Range Of Carbon Isotope Values Research Articles

Highly enriched carbon and oxygen isotopes in carbonate-derived CO2 at Gale crater, Mars

Carbonate minerals are of particular interest in paleoenvironmental research as they are an integral part of the carbon and water cycles, both of which are relevant to habitability. Given that these cycles are less constrained on Mars than they are on Earth, the identification of carbonates has been a point of emphasis for rover missions. Here, we present carbon (δ13C) and oxygen (δ18O) isotope data from four carbonates encountered by the Curiosity rover within the Gale crater. The carbon isotope values range from 72 ± 2‰ to 110 ± 3‰ Vienna Pee Dee Belemnite while the oxygen isotope values span from 59 ± 4‰ to 91 ± 4‰ Vienna Standard Mean Ocean Water (1 SE uncertainties). Notably, these values are isotopically heavy (13C- and 18O-enriched) relative to nearly every other Martian material. The extreme isotopic difference between the carbonates and other carbon- and oxygen-rich reservoirs on Mars cannot be reconciled by standard equilibrium carbonate-CO2 fractionation, thus requiring an alternative process during or prior to carbonate formation. This paper explores two processes capable of contributing to the isotopic enrichments: 1) evaporative-driven Rayleigh distillation and 2) kinetic isotope effects related to cryogenic precipitation. In isolation, each process cannot reproduce the observed carbonate isotope values; however, a combination of these processes represents the most likely source for the extreme isotopic enrichments.

Geochemical characteristics and, origin of heavy oil in the periphery of Wenchang-B depression in Zhu III Sub-basin, Pearl River Mouth Basin, China

Heavy oil reservoirs have been discovered in the periphery of Wenchang-B depression in Zhu III sub-basin, but there is no relevant research on the geochemical characteristics and genesis of heavy oil. The characteristics of source rocks and crude oil were analyzed by various geochemical experiments. The results indicated that: (1) Second member of the Wenchang Formation (WC2) source rock has the largest hydrocarbon generation potential, WC1 and EP source rocks have similar characteristics, but EP source rocks have low maturity and Pr/nC17 greater than one; (2) Its heavy oil-density ranges from 0.91–0.97 g/cm3, with low sulfur content. The saturated hydrocarbon content in well Q4 is high, whereas it has low the non-hydrocarbon content. The carbon isotope values range from −28.2 ‰ and −25.4 ‰. Combined with the biomarker characteristics, the source of heave oil corresponds to WC2 and WC1 source rocks. Hence, there are two origins of heavy oil in the Wenchang-B depression, low-mature heavy crude oil (well Q4), and biodegradable heavy oil (well Q1). Clarifying the origin and source of heavy oil can provide more references for exploration and development.

Division and Correlation of the Upper-Middle Cambrian Evaporate Platform Facies Dolomite Formation in Tarim Basin

The Cambrian strata in Tarim basin is mainly evaporate platform facies dolomite. Due to the lack of trilobite fossils, it is hardly to achieve isochronous stratigraphic division and correlation. The Xiaoerbulake outcrop and Fang 1 well are typical surface and subsurface Cambrian sections in Tarim Basin respectively. Shayilike, Awatage and Xiaqiulitage Groups were regarded as the complete Upper -Middle Cambrian previously which composed of dolomite. The carbon isotope values range from –15.5 to 2‰, and three negative excursions are found (N1, N2 and N3). Comparing with the Global Standard Chronostratigraphic Scale for the Middle and Upper Cambrian System, the fifth stage and the Drumian of the third Series, the ninth and the tenth Stage of Furongian Series can be recognized, while the Guzhangian and Paibian are lost. As a result, the Cambrian Formation in Tarim Basin can be divided accurate to stage scale.

Restricted pasturing of domesticated cattle at a Late Neolithic settlement in Central Germany

Domestic cattle (Bos taurus) were one of the most important animal resources exploited by farming communities throughout the Neolithic period in Central Europe. However, little is currently known about cattle husbandry practices in use for Late Neolithic communities located in Central Germany. Here, we present new zooarchaeological and isotopic data in order to better understand cattle management strategies used during the late fourth and early third millennia BCE at the Late Neolithic Bernburg settlement of Hundisburg-Olbetal in Central Germany. Strontium isotope values of cattle tooth enamel reveal that these animals were most likely herded on local pastures. The overall range of carbon isotope values of sequentially sampled cattle teeth suggests the ingestion of graze that grew in open C3-environments all year round. Furthermore, the observed temporal offset between maximum carbon and oxygen isotope values most likely reflect a herding strategy where cattle grazed on open pastures in the floodplains during the growing season and were possibly kept near the settlement on stubble fields after harvest. Altogether, these data indicate Late Neolithic cattle management systems at the Bernburg settlement of Hundibsurg-Olbetal were small in scale and likely focused on meat production for household subsistence.

Freshwater Contributions and Nitrogen Sources in a South Texas Estuarine Ecosystem: a Time-Integrated Perspective from Stable Isotopic Ratios in the Eastern Oyster (Crassostrea virginica)

Freshwater inflows to Texas estuaries vary widely due to regional climate fluctuations and are being substantially altered by human activities. The natural abundance stable isotope ratios of carbon and nitrogen in oyster adductor muscle were used to acquire a time-integrated view of freshwater and nitrogen contributions to the Mission-Aransas National Estuarine Research Reserve in South Texas. The study objective was to determine the influence of inputs from the San Antonio and Guadalupe rivers, which deliver approximately 1.6 km3 of water to the mid-Texas coastal region annually. A transect of sampling stations extending from the head of San Antonio Bay (northeast of the Reserve boundary) to the Aransas Pass ship channel (roughly 70 km to the southwest) was visited multiple times between 2009 and 2011. Carbon isotopic values increased from approximately −25 to −17‰ while δ15N values decreased from approximately +16 to +10‰ between the bay and ship channel. This range of carbon isotope values translates into time-integrated freshwater fractions as high as 0.8 (1 = 100% fresh) at the most inland sampling station to freshwater fractions around zero approaching the Gulf of Mexico. Contributions from the San Antonio and Guadalupe rivers to waters of the reserve vary between wet and dry years, but overall, the data suggest that these rivers are persistent and substantial sources of fresh water and nitrogen to the reserve. This study emphasizes the importance of connectivity and lateral exchanges among bays/lagoons when considering potential sources of fresh water and nitrogen that control ecosystem structure and function.

Towards Incorporating Insect Isotope Analysis Using Cavity Ring-Down Spectroscopy into Area-Wide Insect Pest Management Programs

Measurement of natural stable isotope signatures of insects has proven to be a useful method in determining their natal origin, feeding strategies and mating behavior, and has spawned a new discipline of insect isotope forensics. However the complexity of measuring isotope signatures using elemental analysis-isotope ratio mass-spectrometry (EA-IRMS) has encountered significant technical barriers, which together with costs, have hindered the advancement and wide spread adoption of this potentially useful approach. In the last decade simpler technologies to measure stable isotope ratios in samples based on laser spectroscopy have been developed. We foresee that these instruments will fuel the advancement and adoption of insect isotope forensics into pest management programs because they are easy to use, more affordable, and complement existing marking tools. However prior to the adoption of these new technologies it is necessary to test their fitness for purpose and explore their limitations. In this paper we set out to test the suitability of using combustion module-cavity ring-down spectroscopy (CM-CRDS) for measuring isotope ratios in an entomological context, covering marking of insects, mating behavior and dietary assessment. We carried out a series of experiments to compare the isotope measurements made with the CM-CRDS with the traditional method of EA-IRMS. Initially we compared measurements of carbon isotope values of a variety of common diet components used in insect rearing, which covered a wide range of carbon isotope values. There was good agreement in the isotope values obtained, using the CM-CRDS and the EA-IRMS with similar accuracy and precision. Secondly we compared the 2 methods to measure carbon isotope values of the common cutworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) samples, and again there was good agreement in the values obtained. Thirdly we tested whether the CM-CRDS could be used to determine the paternity of spermataphores, based on the differential isotope signatures of the paternal and maternal moths, as a result of differences in their rearing diets. The CM-CRDS proved to be fit for the purpose of measuring isotope values in the spermataphores of common cutworm and suggested that the CM-CRDS technology would be suitable for these isotope based mating studies in moths if the spermatophores were of sufficient mass. Finally we explored a number of the issues surrounding CM-CRDS and isotope technologies in general, e.g., logistical considerations, economics of operation, sample size, etc. We conclude that CM-CRDS is a suitable instrument for measuring stable isotope carbon signatures in moths, and most probably other sufficiently large insects and that CM-CRDS could be easily used in both operational and research contexts.

Local adoption of animal husbandry in the southern Levant: An isotopic perspective from the Pre-Pottery Neolithic B funerary site of Kfar HaHoresh

Animal husbandry emerged as an important subsistence strategy at various tempos and trajectories across the southern Levant during the Pre-Pottery Neolithic B (ca. 8500–6500 cal bc). Here, we explore temporal variation in the emergence of animal management strategies, in particular those that alter the composition of the animal diet, west of the Jordan Valley, through carbon (δ13C) and nitrogen (δ15N) isotopic analyses of mountain gazelle, bezoar goat and aurochsen bone collagen from the funerary complex of Kfar HaHoresh. Analyses presented here show an extended range of carbon isotope values in the collagens of Late Pre-Pottery Neolithic B (PPNB) goats relative to Middle PPNB goats, which are also enriched in 13C relative to contemporaneous gazelle. This shift may reflect a greater catchment from which morphologically wild goats derived or that some of the goats at Kfar HaHoresh were provided with some fodder. If the latter is the case, then the use of fodder by 7500 cal bc at Kfar HaHoresh is a relatively late development, emerging several hundred years after goat husbandry strategies emphasising a juvenile harvest and fodder provisioning that first came into use in the Mediterranean region of the southern Levant. There is a pronounced enrichment of nitrogen isotopes in Early PPNB aurochsen, ritually important animals derived from a unique feasting deposit, relative to that of gazelle and goats. Though this may reflect more specialised feeding behaviour in aurochsen compared to the other two bovid groups, an alternative interpretation is that the aurochsen ingested enriched 15N from manured pasture, following restriction of their movement by people. These isotopic data support the documented pattern of a delayed adoption of goat husbandry in the lower Galilee region and may point to differential developmental trajectories where some forms of animal management emerged out of ritual rather than subsistence needs.

Stable isotope data from loess malacofauna: Evidence for climate changes in the Pannonian Basin during the Late Pleistocene

Loess is terrestrial clastic sediment, composed dominantly of silt-sized particles formed by the accumulation of wind-blown dust. It is usually inter-bedded with palaeosol horizons forming loess-palaeosol successions (LPS). Due to its characteristics loess represents a high quality record of climate changes in the Late Pleistocene. The thickest and best-studied loess sections in the Republic of Croatia are located in the eastern Croatian province of Baranja, near the Danube River. This area is a southern part of the Pannonian Basin. Zmajevas LPS was selected in this study. Grain-size distribution indicates that the loess from Zmajevac LPS in Baranja is typical loess, comparable with other loess profiles in the Pannonian Basin.The faunal assemblages in the loess samples prove the dominance of transitional palaeoecological assemblage types, whereas uniformly defined types are rare. Helicopsis striata assemblage is dominant, while Columella columella and Chondrula tridens assemblages are present in smaller proportion. For additional information about palaeoclimate and palaeoecology in Pannonian Basin, stable oxygen and carbon isotope ratios were measured in mollusc shells. Stable oxygen values range from −5.76‰ to −2.45‰. Stable carbon isotope values range from −8.83‰ to −6.84‰. Stable oxygen values show significant palaeotemperature changes during the Late Pleistocene, with the average growing season (AGS) temperature range of 13.24 °C or 9.457 °C, depending on which formula is applied. The overall climate was much cooler then present. Stable carbon isotope values point to dominance of C3 vegetation type during the Late Pleistocene in southern part of the Pannonian Basin.

Short-term variability of human diet at Basketmaker II Turkey Pen Ruins, Utah: Insights from bulk and single amino acid isotope analysis of hair

Strands of human hair excavated from Basketmaker II Turkey Pen Ruins in Utah were examined using stable carbon and nitrogen isotope analysis and show that these individuals were heavily reliant on maize but their diet fluctuated over a period of months. Through serial bulk carbon and nitrogen analysis and serial single amino acid carbon isotope analysis of individual strands of hair, this study addresses the questions of how human diets at the site varied over a period of months and if domesticated turkeys (Meleagris gallopavo) were part of the diet during any point of the year. Hair carbon isotope values range from −9.2‰ to −12.4‰ with an average of −10.8±1.2, and nitrogen isotope values range from 6.0‰ to 7.3‰ with an average of 6.6‰±0.5. There was also variability in the single amino acid δ13C values, with an overall pattern of terrestrial C4 signatures of individual amino acids (δ13C for leucine averaged −17.8‰±1.7). Δ13Cvaline-phenylalanine values ranging from −1.6‰ to 2.6‰ also point strongly to a terrestrial diet. From the stable isotope data presented here, we suggest that BMII diet, while being heavily maize based, did vary during the year, and that turkeys were never a diet staple.

Isotopic characteristics of canopies in simulated leaf assemblages

The geologic history of closed-canopy forests is of great interest to paleoecologists and paleoclimatologists alike. Closed canopies have pronounced effects on local, continental and global rainfall and temperature patterns. Although evidence for canopy closure is difficult to reconstruct from the fossil record, the characteristic isotope gradients of the “canopy effect” could be preserved in leaves and proxy biomarkers. To assess this, we employed new carbon isotopic data for leaves collected in diverse light environments within a deciduous, temperate forest (Maryland, USA) and for leaves from a perennially closed canopy, moist tropical forest (Bosque Protector San Lorenzo, Panamá). In the tropical forest, leaf carbon isotope values range 10‰, with higher δ13Cleaf values occurring both in upper reaches of the canopy, and with higher light exposure and lower humidity. Leaf fractionation (Δleaf) varied negatively with height and light and positively with humidity. Vertical 13C enrichment in leaves largely reflects changes in Δleaf, and does not trend with δ13C of CO2 within the canopy. At the site in Maryland, leaves express a more modest δ13C range (∼6‰), with a clear trend that follows both light and leaf height. Using a model we simulate leaf assemblage isotope patterns from canopy data binned by elevation. The re-sampling (bootstrap) model determined both the mean and range of carbon isotope values for simulated leaf assemblages ranging in size from 10 to over 1000 leaves. For the tropical forest data, the canopy’s isotope range is captured with 50 or more randomly sampled leaves. Thus, with a sufficient number of fossil leaves it is possible to distinguish isotopic gradients in an ancient closed canopy forest from those in an open forest. For very large leaf assemblages, mean isotopic values approximate the δ13C of carbon contributed by leaves to soil and are similar to observed δ13Clitter values at forested sites within Panamá, including the site where leaves were sampled. The model predicts a persistent ∼1‰ difference in δ13Clitter for the two sites which is consistent with higher water availability in the tropical forests. This work provides a new framework for linking contemporary ecological observations to the geochemical record using flux-weighted isotope data and lends insights to the effect of forest architecture on organic and isotopic records of ancient terrestrial ecosystems.

Unraveling cementation environment and patterns of Holocene beachrocks in the Arabian Gulf and the Gulf of Aqaba: stable isotope approach

This paper analyses Holocene beachrocks from the Arabian Gulf and the Gulf of Aqaba to explain the mechanisms that influence the cementation process in these areas. Holocene beachrocks in the Gulf of Aqaba are composed of predominantly terrigenous material derived from erosion of adjacent uplifted Precambrian basement, while the beachrocks in the Arabian Gulf are composed mainly of marine bioclasts and wind-blown siliciclastic sands. The cements of beachrocks in both areas show three textural varieties: (1) isopachous phreatic acicular aragonite; (2) a micritic envelope of high-Mg calcite (HMC); (3) meniscus and gravitational vadose HMC. Radiocarbon dating of beachrock samples from the Arabian Gulf yielded ages from ca. 2300 to 660 yr cal BP whereas samples from the Gulf of Aqaba range in age between 5500 and 2800 yr cal BP. Oxygen isotope values range from 2.6 to 4.4‰ respectively for the Arabian Gulf whereas the Gulf of Aqaba values range from 1.2 to 1.5‰. Carbon isotope values range from 3.2 to 5.9‰ for the Arabian Gulf whereas those from the Gulf of Aqaba range from 3.8 to 4.6‰. The values of δ 18 O VPDB and δ 13 C VPDB in the beachrocks of both areas suggest a marine origin. The beachrocks of the Arabian Gulf were precipitated under high evaporation conditions, while beachrocks from the Gulf of Aqaba were precipitated in normal shallow-marine conditions. The mineralogy and textural habits suggest that cementation of these beachrocks started within the shallow-marine phreatic zone.

Understanding specifics in generalist diets of carnivorans by analyzing stable carbon isotope values in Pleistocene mammals of Florida

Within ancient ecosystems, it is generally difficult to determine the specific diets of species from higher trophic levels, which in turn hinders our understanding of trophic relationships and energy flow through these systems. To better understand the ecology of taxa at higher trophic levels, we used analysis of tooth enamel stable carbon isotope values to infer the dietary preferences ofCanis edwardiiandSmilodon gracilisfrom the Leisey Shell Pit 1A (LSP 1A) and Inglis 1A, two Pleistocene localities in Florida. The goals of the analyses were to (1) determine whether these carnivorans specialized in particular prey types or maintained a generalist diet; (2) ascertain whether carbon isotope values support what was previously suggested about the ecology of these species; and (3) establish what ecological details of ancient food webs can be discovered by carbon isotope analyses at higher trophic levels. Results show that the sampled carnivoran carbon isotope values are distributed among suspected prey isotope values, suggesting that varied prey were taken at the study localities. Prey compositions were modeled for each carnivoran species by using Stable Isotope Analysis in R (SIAR). The modeled diets indicate that each studied carnivoran had a generalist diet; however, there are differences in how these taxa achieved dietary generalization. At the glacial Inglis 1A locality, sampled individuals ofC. edwardiiandS. gracilisshow similar isotope values and modeled dietary prey proportions, although both carnivorans do show a preference for grazing prey species. The similar isotopic values, and calculated prey proportions, observed between these species may imply greater interspecific competition for food. At the interglacial LSP 1A locality,C. edwardiishows values similar to those observed at Inglis 1A. In contrast, the data forS. gracilisshows a preference for consuming browsing prey species. Further, its restricted range of carbon isotope values suggests thatS. gracilismay have concentrated its feeding within a particular habitat. Examination of stable carbon isotope values among species at higher trophic levels reveals that some intricacies of ancient food webs can be discerned.

Natural gas constituent and carbon isotopic composition in petroliferous basins, China

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.

Petrography and geochemistry of the magnesites and dolostones of the Ediacaran Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

The Ediacaran deposits (between 635 and 542 Ma) of the Central Zone of the Iberian Massif consist of alternating siliciclastic and carbonate beds. These carbonates are dolostones and magnesites which are interpreted to have been formed by the replacement of primary peritidal limestones. Through petrographic and geochemical analyses, we recognize different types of dolomites (D1 to D4) and magnesites (M1 and M2). Despite distinct petrographic features of the four types of dolomite, their oxygen and carbon isotopes overlap with δ 18O values ranging from + 15.45 to + 17.51‰ (SMOW) and δ 13C from − 0.13 to 3.21‰ PDB. Sr isotope values for D1 and D2 range from 0.7028 to 0.7091. Magnesites (M1 and M2) show oxygen values higher than + 17.87.0‰, and δ 13C values show the same variability as for the dolomites. D3 and D4 oxygen isotope values are between + 18.91 and 19.61, and the carbon isotope values range are similar to the other diagenetic phases. Sr isotope values for the magnesites and late dolomites (D3 and D4) are 0.7095 to 0.7104, being higher than those of the D1 and D2 dolomites. D1 is a relatively early dolomite phase formed by the replacement of fine grained peritidal limestones. The coarser crystal size of D2, which shows similar geochemical features as D1, suggests formation by dolomitization of coarser grained limestones. The replacement of D1 and D2 by M1 and M2 advanced along stylolites, fractures and bedding planes. This replacement is interpreted to have occurred by hydrothermal fluids, which is suggested by the presence of talc and forsterite. D3, a coarse dolomite, completely destroyed any previous texture and D4 (dolomite cement) post-dates magnesite formation. Interactions of hydrothermal fluids with the prior carbonates reset the oxygen isotopes of the earlier dolomite. The study of these magnesites and related dolostones may offer new insights into the model of formation of sparry magnesites hosted by mixed siliciclastic–carbonate platform deposits. The establishment of the factors and mechanism that control the diagenetic evolution of these carbonates has a great importance in order to understand and predicts porosity and permeability variations of rocks formed under similar geological conditions.

Food-web structure of coastal streams in Costa Rica revealed by dietary and stable isotope analyses

Abstract:Food webs of streams draining tropical rain forests on Costa Rica's Pacific and Caribbean coasts were examined in the 1980s via dietary analyses, and the same streams were surveyed again in 2004 to compare trophic structure based on analysis of stable isotope ratios of fish, macro-invertebrate and plant tissues. Estimates of species’ trophic positions (TP) were calculated from stomach-contents data (51 species; 5420 specimens) and compared with TP estimates derived from analysis of nitrogen isotope ratios (82 taxa; 240 samples). Coefficients of determination for TP based on dietary versus isotopic analysis ranged from 0.18 (Quebrada Camaronal, Corcovado) to 0.73 (Quebrada Estacion, Tortuguero). Consumer taxa within all four streams spanned a broad range of carbon isotope values, indicating assimilation of variable proportions of carbon from periphyton and terrestrial vegetation that in all but one of the streams had similar δ13C values. Approximately half the consumers in all four streams had carbon ratios heavier than any of the in situ production sources examined. This pattern could be explained by consumption of other production sources that were not sampled, including periphyton taxa with variable carbon isotope signatures, or migration of prey and/or consumers between these freshwater and coastal marine habitats.

Reconstruction of Jurassic (Bathonian) palaeosalinity using stable isotopes and faunal associations

Abstract: The Bathonian is an interval from which salinity-controlled benthic faunas are particularly well known. This study examines Late Bathonian faunas from Dorset, UK, from the Wattonensis Beds (Fuller's Earth), of known marine origin, the overlying Elongata Beds, and the Boueti Bed marking the base of the Forest Marble. δ 18 O values of belemnites ( Belemnopsis ) from the Wattonensis Beds range from −1.4 to −0.7‰ and carbon isotope values range from −2.3 to −0.5‰ VPDB. Isotope values of well-preserved Rhynchonella from the Wattonensis Beds range from −2.4 to −1.3‰ for oxygen and −1.1 to 0.6‰ VPDB for carbon. The δ 18 O data suggest high palaeotemperatures (possibly up to 28 °C). A similar spread of data is derived from brachiopods ( Goniorhynchia boueti ) from the Boueti Bed. Given the marine origin of the Wattonensis Beds and overlap of data, the reduced faunal diversity of the Boueti Bed is considered unlikely to be the result of reduced salinity. Brachiopods therefore should not be regarded more sceptically as evidence for fully marine conditions, as has been suggested. The oysters ( Praeexogyra hebridica elongata ) and brachiopods ( Rhynchonella ) from the Elongata Beds have isotope values that also imply fully marine conditions and are interpreted as being indicative of slightly cooler conditions.

Carbon sources for the Palaeozoic giant fungus Prototaxites inferred from modern analogues.

A wide range of carbon isotope values in the Devonian fossil Prototaxites has been interpreted to support heterotrophy and the classification of Prototaxites as a giant fungus. This inference remains controversial because of the huge size of Prototaxites relative to co-occurring terrestrial vegetation and the lack of existing fungal analogues that display equally broad isotopic ranges. Here, we show wide isotopic variability in the modern saprotrophic fungus Arrhenia obscurata collected adjacent to shallow meltwater pools of a sparsely vegetated glacial succession in the Washington Cascades, USA. Soils collected specifically around the edges of these pools were up to 5 per thousand higher in delta(13)C than adjacent soils consistent with C(3) origin. Microbial sources of primary production appear to cause these high delta(13)C values, and the environment may be analogous to that of the Early Devonian landscapes, where Prototaxites individuals with extreme isotopic variance were found. Carbon isotopes are also compared in Prototaxites, Devonian terrestrial vascular plants, and Devonian algal-derived lake sediments. Prototaxites isotopic values show little correspondence with those of contemporaneous tracheophytes, providing further evidence that non-vascular land plants or aquatic microbes were important contributors to its carbon sources. Thus, a saprotrophic fungal identity is supported for Prototaxites, which may have relied on deposits of algal-derived organic matter in floodplain environments that were less dominated by vascular plants than a straight reading of the macrofossil record might suggest.

Carbon and oxygen stable isotopes in the Maastrichtian of the Basque Country, N. Spain

Carbon and oxygen stable isotopes have been analysed from 56 bulk sediment samples taken through an 85 m section across the first occurrence of the planktonic foraminiferan Abathomphalus mayaroensis at Zumaya, and from 10 samples across the same level at Sopelana, Basque Country, Spain. At Zumaya, carbon isotope values lie between +0.82 and +2.38‰ VPBD and correlate well with lithology. Oxygen isotope values lie between −4.00 and −2.72‰ VPBD and correlate weakly with lithology. At Sopelana, carbon isotope values range from 1.48 to 1.83; oxygen isotope values from −3.40 to −2.30‰VPBD. A carbon versus oxygen isotope cross-plot indicates some diagenetic overprinting. Oxygen isotope values imply palaeotemperatures of 27–32 °C, confirming some diagenetic influence. A smoothed carbon isotope curve shows a peak in unit 6, two troughs in unit 7, a peak in unit 8, declining values in unit 9 and another trough in the lower half of unit 10. This pattern correlates well with the limited carbon isotope curve for Sopelana. The two minima adjacent to the first occurrence of A. mayaroensis, the lower of which coincides with the first occurrence of the calcareous nannofossil Lithraphidites quadratus, may prove useful in international correlation of the Lower–Upper Maastrichtian boundary.

Fracture sealing and fluid overpressures in limestones of the Jabal Akhdar dome, Oman mountains

AbstractFractures are important conduits for fluid flow in the Earth's crust. To better understand the spatial and temporal relations among fracturing, fracture sealing, and fluid flow, we have studied fractures, faults, and veins in a large dome (Jabal Akhdar) in the Oman mountains. Our work combines the results of meso‐ and microstructural analyses and stable isotope analyses. Seven generations of fractures and veins have been identified in the carbonate‐dominated dome. The earliest generations of veins developed during extension and subsidence of the Mesozoic basin. These veins formed in the inclined segments of bedding‐parallel stylolites and in extensional fractures that are perpendicular to bedding (#1 and #2, respectively). These extension‐related veins are truncated by bedding‐parallel veins (#4) that formed during top‐to‐north bedding‐parallel shear of both the northern and southern limbs of the dome. These veins are consistent with a change in stress regime and may be related to an earlier generation of strongly deformed pinch‐and‐swell veins (#3) that are exposed locally on the southern limb of the dome. Normal faults contain a set of en‐echelon tension gashes (#5) and veins emplaced in dilational jogs along the fault planes (#6). In the northern part of the dome, veins (#7) associated with thrusts post‐date the normal faults. Samples of veins and their host rocks were analyzed to provide information on fluid‐rock interaction in the dome and the scale(s) of fluid movement. Oxygen isotope values range from +16.2 to +29.3‰; carbon isotope values range from 0 to +3.6‰. The results of the structural and isotopic analyses are consistent with the early veins (#2–#5) having precipitated from overpressured fluid in a isotopically rock‐buffered system. During normal faulting (#5 and #6), a more open system allowed external fluid to infiltrate the dome at drained conditions and precipitate the youngest sets of veins (#6 and #7).

Environments of Formation and Controls on Spatial Distribution of Calcite Cementation in Plio-Pleistocene Fluvial Deposits, New Mexico, U.S.A.

ABSTRACT The Sierra Ladrones Formation is the uppermost syn-rift sedimentary unit of the Rio Grande Rift in central New Mexico. In the vicinity of the study site it was deposited in a variety of fluvial subenvironments. Portions of the Formation are strongly cemented by calcite, forming resistant ledges in otherwise poorly consolidated sediment. The calcite exhibits a wide range of macroscopic and microscopic textures and structures, which formed in different hydrologic environments, including vadose nonpedogenic, vadose pedogenic, and phreatic. Phreatic cements are characterized by sparite with uniform, disseminated, isopachous, poikilotopic, and drusy textures; vadose-nonpedogenic by micrite with meniscus textures; and vadose-pedogenic by textures similar to those in the vadose-nonpedogenic environment, plus macroscopic features such as root structures, destruction of primary sedimentary textures, and a red hue. Many of the cements are of mixed vadose/phreatic origin, with vadose micrite formation preceding phreatic sparite. The calcite is near end-member in composition, with only minor substitution of trace elements (Sr, Mg, Mn, and Fe) for Ca in the calcite structure. Vadose micrite cements generally have a higher trace-element content than phreatic sparite, perhaps as a result of more rapid precipitation in the vadose zone. Carbon isotope values range from -4.7 to -3.5‰ PDB and 18O values range from -10.3 to -7.2‰ PDB. Cements that are dominantly vadose have a wider range of 13C values, and are more 13C-depleted overall than phreatic cements. Oxygen values are similar for both vadose and phreatic cements. At large scales (i.e., several meters to decameters vertical dimension), calcite is present preferentially in coarse-grained sediments that had high pre-cementation saturated permeabilities relative to uncemented finer-grained strata. At smaller scales (i.e., several meters or less vertical dimension), however, there is no clear relationship between degree of cementation and pre-cementation permeability. At these scales, calcite cement is present preferentially in areas that contain early vadose calcite. This suggests that at large scales cementation was controlled by pre-cementation permeability, perhaps due to a greater flux of Ca2+ and/or HCO3- in permeable horizons, whereas at smaller scales, within high permeability units, the distribution of cementation was controlled by other factors, such as the presence of nucleating material.