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Marine radiaxial fibrous calcites are common pore-filling carbonate fabrics in Palaeozoic and Mesozoic carbonates, yet recent analogues are scarce. Although ancient marine radiaxial fibrous calcites are often considered a cement phase and used for palaeoenvironmental interpretations, their origin (primary precipitation versus diagenetic alteration of a pre-existing fabric) and their specific formation processes remain ambiguous. This may be related to insufficient information from a few recent analogues and limitations of traditional methods in investigating ancient radiaxial fibrous calcites. This study documents a Holocene occurrence of marine pore-filling radiaxial fibrous Mg calcites and focuses on their origin, formation processes and implications for ancient analogues. The radiaxial fibrous Mg calcites examined in this study share morphological and mineralogical similarities with numerous ancient marine radiaxial fibrous calcites with a Mg calcite precursor. Based on petrological and geochemical data, the radiaxial fibrous Mg calcites are interpreted as secondary porewater products formed during early burial alteration of micritic Mg calcite precursor cement. These findings provide implications for, at least in part, ancient marine radiaxial fibrous calcites in the context of the formation process and mechanism and are relevant for palaeoceanographic and palaeoenvironmental interpretations based on the presence of this carbonate phase.

Marine calcite cement's crystal habit is often considered a function of the fluid Mg/Ca ratio. In contrast, marine aragonite fabrics are commonly described as acicular (needle) cement with pointed terminations and width-to-length ratios in the order of 1:10. Similarly, botryoidal or spherulitic aragonite cements are well-known from Mesozoic (and older) reefal and slope depositional environments but are less common in Recent depositional environments. Here, we explore a wide range of abiogenic aragonite cement habits (morphologies) in caves and coastal marine depositional environments. We propose that the cement habit represents a novel (and underexplored) archive of diagenetic environment, fluid chemistry and precipitation kinetics. Based on SEM imaging, we find the often-described acicular and fibrous fabrics but also a plethora of less well-known morphotypes such as columnar (often pseudo-hexagonal prisms) and lath, tabular and sheet-like (prismatic single crystals) forms. Twinning of (flat) needle aragonite is observed and might point to precursor phases. Based on the data available, the common needle-type aragonite cement typifies normal marine diagenetic (porewater) environments. Increasingly complex habits (polysynthetic twins, flat needles, pseudo-prismatic and prismatic sheet-like) aragonite crystals) are present under increasing levels of restriction and precipitate from high-salinity porewaters. Intriguingly, pseudo-prismatic aragonite crystals are also present in meteoric, gravity-defying cave carbonates, specifically helictites. Aragonite cement habits are documented and placed into context with their depositional and diagenetic environment, and preliminary interpretations are presented.

Funding: This study was supported by the Interreg NWE Programme through the Roll-out of Deep Geothermal Energy in NorthWest Europe (DGE-ROLLOUT) Project (www.nweurope. eu/DGE-Rollout). The Interreg NWE Programme is part of the European Cohesion Policy and is financed by the European Regional Development Fund (ERDF). Further funding was provided by the Bundesministerium für Wirtschaft und Energie of NRW and the Ruhr-University Bochum Research Department under the Closed Carbon Cycle Economy project. A multi-proxy field and laboratory study was conducted to investigate the impact of a regional fault zone on Devonian carbonate geothermal reservoir properties. The outcrop analogue chosen is exposed in the Steltenberg quarry (North Rhine-Westphalia, Germany) and provides access to Devonian lime- and dolostone units affected by branches of the Ennepe Thrust Fault zone. Data presented include (i) field evidence from sedimentological and structural data, (ii) petrographic and mineralogical analyses, and (iii) geochemical and petrophysical data. Although the interpretation of the depositional environment is not trivial, the carbonate facies present generally reflect the shedding of reefal material from large bioherms to the west of the study areas as well as localised, small bioherms prone to storm disintegration. The inherited properties of these carbonates at deposition, and a clear understanding of their diagenetic history, are the prerequisite for extrapolation between outcrop analogues and their corresponding subsurface units. We document a detailed interpretation of the paragenetic sequence that affected these rocks from deposition, burial, subsequent hydrothermal overprint related to rock disintegration and fluid circulation along the fault zone, and finally, late-stage meteoric overprint. Emphasis is placed on various forms of dolomitisation and its effect on rock properties and reservoir quality. The resulting scenario is complex, and when comparing the data from the Steltenberg quarry with nearby outcrops, significant regional heterogeneity is observed. It is important to separate between: (i) diffuse, locally distributed dolomitisation, (ii) replacive dolomitisation (fronts), spatially separating lime- and dolostones, and (iii) fault-bound, hydrothermal dolomitisation. The Devonian carbonates in the Steltenberg quarry examined here have higher thermal conductivity compared to the Jurassic (Malm) carbonate units in the Munich area, with some even higher than the values of massive limestone facies of the Munich area. As a large portion of North Rhine-Westphalia’s territory is underlain by Devonian carbonates, this is considered significant. However, emphasis is placed on the fact, that data shown here document the spatially complex organisation of rock types and their reservoir properties. This presents a warning against oversimplifications when extrapolating observations from outcrops into subsurface rock bodies. Publisher PDF

Direct studies of recent epeiric seawater chemistry and their geological significance are relatively few compared to the far more abundant studies from ancient epeiric marine carbonates. Acknowledging the limitations of analogue studies in the modern glaciated world, we propose that the recent Abu Dhabi offshore and lagoonal areas share important attributes with the shallow-marine coastal zones of a subtropical epeiric carbonate sea. The study area in Abu Dhabi is dwarfed by the vast epeiric seas of the geological past, but its seawater properties and related geochemistry have relevance for paleoenvironmental interpretations for similar ancient systems. This study documents the geochemical properties of seawater along the Abu Dhabi coast, and discusses their trends within a spatial context. Results show a positive correlation between salinity, temperature, elemental concentrations, and oxygen isotopic composition of seawater, indicating a significant influence of evaporation on seawater chemistry. A distal-to-proximal negative correlation between seawater Mg/Ca and Sr/Ca ratios may be related to a landward increase in abundance of high-Mg calcite foraminifera. After normalization to chloride, seawater elemental (Ca2+, Mg2+, Sr2+) concentrations and alkalinity show decreasing trends in a landward direction, consistent with seawater aging from the open marine to restricted lagoonal settings. Lower ΔAlk (alkalinity change) : ΔCa2+ (Ca2+ change) ratios in seawater compared with the theoretical value, and more negative δ13CDIC values relative to typical (sub)tropical oceans in the study area indicate an influence of factors apart from CaCO3 precipitation on properties of aged seawater. An input of alkalinity and 12C-enriched carbon by organic carbon decomposition (via sulfate reduction) during early diagenesis is proposed to account for these deviations from typical values. The input of alkalinity and 12C-enriched carbon by organic carbon decomposition may be further related to reduced seawater pCO2 and early-marine cementation. Findings discussed here suggest that organic carbon decomposition during early-marine diagenesis may influence the geochemical properties of the coastal areas of shallow epeiric seas, and thus assumedly produce significant differences in proxy data compared with those from carbonates deposited in open oceans.

Carbonates that exhibit obvious diagenetic alteration are usually excluded as archives in palaeoenvironmental studies. However, the potential impact of microbial alteration during early diagenesis is still poorly explored. To investigate the sensitivity of sulphur concentration, distribution, oxidation state and isotopic composition in marine aragonite to microbial alteration, Arctica islandica bivalves and Porites sp. corals were experimentally exposed to anaerobic microbial activity. The anoxic incubation media included a benthic bacterial strain Shewanella sediminis and a natural anoxic sediment slurry with a natural microbial community of unknown species. Combined fluorescence microscopy and synchrotron‐based analysis of the sulphur distribution and oxidation state enabled a comparison of organic matter and sulphur content in the two materials. Results revealed a higher proportion of reduced sulphur species and locally stronger fluorescence within the pristine bivalve shell compared to the pristine coral skeleton. Within the pristine bivalve specimen, reduced sulphur was enriched in layers along the inner shell margin. After incubation in the anoxic sediment slurry, this region revealed rust‐brown staining and a patchy S2‐ distribution pattern rather than S2‐‐layers. Another effect on sulphur distribution was rust‐brown coloured fibres along one growth line, revealing a locally higher proportion of sulphur. The δ34S value of carbonate‐associated sulphate remained largely unaffected by both incubation media, but a lower δ34S value of water‐soluble sulphate reflected the degradation of insoluble organic matter by microbes in both experiments. No significant alteration was detected in the coral samples exposed to microbial alteration. The data clearly identified a distinct sensitivity of organically bound sulphur in biogenic aragonite to microbial alteration even when ‘traditional’ geochemical proxies such as δ18OCARB or δ13CCARB in the carbonate did not show any effect. Differences in the intensity of microbial alteration documented are probably due to inherent variations in the concentration and nature of original organic compositions in the samples.

Holocene marine sediment from proto‐Florida Bay has been eroded, suspended, transported landward and has infiltrated downward (to 1 m depth) to fill secondary pores in subaerially exposed Late Pleistocene limestone. Sediment mineralogy (up to 95% aragonite), sediment texture (mostly needle‐like

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.

International audience; Diagenesis of carbonate minerals is ubiquitous throughout the geologic record. Alteration is initiated immediately after deposition, or takes place in the endo- and exoskeletons as early as during the lifetime of a given carbonate-secreting biota, and can continue throughout the burial history of carbonate sediments and rocks. Variations in the diagenetic response of carbonate archives pose challenges for the reconstruction of past environmental conditions based on proxy data. This paper comparatively assesses alteration features of different aragonitic materials by experimentally-induced diagenesis. A multitude of factors lead to different reactivity and responses of a given aragonite archive, and provide insight to the interpretation of diagenetically altered material in the rock record. Chosen materials used in this study include relatively organic-rich samples such as coral skeletons and bivalve shells, and organic-lean abiotic carbonates such as speleothems and aragonite single crystals. Obtained datasets include distributions of elements, organics, carbon and oxygen isotope ratios, and crystallographic features. Observed variations in diagenetic responses include mineralogy of the diagenetic phases, rate and extent of mineral transformation, distribution of foreign ions in the crystal lattice (primarily Sr, Mg, and S), and the number of specific processes and products along diagenetic pathways. Alteration is shown to be primarily controlled by the initial diagenetic susceptibility of the sample (including porosimetry and structural characteristics, concentrations of organic material, and primary amounts of trace elements in the carbonate, such as Mg). Structural characteristics lead to initial internally “fluid-” or “rock-buffered” conditions, with low porosity and permeability resulting in a greater effect of internal fluids and organics. Differences in the amount of organic content and internal fluids affect transformation rates, secondary mineralogy, and isotope equilibria. Samples with relatively high porosity, high mineral transformation rates, and high primary Mg/Ca, may preferentially form secondary aragonite during fast equilibration with the diagenetic environment. Our results suggest that the degree and nature of diagenetic alteration of aragonite materials are strongly controlled by the micro- to nano-scale internal architecture governing the availability and transfer of aqueous fluids. Results of this study provide significant implications for the interpretation of diagenetic signals in carbonate archives, and have direct significance for the mechanistic understanding of carbonate diagenesis and (paleo)environmental conditions.

Live and dead benthic foraminifera assemblages were studied from 50 samples collected in a lagoon located between Yas Island and Ras al Gurhab Island (UAE) in a system dominated by carbonate sedimentation.Living and dead foraminifera tests are present at all of the sampled locations. The foraminifera assemblage is dominated by a high diversity of miliolidae together with epiphytic larger benthic foraminifera belonging to the genera Peneroplis, Spirolina and Sorites. Hyaline foraminifera, such as Ammonia and Elphidium, are commonly found at all the locations while agglutinated foraminifera are uncommon and have a scattered occurrence.The abundance and diversity of benthic foraminifera were calculated for each sample. Four benthic foraminifera ecological indices were applied to the studied samples. For each of the samples we calculated: the total foraminiferal number (number of foraminifera in 1 g of sediment >125 μm); the percentages of agglutinated, porcellaneous and hyaline foraminifera tests; the ratio between living and dead benthic foraminifera; the ratio between larger benthic foraminifera with normal and abnormal test growth. The above-mentioned data have been applied to construct a foraminiferal assemblage database that facilitates the discrimination between inner and outer lagoonal environments.

The deeply buried (>7 km) upper Ediacaran (Sinian) Dengying Formation (ca. 551.1–541 Ma) in the Sichuan Basin, China, is the largest Precambrian dolostone gas reservoir worldwide. Gas exploration from the Dengying Formation, however, is hampered by a limited understanding of its complex dolomitization history and porosity evolution. New petrological, geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation’s complexities. Microbialite, dolo-grainstone, and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo-mudstone and less common microbialite lithologies from the low-energy platform interior. Spatially-variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization, often inducing secondary porosity. The various forms of dolomitization (sabkha, reflux, and burial dolomitization, and cementation) increased the resistivity of these rocks to chemical and physical compaction and porosity destruction. In the deep burial domain, medium- to coarse-crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120–160 °C and 160–220 °C, respectively. The significance of deep burial dissolution is critically discussed and constrained by: (i) corrosion of late diagenetic minerals, (ii) pores cross-cutting (or forming around) stylolites, and (iii) the occurrence of solid bitumen in the center of secondary pores. Vuggy, inter- and intracrystalline pores developed during a late and deep burial stage, and comprise ca. 20% of the overall Dengying reservoir porosity. Mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction. Similar to many carbonate reservoirs in the Phanerozoic, this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis. Data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried Precambrian dolostone reservoirs.

Early marine diagenetic dolomite is a rather thermodynamically‐stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post‐depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U‐Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike‐slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric‐preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto‐Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high‐grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late‐stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically‐complex dolomite archives must be considered with caution., DFG http://dx.doi.org/10.13039/501100001659

Micrite envelopes are common early diagenetic features in marine carbonates throughout the Phanerozoic. This paper documents a continuum of (sub)Recent, constructive and destructive (composite) micrite envelopes and discusses their application for the recognition of ancient marine firm- to hardgrounds. In the modern subtropical carbonate depositional environment of the Abu Dhabi coastal lagoon, micrite envelopes are actively forming and are directly assigned to a range of processes and depositional environments: (i) in the outer, high-energy lagoon, bioclasts and ooids typically contain empty microendolithic borings; (ii) in the unconsolidated sediments of the inner-to-middle lagoon, destructive micrite envelopes yield fragments of their host grains that result from boring/etching activity, and are bound and covered by biofilms; (iii) constructive envelopes are formed by high-Mg calcite micrite cements and coat destructive envelopes in the inner-to-middle lagoon, or may nucleate on aragonite needle cements in the outer lagoon. Micrite cements that form constructive envelopes are typical early marine diagenetic features and are among the main fabrics of firm- to hardgrounds. Calcitic skeletal grains, such as benthic foraminifera and coralline red algae, are less affected by microbioeroders compared to aragonitic ooids, gastropods and bivalves. High-Mg calcite constructive envelopes have a higher preservation potential relative to aragonite host particles and aragonite cements. Hence, the presence and recognition of constructive micrite envelopes in the rock record may be instrumental to identifying ancient aragonitic as well as unconventional firm- to hardgrounds that may otherwise be difficult to recognise.

Biogenic carbonates are important archives for reconstructing the marine sulfur cycle. However, uncertainties exist about the exact location of carbonate associated sulfate (CAS) and a possible biological control on sulfate incorporation. The behavior of CAS in biogenic carbonates during diagenetic alteration is even more poorly constrained. To investigate the mechanisms of sulfur incorporation and the effects of alteration on sulfur in biogenic carbonates, modern marine bivalve shells of Arctica islandica species were hydrothermally altered at 100 degrees C and 175 degrees C. Fluorescence microscopy, element mapping via NanoSIMS and mu-XRF, sulfur XANES analyses, and delta S-34 measurements were performed on the experimentally altered shell segments. Changes in elemental compositions and delta S-34 of sulfate in the post-alteration solutions were also determined. Results indicate clear differences between the delta S-34 values of the CAS (+ 21 parts per thousand, V-CDT) that reflects ambient seawater sulfate and the organically bound sulfur that is isotopically lighter (+ 14.8 parts per thousand, V-CDT or less). Carbonate associated sulfate is primarily incorporated in the mineral phase of the shell, whereas reduced sulfur phases are mainly found within the intrashell organic matter. Hydrothermal alteration experiments at 100 degrees C resulted in minimal changes of sulfur within the bivalve shells. In contrast, the 175 degrees C experiments triggered decomposition of intrashell organic matrices which then led to extensive diagenetic alteration in both the shell microstructure and chemistry. Changes in total concentration, speciation, and spatial distribution of sulfur reflect the diagenetic processes that occurred within the shells. Preferential incorporation of CAS in a neomorphic calcite phase with Mg/Ca ratios of 0.13-0.21 was observed. Due to its presence in both organic and inorganic phases and its multiple oxidation states with different isotopic compositions we conclude that sulfur is a useful and sensitive proxy for diagenetic alteration in biogenic aragonite.

Microbially mediated carbonates form in numerous environments and produce a range of lithologic fabrics and textures. While the environmental and biological factors controlling microbial carbonate precipitation are becoming more established, the influence of precursor microbial textures on diagenetic processes is less understood. Here we describe microbial oncolites from Mare, New Caledonia, to assess their formation, textures, and subsequent diagenetic alteration. Calcified filaments and sheets are interpreted as microbial-mat constituents (bacterial sheaths and EPS sheets) within a constructive micritic cortex of uniform crystal size (5–10 μm) and shape formed around a nucleus grain. Thin-section analyses show trapped and bound grain inclusions and irregular laminations within the cortex, also consistent with a microbial origin. The combination of a dense micrite cortex, subrounded shape, and irregular laminations are consistent with oncoid formation under periodic wave agitation in a back-reef setting. The Mare oncolites also display an unexpected pseudomorphic stabilization of the nucleus grains and cortex, likely due to the original mineralogy and low permeability of the cortex micrite. To our knowledge, this is the first example of microbial coatings leading to pseudomorphic stabilization of aragonite and high-Mg calcite skeletal grains. This textural preservation lies in stark contrast to the extensive dissolution and moldic porosity found outside the oncolite beds and common to many tropical shallow-water carbonates. These findings have implications for development of meteoric porosity, the interpretation of δ13C values in marine carbonates, and potential controls on fluid flow.