Ancient Carbonate Research Articles

Triple Oxygen Isotopes in Silica–Water and Carbonate–Water Systems

Research Article| January 01, 2021 Triple Oxygen Isotopes in Silica–Water and Carbonate–Water Systems Jordan A.G. Wostbrock*; Jordan A.G. Wostbrock* Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA * E-mail: jogibbons7@unm.edu Search for other works by this author on: GSW Google Scholar Zachary D. Sharp Zachary D. Sharp Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA * E-mail: zsharp@unm.edu Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2021) 86 (1): 367–400. https://doi.org/10.2138/rmg.2021.86.11 Article history first online: 04 Jan 2021 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Jordan A.G. Wostbrock*, Zachary D. Sharp; Triple Oxygen Isotopes in Silica–Water and Carbonate–Water Systems. Reviews in Mineralogy and Geochemistry 2021;; 86 (1): 367–400. doi: https://doi.org/10.2138/rmg.2021.86.11 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search The field of stable isotope geochemistry began with the recognition that the oxygen isotope composition of ancient carbonates could be used as a paleothermometer (Urey 1947; Urey et al. 1951). As stated by Urey (1947), “Accurate determinations of the Ol8 content of carbonate rocks could be used to determine the temperature at which they were formed”. This concept was based on the temperature dependence for the oxygen isotope fractionation between calcite and water. Urey realized that if a mass spectrometer with sufficient precision could be built, a method of reproducibly extracting oxygen from... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic characterization of carbonate platforms and reservoirs: an introduction and review

Abstract Improved seismic data quality in the last 10–15 years, innovative use of seismic attribute combinations, extraction of geomorphological data and new quantitative techniques have significantly enhanced understanding of ancient carbonate platforms and processes. 3D data have become a fundamental toolkit for mapping carbonate depositional and diagenetic facies, and associated flow units and barriers, giving a unique perspective on how their relationships changed through time in response to tectonic, oceanographic and climatic forcing. Sophisticated predictions of lithology and porosity are being made from seismic data in reservoirs with good borehole log and core calibration for detailed integration with structural, palaeoenvironmental and sequence stratigraphic interpretations. Geologists can now characterize entire carbonate platform systems and their large-scale evolution in time and space, including systems with few outcrop analogues such as the Lower Cretaceous Central Atlantic ‘pre-salt’ carbonates. The papers introduced in this review illustrate opportunities, workflows and potential pitfalls of modern carbonate seismic interpretation. They demonstrate advances in knowledge of carbonate systems achieved when geologists and geophysicists collaborate and innovate to maximize the value of seismic data from acquisition, through processing to interpretation. Future trends and developments, including machine learning and the significance of the energy transition, are briefly discussed.

Seismic expression of carbonate build-up karstification: karst modelling strategies and insights from synthetic seismic

Abstract Many ancient carbonate build-ups are impacted by meteoric diagenetic processes, including karstification. However, very little is known about the acoustic properties and seismic expression of karstified reservoirs. This paper investigates potential seismic expressions of karstified build-ups by means of synthetic seismic modelling. The overall stratigraphic architecture and physical properties of rock are derived from published examples of SE Asian carbonate build-ups. Various karst systems, corresponding to distinct stages of karstification of varying intensity, have been superimposed to the background facies-related acoustic model. Three-dimensional synthetic seismic cubes have been computed from the karst-bearing acoustic model and by testing various wavelet frequencies. The proposed approach allowed several types of palaeokarst to be reproduced, from dendritic karst and flank-margin caves to cave networks, in amplitude sections. However, geometrical seismic attributes, like coherency, cannot be accurately reproduced with modelling based on the available literature. In addition, the exploration of this synthetic seismic data shows that non-stratiform diagenetic bodies (i.e. leached carbonate wedges in a mixing zone) appear as stratiform even in very-high-resolution seismic. Thus, their detection and characterization require advanced techniques or prior knowledge, such as borehole data.

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

Application of mapping and dating techniques in the study of ancient carbonate reservoirs: A case study of Sinian Qigebrak Formation in northwestern Tarim Basin, NW China

Ancient marine carbonates experienced complex modifications, making it difficult to identify reservoir genesis and effective porosity before hydrocarbon migration. To solve these issues, we used element mapping and carbonate mineral laser U-Pb radiometric dating techniques to study the diagenetic environments based on geochemistry and diagenesis-porosity evolution based on geochronology of the dolomite reservoir of the Sinian Qigebrak Formation, northwest Tarim Basin. Two major understandings were obtained as follows: (1) Supported by petrographic observations, the element mapping, stable isotopes, strontium isotope, and cathodoluminescence tests were performed on different phases of dolomite cements precipitated in vugs and dissolved fissures. The results show that the dolomite reservoirs of the Qigebrak Formation went through freshwater, marine, extremely shallow burial, burial and hydrothermal diagenetic environments after synsedimentary dolomitization; the reservoir spaces were mainly formed in the synsedimentary period (primary pores) and freshwater environment (supergene dissolution pores) before burial; whereas the marine, burial and hydrothermal environments caused the gradual filling of reservoir space by dolomite cements. (2) Based on the above understandings, each phase of dolomite cement precipitated in the reservoir space was dated by the U-Pb radiometric dating technique, and the diagenesis-porosity evolution curves constrained by geochronology were established. The loss of reservoir porosity mainly occurred in the early Caledonian, and during the peak period of hydrocarbon generation of Yuertusi Formation source rock, the reservoirs still maintained at a porosity of 6%–10%. The above understandings provide a certain basis for the evaluation of accumulation effectiveness of the Sinian Qigebrak Formation, northwestern Tarim Basin, and provide a case for the application of mapping and dating techniques in the study of ancient carbonate reservoirs.

Calibration of carbonate-water triple oxygen isotope fractionation: Seeing through diagenesis in ancient carbonates

High precision triple oxygen isotope measurements of carbonates can better constrain temperatures and oxygen isotope compositions of seawater through geologic time than 18O/16O measurements alone, but lack of a definitive calibration has hindered progress. In this study, we fluorinated both carbonate and water samples to measure quantitatively the triple oxygen isotope composition of each phase. We compared the oxygen isotope fractionation between carbonate and water for different carbonate materials: calcite synthesized with and without carbonic anhydrase, abiogenic calcite from Devils Hole, and extant biogenic calcite and aragonite of marine origin. We found similar 1000lnα18Occ-wt values for all materials and combined the results with the high temperature experimental data of O'Neil et al. (1969), resulting in the following fractionation equation (T in Kelvins) 1000lnα18Occ-wt=2.84(±0.02)×106T2-2.96(±0.19). The calcite triple oxygen isotope values yielded a θ-T relationship of θcc-wt = –1.39(±0.01)/T + 0.5305 whereas the aragonite triple oxygen isotope values yielded a θ-T relationship of θara-wt = –1.53(±0.02)/T + 0.5305. The calcite-water triple oxygen isotope equilibrium fractionation equation for natural samples is Δ17′Occ-Δ17′Owt=2.84(±0.02)×106T2-2.96(±0.19)-1.39(±0.01)T+0.5305-λ. The combined 1000lnα18O and 1000lnα17O relationships can be used to assess equilibrium in ancient samples and to evaluate potential secular changes in the δ18O value of seawater. Most of the Phanerozoic samples analyzed in this study, which were determined to be pristine in previous studies, have undergone some level of diagenesis. Two samples appear to preserve their original oxygen isotope compositions and suggest a cool ocean with a δ18O value similar to the modern ocean. Using a fluid-rock interaction model, we can “see through” the diagenetic process and estimate the triple oxygen isotope composition of the carbonate prior to alteration. In doing so, we show that for the time intervals and sample locations measured in this study, Phanerozoic oceans had a comparable range of oxygen isotope compositions and temperatures as modern seawater.

REE Characteristics of Lower Cretaceous Limestone Succession in Gümüşhane, NE Turkey: Implications for Ocean Paleoredox Conditions and Diagenetic Alteration

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca98.35–99.34Mg0.66–1.65(CO3). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/YbN and ave. 0.73 of Pr/YbN), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81; ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02; ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65; ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74; ave. 1.68) and La/Yb ratios (0.68–1.35; ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00; ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history.

LA-ICP-MS U-Pb geochronology and clumped isotope constraints on the formation and evolution of an ancient dolomite reservoir: The Middle Permian of northwest Sichuan Basin (SW China)

Recent advances in laser-ablation inductively coupled-plasma mass spectrometry (LA-ICP-MS) in-situ U-Pb radiometric dating and clumped isotope thermometry (Δ47) of carbonate minerals provide potential for refining the fluid flow and diagenetic history of carbonate successions. In this study, the Middle Permian dolomites proximal to the Longmenshan fold and thrust belt in northwest Sichuan Basin, southwest China, were investigated using combined U-Pb geochronology, clumped isotope thermometry, and routine isotopic (δ13C, δ18O and 87Sr/86Sr) and elemental geochemistry, in an attempt to reveal the possible relevance of carbonate diagenesis and porosity evolution to the basin-scale tectonic/fluid flow events in the framework of absolute time. Overall, formation and evolution of these dolomites temporally correlated well with major tectonic episodes of the Longmenshan fold and thrust belt. Contrary to the previously-assumed volcanic-related model, U-Pb dating and Δ47 analyses suggest a mid- to late-Triassic replacive dolomitization event (U-Pb ages of 240 ± 12 Ma to 233.8 ± 6.4 Ma) by a hot (Δ47 temperatures 88– 104 °C) basinal brine, which was likely driven by thrust-related compression of the Longmenshan fold and thrust belt during the Triassic. Replacive dolomitization was immediately followed by cementation of euhedral dolomites (U-Pb age of 228 ± 10 Ma) and blocky calcites (U-Pb ages of 224.8 ± 1.8 Ma to 213.4 ± 3.3 Ma) precipitated from the basinal brine in the same tectonic regime. Afterwards, there was long-term cessation of diagenesis that was coincident with, and thus may well be attributed to, tectono-thermal quiescence during most of the Mesozoic Era and Paleogene. Finally, saddle dolomite cements yield Miocene ages (U-Pb ages of 16.40 ± 0.74 Ma to 12.3 ± 1.2 Ma) and precipitated in pre-existing vugs, representing a hydrothermal fluid flow event (Δ47 temperatures up to 170 °C) during the reactivation of thrusting. In addition, similarities in U-Pb ages and geochemical signatures between host rocks (replacive dolomites) and the vug-lining dolomite cements suggest that most porosity in these dolomite reservoirs was inherited from the precursor limestones, which probably experienced meteoric leaching during late Permian uplift. This study highlights that there are geodynamic controls on fluid flow and consequent diagenetic evolution of carbonates in tectonically active sedimentary basins. It illustrates that burial alone in this setting is not a sufficient driver for diagenetic alteration and porosity reduction. The combination of U-Pb radiometric dating, clumped isotope thermometry and routine geochemical analyses is a useful approach in refining the diagenetic and porosity evolution history of ancient carbonate successions.

Implications for controls on Upper Cambrian microbial build-ups across multiple-scales, Mason County, Central Texas, USA

The morphological architecture and distribution of modern and ancient carbonate systems has been shown to follow spatial-self-organization, however, limited studies describe the morphometrics of microbial carbonates. Upper Cambrian microbial-build-ups outcropping in Central Texas, are exposed laterally (plan view), enabling a study of their morphological architecture and spatial distribution. Drone imagery was acquired to capture the outcrop features and develop a digital terrain model (cm scale resolution) for a bedding plane outcrop (600 × 200 m in size). Four scales of microbial growth (S1- few dm, S2- few m, S3- few tens of m, and S4- few hundreds of m) were identified and mapped. A series of morphometric analysis including Ripley's k, univariate, multivariate, and grouping were conducted and results demonstrate that, the scales S1, S2, and S3 display clustering and the spatial organization of microbial-buildups is naturally organized and not random. Further, as the size of the build-ups increases (from S1–S4), the anisotropy (length/width) increases, their shape becomes oblong, and they become aligned (S2–S4) with the inferred regional winds and tide-associated currents (NE-SW according to the present geography). The S1 scale does not align itself with the regional currents; instead, the build-ups behaved as a baffle during growth, and modified the currents locally, leading to preferential alignment at the edges within S2. As the scales increases in sizes (S2, S3, S4), there is competition for space, and due to regional currents, the larger scales preferentially align parallel to high-energy currents. The trends and spatial relationships identified in this study are particularly relevant and provide a scenario for sub-seismic scale heterogeneities for subsurface microbial hydrocarbon reservoirs.

U–Pb dating of calcite in ancient carbonates for age estimates of syn- to post-depositional processes: a case study from the upper Ediacaran strata of Finnmark, Arctic Norway

AbstractResults of in situ U–Pb dating of calcite spherulites, cone-in-cone (CIC) calcite and calcite fibres from a calcareous concretion of the upper Ediacaran of Finnmark, Arctic Norway, are reported. Calcite spherulites from the innermost layers of the concretion yielded a lower intercept age of 563 ± 70 Ma, which, although imprecise, is within uncertainty of the age of sedimentation based on fossil assemblages. Non-deformed CIC calcite from the bottom part of the concretion yielded an age of 475 ± 25 Ma, which is interpreted as the age of CIC calcite formation during a period of fluid overpressure induced during burial of the sediments. Deformed CIC calcite from the top part of the concretion yielded an age of 418 ± 23 Ma, which overlaps with a known Caledonian tectono-metamorphic event, and indicates a potential post-depositional overprint at this time. Calcite fibres that grew in small fissures along spherulite rims, which are interpreted as a recrystallization feature during deformation and formation of a cleavage, gave an imprecise age of 486 ± 161 Ma. Our results show that U–Pb dating of calcite can provide age constraints for ancient carbonates and syn- to post-depositional processes that operated during burial and metamorphic overprinting.

Carbonates and cherts as archives of seawater chemistry and habitability on a carbonate platform 3.35 Ga ago: Insights from Sm/Nd dating and trace element analysis from the Strelley Pool Formation, Western Australia

Carbonates and cherts in the 3.35 billion-year-old Strelley Pool Formation (Fm.; Australia) host stromatolites that are among the oldest remnants of life on Earth. However, it is still not entirely clear whether these mineral phases are authigenic precipitates, and whether they represent reliable geochemical archives of early Earth environments. Here we present major/trace-element and Nd-isotope data of stromatolitic carbonates, associated crystal-fan carbonates, and cherts in the Strelley Pool Fm. (i) to assess the reliability of these chemical sediments as geochemical archives of the fluids from which they precipitated, (ii) to date the time of formation of carbonate and silica phases, and (iii) to trace the sources of elements prevailing in microbial habitats 3.35 Ga ago.Stromatolitic carbonates plot together with the stratigraphically underlying Marble Bar cherts on a Sm-Nd regression line yielding 3253 ± 320 Ma. In contrast, associated crystal-fan carbonates together with altered Marble Bar cherts yield 2718 ± 220 Ma, suggesting that their Sm-Nd isotope system was reset after deposition. Both types of carbonates, as well as white cherts, show shale-normalized rare earth element and yttrium patterns (REYSN; with the exception of redox-sensitive Ce and Eu and heavy REYSN to middle REYSN depletion) that are parallel to those of modern seawater, indicating a predominantly seawater-derived origin. Positive EuSN anomalies (2.1–2.4), combined with heterogeneous ɛNd3.35Ga values between −3.2 and +5.8 within individual alternating stromatolite laminae, further support that the dissolved fraction of seawater on the ancient carbonate platform was variably affected by both continental weathering and high-temperature hydrothermal fluids contributing elements of both young mafic or older felsic rocks. In conclusion, trace element and Nd isotope data presented here match well with the depositional environment, as characterized based on lithological, geochemical, and stratigraphic relationships, on an early continent, showing at least episodic emergence above the sea level, supporting microbial life on a shallow marine platform.

Facies model on the modern isolated carbonate platform in the Xisha Archipelago, South China Sea

The Xuande Atoll, with a discontinuous reef rim caused by platform drowning, is a typical ellipse isolated platform on the northern slope of the South China Sea. Based on the data of drilling holes, the analysis of surface sediments and high resolution acoustic data, six sedimentary facies are identified in the Xuande Atoll, including reef outer slope, reef front, reef flat, aeolianite sand island, back-reef slope and lagoon. Facies map shows that the elevated reef-rimmed margins with islands are more noticeable on the windward of the platform. With an elevated and discontinuous reef-rimmed margin as well as coarse-grained sediment in the interior platform, Xuande can serve as a case of a leaky bucket atoll which is characterized by initially drowning scenarios. The facies architecture of the Xuande Atoll is evidently influenced by complex and interacting parameters including antecedent topography, sea level, wind-driven currents, water depth and tectonic subsidence. Different controlling factors exist in the drowning of the windward and leeward sides of the platform margin. The study of modern depositional settings in the Xuande Atoll will provide insight on predictions for ancient carbonate provinces.

Calcium isotopes in deep time: Potential and limitations

Calcium is an essential element in the biogeochemical cycles that regulate the long-term climate state of Earth. The removal of CO2 from the ocean-atmosphere system is controlled by the burial of carbonate sediments (CaCO3), ultimately linking the global calcium and carbon cycles. This fundamental link has driven the development of the stable calcium isotope proxy with application to both ancient skeletal and non-skeletal bulk carbonate sediments. Calcium isotope ratios (δ44/40Ca) have been used to track long-term changes in seawater chemistry (e.g., aragonite vs. calcite seas) and to elucidate short-term climatic perturbations associated with mass extinction events. However, developments in the calcium isotope proxy have shown that δ44/40Ca values in carbonate minerals also are sensitive to changes in precipitation rates, mineralogy and diagenesis, thereby complicating the application of the proxy to the reconstruction of global cycles. First, inorganic carbonate precipitation experiments have demonstrated that carbonate δ44/40Ca values are sensitive to precipitation rates with higher rates generally leading to larger fractionation. Second, δ44/40Ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average ~ 1.5‰ and ~ 0.9‰ depleted relative to contemporaneous seawater, respectively. The effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals, but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment δ44/40Ca values. Third, changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate δ44/40Ca values that are decoupled from changes in global cycles. Nevertheless, when these factors are appropriately considered the application of calcium isotopes in ancient carbonate sediments becomes a powerful tool for understanding biogeochemical processes that operate over many scales; from diagenetic changes within the sediment pore-space, to regional changes across ancient carbonate platforms, and to global changes in seawater chemistry through time. Importantly, the processes that contribute to variability in carbonate δ44/40Ca values are likely to impact other carbonate-bound proxies, highlighting the potential for calcium isotopes as a tool to better understand the variability of other isotope systems.

Experimental stabilization of carbonate sediments to calcite: Insights into the depositional and diagenetic controls on calcite microcrystal texture

Aragonite and high-magnesium calcite are abundant constituents of modern shallow marine carbonate sediments, but are rare in ancient carbonate rocks. Because these minerals are metastable, they tend to stabilize to microcrystalline calcite which is ubiquitous in most Phanerozoic marine and lacustrine limestones. Calcite microcrystals exhibit a range of textures that have been classified in terms of crystal size, morphology, and contact geometry. These textures have the potential to reveal information about the precursor sediments as well as the diagenetic conditions of stabilization, and thus may serve as a proxy for paleoenvironmental conditions. However, despite their ubiquity and importance, the origin of these textures is controversial. Hypotheses regarding the origin of calcite microcrystal textures typically involve inferences regarding the precursor depositional sediments, diagenetic conditions of stabilization, or diagenetic processes. Here, hundreds of laboratory experiments were conducted in which metastable carbonate sediments were stabilized to calcite. The goal of these experiments was to investigate the depositional and diagenetic controls on calcite microcrystal textures. The depositional variables investigated were precursor mineralogy, type, and size, and the diagenetic variables were temperature, fluid chemical composition, and fluid:solid ratio. Our results show that experimental stabilization of carbonate sediments produces calcite with a wide variety of textures depending on the various depositional and diagenetic parameters. These textures are remarkably similar to those commonly observed in the rock record. Calcite microcrystal morphology is controlled by fluid chemistry, fluid:solid ratio, and reactant type. Calcite microcrystal size is controlled by temperature, fluid chemistry, fluid:solid ratio, and reactant size. The degree to which calcite microcrystals are fitted together is controlled by reactant size and reactant microstructure. Our findings challenge most of the existing hypotheses regarding the origin of calcite microcrystal textures. We specifically show that textures that have been interpreted to indicate late diagenetic processes such as dissolution and cementation, may be the result of the early diagenetic processes of stabilization.

Application of laser in-situ U-Pb dating to reconstruct the reservoir porosity evolution in the Cambrian Xiaoerbulake Formation, Tarim Basin 激光铀铅同位素定年技术在塔里木盆地肖尔布拉克组储层孔隙演化研究中的应用

The Laser Ablation (LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, co-developed by the CNPC Key Laboratory of Carbonate Reservoir and School of Earth and Environmental Sciences, The University of Queensland, has huge potential to overcome the difficulty in sampling and dating ultra-low concentration U-Pb minerals in ancient marine carbonates that are relatively smaller in diameter in diagenetic fabric, which was untenable by the conventional isotope dilution (ID) methods. The in-situ analysis by LA combined with MC-ICP-MS has been applied to the study on the diagenesis and porosity evolution of the Cambrian Xiaoerbulake reservoir, Tarim Basin.By dating of dolomitic and calcitic cements from filled vugs, matrix pores and fractures in different stages, we suggested that the burial diagenetic history of dolomite reservoir in the Cambrian Xiaoerblake Formation mainly features the filling of primary pores and vugs generated under karstification. The filling of vugs mainly occurred during the early Caledonian, while the filling of matrix pores mainly occurred in the middle Caledonian and Indosinian. Fractures activated during the Caledonian and Indosinian served as the channels for hydrothermal migration, providing the materials to cement the vugs and pores; while the residual vugs, pores and fractures with no filling of dolomite, calcite and siliceous cements were the main reservoir space. The diagenesis and porosity evolution history of dolomite reservoir in the Cambrian Xiaoerbulake Formation, Tarim Basin, was hence established. The understandings mentioned above are highly consistent with the tectonic-burial history and basin thermal history in the study area, which may be effective tools to date the ancient marine carbonate reservoir cements, to reconstruct diagenesis and porosity evolution history, and to evaluate effective pores before hydrocarbon migration in the ancient marine carbonates.

Geological mapping reveals the role of Early Jurassic rift architecture in the dispersal of calciturbidites: New insights from the Central and Northern Apennines

AbstractThe role of sea‐bottom topography in the dispersal of shallow water‐derived calciturbidites across a submarine rift, as determined by the local extensional architecture, is under‐investigated, namely with pelagic settings along ancient passive continental margins. A comparison with modern carbonate platform/basin analogues, or with siliciclastic systems, is not always feasible, as ancient carbonate systems were commonly home to anachronistic environments (e.g. the Western Tethyan Mesozoic). Our study focuses on: (a) a reconstruction of the palaeotectonic architecture of the Umbria‐Marche Basin in the Jurassic and on (b) how this architecture produced a submarine topography which governed the dispersal of sediment, shed by the neighbouring Lazio‐Abruzzo Carbonate Platform, for >40 million years. A geological mapping project was performed in the Apennines of Central Italy, a region which in the Jurassic displayed a pattern of intrabasinal highs (pelagic carbonate platforms) and intervening basins which was exceedingly complex due to the high density, and oddly variable trends, of faults rooted in a shallow detachment layer corresponding to thick Triassic salt. A map pairing the occurrences of these resedimented beds with an updated palaeogeography becomes the natural descriptor of the itineraries followed by sediment gravity flows. This qualitative method represents a companion, or even alternative, approach to the one strictly based on physical stratigraphy, and it greatly improves our knowledge of regional geology and rift‐basin analysis. Our case history potentially represents the analogue of hydrocarbon fields both inland and in the offshore. Geological mapping shows that the marginal palaeoescarpments of pelagic carbonate platforms formed obstacles to the gravity flows as sediment load was discharged at their toes. While turbidity flows were locally vigorous enough to climb the escarpments, leaving overbank deposits on the pelagic carbonate platform‐tops, a ‘shelter’ effect is evidenced by the resediment‐free nature of those basins lying downflow, which were shielded by the highs.

Influence of paleo-Trade Winds on facies patterns of the Cambrian Shanganning Carbonate Platform, North China

The Shanganning Carbonate Platform (SCP) of the North China Craton (NCC) was located in the Trade Winds belt during the Cambrian, which influenced its facies development. In recent years, numerous drillcores have been recovered from these units, but the depositional environments and facies architecture of this large (~2.0 × 105 km2) carbonate platform system have not yet been reported in detail. In this study, we integrate sedimentological and magnetic fabric data collected from outcrops and drillcores in order to document variation in microfacies and facies associations within the SCP, to infer paleowind directions and constrain the paleogeographic location of the SCP, and to evaluate eolian controls on platform development. This analysis documented numerous carbonate microfacies belonging to six fabric types (mudstone, wackestone, packstone, grainstone, framestone, and dolostone) along with rarer siliciclastic sediments. These microfacies belong to facies associations representing tidal flat, open platform, platform margin, slope, and deep basinal depositional environments. Anisotropy of magnetic susceptibility (AMS) measurements yielded mean paleocurrent directions for the Early, Middle, and Late Cambrian of 114°, 63°, and 83° (± ~30°) from paleo-north, respectively, that were relatively consistent for 12 out of 13 platform-margin sites, all vectors being corrected for a post-Cambrian ~150° clockwise rotation of the NCC. These wind orientations are consistent with the SCP shifting from south of the paleo-Equator in the Early Cambrian to north of it by the Middle Cambrian. Paleo-Trade Winds influenced facies patterns across the platform, producing a distinct windward-leeward zonation. The windward margin is characterized by well-sorted oolitic grainstone, as well as (favositids) coral and (dasycladacean) algal-sponge framestone, and the leeward margin by poorly-sorted oolitic wackestone-grainstone and fine bioclastic sediments. The present study is innovative in 1) integrating microfacies and AMS analyses in order to establish paleowind patterns and influences on an ancient carbonate platform, 2) documenting windward-leeward variation in quantifiable sediment properties across the SCP, and 3) constraining the paleo-hemispheric location of the NCC during the Cambrian.

Archaeal lipid biomarker as a tool to constrain the origin of methane at ancient methane seeps: Insight into subsurface fluid flow in the geological past

The origin of methane discharged at ancient methane seeps is a key to understand subsurface fluid-flow processes in sedimentary basins. The carbon isotopic composition of methane-seep carbonates can be used to estimate the origin of methane, but mixing of multiple carbon sources results in ambiguous interpretation. We here constrained the origin of methane at ancient seeps by using the carbon isotopic composition and the isotope fractionation of lipid biomarkers of anaerobic methane-oxidizing archaea. We estimated the carbon-isotope fractionation between methane and biomarkers as the offset between their δ13C values at modern seep sites in the Japan Sea and other regions of the world. The biomarker pentamethylicosane (PMI) extracted from a modern seep carbonate of the Japan Sea shows a 44‰ offset from the seep methane. On the basis of literature data, we calculated the carbon-isotope offset as mostly ~20–60‰ for PMI at modern seeps across the world. We also assessed the relationship of the δ13C values between methane and biomarkers by regression analysis on the data taken at the modern seeps, obtaining regression formulas to estimate the δ13C values of methane from those of biomarkers. For ancient seeps, we examined Miocene to Pleistocene methane-seep carbonate rocks collected from Japan Sea sediments. PMI extracted from these ancient carbonates have δ13C values ranging from −137‰ to −93‰. Using the δ13C values of PMI and the carbon-isotope offset and regression formulas, we estimated that the methane at the ancient seeps was mainly of biogenic origin and migrated from the shallow subsurface.

Rare earth elements in the terminal Ediacaran Bambuí Group carbonate rocks (Brazil): evidence for high seawater alkalinity during rise of early animals

Rare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambuí Group, São Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive second-order sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce* values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambuí Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms.

Impacts of basin restriction on geochemistry and extinction patterns: A case from the Guadalupian Delaware Basin, USA

Geochemical data from carbonates often constrain the nature of environmental change during biotic turnover events. Many ancient carbonates, however, formed in geographically-isolated basins subject to local environmental factors, resulting in varying extinction rates between open ocean and restricted settings. It follows that high-resolution data from restricted basins may help unravel poorly-understood biotic crises such as the Mid-Capitanian extinction, which had especially high extinction rates in restricted settings.This study examines factors controlling salinity, stratification, and oxygenation in the Capitanian (Middle Permian) Delaware Basin, USA. Elemental and carbon isotope measurements from time-equivalent strata reveal differences between shallow- and deep-water masses, pointing to local controls such as stratification and de-oxygenated bottom water. Basinal dolomites and evaporites mark periods of elevated salinity tied to sea-level lowstands, which correspond with turnovers in fusulinid and brachiopod communities. Faunal turnover in the Delaware Basin demonstrates a fundamental attribute of restricted basins: water chemistry is often tightly coupled to physical process such as sea level change. We suggest that the relationships among sea level fluctuations, chemical changes, and biotic turnover may explain why the Capitanian mass extinction was more severe in isolated basins than the open ocean.