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.     

Microbial mats implicated in the generation of intrastratal shrinkage (‘synaeresis’) cracks

Intrastratal shrinkage (often termed ‘synaeresis’) cracks are commonly employed as diagnostic environmental indicators for ancient salinity‐stressed, transitional fluvial‐marine or marginal‐marine depositional environments. Despite their abundance and use in facies interpretations, the mechanism of synaeresis crack formation remains controversial, and widely accepted explanations for their formation have hitherto been lacking. Sedimentological, ichnological, petrographic and geochemical study of shallow marine mudstone beds from the Ordovician Beach Formation of Bell Island, Newfoundland, has revealed that crack development (cf. synaeresis cracks) on the upper surface of mudstone beds is correlated with specific organic, geochemical and sedimentological parameters. Contorted, sinuous, sand‐filled cracks are common at contacts between unbioturbated mudstone and overlying sandstone beds. Cracks are absent in highly bioturbated mudstone, and are considered to pre‐date firmground assemblages of trace fossils that include Planolites and Trichophycus. The tops of cracked mudstone beds contain up to 2·1 wt% total organic carbon, relative to underlying mudstone beds that contain around 0·5 wt% total organic carbon. High‐resolution carbon isotope analyses reveal low δ¹³C_org values (?27·6‰) on bed tops compared with sandy intervals lacking cracks (?24·4 to ?24·9‰). Cracked mudstone facies show evidence for microbial matgrounds, including microbially induced sedimentary structures on bedding planes and carbonaceous laminae and tubular carbonaceous microfossils in thin section. Non‐cracked mudstone lacks evidence for development of microbial mats. Microbial mat development is proposed as an important prerequisite for intrastratal shrinkage crack formation. Both microbial mats and intrastratal shrinkage cracks have broad palaeoenvironmental distributions in the Precambrian and early Phanerozoic. In later Phanerozoic strata, matgrounds are restricted to depositional environments that are inhospitable to burrowing and surface‐grazing macrofauna. Unless evidence of synaeresis (i.e. contraction of clay mineral lattices in response to salinity change) can be independently demonstrated, the general term ‘intrastratal shrinkage crack’ is proposed to describe sinuous and tapering cracks in mudstone beds.     

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.     

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.     

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures

Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea‐level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite‐rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre‐scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite‐filled fenestrae; (ii) dolomicrite with bacterium‐like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa‐like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm‐like remains occur in voids between the filaments. The dolomite consistently shows negative δ¹³C values (down to ?11·3‰) and very positive δ¹⁸O (mean value 7·9‰) that suggest formation as primary precipitate with a substantial contribution of organic CO₂. Very negative δ¹³C values (down to ?31·6‰) of early diagenetic calcite associated with the dolomite suggest contribution of CO₂ originating by anaerobic methane oxidation. The shale‐normalized rare earth element patterns of Sutera stromatolites show features similar to those in present‐day microbial mats with enrichment in light rare earth elements, and M‐type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation.     

Syntectonic hydrothermal calcite in a faulted carbonate platform margin (Albian of Jorrios, northern Spain)

ABSTRACT A calcite mass more than 1·5 km long and 20 m wide crops outs along the faulted margin of the Albian carbonate platform of Jorrios in northern Spain. The mass contains abundant dissolution cavities up to 7 m long and 1 m high, filled with cross‐stratified quartz sandstone and alternating sandstone–calcite laminae. Similar cavities are also present in a 50‐m‐wide zone of platform limestones adjacent to the calcite mass that are filled with limestone breccias and sandstone. The calcite mass has mean δ¹⁸O values of 19·6‰ (SMOW), whereas platform limestones have mean δ¹⁸O values of 24·4‰ (SMOW). Synsedimentary faulting of the carbonate margin and circulation of heated fault‐related waters resulted in replacement of a band of limestone by calcite. Soon after this replacement, dissolution by undersaturated fluids affected both the calcite mass and the adjacent limestones. Percolating marine quartz sand filled all dissolution cavities, sometimes alternating with precipitating calcite. The resulting cavities and fills, which recall products of meteoric diagenesis, are attributed to a hydrothermal origin based on their geometry, occurrence along the profile and synsedimentary tectonic relationships. The early faulting and diagenesis are related to local extensional tectonism in a large‐scale strike‐slip setting. Movements occurred during the early dispar/appenninica zone of the Late Albian.     

Origins and accumulation of organic matter in expanded Albian to Santonian black shale sequences on the Demerara Rise, South American margin

Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ¹³C_org values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C₃ land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ¹³C_org values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.     

Palaeoceanographic controls on geochemical characteristics of organic-rich Exshaw mudrocks: role of enhanced primary production

Organic-rich source rocks have generally been attributed to enhanced preservation of organic matter under anoxic bottom waters. Here, geochemical analysis of kerogen and whole rock samples of organic-rich (lithofacies B₁) and organic-lean (lithofacies B₂) laminated mudrocks of the Devonian–Carboniferous Exshaw Formation, Alberta, highlight the importance of primary production in governing the quantity and quality of organic matter. Lower Si/Al, K/Al, Ti/Al and quartz/clay ratios in lithofacies B₂, similar maceral types and the laminated fabric of the two lithofacies indicate that the quality and quantity of organic matter are not related to grain size, redox or organic matter source changes. High Total Organic Carbon (TOC) and Hydrogen Index (HI), low Oxidation Index (Ox.I. ratio of oxygen functional groups to aliphatic groups derived by FTIR), lighter δ¹⁵N_tot and heavier δ¹³C_org isotopes indicate that kerogen of lithofacies B₁ accumulated during periods of high organic-carbon production and delivery of relatively fresh, labile, well-preserved organic matter to the sea floor. In contrast, low TOC, HI, high Ox.I., heavier δ¹⁵N_tot and lighter δ¹³C_org isotopes indicate low primary productivity and delivery, high recycling and poor preservation of organic matter during accumulation of lithofacies B₂.     

Compound-specific carbon isotope study on the hydrocarbon biomarkers in lacustrine source rocks from Songliao Basin

Stable carbon isotopic composition of organic matter (δ¹³C_org) and compound-specific δ¹³C values of biomarkers from 15 lacustrine source rocks were analyzed to identify the original paleoenvironment and source organisms. The δ¹³C values of hopanes (δ¹³C_hop) ranged from −68.7‰ to −32‰ and exhibit strongly ¹³C-depleted values in the lower part of Member 1 of the Nenjiang Formation (K₂n¹, up to −68.7‰), suggesting an origin from predominantly methanotrophic bacteria. ¹³C-enriched δ¹³C_Ga values and significantly ¹³C-depleted δ¹³C_hop in K₂n¹, which coincide with water stratification and an intermittent anoxic photic zone, represents a shallow chemocline. The presence of an intermittent anoxic photic zone, which means that the anoxia expanded into the euphotic zone, is beneficial for OM preservation and results in high values of TOC and HI in this section. However, the absence of gammacerane and ¹³C-enrichment of δ¹³C_hop in Member 2 of Nenjiang Formation (K₂n²) reflect a deeper chemocline, corresponding to relatively oxidizing conditions and low values of TOC and HI. Moreover, the negative correlation of TOC vs δ¹³C_org and HI vs δ¹³C_org reflects the control of OM formation by sedimentary environments rather than productivity in the water column. Thus, the depth of the chemocline not only controls the abundance of OM but also affects the development of the microbial community, such as chemoautotrophic bacteria in the deep chemocline and chemoautotrophic and methanotrophic bacteria in the shallow chemocline. Moreover, δ¹³C_Ga and δ¹³C values for 4-methyl steranes are related to water salinity, with a higher salinity accompanied by ¹³C-enrichment in gammacerane and 4-methyl steranes.     

Isotope Geochemistry of Groundwater from Fractured Dolomite Aquifers in Central Slovenia

The hydrogeochemistry and isotope geochemistry of groundwater from 85 wells in fractured dolomite aquifers of Central Slovenia were investigated. This groundwater represents waters strongly influenced by chemical weathering of dolomite with an average of δ¹³C_CARB value of +2.2 ‰. The major groundwater geochemical composition is HCO₃ ^? > Ca²⁺ > Mg²⁺. Several differences in hydrogeochemical properties among the classes of dolomites were observed when they were divided based on their age and sedimentological properties, with a clear distinction of pure dolomites exhibiting high Mg²⁺/Ca²⁺ ratios and low Na⁺, K⁺ and Si values. Trace element and nutrient concentrations (SO₄ ^2?, NO₃ ^?) were low, implying that karstic and fractured dolomite aquifers are of good quality to be used as tap water. Groundwater was generally slightly oversaturated with respect to calcite and dolomite, and dissolved CO₂ was up to 46 times supersaturated relative to the atmosphere. The isotopic composition of oxygen (δ¹⁸O_H2O), hydrogen (δD_H2O) and tritium ranged from ?10.3 to ?8.4 ‰, from ?68.5 to ?52.7 ‰ and from 3.5 TU to 10.5 TU, respectively. δ¹⁸O and δD values fell between the GMWL (Global Meteoric Water Line) and the MMWL (Mediterranean Meteoric Water Line) and indicate recharge from precipitation with little evaporation. The tritium activity in groundwater suggests that groundwater is generally younger than 50 years. δ¹³C_DIC values ranged from ?14.6 to ?9.3 ‰ and indicated groundwater with a contribution of degraded organic matter/dissolved inorganic carbon in the aquifer. The mass balances for groundwater interacting with carbonate rocks suggested that carbonate dissolution contributes from 43.7 to 65.4 % and degradation of organic matter from 34.6 to 56.3 %.     

Formation of modern dolomite in hypersaline pans of the Western Cape,South Africa

Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km² in size. The δ¹⁸O_PDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ¹³C_PDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The ⁸⁷Sr/⁸⁶Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca²⁺) ions with relatively minor amounts of Ca²⁺ derived from the chemical weathering of bedrock.     

Paired δ13Ccarb and δ13Corg records of the Ordovician on the Yangtze platform,South China

During the Ordovician, huge biological revolutions and environmental changes happened in Earth’s history, including the Great Ordovician Biodiversification Event, global cooling and so on, but the cause of these events remains controversial. Herein, we conducted a paired carbon isotopic analysis of carbonate (δ¹³C_carb) and organic matter (δ¹³C_org) through the Ordovician in the Qiliao section on the Yangtze platform of South China. Our results showed that the δ¹³C_carb trend of the Qiliao section can be correlated with local and global curves. The δ¹³C_org trend seems is less clear than the δ¹³C_carb trend for stratigraphic correlations, but some δ¹³C_org positive excursions in the Middle and Upper Ordovician may be used for correlation studies. These carbon isotopic records may have global significance rather than local significance, revealing several fluctuations to the global carbon cycle during the Ordovician. Several known δ¹³C_carb and δ¹³C_org negative and positive excursions have been recognised in this study, including the early Floian Negative Inorganic Carbon (δ¹³C_carb) Excursion (EFNICE), as well as the early Floian Positive Organic Carbon (δ¹³C_carb) Excursion, the mid-Darriwilian Inorganic Carbon (δ¹³C_carb and δ¹³C_org) Excursion (MDICE), and the early Katian Guttenberg Inorganic Carbon (δ¹³C_carb and δ¹³C_org) Excursion (GICE). These positive excursions and a smooth decline trend of δ¹³C_org values during the early to mid-Floian may imply multiple episodes of enhanced organic carbon burial that began at the early Floian stage, probably resulting in further decline in atmospheric pCO₂ and then global cooling.     

Organic geochemical record of increased productivity in Lake Naukuchiyatal, Kumaun Himalayas, India

Organic geochemical proxies have been studied in a 45-cm-long core retrieved from Lake Naukuchiyatal in Kumaun Himalayas, India. Increase in TOC, N, hydrocarbons and pigments concentration from bottom to surface sediments of the core indicates increase in the lake productivity. Stable isotopes (δ¹³C and δ ¹⁵ N), biomarkers (TAR, CPI and n-ΣC_15,17,19) and C/N atomic (between 9 and 12) suggest dominance of algal derived organic matter in these sediments. Decrease in organic δ¹³C values (between ?27 and ?31‰) in surface sediments indicate influence of sewage and land runoff in shifting organic δ¹³C values, whereas low (between ?0.23 and 2.2‰) δ¹⁵N values together with high pigment concentrations (zeaxanthin and echinenone) represent dominance of cyanobacteria in the lake.     

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.     

Hirnantian (Late Ordovician) δ13C HICE excursion in a North Gondwanan (NE Spain) periglacial setting and its relationship to glacioeustatic fluctuations

In the Iberian Chains of northeastern Spain, the Hirnantian Orea Shale comprises three erosive glaciogenic unconformities punctuating two transgressive glaciomarine sequences. These sequences represent ice retreat episodes of grounded ice on a North Gondwanan region of 50–60°S of estimated palaeolatitude. In contrast, the same formation recorded in the Hesperian Chains repeated episodes of extensional tectonic activity: diamictites were associated with slope-related debris flows and slumps commonly interrupted by truncating discontinuities.An analysis of δ¹³C_org in the Orea Shale has revealed that the lower Orea sequence displays isotopically light baseline values (∼ −27‰) punctuated by minor (2–3‰) shifts. In distal parts, the upper sequence is characterised by a rapid rise in δ¹³C_org values, which mark a positive excursion, in the range of 2.5‰ to 7‰ over 40 cm of thickness. The stratigraphic gap involved in the intra-Orea erosive unconformity appears to be greater both in proximal exposures and slope-related (Hesperian) areas, where the chemostratigraphic shift was not recorded. Upsection in the three studied sections, the δ¹³C values display upsection a sharp return to baseline values of −26‰ to −28‰.The Hirnantian strata of the Iberian and Hesperian Chains have recorded three major correlatable events: (i) a the karstic surface that caps upper Katian limestones and reflects the maximum glacial extension; (ii) a single δ¹³C_org positive peak of ∼6‰ that is recognisable in distal ramp settings, marking the beginning of the second ice retreat episode and probably representing the late Hirnantian HICE shift; and (iii) the erosive unconformity marked by the progradation of the Los Puertos shoreline complexes.     

Evaluation of nitrate source in surface water of southwestern China based on stable isotopes

Stable isotope tracing and analysis play an important role in interpretation of hydrological and ecological processes at the watershed scale and can provide information regarding the flow path, water source, nutrient loss and biogeochemical cycles of a system. In this study, environmental isotopes (δ¹⁸O-H₂O, δD, δ¹⁵N-NO₃ ^?, δ¹⁸O-NO₃ ^?) and chemical compositions of surface water in Guizhou Province, China, were measured to evaluate the primary sources of nitrate and characterize the processes affecting nitrate as well as its correlation with vegetation cover in karstic areas. The δ¹⁵N and δ¹⁸O-NO₃ ^? levels ranged from +1.3 to +9.8 ‰ and +4.7 to +16.9 ‰, respectively, which indicated that nitrate in water from the investigated area primarily originated from nitrification of soil organic matter during the sampling period. There was also a wide range of isotopes in the water and high contents of nitrate in karstic areas with poor vegetation cover, indicating that water and nutrient loss were serious problems hindering plant growth in the study areas. For example, there was a positive relationship between isotopic composition and nitrate content in the natural forest and negative relationship in Libo County nearby, which suggested that the nitrate fate was affected by land use and human disturbance.     

Organic geochemistry and depositional environment of the Aptian bituminous limestone in the Kale Gümüşhane area (NE-Turkey): An example of lacustrine deposits on the platform carbonate sequence

The Jurassic–Lower Cretaceous aged carbonate sequence is widely exposed in the southern zone of Eastern Pontides. Aptian black bituminous limestone is found in the upper part of this sequence in the Kale area (Gümüşhane). This limestone contains faunal remains (e.g., gastropod, ostracod, characean stems and miliolid type benthic foraminifera) that indicate a freshwater, lacustrine depositional environment.The total organic carbon (TOC) values of the bituminous limestone samples range from 0.11–1.30% with an average TOC value of 0.54%. The hydrogen index (HI) varies from 119–448 mg HC/g TOC (average HI 298 mg HC/g TOC) indicating that the limestone contains gas prone as well as oil prone organic matter. Pyrolysis data prove that the organic matter content in the bituminous limestone consists of Type II kerogen. The average T_max value for bituminous limestone samples is 438 °C (434–448 °C). Bitumen/TOC ratios for bituminous limestone are 0.05 and 0.04. The T_max values and the ratios indicate that the bituminous limestone samples contain early mature to mature organic matter.Analysis of solvent extracts from the two richest bituminous limestones show a predominance of high carbon number (C₂₆–C₃₀) n-alkanes. The Pr/Ph ratio and CPI value are 1.34 and 0.96, respectively. C₂₉ is the dominant sterane, with C₂₉ > C₂₇ > C₂₈. The bituminous limestone samples have low C₂₂/C₂₁ ratios, high C₂₄/C₂₃ tricyclic terpane ratios and very low C₃₁R/C₃₀ hopane ratios (<0.25). These data are consistent with the bituminous limestones being deposited in a lacustrine environment.     

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.     

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.     

Source rock potential of selected Cretaceous shales,Orange Basin,South Africa

Twenty Cretaceous shale samples from two wells in the Orange Basin of South Africa were evaluated for their source rock potential. They were sampled from within a 1400 m-thick sequence in boreholes drilled through Lower to Upper Cretaceous sediments. The samples exhibit total organic carbon (TOC) content of 1.06–2.17%; Rock-Eval S2 values of 0.08–2.27 mg HC/g; and petroleum source potential (SP), which is the sum of S1 and S2, of 0.10–2.61 mg HC/g, all indicating the presence of poor to fair hydrocarbon generative potential. Hydrogen index (HI) values vary from 7 to 128 mg HC/g organic carbon and oxygen index (OI) ranges from 37 to 195 mg CO₂/g organic carbon, indicating predominantly Type III kerogen with perhaps minor amounts of Type IV kerogen. The maturity of the samples, as indicated by T _max values of 428–446°C, ranges from immature to thermally mature with respect to oil generation. Measured vitrinite reflectance values (%Ro) of representative samples indicate that these samples vary from immature to mature, consistent with the thermal alteration index (TAI) (spore colour) and fluorescence data for these samples. Organic petrographic analysis also shows that amorphous organic matter is dominant in these samples. Framboidal pyrite is abundant and may be indicative of a marine influence during deposition. Although our Rock-Eval pyrolysis data indicate that gas-prone source rocks are prevalent in this part of the Orange Basin, the geochemical characteristics of samples from an Aptian unit at 3318 m in one of the wells suggest that better quality source rocks may exist deeper, in more distal depositional parts of the basin.