Formation Of Authigenic Minerals Research Articles

Revisiting silicate authigenesis in the Pliocene–Pleistocene Lake Tecopa beds, southeastern California: Depositional and hydrological controls

The Pliocene–Pleistocene Lake Tecopa beds present a well-documented example of authigenic silicate diagenesis in an ancient saline, alkaline lake environment. Controls on authigenic mineral formation and distributions were investigated in nine stratigraphic sections aligned along a north-south transect in the Tecopa basin. Specifically, potential depositional and hydrologic controls on mineral assemblages and distributions were addressed by correlating detailed sedimentological data and basin hydrology with authigenic mineral facies distributions. Deposition occurred within the Lake Tecopa basin in environments ranging from alluvial and eolian around the basin margin to lake margin, mudflat, and shallow and perennial lacustrine in the basin center. The authigenic silicate minerals include trioctahedral smectite, phillipsite, clinoptilolite, opal C-T (cristobalite-tridymite), potassium feldspar, illite, albite, and searlesite, as well as many other minor or less commonly observed phases. Authigenic mineral distributions along the margin of the basin are strongly controlled by sediment composition (primarily tuffaceous component) and lake-level variations. Authigenic mineral compositions in the center of the basin are dominated by feldspar, illite, and searlesite, and are less influenced by sediment composition or short-term changes in lake level. The authigenic silicate mineral composition in the central part of the basin is interpreted to be a result of chemical interaction with a saline, alkaline brine that moved in accord with lake-level changes and induced density-driven circulation. The results suggest that distributions of authigenic silicate minerals in saline, alkaline lake deposits are complexly related to depositional and hydrologic processes and may be of limited utility in resolving lake-level changes in ancient lacustrine systems.

Lithotectonic regularities in distribution of Cenomanian phosphorites in the Dnieper-Donets Depression

Peculiarities in the distribution of nodular phosphorites in two largest phosphorite-bearing deposits (Polpino and Shchigry) of the Voronezh anteclise are considered. Section structure, phosphorites occurrence mode, conditions and succession of authigenic mineral formation in Cenomanian sands are analyzed. Various phosphate phyto-and zoomorphs are described. The second part of the paper is dedicated to lithotectonic regularities in the distribution of Cenomanian nodular phosphorite deposits. History of the origination and development of the science of structural-facies analysis is considered. Close relations of local tectonic structures with the distribution of coal, oil, salt, and other mineral resources are demonstrated. The distribution of nodular phosphorite deposits in the Dnieper-Donets Depression depends on local synsedimentary uplifts of two types.

Sm-Nd systematics of fine-grained fractions of the Lower Cambrian blue clay in northern Estonia

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1–0.4, 0.4–0.6, and 0.6–2.0 μm) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 μm) consists of mixed-layer illite-smectite. The illite crystallinity index (I c ) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790±90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Authigenic mineral formation in the marine environment: Pathways, processes and products

Authigenic mineral formation in the marine environment: Pathways, processes and products

Influence of processes of the formation of authigenic clay minerals of terrigenous rocks on reservoir properties: Evidence from the rocks of the Vartov Arch

Filtration-capacity properties of terrigenous reservoir rocks significantly depend on catagenetic transformations of clay minerals. Results of our research revealed the authigenic nature and formation stages of kaolinite and chlorite. The volume and shape of pore space define the morphology of clay particles. Depending on the pore volume, kaolinite crystallizes either as thick tabular aggregates or as fan-shaped intergrowths. The formation of authigenic clays in pores of terrigenous reservoirs decreases filtration-capacity properties of rocks with low permeability and, conversely, increases filtration-capacity properties of rocks with high primary permeability and large pore volume.

The estuarine chemistry and isotope systematics of 234,238U in the Amazon and Fly Rivers

Natural concentrations of 238U and δ234U values were determined in estuarine surface waters and pore waters of the Amazon and Fly (Papua New Guinea) Rivers to investigate U transport phenomena across river-dominated land–sea margins. Discharge from large, tropical rivers is a major source of dissolved and solid materials transported to the oceans, and are important in defining not only oceanic mass budgets, but also terrestrial weathering rates.On the Amazon shelf, salinity-property plots of dissolved organic carbon, pH and total suspended matter revealed two vastly contrasting water masses that were energetically mixed. In this mixing zone, the distribution of uranium was highly non-conservative and exhibited extensive removal from the water column. Uranium removal was most pronounced within a salinity range of 0–16.6, and likely the result of scavenging and flocculation reactions with inorganic (i.e., Fe/Mn oxides) and organic colloids/particles. Removal of uranium may also be closely coupled to exchange and resuspension processes at the sediment/water interface. An inner-shelf pore water profile indicated the following diagenetic processes: extensive (∼1m) zones of Fe(III)—and, to a lesser degree, Mn(IV)—reduction in the absence of significant S(II) concentrations appeared to facilitate the formation of various authigenic minerals (e.g., siderite, rhodocrosite and uraninite). The pore water dissolved 238U profile co-varied closely with Mn(II). Isotopic variations as evidenced in δ234U pore waters values from this site revealed information on the origin and history of particulate uranium. Only after a depth of about 1m did the δ234U value approach unity (secular equilibrium), denoting a residual lattice bound uranium complex that is likely an upper-drainage basin weathering product. This suggests that the enriched δ234U values represent a riverine surface complexation product that is actively involved in Mn–Fe diagenetic cycles and surface complexation reactions.In the Fly River estuary, 238U appears to exhibit a reasonably conservative distribution as a function of salinity. The absence of observed U removal does not necessarily imply non-reactivity, but instead may record an integration of concurrent U removal and release processes. There is not a linear correlation between δ234U vs. 1/238U that would imply simple two component mixing. It is likely that resuspension of bottom sediments, prolonged residence times in the lower reaches of the Fly River, and energetic particle–colloid interactions contribute to the observed estuarine U distribution. The supply of uranium discharged from humid, tropical river systems to the sea appears to be foremost influenced by particle/water interactions that are ultimately governed by the particular physiographic and hydrologic characteristics of an estuary.

Bat guano and preservation of archaeological remains in cave sites

The formation of authigenic minerals in cave sediments can be used to reconstruct the paleochemical conditions that prevailed in the past, and in this way to assess the completeness of the archaeological record. Previous studies indicated that the major driving mechanisms for chemical diagenesis in prehistoric caves that result in the formation of authigenic minerals, are the degradation of bat guano and the local hydrology. We therefore investigated contemporary bat guano deposits in caves and the sediments directly below such deposits. The emerging patterns show that the formation of authigenic minerals occurs under acidic conditions within tens of years. The availability of phosphate, Al, K, and Fe increases with increasing organic matter degradation, while the availability of nitrogen and sulfur decreases. Insectivorous bat guano contains larger amounts of phosphate and is more acidic than fruit bat guano. In the first stages of diagenesis micro-chemical environments form within the degrading bat guano that result in the formation of a wide variety of authigenic minerals. During advanced stages of diagenesis steady state conditions are achieved resulting in authigenic mineral assemblages being dominated by one mineral type at the same locale. This is reminiscent of known distributions of authigenic minerals in prehistoric cave sediments. Under the acidic conditions produced by degrading bat guano deposits, calcareous artifacts and bones are not expected to persist for long periods of time, unless there is a lot of calcite in the sediments that could buffer the sediment water pH. Guano itself is not likely to be preserved. Periods of cave abandonment can be inferred from the presence of authigenic minerals in the sediments as it is during these times that bats mainly occupy the cave. The occurrence of certain authigenic minerals in open-air sites may serve as evidence for areas rich in organic matter containing phosphate (i.e., animal enclosures, latrines, sewers, trash pits) and/or bones. As the degradation process is rapid, the use of authigenic minerals to identify these loci is relevant to almost all archaeological, as well as historical sites.

Evidence for the submarine weathering of silicate minerals in Black Sea sediments: Possible implications for the marine Li and B cycles

The role of sediment diagenesis in the marine cycles of Li and B is poorly understood. Because Li and B are easily mobilized during burial and are consumed in authigenic clay mineral formation, their abundance in marine pore waters varies considerably. Exchange with the overlying ocean through diffusive fluxes should thus be common. Nevertheless, only a minor Li sink associated with the low‐temperature alteration of volcanic ash has been observed. We describe a low‐temperature diagenetic environment in the Black Sea dominated by the alteration of detrital plagioclase feldspars. Fluids expelled from the Odessa mud volcano in the Sorokin Trough originate from shallow (≈100–400 m deep) sediments which are poor in volcanic materials but rich in anorthite. These fluids are depleted in Na+, K+, Li+, B, and 18O and enriched in Ca2+ and Sr2+, indicating that anorthite is dissolving and authigenic clays are forming. Using a simple chemical model, we calculate the pH and the partial pressure of CO2 (PCO2) in fluids associated with this alteration process. Our results show that the pH of these fluids is up to 1.5 pH units lower than in most deep marine sediments and that PCO2 levels are up to several hundred times higher than in the atmosphere. These conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments. We propose that in Black Sea sediments enhanced organic matter preservation favors CO2 production through methanogenesis and results in a low pore water pH, compared to most deep sea sediments. As a result, silicate mineral weathering, which is a sluggish process in most marine diagenetic environments, proceeds rapidly in Black Sea sediments. There is a potential for organic matter‐rich continental shelf environments to host this type of diagenesis. Should such environments be widespread, this new Li and B sink could help balance the marine Li and Li isotope budgets but would imply an apparent imbalance in the B cycle.

Morphology, internal structure and chemical composition of oxidized pyrite framboids from sediments of Lake Baikal, Siberia

Three long sediment cores (BDP-98 – 600 m, BDP-96 – 200 m and BDP-93 – 100 m) drilled in Lake Baikal (Siberia) have been studied with the aims of establishing an absolute chronology and reconstructing paleoclimatic and paleoenvironmental changes in the region. The location of the lake at relatively high latitude and continental interior and a thick continuous sedimentary archive that developed in a rift system tectonic setting provide unique material for this investigation. The cosmogenic isotope 10Be was used for dating and the results indicate time spans of 8 (+0.8\-0.6) Myr for BDP-98, 5.5 (±0.13) Myr for BDP-96 and >0.7 Myr for BDP-93. Two major sedimentary facies (deltaic and hemipelagic) are distinguished by textural geochemical and mineralogical data. Detrital mineral composition suggests negligible change in provenance during the period studied. Formation of authigenic minerals, such as framboidal pyrite, vivianite and siderite, reflects variable environmental conditions in the lake and climate change in the region. Biogenic silica content shows climatic influence, which is modified by the supply of detrital material and postdepositional alterations. 10Be dating, combined with lithological analysis of the sediments, makes it possible to place temporal constrains on climate cooling at the Miocene/Pliocene boundary (5 Myr ago) and at the Early/Late Pliocene boundary (3.6 Myr ago) as well as the beginning of the northern hemisphere glaciation at about 2.5–2.6 Myr ago. The regional east-west tectonic extension of south-east Asia, related to Tibetan Plateau uplift, was confined in the Baikal area to between about 7 and 5 Myr ago, with a rifting rate calculated at 7 mm year-1. Furthermore, the 10Be data suggest that geomagnetic field intensity strengthened around the Miocene/Pliocene boundary.

Sulfur and Oxygen Isotopes in Barite Deposits of the Western Brooks Range, Alaska, and Implications for the Origin of the Red Dog Massive Sulfide Deposits

Sulfur and oxygen isotope analyses have been obtained for barite samples from the giant stratiform sulfide-barite deposits at Red Dog in the western Brooks Range of Alaska, from stratiform barite deposits elsewhere in the Red Dog district, and from stratiform and vein and breccia barite occurrences in the central Brooks Range. Twelve of the 15 deposits studied lie within middle to Upper Mississippian black shale and chert units. The data reveal two different patterns on δ 34 S versus δ 18 O plots. The first, which is best illustrated by the barite deposit at Anarraaq, shows linear trends with slopes that vary with barite texture. For most samples, δ 34 S and δ 18 O values are both higher than the values characteristic of Mississippian marine sulfate. The second pattern, which is evident at the Red Dog deposits, shows no correlation between δ 34 S and δ 18 O. In most samples, δ 18 O is below the value for Mississippian marine sulfate. Comparisons with sulfate in modern marine environments suggest a possible model for the mineralizing process. Anarraaq-type barite formed at sea-floor vents where ascending fluids carrying barium and methane encountered sulfate-bearing pore waters or bottom waters. Barite deposition was accompanied by the reduction of sulfate to H 2 S by means of microbially mediated anaerobic methane oxidation. Red Dog-type barite was formed in a manner similar to Anarraaq-type barite but was overprinted by a massive sulfide-forming event. Red Dog sulfides precipitated where metal-bearing hydrothermal fluids encountered pore waters that had been charged with H 2 S by anaerobic methane oxidation. Textural and isotopic evidence indicates that the sulfide bodies grew by consuming the available H 2 S and then by reductively dissolving barite. Dissolution of barite caused barium to be released to higher stratigraphic levels where it was reprecipitated on encountering sulfate. Isotopic evidence is presented for a link between methane venting and barite formation and raises the possibility that the coexistence of barite and sulfide at Red Dog, and the occurrence elsewhere in the district of barite-only and sulfide-only deposits, can be explained by a spectrum of vent types in the Mississippian basin analogous to the spectrum that is observed today along the modern continental margins. Authigenic barite formed at some but not all methane seeps, perhaps owing to differences in the barium content of vent fluids, differences in the relative proportion of aqueous fluid and gas emanating from vents, or differences in sulfate availability in local bottom waters. Some barite-forming seeps were later replaced by sulfides (Red Dog deposits) whereas others were not (e.g., Anarraaq barite horizon, Gull Creek, Moil). At sulfide occurrences where there is little evidence of preexisting barite (e.g., Anarraaq, Wulik, Suds), methane venting is indicated by fossils suggestive of chemosynthetic fauna. Mammiform sedimentary structures that are widespread in black chert at the top of the Kuna Formation may represent seeps that supported neither authigenic mineral formation nor chemosynthetic megafauna.

Chemical remagnetization of the Upper Carboniferous-Lower Triassic Pyeongan Supergroup in the Jeongseon area, Korea: fluid migration through the Ogcheon Fold Belt

SUMMARY Palaeomagnetic and rock magnetic investigations have been carried out for the Upper Carboniferous‐Lower Triassic Pyeongan Supergroup, exposed in the Jeongseon area in eastern South Korea. A total of 302 independently oriented core samples were drilled from 24 sites for the study. The mean direction in stratigraphic coordinates (D/I = 345.3 ◦ /38.4 ◦ , k = 4.8, α95 = 18.2 ◦ ) is more dispersed than the mean direction in geographic coordinates (D/I = 354.9 ◦ /61.5 ◦ , k = 58.0, α95 = 4.7 ◦ ), and the stepwise untilting of the characteristic remanent magnetization (ChRM) reveals a maximum value of k at 0 per cent untilting. Furthermore, authigenic magnetic mineral grains accompanied with altered clays are identified by electron microscope observations. These results collectively imply that the ChRMs of this study were acquired by the formation of authigenic magnetic minerals after tilting of the strata. The palaeomagnetic pole position (97.3 ◦ E, 82.4 ◦ N, A95 = 6.8 ◦ ) of the Pyeongan Supergroup calculated from the site mean directions of the ChRMs in geographic coordinates is close to those of the Late Cretaceous and Tertiary periods of the Korean Peninsula. The presence of many hydrothermal vein deposits, reportedly formed between 94 and 73 Ma, near the study area and the evidence for extensive alteration of clay minerals under the influence of fluids shown in the X-ray diffraction study strongly suggest that the chemical remanent magnetization was acquired by a fluid-mediated process during the Late Cretaceous‐Early Tertiary. Considering the different remagnetization timing of several areas in the Ogcheon Fold Belt, which extends from NE to SW diagonally across the Korean Peninsula, it is interpreted that the fluids and magmatism associated with this remagnetization originated from the subduction of the Kula/Pacific plates under the Eurasian Plate during the Cretaceous period. The fluids might have migrated northward or northeastward through the fault systems within the Ogcheon Fold Belt.

Fe redox cycling in Iberian continental margin sediments (NE Atlantic)

In this paper, data are presented on the vertical distribution of pore water Fe 2+ , ferrozine-extractable Fe 2+ and solid phase Fe in sediments along four across-slope transects including the Nazare canyon at the Iberian margin. Sorbed Fe 2+ cannot be measured directly and is operationally defined as the fraction that can be extracted with ferrozine. Our objectives were (1) to investigate the potential role of Fe 2+ sorption in the Fe redox cycle, (2) to quantify Fe redox cycling and (3) to determine its rate limiting factors, with emphasis on differences between stations across the slope and in the canyon. In all sediments pore water Fe 2+ and ferrozine-extractable Fe 2+ concentrations increased simultaneously with depth until a maximum was reached and upon which the pore water Fe 2+ concentration rapidly declined, presumably due to precipitation as iron-sulfide. The ferrozine-extractable Fe 2+ concentration, however, either slowly diminished or remained unchanged when going deeper into the sediment, suggesting ongoing reaction at the sorption surfaces or much slower desorption than adsorption kinetics. Upon Fe reduction, Fe 2+ is released into the pore water where it either directly precipitates and/or adsorbs onto available surfaces of the sediment matrix, including organic matter. Through sorption, authigenic ferrous mineral formation is delayed and Fe 2+ may be transported deeper into the sediment. There, sorbed Fe 2+ can act as a deep source for authigenic ferrous mineral formation. A simple steady-state model was formulated that includes Fe 2+ sorption as a first-order kinetic reaction to estimate Fe reaction rates. Fe oxidation and reduction rates were most intense at the shelf, where organic carbon mineralization rates are high, and decreased with water depth. In the canyon, where deposition fluxes were high, Fe reaction rates increased with water depth until a maximum at the foot of the canyon at 3097 m and decreased again to the abyssal at 4280 m. Turnover times estimated for pore water Fe 2+ , ferrozine-extractable Fe 2+ and solid phase Fe both in the oxidized and reduced layer indicated that sediment mixing was the most important rate limiting factor for Fe cycling at all stations of the Iberian margin. The contribution of Fe reduction to organic matter mineralization was 5% at the 104-m and 113-m stations on the main transect and 6% at the 3097-m station at the foot of the canyon. At the other stations Fe reduction contributed less than 4% to the mineralization of organic matter.

K Uptake by Modern Estuarine Sediments During Early Marine Diagenesis, Mississippi Delta Plain, Louisiana, U.S.A.

ABSTRACT Marine sediment porewaters are commonly depleted in K+ relative to conservative mixing trends, but the mineralogical sink for K+ in the sediment has not been well characterized. Results are presented from a geochemical study of surface waters and porewaters and associated muddy sediments in the Mississippi Delta plain estuary spanning the salinity gradient from 0-12‰. Evidence of K+ depletion in sediment porewaters is integrated with scanning transmission and analytical electron microscopy (STEM/AEM) analyses of clay components to determine possible mineralogical sinks for K+ in the sediments. Conservative mixing between the freshwater influx from the Atchafalaya/Mississippi river systems and Gulf of Mexico seawater controls the surface-water major-element composition. Porewaters from the most saline site, however, are depleted in K+ by up to 33% relative to overlying water, implying uptake of K+ by the sediment. Textural characterization of the sediment at the same location by TEM indicates that it is dominated by aggregates of highly disordered smectite-rich clay material, which is mixed with a small component of more-ordered illite-rich material. AEM compositions obtained on the most smectite-like component in these aggregates indicates that the average K+ contents increase by up to 1.0 wt % K2O relative to similar smectite-rich clay in freshwater sediments. Ca2+ concentrations in smectite-rich clay decrease concomitantly, whereas Na+ concentrations are similar between the sites. X-ray diffraction (XRD) analyses confirm that smectite-rich clay from the freshwater site has a larger interlayer spacing than smectite-rich clay from the brackish-water site, consistent with exchange of smaller hydrated interlayer K+ (± Na+) for larger Ca2+ (± Mg2+). AEM data indicate that the discrete illite-rich component also takes up K+. The integrated porewater and sediment analytical data imply that uptake of K+ by smectite-rich clay is a rapid process (decadal) occurring at the earliest stages of diagenesis in estuarine environments upon exposure to brackish water. Our study shows that K+ uptake in marine sediments may not require the formation of authigenic minerals. Instead, exchange of K+ for other interlayer cations in smectite-rich clay and weathered illite may provide a substantial sink for K+ in the marine environment.

Organic carbon accumulation and metabolic pathways in sediments of mangrove forests in southern Thailand

Rates of organic carbon accumulation, mineralization and burial in sediments were examined during dry and wet seasons in four mangrove forests of a shallow-water embayment (Ao Sawi) in southern Thailand. Mass sediment accumulation (MAR), estimated from 210Pb and 137Cs profiles, was rapid at all forests, ranging from 2.9 to 7.6 kg m −2 yr −1; mixed layer sediment thicknesses ranged from 16 to 38 cm. Total inputs of organic carbon ranged from 26.4 to 40.9 mol C m −2 yr −1 and burial rates ranged from 15.3 to 23.4 mol C m −2 yr −1. Total rates of carbon mineralization, estimated from direct measurements of CO 2 gas flux from exposed sediments, from DIC flux from submerged sediments, and from core incubations for ΣCO 2, ranged from 7.0 to 16.4 mol C m −2 yr −1 with few consistent seasonal differences among the four forests. Rates of O 2 gas flux into exposed sediments ranged from 9.4 to 91.4 mmol O 2 m −2 d −1 and were significantly greater than rates of dissolved O 2 flux into submerged sediments (range: 21.9–38.3 mmol O 2 m −2 d −1). Oxic respiration was the largest decomposition pathway, with sulfate reduction (range: 0.6–16.9 mmol S m −2 d −1) the next most important pathway. Other metabolic pathways appeared to be minor, and methane was not detected in the porewater or in the gas flux chambers. The discrepancy between rates of O 2 and CO 2 fluxes implies involvement in geochemical processes (e.g. sulfide oxidation, authigenic mineral formation). Sediment budgets indicate that organic carbon preservation was greatest (71% burial efficiency) in the oldest forest with equivalent burial efficiency (57–59%) in the younger forests. Mineralization efficiency ranged from 27–40% with the lowest efficiency at the oldest forest. These tropical mangrove forests are storage sites for sediment and, on average, retain approximately 60% of total input of organic carbon to the sediment.

Role of Siliceous Fossil Dissolution in Downcore Variations of Grain Size and Water Content: Western Margin of Clarion-Clipperton Fracture Zone, NE Equatorial Pacific

Grain size and water content in box-core sediments from the Clarion-Clipperton fracture zone (C-C zone) in the northeast equatorial Pacific were analyzed in detail to understand the downcore variations across a hiatus between Quaternary and Tertiary layers. Grain-size distributions in the topmost core sediments show two modes: a coarse mode (peaked at 50 μm) and a fine mode (at 2–25 μm). The coarse mode disappears gradually with depth accompanied by the dissolution of siliceous fossil tests, whereas the fine mode coarsens due to the formation of authigenic minerals. Water content increases abruptly across a color boundary between an upper pale brown layer and a lower dark brown layer that is the hiatus between Quaternary and Tertiary layers. Abundant smectites and microvoid molds, which are created by the prolonged fossil dissolution in the underlying sediment, are attributed for the abrupt downcore variation of water content. Overall variations in grain size and water content in the topmost core sediments in the western C-C zone are possibly constrained by the dissolution of biogenic siliceous fossils. Variations in geotechnical properties related to these changes must be considered in the design of nodule collectors.

火山岩貯留岩の成立条件 孔隙の形成と保存

In terms of generation and preservation of porosity, volcanic-rock reservoirs in Japan are studied. A model of generation and preservation of dissolution pores during interaction between volcanic rocks and sea- or formation-water is discussed.The volcanic rock reservoirs can be divided into the primary-porosity type, the dissolution-porosity type and the fracture-porosity type, based on the origin and preservation mechanism of pores. The first type was observed in Miocene pyroclastic rocks in the Yufutsu Oil and Gas field, Hokkaido. They are mainly composed of scoria and fine volcanic glasses which erupted on a volcanic island. Their pores are neither filled with clay minerals nor collapsed by compaction, but supported by unaltered brittle volcanic fragments.The second type was recognized in the Ayukawa Oil and Gas field and in the Akita area, northeastern Honshu. A reservoir in the Ayukawa Oil and Gas field occurs in dolerites which intruded into unconsolidated argillaceous sediments. Its pores were formed by dissolution of feldspar and clay minerals which once replaced mafic minerals. The framework that preserves pores is an initial texture which was strengthened by secondary K-feldspar precipitated around the original feldspar.Reservoirs in the Niigata area occur in Miocene acidic rocks which erupted on a seafloor. Their pores were formed by dissolution of groundmass and feldspar. The framework that preserves the pores is formed by aggregated authigenic quartz such as spherulite.The third type was observed in the Yurihara Oil and Gas field. One of its reservoirs was formed in highly altered basalt which erupted on a seafloor in the Miocene. Its pores were formed by fractures in porous lava flows which were strongly altered to become characteristically brittle.Microscopic observation of the dissolution pores and the experiments by the previous studies in water-rock interaction suggest that the dissolution pores and the framework could be formed with the formation of authigenic minerals during the water-rock interaction when igneous rocks erupted in the seawater or intruded into unconsolidated sediments. If this model is followed, the more water the igneous rock reacts with, the more dissolution pores and authigenic mineralization may occur. Accordingly, constant fluid flows may cause the generation of dense distribution of dissolution pores.

Microbial signatures in peritidal siliciclastic sediments: a catalogue

A catalogue of microbial structural signatures is presented, based upon the coupling of fundamental biogeochemical–microbial processes and local morphogenetic determinants. It summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones (temperate humid: Mellum Island, southern North Sea; subtropical arid: coast of southern Tunisia). Textural geometries reveal a high structural diversity, but their determinants are primarily based upon six major parameters: (1) intrinsic biofactors: structural diversification of sedimentary microbial films and mats inherent in the organisms, i.e. their construction morphology, growth, taxis and behaviour, and local abundance of specific morphotypes. Most prominent are the ensheathed filamentous cyanobacteria Microcoleus chthonoplastes and Lyngbya aestuarii, and the sheathless filamentous cyanobacterium Oscillatoria limosa. (2) Biological response to physical disturbances: sediment supply, erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats. (3) Trapping/binding effects: physicobiological processes give rise to grain orientations and wavy to lenticular lamina, lamina‐specific grain arrangements and ‘sucrose’ calcium carbonate accumulations. (4) Secondary physical deformation of biogenic build‐ups: mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures. (5) Post‐burial processes: textural fabrics that evolve from mechanical effects of gas formation from decaying mats, and features related to the formation of authigenic minerals (calcium carbonates, calcium sulphates, pyrite). (6) Bioturbation and grazing: post‐depositional structures, such as Skolithos‐type dwellings, traces of burrowing insects, gastropod grazing traces and faecal pellets. In synopsis, the catalogue firstly comprises a sound set of ubiquitous signatures. This uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings. The catalogue secondly documents signatures that are extremely sensitive to tidal position, hydrodynamic regime and overall climatic conditions. These kinds of signature indicate narrow facies zones, which often coincide with the activity or dominance zones of certain organisms. An overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings.

Iron and Manganese Minerals in Suspended Matter from the Barents Sea

The mineralogy of suspended matter from surface and bottom waters is studied at two sites in the Barents Sea. Along with terrigenous minerals, the suspended matter samples contain authigenic mineral phases of iron and manganese oxyhydroxides. Mn-feroxyhite, Fe-vernadite, goethite, and proto-ferrihydrite were identified in samples from surface waters, whereas birnessite and nonferruginous vernadite were registered in samples from bottom waters. The formation of suspended manganese minerals in bottom waters is explained by an additional Mn supply from underlying reduced sediments during their early diagenesis and oxygen depletion in the near-bottom nepheloid layer. Bacteria are supposed to take part in the authigenic mineral formation.

Controls on the formation of authigenic minerals in association with decaying organic matter: an experimental approach

Carcasses of the shrimp Crangon crangon were incubated in a marine medium under oxic conditions at 15°C which was inoculated with a consortium of sulfate-reducing, sulfide-oxidizing, and fermentative bacteria. These standard conditions were varied by adding sediment, omitting sulfate, adding glucose, omitting the inoculum, adding phosphate, and enhancing the buffer capacity. The chemical gradients generated by decay were monitored over a period of 29 days with O 2, pH- and sulfide-microelectrodes. In most of the experiments oxygen was depleted, pH decreased and sulfide accumulated around the carcass within a week, creating steep chemical gradients, and decay was predominantly anaerobic. By 29 days maximum change in O 2 concentration was from around 200 to 0 μM, in pH from 7.5 to 6.2, and in sulphide concentration from 0 to 5.6 mM. Although weight loss and general decay were least when only indigenous bacteria were present, only CaCO 3 crystal bundles formed and there was no soft tissue preservation. In contrast, where decay and weight loss were more extensive anaerobic sulphate reduction was intense, pH decreased markedly, and some muscle tissue was replicated in CaPO 4. The pH close to the decaying carcass seemed to determine whether CaCO 3 or CaPO 4 formed. Paradoxically, the exceptional preservation of soft-tissues in fossils requires elevated rather than restricted microbial activity as this leads to anaerobically driven authigenic mineral formation.

Precipitation sealing and diagenesis: 1. Experimental results

During burial and diagenesis of granular aggregates, significant permeability reduction may be induced by the formation of low‐temperature, authigenic minerals. To quantitatively assess the importance of this process, we have conducted a series of hydrothermal flow‐through experiments using deionized water and labradorite/quartz sand. All experiments were conducted in a modified triaxial apparatus, configured to allow continuous permeability measurements. Under most of the conditions tested, significant permeability reduction is observed with no concurrent decrease in porosity. The overall permeability reduction sometimes exceeds 1 order of magnitude over 4 days and is positively correlated to temperature and deviatoric stress. Scanning electron microscope observations together with data from additional experiments show that the observed permeability reduction is entirely a result of secondary mineral growth. Si and Al concentrations in the postexperiment fluids are also correlated to temperature and stress, confirming the link between the chemical state of the system and permeability behavior. In all experiments, permeability reduction is fastest early and levels off in the late stages. To explain the permeability behavior as a function of time, a conceptual model is developed in which precipitation of authigenic minerals is rapid at early times while dissolution of quartz and labradorite is most active. As the system approaches equilibrium, the components necessary for secondary mineral formation are liberated at a lower rate, thereby causing precipitation to slow. Although authigenic mineral formation does not reduce total pore space in these experiments, there is a reduction in effective porosity, which results in permeability reduction.