Diagenetic Regime Research Articles

Diagenetic Impact on High-Pressure High-Temperature Reservoirs in Deep-Water Submarine Fan Sandstone of Qiongdongnan Basin, South China Sea

The diagenetic evolution of sandstone is very complicated under the conditions of high temperatures and pressures in deep-water, deep-buried regimes, which have great influence on reservoir quality. This study investigates the typical reservoir target of Neogene deep-water, submarine-fan sandstones under high-temperature, high-pressure regimes in the Qiongdongnan Basin, South China Sea. Utilizing a thin section, scanning electron microscope (SEM), mineral geochemistry combined with burial history evolution, complex diagenetic events, and main controlling factors of the sandstone in the Neogene Meishan Formation were determined. The results show that the evolution of sandstone reservoirs is initially controlled by depositional framework compositions and subsequently modified by eogenetic and mesogenetic alterations during progressive burial. Eogenetic alterations mainly include the following: (1) mechanical compaction; (2) dissolution of feldspar; (3) low-Fe calcite cementation. Mesogenetic events were identified as the following: (1) dissolution of feldspar; (2) ferroan calcite and ankerite formation; (3) precipitation of quartz and clay mineral. Mechanical compaction is greatly influenced by the original depositional framework composition, and sandstone samples enriched in high contents of detrital clay matrix always experienced extensive mechanical compaction. Different phases of carbonate cement during different diagenetic regimes lead to continuous destruction on reservoir porosity. The dissolution of unstable feldspar minerals during eogenetic and mesogenetic environments leads to the development of secondary porosities and would enhance the quality of the reservoir. Overpressure formation is pervasively developed owing to early disequilibrium compaction and subsequent natural gas charging. Only well-sorted sandstones with low contents of detrital clay matrix could resist early mechanical compaction, lead to ample residual original porosities, and then undergo extensive mineral dissolution to generate sufficient secondary porosities. Subsequently, these porosities would be effectively protected by overpressure formation. Poor-sorted sandstones with high contents of detrital clay matrix would experience strong mechanical compaction and extensive destruction of original porosities. Thus, these sandstones are difficult to have significant dissolution and are unable to be effectively protected by overpressure formation. Therefore, the interplay between the original framework composition and the corresponding diagenetic pathways coupled with overpressure formation would result in strong reservoir heterogeneity for the deep-buried sandstones during progressive burial.

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Carbon and oxygen isotope characteristics of the Clydach Valley Subgroup, Courceyan, South Wales‐Mendip shelf, UK

AbstractThe Clydach Valley Subgroup (Courceyan) records a dip section through an Early Carboniferous shallow marine, carbonate shelf and consists of three oolitic formations separated by paludal/peritidal units with abundant evidence of subaerial exposure in proximal areas. The lower part correlates with the Kinderhookian–Osagean Boundary Excursion, with allochem data indicating a minimum δ13C value for marine carbonate of +4.5‰, with associated δ18O of −4.6‰. Marine carbonate δ13C and δ18O values of +2‰ and −2‰, respectively, were estimated for the younger part. Allochem isotopic data yield a well‐defined mixing line consistent with stabilisation in meteoric water at varied water: rock ratios, the degree of stabilisation increasing up‐dip and up‐stratigraphy. Two distinct diagenetic styles closely correlate with evidence for the presence (Diagenetic Regime 1) or absence (Diagenetic Regime 2) of subaerial exposure, non‐ferroan and ferroan calcite cement dominating respectively. Five cement zones (Zones 2–6) defined by iron content occur in Diagenetic Regime 1, irrespective of formation, indicating a similar sequence of palaeohydrological changes affected repeated depositional cycles. Zones 3–5 are considered meteoric based on their isotopic composition (δ18O: −5.8 to −11.1‰ and δ13C: −3.7 to −6.2‰) and form distinct clusters dependent on zone, age and location. Pedogenic carbonates and meteoric cements record a long term increase in meteoric δ18O values: −7.8‰ during the Kinderhookian–Osagean Boundary Excursion, −6.9‰ during diagenesis of the upper part of the subgroup and −6.3‰ associated with initial deposition of the overlying Llanelly Formation. This is consistent with global sea water trends, but an element of climate change cannot be ruled out. Increases in cement δ18O values as meteoric systems become established (Zone 3–4) indicate repeated short term variation in rainwater composition probably driven by climate change, but also suggests a link between climate and the depositional cycle.

A 13 million-year record of Li isotope compositions in island carbonates: Constraints on bulk inorganic carbonate as a global seawater Li isotope archive

Lithium isotope (δ7Li) variations in marine carbonate sediments and rocks have been widely used to reconstruct global seawater Li isotope compositions and then trace the paleo-weathering processes. However, there are still debates on whether ancient carbonates can faithfully document original seawater δ7Li signals, as δ7Li values of shallow-water carbonates are tightly related to carbonate mineralogy and diagenetic alteration. In this study, we present high-resolution δ7Li, trace element and carbonate mineralogy data of shallow-water carbonate deposits from two shallow drillcores (Jiuzhang A and B) and a deep drillcore (XK-1) in the South China Sea. We compare these new δ7Li data to the ca. 13 Myr history of seawater δ7Li evolution from the middle Miocene to Pleistocene, in order to better constrain the effects of carbonate mineralogy and early diagenesis on δ7Li values of bulk inorganic carbonates. We observe that the δ7Li values of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores (24.6% ± 1.6%, n = 37, 1σ) are significantly lower than modern seawater δ7Li values (Δ7Liprimary-seawater = ∼–6.0%). In contrast, marine diagenetic carbonates in the XK-1 drillcore exhibit δ7Li values of 28.7% ± 0.7% (n = 54, 1σ), approaching coeval seawater values (Δ7Limarine diagenetic-seawater = ∼–2.0%) and much higher than those of primary carbonate deposits. Meteoric diagenetic carbonates in the XK-1 drillcore show δ7Li values of 22.4% ± 1.6% (n = 46, 1σ), close to that of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores, but significantly lower than coeval seawater δ7Li values (Δ7Limeteoric diagenetic-seawater = –5.0% to –9‰ for modern and Miocene seawater). Such δ7Li variations in shallow-water carbonates are attributed to variations in carbonate mineralogy (i.e., aragonite, high-Mg calcite and low-Mg calcite) and diagenetic regimes (fluid- or sediment-buffered conditions of marine and meteoric diagenesis). By comparing δ7Li data from the South China Sea to those from the Bahamas, we suggest strongly fluid-buffered conditions for Li isotopes during marine diagenesis, resulting in δ7Li values of marine limestones and dolostones approaching ambient seawater δ7Li signals. In contrast, meteoric diagenetic carbonates, likely marked by sediment-buffered conditions, may inherit the original δ7Li signals of primary carbonate deposits. Hence, full considerations of the carbonate mineralogy and diagenesis facilitate a better use of bulk inorganic carbonate-archived δ7Li to reconstruct paleo-weathering evolution in deep time lacking the skeletal fossil records.

Evolution of diagenetic fluids in the deeply buried, upper Ediacaran Dengying Formation, Central Sichuan Basin, China

Deeply buried Precambrian carbonate reservoirs host economic petroleum reserves. The upper Ediacaran Dengying Formation (maximum burial depth >8 km) is predominantly composed of dolostone with porosity ranging from 2 to 4%, and minor limestone with a porosity of <2%. Detailed petrological observations and high-resolution geochemical analyses show the evolution from depositional fluids to late diagenetic fluids. Dolostone is subdivided into dolo-mudstone, dolo-laminite, stromatolitic dolostone, thrombolitic dolostone and oncoidal dolostone. These dolostones were precipitated from the Ediacaran seawater and yield rare earth elements and yttrium (REY) patterns similar to those of modern seawater. In the early diagenetic regimes, fibrous dolomite cement, characterised by superchondritic Y/Ho ratios, was first precipitated from marine porewaters, and then fine-crystalline dolomite cement, characterised by low Y/Ho ratios, was formed during meteoric diagenesis. Bladed dolomite cement was subsequently precipitated from (modified) seawater-dominated porewaters. In the intermediate burial regimes, medium- to coarse-crystalline dolomite cement was formed during reduction of Mn oxides. In the deep-burial regimes, saddle dolomite was precipitated from hydrothermal fluids and yields REY patterns characterised by MREE enrichment. Its superchondritic Y/Ho ratios were likely caused by the fractionation between Y and Ho during the co-precipitation of fluorite. Calcite cement has similar parent fluids to those of saddle dolomite. These dolomite phases have similar δ13C values, while their δ18O values gradually decrease during the paragenetic sequence. Multiphase dolomitisation led to the increase in Ca/Mg ratios of formation water and reservoir performance. This study is significant for those concerned with the deeply buried dolostone reservoirs. KEY POINTS Microbialites and five types of dolomite cements have different REY patterns. Co-precipitation of fluorite led to the superchondritic Y/Ho ratios of dolomite. Dolomite and calcite grains show micro-scale geochemical variations. This dolostone reservoir underwent multiphase fluids, including meteoric water, seawater and hydrothermal fluid.

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.

Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

AbstractThis study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.

The transition of very coarse-grained meandering to straight fluvial drainage systems in a tectonized foreland-basement landscape during the Holocene (SE Germany) – A joint geomorphological-geological study

Very coarse-grained meandering to straight fluvial drainage systems evolved during the Holocene (<7950 BP) in a terrain underlain by crystalline basement rocks of Precambrian to Paleozoic age and Mesozoic sedimentary rocks which are separated from each other by an area intersected by fault zones of Cenozoic age. Faulting provoked ridges in the foreland, dykes in the basement and it is accountable for a significant uplift and stream strong gradient. All structural elements show the same NW-SE strike as a consequence of the modern fault tectonic pattern. The Holocene fluvial drainage system under study evolved homoclinal with a presumed paleosurface (relic landscape/peneplain) and represents the upper reaches of the Main River Drainage System in SE Germany. During the Neogene, chemical weathering created a regolith while uplift in the hinterland and subsidence in the foreland provoked tilting of the paleosurface. The coarse-grained meandering drainage system is described by a characteristic trilogy that encompasses (1) intermediate depocenters/sediment traps, (2) subsequent strike streams, and (3) consequent dip streams that upstream grade into straight streams with no braided streams to be. The drainage system developed under temperate humid climates and can be described by five lithofacies types which were investigated with regard to the origin of the individual landform types, their sedimentological, hydrographic, mineralogical and petrographic features. The hydrographic parameters sinuosity, channel density and channel-floodplain ratio are markers for the paleo-gradient and tectonic activity and can also be used as flow rate and fluvial confinement indices. Mass wasting and fluvial processes both shape the morphology of the valleys. Autochthonous and par-autochthonous minerals and organic matters constitute the basis for the physico-chemical facies analysis and for constraining the climatic and early diagenetic regimes. For correlation and chronology of the lithofacies types radiocarbon dating and optically stimulated luminescence proved to be successful.

Diagenesis of sulfur, iron and phosphorus in sediments of an urban bay impacted by multiple anthropogenic perturbations

Solid-phase speciation and porewater chemistry measured by the diffusive gradients in thin films (DGT) technique were used to understand the diagenesis of sulfur (S), iron (Fe), and phosphorus (P) in sediments of Jiaozhou Bay (China), which has been impacted by multiple anthropogenic perturbations. Despite water eutrophication, sediments of the bay are low in organic carbon and sulfide, but high in unsulfidized Fe(II). Dissimilatory iron reduction (DIR) prevails in sediments of the bay, and there is no evidence for responses of S and Fe diagenesis to the water eutrophication, which is largely attributable to unique depositional and diagenetic regimes in association with multiple anthropogenic perturbations. Good coupling of porewater Fe2+ and P in the porewaters suggests that P mobilization is driven mainly by DIR. Low Fe2+/P ratios in porewaters imply that oxidative regeneration of Fe oxides within the upper sediments is incapable of efficiently scavenging upward diffusing P.

Diagenesis of mollusc aragonite and the role of fluid reservoirs

Biogenic carbonate minerals are widely used as archives in paleoenvironmental research, providing substantial information for past depositional and diagenetic regimes. However, nearly all biogenic carbonates undergo post-mortem diagenetic alteration to variable degrees. Diagenetic features are essentially caused by complex fluid-solid interaction including recrystallization and neomorphosis of shell architecture and related geochemical resetting. A common conception is that a given primary shell texture is replaced by a secondary fabric via micro-scale dissolution-reprecipitation reactions that may reach geochemical and/or isotopic equilibrium with the diagenetic fluid. Here we document that the process of petrographic and geochemical alteration of a biogenic carbonate archive progresses stepwise, and the re-equilibration processes can be separated in space and time. More specifically, attached and bound aqueous fluids within the shell organic matter and fluid inclusions likely play a crucial role in the early stages of alteration within a relatively rock-buffered system. Degradation of organics was observed via decreased sulfur concentration and decreased fluorescence. In this early stage of alteration, the conversion of micro-scale aragonite biominerals to metastable diagenetic calcite (meso)crystals is documented by RAMAN spectroscopy, electron backscatter diffraction (EBSD), and a decrease in strontium concentrations. Further stabilization to larger, neomorphosed calcite crystals resulted from re-equilibration with external fluids and was evidenced by EBSD, oxygen isotope data, and further reduction of strontium. Spatial chemical and isotope data were used to decipher the impact of fluid availability and transfer behavior, i.e. carbonate-buffered versus diagenetic-fluid buffered, and isotope and element exchange behavior, and analyzed in context to textural alteration features. These data suggest that during diagenesis, the evolution to more sustainable (diagenetically induced) fabrics is accompanied by individual reaction steps traced by elemental and isotopic signatures. Kinetics, and thus degree of diagenetic alteration of biominerals at a given exposure to physicochemical alteration conditions is initially strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results shown here have significance for those concerned with biogenic carbonate archives and have wider implications for a mechanistic understanding of carbonate diagenesis.

Patterns of soft tissue and cellular preservation in relation to fossil bone tissue structure and overburden depth at the Standing Rock Hadrosaur Site, Maastrichtian Hell Creek Formation, South Dakota, USA

Recovery of soft tissues and cells from fossil bones is becoming increasingly common, with structures morphologically consistent with vertebrate osteocytes, blood vessels, fibrous/collagenous matrix, and potential intravascular contents now recognized from specimens dating back to the Permian. However, it largely remains unclear how bone tissue structure, early diagenetic regimes, and many other taphonomic variables influence or control the preservation potential of soft tissues in vertebrate fossils. To explore the influence of a few of these factors, we tested a suite of fossils from the Standing Rock Hadrosaur Site, a vast Edmontosaurus annectens bonebed in the Maastrichtian Hell Creek Formation of South Dakota, for preservation of cellular and tissue components. Demineralization of bone samples from each specimen yielded abundant microstructures morphologically consistent with vertebrate osteocytes, blood vessels, and collagenous matrix. This includes the first recovery of osteocytes and vessels from a fossil vertebral centrum and ossified tendons. Perhaps surprisingly, no correlation was found between soft tissue/cellular recovery and either bone tissue structure type (cortical vs. cancellous) or overburden depth at the time of discovery. A traditional taphonomic survey of the site, conducted in parallel and reported previously, affords a clear and detailed history of these remains, both pre- and postburial. Cumulative taphonomic evidence indicates the Edmontosaurus individuals died in a mass mortality event and their disarticulated remains were buried rapidly in a shallow floodplain pond during a crevasse splay event. Oxygenated flood waters and/or groundwater oxidized initially sideritic concretions to goethite during early diagenesis, facilitating rapid cementation of portions of the sediment that likely aided stabilization of soft tissues by shielding regions of the bones from prolonged exposure to pore fluids. Our findings support cancellous bone as a viable target for cellular analyses, corroborate previous propositions that iron-rich environments and rapid burial facilitate soft tissue preservation, and provide new details into early diagenetic environments conducive to such preservation.

Manganese carbonates in the Upper Jurassic Georgiev Formation of the Western Siberian marine basin

Manganese (Mn) carbonate rocks are a common lithological constituent of the Upper Oxfordian to Lower Tithonian (Volgian) Georgiev Formation of the Western Siberian marine basin (WSMB). The Mn carbonates in the Georgiev Formation are present in the form of massive sediments, stromatolites, and oncolites, and are associated with glauconite and partly also phosphate-rich clay- and siltstones. Unlike most Mn carbonates, they are not directly associated with organic-rich sediments, but occur below an organic-rich succession (Bazhenov Formation). The Mn carbonate occurrences can be traced from the western central area of the WSMB to its center along a distance of at least 750km. The thickness of the Mn carbonates and their Mn contents becomes reduced in an eastward direction, related to increased detrital input. The geochemical and mineralogical heterogeneity within the Mn carbonates indicates that they were deposited stepwise in a diagenetic regime characterized by steep gradients in Mn, Ca, and Mg. A first step consisted in the replacement of initial sediments within the microbialites during an early diagenetic stage, followed by a second step where massive sediments were transformed into Mn carbonate. During both steps, the decomposition of organic matter was an important source of the newly formed carbonate. During a further step, voids were cemented by Mn carbonates, which are rich in pyrite. This last generation may only have formed once the organic-rich sediments of the overlying Bazhenov Formation were deposited. Accumulation of the Mn carbonates in the Upper Jurassic WSMB was controlled by the proximity of Mn-enriched parent rocks, likely in the Ural, which were subjected to intense geochemical weathering during the Late Jurassic.

In situ, high-resolution DGT measurements of dissolved sulfide, iron and phosphorus in sediments of the East China Sea: Insights into phosphorus mobilization and microbial iron reduction

Dissolved sulfide, iron (Fe), and phosphorus (P) concentrations in sediments of the East China Sea were simultaneously measured in situ by diffusive gradients in thin films (DGT) technique. The results, by combination with solid-phase Fe speciation, were used to characterize the interplays of Fe, S and P. Diverse distributions of dissolved sulfide among the sites are attributable to highly dynamic diagenetic regimes and varying availability of labile organic carbon (OC). The DGT technique provided high-resolution evidence for coexistence of microbial iron reduction (MIR) and sulfate reduction in localized zones, and for Fe-coupled P mobilization. Measured Fe2+/P ratios suggest that Fe2+ reoxidatiion at the oxic zones can serve as an efficient scavenger of P. Empirical estimation indicates that MIR plays an important role in anaerobic OC mineralization in the sediments, which is a combined result of low availability of labile OC, high reactive Fe content, and unsteady diagenetic regimes.

Quantifying the effect of diagenetic recrystallization on the Mg isotopic composition of marine carbonates

The Mg and Sr isotopic compositions (δ26Mg and 87Sr/86Sr) of pore fluids and bulk carbonates from Ocean Drilling Project Site 1171 (South Tasman Rise; 2148.2m water depth) are reported, in order to evaluate the potential of diagenesis to alter carbonate-based geochemical proxies in an open marine system. Given the trace amounts of Mg in marine carbonates relative to coexisting pore fluids, diagenesis can alter carbonate δ26Mg, a promising proxy for seawater δ26Mg that may help elucidate long-term changes in the global Mg cycle. Constraints on the effect of diagenetic recrystallization on carbonate δ26Mg are therefore critical for accurate proxy interpretations. This study provides context for assessing the fidelity of geochemical proxy-reconstructions using the primary components (i.e., foraminiferal tests and nannofossils) of bulk carbonate sediments.We find that pore fluid δ26Mg values (on the DSM3 scale) at Site 1171 increase systematically with depth (from −0.72‰ to −0.39‰ in the upper ∼260m), while the δ26Mg of bulk carbonates decrease systematically with depth (from −2.23‰ to −5.00‰ in the upper ∼260m). This variability is ascribed primarily to carbonate recrystallization, with a small proportion of the variability due to down-hole changes in nannofossil and foraminiferal species composition. The inferred effect of diagenesis on bulk carbonate δ26Mg correlates with down-core changes in Mg/Ca, Sr/Ca, Na/Ca, and 87Sr/86Sr. A depositional reactive-transport model is employed to validate the hypothesis that calcite recrystallization in this system can generate sizeable shifts in carbonate δ26Mg. Model fits to the data suggest a fractionation factor and a partition coefficient that are consistent with previous work, assuming calcite recrystallization rates of ⩽7%/Ma constrained by Sr geochemistry. In addition, either partial dissolution or a distinctly different previous diagenetic regime must be invoked in order to explain aspects of the elemental chemistry and 87Sr/86Sr of relatively deep sediments from Holes A and C. This study indicates that the dynamics of a given sedimentary system can significantly alter bulk carbonate geochemistry, and presents a framework for considering the potential impact of such alteration on picked archives such as foraminiferal tests and nannofossils. Ultimately, this study contributes to the development of δ26Mg as a proxy for seawater δ26Mg by quantifying the susceptibility of carbonate δ26Mg to diagenetic alteration, particularly in sediments in open marine systems. This study suggests that because of the sensitivity of carbonate δ26Mg to diagenetic recrystallization, it can, in certain systems, be used to quantify the impact of diagenesis on carbonate-based geochemical proxies.

Diagenesis in limestone-dolostone successions after 1 million years of rapid sea-level fluctuations: A case study from Grand Cayman, British West Indies

Meteoric diagenesis in young marine carbonate sediments has commonly been linked to fluctuations in Quaternary glacio-eustatic sea levels. The extent to which these sea-level changes are recorded in these carbonate successions, however, remains questionable. This is amply demonstrated by the diagenetic record found in the limestones and dolostones of the Cayman Formation (Miocene) on the Cayman Islands. On the eastern part of Grand Cayman, dolomitization that ceased by 1 million years ago created an architecture whereby the limestones in the central part of the island were surrounded by dolostones in coastal areas of the island. Since then, the upper 90m of the Cayman Formation has been repeatedly cycled through many different marine and meteoric diagenetic zones as large, rapid eustatic oscillations in sea level affected the island. The records of these diagenetic cycles in the dolostones and limestones are, however, different and impossible to match to the cyclic changes in sea level. In the peripheral dolostones, post-dolomitization diagenetic features are sparse. In contrast, the limestones in the interior of the island exhibit a wider variety of meteoric diagenetic features, including extensive dissolution and calcite cementation. The dolostones have low porosity (<10%) and permeability, whereas the limestones are characterized by high porosity (up to 50%), especially in the lower and middle parts of the studied limestone succession. The different phases of diagenesis found in the limestones, however, cannot be specifically matched to any sea-level fluctuations that have affected these successions. This issue is further exemplified by the fact that that the last marine transgression over the last ~16,000years ago appears to have left no tangible record. The analysis of this succession clearly demonstrates that not all diagenetic regimes will be recorded in the fabrics of limestones or dolostones.

Diagenetic regimes in Arctic Ocean sediments: Implications for sediment geochemistry and core correlation

Dark brown sediment layers are a potential stratigraphic tool in Quaternary Arctic Ocean sediments. They are rich in Mn, Fe, and trace metals scavenged from the water column and were most likely deposited during interglacial intervals. In this study, we combine sediment and pore water data from sediment cores taken in different parts of the Arctic Ocean to investigate the influence of early diagenetic processes on sediment geochemistry. In most studied cores, Mn, Co, and Mo are released into the pore waters from Mn oxide dissolution in deeper (>1.5m) sediment layers. The relationship between sedimentary Mn, Co, and Mo contents in excess of the lithogenic background (elementxs) shows that Coxs/Moxs values are a diagnostic tool to distinguish between layers with diagenetic metal addition from the pore waters (Coxs/Moxs<1), layers affected by Mn oxide dissolution and metal release (Coxs/Moxs>10), and unaffected layers (Coxs/Moxs from 1 to 10). Steady-state calculations based on current pore water profiles reveal that in the studied cores, the diagenetic addition of these metals from the pore water pool alone is not sufficient to produce the sedimentary metal enrichments. However, it seems evident that dissolution of Mn oxides in the Mn reduction zone can permanently alter the primary geochemical signature of the dark brown layers. Therefore, pore water data and Coxs/Moxs values should be considered before core correlation when this correlation is solely based on Mn contents and dark sediment color. In contrast to the mostly non-lithogenic origin of Mn in the dark brown layers, sedimentary Fe consists of a large lithogenic (80%) and a small non-lithogenic fraction (20%). Our pore water data show that diagenetic Fe remobilization is not currently occurring in the sediment. The dominant Fe sources are coastal erosion and river input. Budget calculations show that Fe seems to be trapped in the modern Arctic Ocean and accumulates in shelf and basin sediments.The Fe isotopic signal δ56Fe of the solid phase is positive (∼0.2–0.3‰) in samples defined as the lithogenic background without significant Fe enrichments. With increasing non-lithogenic Fe contents in the sediment, δ56Fe becomes more negative, which indicates a shelf-to-basin export of an isotopically lighter Fe fraction. We assume that the same transport process is true for Mn.

Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

The Wonoka–Shuram Anomaly represents the largest negative carbon isotope excursion recognized in the geologic record and is associated with the emergence and diversification of metazoan life ca. 580 million years ago (Ma). The origin of the anomaly is highly debated, with interpretations ranging from primary to diagenetic, each having unique and potentially transformative implications for early life. Here, we apply carbonate clumped isotope thermometry to three sections expressing the anomaly in order to constrain mineral formation temperatures and thus directly calculate water oxygen isotope compositions (δ18Ow) with which carbonate minerals equilibrated. With δ18Ow known, it is possible to address previous hypotheses for the origin of the anomaly. In each section, precipitation temperatures correlate positively with reconstructed δ18Ow. Previous hypotheses, based on the covariance of δ18Ocarb vs. δ13Ccarb (uncorrected for temperature effects), suggested a meteoric diagenetic origin for the anomaly. However, reconstructed δ18Ow values do not covary with carbon isotope compositions (δ13Ccarb) within anomaly facies. Rather, the oxygen isotope and temperature data are consistent with carbonate recrystallization and equilibration under increasingly rock-buffered conditions. Based on simple modeling and comparison to modern formation fluids, recrystallization may have occurred in an environment far removed from the initial depositional or early diagenetic regime. In addition, although clumped isotope temperatures vary significantly and reach elevated values consistent with burial diagenesis, it is unclear to what degree, if at all, carbon isotope values were reset during recrystallization. Ultimately, these new data indicate that Wonoka–Shuram-aged carbonates experienced equilibration with fluids under increasingly closed-system conditions. The clumped isotope data do not provide a means to distinguish previous hypotheses outright, but provide additional context for the evaluation of geochemical signatures within these ancient carbonate rocks.

Microarchitecture of dolomite crystals as revealed by subtle variations in solubility: Implications for dolomitization

Despite their small size, most dolomite crystals in the dolostones of the Miocene Cayman Formation on Grand Cayman are zoned, with a core formed of high-calcium calcian dolomite (HCD) encased by a cortex formed of low-calcium calcian dolomite (LCD). These two different types of dolomite are separated by a sharp, well-defined cortical boundary. Acid etching shows that the HCD cores are characterized by numerous growth defects including twinning, modulations, and dissolution slots whereas the LCD that forms the cortex contains far fewer and less well-developed growth defects. The fact that the HCD is far more soluble than the LCD is attributed to its higher CaCO3 content and greater density of growth defects.The cortical boundary that separates the HCD core from the LCD cortex is a discontinuity that may have developed as a response to a change in diagenetic regimes or a change in the composition of the dolomitizing fluids. Deciding between these two possibilities, however, is difficult because the small size of the crystals precludes collection of data needed to resolve the issue. Irrespective of the underlying cause, the development of the zoning is critical from a diagenetic perspective because the HCD with its high CaCO3 content and high density of growth defects is far more prone to dissolution than the LCD cortex. Thus, zoned crystals of this type are highly susceptible to diagenetic modifications that will lead to the development of hollow crystals, dedolomite, or inside-out dolomite.

Diagenesis of Neogene Delta-Front Patch Reefs: Alteration of Coastal, Siliciclastic-Influenced Carbonates from Humid Equatorial Regions

Abstract This study offers insights into the diagenetic alteration of coastal carbonates that formed coevally with nearly continuous siliciclastic influx in a humid equatorial setting. A multi-disciplinary petrographic, cathodoluminescent, stable-isotope, trace-element and major-element investigation allowed characterization of diagenetic features, paragenetic sequencing, and an interpretation of diagenetic environments from Neogene patch reefs of the Samarinda region, Mahakam Delta, Borneo, SE Asia. Marine cements are absent from the patch reefs, with grain micritization the only marine diagenetic feature recognized. The predominant diagenetic feature within the patch reefs is pervasive neomorphic stabilization and cementation of aragonite reef components to calcite that pre-dates all compaction features. Meteoric aquifer flow derived from the adjacent landmass is inferred as the main parent diagenetic fluid, since δ18O V-PDB values of calcite cements of −3.6 to −11.7‰ are consistent with precipitation from SE Asian freshwater, and inconsistent with a wholly marine origin. Late-stage fracturing, cementation, and chemical compaction are relatively minor features and attest to a changing paleohydrologic and diagenetic environment. Evidence for a shallow to moderate burial diagenetic regime for these later features are maximum temperature of 53 °C and burial depths < 1000 m inferred from stable-isotope values of calcite and late dolomite cements, dolomite crystal fabrics, the onset depth of stylolite or dissolution-seam formation, and regional geothermal gradients. The humid tropical environment and “ever-wet” conditions on the island of Borneo together with rapid Cenozoic uplift likely led to paleoaquifer flow with fluids focused through adjacent deltaic units into the reef carbonates. In these coastal carbonates from the humid Samarinda region, continental groundwater flow has resulted in pervasive stabilization and calcitization, features rare in arid or temperate counterparts.

Unique authigenic mineral assemblages reveal different diagenetic histories in two neighbouring cold‐water coral mounds on Pen Duick Escarpment, Gulf of Cadiz

AbstractAlpha Mound and Beta Mound are two cold‐water coral mounds, located on the Pen Duick Escarpment in the Gulf of Cadiz amidst the El Arraiche mud volcano field where focused fluid seepage occurs. Despite the proximity of Alpha Mound and Beta Mound, both mounds differ in their assemblage of authigenic minerals. Alpha Mound features dolomite, framboidal pyrite and gypsum, whereas Beta Mound contains a barite layer and predominantly euhedral pyrite. The diagenetic alteration of the sedimentary record of both mounds is strongly influenced by biogeochemical processes occurring at shallow sulphate methane transition zones. The combined sedimentological, petrographic and isotopic analyses of early diagenetic features in gravity cores from Alpha Mound and Beta Mound indicate that the contrast in mineral assemblages between these mounds is caused by differences in fluid and methane fluxes. Alpha Mound appears to be affected by strong fluctuations in the fluid flow, causing shifts in redox boundaries, whereas Beta Mound seems to be a less dynamic system. To a large extent, the diagenetic regimes within cold‐water coral mounds on the Pen Duick Escarpment appear to be controlled by fluid and methane fluxes deriving from layers underlying the mounds and forcings like pressure gradients caused by bottom current. However, it also becomes evident that authigenic mineral assemblages are not only very sensitive recorders of the diagenetic history of specific cold‐water coral mounds, but also affect diagenetic processes in turn. Dissolution of aragonite, lithification by precipitation of authigenic minerals and subsequent brecciation of these lithified layers may also exert a control on the advective and diffusive fluid flow within these mounds, providing a feedback mechanism on subsequent diagenetic processes.