Early Diagenetic Reactions Research Articles

Sequential growth of early diagenetic freshwater siderites in the Holocene coastal deposits, Korea

Spherulitic siderite grains ranging from 100 to 250 μm in diameter occur abundantly in a sediment layer, which is almost 8600 years old, in Namyang Bay, on the west coast of Korea. These large siderite grains consist predominantly of FeCO 3 (average 65%) and MnCO 3 (average 22%) and have low Mg/Ca and Mg/Fe ratios. Compositional scatter plots, together with the high Mn and low Mg contents, indicate that meteoric pore water was mainly involved in the siderite precipitation. Examination of the siderite grains with an electron microprobe reveals a systematic, micron-scale, center-to-margin compositional variation within individual grains. Three zones are recognized on the basis of the compositional variations: Zone I (center) in which Mn decreases and Fe increases, Zone II (intermediate) in which Mn increases and Fe decreases, and Zone III (outer) in which both Mn and Fe decrease and Mg increases. This zoning is interpreted as a result of depth-related early diagenetic reactions with different Fe, Mn, and Mg concentrations in the sediment pore waters. Meanwhile, compositional mapping of spherulitic grains implies that a siderite grain consists of numerous tiny siderite precipitates. Such dust-like tiny siderite precipitates appear to be adhered to the large spherulitic siderite grain. In some cases, large siderite grains are adjoined together and occurred as a siderite aggregate. We suggest that a spherulitic siderite grain is growing sequentially by adhering of numerous tiny siderite precipitates formed in each diagenetic zone onto the siderite grain like the formation of a snowball.

High‐resolution analysis of early diagenetic effects on magnetic minerals in post‐middle‐Holocene continental shelf sediments from the Korea Strait

Two studied sediment cores from the Korea Strait contain mud sequences (14 m and 32.62 m in thickness) that were deposited during the last 6,000 years. The sediments have uniform lithology and geochemical properties, however, marked down‐core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties. An expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep‐sea sediments. The studied sediments are divided into four descending intervals, based on magnetic property variations. Interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite, and the lowest solid‐phase sulfur contents. Interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations, which suggest active reductive dissolution of detrital magnetic minerals. Interval 3 is marked by a progressive loss of hematite with depth, and coincides with the minimum magnetite concentration. Interval 4 has an increasing down‐core enhancement of authigenic greigite, which apparently formed at depths of 3–30 m below the sediment‐water interface due to arrested pyritization reactions. These results indicate delays of thousands of years for acquisition of magnetizations carried by greigite, which suggests that studies of geomagnetic field behavior from greigite‐bearing continental shelf sediments should be conducted with care. Also, virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn‐depositional environmental processes.

Sediments, porewaters and diagenesis in an urban water body, Salford, UK: impacts of remediation

AbstractContaminated sediments deposited within urban water bodies commonly exert a significant negative effect on overlying water quality. However, our understanding of the processes operating within such anthropogenic sediments is currently poor. This paper describes the nature of the sediment and early diagenetic reactions in a highly polluted major urban water body (the Salford Quays of the Manchester Ship Canal) that has undergone remediation focused on the water column.The style of sedimentation within Salford Quays has been significantly changed as a result of remediation of the water column. Pre‐remediation sediments are composed of a range of natural detrital grains, predominantly quartz and clay, and anthropogenic detrital material dominated by industrial furnace‐derived metal‐rich slag grains. Post‐remediation sediments are composed of predominantly autochthonous material, including siliceous algal remains and clays. At the top of the pre‐remediation sediments and immediately beneath the post‐remediation sediments is a layer significantly enriched in furnace‐derived slag grains, input into the basin as a result of site clearance prior to water‐column remediation. These grains contain a high level of metals, resulting in a significantly enhanced metal concentration in the sediments at this depth.Porewater analysis reveals the importance of both bacterial organic matter oxidation reactions and the dissolution of industrial grains upon the mobility of nutrient and chemical species within Salford Quays. Minor release of iron and manganese at shallow depths is likely to be taking place as a result of bacterial Fe(III) and Mn(IV) reduction. Petrographic analysis reveals that the abundant authigenic mineral within the sediment is manganese‐rich vivianite, and thus Fe(II) and Mn(II) released by bacterial reactions may be being taken up through the precipitation of this mineral. Significant porewater peaks in iron, manganese and silicon deeper in the sediment column are most probably the result of dissolution of furnace‐derived grains in the sediments. These species have subsequently diffused into porewater above and below the metal‐enriched layer.This study illustrates that the remediation of water quality in anthropogenic water bodies can significantly impact upon the physical and chemical nature of sedimentation. Additionally, it also highlights how diagenetic processes in sediments derived from anthropogenic grains can be markedly different from those in sediments derived from natural detrital material. Copyright © 2003 John Wiley & Sons, Ltd.

A pH plate fluorosensor (optode) for early diagenetic studies of marine sediments

Pore water pH distributions are closely coupled to early diagenetic reactions and transport processes in surficial sediments. In this study, an optical plate fluorosensor for rapid two‐dimensional detection of H+ concentration patterns in sediment pore waters was developed. The dual excitation (405/450 nm) single emission (520 nm) pH fluorophore HPTS (8‐hydroxypyrene 1, 3, 6, trisulfonic acid trisodium salt) was immobilized onto transparent, supporting sensor foils. Excitation/emission spectra of immobilized HPTS exhibited pH dependent shifts in seawater standards, and fluorescence excitation ratios were used for two‐wavelength, ratiometric detection of pH distributions in sediment and bottom water close to the sediment—water interface (pH ~ 6.5 to 8.5). Sensor foils were fixed to the inner sides of plastic box corers or glass plate aquaria containing sediment and overlying seawater. Equilibration with pore water occurred in seconds, and fluorescence response (excitation/emission) was scanned nondestructively using a monochrome, integrating CCD camera. The two‐dimensional sensor system has a vertical and horizontal resolution of 56 × 54 µm (5 × 5 pixels) applied over an image of 34 × 26 mm. Reproducibility and accuracy were best when foils were prepared from solutions of similar ionic strength to samples. Optical sensor pH distributions correlated well with high‐resolution, vertical distributions measured simultaneously with a minicombination pH electrode, and two‐dimensional pH patterns demonstrated directly the transport‐reaction heterogeneity associated with microtopography and biogenic structures in surface deposits and overlying waters close to the sediment—water interface. Sensor foils retained calibration characteristics for at least 50 d.

Origin of tourmaline in the metamorphosed Sikait pelitic belt, south Eastern Desert, Egypt

A Neoproterozoic metapelitic schist belt at the Sikait area of the south Eastern Desert is a favourable environment for localisation of tourmaline mineralisation in the Pan-African Belt in Egypt. Local concentrations of tourmaline in the Sikait area are closely associated with stratiform metapelitic schists. They are confined to the Nugrus shear zone, along which various leucocratic rocks (leucogranite, pegmatite, aplite) are syntectonically emplaced with various stages of hydrothermal quartz veins. Tourmaline occurs either as disseminated isolated clusters of crystals or as discontinuous tourmalinite bands within the metapelitic rocks, pegmatites and quartz veins. Four types of tourmaline-rich rocks were distinguished: (i) fine-grained tourmaline-rich rocks, which are associated with the biotite schist along contacts with gneissose granites (TR1); (ii) tourmaline-rich rocks, associated with the metapelitic and amphibolitic closely to Nugrus thrust zones (TR2); (iii) tourmaline-rich rocks, associated with the pelitic hornfels at the contact between metapelitic schist and leucogranite (TR3); and (iv) tourmaline-rich rocks and quartz veins, associated with chlorite-graphite schist in contact aureoles with leucogranite and pegmatite veins (TR4). Microprobe analyses revealed that tourmalines are Al saturated for the given Fe/Mg alkali-deficient group tourmaline with minor X-site vacancy and substitution of Ca for Na. Tourmalines belong to the schorl-dravite solid solution series and have a wide compositional range, from nearly end member dravite for TR3 tourmalines to schorl for TR4 tourmalines; TR1 and TR2 tourmalines have intermediate compositions. The Fe/(Fe + Mg) varies between 0.02 and 0.89. Variation in composition of Al-rich tourmalines is essentially caused by variations in Mg, Fe, Na, Ca and Ti. The whole rock chemical analyses of tourmaline-rich rocks closely resemble the trends observed for metapelitic schist and leucocratic rock and reflect mixing between phyllosilicate-rich and quartz-rich end members, which indicates that tourmaline-rich rocks do not contain a significant detrital component. Chondrite-normalised patterns of rare earth elements (REE) in tourmaline-rich rocks and quartz-rich tourmalines are similar to those of the surrounding unaltered metapelitic schists and leucocratic rocks, respectively. Minor depletions of LREE and local negative Ce anomalies characterise the chondrite-normalised REE pattern of TR1 tourmaline-rich rocks, suggesting its formation in the presence of seawater-derived fluids. However, TR4 types are characterised by low content ΣREE, such as that of leucocratic rocks. Thus, the geochemical data imply relative immobility of Al, Ti, Cr and HREE during hydrothermal alteration and later metamorphism. The different tourmaline varieties and their respective compositions are interpreted in terms of multistage evolution. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of biotite-pelitic sediments with these fluids and subsequent recrystallisation during late regional deformation and metamorphism to give TR2 tourmaline-rich rocks. The TR3 tourmaline-rich rocks mainly developed by the thermal metamorphic recrystallisation of TR1. Tourmaline-rich rocks and veins adjacent to leucogranite and pegmatite veins (TR4) are the result of B-metasomatism; the primary B having been recycled from tourmalinites during regional metamorphism and magmatism.

Clay and acid catalysed isomerisation and cyclisation reactions of highly branched isoprenoid (HBI) alkenes: implications for sedimentary reactions and distributions

Several highly branched isoprenoid alkenes isolated from the diatom Haslea ostrearia have been subjected to laboratory simulations of early diagenetic reactions under mild acid conditions including addition of K-10 Montmorillonite clay. The products of these reactions have been found to be highly dependent on the degree of unsaturation of the alkenes. For dienes, double bond migration and geometric isomerisation are observed as the only reactions. In contrast, trienes undergo both isomerisation and rapid cyclisation reactions to yield substituted cyclohexenes. Molecular mechanics calculations have been performed to complement the laboratory experiments and to assist in proposing reaction mechanisms. The significance of these findings for the interpretation of the numerous reports of highly branched isoprenoids in Recent and ancient sediments is discussed.

A comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off Uruguay (SW Atlantic)

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mossbauer spectroscopy. Both methods showed good agreement ( <6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Early carbonate diagenesis in slope sequences and at their boundaries, Cenozoic, offshore New Jersey (ODP Leg 150)

This paper reports the genetic links among the depth distribution, mineralogy, and stable isotopic composition of diagenetic carbonates with sedimentation rates and types and preservation of organic matter in the terrigenous and biogenic sediments of Oligocene and Miocene age on the New Jersey slope. Calcites formed close to the sediment surface at sequence boundaries and maximum flooding surfaces, when the profile of early-diagenetic reactions was stabilized in the sediment column for extended periods. Dolomites precipitated in the sulfate reduction zone when diagenetic profiles stabilized during truncation, sequence boundary formation, and the deposition of lowstand sediments that overlie the sequence boundaries. Most dolomites occur in distal slope sediments that were deposited before the shelf had prograded into the study area. Siderites formed during a later stage of burial in the methanogenic zone; they are not directly genetically related to the sequence stratigraphy of the New Jersey slope. The diagene-tic dolomites and siderites occur in widely separated depth intervals below the present sea floor. The distribution of the diagenetic carbonates and their preferential occurrence in separated depth intervals resulted from different combinations of sedimentation rates and organic matter types and preservation.

Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain)

Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ18O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ18O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism.

Element supply to surface sediments and interrelationships with nodules along the Aitutaki-Jarvis Transect, South Pacific

A suite of surface sediment samples has been studied from sites on the Aitutaki-Jarvis Transect, between the equator and 17°S at approximately 160°W in the Central Pacific Ocean. The sediments were analysed by bulk and partition analysis in order to characterize changes in sediment geochemistry with water depth and with latitude, with particular reference to the effects of variation in supply and dissolution of biogenic material. The surface sediments are heterogeneous, ranging from carbonate or siliceous oozes to pelagic red clays. The elements in the sediments are variably partitioned into different sediment fractions. Examination of the sediments along the Transect has also provided a profile of the carbonate compensation depth, which shows it to deepen from approximately 4800 m at 16°S to approximately 5300 m at the equator. The relationships between the bulk sediments and the sediment reducible fraction and associated Mn-Fe nodules has been examined. Variations in the interrelationships between the nodules and the sediment reducible fraction are ascribed to variations in element supply and early diagenetic reactions. A major decoupling of the relationship between nodules and associated sediment reducible fraction is observed in the north of the Transect at water depths close to the position of the CCD. Diagenetic reactions involving the destruction of organic matter which result in the reductive remobilization of Mn and the release of trace metals from labile material are thought to be important causes of this decoupling.

Influence of terrestrial weathering on early diagenetic reactions in continental shelf sediments

Influence of terrestrial weathering on early diagenetic reactions in continental shelf sediments

Geochemical indicators of depositional and early diagenetic facies in Holocene carbonate muds, and their preservation potential during stabilisation

Holocene carbonate muds (<63- μm fraction) and macro-organic matter were collected from marine subtidal mudbanks and supralittoral mud islands of Florida Bay, and freshwater marshes of the Florida Everglades, U.S.A. Geochemistry, sedimentology and mineralogy were used to determine whether carbonate mud formation in specific marine and freshwater environments was geochemically distinct, and to assess the importance of early diagenetic reactions on mud chemistry and mineralogy. Significant variations were found in the relative amounts of carbonate minerals, not just between the marine and freshwater environments, but also within Florida Bay. In particular, Mg-calcite is enriched in Crane Key muds by ∼ 10%, relative to nearby Cross Bank. The Mg-calcite appears to precipitate in the surface-living cyanobacterial mats of the key, and seems to be linked to Mg chelation by the cyanobacterial organic matter. This Mg-calcite has a distinct δ 13C around − 4‰, ∼ 6‰ lighter than Mg-calcite derived from Thalassia epibionts in the Cross Bank muds. Dolomite was found just below the cyanobacterial mats, forming in the only part of the core where reactive organic carbon has been completely removed by sulphate-reducing bacteria. The δ 13C values of this dolomite (at least − 4‰) are consistent with previously published δ 13C values of dissolved inorganic carbon in pore waters at this horizon, which support dolomite formation in situ. Mg supply for dolomite formation may be from dissolution of the overlying cyanobacterial mat Mg-calcite. The various types of organic matter have distinctive δ 13C-values ranging from ∼ − 25‰ (mangrove and sawgrass) to ∼ − 10‰ (cyanobacterial mats) and − 8‰ ( Thalassia seagrass). Values intermediate between these end-members record mixing of the organic matter types, which shows particularly the time when the bank muds became dominated by Thalassia organic matter, i.e. were relatively removed from mangrove vegetation. Five Holocene mud suites (bank, key, cyanobacterial mats, mangrove swamp and freshwater Everglades swamp) show distinct chemical signatures in their mean δ 13C, δ 18O, MgCO 3 and Sr values. Na distinguishes between seawater and non-seawater influenced environments, but Fe, Mn and P are mainly controlled by non-carbonate materials. Comparison with published stable isotope and elemental data for Pleistocene limestones in the south Florida area suggests that only the facies diagnostic signatures of δ 13C and Mg will be resolvable once the muds have been stabilised to micrite. Alteration of the δ 18O and Sr values during stabilisation will probably be more severe, to the extent that original facies variations are destroyed.

Early diagenetic dolomite concretions in the late Cretaceous Herring Formation, eastern Marlborough, New Zealand

Dolomite concretions occur in the upper part of the late Cretaceous Herring Formation of eastern Marlborough, New Zealand. Herring Formation sediments are centimetre-bedded, poorly indurated, clay mineral-rich, siltstones or very fine sandstones, and were deposited in a NW-SE trending trough. Differential compaction of surrounding sediments around concretions is ubiquitous. Concretionary dolomite consists of intergrown anhedral to subhedral crystals up to 150 μm across. Pyrite is common. Insoluble residue contents indicate sediment porosities of 70% or more at the time of concretion formation, which is consistent with an origin in the upper few metres of the sediment column. Generally, concretions have near stoichiometric dolomite compositions, although a few are composed of calcite. Dolomite Fe compositions average about 4 mol% FeCO 3, with Mn contents in the range 623–9543 ppm. All concretions contain less Mn than Fe. The average Sr content ranges from 175 to 710 ppm. Concretions can be separated into a “light carbon group” ( δ 13 C = < − 6ℵ PDB , δ 18 O = < −5ℵ PDB ) in the basin centre, and a “heavy carbon group” ( δ 13 C = > −9ℵ PDB , δ 18 O = > −4ℵ PDB which are found at basin margins. Ca 2+ and Mg 2+ were derived initially from pore waters and ultimately from diffusion of overlying seawater. Palaeotemperatures calculated from δ 18O analyses indicated concretions formed at less than 60°C. Burial depths derived from palaeotemperatures are higher than expected for concretions formed in the upper sediment column. However, Sr concentrations are interpreted as indicating that the bulk of dolomite formed by the dolomitization of early diagenetic calcite. This process, along with other competing early diagenetic reactions, caused depletion in 18O in formation waters. The combination of Sr and δ 18O data are interpreted as indicating wholly marine formation without the influence of meteoric fluids. The presence of pyrite and δ 13C data indicate concretions formed in association with SO 4 2− reduction in the upper sediment column. The distribution and abundance of organic matter, metabolized by SO 4 2− reducing bacteria, was therefore probably important in the distribution and formation of dolomite concretions. Dolomitization continued as concretions were buried into the zone of bacterial fermentation. In addition, ° 13C analyses indicate that concretions formed in the basin centre contain an organic carbon component derived from upward diffusing methane, whereas basin margin concretions do not. Concretions ceased forming when they were buried to depths below which Ca 2+ and Mg 2+ could diffuse and when supplies of SO 4 2− and/or organic material became insufficient to support levels of SO 4 2− reducing bacteria. Continued burial resulted in compaction of sediments around concretions. Some basin margin concretions were later exhumed and phosphatised.

Chemical processes at the sediment-water interface

In natural waters, the sediment-water interface is the site where gradients in physical, chemical and biological properties are the greatest. Both chemical and microbiological transformation processes are responsible for cycling elements between water and sediments. This paper discusses the various chemical transformations that take place during early diagenesis in sediments, and which are fueled by supply rates of organic carbon and electron acceptors. In particular, our current knowledge of the cycling of the electron acceptors O, N, Mn, Fe and S is assessed, and the important role of transport reactions is described. The elemental fluxes across the sediment-water interface can be described from first principles only if the coupling of physical, chemical and biological processes is better understood. Large gradients in chemical potentials of many chemical species across this interface often lead to steep gradients in proton and electron activities. The redox sequence commonly observed in time and space also leads to changes in the partitioning of solutes between carrier and solution phases. Such processes can cause rapid back-diffusion of released species to the overlying water column or their removal onto secondary carrier phases within the sediments. Physical transport systems such as resuspension or bioturbation of particles control the physical boundary conditions for chemical reactions immediately above or below the sediment-water interface. Because of the complex coupling of the different chemical, physical and biological processes, chemical reactions within this region must be considered an integral part of a three-dimensional network of interactions. Hence, the apparent coexistence of chemical species considered ‘incompatible’ by thermodynamic models can be a consequence of the three-dimensional nature of redox gradients. The microbiological, chemical and physical interactions of dissolved, colloidal and particulate organic carbon, and iron and manganese oxides play a crucial role in the early diagenetic reactions at the sediment-water interface. Their effects on diagenetic reactions, and on trace elements in solution and adsorbed to particles, are discussed. Furthermore, we emphasize the way in which hydrodynamics can control elemental fluxes in environments with high carbon rain rates.

Formation and Diagenesis of Weathering Profiles

Weathering reactions mainly involve the transformation of feldspars, phyllosilicates, amphiboles, pyroxenes, and volcanic glass to the secondary mineral groups, kandites, illites, smectites, vermiculites, and/or chlorites. Although mineralogical changes are complex, bulk compositional changes to weathering profiles, resulting from chemical weathering, are simple and predictable from kinetic, thermodynamic, and mass balance considerations. Predicted bulk compositional changes are corroborated by studies of Recent weathering profiles developed on a variety of plutonic and volcanic rocks under different climatic regimes. Unlike the mineralogical compositions of profiles, the bulk compositional trends are not noticeably modified by climate; consequently, the simple, predictable bulk compositional trends observed in recent profiles provide a "norm" to which ancient weathering profiles can be compared. Early diagenetic reactions may occur prior to burial of the profile by reaction of groundwaters with secondary weathering products. These often result in abnormally high accumulations of Si, , Ca, and Mg to form clay minerals (smectites) and carbonates. The accumulations may be used as indicators of (paleo-)water tables. Late diagenetic reactions occur during and following burial through reaction of basin waters, trapped seawater, or brines with minerals of profiles. Metasomatism is common and includes production of illites, smectites, and chlorites at the expense of kaolinite and reconstitution of partially degraded feldspars to form potash feldspar and albite. Reaction with seawater (high Na/K and Mg/K) results in Na- and Mg-metasomatism, yielding albite and chlorite at the expense of partially degraded feldspars and clay minerals. In contrast, K-metasomatism of buried weathering profiles is favored around the periphery of subsiding continental sedimentary basins where dilute continental ground waters display low Na/K values. NaCl-rich brines and high temperatures in the deep central parts of basins favor the formation of albite at the expense of K-feldspar.

Chemical composition of Lower Ordovician ooids from northern Öland, Sweden, and their sedimentary host matrix

Abstract In the Lower Ordovician of northern Oland several beds contain ooids of various mineralogical composition. At Halludden and Horns Udde the ooids are composed of phosphate and calcite; at Hagudden both chamosite and phosphate ooids occur. At Byrum, beds with chamosite ooids have been observed, but also one bed containing goethitic and hematitic ooids. The chemical composition of the ooids has been examined, as well as their sedimentary host matrix, including phosphate intraclasts and calcite crystals. The results indicate that the goethitic ooids form an intermediate state between the chamosite and the hematitic ooids. It is suggested that the chamosite ooids are of primary origin, and that the other two types evolved from them by early diagenetic reactions. The phosphate ooids are probably also of primary origin. The carbonate fraction of all ooids contains much manganese, which might indicate that calcite filled the original ooid porosity at a later stage.

Possible relationships of stratigraphy and clay mineralogy to source rock potential in lacustrine sequences

Summary Petroleum accumulations in lacustrine environments often possess characteristics different from those in marine settings. In general, lacustrine petroleum fields are smaller, the oil has often not migrated far from the source, and oil shales are common. Some of these differences can be attributed to the relative youthfulness of lacustrine basins and to differences in the type of organic matter present. However, additional influences may arise from the stratigraphic framework and clay composition of the source rocks. The scale of the environment may be a major controlling factor. Gradations between large lakes and marginal marine basins exist, but lake basins will more often be areally small, relatively shallow and subject to frequent lateral migrations in depositional environments. These characteristics make the resulting sedimentary sequences much more heterogeneous, both in stratigraphic relationships and in sediment composition. Stratigraphy will ultimately influence the migration and accumulation of the oil; the sediment composition—particularly clay mineralogy—will affect the generating potential of the source rock. Shifting sediment source areas coupled with lake level fluctuations and early diagenetic reactions may produce thin mudstones containing very heterogeneous clay mineral assemblages, which, accordingly, may not undergo significant transformations during burial diagenesis.

Interstitial water studies of Late Quaternary Eastern Mediterranean sediments with emphasis on early diagenetic reactions and evaporitic salt influences

Interstitial waters of five piston cores from the Eastern Mediterranean were recovered by shipboard squeezing and analyzed for their major elements. Additionally, organic carbon, CaCO 3 and HCl extractable Mg were determined in the sediments from these cores. Three cores contain sapropels, which are considered to be the sedimentary expression of anoxic periods during the Quaternary. The pH in the interstitial waters extracted from the sapropels is significantly lower than the pH of the interstitial waters from the surrounding sediment. It is suggested that the low pH in sapropel interstitial waters is induced by bacterial activity. Other dissolved elements are not influenced by these sapropel layers. Core T83-46, taken in the hypersaline anoxic Tyro Basin, shows a strong downward SO 4 decrease, accompanied by a strong NH 4 increase. The interstitial water chemistry of the Tyro Basin sediment is attributed to selective leaching of evaporitic salt; first halite dissolution, later followed by gypsum dissolution. The interstitial water chemistry of core T83-45, taken in the adjacent Kretheus Basin, is dominated by diffusive processes, which became important after oxic conditions returned in this formerly anoxic hypersaline basin.

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins

The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light sulfide (δ 34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ 34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted, semi-stagnant basins (Baltic Sea). pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-, Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation, called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ 13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations. Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the alkalinity maximum. Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite to illite transformation) to start.

Interstitial water studies of Late Quaternary eastern Mediterranean sediments with emphasis on early diagenetic reactions and evaporitic salt influences : Ten Haven, H.L., G.J. De Lange and R.E. McDuff, 1987. Mar. Geol., 75(1–4):119–136

Interstitial water studies of Late Quaternary eastern Mediterranean sediments with emphasis on early diagenetic reactions and evaporitic salt influences : Ten Haven, H.L., G.J. De Lange and R.E. McDuff, 1987. Mar. Geol., 75(1–4):119–136