Stable isotopic compositions of Recent freshwater cyanobacterial carbonates from the British Isles: local and regional environmental controls

Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ¹⁸O value of ?5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (?7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ¹⁸O of lake data, ?4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to ?12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO₂ and aqueous HCO^?₃. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ¹⁸O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.     

Origin of dolomite in Miocene Monterey Shale and related formations in the Temblor Range,California

Dolomites in thick sections of Miocene Monterey Shale and related formations in the Temblor Range of California acquired their isotopic compositions as they formed at shallow depth in the original sediment rich in organic matter, and retained the composition against the vicissitudes of burial diagenesis. The oxygen isotopes of dolomites of successive beds record changes in temperature of bottom water while the carbon isotopes of the same samples indicate changes in the kind of microbial activity (sulfate reduction vs carbohydrate fermentation) that prevailed at shallow depths in the sediment. In an auxiliary study, two samples of dolomite from sediments of Cariaco Basin off Venezuela (DSDP site 147) were found to have δ5C¹³ of ?14.1 and ?9.8 per ml PDB, although they occur in a heavy-carbon zone containing bicarbonate as heavy as +8.4 per ml. These dolomites probably originated at shallow depth in the light-carbon zone of microbial sulfate reducers and were buried under later sediments down into the heavy-carbon zone of microbial fermenters of carbohydrates without losing their original light-carbon composition.     

Dolomite and nahcolite formation in organic rich sediments: isotopically heavy carbonates

High positive C isotopic ratios in many carbonates coincide with high organic carbon contents. Nahcolites in the Green River oil shales have carbon isotopic ratios up to +20%. Some samples of organic rich Monterey and Pismo Formation dolomites also are ¹³C-rich (δ¹³C to +14%.). Combining the observations of unusual isotopic ratios and organic richness has led to proposed mechanisms of formation of nahcolites and dolomites. Bacterial fermentation produces ¹³C-rich carbon dioxide which in the organic rich sediments is formed in large amounts. This leads to chemical reasons for the occurrence of dolomite rather than calcite and nahcolite rather than trona in some shallow burials.     

Carbon and hydrogen isotopic compositions of New Zealand geothermal gases

Carbon and hydrogen isotopic compositions are reported for methane, hydrogen and carbon dioxide from four New Zealand geothermal areas: Ngawha, Wairakei, Broadlands and Tikitere. Carbon-13 contents are between ?24.4 and ?29.5%. (PDB) for methane, and between ?3.2 and ?9.1%. for carbon dioxide. Deuterium contents are between ?142 and ?197%. (SMOW) for methane and between ?310 and ?600%. for hydrogen. The different areas have different isotopic compositions with some general relationships to reservoir temperature.The isotopic exchange of hydrogen with water indicates acceptable reservoir temperatures of 180–260°C from most spring samples but often higher than measured temperatures in well samples. Indicated temperatures assuming ¹³C equilibria between CH₄ and CO₂ are 100–200°C higher than measured maxima. This difference may be due to partial isotopic equilibration or may reflect the origin of the methane. Present evidence cannot identify whether the methane is primordial, or from decomposing sediments or from reduction of magmatic CO₂. The isotopic equilibria between CH₄, CO₂, H₂ and H₂O are reviewed and a new semi-empirical temperature scale proposed for deuterium exchange between methane and water.     

Source of 1,2,3,4-tetramethylbenzene in asphaltenes from the Tarim Basin

1-Alkyl-2,3,6-trimethylbenzenes and a high relative amount of 1,2,3,4-tetramethylbenzene (TTMB) have been previously detected in the marine oils and asphaltenes in the oils from the Tarim Basin. In the present study, the stable carbon isotopic compositions of TTMB and n-alkanes in the pyrolysates of asphaltenes in the marine oils from the northern Tarim Basin and Silurian tar sands from the Tarim Basin were determined. TTMB has stable carbon isotopic compositions in the range from −23‰ to −24‰ and are about 12‰ more enriched in ¹³C than concomitant n-alkanes (−35‰ to −37‰) in the pyrolysates. The results indicate a contribution from green sulfur bacteria (Chlorobiaceae) to TTMB. Thus, the depositional environments of the source rocks for the marine oils and the bitumen in tar sands from the Tarim Basin are characterized by periods of euxinic conditions within the photic zone.     

Stable carbon isotopes in selected granitic,mafic, and ultramafic igneous rocks

Noncarbonate (combustion) and carbonate (acid decomposition) carbon were separately analyzed in 18 granitic rocks from a group of related Tertiary intrusions near Crested Butte, Colorado, and 14 mafic and ultramafic rocks from various localities in the western United States. Among the granites, carbonate carbon ranges from nil to 0.76 per cent with δC¹³-values from ?5.6 to ? 9.0‰ (vs PDB); noncarbonate carbon varies from 32–360 ppm with δC¹³-values from ?19.7 to ?26.6‰, The mafic and ultramafic rocks have carbonate carbon contents ranging from 53 ppm to about 2 per cent with δC¹³-values from + 2.9 to ?10.3‰; noncarbonate carbon varies from 26 to 150 ppm with δC¹³-values of ?22.2 to ? 27.l‰ For these samples, carbonate carbon ranges from 12.0 to 29.4‰ heavier than coexisting noncarbonate carbon. This consistent difference between δC¹³ of carbonate and noncarbonate carbon may be an isotopic fractionation effect. Because the specific indigenous form of noncarbonate (combustion) carbon is in doubt, conclusive interpretations regarding isotopic equilibration and fractionation cannot be made.These results have bearing on the assessment of the isotopic composition of mantle carbon and consequently are germane to the question of the origin (source) and history of crustal carbon. If mantle carbon is isotopically similar to noncarbonate (combustion) carbon, i.e. δC¹³-values from ?19.7 to ? 27.1‰, then a simple mantle degassing source for crustal carbon is improbable. Such a result would indicate an additional source of crustal carbon such as from a primitive atmosphere or extra-terrestrial accretion.     

Carbon isotope heterogeneity in metamorphic diamond from the Kokchetav UHP dolomite marble,northern Kazakhstan

We analysed isotopic compositions of metamorphic microdiamond secondary ion mass spectrometry. Typical microdiamonds in this dolomite marble show star-shaped morphologies (S-type) consisting of single-crystal cores and polycrystalline rims. Four S-type microdiamonds and two R-type microdiamonds (single crystals with rugged surfaces) were analysed using a 5 μm diameter ion beam. S-type microdiamonds have heterogeneous carbon isotopic compositions even in a single grain. Analysis of a typical S-type microdiamond (no. xx01-1-13) revealed clear difference in δ¹³C between core and rim. The rim shows lighter isotopic compositions ranging from??17.2‰ to??26.9‰, whereas the core is much heavier, with δ¹³C ranging from??9.3‰ to??13.0‰. The δ¹³C values of R-type microdiamonds fall into narrow ranges from??8.3‰ to??14.9‰ for no. xx01-1-10 and from??8.3‰ to??15.3‰ for no. xx01-1-16. These δ¹³C values are similar to those of the S-type microdiamond cores. The R-type probably formed at the same stage as the core of the S-type, whereas rim growth at a second stage did not occur or occurred very weakly in R-type microdiamonds. These carbon isotopic data support the two-stage growth of microdiamonds in the Kokchetav ultrahigh-pressure host rock. To explain the second stage growth of S-type microdiamonds, we postulate a simple fluid infiltration of light carbon from neighbouring gneisses into the dolomite marble.     

The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O¹⁸ by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC¹³ values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C¹³ content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.     

Carbon isotopic composition of diamonds from the Archangelsk diamond province

The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ¹³C value of the crystals ranges from ?2.79 to ?9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ¹³C of ?5.8 ?6.96 ‰, ?5.64/ ?5.85 ‰, and ?5.94/ ?5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ¹³C of ?4.05 and ?6.34 ‰ in host diamond crystals with δ¹³C of ?8.05 and ?7.54 ‰, respectively), and two samples of coated diamonds (cores with δ¹³C of ?6.98 and ?6.78‰ and coats with δ¹³C of ?7.51 and ?8.01 ‰, respectively). δ¹³C values were obtained for individual diamond crystals from bort-type aggregates (δ¹³C of ?4.24/ ?4.05 ‰, ?6.58/ ?7.48 ‰, and ?5.48/ ?6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ¹³C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.     

Aliphatic and diterpenoid hydrocarbons and their individual carbon isotope compositions in coals from the Liaohe Basin, China

Abundant tricyclic diterpanes (i.e., pimarane, dehydroabietane and simonellite) and tetracyclic diterpanes (e.g., phyllocladane) were detected in coal samples from the third member of the Shahejie Formation, Lower-Eogene, Liaohe Basin, China. Gas chromatography–isotope ratio mass spectrometry (GC–IRMS) analyses show that the carbon isotopic composition of terrigenous tricyclic and tetracyclic diterpenoid hydrocarbons are about 4–6‰ enriched in ¹³C compared to n-alkanes in the same samples. In addition, the pimaranes and phyllocladane have comparatively narrow stable carbon isotopic compositions among the different samples, with a slightly wider range in δ¹³C compositions for the abietanes (i.e., abietane, dehydroabietane and simonellite). The n-alkanes and triterpenoids reflect the δ¹³C compositions of higher plant wax.     

Carbon and oxygen isotopic compositions of Newania Dolomite Carbonatites, Rajasthan, India: implications for source of carbonatites

The Newania carbonatite complex of Rajasthan, India is one of the few dolomite carbonatites of the world, and oddly, does not contain alkaline silicate rocks thus providing a unique opportunity to study the origin and evolution of a primary carbonatite magma. In an attempt to characterize the mantle source, the source of carbon, and the magmatic and post-magmatic evolution of Newania carbonatites, we have carried out a detailed stable carbon and oxygen isotopic study of the complex. Our results reveal that, in spite of being located in a metamorphic terrain, these rocks remarkably have preserved their magmatic signatures in stable C and O isotopic compositions. The δ¹³C and δ¹⁸O variations in the complex are found to be results of fractional crystallization and low temperature post-magmatic alteration suggesting that like other carbonatites, dolomite carbonatites too fractionate isotopes of both elements in a similar fashion. The major difference is that the fractional crystallization of dolomite carbonatites fractionates oxygen isotopes to a larger extent. The modes of δ¹³C and δ¹⁸O variations in the complex, ?4.5?±?1‰ and 7?±?1‰, respectively, clearly indicate its mantle origin. Application of a multi-component Rayleigh isotopic fractionation model to the correlated δ¹³C versus δ¹⁸O variations in unaltered carbonatites suggests that these rocks have crystallized from a CO₂ + H₂O fluid rich magma, and that the primary magma comes from a mantle source that had isotopic compositions of δ¹³C ~ ?4.6‰ and δ¹⁸O ~ 6.3‰. Such a mantle source appears to be a common peridotite mantle (δ¹³C = ?5.0?±?1‰) whose carbon reservoir has insignificant contribution from recycled crustal carbon. Other Indian carbonatites, except for Amba Dongar and Sung Valley that are genetically linked to Reunion and Kerguelen plumes respectively, also appear to have been derived from similar mantle sources. Through this study we establish that dolomite carbonatites are generated from similar mantle source like other carbonatites, have comparable evolutionary history irrespective of their association with alkaline silicate rocks, and may remain resistant to metamorphism.     

The influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes in pyrolysates of selected terrestrial kerogens

Expulsion of petroleum from source rock is a complex part of the entire migration process. There exist fractional effects on chemical compositions in hydrocarbon expulsion. Does the carbon isotopic fractionation occur during expulsion and to what extent? Here the influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes from pyrolysates of selected terrestrial kerogens from Tuha basin and Fushun, Liaoning Province of China has been experimentally studied. The pyrogeneration-expulsion experiments were carried out under semi-closed system. The carbon isotopic compositions of individual n-alkanes were measured by GC-IRMS. The main conclusions are as follows. First, there is carbon isotopic fractionation associated with hydrocarbon expulsion from Type III kerogens in Tuha Basin. There exist differences of carbon isotopic compositions between the unexpelled n-alkanes and expelled n-alkanes from Tuha desmocollinite and Tuha mudstone. Second, there is almost no carbon isotopic fractionation associated with hydrocarbon expulsion from Type II kerogens in Fushun and Liaohe Basin. Third, carbon isotopic fractionation in hydrocarbon expulsion should be considered in making oil-source correlation of Type III kerogens at least in the Tuha Basin. Further studies need to be carried out to determine whether this is true in other basins. Fourth, oil and source at different maturity levels cannot be correlated directly for Type III kerogens since the carbon isotopic compositions of expelled hydrocarbons at different temperatures are different. The expelled hydrocarbons are usually lighter (depleted in ¹³C) than the hydrocarbons remaining in the source rock at the same maturity.     

Graphite- and diamond-bearing eclogite xenoliths from the Bellsbank kimberlites,Northern Cape,South Africa

 One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bellsbank kimberlites, Cape Province, South Africa. Graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms. Diamond is octahedral. Both Group I and Group II eclogite varieties are represented by the graphite-bearing specimens, while the single diamond-bearing eclogite is of the Group I variety. The carbon isotopic composition of the graphite varies from δ¹³C=−7‰ to δ¹³C=−2.8‰. This is within the range of carbon isotopic compositions for inclusion-free diamonds in kimberlite from this locality, suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source. The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly higher than the bulk earth estimate. Sr isotopic compositions of the clinopyroxene in these xenoliths vary from ⁸⁷Sr/⁸⁶Sr=0.703 to ⁸⁷Sr/⁸⁶Sr=0.706. This could be due to derivation of the xenoliths from a protolith with variable ⁸⁷Sr/⁸⁶Sr isotopic composition or could be the result of mixing between a low-Sr, high ⁸⁷Sr/⁸⁶Sr component and a high Sr, low ⁸⁷Sr/⁸⁶Sr component. Received: 1 June 1994/Accepted: 6 March 1995     

Seasonal variation of the stable isotopic compositions of coastal marine plankton from Woods Hole,Massachusetts and Georges Bank

A study of the isotopic composition of plankton from Woods Hole Harbor was conducted to investigate seasonal variation in carbon and nitrogen stable isotopes in a shallow coastal environment. Stable isotopic ratios of carbon and nitrogen both showed temporal variation on the scale of weeks to months, with heaviest (most positive) values in summer to fall for both isotopes. Particulate organic matter (POM) δ¹³C values were highest (?19‰ to ?21‰) in August to November and lower (?21‰ to ?25‰) at other times of the year, while δ¹³N-POM values were highest (9.5‰ to 12‰) in March to September and lower (7.5‰ to 9.5‰) at other times of the year. Stable isotopic values were significantly correlated with temperature, DI¹³C, and C∶N ratios, but not with [DIC], [POC], [PN], [chlorophyll], or the taxonomic composition of the phytoplankton. There was no direct evidence of allochthonous inputs of carbon and nitrogen to the system. Woods Hole δ¹³C values were virtually identical to Georges Bank plankton values; similar POC: Chlorophyll and C∶N ratios in the two systems further suggest that Woods Hole Harbor is principally a marine system. The high δ¹³C values of net plankton (>20 μm) during summer and early fall are consistent with a smaller degree of photosynthetic isotopic fractionation at that time, related to temperature and/or [CO_2(aq)]. This pattern was not seen, however, in total POM. Plankton δ¹³N values were higher in Woods Hole Harbor than on Georges Bank, especially during warmer periods, possibly due to high rates of nitrification and organic matter recycling in Woods Hole waters. Relatively wide ranges of stable isotopic values from both Woods Hole Harbor and Georges Bank suggest that seasonality should be considered when attempting to establish endmember C and N isotopic values for temperate marine plankton. Preliminary results from size-fractionated samples suggest that cyanobacteria may fractionate carbon isotopes to a greater degree than net phytoplankton.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Isotopic order, biogeochemical processes, and earth history: Goldschmidt lecture, Davos, Switzerland, August 2002

The impetus to interpret carbon isotopic signals comes from an understanding of isotopic fractionations imposed by living organisms. That understanding rests in turn on studies of enzymatic isotope effects, on fruitful concepts of isotopic order, and on studies of the distribution of ¹³C both between and within biosynthetic products. In sum, these studies have shown that the isotopic compositions of biological products are governed by reaction kinetics and by pathways of carbon flow.Isotopic compositions of individual compounds can indicate specific processes or environments. Examples include biomarkers which record the isotopic compositions of primary products in aquatic communities, which indicate that certain bacteria have used methane as a carbon source, and which show that some portions of marine photic zones have been anaerobic. In such studies, the combination of structural and isotopic lines of evidence reveals relationships between compounds and leads to process-related thinking. These are large steps. Reconstruction of the sources and histories of molecular fossils redeems much of the early promise of organic geochemistry by resolving and clarifying paleoenviron-mental signals. In turn, contemplation of this new information is driving geochemists to study microbial ecology and evolution, oceanography, and sedimentology.     

A contribution to the stable carbon isotope geochemistry of iron meteorites

For most iron meteorites studied, the carbon isotopic composition of nodular graphite falls in the range ?4.8 to ?8.2%. vs PDB and shows a mode between ?5 and ?6%.. Fourteen cohenite analyses from the Magura meteorite fall between ?18.1 and ?19.2%. with a pronounced clustering around ?18.5%.. Carbon of a taenite separate from the same meteorite has an isotopic composition of ?18.8%.; compositions between ?19.7 and ?22.1%. were found for taenite carbon in five other octahedrites. It is suggested that the ¹²C enrichment in cohenite and taenite relative to the nodular graphite is a general phenomenon in iron meteorites, and that the study of ¹³C abundances in iron meteorites may aid in the elucidation of their history. To this end an experimental study of carbon isotope fractionations in the system Fe-Ni-C is essential. The ¹³C content of carbon from several silicate inclusions in the Four Corners and ‘El Taco’ (Campo del Cielo) meteorites is generally similar to the nodular graphite, the ¹²C enrichment (?13%.) in one specimen may be interpreted in terms of a mixing model involving an original inclusion carbon and carbon exsolved from the taenite upon cooling.     

Oxygen and carbon isotope compositions of Upper Cretaceous chalk from the Danish sub-basin and the North Sea Central Graben

The oxygen and carbon isotopic compositions of Upper Cretaceous chalk have been studied from a large number of outcrops and boreholes in the Danish sub-basin and the North Sea Central Graben. The carbon isotopic compositions vary from +0.50%_o to +3.00%_oδ¹³C which correspond to the expected carbon isotopic composition of carbonate precipitated from normal sea water. The oxygen isotopic compositions of samples from outcrops and near-surface drilled sections fall in the range from —0.50%_o to — 2.00%_oδ¹⁸O, which is close to the expected values for carbonate produced by coccoliths in the Late Cretaceous sea. A geographic distribution exists in the Danish sub-basin showing relatively heavy oxygen isotope values in the centre of the basin and slightly more negative values in the eastern part of the basin. Primarily the phenomenon is thought to reflect minor variations in the oceanographic parameters in the Late Cretaceous sea. The deep subsurface sections represent a depth interval of 0–3100 m. Mechanical compaction in the uppermost part of the sequences apparently has no influence on the isotopic composition. Chemical compaction dominates at greater depth, leading to temperature-induced isotopic re-equilibration in the water-rock system. In these sequences the oxygen isotope values range from ? 1.50%_o to — 7.50%_oδ¹⁸O and are significantly correlated with depth of burial, cementation and porosity. The data indicate that pressure-dissolution, recrystallization, reprecipitation and ion-exchange between solid carbonate phase and pore water, are all actively involved in the oxygen isotope re-equilibration process. This process is believed to take place in a diagenetically closed system and is tentatively divided into two phases: 1) an early diagenetic phase which takes place at porosities down to approximately 20% in which the chalk possesses a pore-water controlled isotopic re-equilibration system and 2) a late diagenetic phase at porosities below 20% in which the re-equilibration process increasingly becomes influenced by the rock-introduced isotopic change in the composition of the formation water.     

Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four U.S. estuaries

High molecular weight dissolved organic matter (HMW-DOM) represents an important component of dissolved organic carbon (DOC) in seawater and fresh-waters. In this paper, we report measurements of stable carbon (δ¹³C) isotopic compositions in total lipid, total hydrolyzable amino acid (THAA), total carbohydrate (TCHO) and acid-insoluble “uncharacterized” organic fractions separated from fourteen HMW-DOM samples collected from four U.S. estuaries. In addition, C/N ratio, δ¹³C and stable nitrogen (δ¹⁵N) isotopic compositions were also measured for the bulk HMW-DOM samples. Our results indicate that TCHO and THAA are the dominant organic compound classes, contributing 33-46% and 13-20% of the organic carbon in HMW-DOM while total lipid accounts for only <2% of the organic carbon in the samples. In all samples, a significant fraction (35-49%) of HMW-DOM was included in the acid-insoluble fraction. Distinct differences in isotopic compositions exist among bulk samples, the compound classes and the acid-insoluble fractions. Values of δ¹³C and δ¹⁵N measured for bulk HMW-DOM varied from −22.1 to −30.1‰ and 2.8 to 8.9‰, respectively and varied among the four estuaries studied as well. Among the compound classes, TCHO was more enriched in ¹³C (δ¹³C = −18.5 to −22.8‰) compared with THAA (δ¹³C = −20.0 to −29.6‰) and total lipid (δ¹³C = −25.7 to −30.7‰). The acid-insoluble organic fractions, in general, had depleted ¹³C values (δ¹³C = −23.0 to −34.4‰). Our results indicate that the observed differences in both δ¹³C and δ¹⁵N were mainly due to the differences in sources of organic matter and nitrogen inputs to these estuaries in addition to the microbial processes responsible for isotopic fractionation among the compound classes. Both terrestrial sources and local sewage inputs contribute significantly to the HMW-DOM pool in the estuaries studied and thus had a strong influence on its isotopic signatures.     

Further stable isotope investigations of human urinary stones: comparison with other body components

Carbon and sulphur isotope investigations of human urinary stones have been expanded to relate such data to various body components and to diet. Techniques include isotopic determinations for various body components, for example, hair and urine, as well as trace sulphate and sulphide in apatite-struvite stones.Hair from individuals in Calgary was found to be, on average, about 3‰ depleted in¹³C in comparison to samples from Hawaii. Uric acid stones from both locations were found to be 1 to 3‰ enriched in¹³C, compared to hair. Oxalate stones from Calgary had δ¹³C values very close to those of hair. In contrast, oxalate stones from 31 patients from Honolulu fit the regression lineδ¹³C_oxalate= 0.8 δ¹³C_hair− 4.4‰, with a correlation coefficient of 0.77. It remains debatable as to whether the isotopic differences between the stones and hair reflects preferential incorporation of dietary components or kinetic isotope effects during biochemical conversions. There was no evidence in the data from Honolulu that ethnic background significantly influenced the carbon isotope composition of hair or kidney stones. There was a suggestion that recent arrivals had hair and stones slightly depleted in¹³C as compared to longer residents.The δ³⁴S values of cystine stones from Calgary were markedly consistent, near 0‰, and isotopic variations among different body components of individuals were of the order of 1‰.The trace sulphate content of a bladder stone from Papua New Guinea, was 300ppm S, whereas the sulphide content was negligible (determined by in vacuo Kiba extraction). The total S content of three samples from Calgary averaged 250 ppm, whereas 150 ppm was found for two stones each from Quito, Ecuador and Honolulu, Hawaii. For stones other than the speciment from Papua New Guinea, the sulphate-to-sulphide ratio varied from 1 to 4. The source of sulphide is uncertain but degradation of organic S could contribute to this fraction during Kiba extraction. The small range of δ³⁴S values (+3.5 to +7.4‰) for trace total S in the phosphate-containing urinary stones is believed to reflect only a fraction of the global variation of these materials. Trace sulphate was variably enriched in³⁴S (0 to 9‰) as compared to sulphide. Neither these enrichments, nor the sulphate-to-sulphide ratio, could be related to the struvite-to-apatite ratio.There were no significant differences in the carbon and sulphur isotope compositions of hair from patients and non-stone formers. Both the carbon and sulphur isotope variations can be attributed to the isotopic compositions of diets and the superposition of small kinetic isotope effects during biochemical conversions.