Range Of Isotope Values Research Articles

Modern reindeer and mice: revised phosphate–water isotope equations

The oxygen isotope composition of bone and tooth phosphate of 34 mice specimens ( Pitymus sp., Microtus arvalis and Arvicola terrestris) coming from seven different locations in Italy, Germany and Switzerland was measured by means of well-established techniques. These measurements were carried out with the purpose of establishing quantitative relationships between the δ 18O p from different mice genera and the mean δ 18O w values and to compare these data to previous measurements carried out on various specimens belonging to the genus Apodemus. The three genera studied showed a similar behaviour when compared to the mean δ 18O w values. The slope of the equation calculated for these three genera is significantly different from the slope obtained from Apodemus specimens. Reconsidering the δ 18O w values suggested in the case of Apodemus due to the small number of data available at that time, it seems that these values are too negative by 0.5 to about 1.5‰. If so, the Apodemus equation becomes almost identical to the equation calculated for the new mice values and, consequently, one could conclude that several micromammal genera and species might behave in the same way and obey the same relationship with the mean δ 18O w values. A set of 25 samples of modern reindeer skeletal material from Spitzbergen, Russia and Siberia was also studied with the aim of improving the reindeer isotope equation obtained from a previous study. In fact, the slope of that equation was somehow uncertain due to a rather large range of isotope values obtained from each group of reindeers coming from the same location. The new results confirm such ‘anomalous’ behaviour already shown by other mammals and probably related to dietary behaviours, water fluxes with the environment and isotopic composition of ingested food and water rather than to imperfect equilibrium conditions with environmental water. However, the equation calculated from both the old and new, statistically more significant, isotope results is not far from the previous one.

Foraminifera as indicators of methane-rich environments: A study of modern methane seeps in Santa Barbara Channel, California

Methane hydrates have been implicated as a possible forcing mechanism for rapid climate change during the late Quaternary, via the release of methane from marine reservoirs. Carbon isotopic excursions of foraminifera observed in high-resolution sediment records from Santa Barbara Basin (ODP Site 893) have been interpreted as evidence of episodes of methane release during the late Quaternary. However, the potential relationship between methane in the modern environment and its effect on the stable isotopic composition of benthic and planktonic foraminifera is not well understood. Short cores taken inside a ∼500-m-diameter pockmark with active methane seeps in Santa Barbara Channel, CA, USA, yield information on the effect of high concentrations of environmental methane on foraminiferal assemblages and their stable isotopic composition. These seeps reside at ∼120 m water depth in the eastern portion of the channel, associated with faults and fractures in the Miocene Monterey Formation. Planktonic foraminiferal assemblages and radiocarbon ages show that surface sediments are of late Glacial age (∼25 300 calendar years). This finding suggests an absence of Holocene and deglacial sedimentation in the seep area, likely due the formation of the pockmarks and subsequent winnowing or lack of sedimentation. Benthic foraminiferal assemblages are marked by high abundance of Bolivina tumida, a species previously observed during interstadial episodes and the early Holocene in Santa Barbara Basin, interpreted to include episodes of high methane flux from the basin. While oxygen isotopic values of both benthic and planktonic foraminifera appear normal for cool, glacial conditions, negative carbon isotopic values are seen in benthic foraminifera, primarily in the near-surface intervals. In one interval (1–2 cm below surface), both benthic and planktonic species record highly negative carbon isotopic values, implying that isotopically light methane influenced the entire water column. This spike is of similar or larger magnitude than several earlier observed spikes in the late Quaternary record from ODP Site 893. Single specimen isotopic analyses give an indication of the total range of carbon isotopic values recorded (−0.01 to −25.23‰ for benthics, −0.07 to −22.22‰ for planktonics) in the spike interval. Since modern methane seeps are commonly associated with authigenic carbonate precipitation, scanning electron microscopy was used to assess the possibility that post-depositional alteration may have affected the isotopic composition of the test. These experiments indicate that authigenic carbonate is absent and therefore unable to account for the highly negative carbon isotopic signatures. These data improve our understanding of methane-associated environments by showing that foraminiferal isotopic composition and assemblages may be used to discern the flux of methane from the oceanic reservoir in the geologic record.

Stable isotopes, hypsodonty, and the paleodiet ofHemiauchenia(Mammalia: Camelidae): a morphological specialization creating ecological generalization

Morphological adaptations may indicate increased specialization (narrowing of ecolog- ical niche) or expansion of the suite of lifestyles available to an organism (increasing niche breadth). Hypsodonty in mammals generally has been interpreted as a specialization into a grazing niche from a browsing niche. Here I examine the feeding strategy of the extinct hypsodont camel Hem- iauchenia through an analysis of stable carbon isotope values from its tooth enamel, which was used to clarify its feeding strategy and to resolve conflicting interpretations of dental versus muzzle attributes. The paleodiet of Hemiauchenia is then used to test whether hypsodonty correlates to graz- ing within fossil Lamini. This study focuses on fossils from Florida, which is geographically ideal because unlike other regions of the country almost all extant plants on which animals browse use the C3 photosynthetic pathway. In contrast, most of the grasses and sedges utilized by grazers use the C4 photosynthetic pathway. If Hemiauchenia was an obligate grazer, the stable carbon isotope values of tooth enamel should reflect primarily a diet of C4 grass and sedge (.21.3‰). If Hem- iauchenia was mainly a browser, the isotopic value should be considerably more negative reflecting ingestion primarily of C3 browse (,27.9‰). The mean d 13C values for Hemiauchenia during each time interval average more negative than 28.0‰, indicating a dominantly C3 browse diet, and there is no evidence for abandonment of the browsing niche from the Hemphillian through the Ran- cholabrean North American Land Mammal Ages. However, an increase in the range of isotopic values indicates a diet with a higher proportion of C4 grasses and sedges through time. This study therefore suggests that Hemiauchenia was a hypsodont intermediate feeder with preference for browse during the past 5 million years. Hypsodonty is not strictly associated with obligate grazing; instead it may, in this case, represent an adaptation to widen niche breadth that allowed grazing as well as browsing.

Lower crustal granulite xenoliths from the Pannonian Basin, Hungary, Part 2: Sr–Nd–Pb–Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their δ18O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704–0.708) and lower 143Nd/144Nd (0.5128–0.5124), with higher δ18O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in eHf–eNd isotope space has a shallower slope than the crust–mantle array, with the metasedimentary rocks forming the low eHf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in δ18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of δ18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.

Stable isotopes, hypsodonty, and the paleodiet ofHemiauchenia(Mammalia: Camelidae): a morphological specialization creating ecological generalization

Morphological adaptations may indicate increased specialization (narrowing of ecological niche) or expansion of the suite of lifestyles available to an organism (increasing niche breadth). Hypsodonty in mammals generally has been interpreted as a specialization into a grazing niche from a browsing niche. Here I examine the feeding strategy of the extinct hypsodont camelHemiaucheniathrough an analysis of stable carbon isotope values from its tooth enamel, which was used to clarify its feeding strategy and to resolve conflicting interpretations of dental versus muzzle attributes. The paleodiet ofHemiaucheniais then used to test whether hypsodonty correlates to grazing within fossil Lamini. This study focuses on fossils from Florida, which is geographically ideal because unlike other regions of the country almost all extant plants on which animals browse use the C3photosynthetic pathway. In contrast, most of the grasses and sedges utilized by grazers use the C4photosynthetic pathway. IfHemiaucheniawas an obligate grazer, the stable carbon isotope values of tooth enamel should reflect primarily a diet of C4grass and sedge (>−1.3%). IfHemiaucheniawas mainly a browser, the isotopic value should be considerably more negative reflecting ingestion primarily of C3browse (<−7.9%). The mean δ13C values forHemiaucheniaduring each time interval average more negative than −8.0%, indicating a dominantly C3browse diet, and there is no evidence for abandonment of the browsing niche from the Hemphillian through the Rancholabrean North American Land Mammal Ages. However, an increase in the range of isotopic values indicates a diet with a higher proportion of C4grasses and sedges through time. This study therefore suggests thatHemiaucheniawas a hypsodontintermediate feeder with preference for browseduring the past 5 million years. Hypsodonty is not strictly associated with obligate grazing; instead it may, in this case, represent an adaptation to widen niche breadth that allowed grazing as well as browsing.

Modeling the distribution of isotopic ratios in geochemical reservoirs

We present an extension of the conventional geochemical reservoir model for the evolution of the Earth’s crust–mantle system in which we calculate not only the mean isotopic ratios, but also the distribution of those ratios within the reservoirs. Owing to low chemical diffusion rates, subreservoirs that are created by mass transport into and out of the mantle effectively exist as distinct geochemical entities for all time. By tracking these subreservoirs, we obtain a model of the full range of isotopic values represented in the mantle. Using results from numerical calculations of mixing, we also track the length scales associated with each subreservoir. Applying simple statistics, we obtain the distribution of expected measurements as a function of the stirring time, effective melt fraction, sampling volume, and mass transport history. We present calculations of isotopic heterogeneity for two simple mantle evolution models and explore the sensitivity of geochemical observables to the variables mentioned. We focus on the Rb–Sr and Sm–Nd systems and are able to reproduce much of the observed complexity of oceanic basalts. We infer that the stirring time of the mantle falls between 250 and 750 Myr, and that the initial length scale of mantle heterogeneity before stirring is of the same order as the length scale of sampling. We also conclude that the differences between isotopic data from mid-ocean ridge basalts and ocean island basalts cannot simply be due to differences in sampling volume, but must also reflect differences in the source reservoirs and/or melting processes. Increasing the size of the ocean island basalt source region by 45% with respect to the mid-ocean ridge basalt source region reproduces the offset between the two distributions, but still fails to explain the more isotopically extreme measurements. Our results show that the argument suggesting that the absence of samples with a primitive isotopic signature indicates that no primitive material remains in the mantle is not valid.

Assemblage diversity and isotopic signals of modern ostracodes and host waters from Patagonia, Argentina

Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with δ 18O and δ 13C DIC (DIC: dissolved inorganic carbon) values of host water samples, provide a first data set that characterizes a wide range of modern aquatic environments in the Laguna Cari-Laufquen (41°S, 68–69°W) and the Lago Cardiel area (48–49°S, 70–71°W) in Patagonia, Argentina. This data set will ultimately be used to interpret and calibrate data acquired from lake sediment cores with the goal of reconstructing past climate. Species assemblages and isotope values can be assigned to three groups: (1) springs, seeps and streams, (2) permanent ponds and lakes, and (3) ephemeral ponds and lakes. Springs, seeps and streams are characterized by Darwinula sp., Heterocypris incongruens, Eucypris fontana, Amphicypris nobilis and Ilyocypris ramirezi. Ostracode and water isotope values range between −13 and −5‰ for oxygen, and between −15 and −3‰ for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichment. Limnocythere patagonica, Eucypris labyrinthica, Limnocythere sp. and Eucypris aff. fontana are typical species of permanent ponds and lakes. Isotope values indicate high degree of evaporation of lake waters relative to feeder springs and streams and range between −7 and +5‰ for oxygen, and −5 and +4‰ for carbon. Limnocythere rionegroensis is the dominant species in ephemeral ponds and lakes. These systems display the most enriched isotope values in both ostracodes and host waters, extending from −5 to +7‰ for oxygen, and from −5 to +6‰ for carbon. Living ostracodes show a positive offset from equilibrium values of up to 2‰ for oxygen. Carbon isotope values are up to 6‰ more negative than equilibrium values in highly productive pools. Comparison of ostracode and host water isotope signals permits assessment of the life span of the aquatic environments. Valves from dead ostracodes collected from ephemeral ponds and lakes show a wide scatter with each sample providing a snapshot of the seasonal history of the host water. The presence of the stream species I. ramirezi and a wide range of ostracode isotope values suggest that ephemeral ponds and lakes are fed by streams during spring run-off and seasonally dry. A temporary character of waters is also indicated by H. incongruens, a drought-resistant species that occupies most springs and seeps. In addition, L. rionegroensis has adjusted its reproduction strategies to its environment. Whereas only females were collected in fresh host waters, males were found in ephemeral ponds and lakes with higher solute content. Sexual reproduction seems to be the more successful reproduction strategy in waters with high and variable salinities and affected by seasonal droughts. The temporary character of the aquatic environments shows that the availability of meteoric water controls the life span of host waters and underlines the sensitivity of the area to changes in precipitation.

δ18O of carbonate, quartz and phosphate from belemnite guards: implications for the isotopic record of old fossils and the isotopic composition of ancient seawater

Belemnite guards of Cretaceous and Jurassic age were found to contain varying amounts of quartz deposited both on the external surface and inside the rostrum. The oxygen isotopic composition of coexisting carbonate, quartz and phosphate from the same rostrum was measured according to well-established techniques. None of these compounds showed isotopic values in equilibrium with one another. Assuming δ18O values of the diagenetic water within the range of meteoric waters, the δ18O(SiO2) yield temperatures in agreement with the apparent secondary origin of this phase. The δ18O(CO32−) range, with a certain continuity, between −10.8 and +0.97 PDB-1 with most of the intermediate values being within the range of the carbonate isotopic values of Mesozoic fossils. The most positive isotopic results obtained from phosphate are close to +23/+24‰ (V-SMOW). They can hardly be related to a secondary origin of the phosphate, or to the presence of diagenetic effects, since these results are among the most positive ever measured on phosphate. As far as we know there is no widespread diagenetic process determining an 18O enrichment of phosphate. The very low concentration of phosphate did not allow the determination of its mineralogical composition. All the available δ18O(PO43−) values from belemnite and non-belemnite fossils of marine origin of Tertiary and Mesozoic age are reported along with the newly measured belemnites. The following conclusions may be drawn from the data reported: (1) the pristine oxygen isotope composition of fossil marine organisms (either carbonate or phosphate) may easily undergo fairly large changes because of oxygen isotope exchange processes with diagenetic water; this process is apparent even in the case of geologically recent fossils; (2) the δ18O(PO43−) of belemnite rostra seems to be, at least in the case of the most positive results, in isotopic equilibrium with environmental water because of the similarity between the results from Cretaceous belemnites and the results from Cretaceous and Lower Tertiary pelecypods and fish teeth; 3) if so, the only feasible interpretation that can be suggested for the 18O enriched data is the possibility of a relatively large variation of the oxygen isotopic composition of ocean paleowater from Jurassic to recent time.

Relationship between δ 18O values for skeletal apatite from reindeer and foxes and yearly mean δ 18O values of environmental water

The oxygen isotope composition of bone and tooth phosphate of 50 fox specimens and 30 reindeer specimens from various locations with different climatic and environmental conditions was measured. The existing relationship between these values and the mean oxygen isotope composition of local meteoric water has been calculated. In the case of foxes, specimens belonging to two genera ( Vulpes and Alopex) and three different species were measured. The samples fit a straight line whose equation can be used for paleoclimatological studies either in Arctic or in temperate regions. For reindeer ( Rangifer), a relatively large range of isotopic values was obtained from each location, suggesting imperfect equilibrium conditions with environmental water. The calculated equation can be used for semi-quantitative information on local paleowaters at high latitudes only.

North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: Inferences from geochemical and Nd isotopic characteristics of Triassic metasedimentary rocks

New geochemical and Nd isotopic data from Triassic (ca. 220 Ma) clastic metasedimentary rocks assigned to the Quesnel terrane (Quesnellia) in southeastern British Columbia show that detritus was derived from both primitive and evolved sources. Twenty-four samples yield eNd 2 2 0 values between-9.1 and +5.6, independent of sample location. These samples have Eu anomalies between 0.6 and 1.0, La N /Yb N ratios between 3.8 and 11.4, La/Sc ratios ranging from 0.38 to 4.76, and Th/Sc ratios between 0.09 and 1.62. This range of isotopic and geochemical values overlaps that of samples from upper Paleozoic to Mesozoic rocks of the North American miogeocline and Upper Triassic Nicola Group volcanic rocks of the Quesnel terrane. The range in Nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental North America (evolved cratonic source) and a volcanic arc (primitive source) developing on the continental margin. These data do not support previous interpretations of the Quesnel terrane as an intraoceanic arc. The inferred depositional link between North America and the Quesnel arc in Triassic time is ca. 40 Ma older than the hypothesized obduction of the Quesnel terrane onto the North American continental margin and, when combined with existing geological information, strongly supports a pericratonic, nonexotic origin for the Quesnel terrane in southeastern British Columbia.

Sedimentary phosphate formation in warm shallow waters: new insights into the palaeoceanography of the Permian Phosphoria Sea from analysis of phosphate oxygen isotopes

Abstract Oxygen isotopic analyses of the structural phosphate ( δ 18 O P ) in sedimentary phosphorites of the Upper Permian Phosphoria Formation reveal that phosphogenesis occurred across a broad range of palaeoenvironments. The δ 18 O P values from well-preserved hand-picked phosphate-grain separates range from 20.2 to 13.7 (±0.5)‰ SMOW, with the lowest values in the most landward shallow-water deposits and the highest values in the most basinward deeper water deposits. The relationship between the maximum burial depth and δ 18 O P is opposite to that predicted if burial diagenesis was responsible for the isotopic signature of these phosphate grains. This suggests that the regional trend in oxygen isotopic data is not due to burial diagenesis. The range in oxygen isotopic values does correspond to the potential range of palaeotemperatures in the shallow and broad epicontinental Phosphoria Sea. The range is 14–42°C (±3.5°C) assuming a seawater δ 18 O of −2.5 ‰ SMOW. Palaeotemperatures along an offshore to onshore transect indicate that upwelling involved temperate (14–26°C; mean of 21°C) but not cold waters, and that warming across the palaeoshelf resulted in mean shallow-water temperatures of 34–37°C. The latter values approximate those predicted by climate and ocean circulation models. These warm palaeotemperatures provide further evidence that phosphogenesis in the epicontinental Phosphoria Sea occurred under palaeoceanographic conditions that were different from modern phosphorite depositional systems. This suggests that a purely actualistic approach to the interpretation of the Phosphoria phosphorites is not valid.

A new look at the components of the petroleum system of the South Caspian Basin

Detailed knowledge of the petroleum systems of mature petroleum provinces is often lacking. This has often resulted from the early discovery of most of these provinces' reserves. The South Caspian basin represents one such province. The “modern” era of petroleum exploration in the South Caspian began more than 100 years ago. Even with this long history several aspects of the petroleum system have remained largely speculation. Among the key questions are: • What is the primary hydrocarbon source for the region? • Where and when has hydrocarbon generation taken place? • Can exploration be extended beyond known stratigraphic and areal limits? Recent integrated geologic and geochemical studies have attempted to answer these and other questions for the petroleum system of the South Caspian basin. These studies attempted to overcome prior problems associated with limited sample availability, severity of surface weathering, data quality and completeness. This study confirmed the Maykop Suite as the primary oil source, but restricted its stratigraphic distribution. Organic carbon content within the Maykop ranged upward to 14.28 wt.%, with hydrocarbon generation potentials (S 2) approaching 75 mg HC/g rock. Source rock deposition was cyclic and included both oil- and gas-prone episodes. Geochemically, the region's oils are highly variable as a result of their complex and variable migration and alteration histories rather than their derivation from multiple source rock units. The oils display similar molecular and isotopic compositions to the extracted Maykop samples confirming their genetic relationship. The range in carbon isotope values reflects secular changes within the Maykop Suite source. Unlike some earlier studies, this study showed that all of the oils were generated within the conventional “oil-window” ( R o≈0.9–1.0%). This places the source system at depths in excess of 5 km and emphasizes the role that vertical migration played in the development of the basin's hydrocarbon accumulations. There is evidence that migration was episodic, possibly related to punctuated generation or the opening of conduits through the activity of mud diapirs. Model results also suggest that the rapid Plio–Pleistocene subsidence and sedimentation were the primary controls on hydrocarbon generation. These and other geochemical investigations confirm the potential for the extension of the known petroleum system beyond its current stratigraphic and areal limits.

Dietary Evidence for Competing in the New World Order

Many changes were taking place in the terrestrial landscape near the end of the Mesozoic. Angiosperms are first unequivocally recorded in Cretaceous strata; a rapid diversification and increase in abundance occurred during the Campanian-Maastrichtian. This shift from ferns-gymnosperm to angiosperm dominance was of major importance to terrestrial herbivores. Increasing complexity in the dental batteries of hadrosaurs and ceratopsians also occurred simultaneously. Stable carbon isotopes from teeth have provided detailed information on ecosystem and dietary shifts during the Miocene-Pliocene, with the advent and spread of C4 grasses. However, during the Late Cretaceous all plants probably used only the C3 pathway and carbon isotopes are less useful in distinguishing browsing and grazing habits. However, a large range of carbon isotope values is produced by C3 plants, and indicates that environmental parameters (aridity/humidity) are more important than those produced by taxonomic effects.

New Geochemical Support for Mixing-Zone Dolomitization at Golden Grove, Barbados

Dolomite occurs in the Golden Grove reef terrace of late Pleistocene age in southeastern Barbados, where fore-reef lithologies have been partially to completely dolomitized. Interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade. Among the points of contention are the timing of deposition, dolomitization and dolomite cement precipitation, and the relationship of replacement dolomite with dolomite cement. Data from this study indicate that dolomitization occurred during a single relative sea-level fall following reef development at marine oxygen isotope stage 7.3, at approximately 216 ka. Replacement dolomitization affected both micritic matrix and skeletal grains. During or after replacement dolomitization, dolomite and calcite cements filled primary and secondary pores. The cement paragenesis comprises early dolomite cement, complexly banded cements of alternating calcite and dolomite, late dolomite cement of near-stoichiometric composition, and a final phase of blocky calcite cement. Replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids. Stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite, dolomite cements, and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition. Dolomite oxygen isotope values range from +0.18 to +3.85‰ PDB, and become progressively more depleted throughout the paragenesis, indicating a greater influence of meteoric water through time. Remarkably depleted carbon isotope compositions, for both replacement dolomite and dolomite cement (−9.53 to −23.04‰ PDB), originated from oxidation of thermogenic methane advected from the underlying accretionary complex. Methane was oxidized in marine waters that subsequently mixed with meteoric waters, as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis. Dolomite strontium contents are substantially elevated above those expected for marine dolomitization, ranging from 640 to 1650 ppm, and increasing throughout the paragenesis. Meteoric water provided the excess strontium, as aragonite-to-calcite transformation in the exposed reef terrace increased the fluid Sr/Ca ratio. The continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association. These new data provide additional support to the mixing-zone interpretation for Golden Grove, and that the model of mixing-zone dolomitization remains viable.

Age and Pb-Sr-Nd isotopic systematics of plutonic rocks from the Green Mountain magmatic arc, southeastern Wyoming: Isotopic characterization of a Paleoproterozoic island arc system

Three new U-Pb zircon ages and the Pb-Sr-Nd isotopic systematics of 24 whole-rock samples from mainly plutonic rocks of the Sierra Madre and Medicine Bow Mountains near the Colorado-Wyoming border help establish the Green Mountain magmatic arc as a Paleoproterozoic, variably eroded, island arc terrane. The Green Mountain magmatic arc, a terrane composed of variably metamorphosed volcanic and volcaniclastic rocks, minor metasedimentary rocks, high-grade gneisses, and plutons ranging from gabbro to granodiorite, was formed between ca. 1792 and 1744 Ma. It is the northernmost and oldest part of the Colorado province and is separated from Archean rocks to the north by the east-west-trending Cheyenne belt. New U-Pb zircon ages were determined for two dioritic samples of the Mullen Creek complex (1778 ±2 and 1778 ±17 Ma; an ultramafic/mafic layered intrusion) and for a sample of the Rambler granite (1771 ±3.4 Ma); both units are exposed in the Medicine Bow Mountains. A Sm-Nd internal isochron age of 1750 ±24 Ma (ϵ Nd i = + 3.8) was determined that is within error of the Sm-Nd whole-rock isochron age for the entire Lake Owen sample database (1775 ±45 Ma). Initial Nd signatures (+ 3.3 to 4.8) indicate that the bulk of the arc rocks was derived from a depleted mantle source at 1.78 Ga. Although the Rb-Sr systematics appear disturbed, data from extremely low Rb/Sr, non-hydrous, ultramafic layered units indicate an initial 87 Sr/ 86 Sr value of 0.7024. The range of initial Sr isotopic values for these rocks is elevated relative to depleted mantle sources at 1.78 Ga, an isotopic distinction of modern primitive oceanic island arc systems. The U-Pb data on the same mafic rock samples are consistent with the other isotopic results. The values define average initial Pb values of 206 Pb/ 204 Pb = 15.7 and 207 Pb/ 204 Pb = 15.3, indicative of a depleted mantle source at 1.78 Ga. Felsic plutonic arc rocks exhibit disturbed Pb and Sr isotopic behavior. They are characterized by the same depleted mantle signature with initial ϵ Nd values of ∼2.9–4.4, however, indicating little crustal contamination of source magmas for granites and precluding their derivation by subduction of Archean crustal components during collisional accretion of the arc.

Intrapopulation variation in carbon and nitrogen stable isotope ratios in the earthwormAporrectodea longa

The natural abundance variations in carbon and nitrogen stable isotope ratios in a population of the earthwormAporrectodea longa, a species known to feed on both soil and plant litter, is reported in this paper. Worms were collected from a small land area of an old white clover field and body tissue and mucus were analyzed separately. The range of isotopic values was small, but patterns of variation were not random. Tissue carbon and nitrogen isotope ratios were significantly higher in adult than in juvenileA. longaand tissue nitrogen isotope ratios tended to increase with increasing biomass of individuals. Further, carbon and nitrogen isotope ratios were positively correlated in both tissue and mucus. Possible causes of the observed patterns, including physiological effects, body composition and assimilation of C and N from different plant, soil and microbial sources are discussed. It is concluded that the causes of natural variability in isotopic composition must be understood and validated experimentally before natural abundance stable isotope methods can be used for the analysis of trophic relations among detritivorous soil invertebrates.

99/01132 Low cost adsorbents for low-temperature cleaning of flue gases : Rubio, B. and Izquierdo, M. T. Fuel, 1998, 77, (6), 631–637

There are abundant gas resources in petroliferous basins of China. Large to midsize gas fields are found in Eastern, central and Western of China. However, origin, constituents and isotopic composition of natural gas in different gas fields are varied distinctly, and some present strong chemical secondary alteration and show variation both in age and space. Based on the systematic analysis of constituents and carbon isotope of a large number of gas samples, combined with the geological characteristics, this paper classifies the origins of the gases, explores the gas isotope characteristics and evolutionary regulation with the variation time and space, and further discusses the distinctive geochemistry of the gases in China. These gases are dominated by dry gas, its methane carbon isotope values range from −10‰ to −70‰, ethane from −16‰ to −52‰, propane from −13‰ to −43‰, and butane from −18‰ to −34‰. The carbon isotopes of most gases show the characteristics of humic-derived gas and crude oil cracked gas. In addition, large primary biogenic gas fields have been discovered in the Qaidam basin; inorganic-derived alkane gases have been discovered in deep of the Songliao Basin. Half of these gas fields are characterized by the alkane carbon isotope reversal in different degrees. Research indicates there are several reasons can result in carbon isotope reversal. Firstly, gas charge of different genetic types or different source in one gas reservoir may cause carbon isotope reversal. Besides, high-over mature evolution of gas can also lead to the carbon isotopic reversal of alkanes. Thirdly, secondary alteration of hydrocarbons may also result in abnormal distribution of carbon isotope, isotope transforms to unusual light and heavy.

Petrology and geochemistry of syn- to post-collisional metaluminous A-type granites—a major and trace element and Nd–Sr–Pb–O-isotope study from the Proterozoic Damara Belt, Namibia

Leucocratic A-type granites of the Proterozoic Damara orogen, Namibia are a distinctive groups of rocks based on their mineralogical, chemical and isotopic characteristics. A-type granites can be distinguished from the leucocratic S-type granites on the basis of Fe-rich amphibole and biotite and higher abundances of Zr, Nb, Y and LREE but lower values of 87 Sr/ 86 Sr (500 Ma) and δ 18 O in the A-type granites. In comparison with A-type granites described in the literature, the Damaran A-type granites are enriched in CaO, Al 2O 3, MgO, FeO, Ba, Sr and Eu but depleted in Ga and Zn. Low K/Rb, Rb/Sr, Rb/Ba and Sr/Ba ratios indicate that these melts are not fractionation products of calc-alkaline source rocks and therefore, these chemical criteria are interpreted as a feature of the source rocks, i.e., undepleted metatonalites. The A-type granites have high calculated zircon and monazite saturation temperatures mostly in excess of 800°C and it is concluded that these granites represent high-temperature partial melts. The relationship between major- and trace element data and isotopic composition demonstrates that the A-type granites originate from several sources and have undergone different processes. The lack of evidence for shallow crustal contamination (e.g., lack of country rock xenoliths), the low viscosities of the melts (as inferred from their high calculated temperature) and their high Nd concentrations relative to the country rocks suggest that the isotopic characteristics were acquired from an enriched mantle. Initial Sr isotopic compositions vary over a narrow range (0.70486–0.70928) but the initial Nd isotopic compositions vary considerably with initial ε Nd values between −0.03 and −6.35. This wide range in the Nd isotope values is probably the result of interaction of mantle-derived magmas with crustal rocks. A-type granites at Baukwab have negatively correlated initial ε Nd values and 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios indicating interaction of mantle-derived magmas (low initial ε Nd, low 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ) with continental crust (high initial ε Nd, high 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ). A-type granites from Albrechtstal and Oetmoed have positively correlated Pb isotope ratios and show also positive correlations between Nd isotopes and Pb isotopes, suggesting interaction of at least two different source materials. The relationship between Nd isotopes and Nd concentrations precludes simple assimilation, instead mixing of different sources seems to be more realistic. For the A-type granites from Oetmoed, oxygen isotopes are negatively correlated with Sr isotopes indicating that the crustal end-member has high δ 18 O values and rather low initial 87 Sr/ 86 Sr ratios whereas a hypothetical mantle component has lower δ 18 O values but higher initial 87 Sr/ 86 Sr ratios. The chemical and isotope data suggest that the Damaran A-type granites were generated by a variety of processes including partial melting of mantle-derived tonalitic sources, limited crystal fractionation and interaction of these magmas with crustal material. These data indicate that the Damaran A-type suites mark episodes of continental growth during which mantle material, although limited in extent, is added to the crust.

Comment: Use of stable isotopes to determine diets of living and extinct bears

Notes 2300 Isotopic biogeochemistry of ancient bon collagen is a powerful tool for determining the diet of extinct species. It has been increasingly widely used during the last few years (e.g., Bocherens et al. 1994; Matheus 1995; Grocke and Bocherens 1996). This seemingly simple approach is nonetheless useful only when basic requirements are fulfilled. It appears that Hilderbrand et al. (1996), in their study of extinct cave bear diet using collagen carbonand nitrogenstable isotopes, have left unanswered several key points, which precludes valid conclusions, first by misquoting previous work and second by publishing data that are not interpretable as they stand in the paper. The first point is the misquotation of the conclusions of Bocherens et al. (1994) concerning the cave bear diet. Indeed, Hilderbrand et al. (1996) leave the impression that the interpretation of isotopic-analysis results obtained by Bocherens et al. (1994) is incorrect and that the results presented in that paper, combined with their isotopic-analysis results, indicate an omnivorous diet with a significant meat component for extinct European cave bears. In Bocherens et al. (1994), one key result is that in bears, modern and fossil, isotopic values measured on dentine and bone are not at all equivalent from a palaeodietary point of view. In mammals with brachyodont teeth, dentine collagen reflects a milk diet and thus a higher trophic level than that of the adult. Thus, isotopic-analysis results from such dentine will indicate carnivory, even in species where adults are plant eaters. A crucial consequence is that only adult bone collagen can be used as reflecting adult diet. The discussion in Hilderbrand et al. (1996) mixes all values, from dentine and bone, without taking this point into account. Thus, concluding that the values published in Bocherens et al. (1994) indicate omnivory in cave bears is clearly incorrect. A second point is the fact that the approach used by Hilderbrand et al. (1996) presented several flaws in sampling and assessing the validity of the isotopic measurements, which makes the interpretation of these results highly problematical. From the “sample collection” section of that paper, it appears that Pleistocene samples come from numerous different sites whose age is not known and that are spread all over Europe. It is well known that isotopic signatures are strongly influenced by environmental factors, such as aridity, soil acidity, canopy effect, and season of growth (e.g., Heaton et al. 1986; Ambrose 1991; van der Merwe and Medina 1991; van Klinken et al. 1994; Rodiere et al. 1996). These effects lead to a very large range of isotopic values for a given species when individuals come from different places and different times (e.g., Fizet et al. 1995; Bocherens et al. 1997). An illustration of this high variability can be given by comparing the nitrogen-isotope compositions of large bovine species (Bos and Bison) in three western European upper Pleistocene sites in Belgium (Sclayn: δ15N = 4.7 ± 0.4‰; Bocherens et al. 1997), France (Marillac: δ15N = 6.7 ± 0.9‰; Fizet et al. 1995), and Italy (Paglicci: δ15N = 7.9 ± 1.4‰; Iacumin et al. 1997). It is thus clear that the data presented in Hilderbrand et al. (1996) cannot be interpreted in a palaeodietary perspective, because exact geographical and time origins are not provided in the paper. Also, the lack of individual age indication for the bear samples in Hilderbrand et al. (1996) makes it possible that high δ15N values are due to young age and suckling. Another problem with the isotopic-analysis data from Hilderbrand et al. (1996) is the total absence of information on ancient collagen integrity. It is well known that collagen is altered in ancient bones, and only when chemical purity is assessed can isotope-analysis results be considered with confidence to reflect biogenic signatures (DeNiro 1985; Ambrose 1990; Bocherens 1997). Classical ways of checking collagen purity are to use yield of extraction, %C, %N, and C/N ratio of extracted collagen, and in some cases amino acid analyses. None of these data are provided in Hilderbrand et al. (1996), which precludes any serious interpretation of the presented isotope-analysis results. Finally, so far as the isotopic-analysis data on cave bears presented in Hilderbrand et al. (1996) are concerned, valuable palaeodietary information might be obtained if (i) collagen indegeneity was assessed according to the chemical criteria mentioned above, (ii) time and geographical origins were provided for all samples, (iii) comparisons were made only among herbivores, carnivores, and bears from the same site and the same time period, and (iv) only isotopic-analysis values from adult bone collagen were considered.

Isotopic variation in fossils and matrix of the Cretaceous Red Chalk at Speeton and South Ferriby, Yorkshire, England

SUMMARY Macro- and micro-fossils (belemnites, planktonic and benthonic foraminifera) and coccolithic matrix from the middle Cretaceous Red Chalk of Yorkshire, UK, have been examined using petrographic, isotopic, and chemical methods to investigate the conditions within which they were formed or altered. Only the belemnites ( Neohibilites sp.) attain the petrographic and geochemical criteria for being well preserved. In contrast, the foraminifera and matrix reveal evidence of diagenetic alteration. Mean belemnite δ 18 O values are −0.97‰ (PDB) from South Ferriby and −1.28‰ (PDB) from Speeton. Assuming a sea water SMOW value of −1.0‰, mean palaeotemperatures calculated from the oxygen isotopic compositions are 15.9°C and 16.8°C respectively. The range of isotopic values provided by the belemnites in this study suggests that the isotopic values obtained from early studies of the Chalk, that used only petrographic and/or hand specimen descriptions to screen for diagenetic alteration, could largely represent unaltered values. The palaeotemperatures interpreted from the belemnites are comparable to recent estimates of middle Cretaceous temperature from general circulation model predictions.