Variations In Oxygen Isotopic Composition Research Articles

Long-lived shallow crustal magma reservoirs beneath Ulleung Island volcano (South Korea) and their rejuvenation recorded in zircon

The microanalytical data obtained from zircons collected on Ulleung Island, an emergent peak of a Quaternary alkaline magma system in the southwestern East Sea (Sea of Japan) back-arc basin, provide new insights into the origin of magma, magmatic processes, and the trigger of explosive eruptions. The U-Th ages of zircons extracted from a pumice layer formed during the intense Plinian eruption at ∼11 ka, known as the Ulleung-Oki tephra, show a wide range spanning the pre-eruption equilibrium state (>350 ka) up to the eruption event itself. Zircon grains derived from syenite and gabbro fragments entrained within the tephra yield weighted mean U-Th and U-Pb ages of 34.7 ± 2.3 ka and 1378 ± 22 ka, respectively. Under cathodoluminescence, zircons from the syenite reveal dark, featureless, or oscillatory-zoned cores, containing numerous inclusions of britholite-like Y-rich phases and pyrochlore. These cores are partially or entirely replaced by inclusion-free, brighter domains, suggesting that in situ dissolution and precipitation processes significantly influenced the transformation of zircon. The consistently low δ18O values (5.4–2.6‰) of zircons from Middle Pleistocene trachyte lavas, the Ulleung-Oki tephra, and lithic fragments indicate the persistent presence of shallow crustal magma reservoirs beneath Ulleung Island. These reservoirs have undergone substantial assimilation of hydrothermally altered rocks, with assimilation rates typically around 10–30% and reaching a maximum of approximately 50%. Zircon core-to-rim variations in oxygen isotopic composition, along with textural observations, suggest the rejuvenation of magma reservoirs through the injection of new melt, which likely served as a trigger for the explosive eruptions at Ulleung Island. The similarity in the range of εHf values between the core and rim (+7.4 to −4.3) implies the Hf isotopic consistency of the recharge melt.

Protocols for in situ measurement of oxygen isotopes in goethite by ion microprobe

We present protocols for high-spatial resolution measurement of oxygen isotope ratios of goethite (α-FeOOH) with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) and propose a natural sample as a potential goethite reference material (RM) for ion microprobe analysis. We assess the effects of goethite chemical composition, crystallographic orientation, and texture on the accuracy and repeatability of SHRIMP-SI δ18O (δ18OSIMS) results. Synthetic goethites evaluated as potential δ18OSIMS RM are powdery, porous, and finely crystalline; they do not yield repeatable results. A dense colloform goethite from the Capão topaz mine, Minas Gerais, Brazil, fulfills major prerequisites: it is stoichiometrically relatively pure, yields repeatable oxygen isotope results, and occurs in abundance to produce a RM for long-term use. We use an average laser fluorination δ18OLF-VSMOW value of −17.3 ± 0.3‰ (1SD) obtained for five aliquots of this RM to normalize all δ18OSIMS measurements. Multiple δ18OSIMS analyses of a large fragment of the Capão L4 (CL4) RM analyzed in three different runs yield an overall repeatability of −17.3 ± 0.5‰ (2SD, n = 294) for all three runs combined. Natural variability and crystal orientation effects are the main reasons for the excess spread of the δ18OSIMS results compared to the spot internal precision (ca. 0.2‰). All δ18OSIMS analyses (n = 1027) in various aliquots of CL4, randomly oriented and analyzed in 26 sessions during eight distinct runs, yield an overall repeatability of −17.3 ± 0.7‰ (2SD), confirming that CL4 is a suitable SIMS RM. After ascertaining its suitability as a RM, we used CL4 to standardize analyses of other natural goethite samples with the SHRIMP-SI and compared δ18OSIMS and laser fluorination results to test the relationship between natural properties (e.g., porosity, minor elements substituting for Fe), preparation procedures (e.g., polish and relief), instrument conditions, and the overall precision and accuracy of the SIMS analyses. Samples containing minor elements substituting for Fe (e.g., Al, Mn, Cu, Zn, etc.) or as contaminants (e.g., Si, P) require significant matrix corrections. Because we could not find homogenous natural goethite samples showing a large range in metal concentrations, we extrapolate our calibration curves beyond the composition of our calibration goethite samples. δ18OSIMS results corrected for instrument mass fractionation (using CL4) and compositionally dependent matrix effects (using several calibration goethites of known elemental composition) are less precise but statistically indistinguishable from their laser fluorination results. However, porous samples are unsuitable for SHRIMP-SI δ18O analysis. Dense colloform samples yield repeatable results for individual growth bands, showing that the high spatial resolution, moderate precision, and speed of analysis of the SHRIMP-SI can resolve variations in oxygen isotope composition acquired during sample growth. (UTh)/He geochronology of equivalent aliquots from the same goethite samples reveal that the combination of the two methods permits the extraction of temporal variation in the isotopic compositions of meteoric solutions in the geological past.

Fingerprinting environmental conditions and related stress using stable isotopic composition of rice (Oryza sativa L.) grain organic matter

Sea level rise (SLR) is a primary factor responsible for inundation of low-lying coastal regions across the world, which in turn governs the agricultural productivity. In this study, rice (Oryza sativa L.) cultivated seasonally in the Kuttanad Wetland, a SLR prone region on the southwest coast of India, were analysed for oxygen, hydrogen and carbon isotopic ratios (δ18O, δ2H and δ13C) to distinguish the seasonal environmental conditions prevalent during rice cultivation. The region receives high rainfall during the wet season which promotes large supply of fresh water to the local water bodies via the rivers. In contrast, during the dry season reduced river discharge favours sea water incursion which adversely affects the rice cultivation. The water for rice cultivation is derived from regional water bodies that are characterised by seasonal salinity variation which co-varies with the δ18O and δ2H values. Rice cultivated during the wet and the dry season bears the isotopic imprints of this water. We explored the utility of a mechanistic model to quantify the contribution of two prominent factors, namely relative humidity and source water composition in governing the seasonal variation in oxygen isotopic composition of rice grain OM. δ13C values of rice grain OM were used to deduce the stress level by estimating the intrinsic water use efficiency (WUEi) of the crop during the two seasons. 1.3 times higher WUEi was exhibited by the same genotype during the dry season. The approach can be extended to other low lying coastal agro-ecosystems to infer the growth conditions of cultivated crops and can further be utilised for retrieving paleo-environmental information from well preserved archaeological plant remains.

High precision Os isotopic measurement using N-TIMS: Quantification of various sources of error in 186Os/188Os measurements

The 190Pt–186Os system could be useful to study a number of different geologic processes. Over the last decade, this system has received considerable attention owing to its potential to detect core–mantle signatures in plume-derived lavas. Additionally, this system may also be useful as a proxy for melt depletion, examining the role of pyroxenites/sulfides in mantle melting and for dating of Platinum Group Element (PGE) based ores. The total natural variation of 186Os/188Os is limited (~150ppm in the mantle), which thus requires extremely high levels of accuracy and precision to discern natural variations. 186Os/188Os ratios were measured in laboratory standards and natural samples to examine and quantify the sources of error affecting high precision 186Os/188Os measurements and a set of refined analytical procedures to minimize and reduce these various sources of error were recommended.Inter and intra-run variations in oxygen isotopic composition used in oxide correction of OsO3− peaks can produce systematic shifts in 186Os/188Os. Useful analysis time is wasted in measuring O isotopic composition line-by-line and it is sufficient to utilize pre- and post-run determination of O isotopic composition for performing oxide correction. Although the use of 189Os/188Os as a normalizing ratio for mass fractionation results in lower uncertainty in Os isotopes from oxide correction, this benefit is overwhelmed by the increased error propagation from mass fractionation. Instead, usage of the 192Os/188Os normalization generates the least propagated error due to mass fractionation and is recommended. Earlier studies have variably used different O isotopic compositions for oxide correction and different normalizations for mass fractionation correction, which can generate systematic shifts in the reduced data. Use of mutually consistent normalizing values for mass fractionation largely reduces the offsets between reduced data using different normalizing schemes, thus improving data comparison across laboratories.For the quantities of Os and beam intensities of 186Os utilized in previous 186Os/188Os studies (several 10's of ng Os, 80–250mV of 186Os with a 1011 Ω resistor), the largest source of analytical error is derived from Johnson noise on baseline integrations. Increasing the duration of baseline measurement significantly reduces the Johnson noise error, allowing to achieve maximum external precision for a given sample size. Several 186Os/188Os analyses reported in the literature seem to have been compromised by an interference, which may be WO3−, organics or some other unidentified species that affects both the 186Os/188Os and the 184Os/188Os ratios. This makes it extremely critical to monitor and correct PtO2− interferences, which could mask other potential interferences that directly affect 186Os/188Os measurements.

An oxygen isotope study of Wark–Lovering rims on type A CAIs in primitive carbonaceous chondrites

Calcium–aluminium-rich Inclusions (CAIs) and the thin Wark–Lovering (WL) rims of minerals surrounding them offer a record of the nature of changing conditions during the earliest stages of Solar System formation. Considerable heterogeneity in the gas composition in the immediate vicinity of the proto-Sun had previously been inferred from oxygen isotopic variations in the WL rim of a CAI from Allende (Simon et al., 2011). However, high precision and high spatial resolution oxygen isotope measurements presented in this study show that WL rim and pristine core minerals of individual CAIs from meteorites that had experienced only low degrees of alteration or low grade metamorphism (one from Léoville (reduced CV3), two in QUE 99177 (CR3.0) and two in ALHA 77307 (CO3.0)) are uniformly 16O-rich. This indicates that the previously observed variations are the result of secondary processes, most likely on the asteroid parent body, and that there were no temporal or spatial variations in oxygen isotopic composition during CAI and WL rim formation. Such homogeneity across three groups of carbonaceous chondrites lends further support for a common origin for the CAIs in all chondrites. 16O-poor oxygen reservoirs such as those associated with chondrule formation, were probably generated by UV photo-dissociation involving self-shielding mechanisms and must have occurred elsewhere in outer regions of the solar accretion disk.

Reclassification of Villalbeto de la Peña—Occurrence of a winonaite‐related fragment in a hydrothermally metamorphosed polymict L‐chondritic breccia

AbstractThe Villalbeto de la Peña meteorite that fell in 2004 in Spain was originally classified as a moderately shocked L6 ordinary chondrite. The recognition of fragments within the Villalbeto de la Peña meteorite clearly bears consequences for the previous classification of the rock. The oxygen isotope data clearly show that an exotic eye‐catching, black, and plagioclase‐(maskelynite)‐rich clast is not of L chondrite heritage. Villalbeto de la Peña is, consequently, reclassified as a polymict chondritic breccia. The oxygen isotope data of the clast are more closely related to data for the winonaite Tierra Blanca and the anomalous silicate‐bearing iron meteorite LEW 86211 than to the ordinary chondrite groups. The REE‐pattern of the bulk inclusion indicates genetic similarities to those of differentiated rocks and their minerals (e.g., lunar anorthosites, eucritic, and winonaitic plagioclases) and points to an igneous origin. The An‐content of the plagioclase within the inclusion is increasing from the fragment/host meteorite boundary (approximately An10) toward the interior of the clast (approximately An52). This is accompanied by a successive compositionally controlled transformation of plagioclase into maskelynite by shock. As found for plagioclase, compositions of individual spinels enclosed in plagioclase (maskelynite) also vary from the border toward the interior of the inclusion. In addition, huge variations in oxygen isotope composition were found correlating with distance into the object. The chemical and isotopical profiles observed in the fragment indicate postaccretionary metamorphism under the presence of a volatile phase.

Photoabsorption cross‐section measurements of32S, 33S, 34S, and 36S sulfur dioxide for the B1B1‐X1A1 absorption band

We report measurements of the ultraviolet absorption cross‐sections of32SO2, 33SO2, 34SO2 and 36SO2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm−1. This is the first reported measurement of the 36SO2cross‐section. This work improves earlier measurements of the32SO2, 33SO2 and 34SO2cross‐sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO2 samples were produced in an identical process via combustion of isotopically enriched S0, eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32SO2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32SO2was used to estimate the cross‐sections of33,34,36SO2; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak‐to‐valley amplitude of36SO2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32SO2. The cross‐sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B1B1‐X1A1 and the C1B1‐X1A1 bands.

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous 16O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ18O = 5–10‰, which suggests that the domain was formed in a 16O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from 16O‐rich (δ18O = −40‰) to 16O‐poor (δ18O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more 16O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.

Anatomy of garnets in a Jurassic granite from the south-eastern margin of the North China Craton: Magma sources and tectonic implications

The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (i) euhedral garnet in mafic biotite and garnet rich enclave (Grt I), (ii) coarse-grained garnet (Grt II) in the host granite, and (iii) small euhedral garnet in aplite (Grt III). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt II. Grt III has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt II. Grt III has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ18O values of <3.8‰ in most of Grt I, 3.8–4.7‰ in most Grt II (for inclusion-free garnets), and typically >4.7‰ in Grt III. Some of the Grt II and Grt III display two distinct zonings with cores having similar major and trace element compositions to Grt I.Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U–Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (∼160Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210–236Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie–Sulu orogen (230Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events.Based on the zircon U–Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peritectic garnet, whereas Grt II and III are probably the results of magmatic dissolution–precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie–Sulu orogen representing the South China Craton.

Nature of ore-forming fluids of the Dal’negorsk deposit: Isotopic and geochemical parameters of the altered host rocks

Oxygen isotopic composition was studied in the altered host rocks of the Dal’negorsk borosilicate deposit in order to establish a boron source and the origin of ore-forming fluids responsible for deposition of economic borosilicate ore. The relationships between oxygen isotopic composition and geochemistry of the altered igneous rocks occupying various structural and temporal positions in the ore zone were studied, including premineral high-potassium minor intrusions located in the zones of datolite mineralization; alkali basalt, gabbro, and breccia from the sedimentary framework of the deposit; and postmineral basaltic andesite, basalt, and dolerite dikes. It was suggested that interaction of aqueous fluid with host rocks brought about not only variation in oxygen isotopic composition but also shifts in geochemistry of these rocks, especially as concerns the chemical elements contained in ore-forming fluid. The disturbance of oxygen isotopic system is typical of all studied rocks: δ18O values sharply decrease indicating interaction with aqueous fluid at elevated temperatures. The lowest δ18O (from −2.9 to +0.1‰) is characteristic of the premineral high-potassium and ultrapotassium minor intrusions from skarn-datolite zone. Igneous rocks from the sedimentary framework of the deposit have δ18O of +2 to −0.9‰ The δ18O of postmineral basaltic andesite, basalt, and dolerite dikes varies from 0 to +7‰ with increasing distance from the ore zone. The oxygen isotopic composition of aqueous fluid evidences its exogenic origin. The geochemical and isotopic characteristics of ore-forming fluid show that it could have been deep-seated subsurface water similar to the contemporary water of the Alpine fold zone, which contain up to 700–1000 mg/l B and is distinguished by high K, Li, Rb, Cs contents and high K/Na ratio. Similar geochemistry is characteristic of the fluid inclusions in quartz from ore zones. It cannot be ruled out that continental evaporites were a source of boron as well. The relationships between δ18O, K-Ar age, and geochemical parameters of premineral and postmineral altered intrusive bodies allow us to suggest that the subsurface B-bearing water discharged through narrow channels controlled by premineral basaltic bodies. The discharge was probably initiated by emplacement of basalt and dolerite dikes.

Use of δ 18O tracer to identify stream and spring origins of a mountainous catchment: A case study from Liddar watershed, Western Himalaya, India

Summary Water samples from precipitation, streams and springs were collected and analyzed for oxygen isotope composition to identify the stream and spring origins in Liddar watershed, NW Himalaya, India. Samples were collected from eight precipitation sites, 14 streams and six springs on bimonthly basis from January/March 2008 to January 2009. The data generated indicated a strong spatial and temporal variation in oxygen isotope composition. An altitude effect of −0.1‰ to −0.5‰/100 m change in elevation (mean altitude effect: 0.19‰/100 m) was discernible in precipitation isotopic composition. The precipitation was 18O depleted in cooler seasons/months and at higher elevations. The δ18O of precipitation showed a narrow range (2–5‰) during the periods/months of more precipitation and wide range (8–13‰) during the other months. Like precipitation the isotopic composition of streams is also controlled by the elevations of their catchments. The stream water was 18O depleted in the mountain areas with their catchments at higher altitudes than at lower elevations. The streams/tributaries were most 18O depleted in May and least depleted in September. The oxygen isotope composition of springs is similar to that observed in the streams, being most 18O depleted in May and least 18O depleted in September. On the basis of local vertical isotopic gradient of oxygen the mean altitude of the recharge by rainfall infiltration was estimated. The widespread melting of snow, the high discharges of the streams and the springs and 18O depleted stream and spring waters indicate the dominant contribution of snow melt in May; negligible snow melting, precipitation in the form of snow only, less stream and spring discharges and less variance indicate the contribution from baseflow in January; while as exhausted snow, moderate stream and spring discharges and 18O enriched stream and spring water indicate the contribution from rainfall. The best correlation was observed between the δ18O of springs and streams.

Oxygen- and magnesium-isotope compositions of calcium–aluminum-rich inclusions from CR2 carbonaceous chondrites

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates – most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have 16O-rich (Δ 17O = −23.3 ± 1.9‰, 2 σ error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial 26Al/ 27Al ratios, ( 26Al/ 27Al) 0, in 15 of 16 16O-rich CAIs measured are consistent with the canonical value of (4.5–5) × 10 −5 and a short duration (<0.5 My) of CAI formation. These data do not support the “supra-canonical” values of ( 26Al/ 27Al) 0 [(5.85–7) × 10 −5] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite–grossite-rich CAI El Djouf 001 MK #5 has uniformly 16O-rich (Δ 17O = −23.0 ± 1.7‰) composition, but shows a deficit of 26Mg and no evidence for 26Al. Because this inclusion is 16O-rich, like CAIs with the canonical ( 26Al/ 27Al) 0, we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical 26Al abundance. Another 16O-enriched (Δ 17O = −20.3 ± 1.2‰) inclusion, a spinel–melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (∼11 and 23‰/amu, respectively), a deficit of 26Mg, and a relatively low ( 26Al/ 27Al) 0 = (2.0 ± 1.7) × 10 −5. This could be the first FUN ( Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly 16O-depleted compared to most CR2 CAIs and has lower than the canonical ( 26Al/ 27Al) 0, it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over 0.9 + 2.2 - 0.7 My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Δ 17O ranges from −23.5‰ to −1.7‰), suggesting dilution by 16O-depleted chondrule material and possibly exchange with an 16O-poor (Δ 17O > −5‰) nebular gas. The low inferred ( 26Al/ 27Al) 0 ratios of these CAIs (<0.7 × 10 −5) indicate melting >2 My after crystallization of CAIs with the canonical ( 26Al/ 27Al) 0 and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly 16O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an 16O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid–solid isotope exchange on the CV asteroidal body rather than gas–melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.

Carbon and oxygen isotopic composition of carbonate cements of different phases in terrigenous siliciclastic reservoirs and significance for their origin: A case study from sandstones of the Triassic Yanchang Formation, southwestern Ordos Basin, China

Early carbonate cements in the Yanchang Formation sandstones are composed mainly of calcite with relatively heavier carbon isotope (their δ18O values range from −0.3‰–−0.1‰) and lighter oxygen isotope (their δ18O values range from −22.1‰–−19.5‰). Generally, they are closely related to the direct precipitation of oversaturated calcium carbonate from alkaline lake water. This kind of cementation plays an important role in enhancing the anti-compaction ability of sandstones, preserving intragranular volume and providing the mass basis for later dissolution caused by acidic fluid flow to produce secondary porosity. Ferriferous calcites are characterized by relatively light carbon isotope with δ13C values ranging from −8.02‰ to −3.23‰, and lighter oxygen isotope with δ18O values ranging from −22.9‰ to −19.7‰, which is obviously related to the decarboxylation of organic matter during the late period of early diagenesis to the early period of late diagenesis. As the mid-late diagenetic products, ferriferous calcites in the study area are considered as the characteristic authigenic minerals for indicating large-scaled hydrocarbon influx and migration within the clastic reservoir. The late ankerite is relatively heavy in carbon isotope with δ13C values ranging from −1.92‰ to −0.84‰, and shows a wide range of variations in oxygen isotopic composition, with δ18O values ranging from −20.5‰ to −12.6‰. They are believed to have nothing to do with decarboxylation, but the previously formed marine carbonate rock fragments may serve as the chief carbon source for their precipitation, and the alkaline diagenetic environment at the mid-late stage would promote this process.

Seasonal variation in oxygen isotope composition of waters for a montane larch forest in Mongolia

Measurements of water oxygen isotopic composition were conducted in the 2003 growing season for a montane larch (Larix sibirica Ledeb.) forest in northern Mongolia, a transitional area from the south Siberian taiga to the Asian steppe. Oxygen isotopic composition of foliar water and its daily variability were found to be sensitive to atmospheric evaporative demand. During most of the growing season, water sources used by larch trees were from the upper 30-cm surface layer of the soil when precipitation input was large, and were from the deeper layer when the water supply at the upper soil layer was limited. The Keeling plot method suggested that the forest returned soil water to the atmosphere predominantly by means of canopy transpiration during the peak growth period (in August).

Planetary accretion, oxygen isotopes, and the central limit theorem

Abstract— The accumulation of presolar dust into increasingly larger aggregates such as calcium‐aluminum‐rich inclusions (CAIs) and chondrules, asteroids, and planets should result in a drastic reduction in the numerical spread in oxygen isotopic composition between bodies of similar size, in accord with the central limit theorem. Observed variations in oxygen isotopic composition are many orders of magnitude larger than would be predicted by a simple, random accumulation model that begins in a well‐mixed nebula, no matter what size objects are used as the beginning or end points of the calculation. This discrepancy implies either that some as yet unspecified but relatively long‐lived process acted on the solids in the solar nebula to increase the spread in oxygen isotopic composition during each and every stage of accumulation, or that the nebula was heterogeneous (at least in oxygen) and maintained this heterogeneity throughout most of its nebular history. Depending on its origin, large‐scale nebular heterogeneity could have significant consequences for many areas of cosmochemistry, including the application of well‐known isotopic systems to the dating of nebular events and the prediction of bulk compositions of planetary bodies on the basis of a uniform cosmic abundance. The evidence supports a scenario wherein the oxygen isotopic composition of nebular solids becomes progressively depleted in 16O with time due to chemical processing within the nebula, rather than a scenario where 16O‐rich dust and other materials are injected into the nebula, possibly causing its initial collapse.

Regional variations in oxygen isotopic compositions in the Yeomanand Duperow aquifers, Williston basin (Canada-USA)

Oxygen isotope compositions of formation waters from drill stem tests and produced waters are reported from the Yeomanand Duperow aquifers across the Williston basin. Mapped isotopic distributions vary across the basin from <− 24% δ 18O to values over +9.4% δ 18O, relative to Standard Mean Ocean Water (SMOW). Areal distributions of oxygen isotopes support a complex model of paleohydrogeology of the basin. They can be used to trace meteoric recharge into the basin, identify slow or stagnant brine-flow areas, and the presence of glacially induced recharge water deep in the subsurface.

Oxygen-isotope (δ18O) evidence of Holocene hydrological changes at Signy Island, maritime Antarctica

A record of Holocene hydrological changes has been produced from variations in oxygen-isotope composition (δ18O) preserved in freshwater lake sediments from maritime Antarctica. Small amounts (&lt;5%) of authigenic carbonate have been extracted from a non-marl sediment core from Sombre Lake, Signy Island (60°43'S, 45°38'W). Oxygen-isotope and particle-size analysis provide a sensitive record of hydrological events in the lake arising from local and regional climate phenomena. The climate affects δ18O through snowpack volume and glacier activity in the catchment, lakewater temperatures, the input versus evaporation balance and the duration of seasonal lake ice cover. The most depleted (negative) δ18O values are associated with influxes of meltwater at times of glacier advance or retreat. Enriched (positive) δ18O values occur during more arid, warmer conditions with longer periods of open water in summer. This isotope record can be used to determine century-scale to decadal variability in air circulation and moisture origin. Strong similarities with other Holo cene proxy records from the Weddell Sea and Antarctic Peninsula Region are apparent, including the mid-Holocene climate optimum followed by the Neoglacial and, most recently, late twentieth-century climatic warming. The oxygen-isotope record from Sombre Lake illustrates the importance of remote islands in contribu ting to our understanding of teleconnections in atmospheric and oceanographic circulation, sea-ice extent, air temperatures and precipitation in the Southern Ocean.

A multiproxy approach to reconstructing sea surface conditions using coral skeleton geochemistry

We use elemental ratio (Mg/Ca, Sr/Ca, U/Ca and Ba/Ca) and oxygen isotope data from a Porites lutea coral head collected from offshore Amédée Island, New Caledonia (22°29′S, 166°28′E) to assess the degree to which changes in these geochemical variables reflect variations in sea surface conditions. We have assessed the robustness of each geochemical proxy by comparing 25 years (1992–1968) of monthly geochemical variations with a local record of sea surface temperature (SST) and with the appropriate 1° by 1° grid box from a global gridded SST data set. We conclude from our comparison that the most consistent proxies of monthly SST variations at this site are δ18O (r2 = −0.84; p = &lt;.0001) and Sr/Ca (r2 = −0.84; p = &lt;.0001). The fidelity of the coral‐based Sr/Ca‐SST proxy was assessed via calibration‐verification exercises at New Caledonia and Rarotonga. The “paleoclimate” accuracy of the coral Sr/Ca‐SST technique is judged to be robust, and the precision of the technique is estimated to be ∼0.3° ± 0.5 (2σ) based on the calibration‐verification exercise. In contrast, monthly variations in Mg/Ca and U/Ca ratios are only moderately correlated with monthly SST variations (r2 = 0.55; p = &lt;.0001 for Mg/Ca and r2 = 0.47; p = &lt;.0001). The strength of the correlation between SST and Mg/Ca and U/Ca varies as a function of time, suggesting that variations in these ratios are not a simple function of SST variations. This variability in goodness of fit between U/Ca‐SST and Mg/Ca‐SST may ultimately limit the use of these ratios as coral paleothermometers. Averaging or stacking individual proxy SST records (Sr/Ca, Mg/Ca and U/Ca) to generate a composite proxy SST record does not necessarily improve the proxy record if the signal‐to‐noise ratio in each proxy is highly variable. For example, stacking of Sr/Ca‐Mg/Ca‐U/Ca records at New Caledonia produces a proxy SST time series that has greater standard error than the individual Sr/Ca‐SST time series (cf. ordinary least squares standard error of 1.16°C versus 0.86°C). Ba/Ca variations have little correlation with SST, Sr/Ca, Mg/Ca, or U/Ca variations at New Caledonia. In conclusion, variations in oxygen isotopic composition of seawater, calculated by removing the Sr/Ca‐based temperature component of the δ18O signal, agree reasonably well with observed variations in sea surface salinity, especially at the interannual timescale.

Oxygen isotope and trace element zoning in hydrothermal garnets: Windows into large-scale fluid-flow behavior

Single garnet crystals from a large, multicyclic hydrothermal system in Far East Russia are strongly growth zoned. Microscale ion microprobe analysis reveals consistent intracrystalline patterns in oxygen isotope composition and boron and iron contents, reflecting evolution of the hydrothermal system from magmatic- to meteoric-dominated conditions, and repetitive influx of magmatic fluids. Large intracrystalline variations in oxygen isotope composition (>12‰ δ 18 O) delimit temporal evolution of magmatic and meteoric fluid input within different portions of the hydrothermal system, and suggest that fluid-flow regimes differed between central and distal portions of the system. These results provide compelling evidence that microanalytical data can be extrapolated from intracrystalline scales to study macroscale processes in complex, large-scale fluid-rock systems.

Seasonal Variation in Oxygen Isotopic Composition of Two Freshwater Bivalves: Sphaerium striatinum and Anodonta grandis

Oxygen isotopic values have been obtained from microsamples of the aragonitic freshwater bivalves Sphaerium striatinum ( Pisidiidae) and Anodonta grandis ( Unionidae) collected alive from Wellington Creek, OH. To test whether these organisms secrete their shell in isotopic equilibrium, the SO values of shell aragonite are compared to ambient water temperature and δ 18O values monitored for > 1 yr. These bivalves were chosen for study because they are abundant in surface sediments and cores from Lake Erie where they represent a source of information on the environmental history of the lake. The observed mean values are −5.54‰ for A. grandis and −6.16‰ for S. striatinum. The mean δ 18O value expected for bivalve aragonite if equilibrium precipitation is occurring during May–August in Wellington Creek is −5.69‰. The similarity between measured and predicted isotopic values for both species suggests that they are useful sources of paleoenvironmental data. Overall, the isotopic composition of the shells of the two species reflects less than one half of the calculated range of potential biogenic aragonite values for the stream and omits recording evaporative conditions associated with ponded water. Bivalve δ 18O and δ 13C data covary. The δ 13C data are highly negative and values could reflect 12C enrichment of dissolved organic carbon from organic matter oxidation and/or ingestion of food carbon.