Early Diagenetic Processes Research Articles

Molecular indicators of early stage diagenesis in the tropical coastal Bolgoda Lake, Sri Lanka

Bacterial reactions are one of the major processes that control nutrient cycles in sediments. The aim of this study was to trace the bacterial biomarkers (hopanes) to understand early diagenetic processes in tropical mangrove dominant brackish sediments over modern timescales. Hopane homologues were examined by gas chromatography-mass spectrometry (GC-MS). The results suggested that hopane concentrations were high in Bolgoda Lake surface sediments (average ± SD = 1336.5 ng/g TOC ± 748.7) compared to mangrove mud cores (676.4 ng/g TOC ± 268.5). Therefore, the variation of bacterial biomasses can be controlled by the available reactive organic matter type and depositional environments (sulfate concentration). Total hopane concentrations gradually declined with the depth in core samples. In addition, total hopane concentrations showed moderate and weak correlations with organic matter types in upper (0–60 cm) and lower (> 60 cm) sedimentary successions, respectively. These features in core samples suggest the formation of geohopanoids during diagenesis. The variations of C31 hopane isomerisation ratio suggest the minor influence of reworking bacterial biomarkers in the upper sedimentary succession. Therefore, C31 22S (αβ) hopane distribution can indicate soil acidity (chemical origin) in the mangrove dominant swamp.

Petrography, micromorphology and geochemistry of Mio-Pleistocene palustrine limestones in Kuwait, Arabian Gulf

Elongated lenses of palustrine limestone interbedded with Mio-Pleistocene fluvial sands were exposed within a sand quarry in the north of Kuwait. Detailed field investigations, thorough petrographic and nanoscopic examinations and geochemical analysis revealed that they were originally precipitated as carbonate microbialite in isolated freshwater ponds scattered within a fluvial floodplain. These lacustrine microbialites were subjected to intermittent drying and wetting conditions and eventually altered to pseudobrecciated nodular palustrine limestone through several pedogenic and early diagenetic processes. The pedogenesis is exhibited by cracking, nodulization, mottling, pseudomicrokarst and root traces. The desiccation macrocracks are filled with siliciclastic sands derived from short-lived flash floods. Diagenesis is represented by aggradational neomorphism of micrite, cementation of cracks and voids by sparry calcite and precipitation of biogenic needle-fibre calcite. Concurrently, the cracks filled with siliciclastic sands have been calcretized, forming an embedded interapalustrine calcrete. Mottling postdates the early diagenesis and is manifested by pseudoclotted texture and abundant black spots and dendrites, which may have been developed by the marmorization of iron and manganese ions due to the seasonal fluctuation of groundwater. The δ13C and δ18O values are mostly comparable with those of closed freshwater lakes; however, the high negative values of δ18O may be attributed to diagenetic calcite. The occurrence of this palustrine limestone within a fluvial siliciclastic sequence that lacks clearly developed calcrete profiles may present an exceptional setting for palustrine limestone.

Silurian records of carbon and sulfur cycling from Estonia: The importance of depositional environment on isotopic trends

The Llandovery–Wenlock carbonate-marl succession of the Baltoscandian Basin has been analyzed in the Viki drill core, western Estonia, for carbonate carbon (δ13Ccarb), organic carbon (δ13Corg), carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyr) isotopes, along with trace element concentrations. Following the End-Ordovician glaciation and the Hirnantian carbon isotope excursion, δ13Ccarb values rise to an early Llandovery carbon isotope maximum (+2‰). Subsequently, δ13Ccarb and coeval δ13Corg record a large ∼+4.5‰ excursion in the early Wenlock, associated with a regression and biotic turnover known as the Ireviken bioevent (IBE). Overall, δ34SCAS values generally fall within a range (25‰ to 35‰) similar to that reported in other sections of comparable age. Two distinct trends are observed: low and stable δ34SCAS values are associated with deep-water facies, while δ34SCAS becomes higher and more variable in the shallow-water facies. Similarly, δ34Spyr values are low and invariant (∼−15‰) in deep-water facies but become higher and more variable (ranging between −35‰ and 16‰) in shallow-water deposits. Despite abundant recrystallization, no obvious evidence for large-scale alteration of proxies is apparent in concentrations of Fe, Sr, Mn and Mg, or in isotopic correlations, although increased variability and lower values of δ34SCAS are observed where CAS abundance is low. A large, well-resolved positive excursion in δ34SCAS observed during the IBE from a time-correlative section on Gotland (Sweden) is absent in the data presented here. This difference demonstrates that, counter to general understanding, local environmental variations can give rise to divergent δ34SCAS profiles in coeval sections. The S-isotope fluctuations in CAS and pyrite in the Viki drill core can be explained by local processes involving facies changes, early diagenetic processes and associated open vs. closed system behavior that can overprint and potentially mask basin or global signals.

Microbially influenced formation of Neoarchean ooids.

Ooids are accretionary grains commonly reported from turbulent, shallow-water environments. They have long been associated with microbially dominated ecosystems and often occur in close proximity to, or embedded within, stromatolites, yet have historically been thought to form solely through physicochemical processes. Numerous studies have revealed both constructive and destructive roles for microbes colonizing the surfaces of modern calcitic and aragonitic ooids, but there has been little evidence for the operation of these processes during the Archean and Proterozoic, when both ooids and microbially dominated ecosystems were more widespread. Recently described carbonate ooids from the 2.9Ga Pongola Supergroup, South Africa, include well-preserved examples composed of diagenetic dolomite interpreted to have formed from a high-Mg-calcite precursor. Spatial distributions of organic matter and elements associated with metabolic activity (N, S, and P) were interpreted as evidence for a biologically induced origin. Here, we describe exceptionally well-preserved ooids composed of calcite, collected from Earth's oldest known carbonate lake system, the ~2.72Ga Meentheena Member (Tumbiana Formation), Fortescue Group, Western Australia. We used optical microscopy, Raman spectroscopy, XRD, SEM-EDS, LA-ICP-MS, EA-IRMS, and a novel micro-XRF instrument to investigate an oolite shoal deposited between stromatolites that preserve abundant evidence for microbial activity. We report an extremely fine, radial-concentric, calcitic microfabric that is similar to the primary and early diagenetic fabrics of calcitic ooids reported from modern temperate lakes. Early diagenetic silica has trapped isotopically light and thermally mature organic matter. The close association of organic matter with mineral phases and microfabrics related to primary and early diagenetic processes suggest incorporation of organic matter occurred during accretion, likely due to the presence of microbial biofilms. We conclude that the oldest known calcitic ooids were likely formed through processes similar to those that mediate the accretion of ooids in similar environments today, including formation within a microbial biosphere.

Solute Reservoirs Reflect Variability of Early Diagenetic Processes in Temperate Brackish Surface Sediments

Coastal marine sediments are a hotspot of organic matter degradation. Mineralization products of early diagenetic processes accumulate in the pore waters of the sediment, are subject of biological uptake and secondary biogeochemical processes and are released back into the water column via advective and diffusive fluxes across the sediment-water interface. Seven representative sites in the shallow coastal area of the southern Baltic Sea (15–45 m water depth), ranging from permeable sands to fine grained muds, were investigated on a seasonal basis for their key mineralization processes as well as their solid phase and pore water composition to identify the drivers for the variability of early diagenetic processes in the different sediment types. The sandy sediments showed about one order of magnitude lower organic carbon contents compared to the muds, while oxygen uptake rates were similar in both sediment types. Significantly higher oxygen uptake rates were determined in two near-shore muddy sites than in a deeper coastal muddy basin, which is due to higher nutrient loads and the corresponding addition of fresh algal organic matter in the near-shore sites. Pore water concentration profiles in the studied sediments were usually characterized by a typical biogeochemical zonation with oxic, suboxic, and sulfidic zones. An up to 15 cm thick suboxic zone was sustained by downward transport of oxidized material in which dissolved iron and phosphate indicate an intensive reduction of reactive Fe with the release of adsorbed phosphorus. While the geochemical zonation was stable over time in the muds of the studied deeper basin, high variability was observed in the muds of a near-coastal bay probably mainly controlled by sediment mixing activities. The sediments can be characterized by essentially two factors based on their near-surface benthic solute reservoirs: (1) their organic matter mineralization and solute accumulation efficiency and (2) their redox-state. Benthic solute reservoirs in the pore waters of the top decimeter were generally higher in the muddy than in the sandy sediments as the more permeable sands were prone to an intensive exchange between pore water and bottom water. The three studied muddy sites showed great dissimilarities with respect to their predominating redox-sensitive metabolites (dissolved iron, manganese, and sulfide). Surface-near advective transport like irrigation of permeable sands and rearrangement of cohesive muds had a particularly strong influence on early diagenetic processes in the studied sediments and were probably the most important cause for the spatiotemporal variability of their benthic solute reservoirs.

Rare Earth Elements in early-diagenetic foraminifer ‘coatings’: Pore-water controls and potential palaeoceanographic applications

Rare Earth Element (REE) distributions in the ocean bear the fingerprints of several key environmental processes, including vertical particle/organic carbon fluxes, water column/pore-water oxygenation and ocean transports. The use of ‘fossil’ REE analyses in the service of palaeoceanography as redox, water transport or nutrient cycling ‘proxies’ has long been a tantalizing possibility. Here we demonstrate the application of a novel laser-ablation microanalysis approach for the rapid and accurate measurement of the REE composition of early diagenetic ‘coatings’ on fossil foraminifera. By applying this new method to a range of core-top and multi-core samples, we show that ‘authigenic’ REE enrichments on planktonic foraminifer surfaces (REEfs) reflect a primary seawater signature that becomes overprinted during sediment burial due to early diagenetic processes that control the flux of REEs to pore-fluids. Thus ‘light’ REEs (LREEs), and eventually ‘middle’ REEs (MREEs) are generally enriched in foraminifer 'coatings' relative to seawater, while Ce-anomalies (Ce/Ce*) recorded in surface sediments are typically more positive than local seawater values and are further ‘eroded’ during burial with the onset of anoxic conditions in the sediment. Similar patterns have previously been observed in pore-fluid measurements. Indeed, we show that Mn and Fe concentrations measured in foraminifer ‘coatings’ track the availability of these elements in pore-water, indicating that they are not associated with a secondary oxide phase. We propose that these elements, along with REEs are instead adsorbed directly from pore-fluids. In contrast, U in authigenic coatings tracks the removal of this element from solution under sub-oxic conditions, supporting the use of U/Ca in foraminifer coatings as a redox proxy. Although our results confirm a significant early diagenetic influence on REEfs, we also demonstrate the potential utility of ‘Ce-enrichment’ relative to expected seawater values as a palaeo-oxygenation proxy. We support this proposal with down-core measurements of U/Ca and Ce-enrichment spanning the last deglaciation in the sub-Antarctic Atlantic, as well as a global array of LGM measurements, which are found to co-vary with parallel estimates of radiocarbon ventilation age, consistent with a widespread drop in oxygen suggesting an increase in the efficiency of the biological pump. Our results suggest that laser-ablation REEfs measurements may hold some promise for palaeoceanographic reconstruction, in particular through a shift in emphasis away from the reconstruction of primary seawater signals to the analysis of diagenetic impacts that are sensitive to changing hydrographic conditions.

Manganese incorporation in living (stained) benthic foraminiferal shells: a bathymetric and in-sediment study in the Gulf of Lions (NW Mediterranean)

Abstract. Manganese geochemistry in deep-sea sediments is known to vary greatly over the first few centimeters, which overlaps the in-sediment depth habitats of several benthic foraminiferal species. Here we investigated manganese incorporation in benthic foraminiferal shell carbonate across a six-station depth transect in the Gulf of Lions, NW Mediterranean, to unravel the impacts of foraminiferal ecology and Mn pore water geochemistry. Over this transect water depth increases from 350 to 1987 m, while temperature (∼13 ∘C) and salinity (∼38.5) remained relatively constant. Manganese concentrations in the tests of living (rose bengal stained) benthic foraminiferal specimens of Hoeglundina elegans, Melonis barleeanus, Uvigerina mediterranea, and Uvigerina peregrina were measured using laser ablation inductively coupled mass spectrometry (laser ablation ICP-MS). Pore water manganese concentrations show a decrease from shallow to deeper waters, which corresponds to a generally decreasing organic-matter flux with water depth. Differences in organic-matter loading at the sediment–water interface affects oxygen penetration depth into the sediment and hence Mn pore water profiles. Mn ∕ Ca values for the investigated foraminiferal species reflect pore water geochemistry and species-specific microhabitat in the sediment. The observed degree of variability within a single species is in line with known ranges in depth habitat and gradients in redox conditions. Both the Mn ∕ Ca ratio and interspecific variability hence reflect past Mn cycling and related early diagenetic processes within the sediment, making this a potential tool for bottom-water oxygenation and organic-matter fluxes. Dynamics of both in-sediment foraminiferal depth habitats and Mn cycling, however, limit the application of such a proxy to settings with relatively stable environmental conditions.

A case study of Ca-rich fluid flow in Nimura Cu-Co mine. DRC, Central African Copperbelt

The Neoproterozoic Central African Copperbelt is the one of the largest sediment-hosted stratiform Cu–Co provinces in the world, with 140 Mt of copper and 6 Mt of cobalt, including several world-class deposits (P10 Mt copper). The Congo and Zambia stratiform deposits have characteristics indicating that syngenetic and early diagenetic processes have controlled the formation of the central Africa Neoproterozoic Copperbelt and studies related to diagenetic processes in sedimentary diagenetic processes where rocks triggered the diagenetic model for copper orebodies. For the dolomitic gangue major element concentrations associated with the second phase of the mineralization in the Nimura mine; the ore deposit has been determined and compared to those enriched, depleted than those, which would have remained immobile during the alteration processes. Moreover, it has been observed that FeO is often enriched, compared to SiO2, whose mass has been decreasing. The circulated fluid was almost carbonated, characterized by CaO, which has remained immobile during the alteration process. The Cu–Co mineralization is at least mainly based in the lower ore body, controlled by lithology, and tectonic. Gangue minerals are mostly dolomite and quartz with accessory Mg–chlorite and muscovite.

The lower Maastrichtian chalk in the Ghezala-Mateur area, northern Tunisian domain: Sedimentary environments and early diagenetic features

The lower Maastrichtian chalks of the Ghezala-Mateur area, northern Tunisian domain, were investigated with regard to their palaeogeographic implications. Two lateral facies have been identified (i) the white chalk facies occurs to the NW, NE and SE strikethrough of the late Cretaceous synsedimentary El Aouana Graben and, (ii) the chalky limestone facies occupied the central part of the Graben.Sedimentary features, fossil content and trace fossils distribution reveal that sedimentation occurred in a slightly reducing shallow-water environment, probably corresponding to an outer platform setting, characterized by frequently interrupted sedimentation and conspicuous activity of erosional currents. Both features seemingly contributed to the genesis of numerous hiatal surfaces marked by the widespread activities of Thalassinoides trace fossils. The genesis of hiatal surfaces was probably promoted by recurrent burial-early diagenesis-exhumation cycles in which Thalassinoides burrows were preferentially affected by early diagenetic processes. Nodular chalky limestone, intraformational conglomeratic chalky limestone and knoby chalky limestone are the most common lithologies resulted from the early diagenesis of Thalassinoides-omission surfaces.

Phanerozoic shallow marine sole marks and substrate evolution

Research Article| August 01, 2018 Phanerozoic shallow marine sole marks and substrate evolution Lidya G. Tarhan Lidya G. Tarhan * 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA *E-mail: lidya.tarhan@yale.edu Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (9): 755–758. https://doi.org/10.1130/G45055.1 Article history received: 19 Apr 2018 rev-recd: 29 Jun 2018 accepted: 04 Jul 2018 first online: 01 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation Lidya G. Tarhan; Phanerozoic shallow marine sole marks and substrate evolution. Geology 2018;; 46 (9): 755–758. doi: https://doi.org/10.1130/G45055.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract The presence of erosive sole structures such as tool and flute marks in the sedimentary record can be used to interpret flow dynamics of ancient storm-dominated settings, as well as seafloor sediment properties. The formation and preservation of these structures require a hydroplastic and cohesive substrate. Therefore, the temporal distribution of these structures can be employed to track secular changes in seafloor rheology, which can significantly alter early diagenetic processes and thus shape global biogeochemical cycling. Herein I present a global compilation of Phanerozoic shallow marine successions bearing erosive sole structures, derived from assessment of the sedimentological, tectonic, and paleontological literature. The frequency of sole mark preservation in shallow marine environments has dramatically declined through the Phanerozoic. This first-order trend suggests that the rheological nature of the seafloor has experienced major secular transformations over the past 541 m.y. This record provides evidence for a marked decrease in sediment cohesion and increase in sediment porosity of the uppermost seafloor sediment column through time. I attribute this change in seafloor sedimentary properties to increases in the extent of bioturbation through the Phanerozoic, because the activity of sediment-mixing infauna dramatically increases the fluid content of the shallow sediment pile, decreasing the hydroplasticity necessary for sole mark formation and preservation. The gradual decline in the frequency of abiogenic sole marks in shallow marine successions through the Phanerozoic indicates that the development of bioturbation has been a similarly protracted process. You do not currently have access to this article.

Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events

Floods carry sediments to river deltas and the coastal zone, but little is known about the geochemical evolution of this particulate material deposited over a short period of time. Here, we studied two recent contrasting flood deposits in the Rhône River prodelta area (northwestern Mediterranean Sea). We monitored the porewater and solid-phase chemistry over periods ranging from a few days to 6 months after deposition. Non-steady state diagenetic processes associated with episodic deposition promote a wide spectrum of transient redox conditions in the shallow prodelta region of the Rhône. Specific attributes of diagenetic responses depend on the sources of flood material and scale (thickness) of deposition.The first flood unit of 20–30 cm was composed of light gray mud, poor in organic carbon and rich in reactive manganese oxides. The short-term responses of early diagenetic processes contrasted with a rapid consumption of O2 and NO3- over a few hours just after the deposition event, accompanied by a slower build-up of Mn2+ concentration, and a lagged response in Fe2+ concentration over a few days or weeks. This difference was due to the redox capacity of the sediment, evolving from oxidized, during the flood layer deposition, to more reducing conditions, after a few days or weeks, allowing Fe2+ to build up and remain in solution. Sulfate reduction may have started within a few days within the flood deposit and was greatly enhanced just below the former redox front due to a fresh input of organic matter (OM). This large production of H2S probably led to the precipitation of sulfide minerals in close vicinity to the former redox front, limiting the accumulation of Fe2+ and H2S. The unit was sampled repeatedly three times during the six months following the flood event, and showed that manganese oxides were reduced at a rate of 1.8 mmol m−2 d−1, whereas the iron oxide concentration did not vary substantially.The second flood unit was composed of darker sediment, rich in organic carbon and reactive manganese oxides. The first step of OM degradation was enhanced within this dark deposit with a high release of dissolved organic carbon (DOC). A peak in Mn2+ concentration was also measured in association with the peak in reactive manganese oxides, also showing the rapid reduction of manganese after an input of fresh reactive oxides, and its potential increased release into the water column. Finally, a peak in nitrate concentration appeared in the sediment porewater within this anoxic organic/manganese-rich layer, likely resulting from the anaerobic oxidation of ammonium by manganese oxides.

Diagenetic effects on uranium isotope fractionation in carbonate sediments from the Bahamas

Uranium isotope variations (δ238U) recorded in sedimentary carbonate rocks are a promising new proxy for the extent of oceanic anoxia through geological time. However, the effects of diagenetic alteration on the U isotopic composition in carbonate sediments, which are crucial to understand the accurate reconstruction of marine δ238U, are currently poorly constrained. Here we examine the effects of the aragonite-to-calcite transition in the Pleistocene Key Largo Limestone of South Florida, and assess the effects of vadose meteoric, phreatic meteoric, and phreatic marine diagenesis on U isotope fractionation in carbonate sediments from the Bahamas Transect, including the well-studied Clino, Unda, and ODP Site 1006 drill cores.Our results suggest that early diagenetic processes in Bahamas carbonate sediments fractionate U isotopes by an average of 0.27 ± 0.14‰ (1 SD) heavier than contemporaneous seawater. Downcore variations of δ238U in slope and basin sediments display little, if any, correlation with U concentration and common geochemical indicators of diagenesis (δ13C, δ18O, Mn/Sr, Mg/Ca, Sr/Ca), enrichments of redox-sensitive elements, or rare earth elements anomalies. We propose two possible mechanisms to interpret the positive change in the δ238U during carbonate diagenesis: authigenic enrichment of isotopically positive U(IV) in carbonates and preferential incorporation of isotopically positive aqueous U(VI) species into carbonates. These processes likely operate during early (syndepositional) diagenesis on the banktop. Further diagenesis during deeper burial is limited by the low solubility of U(IV) under reducing pore water conditions.The early diagenetic behavior of U isotopes in Bahamas carbonate sediments is likely broadly representative of carbonate diagenesis in the geological past. We suggest that the mean diagenetic offset determined in this study be applied when reconstructing seawater δ238U from ancient carbonates. Furthermore, early diagenesis induces significant statistical variability in sediment δ238U values, pointing to the need for large, high resolution data sets in order to average out stochastic variations in individual bulk sediment samples.

Redox behaviour of arsenic in the surface sediments of the Marque River (Northern France)

The behaviour of arsenic(As) in anoxic sediments was studied in the Marque River (Northern France), where concentrations of As are close to the geochemical background level. The distribution of Fe, Mn, S and As species in the solid phase as well as in the pore waters was investigated during four sampling campaigns (February, April, July and October 2014) in order to better understand the parameters involved in the behaviour of arsenic during early diagenetic processes. In the solid phase, As was found to be mainly present in the exchangeable fraction, and the most probable As carrier phases appeared to be amorphous iron, manganese (hydr)-oxides, calcite, and siderite. In pore waters, only the inorganic forms of arsenic [As(III), As(V), and thio‑arsenical species] were detected. As(III) was the dominant species, but thio‑arsenic species were also evidenced in core depth. No direct interaction between arsenic, iron and manganese cycle has been observed in the pore waters. The behaviour of As is mainly linked to the cycle of sulfur, and especially to the production of sulfides. Overall, this study confirms that the experimental determination of redox speciation is still very useful to understand the behaviour of As during early stages of diagenesis, since application of thermodynamic modelling to redox sensitive environments is not sufficiently constrained yet to provide reliable results.

Coordination environment of Zn in foraminifera Elphidium aculeatum and Quinqueloculina seminula shells from a polluted site

Foraminifera, unicellular organisms that are widespread throughout marine ecosystems, build Ca-carbonate shells that may incorporate trace metals present in the ocean waters because of natural or anthropogenic supply. In this study, we focussed on the trace element Zn, which is abundant in both contaminated and clean waters. We used X-ray and electron spectromicroscopy to investigate the Zn coordinative environment in individual shells of two species of benthic foraminifera, Elphidium aculeatum and Quinqueloculina seminula, that were sampled from a heavy-metal polluted area of Sardinia, Italy. These species synthesise the Ca‑carbonate in extracellular and intracellular spaces, respectively, which implies some diversity in their physiologies and cation transport processes, and they can adapt and survive in metal-polluted environments.Our analyses of X-ray micro-fluorescence (μ-XRF) maps and Zn-K μ-X-ray absorption near-edge spectroscopy (XANES) reveal that although 50% of Zn occurs as a Ca substitute in calcite or as a tetracoordinate oxygen-adsorbed ion, detectable amounts can also be found in other Zn-independent mineral phases, particularly hydrozincite, whose formation is due to foraminiferal cellular processes. Other Zn phases, sphalerite (attributed to early diagenetic process) and Zn-phosphate, were recognised. Moreover, we found distinct differences in the Zn concentration, distribution and chemical speciation at the micro- and nano-metric scales of the investigated E. aculeatum and Q. seminula species. In the calcite needles of Q. seminula, Zn is uniformly distributed and recognised in a disordered local environment, which suggests that it is incorporated in the calcite phase during the intracellular calcification process. These findings offer insight into Zn incorporation in foraminifera and its potential application in biomonitoring and environmental studies.

Constraining the timing of brittle deformation and sedimentation in southern Finland: Implications for Neoproterozoic evolution of the eastern Fennoscandian shield

Sheared sedimentary clay was found in a faulted fracture in crystalline bedrock in a tunnelling site at 60 m depth in southern Finland. Brittle faults are numerous in the Fennoscandian Palaeoproterozoic bedrock, but only some of them have relative age constraints, while absolute ages are nearly lacking. Sedimentary rocks altogether are uncommon in Finland and only sparsely dated by micropaleontological studies. This study reports K–Ar data of fresh, non-weathered authigenic illite and constrains a time framework for the local faulting and sedimentation. The Neoproterozoic Tonian to Cryogenian ages derived from the grains in diminishing grain-size order are c. 967, 947, 809 and 697 Ma. Results indicate that the formation of the extension fracture is related to the collapse of Sveconorwegian orogeny c. 1000 Ma; clay and mature quartz sand were deposited in this extension fracture in shallow water in an intracratonic basin followed by early diagenetic processes and neocrystallization of illite around 967–947 Ma. The Neoproterozoic 1000–700 Ma sedimentation documented in this study is rare in the Fennoscandian shield as a whole. Neocrystallization of authigenic illite in the finest 0.4 and <0.1 µm fractions c. 809–697 Ma ago is interpreted as resulting from reactivation in the fault due to the continental break-up on the western side of the craton as documented by arenite crosscutting relationships. The younger ages may also be attributed to a Caledonian thermal overprint ca. 410 Ma ago that would influence the 967 Ma age if sufficient thermal energy had been present.

Origin of a kaolinite-NH4-illite-pyrophyllite-chlorite assemblage in a marine-influenced anthracite and associated strata from the Jincheng Coalfield, Qinshui Basin, Northern China

The coal from the Jincheng Coalfield, Qinshui Basin, is a medium- to low-ash yield anthracite. The mineral assemblage in the anthracite is dominated by NH4-illite and kaolinite; with varying proportions of pyrophyllite, chlorite, pyrite, and calcite; and minor anatase, rutile, diaspore, florencite, and synchysite. Kaolinite formed during early diagenetic and epigenetic processes has been largely altered to other phyllosilicates by processes associated with the different stages of coal rank advance. The formation of NH4-illite in the Jincheng coals is the result of illitization of an early diagenetic kaolinite precursor by interaction of kaolinite with nitrogen derived from organic matter. This reaction may have occurred at the latest stage of bituminization (subbituminous to bituminous transition). Pyrophyllite was produced by the interaction of kaolinite and quartz formed by metamorphic processes associated with anthracitization. Cookeite is relatively common in the studied coals. It may have been derived from the reaction of kaolinite with Li ions, although the source of Li is not yet clearly understood.Minerals in the non-coal samples (partings and floor strata) appear to have responded differently to the metamorphic conditions than those in the adjacent coals. Possible mechanisms for the formation of NH4-illite in the Jincheng non-coal rocks at least involve isomorphous substitution of NH4 for K in the K-illite structure. The well-crystallized K-illite in the non-coal rocks probably formed from detrital K-illite, and wasrecrystallized with the coal rank advance. Absence or minor proportions of pyrophyllite in non-coal samples may be due to a lack of epigenetic kaolinite in the non-coal horizons. Pyrophyllitization occurred during a late stage of metamorphism. The general absence of chlorite in the thick non-coal interval is probably due to the low permeability of that horizon, which resulted in a lack of infiltrating Fe- and Mg-bearing fluids.

Sedimentary environments and geochemistry of Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus

Upper Eocene and Lower Oligocene rocks in the northeastern Caucasus were examined in the most representative Chirkei section (Sulak River basin). Sharp lithogeochemical distinctions between them were revealed. The results of the study of nannoplankton demonstrated that the Eocene/Oligocene interface occurs slightly below the boundary between the Belaya Glina and Khadum formations. The studied section revealed a series of nannoplankton bioevents facilitating its stratigraphic subdivision. It has been established that organic matter (OM) in rocks of the Khadum Formation is characterized by a relatively high degree of maturity. Probably, the material of mainly marine genesis contains a terrigenous OM admixture. Positive oxygen isotope anomaly in the upper part of the Belaya Glina Formation reflects global climate changes (cooling) near the Eocene/Oligocene interface. Limitation of the anomaly by the upper boundary of the Belaya Glina Formation is likely related to changes in water salinity variations in the Early Oligocene basin and intense early diagenetic processes in rocks therein. Lithological, geochemical, and paleoecological data suggest that the Khadum paleobasin was depleted in oxygen. Such environment was unstable with periodic intensification or attenuation. Paleoecology in the Belaya Glina basin was typical of normally aerated basins.

A fresh look at ancient dungs: Brazilian Triassic coprolites revisited

Terrestrial tetrapod coprolites are abundant in the Middle to Late Triassic of southern Brazil, but only few specimens have been described in just a couple of papers. Here we revisit the already-known specimens and describe new materials, including their size, shape, external modifications, matrix composition and rare inclusions. Regarding size and shape, the measurements of 152 specimens show that the Triassic coprolites of Brazil follow a normal distribution, in which length and width are positively correlated variables, so that they are not useful for taxonomic purposes. Notwithstanding, two ichnotaxa, Santamariacopros elongatus and Rhynchocopros soutoi, were previously described from the Middle/Upper Triassic Santa Maria Formation, based mainly on morphological (length/width ratio) and preservational features, but we discuss here the validity of such taxa. Once the Santa Maria Formation encompasses at least three tetrapod Assemblage Zones (AZ), Dinodontosaurus AZ, Santacruzodon AZ and Hyperodapedon AZ (from the oldest to the youngest, respectively), it is difficult to attribute these coprolites to any specific animal producers. In addition, the rarity of inclusions and the carbonated composition of the coprolites, related to early diagenetic processes, complicate this attribution too. Nevertheless, some rare inclusions as a hair-like structure and a parasite egg were found in this study. Although the coprotaxonomy fails to represent the Santa Maria Formation coprolites and the recognition of their producers is a problematic task, these fossil dungs are important elements in the knowledge of the Middle to Late Triassic environments of southern Brazil.

Early diagenesis driven by widespread meteoric infiltration of a Central European carbonate ramp: A reinterpretation of the Upper Muschelkalk

Meteoric diagenesis of carbonate ramps is often difficult to interpret and can commonly be confused with other coinciding diagenetic processes. The Middle Triassic Upper Muschelkalk of Switzerland provides an insightful case in which the effects of several overprinting diagenetic environments, including matrix dolomitization, can be clearly unravelled. Previous studies suggested that diagenesis took place in connate marine waters, with later meteoric waters being invoked to explain recrystallization of dolomite. In this study, diagenetic analyses (C–O stable isotope ratios, thin-section point counting, cathodoluminescence and UV-fluorescence microscopy) of calcitic bioclastic samples have revealed that early diagenesis (pre-stylolitization) and the accompanying porosity evolution did not occur exclusively in the presence of marine fluids. Five sequential stages of diagenesis have been identified: marine, shallow burial, mixing-zone, meteoric and dolomitization. Marine diagenesis induced precipitation of bladed and inclusion-rich syntaxial cements that fluoresce strongly under UV-light. Both cements account for a mean 7.5vol% reduction in the porosity of bioclastic beds. Shallow burial diagenesis likely induced mouldic porosity and associated fluorescent dog-tooth cementation. Based on light oxygen isotope and elevated strontium isotope ratios, matrix aragonite–calcite neomorphism is interpreted to have occurred in a mixture of marine and meteoric fluids. The combination of shallow burial and mixing-zone processes reduced porosity on average by 4.8vol%. Evidence for subsequent meteoric diagenesis is found in abundant dog-tooth and blocky calcite cements that have mean δ18OVPDB of −9.36‰ and no signs of recrystallization. These meteoric cements reduced porosity by a further 13.4vol%. Percolation of meteoric water through the ramp was driven by hydraulic gradients on an adjacent basement high, which was exposed by a cycle of early Ladinian regressions. Following meteoric diagenesis the Upper Muschelkalk was dolomitized by refluxing brines. This complex history of diagenesis resulted in moderate porosities in dolomitized rocks (up to 20%), and low porosities (<5%) in calcitic bioclastic beds. These results are used to show that the present-day reservoir properties of non-dolomitized carbonate rocks, particularly bioclastic beds, can be largely attributed to early diagenetic processes. Thus, knowledge of the early diagenetic history and its regional controls provides a means to predict reservoir properties over wide areas between and beyond well sites.

Numerical modeling of early diagenetic processes in Haiyang 4 Area in the northern slope of the South China Sea

Early diagenesis affects the distribution of solutes and minerals in unconsolidated sediments. The investigation of diagenesis is critical to understanding the geochemical transformation and benthic fluxes of elements. During the cruise mission SO-177 in 2004, gravity coring samples were recovered in the Haiyang 4 Area of the northern slope of the South China Sea (SCS). The geochemical concentrations in interstitial water were determined onboard. The 1D C.CANDI reactive transport software was used to model the early diagenesis processes at four sites: 56-GC-3, 70-GC-9, 94-GC-11, and 118-GC-13. All of the simulations reproduced concentration profiles that matched the measurements with the implemented geochemical reactions. The degradation of organic carbon and anaerobic oxidation of methane (AOM) primarily determine the distribution of solutes in the working area. The degradation is active in the top 150 cm, and AOM is vigorous at depths below 200 cm. The local advective flux, sediment rate, and kinetic reaction constants of organic matter, methane and sulfate were calibrated based on the existing concentrations of pore water solutes. Geochemical reactions in this area occur in considerably deeper layers compared to depths cited in the literature. The model results provide evidence for the existence of deep hydrocarbon reservoirs that provide methane to the upper sediments.