Sedimentary Diagenetic Processes Research Articles

Contrasting shale oil accumulation in the upper and lower sweet spots of the lacustrine Permian Lucaogou Formation, Junggar Basin, China

Shale oil enrichment and accumulation in lacustrine strata is rather heterogeneous (unlike that in marine strata), which is a challenging issue to study. Here we carried out a case study in the Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwestern China. Based on data from drill cores, thin sections, rock extracts, and crude oils (including well logging, petrophysics, nuclear magnetic resonance, organic geochemistry, and oil test), the differences of lithology, reservoir physical property, oil generation potential, shale oil content, and physical property and geochemistry of oil between the lower and upper sweet spots were comprehensively compared. Results show that a mixed sedimentary system comprising interbedded carbonate, siltstone, and mudstone was developed owing to terrigenous clastic sedimentation, volcanism, and carbonate deposition. The lower sweet spot has a relatively higher content of silt compared with the upper sweet spot. Due to spatial changes in the depositional environment, only the lower sweet spot occurs at the northeastern margin of the study area and only the upper sweet spot occurs at its southeastern margin, but both occur in the central part of the study area. The porosity and permeability of the sweet spots are highly heterogeneous, due to the complex sedimentary–diagenetic processes, and dissolution pores are common in the lower sweet spot. The hydrocarbon generation potential and shale oil content of the sweet spots are both excellent. Comparatively, the shale oil in the lower sweet spot has higher densities and lower wax contents than those in the upper sweet spot. Thus, the shale oil is more mobile in the upper sweet spot. This implies a high-salinity depositional environment for the lower sweet spot and the oils are generated from salt-tolerant planktonic algae. The oil saturation index (OSI) values are not entirely consistent with the test production results. This indicates that the shale oil productivity is comprehensively controlled by multiple factors, e.g., the hydrocarbon generation potential, reservoir physical properties, and shale oil mobility. These are key features that distinguish lacustrine from marine shale oil systems. The exploration and exploitation strategies of shale oil in lacustrine systems need to be carefully developed.

Hematite concretions from the Late Jurassic Jhuran sandstone, Kutch, western India: Implications for sedimentary diagenesis and origin of “blueberries” on Mars

The discovery of iron-bearing concretions is one of the most exciting discoveries in the recent past on Mars. We report here an analog terrestrial site hosting hematite-bearing concretions belonging to Jhuran sandstone, Kutch, India. Based on petrography, mineralogy, geochemistry and spectral studies (Visible Near Infrared: VNIR and Mössbauer), we show very good similarity between the Jhuran concretions and the “Martian blueberries”. The Jhuran and Martian concretions share similar textural attributes and are of comparable size, but are geochemically different. Based on VNIR study, the mineralogy of the concretions is constrained to comprise hematite, goethite and kaolinite; however, Mössbauer spectra could not detect the pure hematite phase, and instead, suggest the presence of a poorly crystalline mixture of goetheite and hematite. Thus, the Jhuran concretions are most commonly mixtures of goethite and hematite, and not pure hematite. The enrichment of Ni within concretions as compared to the host sandstones is also comparable with the reported Ni-enrichment of Martian concretions. We propose sedimentary diagenesis as the most feasible process that can explain incorporation of Ni into the concretions by coupled substitution in the iron oxide structure. This is inconsistent with the cosmic spherule hypothesis to explain Martian concretion formation, and instead, sedimentary diagenetic processes appear more likely.Finally, the morphology, texture, geochemistry and spectroscopic investigations suggest that the Jhuran iron concretions most likely formed due to sedimentary diagenetic processes, similar to inferences from other Martian analogue localities. Therefore, this study has relevance for understanding the formation processes of Martian blueberries and diagenetic processes on Mars. The Kutch area also has geological similarity with other aspects of ancient Mars and could be a potential testing site for carrying out future Mars exploration studies on Earth.

Pore structure evaluation of low permeability clastic reservoirs based on sedimentation diagenesis: A case study of the Chang 8 reservoirs in the Zhenbei region, Ordos Basin

Sedimentary diagenesis is one of the most significant factors in the development of low permeability sandstone reservoirs due to its important influence on the formation of pore structure. This study takes Group 8 of the Yanchang Formation in the Zhenbei region of the Ordos Basin as the research object. Methods including casting thin-sections, mercury injection, and scanning electron microscopy were used to analyse core and casting thin-sections to examine the sedimentary diagenetic processes, authigenic mineral composition, pore structure, and physical properties of the reservoirs in the study area. The influence of sedimentary diagenesis on the physical properties of the reservoirs was analysed, and the pore structures in the study area were divided into four types. By determining the development mechanism and distribution characteristics of each type of pore structure, the favourable reservoir areas were also predicted. The results showed that type I (large mesopores with medium throats) and type II (mesopores with fine throats) were the favourable pore structures in the research area. These were mainly formed by the rim cementation of chlorite, the dissolution of feldspar and other soluble components, and a weak to mid compaction. Areas with type I and type II pore structures are considered to have a high reservoir quality, with a porosity of between 10% and 25% and a permeability of between 1 mD and 50 mD. These areas are suggested as regions for optimised exploration and development.

Isotope alteration caused by changes in biochemical composition of sedimentary organic matter

Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state 13C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of 13C, while the isotope composition of 15N remained nearly the same. Sedimentary δ15N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ13C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ13C values of sedimentary OM, while this trend was not observed for δ15N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes.

Genesis of green sandstone/mudstone from Middle Jurassic Zhiluo Formation in the Dongsheng Uranium Orefield, Ordos Basin and its enlightenment for uranium mineralization

ABSTRACT The middle Jurassic Zhiluo Formation in the Dongsheng is comprised of a big set of green sandstone/mudstone with most of uranium orebodies occurring in close proximity to its footwall. By synthesizing field observations, region analysis, data collected from previous coal and uranium borehole, a regional north-south geological profile across the entire orefield is conducted. Experiments on sandstone/mudstone including rock mineral identification, clastic micromorphology and element geochemistry were carried out. Information from the geological profile indicates that green sandstone/mudstone is widely present in a stable horizon with clear boundaries to the country rock. Microscopic observations and geochemical data on sandstone/mudstone exhibit similar mineral composition with almost identical slightly flat, minor Eu enriched, Ce depleted chondrite-normalized REE patterns. Furthermore, the green clay membrane of the clasts has a complex composition containing chlorite/smectite, green smectite, chlorite, and green kaolinite, with elements including Fe, Mg, Si, and Al. These above results indicate that the green sandstone/mudstone underwent resemble sedimentary diagenetic processes as the country rock without transformation by large-scale regional fluid, while the existence of Fe2+-rich membrane is the main factor to the green sandstone/mudstone. Further concentration of the pre-enrichment uranium during diagenetic process led to the final formation for uranium deposits. The above studies are conducive to enrich the metallogenic mechanism of sandstone type uranium deposits and could provide certain reference for uranium exploration and deployment.

A case study of Ca-rich fluid flow in Nimura Cu-Co mine. DRC, Central African Copperbelt

The Neoproterozoic Central African Copperbelt is the one of the largest sediment-hosted stratiform Cu–Co provinces in the world, with 140 Mt of copper and 6 Mt of cobalt, including several world-class deposits (P10 Mt copper). The Congo and Zambia stratiform deposits have characteristics indicating that syngenetic and early diagenetic processes have controlled the formation of the central Africa Neoproterozoic Copperbelt and studies related to diagenetic processes in sedimentary diagenetic processes where rocks triggered the diagenetic model for copper orebodies. For the dolomitic gangue major element concentrations associated with the second phase of the mineralization in the Nimura mine; the ore deposit has been determined and compared to those enriched, depleted than those, which would have remained immobile during the alteration processes. Moreover, it has been observed that FeO is often enriched, compared to SiO2, whose mass has been decreasing. The circulated fluid was almost carbonated, characterized by CaO, which has remained immobile during the alteration process. The Cu–Co mineralization is at least mainly based in the lower ore body, controlled by lithology, and tectonic. Gangue minerals are mostly dolomite and quartz with accessory Mg–chlorite and muscovite.

Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First‐Order Reversal Curves

AbstractDiagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first‐order reversal curve unmixing method on well‐studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate‐methane transition. Below the sulfate‐methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.

Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments

Abstract. Iron (Fe) plays a key role in sedimentary diagenetic processes in coastal systems, participating in various redox reactions and influencing the burial of organic carbon. Large amounts of Fe enter the marine environment from boreal river catchments associated with dissolved organic matter (DOM) and as colloidal Fe oxyhydroxides, principally ferrihydrite. However, the fate of this Fe pool in estuarine sediments has not been extensively studied. Here we show that flocculation processes along a salinity gradient in an estuary of the northern Baltic Sea efficiently transfer Fe and OM from the dissolved phase into particulate material that accumulates in the sediments. Flocculation of Fe and OM is partially decoupled. This is likely due to the presence of discrete colloidal ferrihydrite in the freshwater Fe pool, which responds differently from DOM to estuarine mixing. Further decoupling of Fe from OM occurs during sedimentation. While we observe a clear decline with distance offshore in the proportion of terrestrial material in the sedimentary particulate organic matter (POM) pool, the distribution of flocculated Fe in sediments is modulated by focusing effects. Labile Fe phases are most abundant at a deep site in the inner basin of the estuary, consistent with input from flocculation and subsequent focusing. The majority of the labile Fe pool is present as Fe (II), including both acid-volatile sulfur (AVS)-bound Fe and unsulfidized phases. The ubiquitous presence of unsulfidized Fe (II) throughout the sediment column suggests Fe (II)-OM complexes derived from reduction of flocculated Fe (III)-OM, while other Fe (II) phases are likely derived from the reduction of flocculated ferrihydrite. Depth-integrated rates of Fe (II) accumulation (AVS-Fe + unsulfidized Fe (II) + pyrite) for the period 1970–2015 are greater in the inner basin of the estuary with respect to a site further offshore, confirming higher rates of Fe reduction in near-shore areas. Mössbauer 57Fe spectroscopy shows that refractory Fe is composed largely of superparamagnetic Fe (III), high-spin Fe (II) in silicates, and, at one station, also oxide minerals derived from past industrial activities. Our results highlight that the cycling of Fe in boreal estuarine environments is complex, and that the partial decoupling of Fe from OM during flocculation and sedimentation is key to understanding the role of Fe in sedimentary diagenetic processes in coastal areas.

Ultrasonic constraint of the microfracture anisotropy of flysch rocks from the Podhale Synclinorium (Poland)

This paper attempts to show the relationship between joints observed in flysch formations in the field and microfracture fabrics invisible to the naked eye in hand specimens. Ultrasonic measurements demonstrate that the intensity and orientations of domains “memorised” by rock specimens are associated with the historical stresses within the rock mass rather than the rock lamination. The spatial orientations of these microfractures have been measured, and their dynamic-elastic properties have been found to correlate with the orientation of macroscopic joint sets measured in the field. The elastic properties measured vary because of sedimentary diagenetic processes that occured during the tectonic deformations of these flysch rocks in the Podhale Synclinorium of Poland. The structural discontinuities detected by ultrasonic measurements can be perceived as an incipient phase of the macroscopic joints already visible in the field and are attributed to the in situ residual tectonic stresses. Such historical stresses impart a hidden mechanical anisotropy to the entire flysch sequence. The microfractures will develop into macroscopic joints during future relaxation of the exposed rock mass. Understanding the nature and orientation of the invisible microfracture anisotropy that will become macroscopic in the future is vital for the safe and efficient engineering of any rock mass.

Environmental magnetism: Principles and applications

In environmental magnetism, rock and mineral magnetic techniques are used to investigate the formation, transportation, deposition, and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes. All materials respond in some way to an applied magnetic field, and iron‐bearing minerals are sensitive to a range of environmental processes, which makes magnetic measurements extremely useful for detecting signals associated with environmental processes. Environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics, chemistry, and biology and environmental science, including research on climate change, pollution, iron biomineralization, and depositional and diagenetic processes in sediments to name a few applications. Magnetic parameters are used to routinely scan sediments, but interpretation is often difficult and requires understanding of the underlying physics and chemistry. Thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities, complexities, and limitations to interpretations. In this review, we evaluate environmental magnetic parameters based on theory and empirical results. We describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations. We also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology, chemistry, and physics. Finally, we discuss directions in which environmental magnetism is likely to develop in the future.

Mercury concentration and solid phase speciation changes in the course of early diagenesis in marine coastal sediments (Southern Baltic Sea)

On reaching the sea, mercury discharged into the environment is deposited on and buried in marine sediments. Diagenetic processes in sediments lead to changes in both mercury concentration and speciation; they may enhance mercury bioavailability and/or lead to mercury remobilisation. Total concentrations and speciation of mercury were measured in dated sediment samples from sites covering a range of different environmental conditions and mercury concentrations. In the course of ~200 year-long diagenetic processes, the dominant changes in mercury speciation have involved diffusion of labile mercury to the overlying water, transformation of organic-bound mercury to HgS under anoxic conditions or to insoluble humin-bound Hg under oxic conditions, and dismutation of HgS into soluble polysulfides under hyperanoxic conditions. Rate constants of labile-to-stable mercury species transformations and the return flux of mercury were calculated on the basis of sediment core geochronology obtained by the 210Pb method. Although the study was performed in a specific area, we believe that the observed processes are valid for other shallow, coastal seas and that the results enable mercury stability in sediments to be assessed in similar areas worldwide.

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (Norg) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the Norg deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 μmol N cm−2 yr−1. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO −3 reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 μmol N cm−2 yr−1 if DNRA is the dominant pathway. The response to salinization includes a short-term release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO −3 concentrations and greater SO 2−4 concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition.

Geochemistry of surface sediments in the southwestern East/Japan Sea

We analyzed 77 surface sediment samples collected in the southwestern East/Japan Sea from the Korea Strait through the Ulleung Basin and the Korea Plateau for grain size, calcium carbonate, organic carbon, and major (Na, Mg, Al, Fe, K, Ca, and Ti) and trace elements (P, Mn, Sr, Li, Sc, V, Cr, Co, Ni, Zn, Cu, and Pb). The chemical composition of the surface sediments was found to be highly variable spatially. Cluster analysis of surface sediment chemical compositions indicated five major geochemical sedimentary environments: basin, lower slope, coast and upper slope, inner shelf, and outer shelf. Continental-shelf sediments were rich in shell fragments and had relict and coarse-grained characteristics. Recent fine-grained sediments were only distributed in coastal, slope, and basin areas. Concentrations of Al, K, Ca, Ti, Cr, and Sc were highest in the coastal and upper slope areas and decreased with water depth. Elemental ratios using major and trace elements indicated that coastal and upper slope detrital sediments were mixtures of sediments derived from the Changjiang (Yangtze) and Nakdong Rivers. Although the concentrations of organic carbon, P, Mn, V, Co, Ni, Cu, and Pb increased with water depth, their distribution patterns indicated authigenic (V, Cu, and Pb) and diagenetic (Fe, P, Mn, Co, and Ni) origins. The distribution pattern with water depth suggested that the chemical composition of surface sediment was determined by sedimentologic and geochemical processes, such as the supply of detrital and biogenic materials, and authigenic and post-depositional diagenetic processes in sediments.

Study of the mineralization effect on the distribution of lipids in sediments from the Cretan Sea: Evidence for hydrocarbon degradation and starvation stress

Sedimentary diagenetic processes alter the composition and distribution of different lipid compounds. In the present study alterations mediated by microbial communities were investigated along a bathymetric gradient (100 m at 35°23′N–25°09′E, 617 m at 35°33′N–25°08′E, 1494 m at 35°44′N–25°08′E) over the continental margin of northern Crete (Greece, Eastern Mediterranean Sea). Bacterial abundances and distribution were studied using phospholipid linked fatty acids (PLFA), in the range of C 8–C 22, released from intact phospholipids. Lipid components (aliphatic hydrocarbons, free fatty acids, glycerides and glycolipids) were studied over a 2-month incubation period. Carbon mineralization rates at all stations indicated an uneven distribution of active aerobic bacteria with values decreasing towards the deeper stations. PLFA homologue profiles denoted that aerobic gram negative and sulfur oxidizing bacteria dominated microbial communities while the anaerobic, gram positive and sulfate reducing bacteria occurred only in traces. The n-alkane (NA) composition revealed a strong predominance of homologues with odd carbon numbers suggesting an important terrestrial contribution to the sediments. The estimated descriptive ratios of NA, the sum of short chain NA (C 15–C 20) and long chain NA (C 21–C 36) to 17 α( H),21 β( H)-C 30-hopane, before and after a two-month incubation period, indicated the occurrence of hydrocarbon degradation processes. Increased ratios of saturated to unsaturated fatty acids were also recorded after the incubation indicating the starvation of bacterial communities by the end of the experiments.

Effect of the brittle star Amphiura filiformis (Amphiuridae, Echinodermata) on oxygen flux into the sediment

O2 plays a key role in early sedimentary diagenetic processes, but the effect of most macrofaunal species on the pathways and rates of supply of O2 into the seabed are not well known. We investigated the effect of the ophiuroid Amphiura filiformis, one of the dominant macrobenthic species on soft bottoms in the northeast Atlantic, at depths of ~15–100 m, in a laboratory environment. We determined how the presence of the ophiuroid changed the total O2 uptake of macrofauna‐free sediment by combining measurements from a microcosm approach and an approach that uses microelectrodes and a flushed aquarium. We suggest that natural populations of A. filiformis can account for 80% of the total flux of O2 into the soft bottom. At least 67% of this portion is due to the diffusion of O2 across additional sediment‐water interfaces excavated by the brittle star.

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 scaling approach to spatial variability in early diagenetic processes

The traditional approach to understanding early diagenetic processes in sediments has generally been to analyze pore water and solid phases from a single core on depth scales of centimeters. The resulting data is then modeled using the approximations of lateral homogeneity and steady-state conditions. However, the continuing advancement of the field of benthic biogeochemistry and development of new microelectrode analytic techniques are clearly demonstrating that in many sedimentary environments a more sophisticated approach to measure the spatial and temporal variability is necessary. Although no one approach is appropriate for all situations, a scaling method is presented, with examples, in this study for determining appropriate sampling intervals in time and space that has considerable utility for investigating early changes in sediment geochemistry in complex natural systems. This approach is derived from scaling techniques that have been developed by physical oceanographers for the study of processes in the water column where many analogous sampling problems are encountered.

Geochemistry of the Manganese Ore Process in the Ocean: Evidence from Rare Earth Elements

Processes governing the formation of rare earth elements (REE) composition are considered for ferromanganese deposits (nodules, separate parts of nodules, and micronodules of different fractions) within the Clarion–Clipperton ore province in the Pacific Ocean. It is shown that ferromanganese oxyhydroxide deposits with different chemical compositions can be produced in sediments under similar sedimentation conditions. In areas with high bioproductivity, the size of micronodules has a positive correlation with the Mn content and Mn/Fe and P/Fe ratios and a negative correlation with Fe, P, REE, and Ce anomaly. The behavior of REE in micronodules from sediments within bioproductive zones is related to increase of the influence of diagenetic processes in sediments as a response to the growth of the size of micronodules. Distinctions in the chemical composition of micronodules and nodules are related to their interrelations with associated sediments. Micronodules grow in sediments using hydrogenous ferromanganese oxyhydroxides. As they grow, micronodules are enriched in the labile fraction of sediments reworked during diagenesis. Sources of the material of ferromanganese nodules are governed by their formation at the water bottom interface. Their upper part is formed by direct settling of iron oxyhydroxides from the bottom water, whereas the lower part is accumulated due to diagenetic processes in sediments. Differences of REE compositions in ferromanganese deposits are caused by the reduction of manganese during diagenesis and its separation from iron. Iron oxyhydroxides form a sorption complex due to the sorption of phosphate-ion from bottom and pore waters. The sorption of phosphate-ion results in an additional sorption of REE.

A Model for Early Diagenetic Processes in Sediments of the Continental Shelf of the Black Sea

A numerical model for early diagenetic processes in the sediments of the north-western continental shelf of the Black Sea is presented. The north-western shelf area is influenced by nutrient, organic matter and reactive iron inputs from major rivers such as the Danube, Dnieper and Dniester. Low oxygen concentrations in the bottom water and high carbon deposition rates have resulted in a dominance of anoxic mineralization processes including manganese, iron and sulphate reduction. The model includes six solid phase variables (labile and less labile organic carbon, iron-and manganese oxides, iron monosulphide and pyrite) and eight solutes (oxygen, nitrate, ammonium, manganese, ferrous iron sulphate, hydrogen sulphide and methane). The cycling of iron and sulphur is explicitly incorporated in the model since these elements play an important role in the mineralization of organic matter on the continental shelf. The model is capable of performing both steady state and dynamic simulations and can be coupled to biogeochemical models for water-column processes. The output of the model is composed of depth profiles of the state variables and their biogeochemical reaction rates and fluxes of the solute substances across the sediment–water interface. The model has been applied to solid-phase and porewater data from a station near the Danube delta. A steady state stimulation reasonably reproduced the main features, but not the distribution of iron and the concentrations of oxygen and total organic carbon in the first 2 cm. A dynamic simulation revealed that a pulse flux of organic matter resulting from the spring bloom could be responsible for the deviation from steady state in the near-surface distribution of total organic carbon and oxygen. The effect of carbon loading on organic matter degradation pathways has been investigated. There is a gradual increase in the relative importance of anoxic mineralization pathways as carbon loadings increase, but the transitions from manganese-based respiration to iron-based respiration and from iron-based respiration to sulphate reduction are rather abrupt. The first jump is due to the positive feedback between manganese reduction coupled to ferrous iron oxidation and iron oxide reduction. The second swap is due to the formation of iron sulphides. This will consume part of the reduced iron, which is consequently not available for re-oxidation to iron oxide, and the iron-reducing bacteria become iron-oxide limited. The cycling efficiency of iron changes abruptly, and a larger part of the iron will be fixed as iron sulphides.

The Lithogenic Nature of Some Gold Deposits in Carbonaceous–Terrigenous Sequences

Carbonaceous–terrigenous sequences hosting the largest gold deposits in Russia are divided for the first time into two lithochemical (normal potassic and anomalous sodic) rock types. It is shown that the type of terrigenous rock alkalinity is governed by peculiarities of the tectonic evolution of sedimentary-rock basins and stages of lithogenesis. The alkalinity type reflects the character of ore-forming processes. In potassic rocks, primarily disseminated-stringer gold–sulfide deposits are formed as a result of sedimentary-diagenetic processes. In sodic rocks, gold–quartz veins and stringer zones are formed as a result of elisional and catagenetic transformations. All these deposits should be identified as lithogenic objects. Processes of the subsequent zonal metamorphism or tectonomagmatic reactivation complicate the structure of gold deposits (sedimentary-diagenetic sulfide and elisional-catagenetic quartz veins) or create new deposits at the expense of primary lithogenic ore material in other lithostructural environments.