Diagenetic Overprint Research Articles

Carbon isotopic record of a platform-to-basin transect through the Permian Reef Complex (Guadalupian) in the Delaware Basin of Texas and New Mexico

The Guadalupian Permian Reef Complex of the Delaware Basin is one of the most studied carbonate reef systems of the Paleozoic. Despite extensive work on the carbonate sedimentology, sequence stratigraphy, and diagenetic history of the Delaware Basin, a high-resolution carbonate carbon isotope record along a platform to basin transect for the Capitanian (264.3–259.5 Ma, the youngest age of the Guadalupian Epoch) does not yet exist. The carbon isotopic record of the Delaware Basin is important because 1) it allows us to test hypotheses about controls on the carbon isotope proxy, 2) it provides constraints on how well modern carbonate platforms like the Great Bahama Banks serve as analogues for ancient carbonate settings, and 3) these types of restricted basins likely played an important role in Permian carbon cycling and the Capitanian extinctions.In this study we present 493 new Capitanian carbonate carbon isotopic values paired with a detailed sedimentological and sequence stratigraphic framework from the platform, slope, toe of slope, and deep basin of the Delaware Basin. The bulk of the new δ13C values fall within the range of previously reported unaltered carbonates from the basin, suggesting that these results record primary environmental processes and were not significantly altered by diagenetic overprinting. With this dataset, we test hypotheses about sources of carbon isotopic variability in shallow carbonate platforms. Our results indicate that in the Delaware Basin there are no systematic and resolvable depth or lateral gradients in carbon isotopic values, that δ13C values do not vary as a function of grain type, and that there is no resolvable relationship between carbon isotopic composition and sea level change. However, we do document statistically significant differences in δ13C distributions among facies associations which we attribute to the isotopic evolution of an upwelling water mass due to direct precipitation of mud along the slope. Our results support the idea that increasing carbon isotopic values through the Capitanian were driven by increased organic carbon burial in restricted basins.

Strontium Isotope Composition of Triassic Coastal and Shallow Marine Carbonates in the Western Balkanides, NW Bulgaria

Whole-rock samples of Triassic coastal and shallow marine carbonates were analyzed for their Sr isotope signatures and trace element concentrations were measured in order to assess the possible impact of diagenetic alteration. The 87Sr/86Sr ratios of Anisian to lower Carnian calcite micrites reproduce the global Triassic seawater variations fairly well. The recorded higher values can be explained by an influence from detrital aluminosilicate phases and a minor postdepositional alteration. The samples of tidal-flat dolomicrites from the base of the Anisian show no significant diagenetic overprint but have considerably more radiogenic 87Sr/86Sr signatures compared to the global Sr isotope curve that corresponds to the Triassic (Mesozoic) maximum. These anomalously high values may have resulted from some interaction between the dolomitizing brines and the underlying fluvial siliciclastics (Buntsandstein facies), but more likely from the high 87Sr/86Sr river waters discharged from a silicate-dominated drainage basin to the coastal area at the beginning of the Middle Triassic. This study demonstrates that bulk micrite can be used as a proxy for seawater 87Sr/86Sr after proper sample selection and diagenetic screening. It also suggests that the 87Sr/86Sr ratios of ancient carbonates precipitated in coastal settings should be interpreted with caution, considering the possible influence of continentally-derived fluids.

New diagenetic constraints for the interpretation of Neoarchaean stromatolitic dolostone geochemical composition and their palaeoenvironmental significance

Abstract The stromatolitic dolostones of the Ramonnedi Formation in the Lower Transvaal Supergroup of Botswana formed along a shallow carbonate platform during the transition between the Archaean and Proterozoic eons. These dolostones are bound to reveal more details of the mechanisms and timing of Earth oxygenation and particularly the role of microbial communities in the formation of carbonate platforms in the Archaean; but only if we are able to discriminate what signature is pristine and what is due to diagenetic overprinting. Here, the focus is on the isotopic composition of stromatolitic dolostone, pyrite grains and chert laminations found within the Ramonnedi Formation. Oxygen and carbon isotopes were investigated from the Ramonnedi Formation stromatolitic dolostones in order to constrain chemo-physical conditions at the time of deposition and, eventually, evaluate to what extent the system was re-set during diagenetic processes. In situ δ34S and δ18O compositions were determined by secondary ionisation mass spectrometry (SIMS) from pyrite and chert, respectively. These novel results complement previous analyses and reveal that: (i) the pyrite grains are not common in the dolostones and when present have sulphur isotope compositions that point toward abiotic formation; (ii) dolostone δ13CVPDB values between -1.2 and -0.7‰ are close to marine values and might reflect pristine Archaean signature; (iii) the dolostone δ18OVPDB values, between -9 and -11‰, on the other hand, might reflect diagenetic resetting or very warm Archaean seawater, in line with the δ18O values from the chert laminae in the upper Ramonnedi Formation. (iv) The correct interpretation of these results has important implications in the interpretation of the geochemical proxies coming from the lower Transvaal Supergroup carbonate platform and for the interpretation of the onset of the Great Oxidation Event (GOE). Aim of this work is to complement the existing data on the Lower Transvaal Supergroup of Botswana and assess to what extent the geochemistry of these dolostone was overprinted during diagenesis/metasomatism. These results indicate that a multi-analytical approach is required to ensure the correct interpretation of geochemical proxy data related to the onset of the GOE.

What is the source of magnesium in hydrothermal dolomites? New insights from coupling δ26Mg - ∆47 isotopes

The occurrence of fault-controlled hydrothermal dolomitization (HTD) is ubiquitous across stratigraphic records and has been extensively studied due to its association with economic resources. The origin of HTD is often evaluated by combining carbonate geochemistry, fluid inclusion thermometry and reactive transport modelling. However, multiple diagenetic overprinting events can obscure original geochemical signatures. Here, we demonstrate the applicability of two combined isotope systems, magnesium and carbonate clumped isotopes (δ26Mg - ∆47), to trace the source of fluid and magnesium in basin-scale HTD from the Western Canadian Sedimentary Basin (WCSB) and Southern China. Extensive studies in these regions, providing tectono-stratigraphic information and dolomitization models, furnish a robust scenario to evaluate the new isotopic data. Our findings reveal that while the previous geochemical data (δ18Ofluid and 87Sr/86Sr) are partly compatible with dolomitization from seawater, the elevated δ26Mg and δ18Ofluid values (up to -0.3‰ and +11‰, respectively) and the high temperatures (up to 320 °C) are not consistent with dolomitization from seawater alone. Considering this, the uniform mean δ26Mg values of silicate rocks (-0.25‰) and the occurrence of basement-rooted faults in the study areas, the hypothesis from prior work that dolomitization was initially driven by seawater that progressively mixed with crustal fluids sourced from underlying basement rocks, was supported. As the δ26Mg values may reflect the isotopic signature of the fluid and the leached host rocks, when combined with ∆47-derived temperatures, it results in an effective tracer of fluid and Mg sources during dolomitization in tectonically complex basins that are otherwise challenging to unravel. A comparison of our results paints a global picture of isotopic imprints in hydrothermal dolomite and further demonstrate the ability of Mg isotopes to differentiate between dolomitization from seawater versus evaporated brines and also through Mg-rich crustal fluids.

Variable Iron Mineralogy and Redox Conditions Recorded in Ancient Rocks Measured by In Situ Visible/Near‐Infrared Spectroscopy at Jezero Crater, Mars

AbstractUsing relative reflectance measurements from the Mastcam‐Z and SuperCam instruments on the Mars 2020 Perseverance rover, we assess the variability of Fe mineralogy in Noachian/Hesperian‐aged rocks at Jezero crater. The results reveal diverse Fe3+ and Fe2+ minerals. The igneous crater floor, where small amounts of Fe3+‐phyllosilicates and poorly crystalline Fe3+‐oxyhydroxides have been reported, is spectrally similar to most oxidized basalts observed at Gusev crater. At the base of the western Jezero sedimentary fan, new spectral type points to an Fe‐bearing mineral assemblage likely dominated by Fe2+. By contrast, most strata exposed at the fan front show signatures of Fe3+‐oxides (mostly fine‐grained crystalline hematite), Fe3+‐sulfates (potentially copiapites), strong signatures of hydration, and among the strongest signatures of red hematite observed in situ, consistent with materials having experienced vigorous water‐rock interactions and/or higher degrees of diagenesis under oxidizing conditions. The fan top strata show hydration but little to no signs of Fe oxidation likely implying that some periods of fan construction occurred either during a reduced atmosphere era or during short‐lived aqueous activity of liquid water in contact with an oxidized atmosphere. We also report the discovery of alternating cm‐scale bands of red and gray layers correlated with hydration and oxide variability, which has not yet been observed elsewhere on Mars. This could result from syn‐depositional fluid chemistry variations, possibly as seasonal processes, or diagenetic overprint of oxidized fluids percolating through strata having variable permeability.

The Chemistry and Mineralogy (CheMin) X-ray Diffractometer on the MSL Curiosity Rover: A Decade of Mineralogy from Gale Crater, Mars

For more than a decade, the CheMin X-ray diffraction instrument on the Mars Science Laboratory rover, Curiosity, has been returning definitive and quantitative mineralogical and mineral–chemistry data from ~3.5-billion-year-old (Ga) sediments in Gale crater, Mars. To date, 40 drilled rock samples and three scooped soil samples have been analyzed during the rover’s 30+ km transit. These samples document the mineralogy of over 800 m of flat-lying fluvial, lacustrine, and aeolian sedimentary rocks that comprise the lower strata of the central mound of Gale crater (Aeolis Mons, informally known as Mt. Sharp) and the surrounding plains (Aeolis Palus, informally known as the Bradbury Rise). The principal mineralogy of the sedimentary rocks is of basaltic composition, with evidence of post-depositional diagenetic overprinting. The rocks in many cases preserve much of their primary mineralogy and sedimentary features, suggesting that they were never strongly heated or deformed. Using aeolian soil composition as a proxy for the composition of the deposited and lithified sediment, it appears that, in many cases, the diagenetic changes observed are principally isochemical. Exceptions to this trend include secondary nodules, calcium sulfate veining, and rare Si-rich alteration halos. A surprising and yet poorly understood observation is that nearly all of the ~3.5 Ga sedimentary rocks analyzed to date contain 15–70 wt.% of X-ray amorphous material. Overall, this >800 m section of sedimentary rock explored in lower Mt. Sharp documents a perennial shallow lake environment grading upward into alternating lacustrine/fluvial and aeolian environments, many of which would have been habitable to microbial life.

Correlation between geomechanical and sedimentary facies and their implications for flow unit definition in the pre-salt carbonate reservoir, Brazil

Carbonate reservoirs are of great economic importance due to their frequent association with high permeability and hydrocarbon storage capacity. One of the most significant discoveries of this type of reservoir in recent decades has been the Brazilian Pre-salt. This deep reservoir has unusual depositional features, consisting of a wide variety of textures with different compositions, influenced by diagenetic overprinting that modify most of its mechanical, elastic and petrophysical properties. Despite the marked heterogeneity, these carbonates are characterized by excellent reservoir quality. The objective of this study is to establish a correlation between geomechanical behavior and sedimentary facies and its implications for the definition of flow units in the Barra Velha Formation, the main reservoir of the Brazilian Pre-salt. Thus, using a database from the Buzios field composed of well logs (conventional and advanced), well tests, thin sections, and laboratory petrophysics, geomechanical facies (GMFs) were defined using dynamic geomechanical modeling and an unsupervised algorithm (K-mean clutering), followed by textural classification of sedimentary facies and definition of flow units (FUs) on the decametric scale. These classifications were compared using frequency histograms. The geomechanical model revealed that the in-situ stress state exhibits a dual regime, between normal and strike slip. In addition, it was found that the direction of the maximum horizontal stress is in the NE-SW azimuth. In this context, four geomechanical facies were defined, which showed a notorious variation in mechanical strength. The strongest GMFs consisted mainly of well-cemented Grainstones and Packstones with no apparent porosity or microcrystalline porosity and correlated with the FU with the worst production rates. On the other hand, the weaker GMFs were mainly composed of Grainstones and Spherulistones with high intergranular porosity generated by dissolution processes, with higher risk of pore collapse and correlated with the FUs with the best production rates. However, this classification also identified laminated mudstones with microporosities associated with FUs that act as flow barriers. Geomechanical facies have also been incorporated into reservoir development by evaluating wellbore stability, fault reactivation risk, and brittleness index for future CO2 injection projects.

Quantitative Analysis of Diagenesis Control on the Spatial Heterogeneity of Sarvak Carbonate Formation, the Dezful Embayment of Zagros Basin, Western Iran

Summary The heterogeneity indices in conventional carbonate reservoirs are usually influenced by various diagenetic processes. The main aim of this study is to build 3D geological models of the diagenetic and heterogeneity indices addressing the role of diagenesis on spatial heterogeneity variations in the Sarvak carbonate reservoir of an Iranian oil field situated in the Dezful Embayment in the Zagros Basin. Vertical heterogeneity has been discussed in many studies, but the literature is limited on the 3D geological modeling of heterogeneity and providing spatial heterogeneity maps. In this study, we determined the main diagenetic parameters influencing the reservoir heterogeneity and constructed 3D geological models for evaluating the spatial heterogeneity. A couple of heterogeneity indices including the coefficients of variation (CVs) of porosity and permeability, Lorenz coefficient, and Dykstra-Parsons coefficient in cored intervals along with some diagenetic parameters were calculated and modeled. This study shows that lateral heterogeneity is mappable and correlatable with different diagenetic overprints along with shale volume, which controls the reservoir quality and spatial heterogeneity. It also indicates that lateral trends of the heterogeneity indices in different zones of the studied reservoir are influenced by other diagenetic parameters such as secondary porosity (SPI), dolomitization, cementation, and velocity deviation log (VDL, as a diagenetic index). Different diagenetic parameters control reservoir heterogeneity in each zone by decreasing or increasing the degree of the heterogeneity, some of which have dual constructive or destructive effects on heterogeneity indices. Moreover, the results show that the effect of shale volume on reservoir heterogeneity is significant, especially when the amount of the shale volume is notable.

Constraining reducing conditions in the Prague Basin during the late Silurian Lau/Kozlowskii extinction event

The Silurian was marked by repeated extinctions, carbon cycle volatility and significant intervals of climatic change. The most notable of these events were the Ludfordian Lau/Kozlowskii extinction and the associated Mid-Ludfordian Lau C isotope excursion, both of which have been linked to a period of global cooling and expanded reducing conditions in the global ocean. We present new data that characterize the marine palaeoredox conditions of the Prague Basin, a peri-Gondwanan terrane. We use I/Ca ratios to assess the local redox conditions in a shallow water carbonate succession and Fe speciation and redox-sensitive trace element concentrations to assess the local redox conditions of a deeper water sequence. Consistently low I/Ca values in the shallow water section suggest either persistent local low-oxygen conditions or possibly diagenetic overprinting. Fe speciation data suggest that the bottom water redox conditions in the deeper shelf setting were consistently anoxic with possible intermittent euxinia. Concentrations of redox-sensitive trace elements consistently higher than upper continental crust values also indicate persistent reducing conditions in the deeper part of the basin. These local redox proxy data from the Prague Basin, including trends in new pyrite S isotope ( δ 34 S pyr ) data, are consistent with previous findings of the expansion of anoxic and/or euxinic oceanic conditions. These data, derived from a mid-palaeolatitude marine setting, fill an important gap in our current global dataset for this interval of the late Silurian. Supplementary material: All the geochemical data reported and discussed herein are available in the Supplemental Data Tables at https://doi.org/10.6084/m9.figshare.c.7008107 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Interactions between Clays and Carbonates in the Aptian Pre-Salt Reservoirs of Santos Basin, Eastern Brazilian Margin

The giant Pre-salt reservoirs represent most of the oil production in Brazil. The main Aptian sag reservoirs were deposited in a unique and highly complex hyper-alkaline lacustrine setting. These deposits are essentially constituted by fascicular and spherulitic calcite precipitated in a magnesian clay matrix (stevensite, kerolite, and saponite/hectorite). Although vital for understanding the origin and main reservoir quality control, the genesis and interactions of clays and carbonates are still poorly constrained. The detailed petrographic description was focused on 812 thin sections from five wells drilled in the Santos Basin Aptian Barra Velha Formation, combined with cathodoluminescence, UV epifluorescence, and X-ray diffraction analyses. The main syngenetic processes were the deposition of finely laminated peloidal and ooidal Mg-clays, the formation of fascicular calcite crusts on the sediment–water interface, and the redeposition of these materials as intraclasts. Abundant clay peloids engulfed in syngenetic shrubs indicate that calcite and clay precipitation was concomitant, though with highly variable rates. Eodiagenetic phases include matrix-replacive and -displacive spherulites and fascicular shrubs; matrix-replacive blocky calcite and dolomite; lamellar carbonates filling matrix shrinkage pores; and microcrystalline calcite, dolomite, and silica replacing the Mg-clay matrix. The preferential dolomitization and calcitization of peloidal layers were most likely due to their higher permeability and larger specific surface. Matrix-replacive saddle dolomite, macrocrystalline calcite, and dawsonite are interpreted as mesodiagenetic or hydrothermal phases after significant matrix dissolution. Unraveling the processes of the formation and alteration of the carbonates and clays and their interactions in the Pre-salt deposits is essential for constraining the depositional and diagenetic conditions in their unique environments and their diagenetic overprinting and for decreasing the exploration risks and increasing the production of those extraordinary reservoirs.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Dolomitization is the most significant diagenetic process to affect Jurassic carbonate reservoirs along the Central Atlantic Margin (CAM). Despite several studies on dolomitization from different parts of CAM, the origin of these dolomites and their influence on the subsequent diagenetic evolution of Jurassic carbonate systems remains enigmatic. In addition, while dolomitization is evident at the surface and in the subsurface of the Moroccan Atlantic Margin, virtually no detailed studies have been conducted to determine the origin, mechanism, and significance of dolomitization in this basin. Therefore, the principal objective of this study is to assess the origin and occurrence of dolomite in the of Upper Sinemurian-Lower Pliensbachian carbonates of the Arich Ouzla Formation in the Essaouira-Agadir Basin by using petrography and geochemistry.The shallow marine carbonates of the Arich Ouzla Formation have been partially dolomitized and are exposed on the salt-cored Amsittene Anticline. The dolomite is stratabound, and predominantly fabric-retentive, although in some parts it is partially replaced by non-stratabound, fabric-destructive dolomites. From petrographic observations and geochemical proxies, the fabric-preserving dolomites show dolomitization by reflux of mesohaline seawater (δ18Odolomite average = −3.5 ‰ VPDB, and δ13Cdolomite average = 2.0 ‰ VPDB). In contrast, petrographic and geochemical characteristics of the fabric destructive dolomites suggest precipitation from modified seawater/formational brines convected along faults and fractures evidenced by depleted δ18O isotopic values (average = −4.1 ‰ VPDB) with high fluid temperatures (average = 78 °C; range = 66–90 °C) where fluids interacted with the basal Triassic evaporites and siliciclastic sediments.Fabric preserving dolomite has higher porosity (average = 6.0 %) than the precursor limestones (average = 0.4 %), whereas permeability in both rock types (average = 0.48 mD, and average = 0.02 mD, respectively) is low. Fabric destructive dolomite has low porosity in proximity to fracture corridors (average = 1.9 %) due to dolomite recrystallization (overdolomitization), whereas porosity increases to an average of 7.4%, away from fracture corridors. The dolomites are post-dated by calcite cement which occludes vugs, intercrystalline pores and fractures. The calcite is interpreted to be meteoric in origin, because of its non-cathodoluminescence and depleted δ18O (average = −4.7 ‰ VPDB) and δ13C (average = −9.3 ‰ VPDB) isotopic values with respect to Jurassic marine carbonates. The meteoric calcites co-exist with bitumen suggesting that hydrocarbon migration in the basin likely occurred at the same time, most likely during basin inversion and exposure. This work considers dolomitization to be a localised process due to salt diapirism and demonstrates that the coincidence of hydrocarbon emplacement with basin inversion results in degradation and probably leakage of hydrocarbons. This emphasises the importance of local and regional tectonics, including salt diapirism, on patterns of diagenetic overprint in sedimentary basins.

Stable isotope composition and concentration systematics of Ca and trace elements (Zn, Sr) in single aliquots of fossil bone and enamel

Measurements of modern bone and tooth enamel show that calcium stable isotopes (δ44Ca), zinc (δ66Zn), and strontium (δ88Sr) have great potential as ecological and dietary tracers for studies on fossil material. Despite the fact that these three elements can be recovered using sequential chromatographic extraction from a single aliquot, a multi-proxy study has not been attempted yet, which would allow a less destructive analysis of precious fossil material. The present study consists of analyzing δ44Ca, δ66Zn, δ88Sr, and 87Sr/86Sr values in fossil bone and tooth enamel of carnivore and herbivore mammals from the Camiac cave (Upper Pleistocene, Gironde, France). Our study aims at (1) developing a protocol for Ca, Zn, and Sr stable isotope analyses from a single fossil bone or enamel aliquot; (2) testing the respective preservation of biological isotopic signatures in bone and tooth enamel; and (3) comparing the patterning of the different isotopic systems in a fossil mammal community. In the present study, fossil enamel samples have not been leached, as it has been done so far for Zn isotope analyses, but fossil bone samples have been leached using diluted acetic acid or were left unleached. Using inclusive statistical analyses, we identify the effect of leaching on bone chemical compositions, and the concentration and isotope composition of Ca, Zn, and Sr in leached bone and unleached enamel show no sizeable diagenetic overprint. We report for the first time δ66Zn values in fossil bone. The results show that bone δ66Zn values mirror the pattern already observed in enamel, e.g., a significant trophic 66Zn depletion between herbivores and carnivores. Bone δ44Ca and δ88Sr values also mirror those in enamel, and the results highlight a clear distinction between ruminants and non-ruminants, the latter being 44Ca-enriched. The present study focuses on the importance of analyzing several isotopic systems in a single fossil bone or enamel aliquot and shows encouraging results for the preservation of biological isotopic signatures in fossil bone, despite the longstanding dread of a so-called associated pervasive diagenesis. The present study will therefore be useful to consider any precious fossil material with limited destructive sampling possibilities.

Strontium isotope compositions of Late Permian evaporites from the northernmost Thuringian Basin (Germany) indicate continental influence on the marine Zechstein Sea

In the Late Permian Zechstein Sea of Central Europe, up to 2000 m of evaporitic rocks were deposited in at least four consecutive cycles. The age of these evaporitic rocks could not yet be precisely determined, because they are virtually fossil-free and do not contain radiometrically datable volcanic layers. A chemostratigraphic age of the succession can be determined by comparing 87Sr/86Sr ratios of marine gypsum and anhydrite to the worldwide marine strontium evolution curve. Unfortunately, published 87Sr/86Sr data of the Zechstein succession are characterized by frequent outliers towards higher ratios, making an age assignment challenging. The scatter in 87Sr/86Sr ratios might be induced by different processes like the contribution of meteoric water to the brine, in-situ Rb decay, or post-depositional hydrothermal or diagenetic overprint. Here, we present a dataset of 26 new gypsum and anhydrite 87Sr/86Sr ratios from drill cores situated at “Alter Stolberg” in the northernmost Thuringian Basin. Evaporites of the Werra-, Staßfurt-, and Leine cycles were sampled. The close proximity of the drillings allows a very accurate assignment of the stratigraphic position of each sample, so that trends and outliers in 87Sr/86Sr ratios can easily be recognized. While the entire Werra Formation obviously revealed non-marine 87Sr/86Sr ratios, the lowermost 87Sr/86Sr ratios in the Staßfurt and Leine Formations can be assumed to represent marine ratios and allow estimating a chemostratigraphic age of 257‒254 Ma. The combination of the 87Sr/86Sr data with the mineral composition of the samples suggests a contribution of meteoric water, probably river water, to the Zechstein Sea as the main reason for the observed increase in 87Sr/86Sr ratios. Additional in-situ Rb decay, related to the riverine input of clay minerals, cannot be excluded. Modelling the amounts of sea water and meteoric water in the brine indicates that 83‒99% of meteoric water would be necessary to explain the highest 87Sr/86Sr ratios observed in the Werra Formation.Graphical abstract

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.

New insights into the palaeoenvironmental–palaeoclimatic significance and sedimentary dynamics of carbonate Lagerstätten: The lower Albian of Pietraroja (Southern Italy)

AbstractThis study reports the first high‐resolution, integrated facies analysis of the lowermost Albian Pietraroja Lagerstätten (Apennine Carbonate Platform) which yields the first dinosaur (Scipionyx samniticus) found in Italy and other terrestrial vertebrates and plants. Aiming to clarify the long‐debated palaeoenvironmental significance of the Pietraroja succession, the following have been carried out: (i) a field survey to establish stratigraphic position and number of the fossil Lagerstätten; (ii) the centimetre‐scale facies analysis of a new section, ca 15 m thick, consisting of two new lithostratigraphic units, the ‘Cyclically organized Limestones’ and the ‘Cherty Limestones’; (iii) the scanning electron microscopy – energy dispersive X‐ray spectroscopy and backscattered electron – energy dispersive X‐ray spectroscopy analyses of clay mineral proxies for palaeoclimate and non‐carbonate grains; and (iv) the regional and supra‐regional investigation of the event stratigraphical context of the fossil Lagerstätten, in order to elucidate the controls on their formation. The section includes two out of the three observed fossil‐rich Lagerstätten, each up to 1.5 m thick. The arrangement of lithofacies and early diagenetic overprint defines shallow‐water depositional cycles suggestive of precession and short‐eccentricity periodicities. The Middle Lagerstätte yielding Scipionyx samniticus consists of three intervals. The lower, paralic interval was deposited during arid conditions and passes gradually to the plant‐rich, coastal wetland carbonaceous marls of the upper interval. The ‘Cherty Limestones’, yielding the Upper Lagerstätte with terrestrial vertebrates, contains two spiculitic intervals suggesting the development of siliceous sponge meadows in a shallow, restricted lagoon. Genetic stratigraphy suggests that the Pietraroja Lagerstätten were formed during glacioeustatic lowstands; interestingly, the Middle Lagerstätte mirrors the earliest Albian sea‐level lowstand (KAl1 event, ca 111.2 Myr), during a semi‐continuous supply of windblown volcaniclastics. Findings herein substantiate the pivotal role of paralic–continental Lagerstätten for deriving high‐frequency palaeoclimatic dynamics and glacioeustacy from carbonate platform archives. The origin of Tethyan continental bridges between Africa and Europe during the late Aptian–earliest Albian cold interval is discussed.

Three‐dimensional stratigraphic architecture, secondary pore system development and the Middle Pleistocene transition, Sandy Point area, San Salvador Island, Bahamas

AbstractThe Bahamian Archipelago has been the subject of extensive studies of stratigraphy, carbonate island morphology and diagenetic overprinting for many decades. A recently‐acquired dataset comprised of high‐resolution light detection and ranging, tightly spaced cored boreholes with image logs, thin sections, porosity and permeability measurements, and isotopic geochemical analyses provides a unique opportunity to perform detailed sequence stratigraphic analysis. This multi‐faceted study first establishes a new high‐resolution sequence stratigraphic framework for the Sandy Point area of San Salvador Island, encompassing six unconformity bound sequences (exposure surfaces) representing varying lengths of missing time. The stratigraphically lowest sequence is Late Pliocene in age and is dominated by reef facies capped by an extensive laminar caliche that represents 1.5 million years of exposure. The overlying Early Pleistocene is split into two sequences, EP1 and EP2, which are both dominated by low‐energy subtidal facies. The upper three sequences are tied to the Marine Isotope Stage 11, 9 and 5e interglacials, and are distinctly different in facies composition and architecture, being dominated by higher energy facies distributed in a more complex mosaic. The new stratigraphic framework highlights significant changes in depositional style and facies architecture that can be linked to increasing sea‐level amplitude oscillations and greater climate gradient at the Mid Pleistocene Transition. The well‐constrained framework is then used to provide key context for observed complex diagenetic evolution through repeated sea‐level inundation and subaerial exposure. These results suggest that well‐developed subaerial exposure surfaces drive increases in non‐matrix features in the subsequent stratigraphic package following exposure as the well‐cemented exposure surface acts to concentrate fluid flow and dissolution. Integration of non‐matrix features within this framework highlights the importance of dissolution/cementation patterns on formation of these modified unconformity surfaces for focusing later meteoric diagenesis and creating enhanced fluid flow pathways for continued multicyclic diagenetic events.

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Early marine carbonate cementation in a relict shelf-edge coral reef North of Miami, Florida: Pseudo-coupled depositional and diagenetic modeling

Carbonate depositional and early diagenetic processes overlap in time because of the high chemical reactivity of calcite. Previous studies have modeled early diagenesis in a static depositional framework. They followed a rule-based modeling approach instead of applying rigorous thermodynamics and reaction kinetics.This study has developed a pseudo-coupled approach based on a dynamic depositional framework and reactive transport modeling for diagenesis. It has been tested for the “Pompano” reef north of Miami, Florida. The model includes five reef parasequences deposited over 9.0 kyr with five facies zones. After deposition of the first parasequence, diagenetic processes are modeled. The resulting framework forms the starting condition for depositing the next parasequence. Subsequently, its diagenetic overprint and the continued overprint in the underlying parasequence(s) are modeled. These workflow steps are repeated until all five parasequences are covered.The results show the spatial and temporal development of calcite cementation and porosity distribution. During the reef catch-up phase, cement mainly developed at the depositional surface (up to 10% of the total sediment volume). During the keep-up phase, cement abundance increases to a maximum value of ∼17%. After the reef's demise, cement forms a crust at the sediment-water interface with up to 38% of the total sediment volume. In general, controlling factors of cement distribution include facies-specific flow velocities, reef facies, relative sea-level changes, depositional gaps, and seawater salinity.The modeling results of this study are well aligned with the actual cement and porosity distribution in the Pompano reef and with other sub-recent and ancient examples described in the literature. Pseudo-coupled depositional-diagenetic modeling represents a suitable approach for quantitative porosity prediction in comparable depositional and diagenetic settings. It reduces uncertainties in prediction at inter-well to prospect scale for oil and gas exploration, geothermal exploration, and subsurface storage of CO2.

Early diagenetic mobilization of rare earth elements and implications for the Ce anomaly as a redox proxy

The Rare Earth Elements (REE) archived in carbonate rocks retain a wealth of information on paleo-seawater chemistry and local-regional redox conditions. However, interpretations are often ambiguous due to the potential for REE remobilization in marine environments. In this regard, many carbonate rocks that retain primary seawater isotopic signatures exhibit non-seawater-like REE patterns, implying either unusual REE behaviour in seawater or diagenetic overprinting of otherwise well-preserved rock samples. Here, we apply sequential leaching to constrain the possible effects of different REE-bearing phases on measured carbonate REE patterns in order to address this quandary. Our results show that the exchangeable phase contains negligible REE, but could be an important host phase for other elements such as Sr, Ba and K. The acidified hydroxylamine hydrochloride leach is shown here not only to dissolve Fe–Mn oxides but also phosphate minerals, which induces middle REE enrichment in corresponding leachates of dolostone samples in our study. The demonstrable Fe–Mn oxide phase in limestone samples is characterized by middle and/or light REE enrichment and positive Eu anomalies, which are attributed to hydrothermal processes and continuing exchange with marine fluids after initial precipitation. The non-seawater-like REE patterns observed in the carbonate phase of otherwise well-preserved limestones resemble those of the co-existing Fe–Mn oxide fraction, and are interpreted to reflect the dissolution of REE carrier phases during early diagenesis. This view is supported by fluid-rock interaction modelling, which shows how REE can be mobilized at relatively low fluid/rock ratios in shallow porewaters. Non-seawater-like REE patterns may therefore be caused by the incorporation of REE from shallow porewaters before final lithification due to an elevated flux of particulate REE carrier phases, e.g. Fe–Mn oxides and organic matter, to the seafloor. In spite of the sensitivity of carbonate REE patterns to early diagenetic exchange, the co-occurrence of non-seawater-like patterns with primary strontium and carbon isotope values suggests that REE should not be viewed as a general indicator for the preservation of other geochemical proxies. Importantly, Ce anomalies of the carbonate phase will also be affected by porewaters, masking primary seawater values. Our results favour a reevaluation of redox interpretations by taking into account REE patterns as a whole.

Linking diagenesis and reservoir quality to depositional facies in marginal to shallow marine sequence: An example from the Campano-Maastrichtian Gombe Sandstone, Northern Benue Trough, NE Nigeria

Fluvio-deltaic strata host significant volumes of hydrocarbons in different basins across the world, and are characterized by depositional heterogeneities and varying diagenetic overprints, owing to complex interactions of different depositional processes in diverse sub-environments. An understanding of the heterogeneities at different scales is critical to the prediction of reservoir quality distribution and architecture in these strata. The Campanian-Maastrichtian Gombe Sandstone is one of the targeted reservoirs in recent hydrocarbon exploration in the Northern Benue Trough, NE Nigeria. However, to date, the diagenetic and reservoir quality evolutions of the sandstone are not well understood. In this study, a multi-technique approach (consisting of thin-section petrography, scanning electron microscopy, and X-ray diffraction analyses) was employed to link depositional facies and diagenesis to reservoir quality. Results of the study indicate that the sandstones are principally arkosic and subarkosic in composition, and were deposited in lower delta front/prodelta, upper delta front/mouth bar, distributary channel, and interdistributary tidal bar/channel depositional environments. The sandstones experienced both shallow burial and some relatively of deep burial diagenesis. Kaolinite is the most abundant eogenetic cement in the sandstones, and was particularly found to be more abundant in the interdistributary tidal bar/channel and lower delta front/prodelta depositional environments due to their significant amounts of reactive detrital feldspars and muscovite precursors, respectively. The interdistributary tidal bar/channel and the distributary channel depositional facies were highly susceptible to siderite cementation, owing to their proximity to organic-rich mudstone lithofacies. Mechanical compaction was more intense in the lower delta front/prodelta than in the interdistributary tidal bar/channel sandstones, due to the presence of high clay matrix content in the former, which enhanced ductility and close packing. The interdistributary tidal bar/channel sandstones form the best reservoir quality because of the very low amounts of detrital pore-filling matrix (average 1%; n = 24) and less compaction compared to the lower delta front/prodelta, upper delta front/mouth bar and distributary channel sandstones. Grain-coating smectitic clays were best developed in the lower delta front/prodelta, upper delta front/mouth bar and distributary channel, whereas they are poorly developed in the interdistributary tidal bar/channel sandstones. The overall low volumes of deep burial, mesogenetic cements in the sandstones (e.g., illite, dickite, and quartz overgrowths) suggest that the Gombe Sandstone spent short residence time in mesogenetic realm before being uplifted. The results of this study would provide useful input data and understanding that can be utilized for forward diagenetic modelling and burial history construction for the subsurface Gombe Sandstone reservoir in the Northern Benue Trough and similar sedimentary formations elsewhere.