Detrital Carbonate Research Articles

Direct evidence of sediment carbonate dissolution in response to bottom-water acidification in the Gulf of St. Lawrence, Canada

Over the past century, dissolved oxygen concentrations have decreased and metabolic CO2 has accumulated in the bottom waters of the Gulf of St. Lawrence (GSL) and Lower St. Lawrence Estuary (LSLE). Oxygen depletion has been attributed primarily to changes in ocean circulation in the northwest Atlantic Ocean, as well as an increase in the flux of organic matter at or near the seafloor and its accompanying biological oxygen demand. The accumulation of metabolic CO2 in these waters has led to their progressive acidification and a decrease in pH (0.3–0.4 pH unit) commensurate to the variation expected for global oceanic surface waters by the end of this century, albeit by a different mechanism (anthropogenic CO2 uptake from the atmosphere). The decrease in bottom-water pH of the GSL and LSLE is accompanied by a decrease in the carbonate ion concentration and the saturation state of the waters with respect to both calcite and aragonite (ΩC and ΩA). Although the Laurentian Trough sediments are mostly devoid of modern calcium carbonate fossils, detrital (Ordovician/Silurian) carbonates, eroded from Anticosti Island, accumulate on the seafloor. Evidence of carbonate mineral dissolution in the sediments of the Laurentian Trough is examined and supported by pore-water data and vertical variations of their inorganic carbon content. Historical, solid-phase profile data are used to estimate temporal variations of the sedimentary calcite dissolution rates and document the anthropogenic modification of the sediment record.

Last glacial maximum ecology and climate from terrestrial gastropod assemblages in Peoria loess, western Kentucky

ABSTRACTThe Rocks loess section, in unglaciated western Kentucky, provides a high‐resolution environmental record during the last glacial maximum onset. The Peoria Silt (9 m thick) contains 26 terrestrial gastropod species, with up to 15 species within a single 5 cm interval. Thirteen radiocarbon ages, using shells or charcoal, range between 30 and 24.5 cal ka; younger loess has been leached or eroded. Stratigraphic shifts in gastropod assemblages imply significant cooling, particularly ~27 cal ka, as solar insolation was decreasing and the southern Laurentide Ice Sheet rapidly advancing. Midwestern to southern species (e.g. Anguispira kochi, Gastrocopta pentodon, Hawaii miniscula, Helicodiscus parallelus, Vallonia perspectiva) occur only in the lowermost Peoria Silt (~30–27 cal ka). In contrast, cold‐tolerant species (Columella alticola, Vertigo modesta, Vallonia gracilicosta) occur only in full glacial Peoria Silt (27–24.5 cal ka). Inferred mean July temperatures, from mutual climatic range methods, range from ~23 °C at 30 cal ka, cooling to ~18 °C by 26 cal ka; about 3–8 °C cooler than today (~26 °C). Superimposed on this cooling trend are multi‐centennial variations in detrital carbonate, fossil shell concentrations, palaeotemperature estimates, and oxygen isotope values (Vertigo, Discus, Helicodiscus). The finer‐scale variations imply relatively synchronous fluctuations in glacial sediment supply, loess sedimentation, and climate.

Quantifying Provenance and Transport Pathways of Holocene Sediments From the Northwestern Greenland Margin

AbstractThe mineralogical and geochemical compositions of three sediment cores from the northwestern Greenland continental margin (AMD14‐204 and AMD14‐210) and Kane Basin (AMD14‐Kane2B) were investigated using quantitative X‐ray diffraction and energy‐dispersive X‐ray fluorescence in order to document the impact of ice‐ocean interactions on the sediment provenance and transport pathways during the Holocene. Unmixing of the sediment composition and ratios such as quartz/clays and K/Fe indicate that detrital sediments in cores from the northwest Greenland margin are derived mainly from Prøven granite and reworked Archean gneiss, whereas sediments from Kane Basin are derived mainly from detrital carbonate (Ellesmere Island) and Proterozoic gneiss (Humboldt Glacier). Mineralogical and geochemical signatures also reveal that changes in detrital sediment provenance and transport in the region are strongly interconnected to regional ice stream dynamics, especially during the early Holocene with strong meltwater discharge from the Greenland Ice Sheet associated with the end of deglaciation. With the establishment of milder conditions during the mid‐Holocene and reduced glacial activity once ice sheets retreated inland, sediment inputs via ocean currents became more effective. Thus, detrital proxies in core AMD14‐204, located in the Upernavik cross‐shelf trough, support the hypothesis of an intensification of the West Greenland Current influence after 7.7 cal ka BP, which eventually affected the regional sediment dynamic by a greater contribution from basalt to the sediment supply. Finally, our results suggest that the interactions between climate, glacial dynamics, and surface ocean circulation controlled the paleoenvironmental changes observed in the three cores.

Authigenic Sanidine as a Mineral Indicator of Gravitation-Brine Catagenesis in Carboniferous Rocks in the Southern Limb of the Moscow Syneclise

The article presents data on numerous findings of authigenic K-feldspars in the Lower–Upper Carboniferous carbonate and terrigenous rocks in the southern limb of the Moscow syneclise, including the Oka–Tsna bar. In terms of the composition and crystalline structure, all feldspar morphotypes (idiomorphic monocrystalline fringes on the surface of sandy clastic grains, fine-crystalline aggregates, pseudomorphously replacing organogenic carbonate detritus, and pseudoskeletal crystals) are composed of K-sanidine with the overwhelming predominance of K over Na and extremely low concentrations of Ca, Ba, and Sr. As can be seen from 40Ar/39Ar dating, the authigenic sanidine mineralization took place during the Permian, i.e., 20–55 Ma later than the accumulation of host Carboniferous rocks. Mineralogical observations, as well as paleofacies and paleogeographic reconstructions, suggest that the formation of authigenic K-feldspars is associated with the gravitation-brine catagenesis in the southern limb of the Moscow syneclise as a result of the seepage of high-mineralization brines from Permian evaporate marine basins into the underlying older rocks.

Mineralogical and geochemical characterisation of alkaline lake sediments to trace origin, depositional processes, and anthropogenic impacts: Inle Lake (Southern Shan State, Myanmar)

Southeast Asia contains a major portion of wetland areas of international significance, but only 14% of these wetlands are under protection. Inle Lake is the second largest freshwater body in Myanmar and the most important for its environmental, economic, and agricultural values. However, the lake is seriously threatened by anthropic activities as they are dramatically decreasing its open water surface. While water-quality studies have assessed the presence of nutrients and contaminants, little is known about the sediment quality. Therefore, the overall objective of the present study was to analyse the natural sedimentation patterns and the anthropogenic disturbances in this lake. To this end, both grab and core sediments were investigated by combining mineralogical, morphological, and geochemical analyses, and their composition was compared to the composition of rocks and soil from the watershed. Mineralogical data indicate that sediment samples collected at or near the main inflow, and at the outflow show a lower abundance (about 43%) of calcite, which generally increases with depth in sediment cores, reaching more than 90% in the centre of the lake. Lower calcite contents correspond to higher percentages of other minerals. These findings allow us to clearly distinguish the allogenic (quartz, mica/illite, kaolinite, and detrital calcite) from the endogenic (mostly calcite) minerals, as confirmed by the soil and rock mineralogy. The origin of the mineral contents at the different sites is further supported by the observation of the grain morphology and composition. The most abundant metal in the lake sediments is Ca, in agreement with the high calcite content. Following in order of abundance are Al, Fe, and K, which are connected to the detrital input to the lake. In the rock and soil samples, the most abundant trace element is Mn, which is significantly correlated with many other trace elements (e.g., Co, Cr, Cu, Ni, V, Zn, and As), suggesting their common lithogenic origin. Similarly, in the lake sediments, the trace elements Co, Cr, Cu, Ni, Pb, V, Zn, and As display fairly homogenous concentration ranges, in the order of tens of ppm, and are strongly correlated with the detrital minerals (mica, kaolinite, and quartz) and negatively correlated with the calcite content. The mineralogical and elemental distribution in the lakebed area and with depth in the sediment cores allowed us to identify the increased soil and bedrock erosion as a major anthropogenic impact to the lake. The sedimentation disturbance is especially evident at the main inflow, the Nanlit stream, which has caused the development of a large alluvial fan, visible in satellite images. This large sediment input is likely to have accelerated the decrease in open water surface and in depth of this shallow lake. Additionally, when the measured concentrations of Potentially Toxic Elements were compared to consensus-based Sediment Quality Guidelines (SQGs), it was shown that As, Cr, Cu, Ni, Pb, and Zn often exceeded the threshold effect concentration (TEC), and that Cr, Ni, and Pb sometimes exceeded the probable effect concentration (PEC). On the other hand, Enrichment Factors (EFs), calculated with respect to soils of the watershed, showed minimal enrichment and indicated that the potential toxicity effect was primarily a result of detrital sedimentation outpacing carbonate sedimentation. Sediment characterisation allowed us to obtain a better understanding of lake dynamics and environmental impacts, which is fundamental in achieving effective management and maintenance of the ecological character of Inle Lake, in line with the objective set by the Ramsar Convention for wetlands of international relevance.

Characteristics and provenance of hazardous trace elements in soil from a typical agricultural region in eastern Anhui, China

With the purpose of investigate the characteristics and provenances of the hazardous trace elements (HTEs) in a typical agricultural land in the undulating plain area of eastern Anhui province, China, a total of 1170 soil samples were collected. Six HTEs, including Cd, Cr, Hg, Ni, Pb, and As, were determined. Statistical methods including box plot, Pearson correlation analysis, principal component analysis, and cluster analysis, were conducted to illustrate the provenances of HTEs. Results indicate that Cd and Hg were influenced by human activities (such as traffic transportation for Cd and Pb, and chemical pesticides for Hg). Cr, Ni, and As mainly originated from the parent materials (e.g., red detrital rocks, carbonate rocks, and late Pleistocene loess). Pb is of dual provenances of both human activity and parent material. In a word, regarding highly toxic HTEs, the soil in the study area is almost free from pollution, and only a small amount of soil samples contains slightly higher contents of As, Cd and Pb.

Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: A case study from palaeo-lake Fucino (central Italy).

The suitability of organic matter (OM) removal pre-treatments in isotopic studies of lacustrine carbonates is currently under debate. Naturally occurring OM seems to have a negligible effect on the bulk isotopic composition of carbonates compared with changes induced by pre-treatments. This study provides further insights into the possible effects induced by commonly used pre-treatments on natural lacustrine carbonates. Sixteen samples from the Fucino F1-F3 lacustrine succession (Abruzzo, central Italy) were characterised for their mineralogical and geochemical composition and each was split into three identical aliquots. One aliquot was left untreated while the remaining two were treated with NaOCl and H2 O2 dilutions. The same treatment was applied to an internal standard consisting of pure Carrara marble. The treated and untreated samples were analysed for their carbon (δ13 C values) and oxygen (δ18 O values) isotope compositions using an Analytical Precision AP2003 isotope ratio mass spectrometer. The samples had variable proportions of endogenic and detrital components, the detrital portion being more (less) abundant during colder (warmer) climate phases. We observed that neither the NaOCl nor the H2 O2 treatment was able to completely remove OM and therefore there was selective removal of compounds within the OM pool. A possible effect of pre-treatment is the loss of carbonates intimately interspersed within the OM, as suggested by the evolution of isotopic ratios towards the local detrital array. Our study highlights sample-specific changes in geochemistry associated with sample pre-treatments; however, such changes do not seem to lead to either systematic and/or predictable isotopic shifts. We suggest that the suitability of NaOCl or H2 O2 pre-treatments for OM removal should be evaluated on a case-by-case basis. In the specific case of lacustrine marls from palaeo-lake Fucino containing relatively low amounts of OM and in which both detrital and endogenic carbonates occur, both pre-treatments should be avoided.

Early Cretaceous calciturbidites facies from Zagros Fold–Thrust belt: a key to paleogeography and environment of northeast Arabian Platform Passive Margin, examples from Kurdistan Region, Northeast Iraq

The study focuses on description, analysis and mapping of the thick succession of calciturbidites facies for the first time on the Early Cretaceous Arabian Platform Passive Margin from Western Zagros Fold–Thrust belt, Northeastern Iraq (Kurdistan Region). Along the margin, the study revealed active source of shedding voluminous calciturbidites sediments to the periplatform and deep basin of Neo-Tethys sea during the Early Cretaceous. Turbidity currents reworked pure coarse and fine detrital carbonates, from the platform, and transported long distance to the basin passing through the slope of the basin. Along this distance, the sediments crossed the marginal slope scoring submarine channels and depositing megabreccias in addition to coarse detrital carbonates (rudstone) before reaching the basin plain. On the plain mostly the fine grain facies of calciturbidites lay down and mixed with pelagic sediments such as radiolarias and calcispheres. The calciturbidite evidence can be tracked for more than 50 km in the studied area from platform to the basin across the slope. These latter facies are occasionally graded, laminated and cross-laminated with deep basin fossil such as planktonic foraminifera, and radiolarians. Stratigraphically, Qamchuqa, as carbonate factory, and Balambo Formations represent the platform and the basin facies, respectively. The periplatform (slope) was northeastward-facing and its toe was the depositional locus of shared lithology between the two aforementioned units. These formations are two Early Cretaceous rocks successions that crop out in Zagros Fold–Thrust Belt and occur in most famous oil fields in Middle East. These calciturbidites are keys for picturing boundary condition of the Passive Arabian Continental (platform) Margin, which is proved to be dynamic, stormy and with contrasted bathymetry (high topography). The study totally changes all characteristics of Balambo Formation from bathyal fine grain deposits to mixture of different carbonate grain of calciturbidite and pelagic origins.

Younger Dryas and Holocene environmental change at the Atlantic fringe of Europe derived from lake‐sediment stable‐isotope records from western Ireland

Western Ireland lies at the Atlantic fringe of Europe and is thus well positioned to record changes in climate linked to North Atlantic circulation. Lake‐sediment oxygen and carbon isotopes are sensitive to changes in the atmosphere, the lake catchment and the lake itself, and thus are valuable proxies for long‐term environmental change. Here we present Younger Dryas and Holocene stable‐isotope records from endogenic and biogenic carbonates from An Loch Mór, a karstic lake on Inis Oírr, Aran Islands, western Ireland. The bulk carbonate and ostracod‐derived stable‐isotope records are a complex response to regional climatic and local factors. Low oxygen‐isotope and high carbon‐isotope values in the Younger Dryas most likely reflect low air temperatures and dominance of bedrock‐derived carbon from the poorly vegetated catchment, but input of detrital carbonate cannot be ruled out. Holocene variations in oxygen‐isotope values are likely a response mainly to changes in the isotopic composition of rainfall, evaporative enrichment of lake water and, during the past millennium, incursion of seawater as a result of regional sea‐level rise. A reduction in carbon‐isotope values at the Younger Dryas/Early Holocene transition reflects increased input of soil‐derived carbon to the lake, modified by changes in aquatic productivity. During the past millennium a shift to less negative values is attributed mainly to increasing saline influence. Broad similarities with stable‐isotope records from other sites in western Ireland suggest regional climatic controls dominated by North Atlantic atmospheric circulation patterns, although abrupt events, that were possibly unique to An Loch Mór, point to local factors also being important.

Effects of metamorphism and metasomatism on manganese mineralogy: Examples from the Transvaal Supergroup

AbstractManganese-bearing minerals in ancient strata provide a particularly informative record of the redox potentials of ancient Earth surface environments due to the high specificity of species that can oxidize Mn(II). However, little is known about how this sedimentary archive might have been altered by processes occurring long after lithification, including the effects of metamorphism, fluid mobilization, and metasomatism. We investigated Mn mineralization across known metamorphic gradients in the Kaapvaal craton, South Africa, in Archean and early Paleoproterozoic age carbonate-, shale-, and iron formation-bearing marine strata. We sampled contemporaneous strata that record the drowning of the Campbellrand-Malmani carbonate platform and a transition to iron formation deposition in a range of localities, from two metamorphosed (greenschist and above, affected by the intrusion of the Bushveld igneous complex) and four better-preserved (sub-greenschist) deep subsurface drill cores. To evaluate the geochemistry and mineralization tied directly to petrographic textures and cross-cutting relationships, we combined bulk geochemistry with light and electron microscopy and synchrotron microprobe X-ray absorption spectroscopy and imaging to produce Mn speciation maps at the requisite micrometer length scales for these textures. Samples with lesser degrees of post-depositional transformation contained minor amounts of Mn(II) in early diagenetic marine carbonate cements and detrital carbonate grains, while metamorphosed samples typically contained Mn concentrated into a combination of coarse-grained and vein-filling carbonate phases (ankerite, siderite, and rhodochrosite), garnet and amphibole. Chemical imaging analyses of these more metamorphosed samples show that Mn is held by phases and textures that mineralized post-deposition and lithification, demonstrating that Mn was mobilized – at least locally – by metasomatic fluids, although it is difficult to distinguish whether this Mn was original to these strata or was introduced secondarily. Our results confirm that Mn can be mobilized and therefore caution should be applied when interpreting Mn enrichments in sedimentary rocks, especially when Mn enrichment is not geographically extensive and coincides with metamorphic processes.

Origin and distribution of dolomite in Permian Rotliegend siliciclastic sandstones (Dutch Southern Permian Basin)

ABSTRACTDolomite is a common and volumetrically important mineral in many siliciclastic sandstones, including Permian Rotliegend sandstones (the Slochteren Formation). These sandstones form extensive gas reservoirs in the Southern Permian Basin in the Netherlands, Germany, Poland, and the UK. The reservoir quality of these sandstones is negatively influenced by the content and distribution of dolomite. The origin and the stratigraphic distribution of the dolomite is not yet fully understood. The aim of this study is to identify the origin of carbonate. The main methods used to achieve those aims are a combination of thin-section petrography, scanning electron microscopy (SEM and EDX), and XRD analyses.The present study shows that the typical dispersed occurrence of the dolomite is a consequence of dispersed detrital carbonate grains that served both as nuclei and source for authigenic dolomite cement. The dolomite cement formed syntaxial outgrowths and overgrowths around detrital carbonate grains. The study also shows that dolomite cement, often in combination with ankerite and siderite, precipitated during burial after mechanical compaction. Most of the carbonate grains consisted of dolomite before deposition. The carbonate grains were affected by compaction and pressure dissolution, and commonly have no well-defined outlines anymore.The distribution of dolomite cement in the Rotliegend sandstones was controlled by the presence of stable carbonate grains. Due to the restricted and variable content of carbonate grains and their dispersed occurrence, the cement is also dispersed and the degree of cementation heterogeneous.Our findings have important implications on diagenesis modeling. The presence of detrital carbonate excludes the need for external supply by any large-scale advective flow of diagenetic fluids. By knowing that the carbonate source is local and related to detrital grains instead of being externally derived from an unknown source, the presence of carbonate cement can be linked to a paleogeographic and sedimentological model.

Sedimentology and history of sediment sources to the NW Labrador Sea during the past glacial cycle

The paleoceanography and sediment dynamics of the northernmost Labrador Sea provides critical records of glacial history, glacially-dominated sedimentation, and paleocirculation. A reference core at 938 m on the SE Baffin Slope was investigated with new oxygen isotope stratigraphy, X-ray fluorescence geochemistry, and 18 14C-AMS dates and correlated to 14 regional deep-water cores. The reference core provides a paleoceanographic and sediment source record over the last 40 ka, overlying a 3-m-thick blocky mass-transport deposit over MIS 5 autochthonous sediment. Detrital carbonate-rich sediment layers H0-H4, based on bulk geochemistry, were derived principally from Hudson Strait. Shortly after H2 and H3, the shelf-crossing Cumberland Sound ice stream supplied dark brown ice-proximal stratified sediments. Minor supply of carbonate-rich sediment from Baffin Bay allows chronologic integration of the Baffin Bay and Labrador Sea detrital carbonate records and implies an open seaway through Davis Strait. The counterparts of H3, H4, and (?)H5 events in the deep Labrador basin are 4–10 m thick units of thin-bedded carbonate-rich mud turbidites from glacigenic debris flows on the Hudson Strait slope. The behavior of the Hudson Strait ice stream changed through the last glacial cycle. In H1 and H2, the ice stream retreated back across the shelf but did not deglaciate Hudson Bay; in H3–H5 it remained at the shelf break long enough to supply thick turbidites, and subsequent retreat after H4 and H5 deglaciated Hudson Bay. The maximum extent of ice streams in Hudson Strait, Cumberland Sound, and Lancaster Sound was not synchronous.

Quantifying Biogenic Versus Detrital Carbonates on Marine Shelf: An Isotopic Approach

The terrigenous sedimentary budget of passive margin records variations in past sedimentary fluxes, and thus can be used to infer past variations of Earth surface deformation processes, or climate change. Accurate estimates of sediment fluxes over various times scale and space-scale are therefore crucial. Traditionally, offshore sediment sequence volumes take into account only siliciclastic accumulation, the carbonate fraction being considered only as in-situ production (i.e. CaCO3). Here we propose a new geochemical methodology to decipher and quantify the amount of detrital carbonates in comparison to in-situ produced biogenic carbonates. This isotopic approach enables taking into account the export of detrital carbonates, and investigating its effect on sediment budgets. This study, located in the Gulf of Lion, is based on a 300 m long sediment borehole located near the shelf break and covering the last 500 000 years (i.e. 5 glacial-interglacial periods). 86Sr/87Sr isotopic data (0.70809 to 0.70858) are significantly less radiogenic than modern seawater (i.e. 0.7092) and show fluctuations in agreement with stratigraphic and climatic variations. These results suggest an unsuspected high export of detrital carbonates from the catchment area during both glacial (between 55 to 85% of the sedimentary carbonate fraction) and interglacial (between 30 to 50%) conditions. Thus, not only do detrital carbonate fluxes need to be factored into sediment flux calculations, these results suggest that detrital carbonate components could potentially have a strong influence on carbonate 86Sr/87Sr ratios when not obtained from microdrilled biogenic carbonates, such as the entirety of the Precambrian Sr chemostratigraphic record.

A sequence stratigraphic model for the organic-rich Upper Devonian Duvernay Formation, Alberta, Canada

The Upper Devonian Duvernay Formation of western Canada is a prolific source rock that in recent years has become an important exploration target for shale gas and oil. We present a sequence stratigraphic model for the Duvernay Formation that characterizes relationships between sea level, basin circulation, lithofacies distribution, and mechanisms for sediment transport and organic-enrichment. The study demonstrates lateral and stratigraphic variability in the character of sequence stratigraphic surfaces and systems tracts and interprets this variability in the context of sea level cycles at varying time scales. Correlation of 24 core descriptions to a network of 759 wells with wireline logs led to the creation of a basin-scale sequence stratigraphic framework.Three 3rd order depositional sequences are identified; the middle sequence straddles a 2nd order maximum flooding surface. Each systems tract shows unique trends in lithofacies, rock properties, and wireline character. Transgressive systems tracts are characterized by the increasingly widespread deposition of clay-poor, siliceous, organic-rich mudstones in the basin, resulting from decreased clastic and carbonate input, concentration of pelagic organic matter and siliceous radiolaria, and low oxygen concentrations in basin center bottom waters. Highstand systems tracts are characterized by sustained radiolarian input and increases in carbonate sediment due to shedding of carbonate detritus during aggradation and progradation of platform margins and reefs. Lowstand systems tracts are more clastic-rich and organic-poor. Near reef complexes, coarse carbonates locally mark the base of the lowstand systems tract. Bottom currents reworked mud- and silt-sized material in toe-of-slope locations and contributed to slope-parallel progradation during regression.

Cements, Waters, and Scales: An Integrated Study of the Szeged Geothermal Systems (SE Hungary) to Characterize Natural Environmental Conditions of the Thermal Aquifer

The study area, Pannonian Basin (Central Europe), is characterized by high heat flow and presence of low-enthalpy geothermal waters. In the Szeged Geothermal Systems (Hungary), having Miocene to Pliocene sandstone aquifers with dominantly Na–HCO3-type thermal water, unwanted carbonate scaling was observed. An integrated approach consisting of host rock and scale mineralogical and petrographic analyses as well as water chemistry led to a better understanding of the characteristic natural (geogenic) environmental conditions of the geothermal aquifers and to highlight their technical importance. Analyses of the reservoir sandstones showed that they are mineralogically immature mixed carbonate-siliciclastic rocks with significant macroporosity. Detrital carbonate grains such as dolomite and limestone fragments appear as important framework components (up to ~20–25%). During water–rock interactions, they could serve as a potential source of the calcium and bicarbonate ions, contributing to the elevated scaling potential. Therefore, this sandstone aquifer cannot be considered as a conventional siliciclastic reservoir. In mudrocks, a significant amount of organic matter also occurs, triggering CO2producing reactions. Correspondingly, framboidal pyrite and ferroan calcite are the main cement minerals in all of the studied sandstone samples which can suggest that calcite saturation state of the thermal fluid is close to equilibrium in oxygen-depleted pore water. Analysis of the dominant carbonate crystals in the scale can suggest that growth of the feather dendrites of low-Mg calcite was probably driven by rapid CO2degassing of CO2-rich thermal water under far-from-equilibrium conditions. Based on hydrogeochemical data and related indices for scaling and corrosion ability, the produced bicarbonate-rich (up to 3180 mg/l) thermal water has a significant potential for carbonate scaling which supports the aforementioned statement. Taking into consideration our present knowledge of geological setting of the studied geothermal systems, temporal changes in chemical composition and temperature of the thermal water during the heating period can indicate upwelling fluids from a deep aquifer. Regarding the pre-Neogene basement, hydrologic contact with a Triassic carbonate aquifer might be reflected in the observed chemical features such as decreased total dissolved solids and increased bicarbonate content with high scale-forming ability. The proposed upflow of basin-derived water could be channeled by Neogene to Quaternary fault zones, including compaction effects creating fault systems above the elevated basement high. The results may help to understand the cause of the high carbonate scale precipitation rates in geothermal systems tapping sandstone aquifers.

Diagenesis and its impact on the reservoir quality of the fourth member of Xujiahe Formation, Western Sichuan Depression, China

Abstract The fourth member of the Xujiahe Formation (T3x4) in the Western Sichuan Depression is typical of the tight gas sandstones of China. The member is composed of three types of sandstone: litharenite, sublitharenite, and feldspathic litharenite. Some of these sandstones contain a large proportion of carbonate rock fragments. These calcarenaceous sandstones (CCSs) can be differentiated from ordinary sandstones (OSs). The reservoir properties of the fourth member are quite poor (average porosity = 5.9%, average permeability = 1.41 mD). These properties were influenced by several diagenetic processes, including compaction, feldspar dissolution, precipitation of carbonate cements, and clay cementation. Mineral distribution patterns and quantitative data show that the authigenic quartz in T3x4 sandstones is mainly sourced from SeI reactions, feldspar dissolution, and kaolinite illitization. Quartz cements occupy only a small fraction of the reservoir space. Carbonate cementation (primarily calcite) is dominant in the research area. Mineral distribution patterns and isotopic composition trends suggest that the carbonate cements in the sandstones originated both inside and outside of the sandstones. Dissolution of inorganic carbonate rock fragments is the main internal carbon source of carbonate cements in CCSs. In OSs, the contribution is much smaller. Organic carbon derived from the thermal evolution of kerogen in the mudstones that migrated into sandstones via diffusion is the main carbon source in OSs. Due to a combination of compaction of ductile fragments and carbonate cementation, T3x4 reservoir sandstones in Western Sichuan Depression exhibit low porosities. In T3x4 sandstones, porosity is highest for samples where both the detrital ductile grain content and carbonate cement content are low.

Diagenesis factor that effect the reservoir properties and gas accumulation in the 4th member of Xujiahe formation, Western Sichuan Basin, China

Reservoir properties of the fourth member of the Xujiahe Formation in the Western Sichuan Depression are quite poor (average porosity =5.9%, average permeability =1.41 mD). These properties were influenced by several diagenetic processes, especially compaction and carbonate cementation. Due to a combination of compaction of ductile fragments and carbonate cementation, sand/mud interfaces of the thick sandstone layer were tightly cemented, pore space also reduced sharply in positions where rich in ductile grains. Thus, space leaving for hydrocarbon is limited, except where both the detrital ductile grain content and carbonate cement content are low.

Authigenic carbonate formation revealed by lipid biomarker inventory at hydrocarbon seeps: A case study from the Okinawa Trough

Abstract Authigenic carbonates were recovered from the northern Okinawa Trough at 540–700 m water depth. Development of microbial communities and seepage dynamics driving the precipitation of authigenic carbonates remains poorly constrained, even though the source of methane-rich fluids, formation of Fe-rich carbonates, and the potential driving forces were previously reported. Here, petrologic observations, stable carbon and oxygen isotopic compositions, mineralogy, and lipid inventories of authigenic carbonates from the northern Okinawa Trough were analyzed. The carbonate minerals were comprised predominantly of aragonite, high-magnesium calcite, and siderite. The presence of molecular fossils diagnostic for anaerobic methane oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB) with pronounced 13C depletions (as low as −134‰), together with highly negative δ13Ccarb values (as low as −55.8‰), suggest that the carbonates precipitated from anaerobic oxidation of methane (AOM) with microbial methane as the predominant carbon source. The difference between measured and calculated δ18Ocarb values (based on 0‰ of sea water δ18O vs V-SMOW) was higher than +3‰ in one of the carbonates (GGD16), reflecting an origin from gas hydrate dissociation. Most of the carbonates revealed δ18O offsets between measured and calculated values within +3‰, or a lower measured δ18O than calculated value, suggesting a mixture of methane-derived carbonates (low δ13C/high δ18O) and detrital origin carbonates (high δ13C/low δ18O). Biomarker patterns varied significantly among samples. A suite of 13C-depleted biomarkers indicated the predominance of an ANME-1 assemblage for all samples, suggesting carbonates formed at low to medium methane flux. The predominance of ANME-1, abundant detrital minerals, and allochthonous biomarkers, as well as low carbonate contents indicate that most samples formed at greater depth within the sediment column. Only one carbonate sample in this study is interpreted to have precipitated at relatively shallower depth, as indicated by the occurrence of crocetane, dominance of aragonite, and higher overall carbonate content.

Hardgrounds of the Ordovician Baltic Paleobasin as a Distinct Type of Sedimentation Induced by Cyanobacterial Mats

The cold-water Volkhov and warm-water Keila hardgrounds of the Ordovician Baltic Basin represent a specialized product of cyanobacterial communities of the seafloor, another member in a group containing stromatolites and microbially induced sedimentary structures. The leading role of cyanobacterial communities in the development of hardgrounds is suggested by their macrofeatures, structure, and elemental composition revealed by SEM equipped with a microprobe microanalyzer. During the formation of the hardgrounds, the cyanobacterial films were of considerable thickness (up to 4–5 mm), but lacked vertical zonation of bacterial communities, thereby being at an intermediate stage between the mature biofilms and true mats. The soft substrate underlying hardgrounds was inhabited by an abundant infauna. Thanks to bioturbation, the cyanobacterial film was in places destroyed, which prevented the development of hardgrounds. In the warm Late Ordovician basin of northern Estonia, cyanobacterial films formed hardgrounds on the surface of ripple marks. Abundant peloids and warm pore waters facilitated further cementation and hardening of the substrate underneath the hardground before the burial of the ripple marks and the hardground. Cyanobacterial films appeared simultaneously over large areas of the seafloor due to settling of planktonic bacteria. The extracellular polymeric substances (EPSs) released by these bacteria were rapidly mineralized within a maximum of one season, apparently at a depth of 15–25 m, below the fair weather wave base. The formation of hardgrounds in bioherms occurred in a shallower environment and was patchy due to the alternation of living and dead cyanobacterial mats, mineralizing mats and resulting hardgrounds. Inhabitants of biohermal hardgrounds were producers of carbonate detritus, including of micritic size, over vast areas of the seafloor surrounding the bioherm.

Features of ice-rafted debris (IRD) at IODP site U1312 and their palaeoenvironmental implications during the last 2.6 Myr

In this paper, we build an age model by tuning the G. ruber δ18O curve to the LR04 stack and U1308 benthic δ18O curve with the aid of the high-resolution lightness parameter L*, which mirrors the marine oxygen isotope stages, and we analyse the major palaeoenvironmental events indicated by different proxies, particularly the >250 μm fraction and the 150–250 μm fraction of ice-rafted debris (IRD) at IODP Site U1312. In general, the input of IRD at Site U1312 can be divided into two stages. In Stage 1, ranging from ~2.6 Ma to ~1 Ma, the number of lithic grains was small, the frequency of ice rafting events was low, and the components were diverse and mainly comprised quartz, feldspar and clasts. In Stage 2, ranging from ~1 Ma to the present, the number and frequency of lithic grains considerably increased and were accompanied by detrital carbonate additions derived from the Hudson Strait after ~650 ka. Regarding the IRD features in the study area, the distinctive layers are as follows. IRD did not occur until 2.52 Ma at Site U1312, implying that icebergs discharged by the Northern Hemisphere glaciers bearing lithic grains were involved in pelagic sedimentation for the first time. Interestingly, a monotonic component of debris, magnetite with some ilmenite and quartz, was observed in a single layer at approximately 1.4 Ma. The source of this component may simply be the erosion of volcanic formations in Iceland during the glacial period. Detrital carbonate played an important role in the IRD component after ~650 ka, and the input and frequency of IRD increased dramatically, revealing glacier enhancements and the instability of the Laurentide ice sheet. The IRD characteristics at Site U1312 during MIS 22, MIS 12 and MIS 6 were different from those at other sites near the IRD belt and may be closely related to the European ice sheets. During MIS 10-8, the input of IRD was high, the dominant warm species G. ruber was too rare for the isotope test to be conducted, and the abundance of planktonic foraminifera severely declined, which may indicate that extreme conditions occurred at Site U1312 during this interval.