A study of oxygen isotopic fractionation during bio-induced calcite precipitation in eutrophic Baldeggersee,Switzerland

In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ¹⁸O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in ¹⁸O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO₃²⁻] (∼50 μmol/l) in the surface waters. The resulting weighted average δ¹⁸O value for the studied period is −9.6‰, compared with a calculated equilibrium δ¹⁸O value of −9.4‰. These data convincingly demonstrate that δ¹⁸O of calcite are, for the most part, a very reliable proxy for temperature and δ¹⁸O of the water.     

Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes

The oxygen (δ¹⁸O, δ¹⁷O) and carbon (δ¹³C, FMOD¹⁴C-the fraction of modern ¹⁴C) isotopic compositions of carbonate were measured for a set of paired Antarctic CM chondrites (EET 96006, EET 96016, EET 96017, and EET 96019). While the oxygen isotopic compositions do not plot on the terrestrial fractionation line and indicate that a component of the carbonate minerals has an extraterrestrial origin, they also do not fall on the array defined for carbonates by CM falls and are thus consistent with the presence of a terrestrial carbonate component. The δ¹³C and FMOD¹⁴C measurements of carbonate suggest the presence of at least two carbon sources: carbonate derived from atmospheric CO₂ that is inferred to have been produced as a result of silicate weathering reactions and carbonate derived from another carbon source that is either old or non-atmospheric. The relationships between oxygen and carbon isotope data provide additional constraints on the weathering process, and allow the possibility that rock-dominated weathering of the meteorite caused the oxygen isotopic composition of Antarctic water added to the meteorite to evolve away from the terrestrial mass-fractionation array, leading to formation of low temperature terrestrial alteration products that do not lie on the terrestrial fractionation line.     

Environmental setting, (micro)morphologies and stable C–O isotope composition of cold climate carbonate precipitates—a review and evaluation of their potential as paleoclimatic proxies

Given the growing interest in carbonate deposits from polar regions as paleoclimatic proxies, this review paper first provides a classification of the various types of cold-climate carbonate precipitates followed by a summary of the ¹³C and ¹⁸O composition of the carbonate deposits and parent water from which the carbonates precipitated. The cold-climate carbonate precipitates were classified into three broad categories: powders, crusts and speleothem. The carbonate powders include those that precipitated in relation to aufeis aggradation (cryogenic aufeis calcite) and in relation to the growth of various annual/perennial ice formations in freezing caves (cryptocrystalline calcite and calcite pearls). The carbonate crusts can be further subdivided based on their lithic environment; those that precipitated on the upper surface of bedrock/clasts (i.e. subglacially precipitated calcite and evaporative calcite crusts); those that are located on the underside of clasts (i.e. pedogenic carbonates); and those that precipitated in rock outcrop fissures (i.e. endostromatolites). The cold-climate carbonate precipitates have a highly variable isotopic composition with δ¹⁸O values ranging between −6.5‰ and 28‰ VSMOW and δ¹³C values in the −10–20‰ VPDB range. However, each type of carbonate precipitates has a specific δ¹³C and δ¹⁸O range, suggesting that their environmental setting and the mechanism by which they formed controls their ¹³C and ¹⁸O signature. It was found that carbonate deposits that precipitated under equilibrium physico-chemical conditions had a δ¹³C value that is in equilibrium with that of the parent water, while its δ¹⁸O composition was more variable, as it is in part controlled by the temperature of reaction and by the δ¹⁸O and calcite saturation state of the parent water. By contrast, the δ¹⁸O composition of biologically precipitated carbonate deposits (endostromatolites) reflect that of the parent water, while its δ¹³C composition was enriched over that of the parent water due to bacterial methanogenesis. In the case of kinetically precipitated carbonate deposits, the δ¹⁸O and δ¹³C values are out-of-equilibrium relative to that of the parent water due to the faster rate of reaction.     

Controls on the stable isotope composition of seasonal growth bands in aragonitic fresh-water bivalves (unionidae)

Water temperature, oxygen isotope composition and the δ¹³C of dissolved inorganic carbon were measured in 2 southern Michigan rivers, the Huron River and Fleming Creek, between late September 1990 and June 1992. The final full year of shell growth in 3 unionids collected in 1992 from these rivers was sampled for stable isotope analysis with a resolution of 30 μm. The δ¹⁸O of both shell nacre and the prismatic layer is accurately predicted by a fractionation relationship developed for biogenic aragonite. High resolution sampling of 3 species and bulk sampling of 3 other species suggest that all unionids adhere to this oxygen isotope fractionation relationship. This relationship is used to show that shell growth ceases below approximately 12°C. In these 2 settings the average δ¹⁸O value of shell (PDB scale) is within 0.5‰ of the average δ¹⁸O of river water (SMOW scale). Unionids can therefore be used in oxygen-isotope-based paleoclimatic and paleohydrologic reconstructions. In contrast, the carbon isotope ratio of shell is not accurately predicted by published fractionation factors between D.I.C. and carbonate. Shell δ¹³C is more negative than predicted values and the offset is highly variable suggesting a significant and variable incorporation of metabolic carbon into the shell carbonate.     

Controls on ostracod valve geochemistry: Part 2. Carbon and oxygen isotope compositions

The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in ¹⁸O of more than 3‰ relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in ¹⁸O for these species of 1.7 to 2.3‰. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens’ δ¹³C values are mainly controlled by seasonal variations in δ¹³C_DIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope ‘vital effects’ in ostracods. Two mechanisms that might enrich the ¹⁸O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation.     

Hydrogen and oxygen isotopic compositions of sedimentary kaolin deposits,Egypt: Paleoclimatic implications

The clay fractions of sedimentary kaolin deposits representing different ages (Carboniferous and Cretaceous), types (pisolitic flint and plastic), and localities (Sinai and Aswan) from Egypt were analyzed for their H and O isotopic compositions to examine the paleoclimate conditions during their formation. The δD values of the Carboniferous deposits in Sinai range between −67‰ and −88‰, while the values for the Cretaceous deposits in Sinai range between −59‰ and −75‰. The δ¹⁸O values of the Carboniferous deposits range from 17.9‰ to 19.4‰ and the values for the Cretaceous deposits range between 19.2‰ and 20.4‰. The relatively low δD and δ¹⁸O values of the Carboniferous deposit at the Abu Natash area (−67‰ and 17.9‰, respectively) compared to other Carboniferous deposits (averages of −83.3‰, and 18.8‰ for δD and δ¹⁸O, respectively) could be due to isotopic exchange between this deposit and the adjacent dolomite and/or the enclosed hydrothermally-formed Mn ores of the Carboniferous Um Bogma Formation. The δD and δ¹⁸O values of the Cretaceous pisolitic flint kaolin deposit from Aswan (averages of −65‰ and 20.3‰, respectively) and plastic kaolin from the same area (averages of −66‰ and 19.5‰, respectively) are almost identical. The differences in the δ¹⁸O values between the clay fractions of the pisolitic flint kaolin (20.3‰) and the previously analyzed bulk kaolin of the same deposit (average of 17.5‰) suggest a significant effect of non-clay minerals on the isotopic compositions of the kaolin deposits.The H and O isotopic compositions plot close to the kaolinite line that marks the isotopic composition of kaolinite in equilibrium with meteoric water at 20 °C. This indicates that the kaolinite from both the Carboniferous and Cretaceous deposits in Egypt formed by meteoric water weathering of the source rock(s). The δD and δ¹⁸O values also suggest that kaolinite of these deposits formed under warm-temperate to tropical conditions. The slight deviations of some samples from the kaolinite line suggest post-depositional modifications of the isotopic compositions of studied deposits probably due to the interaction between earlier-formed kaolinite and downward percolating meteoric water.The δD and δ¹⁸O values of the Cretaceous and Carboniferous deposits from all localities suggest that both deposits formed under similar climatic conditions due to the location of Egypt at almost the same distance from the equator either to the south during the Carboniferous or to the north during the Cretaceous.     

δ<Superscript>18</Superscript>O and δD variations in Holocene massive ice in the Sabettayakha river mouth,northern Yamal Peninsula

The conditions of formation of massive ice near the South Tambey gas-condensate field in northern Yamal Peninsula are studied. It is shown that massive ice bodies up to 4.5 m thick occur in the Holocene deposits of the high laida and the first terrace. Therefore, they cannot be the remains of glaciers; they are ground ice formations. All three types of massive ice have quite various isotopic compositions: the values of δD range from–107 to–199.7, and δ¹⁸O from–15.7 to–26.48‰. Such a significant differentiation in isotopic composition is a result of cryogenic fractionation in a freezing water-saturated sediment. The most negative isotope values are even lower in this Holocene massive ice than in the Late Pleistocene ice-wedge ice of Yamal Peninsula.     

济阳坳陷奥陶系碳酸盐岩及缝洞充填方解石C、O同位素特征及其意义

对济阳坳陷奥陶系碳酸盐原岩及孔洞缝中充填方解石进行了C、O同位素测定,结果表明孔洞缝充填方解石的δ¹³C和δ¹⁸O值比原岩偏负。奥陶系三山子组和马家沟组孔缝中充填的方解石C、O同位素演化有很大区别,前者的δ¹³C和δ¹⁸O值均为负值,δ¹³C向较高负值偏移,δ¹⁸O值向较低负值偏移;马家沟组八陡段孔缝充填方解石的δ¹³C和δ¹⁸O值也多为负值,δ¹³C向较正值方向偏移,δ¹⁸O值向较高负值偏移。奥陶系碳酸盐岩孔缝充填方解石形成于大气淡水环境和埋藏成岩环境,次生孔洞可能主要形成于早期表生阶段,裂缝形成于中-新生代的构造运动,方解石主要充填于埋藏环境中。次生孔缝的主要形成时期早于油气大量运移期,对古潜山油藏的形成有利。     

Carbon,oxygen and strontium isotope geochemistry of Proterozoic carbonate rocks of the Vindhyan Basin,central India

Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ¹³C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ¹³C compositions. In the central Vindhyan section, the carbonates exhibit positive δ¹³C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ¹⁸O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ ¹⁸O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved ⁸⁷Sr/⁸⁶Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ¹³C values from Upper Vindhyan are characterized by lower ⁸⁷Sr/⁸⁶Sr values (0.7068±2) than those with negative δ¹³C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition.     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.     

Carbon and oxygen isotope chemostratigraphies of the Yangtze platform,South China: Decoding temperature and environmental changes through the Ediacaran

Multicellular animals first appeared on the earth during the Ediacaran period. However, the relationship between the abrupt biological evolution and environmental changes is still ambiguous. In order to examine seawater temperature and the carbon cycle through the Ediacaran, we analyzed the carbon and oxygen isotope compositions of carbonate rocks from drill cores from the Three Gorges area, South China. Importantly, the core samples include the Nantuo tillite, corresponding to the Marinoan glaciation, through the Doushantuo to the lower Dengying Fms. in ascending order.The δ¹³C profile displays five positive and five negative anomalies (PI-1 to 5 and NI-1 to 5), and the oxygen isotopes display very high absolute values around 0‰ with the highest at + 1.83‰. The combined δ¹⁸O and δ¹³C chemostratigraphies display both positive and negative correlations between the δ¹⁸O and δ¹³C values. The occurrence of the negative correlations supports the preservation of primary δ¹⁸O and δ¹³C values.The sample NI-4 has a negative correlation of the δ¹⁸O and δ¹³C excursions. The correlation supports a primary signature for both δ¹⁸O and δ¹³C variations. The positive δ¹⁸O excursion, accompanied by evidence of a eustatic sea-level fall, provides direct evidence for global cooling in the mid-Ediacaran; the 580 Ma Gaskiers Glaciation is a potential candidate for this global cooling event. The negative δ¹³C excursion was possibly caused by an increase in remineralization of dissolved organic carbon (DOC) due to enhanced continental weathering during the glaciation.Sample NI-5 is characterized by very low δ¹³C values, down to ? 10‰, corresponding to the Shuram-Wonoka-Pertatataka Excursion. The cause of the δ¹³C negative excursion is still not clear. However, a ubiquitous occurrence in excursions worldwide, and the lower δ¹³C values in deeper sections favor the enhancement of remineralization and respiration rather than secondary alteration, a restricted sea environment and lithification in coastal areas.     

Oxygen isotopic signature of 4.4-3.9 Ga zircons as a monitor of differentiation processes on the Moon

We report oxygen isotopic compositions for 14 zircon grains from a sample of sawdust from lunar breccia 14321. The zircons range in age from ∼4.4 to 3.9 Ga and in U and Th content from a few to several hundred ppm. As such these grains represent a range of possible source rocks, from granophyric to mafic composition, and cover the total age range of the major initial lunar bombardment. Nevertheless, results show that the oxygen isotopic compositions of the zircons fall within a narrow range of δ¹⁸O of about 1 per mil and have δ¹⁸O values indistinguishable from those observed for terrestrial mid-ocean ridge basalts confirming the coincidence of lunar and Earth oxygen isotopic compositions. In the δ¹⁷O vs. δ¹⁸O, coordinates data form a tight group with a limited trend on the terrestrial fractionation line. The zircon oxygen isotopes show minimal evidence of the extreme and variable mineral differentiation and element fractionation that have contributed to the formation of their parent rocks.     

Bio-mineralization and potential biogeochemical processes in bauxite deposits: genetic and ore quality significance

The Parnassos-Ghiona bauxite deposit in Greece of karst type is the 11th largest bauxite producer in the world. The mineralogical, major and trace-element contents and δ¹⁸O, δ¹²C, δ³⁴S isotopic compositions of bauxite ores from this deposit and associated limestone provide valuable evidence for their origin and biogeochemical processes resulting in the beneficiation of low grade bauxite ores. The organic matter as thin coal layers, overlying the bauxite deposits, within limestone itself (negative δ¹²C isotopic values) and the negative δ³⁴S values in sulfides within bauxite ores point to the existence of the appropriate circumstances for Fe bio-leaching and bio-mineralization. Furthermore, a consortium of microorganisms of varying morphological forms (filament-like and spherical to lenticular at an average size of 2 μm), either as fossils or presently living and producing enzymes, is a powerful factor to catalyze the redox reactions, expedite the rates of metal extraction and provide alternative pathways for metal leaching processes resulting in the beneficiation of bauxite ore.     

Variability of Stable Isotope Fingerprints of the Serpulid Ficopomatus enigmaticus Within a Permanently Stratified Estuary: Implications for (Palaeo)environmental Interpretations

This paper examines how the mixing of freshwater and seawater, and related mixing of freshwater and marine particulate organic matter (POM) in the permanently stratified estuary of the River Krka, Croatia, are reflected in the stable isotope fingerprints of soft tissues and tubes of the serpulid Ficopomatus enigmaticus. The carbon stable isotope composition (δ¹³C values) of the river-borne POM is retained over long distances, causing a depletion in ¹³C of POM in brackish waters. A trophic depletion in ¹³C was recorded in serpulid soft tissues. The serpulid carbonate tubes were depleted in ¹³C even at locations with salinity close to that of the sea and were subject to large isotope fractionation between dissolved inorganic C (DIC) and carbonate caused by vital effects, making carbonate depleted in ¹³C by several per mil compared with DIC. These effects, though large in the freshwater zone, fade towards the sea. The carbonate δ¹⁸O values of tubes reflect the δ¹⁸O values of the water. The temperature-related differences in δ¹⁸O values of tubes from different sites are masked by source-related differences in the δ¹⁸O values of water arising from mixing of freshwater and seawater in the estuary. Therefore, in serpulide tubes, the terrestrial component can easily be overestimated because of vital effects during biomineralisation and trophic depletion in ¹³C in freshwater and brackish environments.     

A reconnaissance study of stable isotope ratios in archaean rocks from the yilgarn block,Western Australia

Isotopic compositions of sulphur, carbon, and oxygen have been determined for constituents from a total of 103 samples of sedimentary rocks, mafic and ultramafic igneous rocks, nickel ores, and gold ores from the Archaean Yilgarn Block.

Sulphides in the bulk of the sedimentary rocks have δ³⁴S values close to 0‰ and appear to have precipitated from solutions which incorporated magmatic sulphur (either juvenile or derived from older rocks). There is no evidence for widespread sulphate reduction.

δ³⁴S values of sulphides in the nickel deposits and associated mafic/ultramafic igneous rocks are within the magmatic range. The small, high‐grade deposits of the Kambalda‐Nepean‐Scotia type have small positive δ³⁴S values, and the large, low‐to‐medium grade dunite‐associated deposits of the Mount Keith‐Perseverance type have small negative δ³⁴S values.

Sulphides in the Kalgoorlie gold ores are enriched in ³²S relative to those in their host dolerite, supporting an epigenetic origin for the gold, under moderately high fO₂ conditions.

The δ¹³C values do not provide unequivocal evidence for the source(s) of the reduced carbon (kerogen) in the sedimentary rocks. Whilst they are compatible with biogenic derivation, it is not possible to rule out contributions from pre‐biotic organic ‘soup’ or from hydrothermal solutions of deep‐seated origin.

Carbonate in the sedimentary rocks are predominantly in epigenetic, sulphide‐bearing veinlets. In many cases, their δ¹³C values suggest precipitation from hydro‐thermal solutions containing magmatically derived CO₂. In only two samples are the petrographic features and δ¹³C values compatible with marine carbonates. Talc‐carbonate altered ultramafic igneous rocks have δ¹³C values consistent with their incorporation of magmatically derived CO₂.

The ?δ¹³C (carbonate‐kerogen) values for most of the sedimentary rock studied fall in a narrow range around +10‰, suggesting isotopic exchange between oxidized and reduced carbon species at moderately high temperatures (>250°C).

δ¹⁸O values of carbonate from both sedimentary rocks and igneous rocks are mainly within the range +7.2‰ to +18.0‰. If the values are primary they are consistent with the formation of carbonate from hydrothermal solutions of magmatic and/or metamorphic origin. However, it is also possible the δ¹⁸O values are the result of post‐depositional equilibration with meteoric waters.     

Zinc-lead skarn deposits at Leadville, New South Wales, Australia, and their distinction from volcanic-hosted massive sulphides

Exploration of Zn-rich sulphide deposits at Leadville, northern Lachlan Fold Belt, New South Wales, for over two decades has been largely on the premise that the mineralisation represents felsic volcanic-hosted massive sulphides (VHMS). Deposits are hosted by ?Silurian felsic metavolcanic, psammopelitic and calcareous metasedimentary rocks which have been intruded by the late Carboniferous I-type Gulgong Granite. Evidence for an epigenetic replacement (skarn) origin of the deposits, rather than representing metamorphosed volcanogenic massive sulphides, includes the proximity of evolved granitic intrusives and reactive carbonate rocks, a skarn mineral assemblage (with characteristic prograde and retrograde stages), lack of textural or lithological indications of an exhalative origin, and gossan and sulphide compositions consistent with Zn-Pb skarns and atypical of Lachlan Fold Belt VHMS deposits. Furthermore, sulphide lead isotope ratios are significantly more radiogenic than signatures for VHMS deposits in the Lachlan Fold Belt. Carbonate δ¹³C and δ¹⁸O and sulphide δ³⁴S values are consistent with the interaction of magmatic hydrothermal fluids with Palaeozoic carbonate rocks and a largely magmatic source of sulphur. It is concluded that the Leadville deposits are of skarn type, genetically related to the Gulgong Granite.     

Carbon and oxygen isotopes in the Frasnian–Famennian section of the Kuznetsk basin (southern West Siberia)

The first detailed isotope-geochemical study of carbonate deposits has been performed in the Lower Famennian stratotype section of the northwestern Kuznetsk Basin (Kosoy Utyos), which was localized in the middle latitudes of the Northern Hemisphere in the Late Devonian. The δ¹³C_carb, δ¹³C_org, and δ¹⁸O variation curves were constructed for the section deposits. Geochemical and petrographic studies of carbonates allowed allocation of samples that underwent postsedimentation alteration and exclude them from further interpretation. Compared with coeval sections in the other world's regions, the Kosoy Utyos section is characterized by higher δ¹³C_carb values, up to 5.4‰, whereas the maximum value in subequatorial area sections is 4‰. The isotope shift amplitude of the studied section reaches 4.6‰, which is 1.5‰ higher than those in other regions. The δ¹⁸O values are 3‰ lower than the ones of the world's coeval sections. The results obtained show that δ¹³C and δ¹⁸O variation trends differ from those of coeval subequatorial sections. The high shift amplitude and maximum δ¹³C_carb values in the Kosoy Utyos section are due to the shallow-water carbonate sedimentation environments on the Siberian continental shelf and, probably, the lower temperatures of waters in the middle latitudes as compared with the subequatorial areas.     

Isotopic partitioning between scallop shell calcite and seawater: effect of shell growth rate

The relationship between molluscan shell growth rate and skeletal δ¹⁸O and δ¹³C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ¹⁸O and δ¹³C. At low shell growth rates, shell δ¹⁸O were more positive (of the order +0.4‰) and δ¹³C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ¹⁸O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ¹³C were consistent with incorporation of isotopically depleted respiratory ¹³C (i.e., a metabolic effect). A trend toward more depleted shell δ¹⁸O and δ¹³C values occurred at higher shell growth rates, with negative δ¹⁸O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ¹⁸O and δ¹³C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.     

Evidence for ocean acidification in the Great Barrier Reef of Australia

Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO₂, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ∼200 year δ¹¹B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The δ¹¹B isotopic compositions reflect variations in seawater pH, and the δ¹³C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ¹⁸O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of δ¹¹B values. The internal precision and reproducibility for δ¹¹B of our measurements are better than ±0.2‰ (2σ), which translates to a precision of better than ±0.02 pH units. Our results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. In the periods around 1940 and 1998 there are also rapid oscillations in δ¹¹B compositions equivalent changes in pH of almost 0.5 U. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ¹¹B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940s to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2-0.3 U, the range being dependent on the value assumed for the fractionation factor α_(B3-B4) of the boric acid and borate species in seawater. Correlations of δ¹¹B with δ¹³C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO₂ in surface waters from the rapidly increasing levels of atmospheric CO₂, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO₂ in atmosphere has already caused a significant trend towards acidification in the oceans during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact of Greenhouse gas emissions on coral calcification.     

Negative δO values in Allan Hills 84001 carbonate: Possible evidence for water precipitation on mars

The Martian meteorite ALH84001 contains ∼1% by weight of carbonate formed by secondary processes on the Martian surface or in the shallow subsurface. The major form of this carbonate is chemically and isotopically zoned rosettes which have been well documented elsewhere. This study concentrates upon carbonate regions ∼200 μm across which possess previously unobserved magnesium rich inner cores, interpreted here as rosette fragments, surrounded by a later stage cement containing rare Ca-rich carbonates (up to Ca₈₁Mg₀₇Fe₀₄Mn₀₇) intimately associated with feldspar. High spatial resolution ion probe analyses of Ca-rich carbonate surrounding rosette fragments have δ¹⁸O_V-SMOW values as low as −10‰. These values are not compatible with deposition from a global Martian atmosphere invoked to explain ALH84001 rosettes. The range of δ¹⁸O values are also incompatible with a fluid that has equilibrated with the Martian crust at high temperature or from remobilisation of carbonate of rosette isotopic composition. At Martian atmospheric temperatures, the small CO₂(gas)-CO₂(ice) fractionation makes meteoric CO₂ an unlikely source for −10‰ carbonates. In contrast, closed system Rayleigh fractionation of H₂O can generate δ¹⁸O_H2O −30‰, as observed at high latitudes on Earth. We suggest that atmospheric transport and precipitation of H₂O in a similar fashion to that on Earth provides a source of suitably ¹⁸O depleted water for generation of carbonate with δ¹⁸O_V-SMOW = −10‰.