18O Content Research Articles

Contributions of Lattice Oxygen in Methane Combustion over PdO-Based Catalysts

Zirconia-supported Pd16O and bulk Pd16O were used as methane combustion catalysts for a reaction mixture containing 1% methane and 4% 18O2 in helium. The methane oxidation reaction was performed in pulsed experiments and the distribution of oxygen isotopes among the reaction products was monitored. The 18O content of the catalyst following labeled reaction mixture pulses was determined by catalyst reduction with either diluted hydrogen or diluted methane pulses. The first reaction pulse at each temperature resulted in both CO2 and water containing primarily 16O. As the 18O2 uptake from the gas phase increased, however, the 18O distribution in water and carbon dioxide changed differently. The isotopic composition of water molecules reflected the oxygen isotopic distribution in the bulk of the catalyst particles, as determined by reduction titration experiments carried out after the reaction sequences. The larger concentration of 18O in the carbon dioxide is explained by the differences in residence time and mobility of the products water and CO2 on the catalyst. The hydrogen/water samples the bulk, while the CO2 reflects the isotopic composition of the surface. The behavior of the zirconia-supported catalyst was similar to that of the bulk PdO at the lowest temperature; however, as the temperature was increased above 600 K, oxygen exchange with the support became important. The catalyst behavior is explained by the presence of a single oxygen species at the catalyst surface: a bridge-bound oxygen to two palladium atoms. The surface is involved in the methane reaction mechanism by successive reduction/reoxidation cycles. Reoxidation uses both bulk and gas phase oxygen, and also oxygen from the support in the case of the zirconia-supported catalyst. Under these conditions the gas phase oxygen exchange with the catalyst is limited by the methane oxidation surface reaction.

Seasonal variation of stable isotopic compositions recorded in a laminated tufa, SW Japan

High-resolution stable isotopic analyses of an annually laminated tufa from Shirokawa, SW Japan, show cyclic variations that correspond to mm-scale summer (dense) and winter (porous) laminae. Both δ 18O and δ 13C values are high in winter and low in summer, and correlate well with each other ( r=0.88). Because the δ 18O content of stream water has remained relatively stable over time, the δ 18O curve reflects seasonal changes in water temperature. Cyclic variation in δ 13C suggests that it is controlled by seasonal changes in the δ 13C of dissolved inorganic carbon in groundwater, which is probably caused by groundwater degassing. This degassing effect is normally strongest during winter when subsurface air is warmer (less dense) than the atmosphere. This decreases P CO 2 in cave air, and the associated degassing of 13C-depleted CO 2 from the groundwater increases the δ 13C value of dissolved inorganic carbon in winter. Calcite was deposited in isotopic equilibrium with stream water. Extreme climatic events, such as drought, can be recorded as anomalies in both the δ 18O and δ 13C curves. This study demonstrates that annually laminated tufa can be used for high-resolution analysis of terrestrial paleoclimate.

Warmer or cooler late Holocene marine palaeoenvironments?: interpreting southeast Australian and Brazilian sea-level changes using fixed biological indicators and their δ18O composition

Relic inter-tidal assemblages of sessile organisms, which are referred to as fixed biological indicators (FBIs), are shown to be useful proxies for late Holocene climatic and environmental changes as well as direct indicators of short term fluctuations in sea-level. A detailed comparison of change in the inter-tidal assemblages of two Southern Hemisphere sites at comparable latitudes (Port Hacking, New South Wales, south–east Australia and the Laguna-Imbituba region of southern Brazil) indicates periods of marked similarities in the timing and nature of changes at ∼5200, ∼3800 cal. years BP and differences after ∼2400 cal. years BP. There is sufficient concurrence of cooling phases and falling sea-levels to strongly suggest that in some cases global rather than regional forcing factors are at work. The value of inter-tidal organisms as indicators of changed environments is developed by identifying the past and present distributions of the common marker species: tubeworms, barnacles and mussels. The δ 18O content of these extant specimens at Port Hacking confirmed changes in ocean water temperature and associated sea-level behaviour corresponding to invasion or extinction of temperature-sensitive species. A similar pattern is repeated, although in a less marked fashion, in the Laguna-Imbituba region of southern Brazil.

Influence of carbonic anhydrase activity in terrestrial vegetation on the 18O content of atmospheric CO2.

The oxygen-18 (18O) content of atmospheric carbon dioxide (CO2) is an important indicator of CO2 uptake on land. It has generally been assumed that during photosynthesis, oxygen in CO2 reaches isotopic equilibrium with oxygen in 18O-enriched water in leaves. We show, however, large differences in the activity of carbonic anhydrase (which catalyzes CO2 hydration and 18O exchange in leaves) among major plant groups that cause variations in the extent of 18O equilibrium (theta(eq)). A clear distinction in theta(eq) between C3 trees and shrubs, and C4 grasses makes atmospheric C18OO a potentially sensitive indicator to changes in C3 and C4 productivity. We estimate a global mean theta(eq) value of approximately 0.8, which reasonably reconciles inconsistencies between 18O budgets of atmospheric O2 (Dole effect) and CO2.

Sr/Mg variation during rock-water interaction: implications for secular changes in the elemental chemistry of ancient seawater

Prior studies have recently demonstrated a positive linear covariance in the Sr and Mg contents of abiotic marine carbonates such that the slope of this relation is dependent upon the Sr/Mg ratio of seawater at the time of precipitation. Samples of ancient abiotic marine calcite cements that have undergone diagenesis through alteration by meteoric waters exhibit similar linear trends that range from primary marine ratios to low values that converge to zero. Thus, Sr/Mg ratios in diagenetic precipitates are potential proxies for the reconstruction of past variations in the elemental chemistry of ancient seawater. To expand upon limited existing Sr/Mg data, late Silurian and middle Triassic abiotic calcite cements were analyzed for stable isotope and minor element compositions. Silurian cements from the Pipe Creek Junior Quarry, Grant County, Indiana, show a linear covariance of Sr and Mg concentrations expressed by the following relationship: Sr/Ca = 0.0306 (±0.0091) Mg/Ca + 2.349 × 10 −6 (±5.812 × 10 −5). Triassic cements from the Cozzo di Lupo Formation, Sicily, Italy, exhibit a linear covariance given by: Sr/Ca = 0.0103 (±0.0009) Mg/Ca + 3.818 × 10 −5 (±2.800 × 10 −5). To evaluate the mechanisms by which Sr/Mg ratios are preserved in diagenetic calcite, a progressive rock-water interaction model was employed that traces the simultaneous evolution of δ 13C, δ 18O, Sr, and Mg contents of both fluid and solid phases. Results from modeling indicate that the Sr/Mg ratio of diagenetic calcite is primarily controlled by the ratio of distribution coefficients for Sr and Mg in calcite. When D Sr/D Mg is equal to unity, covariance of Sr/Ca and Mg/Ca in diagenetic calcite is linear, with a slope equivalent to the Sr/Mg value of the original dissolved solid phase. When D Sr/D Mg deviates from unity, the incorporation of Sr/Ca and Mg/Ca in diagenetic precipitates becomes progressively non-linear. A general agreement of theoretical and empirical results for Pliocene abiotic calcite cements of Frank and Lohmann (1996) indicates that model simulations adequately represent natural diagenetic settings. Additionally, at rock-water ratios where δ 13C has converged to and δ 18O has shifted toward original rock composition, elemental contents, and thus the Sr/Mg ratio, are effectively preserved. Thus, retention of original Sr/Mg chemistries is possible across a broad range of rock-water ratios, even where primary δ 18O values have been significantly modified. Given the correlation between theoretical and empirical Sr/Mg results, we suggest that variation in the Sr/Mg composition of abiotic calcite cements reflect fluctuations in the Sr/Mg chemistry of ancient seawater.

Ikaite Tufa Towers in Ikka Fjord, Southwest Greenland: Their Formation by Mixing of Seawater and Alkaline Spring Water

ABSTRACT Ikka Fjord in southwest Greenland is the type locality for ikaite, a metastable hexahydrate of calcium carbonate. Here, ikaite forms submarine tufa columns rising up to 18 m above the fjord bottom. The columns are spectacular examples of abiogenic carbonate precipitation in a cold seawater environment and so far represent the only known formation of ikaite in seawater. We have analyzed the 2H, 13C, 14C, 18O and 87Sr contents and major-element and minor-element compositions of ikaite and of the different water types in and around Ikka Fjord in order to understand the formation of the Ikka columns. Water from inside the columns is fresh but alkaline, rich in ions of sodium, bicarbonate, and carbonate, and has a stable-isotope composition identical to precipitation falling on the nearby Gronnedal-Ika igneous complex, and we conclude that the tufa columns form over submarine springs issuing from the bottom of Ikka Fjord. Dissolution of unidentified sodium carbonate minerals in carbonatites in the igneous complex probably accounts for the peculiar chemistry of the springs. Ikaite supersaturation and precipitation is controlled by mixing of spring water and seawater at the cold (< 6°C) fjord bottom, and precipitation is accelerated by the formation of hydrated CaCO3o (aq) ion pairs, while phosphate ions in the spring water act as inhibitor for precipitation of anhydrous carbonates. We propose that the spring water seeps in Ikka Fjord literally create their own conduits in the form of the vertical, chimney-like columns with ikaite forming continuously at the tops of the columns. The only limits to upward growth are winter ice and the fresh-water layer capping the fjord during summer.

Palaeowater in coastal aquifers of Spain

Abstract In the framework of the PALAEAUX project, three coastal aquifers have been studied in Spain, looking at the possible existence of Pleistocene or Early Holocene fresh groundwater, in addition to the formerly studied Llobregat Delta. They constitute representative situations of the most common coastal aquifers. The main characteristics of these four areas are: The Llobregat Delta system, studied since 1965, is placed to the southwest of Barcelona (NE Mediterranean coast of the Iberian Peninsula). It is a sedimentary fluvial formation of Pleistocene to recent age, which is representative of small size coastal alluvial formations with the deep confined aquifer open to the sea offshore. Marine sea water penetrated the deep Pleistocene layer during the Flandrian sea-level rise but afterwards relatively high freshwater heads upstream allowed its progressive flushing to the sea through the submarine outlet since 6000–8000 a ago. Freshwater turnover time is shorter than the Holocene duration and it has been renovated due to natural discharge of the aquifer to the sea. The renovation process has recently been accelerated due to intensive groundwater exploitation during the past century. Nowadays, only freshwater a few centuries old remains in low permeability areas of the confined Pleistocene aquifer. The Doñana sedimentary aquifer system corresponds to the ancient estuary of the Guadalquivir River (SW Spain). It contains old saline groundwater that has not been flushed away due to the low freshwater head. In this aquifer 39 At, 85 Kr, 3 H, 14 C and 13 C measurements have been used to ascertain the age structure of several mixed groundwater samples. After correction, the oldest fresh groundwater 14 C age is c. 12–15 ka but 39 Ar ages point to somewhat younger water. Neither the stable isotope contents nor the recharge temperature calculated from noble gases show a climatic signature for these waters. The Inca-Sa Pobla carbonate system is the northeast sector of the island of Mallorca. Brackish and salt groundwater in the coastal area has been found along the coastal strip of S’Albufera, displaying what seems a convective-like flow pattern inside the up to 200 m thick aquifer. The most consistent estimated average 14 C ages of the mixed water vary between 9 and 13 ka, i.e. Early Holocene-Late Pleistocene. Water 18 O and 2 H contents do not show a clear climatic signature. The Amurga volcanic Massif is on the southeast of the island of Gran Canaria. Brackish groundwater is found in the thick phonolite rock sequence. The unsaturated zone can be up to 200 m thick. Groundwater salinity is due to recharge under arid conditions and the influence of marine airborne salts. Groundwater 14 C age is possibly c. 11 ka, indicating a Late Pleistocene age water mixture. The small recharge due to aridity and the thick unsaturated zone explains this. Stable-water isotopes show no clear climatic signature

Oxygen isotopes of marine diatoms and relations to opal-A maturation

In order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water, sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures. Freshly grown natural diatoms (phytoplankton samples and sediment trap material) and cultured diatoms reveal similar oxygen isotope fractionation factors. The fresh diatoms have 3 to 10‰ lower isotope fractionation factors than fossil (sedimentary) diatoms. A temperature-related oxygen isotope fractionation could not be established for the laboratory cultures (and the natural phytoplankton samples), and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica-water oxygen isotope fractionation factor. It is possible, however, that slow diatom growth in sea surface water may well lead to a temperature-dependent silica-water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in paleo-surface water studies. FTIR-spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for Si-O-Si/Si-OH correlates with the 3 to 10‰ δ 18O silica increase from fresh to fossil diatoms. Open-system (flow-through) silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in 18O is therefore not due to dissolution of isotopically light surficial Si-OH groups. It is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of Si-O-Si/Si-OH and the 18O content of framework oxygen. These findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable 18O contents of surface sediments.

Selected ion flow tube mass spectrometry analyses of stable isotopes in water: isotopic composition of H 3O + and H 3O +(H 2O) 3 ions in exchange reactions with water vapor

A new method has been developed for the determination of the isotope abundance ratios of deuterium, D, and oxygen-18, 18O, in water vapor (and water) using selected ion flow tube mass spectrometry (SIFT-MS). H 3O + ions are injected into the helium carrier gas where they associate with the H 2O and HDO molecules in a sample of water introduced into the carrier gas. The D and 18O contents of the product cluster ions H 8DO 4 + and H 9 18OO 3 + at m/e = 74 and 75, respectively, are determined by reference to the majority cluster ion H 9O 4 + at m/e = 73. Allowance is made for the contribution of the H 8 17OO 3 + ions to the m/z = 74 ions. Absolute isotopic ratios are measured within seconds without the need for precalibration of the SIFT-MS instrument, currently to an accuracy of better than 2%.

Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis

ABSTRACTThe 18O content of CO2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO2 and 18O‐enriched leaf water and retrodiffusion of most of this CO2 back to the atmosphere, leaves effectively discriminate against 18O during photosynthesis. Discrimination against 18O (Δ18O) is expected to be lower in C4 plants because of low ci and hence low retrodiffusing CO2 flux. C4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C3 plants. Low CA may limit the extent of 18O exchange and further reduce Δ18O. We investigated CO2–H2O isotopic equilibrium in plants with naturally low CA activity, including two C4 (Zea mays, Sorghum bicolor) and one C3 (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ18O in C4, as well as in some C3, plants. Variations in CA activity and in the extent of CO2–H2O isotopic equilibrium (θeq) estimated from on‐line measurements of Δ18O showed large range of 0–100% isotopic equilibrium (θeq= 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ18O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θeq&lt; 1 (incomplete isotopic equilibrium) on 18O of atmospheric CO2 can be similar for C3 and C4 plants and in both cases it increases with natural enrichment of 18O in leaf water.

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO4–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and 18O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and 18O content. The large ΔSO4–S0 (23.8–26.4‰) measured in the lake suggest that dissolved SO4 forms during disproportionation of magmatic SO2 in the hydrothermal conduit at temperatures of 250∼280°C. The lake δ18OSO4 and δ18OH2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO2-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO4–Cl waters on the lakeshore and neutral pH HCO3–SO4–Cl–Na waters in Blawan village, 17km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO4/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO4–Cl water and ground water.The lakeshore fumarole discharges (T=170∼245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is ∼260°C. The proportions of the oxidized, SO2-dominated magmatic vapor and of the reduced, H2S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO2-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H2S/SO2 ratio deduced for the gas producing the SO4–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H2S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H2S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ34S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.

Effect of Concomitant18O in13C-Urea on the Urea breath test

Helicobacter pylori infection in stomach has been extensively assessed by the 13C-urea breath test (UBT) with infra-red or mass spectrometer. However, analyses of commercially available 13C-urea for UBT showed about 10 atom% of 18O abundance. It is therefore very important to investigate the 18O-isotope effect on UBT. Using a mass spectrometer, breath samples of UBT were analyzed to investigate the change of mass spectra by the isotopic exchange reaction between 18O of carbon dioxide and 16O of water. As the result, the 18O content of 13C, 18O-carbon dioxide exhaled in UBT reached its natural abundance level immediately after the administration of 13C, 18O-urea. It was concluded that the 18O content of 13C, 18O-urea has no effect on UBT using either infra-red or mass spectroscopy. Copyright © 2000 John Wiley & Sons, Ltd.

Small-scale oxygen isotope variations and petrochemistry of ultradeep (>300 km) and transition zone xenoliths

The oxygen isotopic composition of 6 xenoliths for which an origin between 310 to 450 km has been deduced was analyzed. An eclogite from Koidu, Sierra Leone, is composed of garnet with δ 18O gt = 5.67‰ vs SMOW (standard mean ocean water) ± 0.06‰ SEM (standard error of the mean) and clinopyroxene with δ 18O cpx = 5.52 ± 0.18. Results for xenoliths from Jagersfontein, South Africa, are; harzburgite: garnet δ 18O gt = 5.78 ± 0.16, clinopyroxene δ 18O cpx = 5.24 ± 0.27, orthopyroxene δ 18O opx = 4.39 ± 0.21, olivine δ 18O ol = 5.69 ± 0.08; lherzolite: δ 18O gt = 5.63 ± 0.07, δ 18O cpx = 4.3, δ 18O opx = 5.62 ± 0.27; a bimineralic gt/cpx xenolith with minor opx: δ 18O gt = 5.08 ± 0.13, δ 18O cpx = 4.32 ± 0.08, δ 18O opx = 4.21 ± 0.31. Two single crystal garnets with minor associated pyroxenes and in one case, olivine, gave the following results: #1 δ 18O gt = 5.62 ± 0.05, δ 18O cpx = 5.09 ± 0.07, δ 18O opx = 5.62 ± 0.12, δ 18O ol = 5.59; #2 δ 18O gt = 5.62 ± 0.09. While evidence for isotopic equilibrium is observed between some gt/ol, gt/opx and cpx/opx pairs, the 18O/ 16O fractionation between garnet and clinopyroxene tends to be out of equilibrium. Based on the observed oxygen isotope variability, a diffusion model for the data, and published radiogenic isotope data for the samples, it is concluded that these xenoliths experienced a series of metasomatic alterations, including an event which lowered the 18O content of the pyroxenes a few Ma prior to the eruption of the kimberlite.

Ozone-induced hypothermia and bradycardia in rats and guinea pigs in nose-only or whole-body inhalation systems

1. Ozone (O 3) inhalation induces a hypothermic response in rodents characterized primarily by core body temperature ( T CO) and heart rate (HR) decreases. 2. This preliminary study examined T CO, HR, and electrocardiographic changes in rats and guinea pigs during whole-body or nose-only exposure to 1 ppm 18O 3. 3. Rats exposed whole-body demonstrated an immediate hypothermic response, while that of the nose-only exposed rats was delayed ≈60 min; guinea pigs showed no hypothermic response to 18O 3. 4. 18O content in lavage fluid samples suggests that dose is greater in nose-only exposures than whole-body exposures, and that guinea pigs received higher doses than rats. 5. These initial findings suggest that dosimetric differences may result from differences in species responses and exposure scenarios.

Isotopic Exchange of Oxygen between Proton-Exchanged Zeolites and Water

The time courses of the title reaction were analyzed by using a closed circulation system. It was found that there are three kinds of framework oxygen, OI, OII, and OIII, that is, easily, moderately, and hardly exchanging oxygens. The respective rate constants of the exchange reaction, k1, k2, and k3, were almost constant, or independent of the zeolite structures and silica/alumina ratios. In contrast, the numbers of OI and OII (n1 and n2) decreased with increasing silica/alumina ratios, whereas that of OIII (n3) increased. n1 + n2 and n3 are approximately in agreement with the numbers of bridging oxygens of Si−O−Al and Si−O−Si, respectively. Next, IR measurement was applied to confirm the origin of the active framework oxygens. Upon the exchange reaction on H-MFI-23, one acidic OH band at 3612 cm-1 readily shifted to lower frequency while the silanol band at 3735 cm-1 and ν(Si−O−T (T = Si or Al)) bands at 1219 and 1092 cm-1 shifted slightly. The 18O contents in the bands, evaluated by the deconvolution of the spectra, decreased in the following order: acidic OH ≫ Si−O−T = silanol. A similar order was observed on H-FAU-5.6, H-MOR-16, or H-LTL-6.0. Most of oxygens corresponding to the acidic OH bands could exchange with 18O upon treatments in a large excess of H218O. It follows that OI and OII are the bridging oxygens bound to Al and exchangeable with water by the contribution of neighboring proton, and OIII is the bridging oxygen between two Si atoms. It was further found that the exchange reaction, Si−18O−Al + Si−16O−Si → Si−16O−Al + Si−18O−Si, starts to proceed above 673 K and is dominant at 873 K.

Diagenesis by burial fluids, Middle Ordovician platform to platform-margin limestones, East Tennessee; relationship to mississippi valley-type deposits

ABSTRACT We compare relative timing and geochemistry of different paragenetic phases in platform-margin and platform-interior limestones within a nearly 400-m-thick succession of Middle Ordovician strata. Emphasis is on late-stage cements that include ferroan, coarse sparry calcite and ferroan baroque dolomite. Platform-interior rocks contain two late cements (clear, sparry calcite and nonferroan dolomite) not observed in platform-margin rocks. Depleted 18O and high iron content of these four calcite and dolomite phases indicate a fluid that had interacted with siliciclastic material. Late-stage diagenetic cements in the platform-margin limestones generally have more lower values of 18O, higher iron and manganese contents, and more radiogenic 87Sr than their counterparts from the platform. Multiple interpretations for the different diagenetic phases and geochemistry are possible. We favor a water-rock interaction hypothesis because it requires a single source and is therefore the simplest explanation. Ferroan, coarse sparry calcite and ferroan baroque dolomite could have been generated from brines expelled from the Sevier Basin before the end of the Middle Ordovician (ca. 450 Ma). These fluids could have moved up dip into platform carbonates under pressures induced by burial alone. This is based on an analysis of burial curves, possible temperatures, and an assumed range of likely brine compositions. Bitumen associated with some ferroan baroque dolomite may be synchronous with Mississippi Valley-type, main ore-stage sphalerite. The timing for fluid migration that we suggest here contrasts with conventional thinking, which places the major fluid migration during the Alleghenian orogeny (332-250 Ma) at the end of the late Paleozoic.

Measurement of 18O/16O in the soil‐atmosphere CO2 flux

Measurements of 18O in atmospheric CO2 can be used to trace gross photosynthetic and respiratory CO2 fluxes between the atmosphere and the terrestrial biosphere. However, this requires knowledge of the 18O signatures attributable to the fluxes from soil and leaves. Newly developed methods were employed to measure the 18O of soil‐respired CO2 and depth profiles of near‐surface soil CO2, in order to evaluate the factors influencing isotopic soil‐atmosphere CO2 exchange. The 18O of soil‐respired CO2 varied predominantly as a function of the 18O of soil water which, in turn, changed with soil drying and with seasonal variations in source water. The 18O of soil‐respired CO2 corresponds to full isotopic equilibrium with soil water at a depth ranging between 5 and 15 cm. The 18O of respired CO2, in reality, results from a weighted average of partial equilibria over a range of depths. Soil water isotopic enrichment of up to 10‰ in the top 5 cm did not appear to strongly influence the isotopic composition of the respired CO2. We demonstrate that during measurements “invasion” of atmospheric CO2 (the diffusion of ambient CO2 into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the 18O of the soil‐respired CO2. The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO2 diffusion out of the soil to be 7.2 ± 0.3‰. High‐resolution (1 cm) depth profiles of 18O of near‐surface (top 10 cm) soil CO2 were carried out by gas chromatography—isotope ratio mass spectrometry (GC‐IRMS). This novel technique allowed us to observe the competitive diffusion‐equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO2 18O content.

Variations of the stable isotopic compositions of rainfall events from the Cameroon rain forest, Central Africa

This paper discusses the factors controlling the variations of heavy isotope contents of 57 rain events collected throughout the rain forest regions of Cameroon, Central Africa, during the great rainy season (August–November) of 1994. Our data display a wide range of values, from +1.69 to −10.80‰ for 18O and from +23.2 to −85.5‰ for 2H. The data follow closely the global meteoric water line suggesting that rain formation processes occurred under isotopic equilibrium conditions between both the condensate and the corresponding vapour. This also indicates that the observed heavy isotope contents have not been altered by evaporation during the descent of raindrops to the ground. The low heavy-isotope contents observed for some precipitation events are assumed to be controlled by the amount of rainfall and/or by the low condensation temperature. The former effect is related to the northward passage of the intertropical convergence zone (ITCZ) over the region under study, whereas the second effect is linked to high altitudes of condensation. Moreover, the latitudinal variations of the weighted mean 18O contents indicates that in coastal areas (0–230 km inland from the Atlantic coast) the stable isotopic compositions of precipitation are controlled by a continuous extraction from the atmospheric reservoir of vapour as rains according to the Rayleigh distillation model, whereas at distances >230 km, recycling of continental moisture is thought to have influenced the isotopic compositions of rains.

Surface accumulation on Foundation Ice Stream, Antarctica

AbstractThis paper presents new data, derived from an analysis of 16 firn cores, on the distribution of the accumulation rate and 18O content of near-surface layers in the eastern part of the Ronne Ice Shelf, Antarctica. The firn cores were drilled along the traverse route of the Filchner-V-Campaign in 1995. The traverse followed an ice flowline of the Foundation Ice Stream and reached the margin of the inland ice, an area which has not yet been investigated. On the ice shelf the accumulation rates decrease with distance from the coast. Ascending to the inland ice the accumulation rates again reach almost coastal values. This regional distribution is in agreement with the temperature gradient along the traverse. The 18 content of the near-surface layers is closely related to the 10 m firn temperature. They strongly decrease from the grounding line towards the inland ice. At the southernmost site at 1100 ma.s.l, the mean δ18O value of the firn decreases to –40‰. Ice with that isotopic signature was found in cores from the central part of the Ronne Ice Shelf just above the marine ice layer, indicating that it originates from this area. All ice deposited as snow further south was melted beneath the ice shelf after passing the grounding-line area. The time series of accumulation rate and 18O content reveal no climatic trend during the last 30-50 years.

Non‐climatic variations in the oxygen isotopic compositions of plants

AbstractThe 18O content of leaf water strongly influences the 18O contents of atmospheric CO2 and O2. The 18O signatures of these atmospheric gases, in turn, emerge as important indicators of large‐scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The 18O enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the 18O content of leaf water are influenced by nonclimatic, species‐specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9‰ in the δ18O values of stem water, δs, and of about 14‰ in the mid‐day δ18O enrichment of bulk leaf water, δLW–δs. Differences between δ18O values predicted by a conventional evaporation model, δM, and δLW ranged between – 3.3‰ and + 11.8‰. The δ18O values of water in the chloroplasts (δch) in leaves of 10 selected plants were estimated from on‐line CO2 discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that δch can significantly deviate from δM in species with high leaf peclet number. The δ18O values of bulk leaf water significantly correlated with δ18O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in δ18O and δ13C values of cellulose and in their peclet numbers, and from deciduous woodland species in their δ18O and δ13C values of cellulose. The results indicated that predictions of the δ18O values of leaf water (δLW, δM and δch) could be improved by considering plant species‐specific characteristics.