Composition Of Carbonate Concretions Research Articles

Composition and Genesis of Carbonate Concretions in the Region of Paramushir Hydroacoustic Anomalies (Sea of Okhotsk)

Composition and Genesis of Carbonate Concretions in the Region of Paramushir Hydroacoustic Anomalies (Sea of Okhotsk)

Life cycle assessment of carbon concrete composites: a circular economy path beyond climate mitigation?

Sustainable construction and materials play an ever-important role to stay within our planetary boundaries. In support, innovative carbon concrete composites (CCC) promise significant raw material savings by integral design. We aim to illustrate current environmental hotspots and a feasible recycling scenario of CCC that meets circularity requirements. We modelled a cradle-to-grave life cycle assessment for two potential building structural applications (sandwich wall, ceiling reinforcement) made of CCC. We based our recycling scenario on previously conducted large-scale experiments. Results show a relative larger energy intensity and abiotic depletion of fossil fuels for variants of CCC but lower global warming. Yet, recycling is, second to embodied emissions of basic materials, the driving force of total environmental impacts. The presented recycling path (demolition, pyrolysis for carbon fabric, reuse in fiber fleece) offers less "green credentials" than steel.

The Composition, Structure, and Origin of Carbonate Concretions Sampled in the South Kambalnyi Central Thermal Field, Kamchatka

Carbonate concretions are formed at the base of a sequence of hydrothermal clay in the South Kambalnyi Central Thermal Field situated in the southern part of the Kambalnyi volcanic mountain range, Kamchatka. The concretions have complex chemical and mineral compositions: apart from aragonite which is the main component of each layer, the chemical compounds identified there include oxides of iron and silicon, sulfates of calcium and barium, sulfides of iron and other metals, carbonates of iron and manganese, siliceous ferromanganese formations, nitrogen compounds, and phosphates of calcium and rare metals. The concretions have diverse structures and textures that indicate a multiphase character of formation for these mineral aggregates. It is thought that their formation was due to the discharge of deep-seated alkaline metalliferous solutions in a zone of rock argillization of the South Kambalnyi Central Thermal Field.

Aiming for life cycle sustainability assessment of cement‐based composites: A trend study for wall systems of carbon concrete: Dresden Nexus Conference 2020—Session 4—Circular economy for building with secondary construction materials to minimise resource use and land use

AbstractThe narrative sustainable building is core message of European politics. Strategies discussed in this regard concern above all material minimization by using new more efficient manufacturing technologies, new form finding approaches for load‐bearing structures or new high‐performance materials. The goal of the research project V2.10, funded by the German Federal Ministry of Education and Research, refers to the latter by assessing the sustainability potential of building components made of innovative textile reinforced concrete. We conceptualized a life cycle sustainability assessment framework and applied it to variants of sandwich wall systems made of carbon concrete composites and steel‐reinforced concrete. Results indicate hotspots in technology and material choices that could be addressed by circular strategies, for example, refuse, reduce or recycling. Overall, one design variant made of carbon concrete composites is the best performing with respect to all dimensions of sustainability.

Leaching of Carbon Reinforced Concrete—Part 1: Experimental Investigations

The composite material ‘carbon concrete composite (C3)’ is currently capturing the building sector as an ‘innovative’ and ‘sustainable’ alternative to steel reinforced concrete. In this work, its environmental compatibility was investigated. The focus of this research was the leaching behavior of C3, especially for the application as irrigated façade elements. Laboratory and outdoor exposure tests were run to determine and assess the heavy metal and trace element emissions. In the wake of this work, the validity of laboratory experiments and the transferability to outdoor behavior were investigated. The experimental results show very low releases of environmental harmful substances from carbon concrete composite. Most heavy metal concentrations were in the range of <0.1–8 µg/L, and higher concentrations (up to 32 µg/L) were found for barium, chromium, and copper. Vanadium and zinc concentrations were in the range of 0.1–60 µg/L, boron and nickel concentrations were clearly exceeding 100 µg/L. Most of the high concentrations were found to be a result of the rainfall background concentrations. The material C3 is therefore considered to be environmentally friendly. There is no general correlation between laboratory leaching data and outdoor emissions. The results depend on the examined substance and used method. The prediction and evaluation of the leaching of building elements submitted to rain is therefore challenging. This topic is debated in the second part of this publication.

EXPERIMENTAL INVESTIGATION ON STRENGTHENING OF REINFORCED CONCRETE COLUMNS WITH CARBON CONCRETE COMPOSITE

The application of textile reinforced concrete is well-approved technique for strengthening of reinforced concrete members. When using carbon fiber meshes and carbon fiber reinforced polymer bars as reinforcement, this material is called carbon concrete composite. Based on the outstanding properties of carbon fibers, carbon concrete composite is characterized by high bending and tensile strength, and good durability. Therefore, carbon concrete composite is increasingly applied as replacement for ordinary steel bar or steel mesh reinforced concrete. It is favorable building material for production of new buildings and for strengthening of existing reinforced concrete members. In the context of strengthening of existing reinforced concrete columns, it is a usual procedure to cover the member’s surface with a thin layer of carbon concrete composite aiming on reduction of lateral strains of the core concrete when load is increasing. The result is an increased load-bearing capacity of the strengthened column. However, there is insufficient knowledge about the influence of curvature of the carbon meshes in circular cross-sections and in the corners of rectangular cross-sections on their load-bearing capacity. For this reason, an experimental program started to study the influence of curvature, number and type of mesh layers and specimen dimensions on structural behavior of strengthened columns under axial loading. As main outcome it can be stated that besides the curvature other parameters like yarn properties are of essential importance.

Environmental Compatibility of Carbon Reinforced Concrete: Irrigated Construction Elements

To foster a sustainable deployment of the innovative composite material ‘carbon concrete composite’ in the building sector, it is necessary to ensure its resource efficiency and environmental compatibility. The Institute for Building Materials Research of the RWTH Aachen University is therefore investigating the leaching behavior of this material, especially for the case of irrigated façade elements. Laboratory and outdoor exposure tests are run to determine and assess the heavy metal and trace element emissions by leaching. Feasible interconnections between laboratory and outdoor examinations can be used to develop a faster testing of future composite materials. Current results show no critical release of environmental harmful substances from carbon concrete composite.

Evaluating the Performance of Functionalized Carbon Structures with Integrated Optical Fiber Sensors under Practical Conditions.

An Optical Frequency Domain Reflectometry (OFDR) based fiber optic sensor scheme “embedded” in concrete for the purpose of structural health monitoring (SHM) of carbon concrete composites (C3) is presented. The design, while strengthening the concrete structure, also aims to monitor common SHM parameters such as strain and cracks. This was achieved by weaving the carbon fiber together with optical fiber, based on a specialized technique that uses an embroidery setup where both the carbon and optical fiber are woven on a water dissolvable polymer substrate. The performance of the sensing scheme was characterized in-situ utilizing the OFDR based technique and the results presented. The sensors embedded on a custom made concrete block were subjected to varying strain via a three point bending test to destruction and the results discussed. The intended dual-achievement of the scheme thus proposed in SHM and strengthening the C3 is demonstrated. The suitability of the OFDR scheme for C3 is combined with a fibre Bragg grating (FBG)-based approach, and discussed in detail.

Reinforcement Systems for Carbon Concrete Composites Based on Low-Cost Carbon Fibers

Carbon concrete polyacrylonitrile (PAN)/lignin-based carbon fiber (CF) composites are a new promising material class for the building industry. The replacement of the traditional heavy and corroding steel reinforcement by carbon fiber (CF)-based reinforcements offers many significant advantages: a higher protection of environmental resources because of lower CO2 consumption during cement production, a longer lifecycle and thus, much less damage to structural components and a higher degree of design freedom because lightweight solutions can be realized. However, due to cost pressure in civil engineering, completely new process chains are required to manufacture CF-based reinforcement structures for concrete. This article describes the necessary process steps in order to develop CF reinforcement: (1) the production of cost-effective CF using novel carbon fiber lines, and (2) the fabrication of CF rebars with different geometry profiles. It was found that PAN/lignin-based CF is currently the promising material with the most promise to meet future market demands. However, significant research needs to be undertaken in order to improve the properties of lignin-based and PAN/lignin-based CF, respectively. The CF can be manufactured to CF-based rebars using different manufacturing technologies which are developed at a prototype level in this study.

Mechanisms of Late Pleistocene authigenic Fe–Mn-carbonate formation at the Laptev Sea continental slope (Siberian Arctic)

Study of the microstructure and isotopic composition of authigenic tubule-shaped carbonate concretions from sediment core PS51/154-11 on the western Laptev Sea continental slope (present water depth 270 m) has allowed for reconstruction of the conditions prevailing during their formation and identification of the mechanisms controlling their genesis. Concretions were collected from the basal sediment unit with an extrapolated age estimate of 16.3–17.6 cal.ka. Crystallization of carbonate tubules occurred at the beginning of the last deglaciation when the site was located in the proximity to the former coastline and the mouths of the Olenek and Anabar-Khatanga rivers in water depths of about 150–170 m. Microprobe analysis showed that the studied carbonate tubules consist of the minerals belonging to the siderite–rhodochrosite isomorphic series. The measured isotopic composition of δ13С and δ18O in the carbonates varies between − 21.0 and − 17.0‰ and between − 9.86 and 1.72‰ VPDB, respectively. The δ18O values in the authigenic carbonates give evidence for the gradual transition from a freshwater affected to modern-like marine sedimentation environment during carbonate crystallization. Water freshening is confirmed by the co-occurrence of authigenic Fe–Mn carbonates and Fe-phosphate vivianite that is a typical mineral of freshwater environments. The dominant source of dissolved inorganic carbon in the pore water was the isotopically light carbon derived from the diagenetic decomposition of organic matter. Two possible scenarios of authigenic carbonates formation are proposed: penetration of freshened ground waters and/or enhanced freshwater influence during short seasonal floods in combination with geochemical processes in a narrow marginal filter zone that was located extremely close to the Laptev Sea continental slope and the studied core site.

Multifunctional components from carbon concrete composite C3 – integrated, textile-based sensor solutions for in situ structural monitoring of adaptive building envelopes

This contribution will introduce carbon-reinforced concrete components (so-called carbon concrete composites, or C3) with sensor functionalities for innovative building envelopes. For a continuous in situ structural monitoring, these textile-reinforced concrete components are equipped with textile sensor networks consisting of resistive carbon fiber sensors (CFSs), which are integrated into the carbon fiber non-crimp fabrics of the concrete reinforcement by multiaxial warp-knitting. The in situ CFSs, consisting of 1 k or 50 k carbon fiber roving with added staple fiber/multifilament dielectric cladding, are later integral to the load-distributing elements of the concrete component, and elongations within these are easy to record with good correlation to ohmic resistance changes. Gage factors of k = 0.52–1.23 at linearity deviations of ALin = 4.0–8.7% are feasible. This allows a monitoring of C3 building envelopes for structural mechanical changes caused by physical changes within the component through mechanical or thermal loads or deformation and cracks.

Mineral-Based Coating of Plasma-Treated Carbon Fibre Rovings for Carbon Concrete Composites with Enhanced Mechanical Performance.

Surfaces of carbon fibre roving were modified by means of a low temperature plasma treatment to improve their bonding with mineral fines; the latter serving as an inorganic fibre coating for the improved mechanical performance of carbon reinforcement in concrete matrices. Variation of the plasma conditions, such as gas composition and treatment time, was accomplished to establish polar groups on the carbon fibres prior to contact with the suspension of mineral particles in water. Subsequently, the rovings were implemented in a fine concrete matrix and their pull-out performance was assessed. Every plasma treatment resulted in increased pull-out forces in comparison to the reference samples without plasma treatment, indicating a better bonding between the mineral coating material and the carbon fibres. Significant differences were found, depending on gas composition and treatment time. Microscopic investigations showed that the samples with the highest pull-out force exhibited carbon fibre surfaces with the largest areas of hydration products grown on them. Additionally, the coating material ingresses into the multifilament roving in these specimens, leading to better force transfer between individual carbon filaments and between the entire roving and surrounding matrix, thus explaining the superior mechanical performance of the specimens containing appropriately plasma-treated carbon roving.

PEDOGENIC CARBONATE CONCRETIONS IN THE RUSSIAN CHERNOZEM

Pedogenic carbonate concretions are commonly found in grassland soils, but their origin is not fully understood. This study was conducted to determine the radiocarbon age, the stable isotope geochemistry, and chemical composition of carbonate concretions in the Russian Chernozem, one of the typical soils in grasslands. Three sites were sampled: a native grassland field (not cultivated for at least 300 years), an adjacent 50-year continuous fallow field in the V. V. Alekhin Central-Chernozem Biosphere State Reserve in the Kursk region of Russia, and a continuously cropped field in the Experimental Station of the Kursk Institute of Agronomy and Soil Erosion Control. All sampled soils were classified as fine-silty, mixed, frigid Pachic Hapludolls. The mineralogical composition of concretions varies from low-magnesium calcite to pure calcite. The concretion contains 0.05% N, 6.4% C, and has δ 13 C and δ 18 O values of -10.9%o (the per mill symbol, parts per thousand) and -7.8%o, respectively. The outside part of the carbonate concretion is 1909 ± 40 14 C age Before Present (B.P.) compared with 1693 ± 40 14 C age B.P. for the inside part of the same concretion, even though the concretion is found in the C horizon of much older age (10,902 ± 63 14 C age B.P.). Remnants of soil organic matter in concretions are closely associated with the cropped and fallow/plowed soils by pyrolysis molecular beam mass spectrometry.

Isotopic Composition (δ13C, δ18O) of Carbonate Concretions from Terrigenous Deposits in the Northern Caucasus

The isotopic composition of carbon and oxygen was studied in carbonate concentrations with different compositions (calcite, dolomite, and siderite) from Jurassic, Cretaceous, and Tertiary terrigenous deposits of the Northern Caucasus. Wide variations in the isotopic composition (from 41.4 to 18.1‰ for δ13C and from 11.7 to 33.5‰ for δ18O) point to different formation conditions in the early diagenesis zone and the later catagenesis zone.

Karelian shungite—an indication of 2.0-Ga-old metamorphosed oil-shale and generation of petroleum: geology, lithology and geochemistry

The ca. 2.0-Ga-old, 600-m-thick upper Zaonezhskaya Formation near Lake Onega, NW Russia, contains unusually high concentrations of C org (up to 98%), averaging around 25%. The formation contains an estimated 25×10 10 tonnes of organic carbon accumulated within an area of 9000 km 2. Organic material is represented by shungite, which forms a black, dense, amorphous or nanocrystalline mass consisting of C with traces of N, O, S, and H. Autochthonous shungite occurs as disseminated organic material (0.1–50% C org) which, when mixed with migrated bitumen (now pyrobitumen), appears as coal-like seams and lenses of semilustrous and semimat layer-shungite rocks (oil shales, 50–75% C org). The migrated bitumen (originally petroleum), represented by the lustrous vein- and layer-shungite, conformably fills interbedding spaces or cross-cutting joints and usually contains 80–98% C org. The shungite-bearing rocks of the upper Zaonezhskaya Formation represent one of the most richest accumulations of organic material reported from the Palaeoproterozoic, and one of the geologically earliest stages of petroleum generation. The sediments of the Zaonezhskaya Formation were initially deposited in brackish water in a non-euxinic, lagoonal environment. The high C/S ratio (8–1000) with a zero intercept on the C–S cross-plot indicates that deposition occurred in sulphur-poor water. Intensive synchronous volcanism may have contributed to both the enhanced delivery of nutrients and elevated sedimentation rate, and eventually to the high degree of preservation of organic material. The integrated data suggest that the organic material has a biogenic origin, most likely algal or bacterial. The organic material suffered complex catagenetic and metamorphic alteration which is reflected in: (1) the four-modal distribution of C org content (with maxima at 5, 30, 65 and 95%); (2) highly variable δ 13 C org (−45‰ to −17‰); (3) bimodal distribution of δ 13 C org (with maxima at −28 and −39‰); and (4) low H/C ratios (0.005–0.2). Abundant diagenetic carbonates associated with shungite rocks ( δ 13 C carb =−5 to −26‰) and the presence of pyrite ( δ 34 S −22 to +31‰), reflects substantial loss of organic matter via bacterial reduction of sulphate during diagenesis. The shungite rocks are characterised by a further substantial loss (>50%) of biologically produced organic material in the course of thermal maturation and by a depletion in 12 C (>10‰). The isotopic composition of carbonate concretions does not indicate the involvement of fermentative diagenesis. Conservative estimates give δ 13C org of −34‰ as the best value of the initial biomass. Lustrous vein- and layer-shungite containing more than 80% C org are considered to be allochthonous, migrated bitumen (originally petroleum). The semilustrous and semimat layer shungite rocks containing 55–75% C org represent oil shales with both migrated bitumen (originally petroleum) and autochthonous kerogen residues. The oil source rocks were apparently hosted in the Zaonezhskaya Formation. The generated oil has migrated both vertically and laterally with the highest concentration in cupola structures. The locality at Shunga represents the most significant volume of trapped petroleum from the study area.

Abstract: Composition and Origin of Carbonate Concretions in Reservoirs of Marlim Field, Campos Basin, Brazil&amp;nbsp;

Abstract: Composition and Origin of Carbonate Concretions in Reservoirs of Marlim Field, Campos Basin, Brazil&amp;nbsp;

Isotopic composition of diagenetic carbonate concretions at the Cretaceous-Tertiary boundary in south-central Texas, USA: Implications for their growth

Carbonate concretions 10–20 cm below and immediately above the Cretaceous/Tertiary boundary at the Littig Quarry, Elgin, Texas, differ significantly from each other morphologically, petrographically, and isotopically. The Cretaceous Kemp Formation concretions consist of oblate spheroidal nodules made up of microspar and pseudospar. The δ13C and δ18O values of the microspar and pseudospar are relatively invariant (mean ± standard deviation: δ13Cav = −19.4 ± 0.4‰; δ18Oav = −1.6 ± 0.2‰, n=16), indicating concretion grew in a restricted zone beneath the sediment/water interface. δ18Oav coincides with that of Cretaceous/Tertiary planktonic forams reported from North American DSDP foram data, suggesting that the concretion formed from marine-derived waters and was not altered by freshwater. The occurrence of pyrite framboids and the low δ13C values suggest that the concretions grew under sulfate reducing conditions. In contrast, concretions from the Paleocene Littig Member of the Kincaid Formation consist of scattered microspar/pseudospar nuclei cemented by variably compacted, fine to very fine sandy, glauconitic, phosphatic, starved-basin sediment. The Littig Member concretions range in isotope composition from δ18O of −2.0 to −4.8‰. and δ13C of −20.5 to −10.3‰. The trend from high δ18O and low δ13C, to low δ18O and high δ13C, corresponds to increasing compaction and suggests that the Littig Member concretions grew continuously during burial rather than at a single depth like the Kemp Formation concretions. The increase in the13C isotope with increasing compaction is interpreted to indicate that the concretions formed during the shift from sulfate reducing to methanogenic conditions and in the early stages of methanogenesis. *** DIRECT SUPPORT *** A00QA020 00006

Oxygen and carbon isotopic composition of marine carbonate concretions; an overview; discussion and reply

In their review of isotopic data from marine carbonate concretions, Mozley and Burns (1993, p. 8 i) incorrectly associate our work (Dix and Mullins 1987) with their interpretation of the effects of recrystallization on isotopic compositions in concretions. Mozley and Burns (1993, p. 81) state, Late-stage calcite and siderite concretions are characterized by a combination of 'normal marine' 6~3C values and depleted 6~sO values. Such isotopic values have been interpreted by some authors as indicating recrystallization (i.e., that the concretion initially had normal marine carbon and oxygen isotopic compositions and that the oxygen isotopic compositions were reset at higher temperatures; Dix and Mullins 1987 . . . . ). The association of our work with this statement is incorrect because we illustrated that: (1) the bulk of the concretions in the Middle Devonian Hamilton Group appear to have formed during very early stages of burial within the upper 10 m (p. 147); (2) the very negative 5~3C values of most concretion samples and systematic shifts with concretion growth illustrate the role of organic diagenesis within the zone of sulfate reduction, and transitional into the upper zone of methanogenesis (our Fig. II); and (3) the only late-stage concretions (containing ferroan calcite) have 6'3C values ranging between 0 and 2%, and this due to a subsurface shift in porewater 6'3C related to organic diagenesis (our Fig. 11), not recrystallization (p. 150). In their discussion of causes for consistently depleted 5'80 values in shallow-burial calcite concretions, Mozley and Bums (1993, p. 77) appear to dismiss the effect of recrystallization partly through their assumption that the original composition of concretions must be stable low-magnesium calcite. They cite, however, only three studies of concretions that unequivocally remove any possibility for recrystallization. Are these the exceptions or the rule? Combined petrographic and isotopic observations still lead us to the interpretation that recrystallization of Hamilton Group concretions (excluding late-stage concretions) was very important. 6180 data fall within the middle of an isotopic group defined by late-stage septarian calcites, and our samples of septarian calcites also fall within this group with ~laO values that are not too dissimilar to the bulk of concretion calcite (our Fig. 12). We presented petrographic evidence illustrating that two stages of septafian calcite have been replaced by subsequent calcite, including ferroan calcite (our Fig. 7), and that replacement calcite is associated with silica diagenesis in the concretion bodies and migration of petroleum-bearing fluids. If septarian calcite can be replaced, why not the concretion-body calcite too? With so many calcite concretions appearing to form under near-surface burial conditions, could the mineral-water interaction that Mozley and Burns (1993) call upon to account for net shifts in ~aO include very shallow recrystallization ofa metastable magnesium-beating carbonate? Given the diverse mechanisms for causing a shift in 6'80 (Mozley and Burns 1993), recrystallization seems to be a plausible factor (see Desrochers and AI-Aasm 1993) that should not be discarded so readily.

Microbial processes: Controls on the shape and composition of carbonate concretions

Shape, size and mineralogical/chemical compositions of carbonate concretions vary considerably. The questions to be answered are—Why do they form at all? What processes are involved? Why do they so often form as regular spheroids? Why do they sometimes form laterally-extensive bands? What controls whether the mineral composition is calcite, dolomite, rhodochrosite or siderite (or mixtures of them)? The questions are addressed and the answers illustrated by selected examples. Carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents (organic matter). Microbes take advantage of the energy source and are believed to be involved in most of the various processes. Pore waters in steadily-accumulated sediments show a depth-related succession of chemical and carbon stable-isotope compositions. This results from degradation of organic matter proceeding via an orderly succession of processes during burial, each mediated by a specific microbial group. Aerobic oxidation, using dissolved oxygen occurs first, followed by Mn(IV), reduced to Mn(II), as the oxidant. Fe(III) reduction and sulphate reduction are probably more significant. Methanogenesis is the final, deepest microbial process. The common product of all these reactions is CO 2 or dissolved HCO 3 − which can result in carbonate supersaturation. The specific process controls the chemical and isotopic compositions of pore-water and leads to a characteristic mineralogy of any carbonate precipitated. However, the processes may not proceed successively and localised micro-environments occur. Recent work on siderite concretions in very young salt-marsh sediments (Norfolk, U.K.) has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth. Concretions in ancient rocks can be described in terms of the same processes observed in modern sediments. Laterally-extensive siderite bands in Bullhouse Quarry (Westphalian, U.K.) resulted from interaction of two sets of complementary processes: shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation. The model is supported by a mass and charge balance calculation. Similar, complementary processes formed spheroidal concretions (Gammon Shale, USA) but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction. The spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes. Calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model. Kimmeridge Bay (UK) dolomite ledges are marine analogues to the Bullhouse examples. For the pyrite-rimmed calcite Jet Rock concretions (Early Jurassic, Yorkshire, U.K.) the rate of production of sulphide (from sulphate reduction) locally exceeded availability of Fe(II), to precipitate pyrite. Outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion. Because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore-water evolution. However, all represent an expression in ancient rocks of microbiological processes similar to those operating today.

Oxygen isotopic composition of carbonate concretions from the lower Cretaceous of Victoria, Australia: implications for the evolution of meteoric waters on the Australian continent in a paleopolar environment

Oxygen isotopic data from carbonate cements in concretions have been used to infer the isotopic composition of meteoric fluids present at the time of concretion growth in terrestrial sediments that were deposited within the early Cretaceous South Polar Circle at 75–80°S. Carbon and oxygen isotope compositions have been determined on over 135 samples of carbonate from 45 concretions taken from 24 localities (Aptian-Albian in age) in the terrestrial sedimentary basins associated with the Otway and Strzelecki groups, southeastern Australia. The carbonate cements include calcite having−26.4 ⩽ δ 13C ⩽ 19.6and3.6 ⩽ δ 18O ⩽ 29.6 or siderite having17.6 ⩽ δ 18O ⩽ 30.8. Calcite-cemented concretions are more abundant and are interpreted to represent early near-surface cementation events on the basis of textural evidence such as high ( > 30%) porosities at the time of cementation and mineralogical evidence such as the preferential preservation within concretions of labile detrital grains including plagioclase, pyroxene, and amphibole. The oxygen isotopic data indicate that meteoric fluids with very lowδ 18O, certainly less than −15‰ and probably on the order of −20‰, were involved in the precipitation of the early calcites. The extremely lowδ 18O values of the fluids involved in the early diagenesis of both the Otway and Strzelecki groups suggest that the catchment area of the river system that carried sediments to these basins had a cold high-latitude climate (with mean annual temperatures less than 5°C and quite possibly below freezing). By analogy with the relationship between modern 18O distribution of meteoric fluids and climate, these new data suggest that the early Cretaceous polar regions may not have been ice-free.