Oxygen Isotope Ratio Measurements Research Articles

Identifying the Origin of Southwestern Shell: A Geochemical Application to Mogollon Rim Archaeomolluscs

Archaeological marine shell artifacts moving over long distances may reveal the remnants of social networks, social currency, and the nuances of exchange. For the southwestern United States and northern Mexico, potential sources of marine shell are predominantly the Gulf of California and Pacific Ocean. There exists some taxonomic overlap between molluscan communities of these regions and the Gulf of California, necessitating non-biogeographic methods to distinguish their origins. Combined oxygen and carbon stable isotope ratio measurements demonstrate that modern shells from these water bodies have distinct isotopic ranges. Molluscan isotopie composition within the Gulf of California varies, which allows for the identification of distinct source regions. Archaeological marine shell from Pueblo III and IV sites in the Mogollon Rim region of east-central Arizona are sourced, demonstrating that archaeological shell was obtained from a northeastern subregion of the Gulf of California. This is the closest source for the Puebloan communities, but it is not consistent with previous hypotheses concerning the origin of marine shell in the Colorado Plateau and Mogollon Highlands, which suggested an exchange route via Paquime originating south of Isla Tiburon. We discuss the implications of these results in the context of previous research and draw conclusions about the meaning of shell use in the region.

Identifying the Origin of Southwestern Shell: A Geochemical Application to Mogollon Rim Archaeomolluscs

Archaeological marine shell artifacts moving over long distances may reveal the remnants of social networks, social currency, and the nuances of exchange. For the southwestern United States and northern Mexico, potential sources of marine shell are predominantly the Gulf of California and Pacific Ocean. There exists some taxonomic overlap between molluscan communities of these regions and the Gulf of California, necessitating non-biogeographic methods to distinguish their origins. Combined oxygen and carbon stable isotope ratio measurements demonstrate that modern shells from these water bodies have distinct isotopic ranges. Molluscan isotopie composition within the Gulf of California varies, which allows for the identification of distinct source regions. Archaeological marine shell from Pueblo III and IV sites in the Mogollon Rim region of east-central Arizona are sourced, demonstrating that archaeological shell was obtained from a northeastern subregion of the Gulf of California. This is the closest source for the Puebloan communities, but it is not consistent with previous hypotheses concerning the origin of marine shell in the Colorado Plateau and Mogollon Highlands, which suggested an exchange route via Paquime originating south of Isla Tiburon. We discuss the implications of these results in the context of previous research and draw conclusions about the meaning of shell use in the region.

An Investigation of Cattle Birth Seasonality using δ13C and δ18O Profiles within First Molar Enamel

Cattle (Bos taurus) are biologically able to breed year‐round, potentially giving farmers the freedom to choose a calving strategy to best meet their economic goals. Thus, an accurate method to determine cattle birth seasonality from archaeological remains would prove to be a valuable tool when investigating a prehistoric farming community. This paper presents the results of intra‐tooth isotope ratio analysis (δ18O, δ13C) of first, second and third molars from 15 cattle. The principal outcome is a possible new approach to determining cattle birth seasonality utilizing both carbon and oxygen isotope ratio measurements of first molar enamel. Although this technique requires verification through more extensive testing, particularly of modern material, initial results suggest that it may produce more accurate predictions of birth seasonality than techniques based on intra‐tooth δ18O measurements of second and third molars.

Partitioning evapotranspiration – Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes

Summary Stable oxygen isotopes of water provide a valuable tracer for water movements within ecosystems and are used to estimate the contribution of transpiration to total ecosystem evapotranspiration ( ft ). We tested the Craig and Gordon equation against continuous field measurements of isotopic composition of evaporation and assessed the impact for partitioning evapotranspiration. Therefore, evaporation ( E ) and its isotopic signature ( δ 18 O E ) on bare soil plots, as well as evapotranspiration ( ET ) and its corresponding isotopic composition of ( δ 18 O ET ) of an herbaceous layer was measured with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We quantified the variation in δ 18 O E arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature ( δ 18 O e ) and the temperature at the evaporating site ( T e ), and the kinetic fractionation factor ( α k ). We could hence quantify ft based on measured δ 18 O ET , modeled δ 18 O E from observed soil water isotopic composition at the evaporating site ( δ 18 O e ), and modeled δ 18 O of transpiration ( δ 18 O T ) from observed total soil water isotopic composition. Our results demonstrate that predicting δ 18 O E using the Craig and Gordon equation leads to good agreement with measured δ 18 O E given that the temperature and 18 O isotope profiles of the soil are thoroughly characterized. However, modeled δ 18 O E is highly sensitive to changes in T e and δ 18 O e as well as α k . This markedly affected the partition results of transpiration and evaporation from the total ET flux: The fraction of transpiration ( ft ) varied strongly using different formulations for α k and assuming steady or non-steady state transpiration. These findings provide a first comparison of laser-based and modeled isotopic compositions of evaporation based on the Craig and Gordon equation under field conditions. This is of special interest for studies using stable isotopes to separate soil evaporation and plant transpiration fluxes and highlights the need for a thorough characterization of the micrometeorological and isotopic constitution of the upper soil layer to locate the evaporating front with a resolution of a few cm soil depths. We also call on a better characterization of the kinetic fractionation factor of soil evaporation.

Canadian Basin freshwater sources and changes: Results from the 2005 Arctic Ocean Section

We present measurements of oxygen isotope ratios and nutrient concentrations along the 2005 Arctic Ocean Section aboard the icebreaker Oden. The data are used to estimate freshwater contributions from meteoric water (mainly river runoff), sea‐ice meltwater, and Chukchi Sea shelf water, itself a combination of Pacific and indigenous Arctic water types. Nutrients ratios are combined to form quasi‐conservative water‐mass tracers (phosphate‐star, N‐star, and the empirical Arctic N‐P relationship) and used along with salinity and δ18O, which are conservative in the ocean interior. Disagreements between two different freshwater analyses in the Western Arctic are largely resolved using a salinity‐dependent Redfield ratio, a new estimate of the Pacific end‐member, and an analysis of the Bering Strait inflow contribution to detraining shelf waters. Freshwater components from 2005 are placed into the context of the overlapping 1994 Arctic Ocean Section (aboard the Louis St. Laurent) and a time series of hydrographic/tracer casts between 1987 and 1992 in the Canada Basin. Compared to 1987–1994; the 2005 transect exhibits increased meteoric water concentrations in the northern part of the Canadian Basin and a decrease in the southern part. This pattern is related to changes in the distribution of wind‐stress curl during the several years prior to each sampling campaign. In addition, a previously observed correlation between sea‐ice formation and river runoff disappears over the Central Arctic in 2005, a change that we attribute to a northward shift of sea‐ice formation. Resampling approximately every 3 years should resolve the dynamics driving changes in freshwater and nutrient distributions.

Position-specific measurement of oxygen isotope ratios in cellulose: Isotopic exchange during heterotrophic cellulose synthesis

We describe the first reported method for the measurement of oxygen isotope ratios at each position in the glucose units of the cellulose molecule. The overall process comprises a series of synthetic organic sequences, by which α-cellulose is hydrolysed to glucose, and oxygen atoms at specific positions in the glucose molecule are removed in samples of benzoic acid for measurement of δ18O. Values of δ18O at specific positions in cellulose are calculated from these δ18O values and the overall δ18O value of the cellulose. We apply the method to determine the degree to which oxygen atoms at each position undergo isotopic exchange with water during heterotrophic cellulose synthesis, such as occurs in the cambium of trees. To do this we extract α-cellulose from wheat seedlings germinated in the dark in aqueous media of differing oxygen isotope ratios. Results indicate that oxygen atoms at positions 5 and 6 (O-5 and O-6 respectively) undergo around 80% exchange with medium water, O-3 undergoes around 50% exchange, and O-2 and O-4 do not undergo isotopic exchange. The results have important implications for extracting palaeoclimatic records from oxygen isotope time series obtained from tree ring cellulose. As O-5 and O-6 undergo significant exchange with medium water during heterotrophic cellulose synthesis, oxygen isotopes at these positions in tree ring cellulose should carry a predominantly trunk (source) water signal. On the other hand, O-2 and O-4 should retain the isotopic signature of leaf water in tree ring cellulose. Our method therefore potentially enables the separate reconstruction of past temperature and humidity data from oxygen isotope ratios of tree ring cellulose – something that has hitherto not been possible. The measured degrees of isotopic exchange are to some extent unexpected and cannot be fully explained using current biochemical mechanisms, suggesting that knowledge of these processes is incomplete.

Measurement of δ18 O values in arsenic and selenium oxyanions

Past stable isotopic studies of selenium and arsenic oxyanions within a biogeochemistry context have focused solely on selenium isotopes due to monoisotopic nature of arsenic and lack of attention to oxygen. A modern quantification method of δ(18)O values in selenate, selenite, arsenate, and arsenite is presented here, similar to methods for other inorganic oxyanions such as phosphate. Dissolved oxyanions were recovered after precipitation with silver or barium cations within controlled pH ranges and purified by drying and roasting under vacuum. Oxygen isotope ratio measurements were performed on 250-450 micrograms of solids using continuous flow high-temperature conversion elemental analyzer/isotope ratio mass spectrometry (CF-TCEA/IRMS). Each Se and As solid was evaluated for reproducibility and stability of δ(18)O values and the usefulness of this method for evaluating aqueous phase Se and As oxyanion reactions was addressed. Only silver arsenate and barium selenate were shown to have stable, reproducible δ(18)O values over 5 months and were calibrated to the VSMOW scale. The method was applied to confirm complete exchange of oxygen isotopes between arsenate and solvent water at pH 7 which produced an O-isotope fractionation between arsenate and water of 6.0‰. Furthermore, the method revealed the ability of oxygen in selenate to kinetically fractionate during reduction to elemental selenium by a green rust compound which produced a fractionation (20.9‰) larger than reported δ(80/76)Se values for similar reactions. The experimental and analytical conditions for δ(18)O analysis of Se and As oxyanions were detailed. δ(18)O analyses of As and Se oxyanions may be useful in biogeochemical studies for tracing chemical reaction pathways, determining environmental sources of Se, or diagnosing specific reaction mechanisms for As and Se oxyanions.

Evaluation of the effects of composition on instrumental mass fractionation during SIMS oxygen isotope analyses of glasses

Significant instrumental mass fractionation (IMF) occurs during measurements of oxygen isotope ratios in magmatic glasses by SIMS. In order to characterise and correct for this fractionation, we measured oxygen isotope ratios in a range of international and internal glass standards ranging in composition from basalt (47wt.% SiO2) to rhyolite (72wt.% SiO2) and with known major element compositions. Oxygen isotope ratios were determined by laser fluorination at SUERC, East Kilbride, or taken from previously published values. A total of 1105 δ18O measurements were made over nine sessions on a Cameca IMS-1270 ion microprobe at the University of Edinburgh. SIMS measurements on glass standards had external precision better than ±0.36‰ (1σ), and the reference material analysed alongside the unknown samples, USGS synthetic glass GSA-1G, had an average external precision of ±0.14‰. The selected standards are thus sufficiently homogeneous in δ18O to be suitable calibration standards. In terms of δ18O, the SIMS measurements show that, within a single session, IMF may vary by up to 4.7‰ from one glass standard to another. IMF is strongly correlated with SiO2 and CaO. A least squares regression calculation was used to explore potential univariate and multivariate correction schemes. For each correction scheme, the correction coefficients determined for each session were then used to calculate the IMF and correct the measured isotopic ratio of each glass standard. A univariate correction scheme using only SiO2 to correct for IMF reproduced 75% of the glass standards to within ±0.2‰ of their true δ18O, and 95% to within ±0.4‰. Bivariate correction schemes using SiO2–CaO and FeO–CaO produced similar results, but did not significantly improve on the SiO2 correction. The correction schemes were applied to δ18O measurements made on melt inclusions and glasses from the Askja volcanic system, North Iceland. The uni- and bivariate correction schemes tested produced δ18O values within the published range for Icelandic basalts. We recommend a simple correction scheme based on the SiO2 content of appropriate standards, which should span a suitable compositional range from basalt to rhyolite.

Tracing sources and pathways of dissolved nitrate in forest and river ecosystems using high‐resolution isotopic techniques: a review

AbstractNitrogen inputs into stream and river ecosystems, and the factors influencing those inputs, are important for various ecological and environmental concerns. Reliable information on where and how nitrogen compounds flow into aquatic ecosystems is indispensable to understanding the nutrient status of these ecosystems. Such information should include the biogeochemical mechanisms and hydrological controls of nutrient leaching into rivers from terrestrial systems such as forests, agricultural fields, and urbanized areas. Advancements in stable isotopomer measurements over the past two decades have expanded the variety of target substances and the precision with which they can be investigated. The high‐throughput microbial denitrifier method allows for simultaneous measurement of nitrogen and oxygen isotope ratios and can provide high‐resolution spatiotemporal information on both nitrate sources and biogeochemical processes. Although advanced techniques of stable isotope analysis have been used extensively to detect sources and estimate the relative contributions of multi‐source systems in various rivers, there are still new horizons in investigating nitrogen transformations. For example, stable isotopes of oxygen (18O and 17O) occurring in nitrate due to atmospheric deposition can be used as natural tracers for evaluating internal nitrogen cycling; these isotopes are distinct from the oxygen within microbially generated nitrate in soils and water bodies. Another future challenge is improved use of nitrous oxide isotopomers in evaluating the relative contributions of nitrification and denitrification. Such analysis could provide insight into the nitrogen transformation that occurs under redox conditions at the boundary between terrestrial and aquatic systems, where nitrification and denitrification often occur simultaneously in soil and aquatic environments.

High‐temperature pyrolysis/gas chromatography/isotope ratio mass spectrometry: simultaneous measurement of the stable isotopes of oxygen and carbon in cellulose

Stable isotope analysis of cellulose is an increasingly important aspect of ecological and palaeoenvironmental research. Since these techniques are very costly, any methodological development which can provide simultaneous measurement of stable carbon and oxygen isotope ratios in cellulose deserves further exploration. A large number (3074) of tree-ring α-cellulose samples are used to compare the stable carbon isotope ratios (δ(13)C) produced by high-temperature (1400°C) pyrolysis/gas chromatography (GC)/isotope ratio mass spectrometry (IRMS) with those produced by combustion GC/IRMS. Although the two data sets are very strongly correlated, the pyrolysis results display reduced variance and are strongly biased towards the mean. The low carbon isotope ratios of tree-ring cellulose during the last century, reflecting anthropogenic disturbance of atmospheric carbon dioxide, are thus overestimated. The likely explanation is that a proportion of the oxygen atoms are bonding with residual carbon in the reaction chamber to form carbon monoxide. The 'pyrolysis adjustment', proposed here, is based on combusting a stratified sub-sample of the pyrolysis results, across the full range of carbon isotope ratios, and using the paired results to define a regression equation that can be used to adjust all the pyrolysis measurements. In this study, subsamples of 30 combustion measurements produced adjusted chronologies statistically indistinguishable from those produced by combusting every sample. This methodology allows simultaneous measurement of the stable isotopes of carbon and oxygen using high-temperature pyrolysis, reducing the amount of sample required and the analytical costs of measuring them separately.

ASTRONOMICAL OXYGEN ISOTOPIC EVIDENCE FOR SUPERNOVA ENRICHMENT OF THE SOLAR SYSTEM BIRTH ENVIRONMENT BY PROPAGATING STAR FORMATION

New infrared absorption measurements of oxygen isotope ratios in CO gas from individual young stellar objects confirm that the solar system is anomalously high in its 18O/17O ratio compared with extra-solar oxygen in the Galaxy. We show that this difference in oxygen isotope ratios is best explained by 1 per cent enrichment of the proto-solar molecular cloud by ejecta from type II supernovae from a cluster having of order a few hundred stars that predated the Sun by at least 10 to 20 Myr. The likely source of exogenous oxygen was the explosion of one or more B stars during a process of propagating star formation.

High‐precision SIMS oxygen, sulfur and iron stable isotope analyses of geological materials: accuracy, surface topography and crystal orientation

AbstractA high‐precision SIMS analysis technique has been established for oxygen, sulfur, and iron isotope ratios and applied to a wide range of geoscience research areas using a Cameca IMS‐1280 at the Wisconsin Secondary Ion Mass Spectrometer Laboratory (WiscSIMS). Precision and accuracy of 0.3‰is achieved routinely for the measurement of 18O/16O ratio using multicollection Faraday Cup (FC) detectors and primary Cs+ beam size of 10 µm. Smaller beam sizes of 3 µm to < 1 µm yield precisions of 0.7–2‰using a multicollection Electron Multiplier (EM) in pulse‐counting mode for 18O. We evaluate small SIMS analytical biases at the level of a few ‰or less using standard minerals with homogeneous oxygen isotope ratios: (i) topography of samples related to polishing relief of grains and location of analysis in a sample holder; and (ii) crystal orientation effects in magnetite (Fe3O4). The latter effect has not been detected for oxygen isotope ratio measurements in other minerals including a variety of silicate, oxide, and carbonate minerals at WiscSIMS. However, similar analytical biases that are correlated with crystal orientation have been identified from Fe isotope analyses in magnetite and S isotope analysis in sphalerite (ZnS), and many minerals have not yet been evaluated. The total range of analytical bias among randomly oriented magnetite grains becomes smaller by reducing the sputtering energy of the primary ions (from 20 to 10 keV), which may help reduce crystal orientation effects. Copyright © 2010 John Wiley & Sons, Ltd.

Basin‐wide photosynthetic production rates in the subtropical and tropical Pacific Ocean determined from dissolved oxygen isotope ratio measurements

We used the oxygen isotope anomaly (17Δ) to determine gross photosynthetic O2 production (GOP) on Pacific Ocean transits of a cargo ship between United States and Australia and New Zealand in August 2004 and February, August, and November 2005. The 17Δ approach yielded robust regional GOP averages of 98 ± 8, 200 ± 11, 115 ± 7, and 182 ± 32 mmol O2 m−2 d−1 for the northern subtropics, tropics, southern subtropics, and South Coral and Tasman Sea, respectively. When converted to 14C primary production (PP) rate equivalents, the 17Δ‐based rates compare favorably with prior field data and indicate that satellite algorithms (i.e., vertically generalized productivity model (VGPM) and carbon‐based model (CBM)) underestimate in situ rates by a factor of 1.3–3, with greatest underestimation in the tropics. We determined empirical relationships between chlorophyll, nitrate, and GOP that explain 81% of variance in observed GOP rates. Integrating over the Pacific basin from 30°N to 30°S (excluding coastal areas) yields a gross production of 37.3 Gt C yr−1 and a 14C PP equivalent of 20.3 Gt C yr−1, 1.3–1.8 × higher than the 11.4 and 15.9 Gt C yr−1 estimated by the VGPM and CBM, respectively.

Identification of the parent bodies of micrometeorites with high-precision oxygen isotope ratios

Oxygen isotopic compositions allow identification of potential parent bodies of extraterrestrial materials. We measured oxygen isotope ratios of 33 large (diameter >500μm) silicate melted micrometeorites (cosmic spherules) from Antarctica, using IR-laser fluorination coupled with mass spectrometry. It is the first time that this high-precision method is used on individual micrometeorites. The selected micrometeorites are representative of the influx of extraterrestrial materials to the Earth. Our results show that most micrometeorites are related to carbonaceous chondrites, which is consistent with previous studies. However, 20–50% of them seem to be related to CO/CV carbonaceous chondrites, whereas CM/CR carbonaceous chondrites were thought to be the main source for micrometeorites. Furthermore, ∼30% of measured samples have oxygen isotope ratios lying above the terrestrial fractionation line, which relates them to ordinary chondrites or other, as yet, unsampled parent bodies.

Freshwater transport at Fimbulisen, Antarctica

The intricate near‐circumpolar system of fronts and currents surrounding Antarctica isolates much of Earth's freshwater from the saline oceans immediately north. The Antarctic Slope Front sustains bathymetrically steered flow at the shelf break, whereas the shallow Coastal Current travels rapidly alongside the ice front. A hydrographic survey of the southeastern Weddell Sea finds these two features to have merged near the narrow (<40 km wide) continental shelf at Fimbulisen. On the prime meridian, its Trolltunga ice tongue overshoots the shelf break northward into this slope current. Observations on either side of the ice tongue demonstrate its retarding effect on the westward‐flowing waters it overhangs and its contribution to the poorly understood freshwater budget. From oxygen isotope ratio measurements and referenced geostrophic shears, we find the combined glacial meltwater and sea ice melt transport upstream of Trolltunga at 0.6°E to account for 18.0 ± 5.8 mSv of the total 1.6 ± 0.2 Sv westward transport (Sv = 106 m3 s−1). In Trolltunga's lee and downstream at 2.8°W, we find this figure to ultimately increase to 23.8 ± 15.5 mSv of a total 2.8 ± 0.4 Sv transport. Each of these sections was impeded by sea ice cover, so these estimates of westward transport are probably lower limits. The westward glacial ice meltwater transport of 10 ± 3 mSv at 2.8°W highlights the role that Fimbulisen plays in preconditioning shelf waters before they reach broad continental shelves in the southwestern Weddell Sea where they transform to bottom waters.

Primitive oxygen-isotope ratio recorded in magmatic zircon from the Mid-Atlantic Ridge

The oxygen-isotope composition of the Earth’s upper mantle is an important reference for understanding mantle and crust geochemical cycles. Olivine is the most commonly used mineral for determining the influence of crustal processes on the oxygen-isotope ratio (δ18O) of primitive rocks, however it is an uncommon mineral in continental crust and readily alters at or near Earth’s surface. Here we report the first measurements of oxygen-isotope ratios in zircon from oceanic crust exposed at a mid-ocean ridge. Measurements of δ18O and trace elements were made by ion microprobe on zircon in polished rock chips of gabbro and veins in serpentinized peridotite drilled from the Mid-Atlantic Ridge. The zircon grains contain both oscillatory and sector growth zoning, features characteristic of magmatic zircon. Values of δ18O (zircon) = 5.3 ± 0.8‰ (2 st. dev., n = 68) for the population are consistent with the interpretation that these grains are igneous in origin and formed in high-temperature isotopic equilibrium with mantle oxygen. The δ18O values demonstrate that zircon in oceanic crust preserves primitive δ18O in spite of sub-solidus alteration of the whole rock. The fact that the primitive δ18O (zircon) values fall in a narrow range (5.3 ± 0.8‰) strengthens the use of oxygen isotopes in zircon as a tracer to identify processes of exchange in a wide range of modern and ancient crustal environments, including subducted oceanic crust (eclogite), and also in the oldest known pieces of Earth, >3900 million-year-old detrital zircon grains from Western Australia.

Life form-specific variations in leaf water oxygen-18 enrichment in Amazonian vegetation

Leaf water (18)O enrichment (Delta(o)) influences the isotopic composition of both gas exchange and organic matter, with Delta(o) values responding to changes in atmospheric parameters. In order to examine possible influences of plant parameters on Delta(o) dynamics, we measured oxygen isotope ratios (delta(18)O) of leaf and stem water on plant species representing different life forms in Amazonia forest and pasture ecosystems. We conducted two field experiments: one in March (wet season) and another in September (dry season) 2004. In each experiment, leaf and stem samples were collected at 2-h intervals at night and hourly during the day for 50 h from eight species including upper-canopy forest trees, upper-canopy forest lianas, and lower-canopy forest trees, a C(4) pasture grass and a C(3) pasture shrub. Significant life form-related differences were detected in (18)O leaf water values. Initial modeling efforts to explain these observations over-predicted nighttime Delta(o) values by as much as 10 per thousand. Across all species, errors associated with measured values of the delta(18)O of atmospheric water vapor (delta(v)) appeared to be largely responsible for the over-predictions of nighttime Delta(o) observations. We could not eliminate collection or storage of water vapor samples as a possible error and therefore developed an alternative, plant-based method for estimating the daily average delta(v) value in the absence of direct (reliable) measurements. This approach differs from the common assumption that isotopic equilibrium exists between water vapor and precipitation water, by including transpiration-based contributions from local vegetation through (18)O measurements of bulk leaf water. Inclusion of both modified delta(v) and non-steady state features resulted in model predictions that more reliably predicted both the magnitude and temporal patterns observed in the data. The influence of life form-specific patterns of Delta(o) was incorporated through changes in the effective path length, an important but little known parameter associated with the Péclet effect.

Unravelling contamination signals in biogenic silica oxygen isotope composition: the role of major and trace element geochemistry

AbstractThe oxygen isotope composition of diatom silica (δ 18Odiatom) provides valuable information for palaeoclimate studies where carbonate proxies are either rare or absent in many lakes and ocean sediments. Unfortunately, mineral and rock fragments found alongside diatoms in most sediments can be problematic as the method used will liberate oxygen from all components within the sediment, producing both high‐frequency noise and low‐frequency excursions that can resemble climate signals. Removal of mineral contamination to date has largely relied upon the combination of chemical leaching and physical separation techniques (e.g. sieving, density). This combination can be inefficient and often significant proportions of contaminants are present in the ‘purified’ diatom sample. Using electron optical imaging and whole‐rock geochemistry on previously ‘purified’ diatom samples, a mass balance approach has been developed whereby the types and proportions of residual contaminants are identified. By integrating this information with measured oxygen isotope ratios of the contaminants, it is then possible to remove contamination effects from the δ18Odiatom record. Contamination effects relating to carbonates, tephra and silt are modelled for cores from Lake Tilo (Ethiopia) and Lake Baikal (Siberia). In both lakes the new modelled δ18Odiatom curves show less high‐frequency noise, thus enabling better resolution of low‐frequency climate signals. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons Ltd.

Ice-shelf – ocean interactions at Fimbul Ice Shelf, Antarctica from oxygen isotope ratio measurements

Abstract. Melt water from the floating ice shelves at the margins of the southeastern Weddell Sea makes a significant contribution to the fresh water budget of the region. In February 2005 a multi-institution team conducted an oceanographic campaign at Fimbul Ice Shelf on the Greenwich Meridian as part of the Autosub Under Ice programme. This included a mission of the autonomous submarine Autosub 25 km into the cavity beneath Fimbul Ice Shelf, and a number of ship-based hydrographic sections on the continental shelf and adjacent to the ice shelf front. The measurements reveal two significant sources of glacial melt water at Fimbul Ice Shelf: the main cavity under the ice shelf and an ice tongue, Trolltunga, that protrudes from the main ice front and out over the continental slope into deep water. Glacial melt water is concentrated in a 200 m thick Ice Shelf Water (ISW) layer below the base of the ice shelf at 150–200 m, with a maximum glacial melt concentration of up to 1.16%. Some glacial melt is found throughout the water column, and much of this is from sources other than Fimbul Ice Shelf. However, at least 0.2% of the water in the ISW layer cannot be accounted for by other processes and must have been contributed by the ice shelf. Just downstream of Fimbul Ice Shelf we observe locally created ISW mixing out across the continental slope. The ISW formed here is much less dense than that formed in the southwest Weddell Sea, and will ultimately contribute a freshening (and reduction in δ18O) to the upper 100–150 m of the water column in the southeast Weddell Sea.

Oxygen isotope ratio measurements in CO_2 by means of a continuous-wave quantum cascade laser at 43 μm

A mid-infrared laser spectrometer was developed for simultaneous high-precision (18)O/(16)O and (17)O/(16)O isotope ratio measurements in carbon dioxide. A continuous-wave, liquid-nitrogen cooled, distributed feedback quantum cascade laser, working at a wavelength of 4.3 microm, was used to probe (12)C(16)O(2), (16)O(12)C(18)O, and (16)O(12)C(17)O lines at ~2311.8 cm(-1). High sensitivity was achieved by means of wavelength modulation spectroscopy with second-harmonic detection. The experimental reproducibility in the short and long terms was deeply investigated through the accurate analysis of a large number of spectra. In particular, we found a short term precision of 0.5 per thousand and 0.6 per thousand, respectively, for (18)O/(16)O and (17)O/(16)O isotope ratios. The occurrence of systematic deviations is also discussed.