Formation of siderite-Mg-calcite-iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England

Concretions cemented mainly by siderite, Mg-calcite and iron monosulphide are common in late Holocene marsh and sandflat sediments on parts of the north Norfolk coast. Field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate-reducing bacteria are active. δ¹³C values ranging from ?3 to ?11·8% (mean ?5·9%0) suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter. δ¹⁸O values ranged from ?6·4% to + 0·8% (mean ?1·0%) suggesting that carbonate precipitated in porewaters ranging from pure sea water to-sea water diluted with meteoric water. Chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation. Some concretions have formed around fragments of wood or metal, but others contain no apparent nucleus. In field experiments siderite, FeS and Mg-calcite were precipitated around several different nuclei within a period of six months. We suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction, such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron.     

Primary and diagenetic isotopic signals in fossils and hemipelagic carbonates: the Lower Jurassic of northern Spain

Stable isotope and trace element analyses of 230 Jurassic (Pliensbachian–Toarcian) samples from northern Spain have been performed to test the use of geochemical variations in fossils (belemnites and brachiopods) and whole‐rock hemipelagic carbonates as palaeoceanographic indicators. Although the succession analysed (Reinosa area, westernmost Basque–Cantabrian Basin) has been subject to severe thermal alteration during burial diagenesis, the samples appear to be well preserved. The degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic, chemical and cathodoluminescence analyses. It is demonstrated that brachiopods and whole‐rock carbonates, although widely used for palaeoceanic studies, do not retain their primary marine geochemical composition after burial diagenesis. In contrast, there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock. Well‐preserved belemnite shells (non‐luminescent to slightly luminescent) typically show stable isotope values of +4·3‰ to –0·7‰δ¹³C, +0·7‰ to –3·2‰δ¹⁸O, and trace element contents of <32 μg g^–1 Mn, <250 μg g^–1 Fe, >950 μg g^–1 Sr and Sr/Mn ratios >80. This study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low‐Mg calcite fossil shells and hemipelagic bulk carbonates, behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated. Multiple non‐luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders, yet their δ¹³C and δ¹⁸O values exhibited moderate scatter. Such variability is likely to be related to the palaeoecological behaviour of belemnites and/or high‐frequency secular variations in sea‐water chemistry superimposed on the long‐term isotopic trend. A pronounced positive carbon‐isotope excursion (up to +4·3‰) is documented in the early Toarcian serpentinus biozone, which correlates with the Toarcian δ¹³C maximum reported in other European and Tethyan regions.     

Carbon, oxygen, strontium, and sulfur isotopic compositions in late Precambrian rocks of the Patom Complex, central Siberia: Communication 2. Nature of carbonates with ultralow and ultrahigh δ13C values

The Patom Complex is characterized by a unique association of carbonate rocks with ultralow (≤8‰) and ultrahigh (>6‰) δ¹³C values. The thickness, stable isotopic composition along the strike, and lithological and geochemical parameters suggest that these rocks could not form as a result of short-term local events or epigenetic processes. Ultralow δ¹³C values (less than ?8‰) in carbonate rocks of the Zhuya Group, which substantially exceed all the known negative C isotope anomalies in thickness (up to 1000 m) and amplitude (δ¹³C = ?10 ± 2‰), point to sedimentation under conditions of extreme “contamination” of water column by oxidized isotopically light organic (hereafter, light) carbon. The decisive role in this contamination belonged to melting and oxidation of huge volumes of methane hydrates accumulated in sediments during the powerful and prolonged Early Vendian glacial epoch. The accumulation of δ¹³C-depleted carbonates was preceded by the deposition of carbonates with anomalously high δ¹³C values. These carbonates formed at high rates of the burial of organic matter and methane in sediments during periods when the sedimentation basin consumed carbon dioxide from the atmosphere and organic carbon was conserved in sediments.     

Factors influencing methane-derived authigenic carbonate formation at cold seep from southwestern Dongsha area in the northern South China Sea

This paper carried out mineral and geochemical studies on a profile through a diagenetic methane-derived authigenic carbonate sample that was collected from southwestern Dongsha area of the northern South China Sea. Five samples locating in the cross-sectional middle mainly consist of dolomite and quartz, and two samples close to the surface have a small amount of Mg-calcite. The δ¹³C values of the samples vary between ?30.59 and ?0.30 % VPDB, with δ¹⁸O values ranging from 3.07 to 3.59 % VPDB, δ^44/40Ca values ranging from 1.35 to 1.47 % SRM915a, indicating a contribution of methane to the carbon pool where the precipitation of authigenic carbonates occurred. Based on the isotope values alone, it can not be distinguished if the carbon source is thermogenic gas or a mixture of biogenic methane and marine dissolved inorganic carbon. The δ¹⁸O values are in general consistent with dolomite precipitation from a fluid similar to present seawater. The observed small variation might be related to the oxygen isotope composition of seep fluid. The relative small range in calcium isotope values suggests that relatively constant growth conditions and precipitation from seawater. The central part of the carbonate nodule formed under the strong influence of methane seepage, and the external part is less influenced by methane, either due to reduced methane flux to the surface or caused by erosional exhumation of the carbonate nodule from greater depth to the sediment surface.     

Carbon and oxygen stable isotopes in the middle-upper miocene and lower pliocene carbonates of the Eastern Paratethys (Kerch-Taman Region): Palaeoenvironments and post-sedimentation changes

C and O isotope composition of Middle-Upper Miocene and Lower Pliocene carbonates from Kerch-Taman Region (Eastern Paratethys) have been studied in order to reconstruct palaeoenvironmental variability and post-sedimentation changes. The δ¹³C and δ¹⁸О values of the Upper Sarmatian to Lower Pliocene organogenic carbonates reflect the desalinization of paleobasins, global Late Miocene Cooling, and increase in seasonal temperature fluctuations. Isotopic composition of the Middle Sarmatian organogenic carbonates was strongly influenced by evaporation processes, high bioproductivity, and local submarine methane emissions. Warm climate and low bioproductivity together with unstable hydrological regime during the Late Chokrakian and the Karaganian times influenced the isotope composition of primary carbonates. Calcite shell of Spiratella sp. (δ¹³C =–0.4‰ and δ¹⁸О =–0.4‰) from Tarkhanian sediments was formed in warm marine environment. Dolomitization prevails over other secondary mineralization in the studied carbonate rocks. Two groups of secondary dolomites that are characterized by negative and positive δ¹³C values have been recognized. Lowe δ¹³C values (up to–31.4‰) in dolomites indicate the influence of both dissolved inorganic carbon (DIC) from oxidized organic matter (С_org) and methane. Dolomites with positive δ¹³C values (7.0 and 7.8‰) associat with migration of CO₂- and CH₄-containing saline groundwater.     

Depositional environment of a Kimmeridgian carbonate 'black band' (Akkuyu Formation, south-western Turkey)

The Upper Jurassic Akkuyu Formation of south-western Turkey consists of pale-coloured carbonate facies, but includes a peculiar ‘black band’ at its base. In order to characterize the depositional environment of this level, a sedimentological, organic and inorganic geochemical study was performed on 30 samples. Three units are distinguished. Unit 1 consists of black marlstones, very rich in organic carbon (22–30%) with HI (hydrogen index) values around 600. The organic matter is an amorphous homogeneous material even at nanoscopic scale. This unit is also characterized by a very high concentration of V ( >1000 p.p.m.) and a δ¹³C_org value around ?26·5‰. Unit 2 shows thin-bedded black limestones with chert layers. Its organic content varies from 2% to 6%. As in unit 1, the HI values are uniformly high (600), and gas chromatography shows a high quantity of hopanes. The V concentration is also high (≈400 p.p.m.). The unit has a high Sr/Ca ratio, an organic carbon isotopic ratio close to ?28·7‰ and diffuse, nanoscopically amorphous organic material that is closely associated with pyrite. Unit 3 corresponds to the white limestones overlying the ‘black band’. This calcitic facies is almost devoid of organic carbon (<0·1%). The δ¹³C_org values are similar to those of unit 1 (?25·5‰ to ?26‰). Deposition occurred in a basinal environment surrounded by a carbonate platform. Primary productivity was high but not exceptional; terrestrial organic inputs were scarce. Bottom waters were probably euxinic during the deposition of unit 1. The presence of both hopane-rich and sulphur-rich nanoscopically amorphous organic matter may be the result of strong sulphate-reducing bacterial activity. Coupled with low amounts of iron, this led to intense vulcanization of the organic matter and thus good preservation. With the onset of unit 2 deposition, environmental conditions became less extreme, perhaps because of cooler water temperatures. Finally, the depositional conditions became oxic, leading to the accumulation of pale-coloured organic-poor limestones of unit 3.     

Methane-derived high-Mg calcite submarine cement in Holocene nodules from the Fraser Delta, British Columbia, Canada

Carbonate nodules and slabs in late Holocene shelly terrigenous deposits of the modern Fraser River delta (～49°N) are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high-Mg (～ 10–20 mol% MgCO₃) calcite cement as coalescing isopachous crusts on grains. Previous reports that the cement is low-Mg calcite are not supported by this study. Highly negative δ¹³C values of ? 7 to ? 59‰ for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial. In particular, the production of biogenic methane (CH₄) by anaerobic bacterial fermentation, its upward migration, chemical or biological oxidation to CO₂ and neutralization in the near-surface sediment, and diffusion to microenvironments relatively enriched in organic components, are a possible set of conditions influencing the process and sites of carbonate cementation. Methane-derived Mg-calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow-water terrigenous sands and muds at non-tropical latitudes.     

Dolomitization of the Waulsortian Limestone (Lower Carboniferous) in the Irish Midlands

The Waulsortian Limestone (Lower Carboniferous) of the southern Irish Midlands is dolomitized pervasively over a much larger region than previous studies have documented. This study indicates a complex, multistage, multiple fluid history for regional dolomitization. Partially and completely dolomitized sections of Waulsortian Limestones are characterized by finely crystalline (0·01–0·3 mm) planar dolomite. Planar replacive dolomite is commonly followed by coarse (≥0·5 mm) nonplanar replacive dolomite, and pervasive void‐filling saddle dolomite cement is frequently associated with Zn–Pb mineralization. Planar dolomite has average δ¹⁸O and δ¹³C values (‰ PDB) of –4·8 and 3·9 respectively. These are lower oxygen and slightly higher carbon isotope values than averages for marine limestones in the Waulsortian (δ¹⁸O=–2·2, δ¹³C=3·7). Mean C and O isotope values of planar replacive dolomite are also distinct from those of nonplanar and saddle dolomite cement (–7·0 and 3·3; –7·4 and 2·4 respectively). Fluid inclusions indicate a complex history involving at least three chemically and thermally distinct fluids during dolomite cementation. The petrography and geochemistry of planar dolomites are consistent with an early diagenetic origin, possibly in equilibrium with modified Carboniferous sea water. Where the Waulsortian was exposed to hydrothermal fluids (70–280 °C), planar dolomite underwent a neomorphic recrystallization to a coarser crystalline, planar and nonplanar dolomite characterized by lower δ¹⁸O values. Void‐filling dolomite cement is isotopically similar to nonplanar, replacive dolomite and reflects a similar origin from hydrothermal fluids. This history of multiple stages of dolomitization is significantly more complex than earlier models proposed for the Irish Midlands and provides a framework upon which to test competing models of regional vs. localized fluid flow.     

Formation of carbonate concretions in surface sediments of two mud mounds,offshore Costa Rica: a stable isotope study

The surface sediments of two mud mounds (“Mound 11” and “Mound 12”) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO₃). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded δ¹⁸O_carbonate values ranging between 34.0 and 37.7 ‰ Vienna standard mean ocean water (VSMOW) and δ¹³C_carbonate values from ?52.2 to ?14.2 ‰ Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The δ¹⁸O_carbonate values suggest mineral formation from seawater-derived pore fluid (δ¹⁸O_porefluid = 0 ‰ VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (δ¹⁸O_porefluid ≈5 ‰) in Mound 11. A positive correlation between δ¹³C_carbonate and δ¹⁸O_carbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (δ¹³C_porefluid ≈0 ‰) and (2) bicarbonate which formed during the AOM (δ¹³C_porefluid ≈?70 ‰). Furthermore, the δ¹⁸O_porefluid composition, with values up to +4.7 ‰ Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of ¹⁸O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (δ¹³C_CH4 = ?38 ‰). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).     

Traces of brine and hydrocarbon migration in carbon,oxygen, and sulfur isotopic compositions of reservoir rocks in the Talakan petroleum field,Southwestern Yakutia

Dolomites from the productive Osa horizon (upper subformation of the Lower Cambrian Bilir Formation) in the Talakan petroleum field show a prominent 1–2‰ decrease in δ¹⁸O (from 23–24 to 21–22‰), which presumably marks a zone of relatively high water/rock ratios. Productive boreholes are characterized by moderate δ³⁴S values (from 25.1 to 30.6‰) and negative correlation between δ³⁴S in anhydrite and δ¹⁸O in associated dolomite, which points to a partial sulfate reduction during catagenesis. In nonproductive borehole, δ³⁴S values increase significantly (from 31.4 to 35.6‰) and show positive correlation with δ¹⁸O in dolomite. Rocks recovered by nonproductive borehole possibly recrystallized during early diagenesis, and, correspondingly lost their permeability and capacity to form pores. Limestones and dolomites of the Osa horizon have a carbon isotopic composition within the range of normal marine carbonates (δ¹³C = 0 ± 1 ‰), which does not indicate a significant role of organic matter in postsedimentary recrystallization of carbonate sediments. A positive δ¹³C excursion up to 4.5‰ recorded in the lower subformation of the Bilir Formation presumably occurred at the sedimentation stage under conditions of high rates of bioproductivity and organic matter burial in sediments.     

Geochemical evidence of microbial activity within ooids

Ooid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget. Based on stable isotopes, nutrient and elemental analyses on solid components and ooidal leachates, this study supports the notion of microbial involvement in the development of ooids from Great Bahama Bank. Carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity. The δ¹³C values for organic carbon in the bulk (?11·9 to ?16·9‰); intercrystalline/intracrystalline (?11·9 to 16·7‰); and intracrystalline phases (?12·4 to ?17·7‰) were similar and, except for the more enriched values of ooids from Butterfly Beach, were within the range of photosynthesisers. The δ¹⁵N values for the bulk (+0·5 to ?0·2‰); intercrystalline/intracrystalline (?0·3‰ to ?0·7‰) and intracrystalline organic matter (?0·3 to ?1·7‰) showed a narrow range consistent with nitrogen fixation. While positive δ¹⁵N and δ¹⁸O values of the leached from the ooids provided evidence of denitrification, the carbonate associated sulphate δ³⁴S_CAS of the bulk sediments (+19·2 to +19·6‰) and δ³⁴S of the leachates (+16·6 to +18·3‰) provided weak indication of sulphate reduction, suggesting either that high concentrations of isotopically enriched S are overriding bio‐signatures of sulphate reduction or that microbes are preferentially using as an electron acceptor. In contrast, the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids. The high Mg/Ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in CaCO₃ precipitation. Together, these findings indicate that a redox dependent microbial consortium may influence CaCO₃ precipitation in the form of ooid accretion, cementation and micritization. It is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets.     

Subaerial exposure and deposition of shallowing upward sequences: evidence from stable isotopes of Purbeckian peritidal carbonates (basal Cretaceous), Swiss and French Jura Mountains

Purbeckian (lowermost Cretaceous) peritidal carbonates are characterized by open marine, lagoonal, intertidal and lacustrine facies arranged in Milankovitch-type shallowing upward sequences. Shallowing upward sequences typically consist of 2–6 individual beds. The sequences may be (i) complete, (ii) incomplete or (ii) pedogenetically overprinted, reflecting the duration of subaerial exposure and/or the extent of erosion and pedogenetic modification at the cycle tops. The stable isotopic composition of the peritidal micrites reveals homogenous δ¹⁸O values attributed to diagenetic stabilization in a meteoric, water-buffered system. Carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences. Complete shallowing upward sequences consist of 4–6 individual carbonate beds. The carbon isotope values show a facies-dependent pattern: open marine carbonate muds record enriched δ¹³C values of +0·28‰ while lagoonal (−0·82‰), intertidal (−2·46‰) and lacustrine micrites (−2·96‰) are increasingly depleted. This distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence. Incomplete sequences (2–5 carbonate beds) are characterized by depleted δ¹³C values below subaerial exposure surfaces that become progressively enriched in ¹³C with increasing depth. Pedogenetically overprinted sequences (1–3 carbonate beds) show strong ¹³C depletion throughout the sequence with little variation in the carbon isotopic composition. The depleted values (−4·5‰) of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas CO₂. The duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations, the completeness of the shallowing upward sequences and the carbon isotope pattern. The recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief. Otherwise, the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time-stratigraphic interpretation.     

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA)

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ¹³C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ¹³C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ¹³C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ³⁴S) and silica (δ¹⁸O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.     

Physical and chemical growth conditions of Ordovician organogenic deep-water dolomite concretions: implications for the δ18O of Early Palaeozoic sea water

The estimated depth of formation of authigenic dolomite concretions in the Middle Ordovician Cloridorme Formation, Quebec, ranges from < 1 m to 150–200 m below sea floor (mbsf) (mostly between < 1 and 25 mbsf), based on centre‐to‐margin variations in minus‐cement porosity (80–90% to 45–75%). Formation depths are > 350 mbsf (25–17% porosity) in the Lower Ordovician Levis Formation. Outward‐decreasing δ¹³C_VPDB values (10·2–0·8‰) suggest precipitation in the methane generation zone with an increasing contribution of light carbonate derived by advection from thermocatalytic reactions at depth. Anomalously low δ¹⁸O_VPDB values (centre‐to‐margin variations of ?0·4 to ?7·5‰) give reasonable temperatures for the concretion centres only if the δ¹⁸O of Ordovician sea water was negative (?6‰) and the bottom water was warm (> 15 °C). The 3–5‰ lower values for the concretion margins compared with the centres can be explained if, in addition, volcanic‐ash alteration, organic‐matter decomposition and/or advection of ¹⁸O‐depleted water lowered the δ¹⁸O of the pore water further by 2·0–4·0‰ during the first 25–200 m of burial. Reasonable growth temperatures for the margins of 17–20 °C are compatible with a lowering of the isotopic ratios by 1 to < 1·3‰ as a temperature effect. The systematic concentric isotope zonation of the concretions suggests that the well‐ordered near‐stoichiometric dolomite is a primary feature and not the result of recrystallization. Diagenetic dolomite beds of the Cloridorme Formation appear to have formed by coalescence of concretions, as shown by randomly sampled traverses that indicate formation at different subsurface depths. Growth of the Cloridorme dolomites was probably limited by calcium availability, at least 50% of which was derived from connate water, and the remainder by diffusion from sea water. Dolomite precipitation was favoured over calcite by very high sedimentation rates, the abundance of marine organic matter in the host sediment and a correspondingly thin sulphate reduction zone. Deep‐seated concretion growth in the Levis Formation required either internal sources for the participating ions (carbonate dissolution event) or porewater advection along faults.     

Petrography and geochemistry of early-stage, fine- and medium-crystalline dolomites in the Middle Devonian Presqu'ile Barrier at Pine Point, Canada

The petrography and geochemistry of fine- and medium-crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal fluids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back-barrier facies in the Elk Point Basin. Fine-crystalline dolomite has δ¹⁸Ο values between ?1·6 to –3·8‰ PDB and ⁸⁷Sr/⁸⁶Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine-crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor. Medium-crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has δ¹⁸O values between ?3·7 to ?9·4‰ PDB (mean ?5·9‰ PDB) and ⁸⁷Sr/⁸⁶Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.     

Early diagenetic recrystallization of Holocene (<3000 years old) peritidal dolomites, Ambergris Cay, Belize

Three peritidal carbonate crusts and associated intercrust sediments (total thickness of ～30cm; aged <3000 years BP) on Ambergris Cay, Belize, contain 32–100% calcian dolomite (δx=72·5% dolomite) ranging in composition from 40 to 46 mol% MgCO₃ (δx=43·3). Dolomite replaced high Mg calcite foraminiferal muds penecontemporaneously with sedimentation, forming partially dolomitized sediments and lithified crusts. Dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present. Detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth; 0·4 μm near the top of the section to 1·0 μm near the base of the dolomitized section. This size increase is not accompanied by any significant decrease in porosity. Crystal size distributions appear to be log-normal and become increasingly broad and flat with depth. Rietveld X-ray pattern-fitting structure refinements indicate increasing Ca and Mg concentrations on their respective sites (cation ordering) as a function of increasing depth. Most of the ordering occurs within the first 15 cm of the surface. Stoichiometry does not increase with depth indicating no relationship between the Ca/Mg ratio and cation ordering. Strong geochemical trends were observed down-section in the dolomite, including: (1) increasing Mn content (44 to 274 ppm), and (2) decreasing δ¹³C values (?0·9 to ?5·5‰ PDB). Oxygen isotope values range from δ¹⁸O = 1·3‰ PDB in the upper part of the section to 2·6‰ PDB in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend. Down-section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy-driven dissolution-reprecipitation process (Ostwald ripening). The observed trends in carbon isotopes and Mn content probably result from geochemical re-equilibration during recrystallization and reflect reducing conditions and an isotopically light, organically derived, carbon source. Oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level.     

Carbon isotopic change at the base of the upper permian zechstein sequence

Fluctuation of the carbon isotope composition of marine carbonates has recently been developed as a powerful tool for the identification of ocean-wide anoxic conditions and changes in the world budget of carbon and oxygen. A change in δ¹³ from the normal marine values (0 to + 2%) to values highly enriched in ¹³C (+3·5 to 4·5%) is recorded at the base of the Zechstein sequence both in central Germany and northeastern England. The change occurred over a relatively short period of time indicating a rapid and pronounced change in the organic carbon/carbonate budget. Evidence from other Permian basins show similar highly enriched δ¹³C values. This change may correspond to that in carbon balance distinguished by the Garrels and Perry (1975) model and based on the change in sulphur isotopic composition during the Permian.     

Early diagenetic siderite as an indicator of depositional environment in the Triassic Rewan Group, southern Bowen Basin, eastern Australia

Early concretionary and non-concretionary siderites are common in subsurface Triassic sandstones and mudrocks of the Rewan Group, southern Bowen Basin. A detailed petrological and stable isotopic study was carried out on these siderites in order to provide information on the depositional environment of the host rocks. The siderites are extremely pure, containing 85–97 mol% FeCO₃, and are commonly enriched in manganese. δ¹³C (PDB) values are highly variable, ranging from - 18·4 to +2·9‰, whereas δ¹⁸O (PDB) values are very consistent, ranging from - 14·0 to - 10·2‰ (mean= - 11·9 ± 1·0‰). The elemental and oxygen isotopic composition of the siderites indicates that only meteoric porewaters were involved in siderite formation, implying that host rocks accumulated in totally non-marine environments. The carbon isotopic composition of the siderites is interpreted to reflect mixing of bicarbonate/carbon dioxide generated by methane oxidation and methanogenesis. Very low δ¹³C values demonstrate that, contrary to current views, highly ¹³C-depleted siderite can be produced at shallow burial depths in anoxic non-marine sediments.     

Dolomitization of Quaternary reef limestone, Aitutaki, Cook Islands

Six holes were drilled to depths of 30–69 m in the shallow lagoon of Aitutaki in the southern Cook Islands. One hole encountered pervasively dolomitized reef limestones at 36 m subbottom depth, which extended to the base of the drilled section at 69·3 m. This hole was drilled near the inner edge of the present barrier reef flat on the flank of a seismically defined subsurface ridge. Both the morphology and biofacies indicate that this ridge may represent an outer reef crest. Mineralogy, porosity and cementation change in concert downhole through three zones. Zone 1, 0–9 m, is composed of primary skeletal aragonite and calcite with minor void-filling aragonite and magnesian calcite cement of marine phreatic origin. Zone 2, 9–36 m, is composed of replacement calcite and calcite cement infilling intergranular, intragranular, mouldic and vuggy porosity. Stable isotopes (mean δ¹⁸O=—5·4‰ PDB for carbonate; δD =—50‰ SMOW for fluid inclusions) support the petrographic evidence indicating that sparry calcite cements formed in predominantly freshwater. Carbon isotope values of —4·0 to —11·0‰ for calcite indicate that organic matter and seawater were the sources of carbon. Zone 3, 36–69·3 m, is composed of replacement dolostone, consisting of protodolomite with, on average, 7 mol% excess CaCO₃ and broad and weak ordering X-ray reflections at 2·41 and 2·54 A. The fine-scale replacement of skeletal grains and freshwater void-filling cements by dolomite did not significantly reduce porosity. Stable isotopes (mean δ¹⁸O=+2·6‰₀ PDB for dolomite; maximum δD =—27‰ SMOW for fluid inclusions) and chemical composition indicate that the dolomite probably formed from seawater, although formation in the lower part of a mixed freshwater-seawater zone, with up to 40% freshwater contribution, cannot be completely ruled out. The carbon (δ¹³C=2·7‰) and magnesium were derived from seawater. Low-temperature hydrothermal iron hydroxides and associated transition metals occur in void space in several narrow stratigraphic intervals in the limestone section that was replaced by dolomite. The entire section of dolomite is also enriched in these transition metals. The metals dispersed throughout the dolostone section were introduced at the time of dolomitization by a different and later episode of hydrothermal circulation than the one(s) that produced the localized deposits near the base of the section. The primary reef framework is considered to have been deposited during several highstands of sea level. Following partial to local recrystallization of the limestone, a single episode of dolomitization occurred. Both tidal and thermal pumping drove large quantities of seawater through the porous rocks and perhaps maintained a wide mixing zone. However, the isotopic, geochemical and petrographic data do not clearly indicate the extent of seawater mixing.     

Organic Carbon Isotope Geochemistry of the Neoproterozoic Doushantuo Formation, South China

The Neoproterozoic Doushantuo Formation on the Yangtze Platform, South China, documents a sedimentary succession with different sedimentary facies from carbonate platform to slope and to deep sea basin, and hosts one of the world-class phosphorite deposits. In these strata, exquisitely preserved fossils have been discovered： the Weng＇an biota. This study presents carbon isotope geochemistry which is associated paired carbonate and organic matter from the Weng＇an section of a carbonate platform （shelf of the Yangtze Platform, Guizhou Province） from the Songtao section and Nanming section of a transition belt （slope of the Yangtze Platform, Guizhou Province） and from the Yanwutan section （basin area of the Yangtze Platform, Hunan Province）. Environmental variations and bio-events on the Yangtze Platform during the Late Neoproterozoic and their causal relationship are discussed. Negative carbon isotope values for carbonate and organic carbon （mean δ^13Corg = -35.0‰） from the uppermost Nantuo Formation are followed by an overall increase in δ^13C up-section. Carbon isotope values vary between -9.9‰ and 3.6‰ for carbonate and between -35.6‰ and -21.5‰ for organic carbon, respectively. Heavier δ^13Ccarb values suggest an increase in organic carbon burial, possibly related to increasing productivity （such as the Weng＇an biota）. The δ^13C values of the sediments from the Doushantuo Formation decreased from the platform via the slope to basin, reflecting a reduced environment with minor dissolved inorganic carbon possibly due to a lower primary productivity. It is deduced that the classical upwelling process, the stratification structure and the hydrothermal eruption are principally important mechanisms to interpret the carbon isotopic compositions of the sediments from the Doushantuo Formation.