Diagenetic Recrystallization Research Articles

New evidence from the Paleoproterozoic Gunflint Iron Formation for microbially-driven, early diagenetic precipitation of siderite

Petrographic, geochemical, C-, O-, and clumped isotope measurements of drill core samples from the Paleoproterozoic (1.88 Ga) Gunflint Iron Formation containing siderite, ankerite, Fe-silicate clays (dominantly greenalite) and silica, reveal variable but light carbonate δ13C values (-21.3 to -7.87‰, VPDB), with heavy and less variable carbonate δ18O values (20.6 to 24.9‰, VSMOW). These measurements are consistent with a model in which siderite is formed during early diagenesis as a by-product of the metabolism of dissimilatory iron reducing (DIR) microbes that consume Fe(III)-oxide and organic carbon delivered from the water column to the sediment-water interface. This model indicates that the oxygen isotope compositions of reactant Fe-oxide, organic matter and dissolved inorganic carbon were set in equilibrium with seawater at T = 5–35 °C and δ18O = -1.0 to -6.5‰, prior to being metabolized to form siderite. Clumped isotope measurements on Fe-bearing carbonates yield temperatures (T(Δ47)) between 40 °C-132 °C, which constrain δ18Ofluid to values between -6.22 to 7.32‰. These measurements record the onset temperature of DIR followed by low water-to-rock ratio diagenetic recrystallization at elevated temperature. Combined petrographic, chemical, and isotopic measurements reveal that the major phases delivered to the shallow seafloor were a disequilibrium assemblage of Fe-oxide, greenalite, silica, and organic matter that underwent microbially mediated modification to form an assemblage of siderite, greenalite, silica, and later diagenetic ankerite. We contend that observed differences between carbonate derived from Gunflint core and outcrop may be reconciled by removal of siderite during exposure and weathering, leaving outcrop enriched in late diagenetic ankerite.

Mineralogical and Geochemical Composition of Late Permian Coals from Dengfeng Coalfield, North China: Conversion of Clay Minerals in Coal during Coalification

Dengfeng Coalfield represents a significant coalfield in Henan Province, North China. It is therefore essential to gain an understanding of the mineralogy and geochemistry of the Dengfeng coal, both from a geochemical perspective and in terms of the wider environmental context. In this study, a total of 27 coal bench samples were collected from the No. II1 coal of the Dengfeng Coalfield. The mineral species and major elements were quantitatively analysed using the X-ray diffraction and X-ray fluorescence methods, respectively. The minerals in the Dengfeng coal are dominated by ammonian illite and kaolinite with average contents of 3.73% and 7.47%, respectively. These are followed by calcite (2.74% on average) and ankerite (0.49%). The mean value of the kaolinite Hinkley index, which is a quantitative measure of kaolinite crystallinity, is 1.26. This suggests that kaolinite formation is primarily driven by diagenetic recrystallisation. The ammonian illite exhibits an average d001 of 10.2995 Å, indicative of a prevalence of NH4+ interlayer cations, with K+ also present in notable quantities. The ratio of NH4⁺ to (NH4⁺ + K⁺) has an average value of 0.90, which is indicative of the predominance of NH4⁺. The mean value of the illite Kübler index, which is a quantitative measure of illite crystallinity, is 0.264. This suggests that the diagenetic conditions correspond to the rank of the Dengfeng coal. The kaolinite present in the Dengfeng coal is suggested to have been derived from terrigenous detritus and subsequently subjected to diagenetic recrystallisation, resulting in a relatively high Hinkley index. The ammonian illite in the Dengfeng coal was predominantly formed through the conversion of the precursor kaolinite, with the influence of seawater during peat accumulation favouring the conversion of kaolinite to ammonian illite.

Evaluation of Reservoir Properties of Sylhet Limestone of Jaintia Group, North-Eastern Sylhet, Bangladesh

The Sylhet Limestone in the Bengal Basin, formed in the Eocene Epoch and known for its fossil content, is significant in understanding the reservoir characteristics. Despite exposure in Jaflong and Takerghat of the Bengal Basin, little research has been conducted on reservoir characteristics. This study aimed to comprehensively examine the Sylhet Limestone Formation, encompassing its crystalline upper and fossiliferous lower sections. It utilized field investigations and laboratory analyses to address the gaps of sporadic or insufficient earlier studies. A thorough examination of thin sections from the Sylhet Limestone, exposed in the Dauki River area, provides insights into the textural and mineralogical attributes and the presence of skeletal fossils within the limestone. Based on the analysis of thin sections, the limestones are categorized as Rudestones and Packstones. The porosity observed in the exposed rocks ranges from 5% to 12%, with most pores associated with interconnected fractures and joints. However, thin-section studies also indicate evidence of diagenetic recrystallization and calcite cementation. Hence, closely spaced, interconnected joints and fractures filled with diagenetic calcite might deteriorate the reservoir quality. Notably, this limestone exhibits fossilized specimens such as Nummulite, Discocyclina, Alveolina, Assilina, and Ostracoda, among others. The combination of the fossil assemblage, limestone texture, and composition strongly suggests that this limestone formation was deposited in a shallow marine environment with minimal sediment input under a warm and humid climate. The petrographic analysis of the limestones concludes that the upper portion of the formation is fine-grained while the lower part is coarse-grained. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 119-137

Calcitized evaporites in the Mesoproterozoic Atar and El Meriti Groups, Mauritania, Africa

One of the most profound byproducts of the Great Oxidation Event (GOE) was the onset of oxidative weathering on land and the riverine delivery of sulfate to marine systems. Although the exact nature of marine oxygenation in the aftermath of the GOE remains uncertain, marine sulfate likely remained below 2 mM until the late Mesoproterozoic and may have remained <10 mM through the Neoproterozoic. Prior to the late Mesoproterozoic, precipitation of sulfate minerals were inhibited by low marine sulfate and possibly by elevated carbonate saturation. By ∼1.1 Ga marine sulfate concentrations were sufficient to sustain the earliest widespread evaporite deposition. Low sulfate concentrations, however, influenced both the deposition and diagenesis of evaporite deposits. Here we investigate unusual limestone deposits in the 1.1 Ga Atar Group, Mauritania, that contain both macroscopic and microscopic features that suggest a primary origin as gypsum. Elevated concentrations of carbonate-associated sulfate (CAS), sodium, and potassium are consistent with primary deposition as a sulfate evaporite and subsequent calcification under low water-rock ratios. Isotope and elemental compositions further indicate diagenetic recrystallization under marine to hypersaline marine conditions. Combined, these observations show the potential for recognizing key evaporite facies in the rock record and support a growing recognition of late Mesoproterozoic evaporite deposits.

Porosity characterization using medical computed tomography scans of two deeply buried paleokarst aquifers in Silurian carbonates, southern Ontario, Canada

Paleokarst and paleokarst aquifers are not as well-documented as shallow karst, and studies of pores and pore networks are rare. Paleokarst lacks the open conduits typical of karst due to compaction, infilling, and diagenetic recrystallization by deep burial, so groundwater movement is through matrix porosity. In this case study, porosity networks in two saline aquifers in carbonate paleokarst of the Silurian Guelph Formation and Salina A-1 Carbonate Unit in southern Ontario (Canada) have been studied in drill cores at microscopic to macroscopic scales, utilizing medical computed tomography (CT) scans, optical petrography, macroscopic core examination, and scanning electron microscopy. The CT scans provided nondestructive three-dimensional visualization and quantification of pore distribution, size and volume, pore connections, and estimates of total porosity, similar to gas porosimetry. In the A-1 Carbonate, 50% of the pore volume comprises layers of macropores associated with algal laminations, with good horizontal connections. In the Guelph Formation, vuggy macropores contribute most of the pore volume. They are connected through subvertical fractures and rubble-filled karst conduits and by abundant small pores and micropores with poor horizontal connections. The Guelph paleokarst represents a longer period of subaerial exposure as evidenced by its greater thickness and geographic extent, former karst conduits, and near-total destruction of primary sedimentary fabrics. The findings provide real-world examples for construction of laboratory scale models of paleokarst aquifers, demonstrate the value of a multi-scale approach to porosity studies, and showcase the value of medical CT scans with its unique ability to visualize pore connections within drill cores.

In‐Situ and Triple Oxygen Isotope Characterization of Seafloor Drilled Cherts: Marine Diagenesis and Its Bearing on Seawater Reconstructions

AbstractCherts are used to reconstruct the evolution of seawater δ18O and temperature over geological time. However, given the influence of marine diagenesis, reconstructing seawater from the isotope composition of cherts is not straightforward, resulting in ambiguity of interpretation. Here, we present a detailed isotope and petrographic investigation of deep‐sea drilled 135–40 Ma cherts with focus on the effects of marine diagenesis. We combined triple O‐isotope data with in‐situ δ′18O‐16OH/16O measurements using secondary ion mass spectrometry (SIMS). We also provide electron microprobe maps, traditional δ′18O measurements from petrographically diverse domains, and δD and H2O wt.% values. The bulk δ′18O values range between 29‰ and 38‰ in our collection, while SIMS δ′18O data reveal significant intra‐sample heterogeneities up to 6‰ related to distinct petrographic features (e.g., filled radiolarian tests) and to micrometer‐scale variations in silica forms. Further, the δ′18O—Δ′17O values of these seafloor‐drilled cherts plot near and under equilibrium curve. Both triple‐O and SIMS δ′18O results reflect diagenesis in presence of marine pore waters at temperatures higher than ambient seawater, which is especially appreciable in cherts deposited on young oceanic crust. Despite the relatively constant δ18O seawater values over last 135 Ma, the marine silica spanning between 0 and 135 Ma occupies a wide compositional space in the δ′18O—Δ′17O rather than an equilibrium curve. The δ′18O values of cherts from modern‐seafloor positively correlate with the oceanic crustal age at the time of deposition, hinting at the importance of the heat flux in the diagenetic recrystallization of marine silica.

Triple oxygen isotope systematics of diagenetic recrystallization of diatom opal-A to opal-CT to microquartz in deep sea sediments

The oxygen-18 isotopic composition (δ18O) of silica preserved in oceanic sediments is an important archive of Earth’s temperature and/or seawater δ18O from the Archean to present. Recent advances in high-precision measurements of both δ18O and δ17O values have been used to provide additional constraints on what the oxygen isotopic composition of chert reflects about past conditions. Here, we examine the effects on the triple oxygen isotopic composition of chert that occurs during transformation and recrystallization of biogenic opal-A to opal-CT to microquartz in deep sea sediments. We studied late Miocene to present samples from the Sea of Japan at ODP Site 795 and measured biogenic diatom opal-A, opal-CT, microquartz chert, and ‘altered’ opal-A samples—previously measured for δ18O values only—for both δ18O and δ17O values. We find that δ18O decreases and Δ′17O increases (where Δ′17O = δ17O – 0.528 × δ18O) with depth, coincident with the conversions from diatom opal-A to opal-CT to microquartz. Silica samples deviate from the trend expected for triple oxygen isotopic equilibrium with modern seawater. To explain these data, we developed a model that shows that local temperature gradients and pore fluid δ18O profiles in combination lower the measured opal-CT and microquartz δ18O values and raise the Δ′17O values relative to the initial opal-A, but deviate from triple oxygen isotopic equilibrium with seawater. We find that a steeper local temperature gradient and a larger influence of hydrothermal alteration of basalt at the base of the sediment column (which lowers pore fluid δ18O values) in the past explain both the measured δ18O and Δ′17O values of the opal-CT and microquartz.These data and our modeling show that the transformation of opal-A to microquartz in marine sediments at elevated temperatures and in the presence of lowered pore water δ18O values leads to an array that falls below the theoretical triple oxygen isotope line of equilibrium for SiO2 with modern seawater. Further, our model indicates that opal-CT and microquartz do form in triple oxygen isotopic equilibrium with pore fluids that are offset in their triple oxygen isotopic composition compared to seawater due to fluid-rock alteration of igneous rocks at the base of the sediment column. The diagenetic processes taking place in the Japan Sea do not explain a large portion of the existing Archean to present triple oxygen isotope chert data, which likely require either changes in the oxygen isotopic composition of the source water (i.e., ocean water) and/or alteration by meteoric fluids. Further, our data demonstrate that the triple oxygen isotopic composition of preserved chert need not represent surface conditions, but instead may reflect processes that occur in subsurface sediments at elevated temperatures and with modified pore fluid oxygen isotopic compositions.

Mineral paragenesis and stratigraphic setting of the Neoproterozoic volcano-sedimentary succession of Samran Group, Jabal Farasan Area, West central Arabian Shield, Saudi Arabia

Jabal Farasan is located in the west central part of the Arabian Shield. The present study aims to reveal the stratigraphic setting and mineral paragenesis of the exposed volcano-sedimentary succession of the Samran Group in Jabal Farasan area. The succession of Samran Group is composed of three successive facies: the lower interbedded andesitic tuffs and tuffaceous andesites and trachyte’s (F1), the middle-altered quartz diorite/andesite (F2) and, an upper interbedded andesitic tuff, tuffaceous andesites and trachytes (F3). The vertical variations in the petrographic lithotypes depend mainly on the position of the depositional sites relative to the volcanic centers (proximal and distal). Both the volcanic rocks and the associated units are subjected to syn-and post-depositional diagenetic/metamorphic processes. These processes include the formation of the Fe2+-silicates: (i.e., chlorite instead of the volcanic ashes and the formation of plagioclase minerals). Post diagenetic recrystallization of the tuffaceous mudstones and the formation of microcrystalline quartz. The diagenetic oxidation of the Fe2+-silicates led to the formation of volcaniclastic red beds in certain stratigraphic horizons.

The fauna of a Danian fossil Conservation Lagerstätten from the cold-water coral mound complex at Faxe, Denmark

Fossil cold-water coral mound ecosystems are rare in geological history. One of the best known examples are the Danian coral mounds of Faxe comprising a complex interplay between corals, bryozoans, and a diverse associated fauna. The Baunekule facies is a special type of weakly consolidated coral rudstone to floatstone found in the Faxe Formation. The facies has a rich fauna of 220 species that are extremely well-preserved due to an early diagenetic recrystallization of aragonite to calcite prior to dissolution and is regarded as a fossil conservation Lagerstätten. The fauna lived between dead and living coral fragments on the flank of a growing Dendrophyllia coral mound. The fauna is authocthonous and dominated by small gastropods, polychaete tubeworms, bivalves and brachiopods. The fauna from Baunekule facies are compared to other parts of the Faxe Formation which have experienced a significantly different diagenesis. More than 57% of the fauna are not described from others parts of the Faxe Fm. More than 80% of the studied species from the coral limestone are unknown from other parts of Danian localities worldwide. The study of the Baunekule facies compares well with data from modern living coral mounds of Lophelia with a low species richness of framebuilding corals and a high species richness of the associated highly specialised fauna that show a dominance of attached suspension feeders, mobile carnivores, and grazers. The small gastropods are apparently lacking on the modern mounds, probably related to the sampling methods in the deep ocean. In general, there is agreements between the ecology of the fossil and the modern cold-water coral ecosystems with evidence for similar environments from Danian to modern time. The fossil cold-water coral mounds probably acted as diversity hotspots in the outer shelf – slope settings in the Danian epicontinental sea. • Palaeoecological studies of 21,000 specimens from rare fossil cold-water coral mounds • Identifying a rich fauna of 220 well-preserved small species (mainly gastropods) • Preservation is due to an early diagenetic recrystallization of aragonite to calcite. • The ecology of the fossil and the modern cold-water coral ecosystems are comparable. • The fossil cold-water coral mounds acted as diversity hotspots on the Danian shelves.

Controls on metal enrichment in ferromanganese crusts: Temporal changes in oceanic metal flux or phosphatisation?

Oceanic hydrogenetic ferromanganese (Fe-Mn) crusts are a major repository for many metals, such as Co, Ni, Cu, Pt, Te and REE, which are essential for decarbonisation of transport and energy systems. Secondary mineralisation processes, occurring during phosphatisation episodes, commonly impregnate the shallower deposits with carbonate fluorapatite (CFA). The suboxic oceanic conditions during such events are frequently invoked to explain the lower Co content and unusually high Ni, Cu, Zn and Pt content of older phosphatised crusts. Here, the hypothesis of suboxic diagenetic recrystallization induced by phosphatisation episodes as a driving mechanism for Ni, Cu, Zn and Pt enrichment and Co depletion is evaluated. Accurately dated geochemical profiles, spanning 75 Ma of depositional history, for a shallow (1100 mbsl) phosphatised sample and a deeper (3100 mbsl) unphosphatised sample from Tropic Seamount in the north-east Atlantic, are compared. An isocon analysis, which allows to quantitively evaluate chemical gains and losses in mass transfer and therefore permits compensation for the dilution effect induced by the addition of CFA in the Fe-Mn crusts, demonstrates that no loss of Co has occurred in the phosphatised crust, whilst Pt, Te, Cu, Ni and Zn are enriched relative to younger, unphosphatised Fe-Mn crust. Both geochemical profiles show sympathetic trends and similar amplitudes of variation in concentration. This excludes phosphatisation as the driving mechanism for the metal enrichment and depletion. Systematic differences in metal content between the two samples, such as higher Cu and lower Co content in the deeper sample, are consistent with the depth profile of dissolved metal concentrations in the water column. The variability observed in the geochemical profiles is consistent with temporal changes in metal fluxes to the ocean, as a result of the evolving climate and oceanographic configuration of the north-east Atlantic Ocean through the Cenozoic. It is concluded that changing metal fluxes, rather than secondary mineralisation process associated with phosphatisation, is the dominant control on the primary metal content in Fe-Mn crust deposits at Tropic Seamount.

Chromium isotope heterogeneity on a modern carbonate platform

Chromium isotope (δ53Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in δ53Cr values within a carbonate platform and slope system and assess the preservation potential of marine δ53Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of δ53Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new δ53Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy δ53Cr values from +0.77 to +1.07‰ while drill core samples record a larger range of δ53Cr values from +0.51 to +0.97‰. Down-core variability in δ53Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive δ53Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of δ53Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the δ53Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of δ53Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.

Climate variation during the Holocene influenced the skeletal properties of Chamelea gallina shells in the North Adriatic Sea (Italy).

Understanding how marine taxa will respond to near-future climate changes is one of the main challenges for management of coastal ecosystem services. Ecological studies that investigate relationships between the environment and shell properties of commercially important marine species are commonly restricted to latitudinal gradients or small-scale laboratory experiments. This paper aimed to explore the variations in shell features and growth of the edible bivalve Chamelea gallina from the Holocene sedimentary succession to present-day thanatocoenosis of the Po Plain-Adriatic Sea system (Italy). Comparing the Holocene sub-fossil record to modern thanatocoenoses allowed obtaining an insight of shell variations dynamics on a millennial temporal scale. Five shoreface-related assemblages rich in C. gallina were considered: two from the Middle Holocene, when regional sea surface temperatures were higher than today, representing a possible analogue for the near-future global warming, one from the Late Holocene and two from the present-day. We investigated shell biometry and skeletal properties in relation to the valve length of C. gallina. Juveniles were found to be more porous than adults in all horizons. This suggested that C. gallina promoted an accelerated shell accretion with a higher porosity and lower density at the expense of mechanically fragile shells. A positive correlation between sea surface temperature and both micro-density and bulk density were found, with modern specimens being less dense, likely due to lower aragonite saturation state at lower temperature, which could ultimately increase the energetic costs of shell formation. Since no variation was observed in shell CaCO3 polymorphism (100% aragonite) or in compositional parameters among the analyzed horizons, the observed dynamics in skeletal parameters are likely not driven by a diagenetic recrystallization of the shell mineral phase. This study contributes to understand the response of C. gallina to climate-driven environmental shifts and offers insights for assessing anthropogenic impacts on this economic relevant species.

Archaeological Ceramic Diagenesis: Clay Mineral Recrystallization in Sherds from a Late Byzantine Kiln, Israel

The pseudo-amorphous clay components of some of the pottery sherds that formed a surface in the firing chamber of a Late Byzantine kiln were shown by Fourier Transform Infrared Spectroscopy to have undergone almost complete recrystallization. Powder X-ray diffraction showed that the crystalline montmorillonite component of these sherds increased and kaolinite formed de novo. As this recrystallization process only occurred in the center of the firing chamber, we infer that the recrystallization process was due to repeated exposure of the sherds to high temperatures. The zeolite gonnardite was identified by X-ray diffraction. The chemical compositions of sodium-rich minerals, determined by energy dispersive X-ray spectroscopy (EDS), are consistent with the presence of gonnardite and analcime, and showed that the sodium was partially substituted by calcium and other cations. As these zeolites were also present in sherds from the upper pottery chamber, they did not form only as a result of repeated exposure to high temperatures. The demonstration that the clay mineral component of ceramics can undergo diagenetic recrystallization supports the possibility that provenience studies based on elemental analyses, especially of cooking pots that are repeatedly exposed to high temperatures, may be affected by recrystallization.

Geochemical controls on the elemental composition of siderite: Implications for palaeo-environmental reconstructions

The elemental composition of siderite (FeCO3) has been widely used as a palaeo-environmental proxy, and commonly utilised in the diagnosis of marine influences on Phanerozoic coastal wetland sediments. That siderite might reflect marine or freshwater sources is based on the premise that a marine origin should show an enrichment of Ca and/or Mg, while high concentrations of Mn should reflect precipitation from freshwater. However, the main controls on the elemental composition of siderite, and the degree to which siderite composition reflects parent water chemistry, are poorly understood. To address these issues, we conducted siderite nucleation and seeded growth experiments to examine element partitioning at a variety of saturation states and solution compositions (i.e., cation concentrations, pH, and pCO2). Results indicate a strong preference for Mn uptake over Ca and Mg, even when Mn is present at concentrations far below those of Ca or Mg. This uptake is enhanced at slower growth rates and lower saturation states, which may, in part, reflect surface structural controls on uptake and or a change in the dominant growth mechanism. Ca uptake only occurred during nucleation experiments, while measurable Mg uptake was not observed in experiments conducted at 20 °C, but increased slightly at 55 °C. These results suggest that the chemical composition of siderite does not directly reflect aqueous cation chemistry as previously assumed and is strongly influenced by kinetics rather than equilibrium behaviour. This leads us to conclude that variations in pore water chemistry alone cannot explain the range of Ca and Mg concentrations reported from geological siderites, and that other factors must be considered (such as physical admixtures of multiple carbonate minerals and/or the products of diagenetic recrystallization).

Nd and Sr Isotope Composition in the Tooth Enamel from Fe–Mn Nodules of the Cape Basin (Atlantic Ocean): Age and Sources

Abstract—The Nd and Sr isotope compositions were determined for the first time in biogenic apatite sampled throughout the tooth section (from the base to tip) of predatory fish in the nucleus of Fe–Mn nodules from the Cape Basin. The results showed that diagenetic recrystallization of apatite does not affect the 87Sr/86Sr ratio in the tooth enamel, but leads to the decrease of Sr content. The age of tooth was determined using Sr isotope stratigraphy at 5.2 ± 0.2 Ma for sample 2188/4 and 6.6 ± 0.3 Ma for sample 2188/5. The calculated growth rate of Fe and Mn oxyhydroxide layers varies within 0.4–2.8 mm per 1 Ma. The 143Nd/144Nd ratio in the tooth enamel varies within single station and depends on the local Nd sources in pore water. The value of eNd varies from –5.2 to –6.9 in the enamel of tooth 2188/4 and remains constant at –8.7 ± 0.1 in sample 2188/5. A change of Nd isotope composition in sample 2188/4 likely reflects temporal variations of Nd fraction from bottom and pore waters that penetrated inside the enamel during REE diffusion. The value of eNd in the oxyhydroxide layers of Fe–Mn nodule 2188/4 (from –7.8 to –7.9) is homogenous for the external and internal parts of the tooth. In order to use eNd in apatite enamel and authigenic Fe and Mn oxyhydroxides in sediments for paleoreconstructions of thermohaline water circulation, it is necessary to develop additional criteria for selecting diagenetically unaltered matter.

Formation mechanism of carbonates in the lacustrine muddy shale and it implication for shale oil and gas: A case study of source rocks in Member 4 and Member 3 of Shahejie Formation, Dongying sag

Through analysis of microscopic characteristics, mineral components, elements and isotopes, the genetic mechanism of carbonates in the deep lacustrine source rocks in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation in Dongying sag, is well investigated. The results show that four types of carbonates in the deep lake, i.e., lenticular coarse crystalline carbonate, lamellar micro-fine crystalline carbonate, lamellar cryptocrystalline carbonate and massive cryptocrystalline carbonate. Of which, the lenticular coarse crystalline carbonate is formed by diagenetic recrystallization. For the lamellar micro-fine crystalline carbonate and the lamellar cryptocrystalline carbonate, through the alga photosynthesis, the carbon dioxide (CO2) is constantly extracted from water, thus the concentration of CO3−2 ion in water increases, and then the CO3−2 ion reacts with Ca2+ ion in lake water surface to form the carbonates; the saline water environment is favorable for preservation of carbonate particles which mostly occur in lamellar micro-fine crystalline; in the brackish water environment, the water is deep, and the carbonate crystalline beneath the carbonate compensation depth surface is usually is dissolved, and most of lamellar cryptocrystalline are preserved. The massive cryptocrystalline carbonate is formed by the sedimentary carbonate which transport from shallow water to deep water by gravity flow. To some extent, the carbonates control reservoir property and compressibility of muddy shale in the upper submember of Member 4 and lower submember of Member 3 of Shahejie Formation in Dongying sag, and provide important information for reconstruction of sedimentary environment of the ancient lake.

Stratigraphy and deformation of Pleistocene talus in relation to a normal fault zone (central Apennines, Italy)

In studies of neotectonism, alluvial-fan and talus deposits commonly are used as deformation markers, but rarely are studied themselves and in relation to adjacent faults. Herein we report on the facies, diagenesis and deformation of a talus succession and fault cataclasites in the central Apennines of Italy. The study site is located near the western end of the Assergi normal fault zone that accommodates >2000 m of vertical throw, but was dormant since a longer interval of time.The preserved talus succession is confined to the fault hangingwall. Deposition and deformation of the talus overlapped with the terminal phase of fault activity. The talus accumulated mainly from grain flows, cohesive debris flows and ephemeral fluid flows; it comprises (i) two superposed units of scree breccias, partly cemented before deformation, and intercalated with (ii) an interval of unlithified scree and soils. The exposed succession accumulated between ≥33–30 cal ka BP to less than ~22 cal ka BP. Talus breccias record complex diagenetic successions including eluviation/dissolution of primary matrix, growth of interstitial cements interrupted by episodes of dissolution and/or fracturation, and late-stage dissolution porosity development. Downthrow by faulting produced two types of folds, (i) a syncline-anticline pair with inclined fold axes roughly normal to the fault plane, and (ii) a recumbent fold with an axis subparallel to depositional strike of the scree slope; the complicated stratigraphic architecture of the deformed succession is truncated along and hidden beneath the topsoil of the present, uniform slope surface. Further deformation structures include conjugate fractures locally coated by speleothem flowstones, deformation bands, sediment fabrics characterized by planar clast contacts, and lithoclasts crushed in situ while embedded in the sediment. Cementation overlapped with and post-dated in-situ crushing of clasts.The core of the normal fault is an ultracataclasite that (i) shows different degrees of diagenetic recrystallization into micro- to pseudospar, and (ii) that is riddled with solution pores fringed or filled with successive ‘generations’ of calcite cement. These cements are locally sharply capped along discrete levels, and overlain by ultracataclasite with floating chunks of calcite cement crystals ripped off during increments of faulting. Stable isotopes of oxygen and carbon indicate that the ultracataclasites lithified under influence of meteoric waters, but at variable degrees of rock buffering; the calcite cements within the talus breccias and the flowstones along fractures, in turn, precipitated from meteoric-derived waters with low to negligible rock buffering. The structural juxtaposition of ultracataclasites (probably formed in 1–2 km depth) with talus breccias indicates that the preserved fault/talus ensemble records only the terminal phase of total faulting. The interstitial cements in talus, the flowstones on fracture walls, and the cements in ultracataclasites should be datable with the 234U/230Th errorchron method. This promises to be a new approach to derive age constraints on talus cementation and fracturation, and on palaeoactivity and final dormance of normal faults.

Chemical taphonomy and preservation modes of Jurassic spinicaudatans from Patagonia: a chemometric approach

AbstractSpinicaudatans (‘clam shrimps’) are small branchiopod crustaceans enclosed in a chitinous bivalved carapace that is often the only preserved element in the fossil record. However, few studies have analyzed the preservation of these carapaces, which have been found in continental facies from the Devonian to the present. The aim of this study was to contribute to a better understanding of the chemical preservation of fossil spinicaudatan carapaces, and it focused on spinicaudatan carapaces of the Cañadón Asfalto Formation from the Jurassic of Argentina. Semiquantitative energy-dispersive X-ray spectrometry (EDS) analysis provided elemental composition data that were interpreted using principal component analysis (PCA). The results showed a complex chemical mode of preservation for spinicaudatan carapaces. In some parts, EDS spectra of the specimens exhibit peaks of calcium, phosphorous, aluminum, and fluorine, representing the retention of original carapace material with some diagenetic recrystallization. Certain zones of the carapace show low-intensity peaks of the elements mentioned, while silicon and oxygen peaks (from the rock matrix) become the dominant spectral signals. These modes of preservation modify the interpretations and observations of the ornamentation of the carapace, which are used as taxonomic features. Our results suggest that specific diagenetic processes play a fundamental role in the preservation of spinicaudatans.

The effects of diagenesis on geochemical paleoredox proxies in sedimentary carbonates

Metal and metal isotope records in carbonates have the potential to provide novel insights into ancient ocean–atmosphere redox conditions, paleoenvironmental conditions, and biogeochemical cycling. However, trace element geochemical signatures in carbonates can record either diagenetic or depositional signatures. Here we explore the variability in uranium isotopes, trace metal and rare earth element + yttrium (REY) concentrations in carbonate successions that have undergone several common types of diagenetic alteration. Case studies include the Cryogenian Balcanoona Reef, Australia (marine dolomitization, neomorphism); the Devonian Canning Basin reefs, Australia (burial dolomitization, karstification); the Paleozoic of the Great Basin, USA (high temperature and regional burial dolomitization) and the Carboniferous Waulsortian Limestone, Ireland (Pb-Zn mineralization). In all of the examined cases there are significant heavier and/or lighter shifts in U isotope values between the most petrographically pristine marine depositional components and altered or late-stage diagenetic phases. Although we also found that REY patterns can be overprinted during diagenesis, normalized REY profiles appear to be commonly retained through diagenetic recrystallization, consistent with previous studies. The direction and magnitude of the change in metal isotope systems during diagenesis cannot be generalized between case studies, consistent with carbonate alteration being controlled by a wide range of factors including the composition and source of the alteration fluids. In this light, we build on framework developed from traditional isotope systems and support the view that future work on metal and metal isotope compositions of carbonates should include integrated sedimentological and petrographic analysis. Petrographic work not only represents a screening procedure for sedimentary geochemical work, but can also significantly enhance the paleo-environmental interpretation of metal isotope data.

A biofilm and organomineralisation model for the growth and limiting size of ooids

Ooids are typically spherical sediment grains characterised by concentric layers encapsulating a core. There is no universally accepted explanation for ooid genesis, though factors such as agitation, abiotic and/or microbial mineralisation and size limitation have been variously invoked. Here we examine the possible influence of microbial organomineralisation on the formation of some naturally occurring ooids. We develop a mathematical model for ooid growth, inspired by work on avascular brain tumours, that assumes mineralisation in a biofilm to form a central core which then nucleates the progressive growth of concentric laminations. The model predicts a limiting size with the sequential width variation of growth rings comparing favourably with those observed in experimentally grown ooids generated from biomicrospheres. In reality, this model pattern may be complicated during growth by syngenetic aggrading neomorphism of the unstable mineral phase, followed by diagenetic recrystallisation that further complicates the structure. Our model provides a potential key to understanding the genetic archive preserved in the internal structures of some ooids.