Changes In Carbon Isotope Ratios Research Articles

Lake Ivanpah: An overlooked pluvial lake in the southern Great Basin, U. S. A.

A long-held but untested hydroclimatic model maintains that, in the arid southern Great Basin, U.S.A., hydrographically isolated watersheds held no late Quaternary paleolakes, even during pluvial periods. We test this model at Ivanpah Dry Lake, which has a watershed lying wholly within the mountains of the Mojave Desert, and no obvious ancient shorelines. Remote imagery was used to identify relict geomorphic features attributable to wave action during high-lake stands, and excavations in different parts of the basin margin revealed beach and lacustrine sedimentary sequences, some truncated by relict playa surfaces, most >2.5 m above the current playa. In the southern Ivanpah basin exposed lacustrine sediment grades upward to near-shore muds, capped by alluvium of the Cima Wash Delta-Fan. Time-dependent changes in carbon isotope ratios indicate that saltscrub (Atriplex polycarpa), the desert vegetation surrounding the dry lake today, did not become dominant until after 5.4 cal ka. A comparison of Paleolake Ivanpah’s initial radiocarbon chronology to regional hydroclimatic proxies demonstrates its potential to inform on changing sources of moisture contributing to pluvial events. This study shows not only that pluvial lakes occurred in the isolated watersheds of the Mojave Desert, but also that the margins of these basins are so geomorphically active as to entirely obscure the evidence of their occurrence. However, this process often buries and protects, rather than obliterates, geomorphic and stratigraphic evidence for past high-lake stands. It also shows that the basin margin, and not its depocenter, is where evidence of pluvial lakes should be sought. In the Ivanpah basin, some Holocene paleolakes appear to have persisted for decades to centuries.

Changes of carbon-isotope ratios in soil organic matter relative to parent vegetation and site specificity

Investigating the correlation between biodiversity and ecosystem function in natural environments using carbon-isotope composition (δ13C) allows distinguishing the nutrient cycling pattern and anthropogenic effects incorporation in plants and soil processes. The mechanisms behind the isotopic composition of soil organic matter (SOM) and parent vegetation in relation to the context of site-specificity was approached in this work. Formation of SOM can be affected by the presence of a high concentration of heavy metals in soils. Still, no systematic studies were performed in most of the industrial sites to support this hypothesis. In order to explore this incomplete understood influence, investigation of carbon isotope signatures (d13C) variations in soil organic matter were performed in two industrial areas from Romania (Copșa Mică industrial platform and Baia Sprie mining zone). The current study, also, investigated the C:N ratio variation, as well as the influence of N speciation regarding d13C values of SOM. The decrease in C:N ratio indicated an increasing effect of the microbial products on SOM matter at increasing depth, for both regions, while an increase of the denitrification processes with depth was found for both areas. For the most appropriate depth (20-40 cm), the soil from Baia Sprie region was more enriched in 13C comparing with the soil from Copsa Mica region, and this higher isotope fractionation of SOM might be due to a higher carbon content, respectively a higher nitrogen content of Baia Sprie soil. It was concluded that the SOM of the surface soil in the two investigated regions has an 13C isotopic composition similar to the plant remains from which it was formed, offering an integrated value of plant material, time and the local origin and providing useful markers of tree isotopic composition.

Dissipation of S-metolachlor and butachlor in agricultural soils and responses of bacterial communities: Insights from compound-specific isotope and biomolecular analyses

The soil dissipation of the widely used herbicides S-metolachlor (SM) and butachlor (BUT) was evaluated in laboratory microcosms at two environmentally relevant doses (15 and 150 μg/g) and for two agricultural soils (crop and paddy). Over 80% of SM and BUT were dissipated within 60 and 30 days, respectively, except in experiments with crop soil at 150 μg/g. Based on compound-specific isotope analysis (CSIA) and observed dissipation, biodegradation was the main process responsible for the observed decrease of SM and BUT in the paddy soil. For SM, biodegradation dominated over other dissipation processes, with changes of carbon isotope ratios (Δδ13C) of up to 6.5‰ after 60 days, and concomitant production of ethane sulfonic acid (ESA) and oxanilic acid (OXA) transformation products. In crop soil experiments, biodegradation of SM occurred to a lesser extent than in paddy soil, and sorption was the main driver of apparent BUT dissipation. Sequencing of the 16S rRNA gene showed that soil type and duration of herbicide exposure were the main determinants of bacterial community variation. In contrast, herbicide identity and spiking dose had no significant effect. In paddy soil experiments, a high (4:1, V/V) ESA to OXA ratio for SM was observed, and phylotypes assigned to anaerobic Clostridiales and sulfur reducers such as Desulfuromonadales and Syntrophobacterales were dominant for both herbicides. Crop soil microcosms, in contrast, were associated with a reverse, low (1:3, V/V) ratio of ESA to OXA for SM, and Alphaproteobacteria, Actinobacteria, and Bacillales dominated regardless of the herbicide. Our results emphasize the variability in the extent and modes of SM and BUT dissipation in agricultural soils, and in associated changes in bacterial communities.

Effects of acidification on the isotopic ratios of Neotropical otter tooth dentin.

Stable carbon and nitrogen isotope ratios are widely used in ecological studies providing important information on the trophic ecology and habitat use of consumers. However, some factors may lead to isotopic variability, which makes difficult the interpretation of data, such as the presence of inorganic carbon in mineralized tissues. In order to remove the inorganic carbon, acidification is a commonly used treatment. The effects of two methods of acidification were tested: (i) dentin acidification with 10% HCl using the 'drop-by-drop' technique, and (ii) dentin acidification in an 'HCl atmosphere', by exposing the dentin to vaporous 30% hydrochloric acid. Results were compared with untreated subsamples. The stable carbon and nitrogen ratios of untreated and acidified subsamples were measured using an elemental analyzer coupled to an isotope ratio mass spectrometer. The nitrogen isotopic ratios were statistically different between the two acidification treatments, but no significant changes in carbon isotopic ratios were found in acidified and untreated samples. The results indicated that acidification had no effect on carbon isotopic ratios of Neotropical otter tooth dentin, while introducing a source of error in nitrogen isotopic ratios. Therefore, we conclude that acidification is an unnecessary step for C and N stable isotope analysis.

Detecting the fingerprint of drought across Europe\u2019s forests: do carbon isotope ratios and stem growth rates tell similar stories?

BackgroundUnderstanding how trees respond to drought is critical to forecasting both short and long-term impacts of climate change on forests. The isotopic ratio of 13C to 12C stored in wood – referred to as δ13C – is widely used as an indicator of plant water status. Yet whether changes in δ13C linked to drought are also associated with declines in annual carbon assimilation and allocation to stem growth remains unclear.MethodsHere we used tree ring data from over 3000 trees – representing 26 populations of 16 common European tree species sampled at six locations that span more than 20° in latitude – to test whether drought induces coordinated changes in carbon isotope ratios and stem basal area increments (BAI).ResultsWe found that δ13C is a reliable indicator of drought across a wide range of species and environmental conditions. All but one of the populations sampled in this study showed a statistically significant increase in δ13C under drought conditions. However, when considering the effects of these same drought events on BAI, we found no evidence to suggest that increases in δ13C were coupled with significant declines in stem growth. While BAI was 11.9% lower on average in drought years, this decline in BAI was not significant when analysed across species. In fact, only seven of the 26 populations we sampled exhibited significant declines in BAI under drought conditions – four of these from a single study site in the Carpathian Mountains of Romania.ConclusionsWhile δ13C responded strongly and consistently to drought across a diverse group of tree species and environmental conditions, we found that most tree species were able to sustain growth even under conditions of low soil water availability. Consequently, while δ13C provides a powerful indicator of past drought occurrence, by themselves carbon isotope ratios tell us little about how carbon sequestration and allocation to wood are affected by conditions of low water availability across Europe’s forests.

Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces

Research Article| January 01, 2015 Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces David S. Jones; David S. Jones Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Roger C. Creel; Roger C. Creel Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Bernardo Rios; Bernardo Rios Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA Search for other works by this author on: GSW Google Scholar Danielle P. Santiago Ramos Danielle P. Santiago Ramos Geology Department, Amherst College, 11 Barrett Hill Road, Amherst, Massachusetts 01002, USA *Current address: Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA. Search for other works by this author on: GSW Google Scholar Geology (2015) 43 (1): 59–62. https://doi.org/10.1130/G36236.1 Article history received: 02 Sep 2014 rev-recd: 30 Oct 2014 accepted: 05 Nov 2014 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David S. Jones, Roger C. Creel, Bernardo Rios, Danielle P. Santiago Ramos; Chemostratigraphy of an Ordovician–Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces. Geology 2015;; 43 (1): 59–62. doi: https://doi.org/10.1130/G36236.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (δ13C) to be preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative δ13C anomalies developed in Pliocene–Pleistocene carbonate platforms following glacioeustatic sea-level fall due to remineralization of terrestrial biomass during meteoric diagenesis. These anomalies are similar in structure and magnitude to some Neoproterozoic δ13C records, opening the possibility that the Neoproterozoic δ13C anomalies have a meteoric origin derived from a large terrestrial biosphere. Here we test the hypothesis that a large terrestrial biosphere existed prior to the Silurian–Devonian land-plant radiation by examining δ13C records of subaerial exposure surfaces formed in a shallow-water carbonate platform during the Ordovician–Silurian icehouse. The exposure surfaces include an unconformity at the Ordovician-Silurian boundary with terra rossa and dissolution-collapse breccia, and a lower Silurian quartz sand layer feeding a 50-m-deep system of karst pipes. There is no evidence for δ13C depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10–120 ppm) and covary with δ18O; oxygen isotope ratios, however, do not positively correlate with δ13C. Our results suggest that there was no significant terrestrial biosphere during Ordovician–Silurian time, and by extension, that Neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Validation of user- and environmentally friendly extraction and clean-up methods for compound-specific stable carbon isotope analysis of organochlorine pesticides and their metabolites in soils

In order to evaluate the potential of compound-specific stable carbon isotope analysis (CSIA) for tracking organochlorine pesticides in soil systems, sample pre-treatment methods have to be developed, which can provide recoveries sufficient for low detection limits without altering the isotope ratio of the target compounds. In this study we tested the compatibility of CSIA with user- and environmentally friendly extraction methods, including the Quick, Easy, Cheap, Effective, Rugged and Safe procedure (QuEChERS), Ultrasonic Assisted Extraction (USE) and Focused Ultrasonic Extraction (FUSE), as well as clean-up methods, including sulfuric acid clean-up and Florisil® column chromatography for hexachlorocyclohexanes (HCHs), p,pʹ-dichlorodiphenyltrichloroethane (DDT) and their environmental metabolites (chlorinated benzenes, dichlorodiphenyldichloroethylene – DDE and dichlorodiphenyldichloroethane – DDD) in soils. We optimized the extraction methods for maximum recovery and pre-concentration. At optimal conditions, all extraction methods and clean-up procedures, as well as the pre-concentration of the extract by solvent evaporation, led to insignificant changes in carbon isotope ratios of the target compounds. We modified the USE procedure to increase the volume of withdrawn organic extract, resulting in a higher pre-concentration of the target compounds by the subsequent solvent evaporation step. This Modified Ultrasonic Assisted Extraction (MUSE) was the most suitable procedure, and it was validated for the determination of carbon isotope ratios of the target compounds using two different types of soil matrices. The method could be applied to analyze carbon isotope ratios of HCHs, DDT, and their chlorinated metabolites in soil samples with concentrations ranging from 0.3 to 0.8mg/kg. The analytical uncertainty of MUSE, incorporating both accuracy and precision, was ≤0.4‰.

Influence of Different Proton Pump Inhibitors on Activity of Cytochrome P450 Assessed by [13C]‐Aminopyrine Breath Test

Aminopyrine is metabolized by cytochrome P450 (CYP) in the liver. The investigators evaluated influences of different PPIs on CYP activity as assessed by the [(13)C]-aminopyrine breath test ([(13)C]-ABT). Subjects were 15 healthy volunteers with different CYP2C19 status (5 rapid metabolizers [RMs], 5 intermediate metabolizers [IMs], and 5 poor metabolizers [PMs]). Breath samples were collected before and every 15 to 30 minutes for 3 hours after oral ingestion of [(13)C]-aminopyrine 100 mg on day 8 of each of the following regimens: control; omeprazole 20 mg and 80 mg, lansoprazole 30 mg, and rabeprazole 20 mg. Changes in carbon isotope ratios in carbon dioxide ((13)CO(2)/(12)CO(2)) in breath samples were measured by infrared spectrometry and expressed as delta-over-baseline (DOB) ratios (‰). Mean areas under the curve of DOB from 0 to 3 h (AUC(0-3h) of DOB) were significantly decreased by omeprazole 20 mg and lansoprazole 30 mg but not by rabeprazole 20 mg. Conversely, higher PPI dose (ie, omeprazole 80 mg) seemed to further decrease AUC(0-3h) of DOB in RMs but increased it in PMs. Omeprazole and lansoprazole at the standard doses inhibit CYP activity but rabeprazole does not, whereas high-dose omeprazole seems to induce CYPs.

Testing the growth limitation hypothesis for subarctic Scots pine

Summary 1. We tested if tree line trees are sink or resource limited by comparing the reactions in Scots pine trees (Pinus sylvestris L.) to changes in resource availability (through nutrient addition and defoliation), and in sink strength (through debudding) in north‐eastern Finland. 2. Height extension growth increased by over 200% and shoot extension growth by 27%, as a result of nutrient addition, whereas changes in radial increment were smaller and not statistically significant. 3. Although in defoliated trees height and shoot extension were not significantly affected, defoliated trees decreased their radial growth by 50%. 4. Branch extension growth of the remaining buds was increased due to debudding treatment by 47%, but there were no changes in radial increment. 5. Non‐structural carbohydrate concentrations in twigs, however, remained relatively constant in all treatments. 6. There were no statistically significant changes in carbon isotope ratio due to any of the treatments, indicating that the manipulation treatments did not modify tree water relations. 7. Synthesis. Our results do not support the growth limitation theory for tree line trees and suggest that resource limitations might be more important.

Evaluating high time‐resolved changes in carbon isotope ratio of respired CO2 by a rapid in‐tube incubation technique

Recent insights into fractionation during dark respiration and rapid dynamics in isotope signatures of leaf- and ecosystem-respired CO(2) indicate the need for new methods for high time-resolved measurements of the isotopic signature of respired CO(2) (delta(13)C(res)). We present a rapid and simple method to analyse delta(13)C(res) using an in-tube incubation technique and an autosampler for small septum-capped vials. The effect of storage on the delta(18)O and delta(13)C ratios of ambient CO(2) concentrations was tested with different humidity and temperatures. delta(13)C ratios remained stable over 72 h, whereas delta(18)O ratios decreased after 24 h. Storage at 4 degrees C improved the storage time for delta(18)O. Leaves or leaf discs were incubated in the vials, flushed with CO(2)-free air and respired CO(2) was automatically sampled within 5 min on a microGas autosampler interfaced to a GV-Isoprime isotope ratio mass spectrometer. Results were validated by simultaneous on-line gas-exchange measurements of delta(13)C(res) of attached leaves. This method was used to evaluate the short-term (5-60 min) and diurnal dynamics of delta(13)C(res) in an evergreen oak (Quercus ilex) and a herb (Tolpis barbata). An immediate depletion of 2-4 per thousand from the initial delta(13)C(res) value occurred during the first 30 min of darkening. Q. ilex exhibited further a substantial diurnal enrichment in delta(13)C(res) of 8 per thousand, followed by a progressive depletion during the night. In contrast, T. barbata did not exhibit a distinct diurnal pattern. This is in accordance with recent theory on fractionation in metabolic pathways and may be related to the different utilisation of the respiratory substrate in the fast-growing herb and the evergreen oak. These data indicate substantial and rapid dynamics (within minutes to hours) in delta(13)C(res), which differed between species and probably the growth status of the plant. The in-tube incubation method enables both high time-resolved analysis and extensive sampling across different organs, species and functional types.

Determination of 13C/12C-ratios of anthropogenic organic contaminants in river water samples by GC-irmMS

This study describes the application of a common analytical procedure adapted for compound-specific stable carbon isotope analyses of riverine contaminants. To evaluate the sensitivity of the analytical method and the precision of the isotopic data obtained, a set of numerous substances at different concentration levels were measured. For most of the anthropogenic contaminants investigated (including chlorinated aliphatics and aromatics, musk fragrances, phthalate-based plasticizers and tetrabutyl tin) acceptable carbon isotope analyses could be obtained down to amounts of approximately 5 ng absolutely applied to the gas chromatograph. These amounts correspond to concentrations in water samples at a natural abundance level of approximately 50–200 ng L−1 (low to medium contaminated river systems). However, it has to be considered that the precision and the sensitivity of the analytical method depend partially on the chemical properties of the substances measured. Five recovery experiments were conducted to assess changes in carbon isotope ratios during sample preparation and measurement. The compounds selected for these experiments are known riverine contaminants. Isotopic shifts or higher variations of the isotope ratios as a result of the analytical procedures applied were observed only for a couple of contaminants. Furthermore, compound-specific carbon isotope analyses were performed on eight water extracts of the Rhine river. By comparing the variation of the data of several individual compounds with the deviations obtained from the recovery experiments, it was possible to differentiate contaminants with unaffected isotope ratios and substances with significant alterations of the δ13C-values.

An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela

Other| October 01, 2003 An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela LINDA M. DE ROMERO; LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu Search for other works by this author on: GSW Google Scholar IRENE M. TRUSKOWSKI; IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar TIMOTHY J. BRALOWER; TIMOTHY J. BRALOWER 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. Search for other works by this author on: GSW Google Scholar JAMES A. BERGEN; JAMES A. BERGEN 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. Search for other works by this author on: GSW Google Scholar OSCAR ODREMAN; OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela Search for other works by this author on: GSW Google Scholar JAMES C. ZACHOS; JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 Search for other works by this author on: GSW Google Scholar FRANCIA A. GALEA-ALVAREZ FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela TIMOTHY J. BRALOWER * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 JAMES A. BERGEN ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 349–366. https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Article history Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LINDA M. DE ROMERO, IRENE M. TRUSKOWSKI, TIMOTHY J. BRALOWER, JAMES A. BERGEN, OSCAR ODREMAN, JAMES C. ZACHOS, FRANCIA A. GALEA-ALVAREZ; An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela. PALAIOS 2003;; 18 (4-5): 349–366. doi: https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The biostratigraphy of the La Luna Formation has long been in dispute, despite the importance of this unit as the most important source rock for hydrocarbons in Venezuela. In this paper, calcareous microfossil biostratigraphy combined with carbon-isotope stratigraphy provides a stratigraphic framework for the formation, permitting revision of temporal and spatial patterns of deposition of organic-rich sediments. Detailed studies were conducted on a cored borehole and five outcrop sections distributed across the Maracaibo Basin of western Venezuela.Planktic foraminifera have fair to good preservation, and nannofossils are poorly preserved. Many of the Cenomanian to Campanian planktic foraminiferal marker species are present, permitting the application of a traditional zonal scheme. An informal nannofossil biostratigraphic zonal scheme, based primarily on dissolution-resistant species, has been developed. Integration of these zonal schemes has enabled the correlation of changes in carbon-isotope ratios to the global C-isotope stratigraphy. The results have been used to estimate temporal variation in sedimentation rates as well as to reconstruct depositional patterns across the Maracaibo Basin. Deposition began in the eastern and northwestern part of the Maracaibo Basin in the middle Cenomanian and progressed towards the south and west, reaching the southwestern corner by the middle Coniacian. Although the uppermost part of the formation could not be dated, deposition in the eastern basin continued at least into the Coniacian and in the western basin at least until the middle–late Santonian. Sedimentation rates were highly variable with a period of condensed sedimentation at the Cenomanian–Turonian boundary. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Modeling thermogenic gas generation using carbon isotope ratios of natural gas hydrocarbons

An isotopic model to predict δ 13C values for methane (C 1), ethane (C 2), and propane (C 3) as a function of fractional conversion of kerogen and/or oil to gas is proposed for gas generation from Type II kerogen. Fractional conversion is then correlated to gas generation temperature by coupling the isotopic model to a multicomponent compositional kinetic model for gas generation. The relationships between δ 13C 1, δ 13C 2 and δ 13C 3 are constrained by the natural gas plot model of thermal cracking. The isotopic model has been tested and calibrated using a data set of gas samples derived from sapropelic marine shales (Type II kerogen) in the Delaware and Val Verde basins, West Texas. These gases are relatively unaffected by migration or mixing of biogenic gas, so that changes in carbon isotope ratios of the gas components primarily reflect the effect of maturation. This data set is therefore useful for illustrating the relationship between gas generation temperature and δ 13C values for gases derived from Type II kerogen. Several applications of the isotopic /compositional kinetic model in petroleum system evaluation are discussed. The effect of burial history on the relationship between δ 13C 1δ 13C 3 and gas generation temperature can be determined from the coupled models. Also, δ 13C values of C 1C 3 observed in the reservoir will depend on whether gas accumulates from source to trap, or whether it is expelled at various stages into several traps. Both trapping scenarios can be examined using the isotopic/compositional kinetic model. Separate isotopic models are needed for gases derived from Type II versus Type III kerogens because of the larger difference between δ 13C 1 and δ 13C 2 observed for gases generated from terrestrial organic matter. This larger difference may result from greater molecular or isotopic heterogeneity in Type III kerogens, or may reflect differences in generation and/or accumulation of gases from the differing kerogen types.

Growth increments in teeth of &lt;i&gt;Diictodon&lt;/i&gt; (Therapsida)

Growth increments circa 0.02 mm in width have been observed in sectioned tusks of Diictodon from the Late Permian lower Beaufort succession of the South African Karoo, dated between about 260 and 245 million years ago. Mean growth increments show a decline from relatively high values in the Tropidostoma/Endothiodon Assemblage Zone, to lower values in the Aulacephalodon/Cistecephaluszone, declining still further in the Dicynodon lacerficeps/Whaitsia zone at the end of the Permian. These changes coincide with gradual changes in carbon isotope ratios measured from Diictodon tooth apatite. It is suggested that the decline in growth increments is related to environmental changes associated with a decline in primary production which contributed to the decline in abundance and ultimate extinction of Diictodon.

Changes in carbon isotope ratios in the late Permian recorded in therapsid tooth apatite

THE end of the Permian witnessed global extinctions of marine and terrestrial fauna1. As marine biogenic carbonate and atmospheric carbon dioxide are in long-term isotopic equilibrium, changes in carbon isotope ratios in marine carbonate strata should record changes that occurred at that time in atmospheric ratios; one might then expect a correlation between 13C/12C ratios in such marine sequences and those in terrestrial records that derive from carbon-bearing materials that incorporate atmospheric CO2. An example of the latter is fossil tooth apatite from herbivorous fauna. Correlations of this sort between marine and terrestrial records are, however, difficult to establish as they rely on particular characteristics of the isotope curves. Here we show that stable-carbon isotope ratios in tooth apatite from Diictodon (a mammal-like reptile), in the late Permian Lower Beaufort succession of the South African Karoo (∼260–245 Myr), show a progressive depletion in 13C that is very similar to trends in marine carbonates from late Permian deposits in Eurasia2–7. These results suggest that global changes in total biomass occurred towards the end of the Permian: primary production decreased and continental aridity increased8, leading to a depletion of 13C in atmospheric and oceanic CO2.

Seasonal variations of carbon and nitrogen isotope ratios of plankton and sinking particles in Lake Kizaki.

Seasonal variations in carbon and nitrogen isotope ratios of plankton and sinking particles in Lake Kizaki were observed from April to November 1985 and May to October 1986. Carbon isotope ratio ( δ13C) of plankton with the size of 58 to 100 μm, which was mostly composed of phytoplankton, showed the highest value in June (-19 to -15 ‰) and the lowest value in April (-35.3 ‰). The δ13C value of plankton larger than 100 μm (mainly zooplankton) was lower by 1 to 5 ‰ than those of 58100 μm plankton. The nitrogen isotope ratios ( δ15N) of phytoplankton samples ranged from 1 to 6 ‰, and those of zooplankton samples were from 5 to 10 ‰. These changes in carbon and nitrogen isotope ratios of plankton in Lake Kizaki seemed to reflect the growth conditions of phytoplankton, such as water temperature, nutrient concentration, and light intensity. It was also suggested that the relative contributions of NH4+ and NO3- uptakes to total nitrogen uptake might affect the nitrogen isotope ratio of phytoplankton. The seasonal change in carbon isotope ratios of sinking particles collected by sediment trap was not so large, compared with that of phytoplankton. This indicated that the decomposition of autochthonous organic carbon occurred actively in the water column and that the contribution of allochthonous organic carbon to the bottom sediment was relatively large in Lake Kizaki.