Analysis Of Oxygen Isotope Ratios Research Articles

A comparison of the palaeoclimate signals from diatom oxygen isotope ratios and carbonate oxygen isotope ratios from a low latitude crater lake

The analysis of oxygen isotope ratios ( δ 18O) from authigenic lake carbonates has become a well-established palaeoclimate technique. Less common is the use of δ 18O in biogenic silica (e.g. diatoms, sponge spicules, phytoliths), but the technique is being increasingly utilised for non-carbonate lakes. This paper aims to compare δ 18O diatom with δ 18O carbonate in a low latitude, closed basin lake. Due to the influence of pH, rarely are both carbonate and diatoms preserved in lake sediments in concentrations high enough to measure oxygen isotopes. The δ 18O composition of both carbonate and silica should reflect the isotope composition and temperature of the lake water at the time of precipitation and hence theoretically they should be equivalent. Only one comparative study of δ 18O diatom and δ 18O carbonate currently exists and that found differences between their δ 18O compositions, probably because they precipitated in different seasons. Unless there is evidence to suggest that silica and carbonate precipitate at the same time, their δ 18O records are likely to be different but complementary. In this study, we show that δ 18O diatom and δ 18O calcite from Lake Tilo, Ethiopia show some similar broad climate trends. However, the δ 18O diatom curve is generally more variable and does not record two regional arid events, picked up by the δ 18O calcite data. Several possible reasons for this are discussed: the precision of the δ 18O diatom extraction technique, contamination of the diatom samples from tephra (volcanic glass), vital effects in the diatom samples, differences in their respective equilibrium isotope fractionation rates and the possibility that the dominant diatom, Aulacoseira granulata, and calcite precipitated in different seasons during this time. Currently, the problems associated with the cleaning and extraction of δ 18O diatom suggests that details found in δ 18O calcite records may be lost in δ 18O diatom records where contaminants such as tephra and clay minerals are difficult to remove.

The Online 18O/ 16O Analysis: Development and application

Oxygen is a globally important element, in inorganic compounds it is involved in key structural compounds of the earth, and it is providing functional groups for most organic compounds. Oxygen isotope ratio analyses on these compounds provide information on the oxygen sources and (formation) reactions in which the compound was involved. Therefore generally applicable analytical methods for the δ 18O determination are highly desirable. After a description of online high temperature "carbon reduction" methods for the conversion of oxygen in the compounds mentioned into CO and discussion of problems involved, especially in analysis of inorganic substances like carbonates and silicates, selected examples for the application of these carbon reduction methods will be presented. The well-known correlation of δ 18O values of cellulose (in tree rings) to that of (leaf) water is used since the 1970's as climate indicator. The observed enrichment in 18O in cellulose by about 27 in comparison to leaf water is attributed to an oxygen equilibrium isotope effect between carbonyl groups and water. But not all carbohydrates show this "general" 18O enrichment relative to water. By means of a recently published model a non-statistical oxygen isotope distribution in carbohydrates is postulated. Oxygen isotope discrimination has been recognized long ago as an important principle of authenticity evaluation in food and beverages. Though many basic mechanisms of isotope discrimination in nature are known, the corresponding application of data is often based only on empirical observations in this field. The fact that oxygen in natural organic plant material is derived from three sources (CO2, H2O, O2) with distinct differences in δ 18O and that their incorporation by different reactions implies oxygen isotope effects of different sizes, is therefore of special importance, and it is attempted by means of individual isotopic increments of functional groups to predict global δ 18O values of natural and nature-identical compounds. Vice versa δ 18O values can also give hints on the elucidation of biosynthetic pathways.

Oxygen Isotopes in Zircon

Isotopic and trace element analysis of zircons can provide reliable and robust estimates of age, compositions of coexisting minerals and melts, and constraints on the genesis and protoliths of host rocks. Recent technological developments facilitate analysis of oxygen isotope ratios in zircon with high accuracy and precision by laser heating/gas-source mass-spectrometry and in situ from thin sections or grain mounts by ion microprobe/secondary ion mass-spectrometer. A large number of studies have shown that non-metamict zircons preserve their δ18O value from the time of crystallization; hence oxygen isotope ratio can be correlated with age (U-Pb) or trace element composition. The zircon δ18O record is generally preserved despite other minerals that have been reset by high-grade metamorphism or intense hydrothermal alteration. Thus the refractory nature and robust inheritance of zircon offers a potential means to sort out magmatic equilibration and reequilibration, and post-magmatic alteration, an eternal problem for igneous rocks. New processes and interpretations for igneous events have been proposed when the effects of post-magmatic exchange are fully recognized. Crustal recycling can be recognized from magmatic values of δ18O(zircon), and if source rocks are igneous and young at the time of melting, δ18O will often be the best geochemical signature. Microanalytical techniques are increasingly useful for stable isotope analysis of silicates. Both the laser fluorination/mass-spectrometer and the ion microprobe/secondary ion mass-spectrometer offer significant advantages over conventional techniques for analysis of zircon. ### Laser fluorination Accurate analysis of δ18O in refractory minerals such as zircon is optimized by use of a CO2 laser (λ = 10.6 μm). The best precision has been obtained when zircons are powdered before analysis. Accuracy and precision of ±0.05 to ±0.1‰ (1 standard deviation = 1sd) are reported for homogeneous samples of ~2 mg (~0.5 mm3) (Valley et al. 1994, …

Regime Shift Signatures from Stable Oxygen Isotopic Records of Otoliths of Atlantic Cod (Gadus morhua)

The sudden collapse of Atlantic cod (Gadus morhua) may relate to ocean climate, or regime shifts as demonstrated in production of Pacific salmon. This paper reports the results of stable oxygen isotope ratio analyses (18O/16O or δ18OA) from 91 otoliths of cod over a period of about 20 years. Seasonal δ18OA variations of individual otoliths started at an initial value of about −0.5 to 0‰ VPDB, and then reached a stable level in the range of +2.5 to +3.5‰ VPDB after 4–5 years. The initial low values correspond to the natal sources of mature cod, while the higher δ18OA values represent the water conditions before the cod was caught. This pattern of δ18OA variation was observed over the life history of all cod examined. Furthermore, the calculated isotopic temperatures agreed with those obtained from summer bottom trawl survey, indicating that δ18OA of otoliths could be used as a thermometer in determining the ambient seawater temperature where the cod lived. Comparison of long-term δ18OA records and biological and meteorological observations suggested that decadal-scale ecosystem changes did occur in the late 1970s and early 1990s in Atlantic Canada, comparable to regime shifts occurred in the North Pacific.

Geographic Identities of the Sacrificial Victims from the Feathered Serpent Pyramid, Teotihuacan: Implications for the Nature of State Power

This study addresses the political and military structure of early Teotihuacan through the analysis of oxygen-isotope ratios in skeletal phosphate from 41 victims of a sacrifice associated with the Feathered Serpent Pyramid. Oxygen-isotope ratios are markers of geographic identity. A comparison of bone and enamel values, which provides a contrast between environments experienced during growth and those of adulthood, illustrates that at least four different regions are represented in this sample. Those identified as soldiers had either lived locally since childhood or had moved to Teotihuacan from several foreign locations. Most had lived in Teotihuacan for a prolonged period before their death. This pattern suggests foreign “recruitment” or mercenary behavior. The women had either lived all their lives in Teotihuacan or had moved from there to a foreign location. Most of the individuals in the center of the pyramid (burial 14) did not come from Teotihuacan, nor had they lived in the city long before their deaths. We suggest that the choice of victims was meant to demonstrate Teotihuacan's powerful ideology to the rest of the Mesoamerican world. Notably, this isotopic evidence of physical interaction between Teotihuacan and foreign regions considerably predates the currently existing archaeological evidence.

REVISITING THE TEOTIHUACAN CONNECTION AT ALTUN HA

An analysis of oxygen-isotope ratios in skeletal phosphate was used to assess the possibility that the Early Classic period (a.d. 280–550) Maya elite male in Tomb F/8-1 from the eastern Belizean site of Altun Ha had originally come from Teotihuacan, Mexico. When compared with four other individuals used as controls for spatial and temporal variability in δ18Op values at Altun Ha, this individual falls at the high end of the expected range of local intrasite variation but does not have a δ18Op value consistent with that of Teotihuacan. The mortuary and isotopic data have been compared with those from the previously analyzed Maya site of Kaminaljuyu, Guatemala, in order to examine regional and temporal differences in the influence of the powerful state of Teotihuacan. It appears that Teotihuacan was not the homeland of any of the tomb individuals analyzed from either site. Thus, models of ideological or symbolic power are supported over those of political or military imperialism.

Sample vial influences on the accuracy and precision of carbon and oxygen isotope ratio analysis in continuous flow mass spectrometric applications

Sample vial influences on the accuracy and precision of carbon and oxygen isotope ratio analysis in continuous flow mass spectrometric applications

Sample vial influences on the accuracy and precision of carbon and oxygen isotope ratio analysis in continuous flow mass spectrometric applications

Due to the small amounts of sample gas involved in continuous flow mass spectrometric analysis, care should be taken to evaluate the influence of sample containers on the carbon and oxygen isotope ratios of samples. Data indicate that Na-glass and borosilicate glass vials, equipped with butyl rubber septa, can cause significant changes in the isotopic composition of CO(2) gas, even where sample gases are stored within the vial for less than one day. The magnitude of these changes varies from vial to vial. Given the leverage that contamination can potentially exert on small gas samples, each researcher should carefully evaluate the effect of sample vials in order to eliminate unknown and unwanted changes in the composition of samples. Copyright 2000 John Wiley & Sons, Ltd.

An early Holocene oxygen isotope record from the Olson buried forest bed, southern Lake Michigan

Analysis of a 3.5 m vibracore from the Olson buried forest bed in the southern Lake Michigan basin provides new paleolimnological data for the early Holocene. The core records a rise in lake level from the Chippewa low water phase toward the Nipissing high water phase. Deepening of the water level at the core site is suggested by a trend toward decreasing organic carbon content up core that is interpreted as a response to increasing distance between terrestrial debris sources and the core site.Published data from deep water cores from the southern Lake Michigan basin suggest there had been an inflow of isotopically light water from glacial Lake Agassiz into the southern basin between 10.5-11 ka (A1 event). The data also indicate a second flood of isotopically light water between 8-9 ka (A2 event).Three new 14C dates from the Olson site core suggest that most of the sediment was deposited between 8.45 ka and 8.2 ka, an interval roughly coeval with the second pulse of 18O-depleted water (A2) from Lake Agassiz into the southern basin. Oxygen isotope ratio analysis of shell aragonite from the gastropods Probythinella lacustris and Marstonia deceptashows increasingly negative values up core. This trend inσ18O values suggests that 18O - depleted water entered the southern basin about 8.4 ka. The Olson site core thus provides a chronology of events in the southern Lake Michigan basin associated with the draining of glacial Lake Agassiz.

Carbon and oxygen isotope ratios of ecosystem respiration along an Oregon conifer transect: preliminary observations based on small-flask sampling.

Isotope ratio analyses of atmospheric CO(2) at natural abundance have significant potential for contributing to our understanding of photosynthetic and respiration processes in forest ecosystems. Recent advances in isotope ratio mass spectrometry allow for rapid, on-line analysis of small volumes of CO(2) in air, and open new research opportunities at the ecophysiological, whole-organism, and atmospheric levels. Among the immediate applications are the carbon and oxygen isotope ratio analyses of carbon dioxide in atmospheric air. Routine analysis of carbon dioxide in air volumes of approximately 50-300 &mgr;l is accomplished by linking a commercially available, trace gas condenser and gas chromatograph to an isotope ratio mass spectrometer operated in continuous-flow mode. Samples collected in the field are stored in either gas-tight syringes or 100-ml flasks. The small sample volume required makes it possible to subsample the air in flasks for CO(2) and then to sample the remaining air volume for the analysis of the isotopic composition of either methane or nitrous oxide. Reliable delta(13)C and delta(18)O values can be obtained from samples collected and stored for 1-3 days. Longer-term storage, on the order of weeks, is possible for delta(13)C measurements without drift in the isotope ratio signal, and should also be possible for delta(18)O measurements. When linked with an infrared gas analyzer, pump and flask sampling system, it is feasible to sample CO(2) extensively in remote forest locations. The air-sampling system was used to measure the isotope ratios of atmospheric CO(2) and to conduct a regression analysis of the relationship between these two parameters. From the regression, we calculated the delta(13)C of ecosystem respiration of four coniferous ecosystems along a precipitation gradient in central Oregon. The ecosystems along the coast-to-interior Oregon (OTTER) gradient are dominated by spruce-hemlock forests at the wet, coastal sites (> 200 cm precipitation annually) to juniper woodlands (20 cm precipitation) at the interior, dry end of the transect. The delta(13)C values of ecosystem respiration along this transect differed by only 1.3 per thousand (range of -25.2 to -23.9 per thousand ) during August at the peak of the summer drought. Following autumn rains in September, the delta(13)C of ecosystem respiration in the four stands decreased; overall the difference in the carbon isotope ratio of ecosystem respiration among sites increased to 3.9 per thousand (-26.8 to -22.9 per thousand ).

A rapid method for the preparation of O2 from CO2 for mass spectrometric measurement of17O/16O ratios

Following the discovery of strong oxygen isotope enrichment in stratospheric O3 and in particular the puzzling concomitant large excess enrichment in 17O, more atmospheric gases, importantly CO2 and CO, have been found to suffer from this effect, known as mass independent fractionation. Understanding this new category of isotope effects, which for O3 appears to be a rare, symmetry related kinetic isotope effect, is of fundamental importance. Moreover, the search for the presence of mass independent fractionation effects in atmospheric trace gases and understanding their causes is of interest to atmospheric chemistry. Conventional accurate oxygen isotope ratio analysis is based on mass spectrometry of CO2. However, using CO2 does not allow measurement of the 17O/16O ratio due to isobaric interference from 13C variations. Therefore, conversion to O2 is necessary, for which a fast and safe method has been developed. The essence of the new method is that CO2 is first converted to H2O and CH4 by reacting it with H2. To increase reaction speed and to achieve quantitative conversion, a Ni powder catalyst is used at its Curie point. Subsequently, the formed H2O readily reacts with F2 to form the desired O2 with HF as waste product. A safe reaction is assured by using a mixture of only 5% F2 in He. The validity of the produced isotope ratios is verified independently. © 1998 John Wiley & Sons, Ltd.

Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment

Abstract The very limited instrumental record makes extensive analyses of the natural variability of global tropical cyclone activities difficult in most of the tropical cyclone basins. However, in the two regions where reasonably reliable records exist (the North Atlantic and the western North Pacific), substantial multidecadal variability (particularly for intense Atlantic hurricanes) is found, but there is no clear evidence of long-term trends. Efforts have been initiated to use geological and geomorphological records and analysis of oxygen isotope ratios in rainfall recorded in cave stalactites to establish a paleoclimate of tropical cyclones, but these have not yet produced definitive results. Recent thermodynamical estimation of the maximum potential intensities (MPI) of tropical cyclones shows good agreement with observations. Although there are some uncertainties in these MPI approaches, such as their sensitivity to variations in parameters and failure to include some potentially important interac...

Ion microprobe analysis of oxygen isotope ratios in quartz from Skye granite: healed micro-cracks, fluid flow, and hydrothermal exchange

Quartz grains in hydrothermally altered granites from the Isle of Skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 μm scale. Ion microprobe analysis of one grain shows a gradient of 13‰ over 400 μm and a greater range in δ 18O than all quartz previously analyzed on the Isle of Skye. Other crystals from the same outcrop are homogeneous. Digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable. The ion probe analysis correlates low δ 18O values on a micro-scale to one set of healed cracks. Thus, quartz exchanges oxygen isotopes primarily by solution and reprecipitation along fractures, in contrast to more reactive feldspar that appears to exchange from the grain boundary inward. Macroscopic models of isotope exchange are not realistic for these rocks; the flow of hydrothermal fluids was heterogeneous, anisotropic and crack controlled.

Carbon and Oxygen Isotope Ratio Analysis of Small Carbonate Samples by Conventional Phosphoric Acid Digestion: Sample Preparation and Calibration

Carbon and Oxygen Isotope Ratio Analysis of Small Carbonate Samples by Conventional Phosphoric Acid Digestion: Sample Preparation and Calibration

Carbon and Oxygen Isotope Ratio Analysis of Small Carbonate Samples by Conventional Phosphoric Acid Digestion: Sample Preparation and Calibration

Carbon and Oxygen Isotope Ratio Analysis of Small Carbonate Samples by Conventional Phosphoric Acid Digestion: Sample Preparation and Calibration

An excimer laser-based micro analytical preparation technique for in-situ oxygen isotope analysis of silicate and oxide minerals

A new excimer laser system for in-situ analysis of oxygen isotope ratios of silicates and oxides is described. In contrast to IR lasers the UV laser does not cause any fractionation during laser ablation due to edge or other effects. Analytical reproducibility and precision is equivalent to the conventional method for sample sizes of 0.1 mg, which is the minimum amount required by our mass-spectrometer system. Refractive minerals like olivine and garnet can be analyzed with a precision comparable with conventional analyses of highly reactive minerals, quartz, however, which is transparent at 248 nm does cause problems. Other advantages and disadvantages of our UV laser system are discussed.

Isotopic fractionation in secondary ionization mass spectrometry

AbstractWe have studied isotopic fractionation effects which occur during the analysis of oxygen isotope ratios by secondary ionization mass spectrometry. Variable instrumental isotopic fractionation has been well documented in the past as making reliable ion microprobe analyses of oxygen isotopes elusive. We report here techniques for minimizing these effects by careful optimization of an Isolab 54 ion microprobe and a method of integrating the secondary‐ion beam to eliminate a major source of fractionation caused by unequal focusing of isotopes on the source slit of the mass spectrometer. A further improvement has been the installation of new ion detectors for measuring 18O− and 17O− secondary ion beams in which the ions strike a conversion dynode (Conversion dynode system—CDS) to release electrons into a channeltron. The CDS detectors have a uniform, high and stable gain which, coupled with the beam integration techniques referred to above, yield reproducible oxygen isotope ratios on both conducting and insulating mineral standards (⩽1.2‰ on 18O/16O during a day and <2‰ over periods of days and weeks).

In-Situ Analysis of Oxygen Isotope Ratios in Silicates and Carbonates by Ion Microprobe

In-Situ Analysis of Oxygen Isotope Ratios in Silicates and Carbonates by Ion Microprobe

Submillimeter scale zonation of δ18O in quartz and feldspar, Isle of Skye, Scotland

Laser probe analysis of oxygen-isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains. Two samples collected within 10 m of each other in the Maol na Gainmhich Epigranite (MGE), Isle of Skye, Scotland, are similar petrographically, yet display very different δ 18 O zonation patterns. SK1 has isotopically zoned quartz ( δ 18O = 1.8–8.2‰ ), with intragranular gradients of δ 18 O as great as 5 ‰/mm and individually homogeneous grains of K-feldspar (±0.2‰) which also display a large range of values ( δ 18O = −1.7‰ to −7.3‰ ). Oxygen-isotope gradients of more than 11‰/mm were measured in this sample. In contrast, SK4 has homogeneous quartz ( δ 18 O = 5.7 ± 0.3, n = 14) and a zoned K-feldspar phenocryst ( δ 18O = − 3.3‰ to −6.9‰ ). Isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains. The isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the MGE pluton, although the average values of δ 18 O obtained by laser probe are identical to the average bulk values. Values in K-feldspar suggest exchange at 400°C with infiltrating meteoric water ( δ 18O $ ̃ −10 ). The marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous. These microanalytical data for quartz and feldspar provide the first oxygen-isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures.

Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites: diffusive exchange as a guide to cooling history

Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 μm and precision in the range of 1‰ (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of δ18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with δ18O=8.9±1‰ SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9‰ depleted in δ18O. These low δ18O values increase in smooth gradients across the outer 10 μm of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 μm diameter magnetites which average 1.2‰ lower in δ18O than coarse magnetites due to low δ18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk Δ18O (Cc-Mt)=9.3‰ and yielding an apparent equilibration “temperature” of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low δ18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low δ18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.