115 results found
Bernasconi SM, Daeron M, Bergmann KD, et al., 2021, InterCarb: A Community Effort to Improve Interlaboratory Standardization of the Carbonate Clumped Isotope Thermometer Using Carbonate Standards, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 22
Anderson NT, Kelson JR, Kele S, et al., 2021, A Unified Clumped Isotope Thermometer Calibration (0.5-1,100 degrees C) Using Carbonate-Based Standardization, GEOPHYSICAL RESEARCH LETTERS, Vol: 48, ISSN: 0094-8276
Cruset D, Vergés J, Benedicto A, et al., 2021, Multiple fluid flow events from salt-related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees), Basin Research, ISSN: 0950-091X
Fluid systems in inverted rifted margins are challenging to interpret because fractures formed before compression were often reactivated acting as fluid pathways as new ones formed. Deciphering the fracture and fluid flow history in such complex settings has key implications for the prediction of the distribution of mineral resources. As an example, we reconstruct the fluid flow evolution of a portion of the inverted Pyrenean rift, the Upper Pedraforca thrust sheet, from the Mesozoic extension to the Alpine orogeny. We combine structural analysis and petrographic, geochemical and geochronological data obtained from 87 samples of fracture-filling carbonate cements. During the Late Jurassic-Early Cretaceous, low-temperature seawater produced dolomitization of Jurassic and Lower Cretaceous limestones in an extensional setting. During the Early Cretaceous salt-related extension, formation waters, probably evolved seawater or fluids that interacted with Triassic evaporites, at temperatures from 125 to 149°C migrated through fractures. The formation of breccias within post-salt rocks in primary weld zones facilitated the upward migration of formation waters that interacted with pre-salt rocks with high 87Sr/86Sr ratios. Formation waters at temperatures of 80°C migrated during the emplacement of the Upper Pedraforca thrust sheet in the Late Cretaceous-Palaeocene. These fluids interacted with Upper Cretaceous Carbonates and/or Triassic evaporites. In contrast, the influence of meteoric fluids increased in shallower positions due to the exhumation of the SE Pyrenees during the Eocene-Oligocene. Coevally, hot dolomitizing fluids migrated along diapir walls during the formation of secondary welds. Supergene ores documented in diapiric areas worldwide related to meteoric fluids, and the similar meteoric percolation occurred in the Upper Pedraforca thrust sheet, suggest that supergene mineralization could be found in the Pyrenees. Brecciation of rocks in primary weld zon
Cruset D, Cantarero I, Benedicto A, et al., 2020, From hydroplastic to brittle deformation: Controls on fluid flow in fold and thrust belts. Insights from the Lower Pedraforca thrust sheet (SE Pyrenees), MARINE AND PETROLEUM GEOLOGY, Vol: 120, ISSN: 0264-8172
Cruset D, Cantarero I, Benedicto A, et al., 2020, Geochronological and geochemical data from fracture-filling calcites from the Lower Pedraforca thrust sheet (SE Pyrenees), DATA IN BRIEF, Vol: 31, ISSN: 2352-3409
Cruset D, Ibanez-Insa J, Cantarero I, et al., 2020, Significance of Fracture-Filling Rose-Like Calcite Crystal Clusters in the SE Pyrenees, MINERALS, Vol: 10
Adlan Q, Davies AJ, John CM, 2020, Effects of oxygen plasma ashing treatment on carbonate clumped isotopes, RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Vol: 34, ISSN: 0951-4198
Garfi G, John CM, Lin Q, et al., 2020, Fluid Surface Coverage Showing the Controls of Rock Mineralogy on the Wetting State, GEOPHYSICAL RESEARCH LETTERS, Vol: 47, ISSN: 0094-8276
Lukoczki G, Haas J, Gregg JM, et al., 2020, Early dolomitization and partial burial recrystallization: a case study of Middle Triassic peritidal dolomites in the Villany Hills (SW Hungary) using petrography, carbon, oxygen, strontium and clumped isotope data, INTERNATIONAL JOURNAL OF EARTH SCIENCES, Vol: 109, Pages: 1051-1070, ISSN: 1437-3254
Le Blevec T, Dubrule O, John C, et al., 2020, Geostatistical Earth modeling of cyclic depositional facies and diagenesis, AAPG Bulletin, Vol: 104, Pages: 711-734, ISSN: 0149-1423
In siliciclastic and carbonate reservoirs, depositional facies are often described as being organized in cyclic successions that are overprinted by diagenesis. Most reservoir modeling workflows are not able to reproduce stochastically such patterns. Herein, a novel geostatistical method is developed to model depositional facies architectures that are rhythmic and cyclic, together with superimposed diagenetic facies. The method uses truncated Pluri-Gaussian random functions constrained by transiograms. Cyclicity is defined as an asymmetric ordering between facies, and its direction is given by a three-dimensional vector, called shift. This method is illustrated on two case studies. Outcrop data of the Triassic Latemar carbonate platform, northern Italy, are used to model shallowing-upward facies cycles in the vertical direction. A satellite image of the modern Bermuda platform interior is used to model facies cycles in the windward-to-leeward lateral direction. As depositional facies architectures are modeled using two Gaussian random functions, a third Gaussian random function is added to model diagenesis. Thereby, depositional and diagenetic facies can exhibit spatial asymmetric relationships. The method is applied in the Latemar carbonate platform that experiences syn-depositional dolomite formation. The method can also incorporate proportion curves to model non-stationary facies proportions. This is illustrated in Cretaceous shallow-marine sandstones and mudstones, Book Cliffs, Utah, for which cyclic facies and diagenetic patterns are constrained by embedded transition probabilities.
Garfi G, John CM, Berg S, et al., 2019, The sensitivity of estimates of multiphase fluid and solid properties of porous rocks to image processing, Transport in Porous Media, Vol: 131, Pages: 985-1005, ISSN: 0169-3913
X-ray microcomputed tomography (X-ray μ-CT) is a rapidly advancing technology that has been successfully employed to study flow phenomena in porous media. It offers an alternative approach to core scale experiments for the estimation of traditional petrophysical properties such as porosity and single-phase flow permeability. It can also be used to investigate properties that control multiphase flow such as rock wettability or mineral topology. In most applications, analyses are performed on segmented images obtained employing a specific processing pipeline on the greyscale images. The workflow leading to a segmented image is not straightforward or unique and, for most of the properties of interest, a ground truth is not available. For this reason, it is crucial to understand how image processing choices control properties estimation. In this work, we assess the sensitivity of porosity, permeability, specific surface area, in situ contact angle measurements, fluid–fluid interfacial curvature measurements and mineral composition to processing choices. We compare the results obtained upon the employment of two processing pipelines: non-local means filtering followed by watershed segmentation; segmentation by a manually trained random forest classifier. Single-phase flow permeability, in situ contact angle measurements and mineral-to-pore total surface area are the most sensitive properties, as a result of the sensitivity to processing of the phase boundary identification task. Porosity, interfacial fluid–fluid curvature and specific mineral descriptors are robust to processing. The sensitivity of the property estimates increases with the complexity of its definition and its relationship to boundary shape.
Tagliavento M, John CM, Stemmerik L, 2019, Tropical temperature in the Maastrichtian Danish Basin: Data from coccolith Delta(47) and delta O-18, GEOLOGY, Vol: 47, Pages: 1074-1078, ISSN: 0091-7613
Petersen SV, Defliese WF, Saenger C, et al., 2019, Effects of Improved O-17 Correction on Interlaboratory Agreement in Clumped Isotope Calibrations, Estimates of Mineral-Specific Offsets, and Temperature Dependence of Acid Digestion Fractionation, GEOCHEMISTRY GEOPHYSICS GEOSYSTEMS, Vol: 20, Pages: 3495-3519
MacDonald JM, Faithfull JW, Roberts NMW, et al., 2019, Clumped-isotope palaeothermometry and LA-ICP-MS U-Pb dating of lava-pile hydrothermal calcite veins, Contributions to Mineralogy and Petrology, Vol: 174, ISSN: 0010-7999
Calcite veins are a common product of hydrothermal fluid circulation. Clumped-isotope palaeothermometry is a promising technique for fingerprinting the temperature of hydrothermal fluids, but clumped-isotope systematics can be reset at temperatures of > ca. 100 °C. To model whether the reconstructed temperatures represent calcite precipitation or closed-system resetting, the precipitation age must be known. LA-ICP-MS U–Pb dating of calcite is a recently developed approach to direct dating of calcite and can provide precipitation ages for modelling clumped-isotope systematics in calcite veins. In this study, clumped-isotope and LA-ICP-MS U–Pb calcite analyses were combined in basalt-hosted calcite veins from three settings in Scotland. Samples from all three localities yielded precipitation temperatures of ca. 75–115 °C from clumped-isotope analysis, but veins from only two of the sites were dateable, yielding precipitation ages of 224 ± 8 Ma and 291 ± 33 Ma (2σ). Modelling from the dated samples enabled confident interpretation that no closed-system resetting had occurred in these samples. However, the lack of a precipitation age from the third location meant that a range of possible thermal histories had to be modelled meaning that confidence that resetting had not occurred was lower. This highlights the importance of coupling clumped-isotope thermometry and LA-ICP-MS U–Pb calcite dating in determining the temperature of hydrothermal fluids recorded in calcite veins. This paired approach is shown to be robust in constraining the timing and precipitation temperature of calcite formation, and thus for tracking hydrothermal processes.
McDougall K, John CM, 2019, Benthic foraminiferal biotic events related to the Paleocene-Eocene Thermal Maximum along the California margin, MARINE MICROPALEONTOLOGY, Vol: 150, ISSN: 0377-8398
Veillard C, John C, Krevor S, et al., 2019, Rock-buffered recrystallization of Marion Plateau dolomites at low temperature evidenced by clumped isotope thermometry and X-Ray diffraction analysis, Geochimica et Cosmochimica Acta, Vol: 252, Pages: 190-212, ISSN: 0016-7037
Much debate exists on the extent to which early dolomites recrystallize and preserve the signature of their primary diagenetic setting. Here, we combine clumped isotopes thermometry with X-ray diffraction and thin section petrography to study dolomite recrystallization under shallow burial (<1 km) conditions. We analysed 26 dolomite samples from two Miocene carbonate platforms on the Marion Plateau, NE Australia. Marion Plateau dolomites provide an ideal case study to examine the effects of recrystallization because of the relative simplicity of the geological setting, with simple subsidence, and several episodes of early dolomitization by normal Miocene sea water. Results show that Marion Plateau dolomites are very rich in calcium and their formation temperature inferred from clumped isotopes T(Δ47dol) ranges between 12 and 35°C. The apparent fluid composition (δ18Ow (app)) falls in the range of sea water composition, but a correlation between T(Δ47dol), δ18Odol, and δ18Ow (app) exists: the higher the crystallization temperature, the more negative the fluid composition is. T(Δ47dol) and δ18Ow (app) increase with depth, whereas δ18Odol and δ13Cdol tend to both decrease with depth. We interpret the negative correlation between T(Δ47dol) and δ18Ow (app) as evidence of shallow burial recrystallization via dissolution/re-precipitation. Modelling of the T(Δ47dol), δ18Odol, and δ18Ow (app) indicates that the recrystallization happened at very low water to rock ratio. Carbon isotopes are inherited from the dolomitization process, and not reset during recrystallization. This study shows that dolomite recrystallization has the potential to affect T(Δ47dol) at depths shallower than previously demonstrated. It emphasizes the fact that high calcium dolomites (and possibly aragonite and high Mg-calcite) can have a range of T(Δ47dol) before entering the solid-state reordering re
Lukoczki G, Haas J, Gregg J, et al., 2019, Multi-phase dolomitization and recrystallization of Middle Triassic shallow marine–peritidal carbonates from the Mecsek Mts. (SW Hungary), as inferred from petrography, carbon, oxygen, strontium and clumped isotope data, Marine and Petroleum Geology, Vol: 101, Pages: 440-458, ISSN: 1873-4073
Shallow marine to peritidal carbonates of the Triassic Csukma Formation in the Mecsek Mts. of SW Hungary are made up of dolomites, limestones and dolomitic limestones that show evidence of a complex diagenetic history. Integration of petrographic, conventional stable oxygen and carbon isotope, clumped isotope, and strontium isotope data with the paleogeography, paleoclimate, and burial history of the region revealed four major diagenetic stages. Stage 1: The peritidal carbonates were dolomitized penecontemporaneously during the Middle Triassic by refluxing evaporatively concentrated brines. Stage 2: Increasing burial during the Late Triassic–Jurassic resulted in recrystallization of the Kán Dolomite Member in an intermediate burial setting. Stage 3: During the Early Cretaceous seawater was drawn down and circulated through rift-related faults, causing renewed recrystallization of the Kán Dolomite Member as well as dolomitization of the Kozár Limestone Member and the underlying limestones in a deep burial setting, but only in the vicinity of the faults. Stage 4: During the Late Cretaceous and Cenozoic thrusting resulted in tectonic expulsion of basinal fluids and precipitation of multiple saddle dolomite cement phases near the faults.The results of this study imply that the clumped isotope method integrated with other geochemical data can successfully be applied to identify the nature and potential sources of extra-formational diagenetic fluids responsible for dolomitization and recrystallization. This study provides conclusive evidence for multi-phase dolomitization and dolomite recrystallization over several millions of years (Middle Triassic through Early Cretaceous) and several thousands of meters of burial in the Csukma Formation in SW Hungary. Furthermore, this study is the first to identify fault-controlled dolomitization by circulating Cretaceous seawater within Triassic carbonates of central Europe, further supporting the viability
Nardini N, Munoz-Lopez D, Cruset D, et al., 2019, From early contraction to post-folding fluid evolution in the frontal part of the boixols thrust sheet (southern pyrenees) as revealed by the texture and geochemistry of calcite cements, Minerals, Vol: 9, ISSN: 2075-163X
Structural, petrological and geochemical (δ13C, δ18O, clumped isotopes, 87Sr/86Sr and ICP-MS) analyses of fracture-related calcite cements and host rocks are used to establish a fluid-flow evolution model for the frontal part of the Bóixols thrust sheet (Southern Pyrenees). Five fracture events associated with the growth of the thrust-related Bóixols anticline and Coll de Nargó syncline during the Alpine orogeny are distinguished. These fractures were cemented with four generations of calcite cements, revealing that such structures allowed the migration of different marine and meteoric fluids through time. During the early contraction stage, Lower Cretaceous seawater circulated and precipitated calcite cement Cc1, whereas during the main folding stage, the system opened to meteoric waters, which mixed with the connate seawater and precipitated calcite cement Cc2. Afterwards, during the post-folding stages, connate evaporated marine fluids circulated through newly formed NW-SE and NE-SW conjugate fractures and later through strike-slip faults and precipitated calcite cements Cc3 and Cc4. The overall paragenetic sequence reveals the progressive dewatering of Cretaceous marine host sediments during progressive burial, deformation and fold tightening and the input of meteoric waters only during the main folding stage. This study illustrates the changes of fracture systems and the associated fluid-flow regimes during the evolution of fault-associated folds during orogenic growth.
Davies A, John C, 2019, The clumped (13C-18O) isotope composition of echinoid calcite: further evidence for “vital effects” in the clumped isotope proxy, Geochimica et Cosmochimica Acta, Vol: 245, Pages: 172-189, ISSN: 0016-7037
Carbonate clumped isotope thermometry is based on thethermodynamically dependent relative abundance of 13C-18O bonds(quantified as Δ47) within the carbonate crystal lattice. The clumping of13C-18O in carbonates is based on a self-reaction of isotope exchangethat occurs rapidly at near neutral pH. Similar Δ47-temperaturerelationships between biogenic and inorganically precipitated carbonatein initial studies led to the promise of a proxy free of biologicallydriven disequilibrium effects, commonly referred to as "vital effects".This has been largely the case for most organisms investigated.Biologically mediated disequilibrium precipitation has however beenreported in corals and cephalopods and brachiopod molluscs. Echinoderms,despite their complex inter-cellular bio-mineralization strategy, largeinter-skeletal fractionation of δ18O, δ13C and rapid calciteprecipitation have however not been previously investigated with regardsto their clumped isotope composition. We present clumped isotopiccomposition (Δ47) of 25 inter-skeletal elements of 5 echinoid specieswith varying growth temperatures. We found no statistically significantinter-skeletal variation in Δ47 in all echinoid species measured, asurprising find given the inter-skeletal variability reported for δ13Cand δ18O. Our echinoid Δ47-temperature calibration however shows astatistically significant positive offset from Δ47-temperaturecalibration for inorganic calcite of 0.015‰. The pattern of isotopicfractionation in δ18O and Δ47 of echinoderms is not consistent with CO2hydration or hydroxylation, diffusion or high-Mg composition of echinoidcalcite. Positive offsets in the Δ47 of echinoid calcite may howeverrelate to deviations in the pH of the calcifying fluid from the pH atwhich equilibrium calcite is precipitated.
Al-Salmi M, John C, Hawie N, 2019, Quantitative controls on the regional geometries and heterogeneities of the Rayda to Shu'aiba formations (Northern Oman) using forward stratigraphic modelling, Marine and Petroleum Geology, Vol: 99, Pages: 45-60, ISSN: 1873-4073
The complex geometry of carbonate systems is influenced by a multitude of physical as well as biological processes. The Lower Cretaceous carbonates of Northern Oman are characterised by a variability of regional-scale geometries with expected vertical and lateral facies variations. The main environmental and tectonic controls acting on the depositional processes of the Lower Cretaceous ramps and platforms through space and geological time (in 4 dimensions) are only partially understood. In this study, we use a 4D DionisosFlow Forward Stratigraphic Modelling (FSM) approach to explore the role of: (i) eustasy; (ii) subsidence; (iii) initial paleobathymetry, and (iv) wave energy, to generate carbonate stacking patterns and heterogeneities. Carbonate production was maintained constant through deposition. Multi-disciplinary and multi-scale datasets were used (i.e. seismic, well and field data) to constrain the FSM input parameters and sensitivity analysis was carried out to validate or refute some depositional model hypotheses. Results show that basement topography and eustasy have the greatest influence on the progradational geometries and the lateral continuity of clinoform architectures during the Tithonian to Valanginian second-order super-sequence. In the Valanginian to Aptian super-sequence, subsidence was the primary control for the observed aggradational stacking pattern. Lateral and vertical stacking of carbonate lithologies, textures as well as facies are thus apprehended through this FSM approach, leading to a better assessment of petroleum systems elements as reservoir, seal and trap.
Cruset D, Cantarero I, Verges J, et al., 2018, Changes in fluid regime in syn-orogenic sediments during the growth of the south Pyrenean fold and thrust belt, GLOBAL AND PLANETARY CHANGE, Vol: 171, Pages: 207-224, ISSN: 0921-8181
Hirani J, Bastesen E, Boyce A, et al., 2018, Controls on the formation of stratabound dolostone bodies, Hammam Faraun Fault block, Gulf of Suez, Sedimentology, Vol: 65, Pages: 1973-2002, ISSN: 0037-0746
Dolomitization is commonly associated with crustal‐scale faults, but tectonic rejuvenation, diagenetic overprinting and a fluid and Mg mass‐imbalance often makes it difficult to determine the dolomitization mechanism. This study considers differential dolomitization of the Eocene Thebes Formation on the Hammam Faraun Fault block, Gulf of Suez, which has undergone a simple history of burial and exhumation as a result of rifting. Stratabound dolostone bodies occur selectively within remobilized sediments (debrites and turbidites) in the lower Thebes Formation and extend into the footwall of, and for up to 2 km away from, the Hammam Faraun Fault. They are offset by the north–south trending Gebel fault, which was active during the earliest phases of rifting, suggesting that dolomitization occurred between rift initiation (26 Ma) and rift climax (15 Ma). Geochemical data suggest that dolomitization occurred from evaporated (ca 1·43 concentration) seawater at less than ca 80°C. Geothermal convection is interpreted to have occurred as seawater was drawn down surface‐breaching faults into the Nubian sandstone aquifer, convected and discharged into the lower Thebes Formation via the Hammam Faraun Fault. Assuming a ca 10 Myr window for dolomitization, a horizontal velocity of ca 0·7 m year−1 into the Thebes Formation is calculated, with fluid flux and reactivity likely to have been facilitated by fracturing. Although fluids were at least marginally hydrothermal, stratabound dolostone bodies do not contain saddle dolomite and there is no evidence of hydrobrecciation. This highlights how misleading dolostone textures can be as a proxy for the genesis and spatial distribution of such bodies in the subsurface. Overall, this study provides an excellent example of how fluid flux may occur during the earliest phases of rifting, and the importance of crustal‐scale faults on fluid flow from the onset of their growth. Furthermore, this article presents a m
John CM, Khan SB, 2018, Mental health in the field, Nature Geoscience, Vol: 11, Pages: 618-620, ISSN: 1752-0894
Field work is an important and valued part of geoscience research, but can also serve as a source of stress. Careful planning can help support the mental health and wellness of participants at all career stages.The satirical poet Juvenal (ad 55–140) wrote: “Orandum est ut sit mens sana in corpore sano”, “A man should pray for a healthy mind in a healthy body”1. Field work and field courses are central to geosciences, and typically require the corpore sano (healthy body)2,3. Health, safety and environmental practices have improved to better protect the geologist’s body from harm4. But what about the mens sana, the need for a healthy mind to be able to cope with the mental strain of field work? Mental health in academia is taking central stage because of the realization that poor mental health adversely impacts our student population5. But the potential positive and negative impacts of field work on mental health remain less well explored.
Le Blévec T, Dubrule O, John CM, et al., 2018, Geostatistical modelling of cyclic and rhythmic facies architectures, Mathematical Geosciences, Vol: 50, Pages: 609-637, ISSN: 1874-8961
A Pluri-Gaussian method is developed for facies variables in three dimensions to model vertical cyclicity related to facies ordering and rhythmicity. Cyclicity is generally characterised by shallowing-upward or deepening-upward sequences and rhythmicity by the repetition of facies at constant intervals along sequences. Both of these aspects are commonly observed in shallow-marine carbonate successions, especially in the vertical direction. A grid-free spectral simulation approach is developed, with a separable covariance allowing a dampened hole-effect to capture rhythmicity in the vertical direction and a different covariance in the lateral plane along strata, as in space-time models. In addition, facies ordering is created by using a spatial shift between two latent Gaussian functions in the Pluri-Gaussian approach. Rapid conditioning to data is performed via Gibbs sampling and kriging using the screening properties of separable covariances. The resulting facies transiograms can show complex patterns of cyclicity and rhythmicity. Finally, a three dimensional case study of shallow-marine carbonate deposits at outcrop shows the applicability of the modelling method.
Lu Y-C, Song S-R, Taguchi S, et al., 2018, Evolution of hot fluids in the Chingshui geothermal field inferred from crystal morphology and geochemical vein data, Geothermics, Vol: 74, Pages: 305-318, ISSN: 0375-6505
The Chingshui geothermal field once hosted the first geothermal power plant in Taiwan from 1981 to 1993. After a long period of inactivity, this field is attracting renewed interest to meet the need for clean energy. A 213-m length of cores (IC-21) with continuous recovery, the longest in the Chingshui geothermal field, was recovered from 600 m to 813 m below the surface in 2010. Three types of calcite crystal morphologies have been identified in the veins of the cores of well IC-21: bladed, rhombic and massive crystals. Bladed calcites are generated via degassing under boiling conditions with a precipitation temperature of ∼165 °C and calculated δ18O value of −6.8‰ to −10.2‰ VSMOW for the thermal water. Rhombic calcites grow in low concentration Ca2+ and CO32− meteoric fluids and precipitate at approximately ∼180 °C. Finally, massive calcites are characterized by co-precipitation with quartz in the mixing zone between meteoric water and magmatic or metamorphic fluids with calculated δ18O value of up to 1.5 ± 0.7‰ VSMOW. Furthermore, the scaling and hot fluids at a nearby pilot geothermal power plant confirm a meteoric origin. Based on these observations, we propose that the current orientations of the main conduits for geothermal fluids are oriented at N10°E with a dip of 70°E. This result provides the basic information needed for deploying production and injection wells in future developments of the geothermal power plant in this region.
Jacquemyn C, Jackson MD, Hampson GJ, et al., 2018, Geometry, spatial arrangement, and origin of carbonate grain-dominated, scour-fill and event-bed deposits: Late Jurassic Jubaila Formation and Arab-D Member, Saudi Arabia, Sedimentology, Vol: 65, Pages: 1043-1066, ISSN: 0037-0746
Outcrop analogues of the Late Jurassic lower Arab‐D reservoir zone in Saudi Arabia expose a succession of fining‐upward cycles deposited on a distal middle‐ramp to outer‐ramp setting. These cycles are interrupted by erosional scours that incise up to 1·8 m into underlying deposits and are infilled with intraclasts up to boulder size (1 m diameter). Scours of similar size and infill are not commonly observed on low‐angle carbonate ramps. Outcrops have been used to characterize and quantify facies‐body geometries and spatial relationships. The coarse grain size of scour‐fills indicates scouring and boulder transport by debris or hyperconcentrated density flows strengthened by offshore‐directed currents. Longitudinal and lateral flow transformation is invoked to produce the ‘pit and wing’ geometry of the scours. Scour pits and wings erode up to 1·8 m and 0·7 m deep, respectively, and are on average 50 m wide between wing tips. The flat bases of the scours and their lack of consistent aspect ratio indicate that erosion depth was limited by the presence of cemented firmgrounds in underlying cycles. Scours define slightly sinuous channels that are consistently oriented north–south, sub‐parallel to the inferred regional depositional strike of the ramp, suggesting that local palaeobathymetry was more complex than commonly assumed. Weak lateral clustering of some scours indicates that they were underfilled and reoccupied by later scour incision and infill. Rudstone scour‐fills required reworking of material from inner ramp by high‐energy, offshore‐directed flows, associated with storm action and the hydraulic gradient produced by coastal storm setup, to generate erosion and sustain transport of clasts that are generally associated with steeper slopes. Quantitative analysis indicates that these coarse‐grained units have limited potential for correlation between wells as laterally continuous, highly permeable reservoir flow units, but the
Kluge T, John CM, Boch R, et al., 2018, Assessment of factors controlling clumped isotopes and delta O-18 values of hydrothermal vent calcites, Geochemistry, Geophysics, Geosystems, Vol: 19, Pages: 1844-1858, ISSN: 1525-2027
The clumped isotope composition of CaCO3 (Δ47) is a geochemical proxy that can provide mineral formation temperatures and, together with measured carbonate δ18O, inferred fluid δ18O values. Under natural conditions, carbonates form within a relatively wide pH range and varying growth rates which are typically not reflected in laboratory‐based calibrations (mostly ∼pH 8, moderate growth rates). A pH and growth‐rate dependence is known for oxygen isotopes and was also postulated for clumped isotopes. Theoretical predictions suggest that Δ47 values could lie between the carbonate equilibrium value and the value inherited from the dissolved inorganic carbon (predicted offset: +0.04‰ pH < 4 and −0.025‰ at high pH > 12). Here we test whether pH (in addition to temperature) is recorded in the carbonate clumped isotope composition using modern calcites from natural travertine‐forming streams and scales precipitated in pipes of deep geothermal wells from Italy, Hungary, and Turkey (pH: 6.1–7.5, T: 33–100°C). Although a comparison of all samples with expected equilibrium values in this pH range and known formation temperatures reveals only an insignificant Δ47 offset (0.006 ± 0.004‰, 1SE, n = 9), the clumped isotope values of samples with the highest growth rates (0.014 ± 0.007‰, 1SE, n = 5) are consistent with the theoretical prediction attributable to pH of 0.01‰. Similarly, deviations in δ18O of up to −2‰ follow a growth‐rate dependence. This field‐based study shows that pH‐related effects are mostly small for Δ47 in the subsurface environment at lower pH and that high mineral growth rates control the magnitude of this disequilibrium.
James CA, John CM, Menon R, 2018, Assessment of spatio-temporal changes in terrestrial carbon sequestration due to Kochi metro rail project in India, URBAN CLIMATE, Vol: 24, Pages: 703-713, ISSN: 2212-0955
Honlet R, Gasparrini M, Muchez P, et al., 2018, A new approach to geobarometry by combining fluid inclusion and clumped isotope thermometry in hydrothermal carbonates, Terra Nova, Vol: 30, Pages: 199-206, ISSN: 0954-4879
This study presents a new approach to geobarometry by combining fluid inclusion and clumped isotope (Δ47) thermometry on carbonate minerals. The offset between homogenisation temperatures of primary fluid inclusions with known composition and Δ47 temperatures of the host mineral allows a direct estimation of the fluid pressure at the time of carbonate crystallisation. This new approach is illustrated via hydrothermal dolomite samples from the Variscan foreland fold‐and‐thrust belt in northern Spain. Clumped isotope analyses yield crystallisation temperatures (107–168°C) which are higher than homogenisation temperatures in corresponding samples (95–145°C). The calculated pressure values suggest that dolomitizing fluids were overpressured during formation of zebra dolomite textures, whereas lower pressures are obtained for dolomite cement from breccia textures. This new approach to geobarometry opens up the possibility of estimating the pressure of carbonate crystallisation and has potential applications in diagenesis, basin analysis, ore geology and tectonics.
Beckert J, Vandeginste V, McKean TJ, et al., 2018, Ground-based hyperspectral imaging as a tool to identify different carbonate phases in natural cliffs, International Journal of Remote Sensing, Vol: 39, Pages: 4088-4114, ISSN: 0143-1161
Recent research has shown hyperspectral imaging to be a powerful tool to distinguish carbonate phases with slight compositional differences on quarry cliff faces. The traditional remote sensing set-up uses an optimal short distance between the hyperspectral camera mounted on a tripod and a quarry wall characterized by a planar, mostly unweathered surface. Here we present results of a modified workflow geared to the application of ground-based hyperspectral imaging of rough and weathered cliff faces in order to map large scale dolomite bodies from a distance of up to several kilometres. The goal of the study was to determine unique spectral properties of fracture-controlled dolomite bodies in order to be able to distinguish them from a dolomitic host rock. In addition, the impact of weathering on carbonate phases and thus, the modification of the spectral signature between altered and unaltered carbonates is assessed. The spectral analysis is complemented by ICP-AES (inductively coupled plasma atomic emission spectroscopy) measurements of the spectrally measured powders. Furthermore, we examined the detection limits and characterisation potential of dolomite bodies from hyperspectral images captured at varying distances from cliff faces in the study area. Hyperspectral images of 10 natural cliffs distributed across the Central Oman Mountains were obtained with a Push broom scanner system. The high resolution of 5.45 nm (288 bands in total) enabled the visualization of small-scale changes in the near infrared continuous spectrum of all present lithofacies types. The determination of dolomite bodies of varying sizes (metre to hundreds of metres) on natural cliffs was achieved with the hyperspectral mapping approach and mapping results have been tested with the position of visually defined dolomite bodies on field panoramas. Spectra of natural cliffs contain a strong absorption peak indicative for iron which is absent in spectra of unweathered sample powders. However, I
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