if a hierarchical model is used.

Isotopic study of the source and cycle of sulfur in the Yamdrok Tso basin, Southern Tibet, China

This study focuses on the inland drainage basin of Yamdrok Tso (Tso means ‘lake’ in the Tibetan language), which lies at altitudes between 4394 and 4989 m, in southern Tibet. The basin is located in the Tethys Himalayan zone, and consists of seven lakes with different hydrological characteristics. The main purpose of the study was to investigate the source of dissolved SO42− and the sulfur cycle in the groundwater-river-lake system. We measured the hydrogeochemical composition, δ18OH2O, δ34SSO4 and δ18OSO4 of sulfate in river, spring and lake water; the total sulfur content (STotal) and δ34STotal of total sulfur in a lake sediment core; and the δ34Spyrite of pyrite in the bedrock. The results indicate that most of the rivers in the study area are characterized by a high sulfate content with values of up to 270 mg/L. In addition, the δ34Spyrite of pyrite in the bedrock ranges between −4.31‰ and −5.77‰, while the sulfate in river and spring waters has mainly negative values of δ34SSO4 (−7.14‰ to −2.02‰) and δ18OSO4 (−13.2‰ to −4.04‰). In contrast, the waters of closed lakes in the study basin have high values of δ34SSO4 (3.44‰–8.58‰) and δ18OSO4 (7.26‰–10.3‰). The negative δ34SSO4 and δ18OSO4 dual-isotopic composition of sulfate indicates that pyrite weathering controls the dissolved sulfate and other solutes in rivers and spring water, while the anthropogenic pollutant sulfur flux is negligible. Water is the main oxygen source for sulfate derived from the oxidation of pyrite, while Fe3+ is the main direct agent of pyrite S oxidation. The lake water and sediments in the study basin are important sinks/stores of sulfur. The δ34STotal values of total sulfur in a sediment core from a closed and holomictic lake ranges between −8.5‰ and −44.9‰. As evidenced by the strong 34S and 18O enrichment in the residual pool of SO42− in the lake waters, and the marked depletion of 34S in the sediment core, the dissimilatory microbial sulfate reduction (MSR) in the lake sediments generally results in the preferential sequestration of sulfur enriched in 32S. In addition, the magnitude of 34S/32S fractionation between sulfate in the lake water and sedimentary total sulfur can reach up to 48.3‰. This abnormally high fractionation value can be explained mainly by the reoxidation of reduced inorganic sulfur compounds, together with the dispropotionation pathway in the sediments, caused by lake level fluctuations. The changes in STotal and δ34STotal values with depth in the sediment core can also be explained by the same mechanism.

Ostracod calcite records the 18O/16O ratio of the bicarbonate and carbonate ions in water

The δ18O of ostracod valves is widely used to infer water δ18O and temperature. However, ostracod δ18O appears sensitive to other environmental variables. In addition, there is species-dependent ostracod calcite 18O enrichment, relative to slowly precipitated inorganic calcite under the same conditions. Together these uncertainties complicate ostracod paleoclimate reconstructions. This study presents a new understanding of the causes of ostracod δ18O variations based on a global database of published ostracod δ18O values in lake, marine and coastal environments, and from culture experiments. The database includes associated field/experiment host water parameters including temperature (−1 to 32°C), water δ18O (−13.2‰ to 4.3‰ VSMOW), pH (6.9–10.4), salinity (0–72g/kg), calcite saturation states (0.6–26), and dissolved inorganic carbon concentration [DIC] (0.9–54.3mmol/kg). The data show that: (1) the δ18O of marine and non-marine ostracods reflects the 18O/16O of the sum of host water CO32− and HCO3− ions. For example, at a given temperature, the δ18O of non-marine ostracods decreases by 4‰ to 6‰ as [CO32−]/[DIC] reaches 70%, depending on the ostracod species. In low [CO32−]/[DIC] settings (i.e. high HCO3−/CO32−), ostracod 18O/16O is close to the 18O/16O of HCO3− ions, which explains why on average ostracod δ18O is higher than the δ18O of inorganic calcite precipitated slowly under the same conditions. (2) Taxonomic offsets in ostracod δ18O vary with the host water [CO32−]/[DIC]. In environments where HCO3−≫CO32− (i.e. most freshwater lakes), the 18O/16O of Candonids is indistinguishable from the 18O/16O of HCO3− ions (difference of 0.10±0.16‰) while the 18O/16O of Cyprids is lower than the 18O/16O of HCO3− ions by −0.77‰ to −0.32‰, Cytherids by −0.88±0.29‰, and Limnocytherids by −1.12±0.05‰. (3) The sensitivity of ostracod δ18O to [CO32−]/[DIC] also varies with taxonomy. For each percent increase in [CO32−]/[DIC], Candonids δ18O decreases by −0.098±0.024‰, Cyprids by −0.075±0.004‰, Cytherids by −0.057±0.012‰, and Limnocytherids by −0.058±0.005‰. (4) The similarity in 18O/16O between ostracod calcite and the sum of host water ‘HCO3− and CO32−’ (despite species-specific offsets) suggests rapid precipitation of a finite DIC pool in the ostracod calcifying fluid. We propose an ostracod δ18O model whereby the more negative the 18O/16O of a given species relative to the sum of CO32− and HCO3−, the greater the isotopic equilibrium between the DIC pool and H2O in the ostracod calcifying fluid. (5) Higher host water salinities and pH induce higher [CO32−]/[DIC], resulting in lower ostracod δ18O, which explains the variable salinity and pH effects on ostracod δ18O. Our database and ostracod δ18O model shows that ostracod δ18O records from closed basin environments likely reflect high or variable [CO32−]/[DIC], rather than water temperature and δ18O alone. Our study reconciles contradictory observations of controls on ostracod δ18O and paves the way for improved paleo-environmental interpretations and reconstructions of past water [CO32−]/[DIC].

The influence of oxygen isotope exchange between CO2 and H2O in natural CO2-rich spring waters: Implications for geothermometry

Oxygen isotope ratio (δ18O) value deviations from the Meteoric Water Line with no significant change in the hydrogen isotope (δ2H) composition have been reported in naturally occurring CO2-rich waters from around the world. Here we review the effects of oxygen isotope exchange with CO2, high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the CO2-rich water isotopic composition. We present two case studies from Daylesford (Australia) and Pah Tempe (Utah, USA) mineral springs, where we use a numerical geochemical modelling approach to resolve the influence of low temperature water-rock interactions and CO2 equilibration to the oxygen isotope ranges observed in the mineral waters. In both cases, we find that mineral dissolution – precipitation reactions are unlikely to have a significant effect on the groundwater isotopic compositions, and that the observed δ18O values in natural CO2 springs can be simply explained by equilibrium fractionation between water and free phase CO2. Traditionally, the interaction of CO2 and water in a natural CO2-rich groundwater setting has only been associated with water 18O depletion and this is the first study to consider 18O enrichment. We establish that in a natural setting, CO2 and water equilibration can result in water 18O depletion or enrichment, and that the change in the oxygen isotope composition ultimately depends on the initial CO2 and water δ18O values. Our new conceptual model therefore provides a mechanism to explain water 18O enrichment at ambient temperatures. This finding is critical for the use of δ18O in groundwater geothermometry and for the interpretation of natural water circulation depths: we argue that in some cases, natural waters previously interpreted as geothermal based on their oxygen isotope composition may actually have acquired their isotopic signature through interaction with CO2 at ambient temperatures.