Oxygen Isotope Signatures Research Articles

Perspectives in carbonate paleoclimatology

Sedimentary carbonate is the most successful material in paleoclimatological researches. Researches in this field are now challenging to extract high-resolution and quantitative records of millennial-century scales in order to predict the climatic changes due to global warming. The most promising study material is a reef coral, which records temperature (and salinity) changes in its annually-laminated aragonitic skeleton. Data from coral climatology and annually-laminated polar ice cores would contribute to climate prediction although the information is geographically limited in only tropical and polar areas.For the information from temperate-subtropical climatic belts, carbonate paleoclimatology using speleothems is effective. Oxygen and carbon isotopic signatures of speleothems are now treated as possible proxies of water temperature and vegetation. Another promising material for the study on temperate-subtropical areas is a tufa, a calcitic deposit in freshwater environments of limestone areas. The tufa commonly shows annual laminations and advances to speleothems in its larger depositional rate (several millimeters per year) that allows higher analytical resolution. This study represents that rainfall events were recoded in a Japanese tufa deposit.

Correcting the Cenozoic δ18O deep-sea temperature record for Antarctic ice volume

The oxygen isotope signal in benthic foraminifera from deep-sea cores is mainly determined by deep-ocean temperature and land ice volume. Separating the temperature and ice volume signals is a key step in understanding the evolution of Cenozoic climate. Except for the last few million years, fluctuations in land ice volume were determined mainly by the size of the Antarctic ice sheet. Here a simplified Antarctic ice-sheet model and composite deep-sea oxygen isotope records are used to derive a deep-sea temperature history for the Cenozoic. The ice-sheet model is forced with the oxygen-isotope record, which is instantaneously corrected for the calculated ice volume. Therefore, the resulting deep-sea temperature and Antarctic ice volume curves are mutually consistent. The temperature record of Lear et al. [Science 287 (2000) 269], derived from the Mg/Ca ratio in benthic calcite, is also used to force the ice-sheet model. The resulting ice-volume does not agree well with the ice-volume history inferred by Lear et al. from a comparison of Mg/Ca and δ 18O records. The most important points emerging from this study are: • The Cenozoic δ 18O record from benthic forams is a truly mixed signal (ice volume and deep-sea temperature contribute equally to the variance). • For the period of 45 to 30 Ma ago, δ 18O temperatures are significantly lower than Mg/Ca temperatures. • Rapid nonlinear build-up of a continental-scale ice sheet on the Antarctic continent can easily occur for a linear cooling rate. • It is unlikely that the method of Lear et al. provides a reliable measure of ice volume. • The cooling rate at the beginning of the Pliocene is much larger in the δ 18O temperature curves than in the Mg/Ca temperature curve.

Carbon and Oxygen Isotopic Signatures in Albian-Danian Limestones of Cauvery Basin, Southeastern India

The Albian-Danian limestones of Cauvery Basin show a wide range of d 13C and d 18O values (–13.2 to +1.1% and –9.0 to –2.5%, respectively). The cement samples show negative carbon and oxygen isotope values (–18.9 to –3.9% and –9.0 to –4.3%, respectively). The petrographic study reveals the presence of algae, molluscs, bryozoans, foraminifers and ostracods as major framework constituents. The limestones have microspar and equant sparry calcite cements. The pore spaces and vugs are filled with sparry calcite cement. The bivariate plot of d 13C and d 18O suggests that most of the samples fall in the freshwater limestone and meteoric field, while few samples fall in the marine limestone and soil calcite fields. The presence of sparry calcite cement, together with negative carbon and oxygen isotope values, indicates that these limestones have undergone meteoric diagenesis.

Resolving seasonality in tropical trees: multi-decade, high-resolution oxygen and carbon isotope records from Indonesia and Thailand

Dendrochronological techniques have found limited applications in the tropics because of invisible or indistinct banding in wood. The seasonal cycles of rainfall and relative humidity in these regions, while not sufficient to produce distinct visible rings, may still generate seasonal signals in the oxygen isotopic composition of tree cellulose which can be used for climate reconstruction and chronology. We explore this approach using trees from Indonesia and Thailand, from three different families. Multi-decade δ 18O records from Javanese cross-dated teak rings and bomb radiocarbon-dated Suar wood lacking visible rings demonstrate the reproducibility of the signal between trees grown at the same locality as well as from wider geographical regions. These results confirm predictions that the trees oxygen isotopic signatures reflect an external climatic forcing. High-resolution δ 18O records reveal large seasonal cycles: up to 4‰ for Javanese Suar samples and up to 18‰ for a Thai Podocarpus sample. We show that the six δ 18O and δ 13C cycles measured on a Podocarpus match the number of growth years for the period spanning the time of wounding and cutting of the wedge section. This result demonstrates that the isotopic cycles found in this tree with indistinct annual rings are indeed seasonal and could be used for chronology. We present evidence that stable isotope chronologies of tropical trees also contain insights in tropical tree physiology and growth dynamics.

これからの炭酸塩古気候学

Sedimentary carbonate is the most successful material in paleoclimatological researches. Researches in this field are now challenging to extract high-resolution and quantitative records of millennial-century scales in order to predict the climatic changes due to global warming. The most promising study material is a reef coral, which records temperature (and salinity) changes in its annually-laminated aragonitic skeleton. Data from coral climatology and annually-laminated polar ice cores would contribute to climate prediction although the information is geographically limited in only tropical and polar areas.For the information from temperate-subtropical climatic belts, carbonate paleoclimatology using speleothems is effective. Oxygen and carbon isotopic signatures of speleothems are now treated as possible proxies of water temperature and vegetation. Another promising material for the study on temperate-subtropical areas is a tufa, a calcitic deposit in freshwater environments of limestone areas. The tufa commonly shows annual laminations and advances to speleothems in its larger depositional rate (several millimeters per year) that allows higher analytical resolution. This study represents that rainfall events were recoded in a Japanese tufa deposit.

Long‐Term Disturbance of Ground Water Chemistry Following Well Installation

Ground water samples collected from a multilevel sampler shortly after its construction showed significantly higher alkalinity and concentrations of calcium and magnesium than those from nearby wells installed 10 years earlier. The sampler was drilled using a conventional hollow-stem power auger in a sandy, silicate aquifer lying beneath an isthmus between two lakes in northern Wisconsin. Ground water in the study area is of low ionic strength and its chemistry is dominated by silicate mineral weathering. Periodic sampling over two years following installation of the sampler showed that the higher solute concentrations had subsequently decreased. Oxygen isotope signature and other solute species, such as sulfate and chloride, were comparable to those of older wells and did not show any notable trends over time. Independent variation of other chemical species that cannot be derived from aquifer minerals, and the similarly high concentrations in older wells shortly after their installation, suggest that rapid dissolution of fresh mineral surfaces and hyperfine particles generated during drilling has induced the enhanced concentrations. This observation is consistent with the field equivalent of laboratory mineral dissolution experiments that show initially increased dissolution rates that decay over time. Well installations for geochemical sampling in dominantly silicate material may require longer times to reach an equilibrium state than has been previously thought.

Mean oxygen-isotope signatures in Porites spp. corals: inter-colony variability and correction for extension-rate effects

The assessment of inter-colony variability in the mean skeletal δ18O signatures of modern Porites spp. corals is a prerequisite for the estimation of past mean climate conditions based on fossil colonies. Here we show that the mean δ18O signatures of Porites spp. corals from the northern end of the Gulf of Aqaba (Red Sea) with mean extension rates between 0.2 and 1.5 cm/year can have an inter-colony variability as large as 1.28‰. At extension rates of less than 0.6 cm/year the mean coral δ18O values of the individual colonies are strongly dependent on the mean extension rate, with increasingly higher δ18O values corresponding to decreasing extension rate. This suggests that extension-rate-related kinetic isotope disequilibrium effects are responsible for a large proportion of the inter-colony differences in the mean coral δ18O signatures. A correction procedure for these effects based on the relationship between mean δ18O values and mean extension rate reduces the variability of mean coral δ18O values among the individual colonies to 0.43‰. Although certainly not perfect, the correction procedure enables a better assessment of mid-Holocene climate conditions at this location based on Porites spp. with mean extension rates of less than 0.6 cm/year.

Remnants of premetamorphic fluid and oxygen isotopic signatures in eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes, eastern China

AbstractA combined oxygen‐isotope and fluid‐inclusion study has been carried out on high‐ and ultrahigh‐pressure metamorphic (HP/UHPM) eclogites and garnet clinopyroxenite from the Dabie‐Sulu terranes in eastern China. Coesite‐bearing eclogites/garnet clinopyroxenite and quartz eclogites have a wide range in whole‐rock δ18OVSMOW, from 0 to 11‰. The high‐T oxygen‐isotope fractionations preserved between quartz and garnet preclude significant retrograde isotope exchange during exhumation, and the wide range in whole‐rock oxygen‐isotope composition is thought to be a presubduction signature of the precursors. Aqueous fluids with variable salinities and gas species (N2‐, CO2‐, or CH4‐rich), are trapped as primary inclusions in garnet, omphacite and epidote, and in quartz blebs enclosed within eclogitic minerals. In high‐δ18O HP/UHPM rocks from Hujialin and Shima, high‐salinity brine and/or N2 inclusions occur in garnet porphyroblasts, which also contain inclusions of coesite, Cl‐rich blue amphibole and dolomite. In contrast, in low‐δ18O eclogites from Qinglongshan and Huangzhen, the Cl concentrations in amphibole are very low, < 0.2 wt.%, and low‐salinity aqueous inclusions occur in quartz inclusions in epidote porphyroblasts and in epidote cores. These low‐salinity fluid inclusions are believed to be remnants of meteoric water, although the fluid composition was modified during pre‐ and syn‐peak HP/UHPM. Eclogites at Houshuichegou and Hetang contain CH4‐rich fluid inclusions, coexisting with high‐salinity brine inclusions. Methane was probably formed under the influence of CO2‐rich aqueous fluids during serpentinisation of mantle‐derived peridotites prior to or during plate subduction. Remnants of premetamorphic low‐ to high‐salinity aqueous fluid with minor N2 and/or other gas species preserved in the Dabie‐Sulu HP/UHPM eclogites and garnet clinopyroxenite indicate a great diversity of initial fluid composition in the precursors, implying very limited fluid–rock interaction during syn‐ and post‐peak HP/UHPM.

Low δ18O eclogites from the Kokchetav massif, northern Kazakhstan

AbstractOxygen isotopic compositions of silicates in eclogites and whiteschists from the Kokchetav massif were analyzed by whole‐grain CO2‐laser fluorination methods. Systematic analyses yield extremely low δ18O for eclogites, as low as −3.9‰ for garnet; these values are comparable with those reported for the Dabie‐Sulu UHP eclogites. Oxygen isotopic compositions are heterogeneous in samples of eclogite, even on an outcrop scale. Schists have rather uniform oxygen isotope values compared to eclogites, and low δ18O is not observed. Isotope thermometry indicates that both eclogites and schists achieved high‐temperature isotopic equilibration at 500–800 °C. This implies that retrograde metamorphic recrystallization barely modified the peak‐metamorphic oxygen isotopic signatures. A possible geological environment to account for the low‐δ18O basaltic protolith is a continental rift, most likely subjected to the conditions of a cold climate. After the basalt interacted with low δ18O meteoric water, it was tectonically inserted into the surrounding sedimentary units prior to, or during subduction and UHP metamorphism.

Petrology of sapphirine granulite and associated sodic gneisses from the Indian Head Range, Newfoundland

Migmatitic granulites from the Indian Head Range (IHR) are dominated by granoblastic, Opx-bearing (quartz) dioritic gneiss with subordinate garnet+orthopyroxene+biotite+albite (±quartz±microcline±cordierite±sillimanite) gneiss and comparatively biotite-rich, sapphirine+cordierite+orthopyroxene+albite (±microcline±sillimanite±corundum) gneiss. The latter contains at least two generations of sapphirine and cordierite, one apparently predating migmatization, the other associated with or following this event. Mineral thermobarometers yield temperatures up to ∼800 °C at 8 kbar. The IHR granulites have very high δ 18O values (+10.8 to +14.5) which most likely indicate a sedimentary precursor. Their REE and HFSE contents resemble the trace element signature of post-Archean Australian shale (PAAS). Lithological analogues from other Grenvillian inliers in western Newfoundland have similar oxygen isotopic and immobile-element signatures. They too are interpreted as metasedimentary rocks. The IHR rocks, however, have unusually sodic compositions (e.g., Na 2O/CaO and Na 2O/K 2O=2.1–22.5 and 0.93–13.8, respectively) and contain albite or sodic oligoclase despite their high metamorphic grade. The geochemical data indicate that these rocks were albitized prior to high-grade metamorphism. No counterpart of this event is recorded in granulites from the other inliers, indicating that the IHR may be highly allochthonous with respect to these other Grenvillian terranes.

Hydrochemical and isotopic characteristics of groundwater in the Souss Upstream Basin, southwestern Morocco

Heterogeneous shallow Plio-Quaternary formations of the Souss Plain represent the most important aquifer in southern High Atlas Mountains in Morocco. The present work was conducted in the Souss Upstream Basin to identify the chemical characteristics and the origin of groundwater in an aquifer under semi-arid climate. Isotopic and hydrochemical compositions combined with geological and hydrogeological data were used for this purpose. The total dissolved solids vary from 239 to 997 mgl−1, and the following groundwater types are recognized: Ca2+–Mg2+–HCO3−, Ca2+–Mg2+–SO42− and Ca2+–Mg2+–Cl−. The groundwater is saturated and slightly supersaturated with respect to carbonate minerals and undersaturated with respect to evaporite minerals, which means that the groundwater composition is largely controlled by the dissolution of carbonate rocks known in the basin. The isotopic contents of groundwaters ranged from −8‰ to −5.2‰ for δ18O, from −52‰ to −34‰ for δD, and from 0 to 5.5 TU for tritium. The hydrogen (δD) and oxygen (δ18O) isotope signatures reveal a significant infiltration before evaporation takes place, indicating a major recharge directly from fractures in the crystalline and limestone formations of Atlas Mountains (above 800 m a.s.l.) and infiltration of surface water in the alluvial cones at the border of the Atlas basins. The very low tritium values suggest that the groundwater recharge follows a long flow path and a mixing between old and modern water is shown. However, a slight evaporation effect is noted in the southern part of the basin close to the Anti-Atlas Mountains.

Extreme crustal oxygen isotope signatures preserved in coesite in diamond.

The anomalously high and low oxygen isotope values observed in eclogite xenoliths from the upper mantle beneath cratons have been interpreted as indicating that the parent rock of the eclogites experienced alteration on the ancient sea floor. Recognition of this genetic lineage has provided the foundation for a model of the evolution of the continents whereby imbricated slabs of oceanic lithosphere underpin and promote stabilization of early cratons. Early crustal growth is thought to have been enhanced by the addition of slab-derived magmas, leaving an eclogite residuum in the upper mantle beneath the cratons. But the oxygen isotope anomalies observed in eclogite xenoliths are small relative to those in altered ocean-floor basalt and intermediate-stage subduction-zone eclogites, and this has hindered acceptance of the hypothesis that the eclogite xenoliths represent subducted and metamorphosed ocean-floor basalts. We present here the oxygen isotope composition of eclogitic mineral inclusions, analysed in situ in diamonds using an ion microprobe/secondary ion mass spectrometer. The oxygen isotope values of coesite (a polymorph of SiO2) inclusions are substantially higher than previously reported for xenoliths from the subcratonic mantle, but are typical of subduction-zone meta-basalts, and accordingly provide strong support for the link between altered ocean-floor basalts and mantle eclogite xenoliths.

Geochemical and Oxygen Isotope Signature of Sea-FloorAlteration Associated with a Polydeformed and Highly Metamorphosed MassiveSulfide Deposit, Ruostesuo, Central Finland

Massive sulfide deposits in the Vihanti-Pyhasalmi district, central Finland, are hosted by a highly deformed and metamorphosed bimodal volcanic sequence. Alteration zones that envelop these deposits are composed of cordierite + orthoamphibole ± garnet-bearing rocks (referred to as COR rocks) belonging to upper amphibolite to granulite facies. Outcrops of mafic and felsic COR rocks (MCOR and FCOR) in the vicinity of the Ruostesuo Zn-Cu deposit define mappable layers with distinctive metamorphic mineral assemblages: orthoamphibole + cordierite ± plagioclase ± quartz (M1COR); garnet + orthoamphibole + cordierite ± plagioclase ± quartz (M2COR); quartz + cordierite + biotite ± orthoamphibole ± cummingtonite ± garnet ± plagioclase (F1COR); and quartz + cordierite + garnet + sillimanite (F2COR). These rocks are interlayered with, or grade into, typical amphibolites (i.e., hornblende + plagioclase) and quartz + plagioclase-bearing rocks that are interpreted as least altered mafic and felsic metavolcanic rocks, respectively. Whole-rock geochemical data for least altered metavolcanic rocks and COR rocks at Ruostesuo define three mafic suites (primitive basalts, basaltic andesites, and ferrobasalts) and two felsic suites (R1 and R2 rhyolites). Using a combination of high field strength element ratios and rare earth element profiles, COR rocks can be assigned to all five volcanic suites. Calculated mass changes for the majority of exposed COR rocks at Ruostesuo are similar to those reported for chlorite + quartz assemblages found within the inner core of alteration pipes located below some sea-floor massive sulfide deposits. Relative to least altered precursors, mafic COR rocks have gained Fe, Mg ± S and lost variable amounts of Si, Ca, Na, K, Rb, Ba, Sr, and Eu, which resulted in an average net mass loss of about 10 percent. The bulk composition of most altered mafic COR rocks (near total loss of Ca, Na, K) reflects the proportion of chlorite to quartz within the premetamorphic alteration assemblages and the composition of the chlorite. Mass-change results for altered mafic rocks indicate that basalts at Ruostesuo were altered by an evolved, Fe-rich hydrothermal fluid at high water/rock ratios. Mass changes for COR rocks of the R2 felsic volcanic suite are similar to those of the mafic rocks (enriched in Fe ± Mg and depleted in Ca, Na, K, Rb, Ba, Sr, and Eu) except for significant additions of Si that resulted in net mass gains of 20 to 30 percent. Felsic COR rocks of the R1 volcanic suite show gains in K, Rb, and Ba that are attributed to the formation of premetamorphic sericite during an alteration stage dominated by hydrothermal seawater. In comparison to least altered metavolcanic rocks, mafic and felsic COR rocks are depleted in 18O, with {Delta}18O values 2 to 5 per mil lower than precursor rocks. Estimated temperatures of alteration were 270° to 350°C. These data indicate that the signature of a high-temperature (~300°C) sea-floor hydrothermal upflow zone is preserved within upper amphibolite to granulite facies rocks at Ruostesuo. The stratigraphic boundary between strongly chloritized mafic volcanic rocks and silicified felsic volcanic rocks marks a transition from net mass losses associated with hydrothermal fluid-dominated alteration to net mass gains associated with seawater-dominated alteration. Silicification at or near the boundary between mafic and felsic volcanic rocks may have been the result of conductive cooling or mixing with seawater from above. In the absence of textural or crosscutting relationships indicative of a younging direction, a stratigraphic model is proposed on the basis of the above geochemical constraints. Altered basalts at Ruostesuo represent the exposed base of a chloritic stockwork zone that passes upsection into altered felsic volcanic rocks that host massive Zn-Cu sulfides. This stratigraphic model has significance for the interpretation of other massive sulfide deposits in the Vihanti-Pyhasalmi district and exploration strategies that couple chemostratigraphy with mass-change results. More significantly, the preservation of premetamorphic oxygen isotope values for strongly metamorphosed and locally migmatitic COR rocks indicate that oxygen isotope mapping is a viable exploration tool in the district and in other mineralized upper amphibolite to granulite facies terranes worldwide.

Geochemical and Oxygen Isotope Signature of Sea-Floor Alteration Associated with a Polydeformed and Highly Metamorphosed Massive Sulfide Deposit, Ruostesuo, Central Finland

Geochemical and Oxygen Isotope Signature of Sea-Floor Alteration Associated with a Polydeformed and Highly Metamorphosed Massive Sulfide Deposit, Ruostesuo, Central Finland

The use of stable isotope ratios in whitebait otolith carbonate to identify the source of prey for Western Australian penguins

A methodology has been developed and used to investigate a direct link between juvenile whitebait ( Hyperlophus vittatus), the key prey species sampled from the stomachs of Little Penguins ( Eudyptula minor), and the specific nursery area of that prey. A unique application of existing methodology initially involved the measurement of the stable isotopes of oxygen ( δ 18O: δ 16O) and carbon ( δ 13C: δ 12C) in sagittal otolith carbonate, using standard mass spectrometric techniques. The results indicated that the inshore distribution of juvenile (0+ year old) whitebait between Cockburn Sound and Koombana Bay, Western Australia consisted of a number of separate assemblages. Measured differences in stable oxygen isotope ratios were attributed to variations in freshwater input to the embayments that provided the whitebait habitats. In contrast, the measured stable carbon isotope ratios probably resulted from the different isotopic compositions of the food webs in the various habitats. Secondly, a comparison of the average value of carbon and oxygen isotope signatures of pooled otoliths from samples of whitebait from a number of different nearshore coastal sites (assemblages), with that of whitebait obtained from the stomachs of penguins at their main breeding site (Penguin Island) indicated that the values from the penguins resemble most closely those of the average otolith values obtained from whitebait from only one site (Becher Point). Assuming that the whitebait sampled were representative of the whitebait in the nearshore habitats and the diets of the penguins, then these results imply that at the time of sampling the penguins were feeding on whitebait from only one site.

Seasonality of δ18O in needles and wood of Picea abies

○ The temporal variability in δ 18 O of organic matter (δ 18 O org ) in needles and in early- and late-wood of mature spruce trees (Picea abies) was related to changes in climate under field conditions, including soil water δ 18 O (δ 18 O wso ), air humidity, temperature and global radiation. Measured δ 18 O org was compared with values calculated with the Dongmann model extended by a dampening factor (f org ) and a biochemical fractionation parameter (e org ). ○ The short-term variations in δ 18 O org of current-year spruce needles could be explained by the δ 18 O wso in the topsoil layer (r 2 = 0.43). The difference in δ 18 O org between the years was influenced by differences in humidity. A highly significant agreement between measured and modeled δ 18 O org was achieved with f org = 0.2, yet the dampening was not as strong (0.66), if only the δ 18 O org during needle development was considered. The temporal δ 18 O org signal of current-year needles correlated with the corresponding early- or late-wood δ 18 O, although strongly dampened in the tree rings. ○ These results show that the short-term climate is much more strongly reflected in the seasonal oxygen isotope signal of current-year needle organic matter than early- and late-wood. Yet the temporal variations in both tissues, leaf and wood are the same, showing how precisely the isotopic pattern is translated to the tree rings.

Precise dating of Dansgaard-Oeschger climate oscillations in western Europe from stalagmite data.

The signature of Dansgaard-Oeschger events--millennial-scale abrupt climate oscillations during the last glacial period--is well established in ice cores and marine records. But the effects of such events in continental settings are not as clear, and their absolute chronology is uncertain beyond the limit of (14)C dating and annual layer counting for marine records and ice cores, respectively. Here we present carbon and oxygen isotope records from a stalagmite collected in southwest France which have been precisely dated using 234U/230Th ratios. We find rapid climate oscillations coincident with the established Dansgaard-Oeschger events between 83,000 and 32,000 years ago in both isotope records. The oxygen isotope signature is similar to a record from Soreq cave, Israel, and deep-sea records, indicating the large spatial scale of the climate oscillations. The signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western Europe, an important site in human cultural evolution. We also find evidence for a long phase of extremely cold climate in southwest France between 61.2 +/- 0.6 and 67.4 +/- 0.9 kyr ago.

Validating methods for measuring delta18O and delta13C in otoliths from freshwater fish.

The ability of the phosphoric acid digestion technique to extract carbon dioxide from biogenic carbonates and reliably reproduce delta(18)O and delta(13)C signatures from standard reference materials (NBS-18, NBS-19) was tested and shown to produce accurate, unbiased measurements of non-biologic materials. The effects of roasting preparation methods commonly reported when analyzing biogenic carbonates were also tested in a series of experiments using reference standards and otoliths obtained from aquacultured Arctic charr and rainbow trout. Roasting had no effect on the isotope measurement of reference standards. No significant differences between mean oxygen isotope signatures from paired experiments with roasted and non-roasted fish otoliths were found. However, otolith oxygen isotope measurements were significantly enriched in comparison to rearing water-based measurements for both species. Agreement between expected isotopic equilibrium and measured otolith delta(18)O values varied as a function of roasting temperature and between species. Criteria for the selection of appropriate roasting temperatures are suggested and favour 350 degrees C in freshwater fish where unbiased estimates of average rearing water temperatures and known differences in rearing temperatures were obtained. Carbon isotopic disequilibria were observed for both species. A mixing model analysis established differences in the percentage of metabolically derived carbon in studied otoliths, with Arctic charr deriving a greater proportion of otolith delta(13)C from metabolism as a result of higher metabolic rates.

Early petrologic processes on the ureilite parent body

Abstract— We present a petrographic and petrologic analysis of 21 olivine‐pigeonite ureilites, along with new experimental results on melt compositions predicted to be in equilibrium with ureilite compositions. We conclude that these ureilites are the residues of a partial melting/smelting event. Textural evidence preserved in olivine and pigeonite record the extent of primary smelting. In pigeonite cores, we observe fine trains of iron metal inclusions that formed by the reduction of olivine to pigeonite and metal during primary smelting. Olivine cores lack metal inclusions but the outer grain boundaries are variably reduced by a late‐stage reduction event. The modal proportion of pigeonite and percentage of olivine affected by late stage reduction are inversely related and provide an estimation of the degree of primary smelting during ureilite petrogenesis. In our sample suite, this correlation holds for 16 of the 21 samples examined.Olivine‐pigeonite‐liquid phase equilibrium constraints are used to obtain temperature estimates for the ureilite samples examined. Inferred smelting temperatures range from ∼1150°C to just over 1300°C and span the range of estimates published for ureilites containing two or more pyroxenes. Temperature is also positively correlated with modal percent pigeonite. Smelting temperature is inversely correlated with smelting depth—the hottest olivine‐pigeonite ureilites coming from the shallowest depth in the ureilite parent body. The highest temperature samples also have oxygen isotopic signatures that fall toward the refractory inclusion‐rich end of the carbonaceous chondrite‐anhydrous mineral (CCAM) slope 1 mixing line. These temperature‐depth variations in the ureilite parent body could have been created by a heterogeneous distribution of heat producing elements, which would indicate that isotopic heterogeneities existed in the material from which the ureilite parent body was assembled.

Oxygen isotope signatures in olivines from São Miguel (Azores) basalts: implications for crustal and mantle processes

Oxygen isotope ratios were measured in olivines from eight São Miguel basalt lavas. With one exception (4.57‰), the olivines are indistinguishable from one another with an average δ 18O of 4.92±0.03‰ (1 σ). This value is slightly lower than that characteristic of upper mantle peridotite and MORB olivines (5.2±0.2‰). Assimilation of ≥10–20% of high-temperature altered lower oceanic crust or 4–9% hydrothermally altered volcanic edifice rocks could produce the low δ 18O signatures in the São Miguel olivines; both of these assimilation models are permitted by the trace element and radiogenic isotope variations in the São Miguel basalts. However, the limited variation in δ 18O despite eruption of the basalts through compositionally and tectonically variable lithosphere, and the lack of correlation of δ 18O with olivine forsterite content, are more easily explained if the olivine δ 18O signatures are inherited from their mantle source. If the δ 18O signatures reflect mantle source compositions, then the relatively low and uniform δ 18O signatures allow constraints to be placed on the origin of the mantle sources beneath São Miguel. Extreme variations in radiogenic isotope signatures have previously been attributed to two component source mixing between a predominant Azores plume source with mild HIMU-like characteristics, and an EMII-type mantle with very radiogenic Sr. The low δ 18O signatures in the São Miguel basalt olivines suggest that the predominant Azores plume source contains >10% hydrothermally altered recycled oceanic crust. The limited variation in δ 18O is consistent with a component of recycled sediment in the São Miguel EMII-type source, although, unlike the case for other EMII OIB (e.g. Samoa and Society), the relatively low δ 18O signatures in São Miguel restrict any involvement of recycled sediment to <2% of a relatively low δ 18O and very radiogenic Sr or high Rb/Sr sediment. Involvement of several percent metasomatized subcontinental lithospheric mantle could alternatively produce the EMII-type Sr–Nd–Pb isotope signatures without significantly affecting the plume-related low δ 18O signatures. The São Miguel δ 18O data are thus consistent with mixing between a low δ 18O Azores plume source with a component of subducted, hydrothermally altered lower oceanic crust, and either minor recycled sediment or localized EMII-rich delaminated subcontinental lithospheric mantle. The latter could have been introduced into the lithosphere or shallow asthenosphere during opening of the Atlantic ocean basin.