High 18O Research Articles

Dolomitization and Dolomite Neomorphism: Trenton and Black River Limestones (Middle Ordovician) Northern Indiana, U.S.A.

ABSTRACT The Trenton and Black River Limestones are dolomitized extensively along the axis of the Kankakee Arch in Indiana, with the proportion of dolomite decreasing to the south and southeast of the arch. Planar and nonplanar dolomite replacement textures and rhombic (type 1) and saddle (type 2) void-filling dolomite cements are present. Three stages of dolomitization, involving different fluids, are inferred on the basis of petrographic and geochemical characteristics of the dolomites. Nonferroan planar dolomite has relatively high 18O values (-1.8 to -6.1o/oo PDB) and has 87Sr/86Sr ratios (0.70833 to 0.70856) that overlap those of Middle Ordovician seawater. Petrography, geochemistry, and the geometry of the dolomitized body suggest that the planar dolomite was formed in Middle and Late Ordovician seawater during the deposition of the overlying Maquoketa Shale. Ferroan planar and nonplanar dolomite occurs in the upper few meters of the Trenton Limestone, confined to areas underlain by planar dolomite. This dolomite contains patches of nonferroan dolomite with cathodoluminescence (CL) characteristics similar to underlying planar dolomite. Ferroan dolomite has relatively low 18O values (-5.1 to -7.3o/oo PDB) and has slightly radiogenic 87Sr/86Sr ratios (0.70915 to 0.70969) similar to those obtained for the overlying Maquoketa Shale. These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Maquoketa Shale during burial. The absence of ferroan dolomite at the Trenton-Maquoketa contact, in areas where the earlier-formed nonferroan planar dolomite also is absent, indicates that the fluid expelled from the overlying shale did not contain enough Mg2+ to dolomitize limestone. Type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite, suggesting that it also formed from shale-derived burial fluids. CL growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the Kankakee Arch. Type 2 saddle dolomite cements precipitated late; their low 18O values (-6.0 to -7.0o/oo PDB) are similar to those of the type 1 dolomite cement. However, fluid-inclusion data indicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94° to 143°C) than the type 1 cements (80° to 104°C). A trend of decreasing fluid-inclusion homogenization temperatures and salinities from the Michigan Basin to the axis of Kankakee Arch suggests that these fluids emerged from the Michigan Basin after precipitation of type 1 cement.

Oxygen reduction sites and diffusion paths at La 0.9Sr 0.1MnO 3− x/yttria-stabilized zirconia interface for different cathodic overvoltages by secondary-ion mass spectrometry

Oxygen reduction active sites were investigated at the interface of O 2/La 0.9Sr 0.1MnO 3− x (LSM)/yttria-stabilized zirconia (YSZ) for three different overvoltages of cathodic polarization ( η=−0.336 V, η=−0.185 V, and η=−0.090 V versus reference electrode). Isotopic oxygen ( 16O/ 18O) exchange under cathodic polarization and secondary ion mass spectrometry (SIMS) technique were examined to visualize the oxygen incorporation/reduction active sites. The LSM mesh pattern electrode was prepared to define the area and length of contact between LSM mesh and YSZ. Under cathodic polarization of η=−0.336 V, oxide ions can diffuse through the LSM to take part in the electrode reaction. From the SIMS images of YSZ surface, the active sites for oxygen reduction/incorporation are distributed in spots at the position where the LSM mesh was attached. Especially, O 2/LSM/YSZ three phase boundary (TPB) is the main active site for oxygen incorporation/reduction, although some spots of high 18O concentration were observed at the center of the LSM mesh part. Under the lower polarization ( η=−0.185 V, and η=−0.090 V), oxide ions did not diffuse through the LSM mesh. SIMS imaging analysis of YSZ surface showed high 18O concentration spots around the center of the LSM mesh part. The distribution of ionic currents at the surface of YSZ affects the distribution of active sites for oxygen incorporation.

Analysis of concomitant18O in13C-urea

The 13C-urea breath test by infra-red absorption or mass spectra has been extensively used to detect. Helicobacter pylori infection. However, commercially available 13C-urea contains higher 18O, compared to natural urea. It is evidently important for this diagnosis to investigate the 18O-isotope effect on infra-red absorption or mass spectra, based on the quantitative analysis of 18O in 13C-urea. For this purpose, a new analytical method using mass spectroscopy has been established to determine 18O abundance in urea. Copyright © 1999 John Wiley & Sons, Ltd.

Stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the Duluth Complex, Minnesota

Thin (3–20 m), picritic to noritic sills are found in the Proterozoic metasedimentary footwall rocks of the Duluth Complex, Minnesota. The sills were emplaced prior to the major intrusions that comprise the Duluth Complex, and underwent contact metamorphism along with the country rocks. Oxygen, hydrogen, and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals, melts, and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the Midcontinent Rift system. The pre-Duluth Complex sills exhibit a range in δ 18O values from 4.9‰ to 14.8‰, with values between 6‰ and 7‰ generally found in sill interiors. High δ 18O values near sill contacts with high 18O metasedimentary rocks of the pelitic Virginia Formation or Biwabik Iron Formation, coupled with a smooth sigmoidal isotopic profile centered at the contact, suggest that oxygen diffusion was an important exchange mechanism. The elevated δ 18O values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma. Dehydration reactions in the pelitic rocks of the contact aureole liberated high 18O fluids that enhanced subsolidus diffusive exchange. Advective displacement of the diffusion profiles toward the sill interior is less than 40 cm, and suggests that layer parallel flow dominated in the dehydration of the contact aureole. Elevated, but uniform δ 18O values (9.7‰ to 10.5‰) in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity, perhaps related to extensive development of microcracks. Although δD values of the pelitic country rocks record a history of dehydration, systematic variations of δD (−64‰ to −143‰) and H 2O (0.15 to 5.40 wt.%) content are not found in the sills. δ 18O values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium, and are consistent with a diffusion-dominant exchange process at temperatures near 500°C. Results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1.4 Ma, depending on local controls of porosity and permeability, as well as rates of fluid production in the contact aureole. Localized areas of 18O and D depletion in the sills (values as low as 4.9‰ and −143‰, respectively) denote exchange with meteoric water after interaction with the high 18O metamorphic fluids in the contact aureole. Although all of the elevated 18O samples in the contact environment may have suffered 18O depletion, most exchange with meteoric water appears to be spatially localized, and is thought to reflect highly channelized, fracture-controlled fluid flow. Sulfur isotopic values of the sills are variable (−2.7‰ to 11.2‰), and indicative of an evolution involving pre-emplacement contamination of basaltic magma, and sub-solidus exchange with an H 2S-bearing metamorphic fluid. Sulfur contents exceed 3.0 wt.% only within troctolitic to melatroctolitic sills, and δ 34S values of 7.8‰ to 8.3‰ are strongly suggestive of pre-emplacement contamination by sulfur derived from a Proterozoic sedimentary unit. High δ 18O and δ 34S rocks, particularly at sill margins, are consistent with either hydrothermal precipitation of fine-grained sulfide minerals, or isotopic exchange between magmatic sulfides and an H 2S-bearing metamorphic fluid.

Multielement stable isotope ratio analysis of glycerol to determine its origin in wine

The adulteration of wine with glycerol is considered to be a problem in European wine-producing countries. The latest control methods are mainly based on the detection of impurities from commercial products, but suffer from the raising efficiency of the purification processes. As there is little chance of being able to identify glycerol from different sources on the basis of a method which uses only one isotope, a multielement approach was tested. Glycerol from wine showed the lowest relative enrichment with D, mainly in position C-2, a relatively high 18O content, and very negative δ13C values, which significantly correlated with those of ethanol from the same wines. The isotopic data of glycerol samples from different sources were in agreement with those given by indices of origin (impurities). These data allowed us to identify the origin of these glycerol samples, i.e. whether they were produced industrially or synthesised by animals or plants. Glycerol of plant origin was most similar to glycerol found in wine. The combination of several isotopic data by discriminance analysis yielded clusters of data obtained from glycerol samples of similar origin. Taking into account the characteristics of possible mixtures, proof that wine has been adulterated depends on the origin and isotope levels of the added compound. This study showed that it is possible to prove that wine has been adulterated with glycerol from other sources when the latter is present at a concentration of 15% of the total glycerol content.

Geochemical and stable isotope patterns in altered volcaniclastic and intrusive rocks of the Kruidfontein carbonatite complex, South Africa

The inner zone of the Kruidfontein Complex consists largely of carbonatitic volcaniclastic breccia with interbedded, massive to well-bedded, tuffaceous sedimentary units dipping concentrically at > 50° around the margin, the dip becoming shallower towards the centre. A geochemical study has revealed the formation of siderite, ankerite, fluorite, K-feldspar, hematite, quartz, apatite, calcite and chlorite as alteration minerals. Different generations of fluorite have been distinguished by variation in Rare Earth Element (REE) contents and Light REE (LREE) to Heavy REE (HREE) ratios. One or more of these phases correlates positively with the rare-earth minerals, whilst others show a negative correlation. The C and O isotope patterns point to three major stages of development for the inner zone of the complex. Stage one involves carbonatite magma which originated from fractionation with a H 2OCO 2 vapour phase, at 700°C in a closed system with a H 2O/CO 2 molar ratio of 0.5. The second stage of the development is recorded in the intrusive carbonatitic dykes and in at least part of the carbonatitic ash flow tuffs, by a shift to higher δ 18O values without major changes in the δ 13C values. This is a consequence of a secondary alteration for which a temperature of approximately 150°C has been estimated. The isotopic characteristic of the alteration fluid could be determined as δ 18O H2O = 1.2% and δ 13C CO2 = −3.8%; a possible source being a hydrothermal fluid which introduced isotopically light C from the deep subvolcanic area and mixed with high 18O ground water that had equilibrated at about 150°C. The third stage is characterised by siderite mineralisation, with a concomitant precipitation of fluorite. Siderite appears to be secondary to the other carbonates and can thus be assumed to have formed at temperatures at or below 150°C. The secondary nature of the siderite mineralisation is re-inforced by a negative correlation between the whole rock carbonate δ 18O values and the modal siderite content. By application of siderite/water and siderite/CO 2 fractionation factors from Carothers et al. (1988), the C- and O-isotope composition of the siderite precipitating fluid, was approximated as δ 13C CO2 = −8.0% and δ 18O H2O = −1.4%. From these findings it is concluded that siderite found in the Kruidfontein Complex is likely to have been precipitated by a late-stage Fe-bearing hydrothermal fluid, depleted in 18O and 13C. Carbon was provided by a deep-seated source within the subvolcanic region and Fe was leached from rocks that the hydrothermal fluid passed through. The siderite mineralisation and concomitant fluorite formation most probably took place during progressive cooling of the Kruidfontein Complex, as can be deduced from the positive correlation between the δ 18O and δ 13C values of siderite.

Diagenetic Mineralogy, Geochemistry, and Dynamics of Mesozoic Arkoses, Hartford Rift Basin, Connecticut, U.S.A.

ABSTRACT The main event in the diagenetic history of the Hartford basin was the arrival of the rift-associated to its arkosic fill, in the form of both high heat flow and diabase dikes, sills, and basalt flows about 187 ± 3 Ma. Pre-heat-wave (or pre-basalt) diagenetic minerals are widespread throughout the basin, and include hematite cement, quartz and albite overgrowths, and minor euhedral rutile. This widespread distribution suggests that they grew in a regime of generally downward-migrating meteoric water. The silica, aluminum, and sodium needed to make these cements were released probably by tropical weathering of the top of the arkose itself, a few hundred meters overhead. The heat wave of which basalts and dikes were part suddenly heated the basin, and radically determ ned the subsequent diagenesis of the arkoses: (1) it drove pore-water convection through sills and dikes and arkoses, erasing the earlier meteoric-water regime; (2) it caused quick growth of post-basalt diagenetic minerals (chert and mosaic albite cement, illite and chlorite cement, and fibrous laumontite cement with bipyramidal quartz euhedra) mostly at localized occurrences, the updrafts of pore-water convection cells; (3) it modified the texture of both quartz and albite cements from overgrowth to microcrystalline; and (4) the dikes, sills, and basalt flows probably provided much of the magnesium, ferrous iron, potassium, and copper needed to make the illite + chlorite cement and copper sulfide nodules found in the arkoses. The radiogenic age of the illite cement from one locality i 180 (± 10) Ma, close to the age of the basalts and dikes. This temporal proximity warrants linking the growth of illite to the sudden heating and the igneous intrusion. High 18O content of the diagenetic illite suggests an origin from relatively heavy water that could have been produced in surficial evaporative environments of the rift basin, and that later sank. Combined petrographic, K/Ar, geochemical, fission-track, and dynamic-modeling evidence and interpretations lead to a generalized time-temperature profile for the basin. Because of their distinctive tectono-thermal origin, rift basins probably have a distinctive diagenetic history. The diagenesis and dynamics sketched here for the Hartford rift basin, including some of the unusual mineralogical and textural details in its arkoses, may apply to other rift basins.

Description of active sites on molybdenum oxide as detected by isotope exchange between C 18O 2 and Mo 16O 3

The oxygen exchange reaction between C 18O 2 and MO 16O 3 has been studied on well characterized MoO 3 crystallites. Temperature programmed desorption of adsorbed C 18O 2 has shown that the extent of surface oxygen exchange is higher on partially reduced samples exposing the (010) face. This has been related with the easier reducibility of the (010) face. Moreover, experiments in a recycle reactor at moderate reaction temperature (623K) suggest that surface oxygen exchange is more rapid on the basal (010) face. On the other hand, when the experiment is run at higher reaction temperature (773K), using a higher 18O(C 18O 2)/ 16O(MoO 3) ratio and for a longer period, a continuous oxygen exchange reaction occurs, specially with samples presenting lateral faces. In this latter conditions, two contributions to the exchange reaction can be distinguished. The first corresponds to a fast surface oxygen exchange. The second contribution which is more prominent over samples with (100) faces can be assigned to migration of oxygen atoms from lateral towards basal faces and/or to a slow migration of oxygen atoms from the bulk structure of MoO 3 towards the lateral faces. From the results of this study it is speculated on the importancë of a preferential oxygen migration from the bulk MoO 3 towards the lateral faces, which is related with the selective partial oxidation of olefins observed on these faces.

Two-Phase Diagenesis of Quaternary Carbonates, Arabian Gulf: Insights from 13C and 18O Data

ABSTRACT Samples from a linear transect of borings in the shallow marine waters offshore of Al Jubayl, Saudi Arabia, represent dolomitic deposits (dolomitic facies) overlain by low-magnesium calcitic deposits (calcitic facies) and topped by a veneer of aragonitic sediments (aragonitic facies). The dolomitic fades, with its associated evaporites, accumulated and was diagenetically altered as part of Pleistocene sabkha complexes. The calcitic facies is composed of a wide variety of shallow marine Pleistocene carbonates and is petrographically indicative of diagenesis in a meteoric phreatic realm. The aragonitic sediments are Holocene and have undergone only minor shallow marine diagenetic alteration. Whole-rock stable-isotope data (26 aragonitic and 112 calcitic samples) show that the calcitic deposits have undergone two distinct episodes of diagenesis. (1) During Pleistocene sea-level lowstand(s), diagenesis in the meteoric vadose realm altered these originally aragonitic and high-magnesium calcitic sediments, with estimated mean 18O and 13C values of 0.0 and 3.3 PDB, respectively, to low-magnesium calcites, with mean 18O and 13C values of -3.5 and 2.0 PDB, respectively. (2) Under the present sedimentological regime, sabkha-derived evaporative waters, with relatively high 18O values, descended from the modern coast, mixed with the regional groundwater, and flowed laterally gulfward within a confined phreatic aquifer (i.e, the calcitic facies) beneath the marine waters of the Arabian Gulf. Although the calcitic samples display diagenetic trends because of the descent and concomitant lateral migration of waters beneath an exposure surface, these trends differ from those recognized in similar studies. The subsequent diagenetic overprinting by the sabkha-derived surface waters has increased the 18O values as much as 4. Additionally, an overprint of the waters' path can be recognized to the seaward end of the sample transect, almost 10 km from the shoreline, by a gradual decrease gulfward in the mean whole-rock 18O values within the calcitic facies. In contrast to analogous studies, diagenesis of the Al Jubayl carbonates due to the sabkha-derived waters resulted in 18O values that are widely distributed, whereas the 13C values are narrowly distributed, and the sediments show the highest 18O values where the water had the greatest influence in the water-rock interaction; these trends are opposite to those of most other diagenetic sequences.

A Global Analysis of the Oxygen Isotope Composition of Basalts: Evidence for Low and High 18O Reservoirs in the Mantle

It is now well established that the mantle is compositionally heterogeneous on a variety of scales. The ultimate cause for these heterogeneities is the recycling of continental and oceanic crust via subduction into the mantle. Stable isotope studies have the potential to make important contributions to studies of mantle heterogeneity, but their application has lagged behind radiogenic isotopes. In this contribution we examine oxygen isotope variations in basalts and discuss their causes. Oxygen isotopes are strongly fractionated during low-temperature processes such as weathering and hydrothermal alteration. Therefore, subduction related processes should provide an effective mechanism of introducing oxygen of variable composition into the mantle.

Derivation of the formyl-group oxygen of chlorophyll b from molecular oxygen in greening leaves of a higher plant ( Zea mays)

Using mass spectroscopy, we demonstrate as much as 93% enrichment of the 7-formyl group oxygen of chlorophyll b when dark-grown, etiolated maize leaves are greened under white light in the presence of 18O 2. This suggests that a mono-oxygenase is involved in the oxidation of its methyl group precursor. The concomitant enrichment of about 75% of the 13 1-oxygen confirms the well-documented finding that this oxo group, in both chlorophyll a and b, also arises from O 2. High 18O enrichment into the 7-formyl oxygen relative to the substrate 18O 2 was achieved by optimization of the greening conditions in combination with a reductive extraction procedure. It indicates not only a single pathway for Chl b formyl group formation, but also unequivocally demonstrates that molecular oxygen is the sole precursor of the 7-formyl oxygen.

Stable isotope imaging of a dynamic groundwater system in the southwestern Sacramento Valley, California, USA

A dynamic image of a shallow (45–160 m below the surface) aquifer in a 25km 2 area in the southwestern Sacramento Valley, California has been obtained by 18 O 16 O and D/H determinations of groundwater from municipal wells. The regular summertime drawdown of the water table is strongly correlated with municipal-wide increases in the δ 18O values, particularly in wells with shallow perforation levels, thus demonstrating that the high 18O water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates. These 1.5‰ enrichments in the δ 18O values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m. The increased δ 18O values correlate with increased deviation from the meteoric water line and with increased nitrate levels, indicating that the high 18O groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen-fertilized, flood-irrigated cropland. Isotopic gradients provide a new method for qualitatively determining subsurface permeability. Zones of low isotopic gradients, interpreted as high 18O plumes, probably follow coarse-textured deposits of ancient stream channels. The 18O gradients are primarily controlled by subsurface permeability, although additional influences may include: (1) the heterogeneous distribution of the high 18O groundwater; (2) isotopic contributions from deeper groundwater; (3) variable perforation depths of the wells; (4) local differences in the water table level. Deeper, sodium bicarbonate groundwaters (300–650 m below the surface) have lower δ 18O values (to −9.4 ‰) than the more shallow magnesium bicarbonate groundwaters, suggesting derivation from 18O depleted meteoric groundwaters of the Sierra Nevada. Minor mixing of the deeper groundwater into the municipal discharge occurs, even though previous workers considered the deeper groundwater to be confined.

Elemental and Isotopic Composition of Siderite in the Kuparuk Formation, Alaska: Effect of Microbial Activity and Water/Sediment Interaction on Early Pore-Water Chemistry

ABSTRACT The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin. Authigenic siderite is common throughout the formation, either concentrated in distinct layers or as isolated rhombs. Examination of siderite-rich samples with back-scattered electron imaging reveals complex compositional zonation in the siderite. The samples are principally composed of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich siderite, with mean compositions of (Fe77.5Mg13.8Ca7.9Mn0.7 )CO3 and (Fe55.8Mg32.8Ca10.8Mn0.6 )CO3, respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderit s are most abundant in non-glauconitic sandstones and mudrocks. The whole-rock isotopic composition of the siderite is highly variable, with 13C values ranging from -20.3 to 7.9 PDB, and 18O values from 21.4 to 31.1 SMOW. This variation in isotopic composition correlates with the relative proportions of the two siderite types in a given sample. The Fe-rich siderites have low 13C and high 18O values, whereas the Mg-rich siderites have high 13C and low 18O values. Both the elemental and carbon isotopic compositions of the siderite result from modification of the original marine pore waters during successive stages of microbial decomposition of organic matter. The oxygen isotopic compositions of the siderite indicate that early pore waters were depleted in 18O, perhaps as a result of water/sediment interaction. Authigenic ankerite associated with the siderite-bearing samples is unzoned and has a mean composition of Ca1.11 Fe0.35Mg0.53(CO3)2. The ankerites have higher 13C values and lower 18O values than the Fe-rich siderite, but have 13C values less than and 18O values greater than (with one exception) that of the Mg-rich siderite. The isotopic data suggest that ankerite precipitation began prior to precipitation of Mg-rich siderite and ended subsequent to or during Mg-rich side ite precipitation.

Water relations of coastal plant communities near the ocean/freshwater boundary.

Salinity and isotope ratios were determined in water from several wells in the Florida Keys, and tidal inlets. Both D/H and 18O/16O ratios of water from wells and tidal inlets were highly correlated to their salinity. Water from standing pools was enriched in deuterium and oxygen-18 relative to their salinity because of evaporation processes. 18O/16O and D/H ratios of stem water from plants of several different communities at Sugar Loaf Key, ranging from hardwood hammocks to mangroves, were highly correlated to their predawn water potential. The correlation was consistent with the presence of high salinity in waters with high 18O and D content. Most individuals from each community were either utilizing water with isotopic characteristics typical of freshwater or of ocean water, while only a few individuals had stem water with isotopic ratios intermediate to these two water sources.

Paleoclimatic Controls on Stable Oxygen and Carbon Isotopes in Caliche of the Abo Formation (Permian), South-Central New Mexico, U.S.A.

ABSTRACT The stable oxygen and carbon isotopic composition of caliche in fluvial and supratidal rocks of the Abo Formation (Permian), south-central New Mexico, is controlled by paleoclimate and depositional environment. Fluvial caliche consists of low-Mg calcite nodules and vertically oriented tubules that display stage II texture. Micrite matrix support, brecciation, ooids/pisoliths, aveolar-septal texture, and peloids are common in the fluvial caliche and, along with red color and slickensides in the host shale, indicate pedogenesis in a well-oxidized vadose zone. In contrast, periodic waterlogging of the supratidal paleosols, probably due to high water table, is indicated by drab colors, carbonaceous flecks, horizontal rhizoliths, and the paucity of vadose textures in the stage II caliche n dules. Stable oxygen isotopes are similar in the fluvial and supratidal caliches and range from 21.6 to 30.5 (SMOW). The data exhibit a crude bimodality and 18O enrichment with a decrease in age (higher in the section). Consideration of these data in the context or -temperature relations suggests that 1) surface waters responsible for caliche formation increased in 18O (from roughly -8 to + 1) over the 18 m.y. time interval that separated the lowest stratigraphic nodule horizon from the highest, 2) the inc easing 18O values also reflect a warming trend (approximately 15° to nearly 3°C) in the mean monthly temperature over this same time period, with perhaps an associated increase in Permian ocean temperatures, and 3) the significant variation in 18O from oldest to youngest caliche was probably enhanced by the effect, such that as the temperature increased, the amount of precipitation decreased, resulting in high 18O values. Caliches in the Abo are enriched in heavy carbon (-7.2 to -1.5 PDB) compared to that of soil carbonate derived exclusively from C, plants (-12 PDB), and the supratidal caliches contain somewhat heavier carbon compared to the fluvial caliche. The 13C values for both environments increase with a decrease in caliche age. These results indicate that as the temperature increased and rainfall decreased with time, the level of C3 plant productivity apparently declined, allowing a greater influx of atmospheric CO2 into the soil. This can only occur when soil respiration rates are quite low or at very sha low depths (less than 10 cm), or both. Atmospheric CO2 seems to have invaded the supratidal soils to a somewhat greater extent than the fluvial soils.

Cryosphere/ocean interactions at the margin of the Laurentide Ice Sheet during the Younger Dryas Chron: SE Baffin Shelf, northwest Territories

Cores HU82‐034‐057 and HU84‐035‐008, Resolution Basin, SE Baffin Shelf, contain 200 and 450 cm, respectively, of sediment that spans the Younger Dryas chron. In both cores the interval is bracketed by 14C dates on foraminifera or molluscs. These sites were close to the margin of the late Wisconsin (Foxe) ice sheet as it flowed toward the Labrador Sea. Prior to 11 ka, both cores record moderate to high accumulations of foraminifera, relatively high del 18O values in planktonic foraminifera, and low values of detrital carbonate. The diatom and percent opal records imply occasional seasonally open water conditions. During part of the Younger Dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production, suggesting surface water conditions affected by increased sea ice and/or reduced nutrients. In addition, the Younger Dryas interval is marked by an increase in coarse sand and detrital carbonate, a decrease in total organic carbon and foraminifera, and high rates of sediment accumulation. The inferred environment during the Younger Dryas is ice‐proximal. In HU82‐034‐057, the foraminifera and other data suggest a change in conditions during the middle part of the Younger Dryas chron.

Evidence for a Universal Pathway of Abscisic Acid Biosynthesis in Higher Plants from 18O Incorporation Patterns

Previous labeling studies of abscisic acid (ABA) with (18)O(2) have been mainly conducted with water-stressed leaves. In this study, (18)O incorporation into ABA of stressed leaves of various species was compared with (18)O labeling of ABA of turgid leaves and of fruit tissue in different stages of ripening. In stressed leaves of all six species investigated, avocado (Persea americana), barley (Hordeum vulgare), bean (Phaseolus vulgaris), cocklebur (Xanthium strumarium), spinach (Spinacia oleracea), and tobacco (Nicotiana tabacum), (18)O was most abundant in the carboxyl group, whereas incorporation of a second and third (18)O in the oxygen atoms on the ring of ABA was much less prominent after 24 h in (18)O(2). ABA from turgid bean leaves showed significant (18)O incorporation, again with highest (18)O enrichment in the carboxyl group. The (18)O-labeling pattern of ABA from unripe avocado mesocarp was similar to that of stressed leaves, but in ripe fruits there was, besides high (18)O enrichment in the carboxyl group, also much additional (18)O incorporation in the ring. In ripening apple fruit tissue (Malus domestica), singly labeled ABA was most abundant with more (18)O incorporated in the tertiary hydroxyl group than in the carboxyl group of ABA. Smaller quantities of this monolabeled product (C-1'-(18)OH) were also detected in the stressed leaves of barley, bean, and tobacco, and in avocado fruits. It is postulated that a large precursor molecule yields an aldehyde cleavage product that is, in some tissues, rapidly converted to ABA with retention of (18)O in the carboxyl group, whereas in ripening fruits and in the stressed leaves of some species the biosynthesis of ABA occurs at a slower rate, allowing this intermediate to exchange (18)O with water. On the basis of (18)O-labeling patterns observed in ABA from different tissues it is concluded that, despite variations in precursor pool sizes and intermediate turnover rates, there is a universal pathway of ABA biosynthesis in higher plants which involves cleavage of a larger precursor molecule, presumably an oxygenated carotenoid.

Oxygen and hydrogen isotopes in the volcano-sedimentary complex of Huelva (Iberian Pyrite Belt): example of water circulation through a volcano-sedimentary sequence

The minerals of basic and acidic rocks from the volcano-sedimentary sequence in the Huelva area, Spain, Iberian Pyrite Belt, display an extended δ 18O enrichment. Quartz δ 18O values from quartz-keratophyres vary from +10.5 to +17.0 and feldspar δ 18O values from +14.4 to +16.0. For the spilite or spilitized dolerites δ 18O values vary from +9.9 to +13.4 for feldspar, from +6.4 to +9.8 for chlorite, from +3.7 to +4.3 for ilmenite and from +13.6 to +14.0 for quartz, but pyroxene exhibits magmatic values, from +5.3 to +6.1 with an exception at +7.5. The chlorite δD values vary from −34 to −43‰. This is attributed to hydrothermal alteration with seawater enriched in δ 18O by circulation through sediments. The temperatures of interaction determined from isotopic fractionations between minerals range from 400° to 520°C. Calculated δD and δ 18O values for water in equilibrium with the minerals at isotopic temperatures range from −16 to +5 and from +8.3 to +12.8, respectively. A model of circulation of seawater through a pile of sedimentary rocks and then through basaltic rocks is proposed to explain the high 18O compositions of the rocks from the Huelva District. Water/rock mass ratios calculated from this model range between 0.3 and 0.7 for the determined range of temperatures.

Synthesis of regiospecifically labeled [18O]glycolic acid and [18O]acyldihydroxyacetone phosphate.

Methods are detailed for the preparation of [2-18O]glycolate from chloroacetic acid and for the direct conversion of these intermediates to regiospecifically labeled [2-18O]-2-O-acylglycolic acids containing approximately 90% 18O at the C-O-acyl bond. Methods are also detailed for optimization of reaction conditions and yields for each synthetic step in previously published methods for the preparation of 1-O-acyldihydroxyacetone-3-O-phosphate (DHAP) from acyloxyacetic acid (i.e., 2-O-acylglycolic acid), where acyl is tetradecanoyl, hexadecanoyl, or heptadecanoyl. The optimized reaction conditions generate 1-O-acyl DHAP in its acid form, both in high overall yield and in high purity, without requiring a final chromatographic purification of the product, 1-O-acyl DHAP. Combining these new methods, efficient and facile preparations of regiospecifically labeled [1-18O]-1-O-hexadecanoyl DHAP and [1-18O]-1-O-heptadecanoyl DHAP have now been demonstrated, in which approximately 90% 18O is specifically located only at the C-O-acyl position. Some mechanistic postulates are offered to account for the optimized yields, regioselectivities, and high 18O incorporation which are observed in the reactions we have employed to generate 1-O-acyl DHAP from glycolate intermediates.

Combined oxygen isotope – compositional studies of some granitoids from the Grenville Province of Ontario, Canada: implications for source regions

Oxygen isotopic compositions of whole rocks and coexisting quartz–feldspar pairs have been determined for nine pre-, and syn- to late-kinematic granitoid plutons in the Grenville Province of Ontario. These new data demonstrate that granitoid rocks (Algonquin, Mulock) in migmatite terrain of the Ontario Gneiss Segment possess normal δ18O values (<9.0‰), whereas mesozonal to epizonal plutons (Elphin, Coe Hill, Deloro, Barber's Lake) in the Central Metasedimentary Belt (CMB) are characterized by significantly higher 18O contents (δ18O > 9.0‰), in accord with previous results.In the Algonquin sodic suite, a gross covariance of δ18O with compositional indices is present, from 6.4‰, SiO2 = 50.5 wt. % (gabbro) to 8.7‰, SiO2 = 72 wt. % (trondhjemite), resulting from combined assimilation–fractional crystallization. Mafic members of the sodic suite are 18O enriched overall (5.8–7.9‰) relative to fresh tholeiites (5.7 + 0.3‰), implicating some 18O contamination of the protolith. The dispersion of δ18O values in the Algonquin potassic suite, from 4.3 to 9.3‰, is independent of composition and attributed to isotopic exchange with low-18O thermal waters during emplacement. Biotite–hornblende granite of the Mulock batholith is characterized by a limited oxygen isotope compositional range, where the average δ18O = 8.1 ± 0.5‰; δ18O correlates with SiO2 but not with the zonal distribution of Ba, Rb, and Sr abundances.The Union Lake quartz diorite (δ18O = 8.5 ± 0.1‰) and White Lake trondhjemite (δ18O = 7.3 ± 0.6‰) have oxygen isotope compositions comparable to those of other trondhjemitic suites in the CMB. A systematic enrichment of ~1.2‰ in the Union Lake pluton, together with enhanced Ca, Mg, Fe, and Sr, can be accounted for by assimilation of ~5% marbles and 10% amphibolites from the country rock. Uniformly high δ18O values of 11.5 ± 0.8‰ characterize the Elphin granite–syenite complex. The largest values (11.7–12.7‰) and lowest SiO2 (54–56 wt. %) are in the partially assimilated host gabbro–diorite complex, endorsing the presence of 18O-enriched source regions. The Cheddar biotite–hornblende granite, one of a population of intrusions within the alkalic belt of the western CMB, has a restricted isotopic span, where δ18O = 8.8 ± 0.9‰. An unusual concave rare-earth-element (REE) distribution may result from interaction with a heavy rare-earth -element (HREE) enriched volatile phase. The Coe Hill biotite granite (δ18O = 10.4 ± 0.4‰) is isotopically in compliance with other granites and syenites of the CMB. Covariance of δ18O and SiO2, in conjunction with smooth and continuous geochemical trends, is interpreted in terms of assimilation–fractional crystallization.Peralkaline granite of the Deloro pluton includes a hypersolvus phase with high, scattered δ18O values (9.1–11.8‰) and a subsolvus counterpart attributed to late influx of water that induced isotopic reequilibration toward a more constrained range (δ18O = 9.2–10.2‰). REE distributions of a calcic syenite phase are compatible with its evolution by fractional crystallization of a low-K tholeiitic magma, and the high-18O character (δ18O = 11.1–12.6‰) requires 18O enrichment of the protolith and (or) 18O contamination of the magma. Peralkaline rhyolitic volcanics, compositionally coherent with the Deloro pluton and possibly representing extrusive equivalents, possess significantly higher and more variable δ18O values, from 11.7 to 14.2‰; this is attributed to 18O enrichment during low-temperature exchange with thermal waters, superimposed on a primary high-18O magma. The Barber's Lake two-mica granite contains enhanced abundances of U (15 ppm) and Th (36 ppm) in conjunction with systematically elevated δ18O values (10.4 ± 0.5‰). Geochemical constraints are compatible with its evolution from a trondhjemitic magma, but the isotopically enriched nature requires extensive 18O contamination of the protolith and (or) magma. These nine granites variously retain "memory" of primary and (or) secondary features, including δ18O of the source region, covariance of isotopic and compositional parameters, and sporadically superimposed disturbance by exchange with thermal waters. During metamorphism, quartz and feldspar were systematically reset to high-temperature fractionations, but the extent of open-system exchange with rock reservoirs was limited.Despite some probable disturbance by metamorphism and the limited data available, O–Sr isotope systematics of the Grenville granitoids indicate that (1) high-18O granites from the Frontenac Axis were derived from in situ anatexis of Grenville Supergroup metasediments, (2) synkinematic granites were derived by mixing of a primary magma generated at a lower crustal (granulite facies) or upper mantle level with the fusion products generated by partial melting of the Archean–Early Proterozoic type metasediments, and (3) the tonalite–trondhjemite suite in this part of the Grenville Province was derived from a similar lower crustal or upper mantle primary magma by direct fractional crystallization.