Early Diagenesis Processes Research Articles

Response of early diagenesis to methane leakage in the inner shelf of the East China Sea

Shelf seas are experiencing a rise in shallow gas leaks, primarily methane, raising concerns due to their environmental impact. However, the effect of the leaks on early diagenesis remains poorly understood. This study analyzes sediment lithology, organic geochemistry and porewater geochemistry of two short cores collected nearby the pockmarks in the muddy inner shelf of the East China Sea. Our findings clearly demonstrate the impact of methane leakage on early diagenesis, evidenced by the shallower position of the SMTZ (sulfate-methane transition zone), higher concentrations of uranium (U), vanadium (V), and manganese (Mn) in the porewater near and above the SMTZ, and downcore decrease in Mg2+, Ca2+, and Sr2+ concentrations versus increase in Mg2+/Ca2+ and Sr2+/Ca2+ ratios. Their profile variations and the difference between two cores are determined by the intensity of methane leakage. The estimated methane diffusive flux of 619 mmol m-2 yr-1 at YEC7–2 is roughly 8.5 times that at YEC7–1 (73 mmol m-2 yr-1), consistent with a shorter distance of YEC7–2 to the pockmark with active methane leakage. A schematic model is summarized to demonstrate the response of early diagenesis processes to the increasing methane leakages in response to changing sedimentation regimes from accretion to severe erosion. This study undoubtedly improves our understanding mutual promotion effect between seafloor erosion and gas leakage, and their impact on early diagenesis processes and resultant porewater geochemical changes and authigenic mineral records.

Distribution of mercury in modern bottom sediments of the Beaufort Sea in relation to the processes of early diagenesis: Microbiological aspect

This study investigated the contents of total mercury (THg), trace metals, and CH4 and determined the signature microbes involved in various biogeochemical processes in the sediment of the Canadian Beaufort Sea. The THg ranged between 32 and 63 μg/kg and the trace metals such as Fe, Al, Mn, and Zn were significant in distributions. The pH, SO42−, Fe2+, and redox proxy metals were crucial factors in the spatial and vertical heterogeneity of geochemical distributions. CH4 was detected only at the mud volcano site. Microbial analyses identified Clostridium, Desulfosporosinus, Desulfofustis, and Desulftiglans as the predominant Hg methylators and sulfate reducers; Nitrosopumilus and Hyphomicrobium as the major nitrifiers and denitrifiers; Methanosarcina and Methanosaeta as keystone methanogens; and Methyloceanibacter and Methyloprofundus as signature methanotrophs. Altogether, this study expands the current understanding of the microbiological and geochemical features and could be helpful in predicting ecosystem functions in the Canadian Beaufort Sea.

Geochemical balance of the mineral fraction and statistical analyzes of the Ebolowa municipal lake’s sediments (central Africa): Implication for early diagenesis process

The Ebolowa Municipal Lake (EML) (South Cameroon) in order to identify the early diagenesis processes taking place in the lake and the factors influencing them. To this end, 21 samples were collected. In situ, hydrogen potential, redox potential, conductivity, dissolved oxygen content, and turbidity were measured. In the laboratory, the samples were subjected to mineralogical analysis by X-ray diffraction, geochemical analysis by X-ray fluorescence and ICP-MS, and statistical analysis. The coefficient of variation (Qi) was calculated from the geochemical data. In the water column, OD > 2 mg/L, pH > 7 and Eh < 0 mV. In sediments: pH < 7, Eh values are lower. The contents of 2.08 ≤ TOC ≤ 12.65%. The mineralogical procession consists of quartz, kaolinite, gibbsite, goethite, and siderite. The latter is only present in the EML. The sediments are dominated by SiO2 (60.44–89.47%), Al2O3 (6.55–18.17%), and Fe2O3 (1.15–6.21%). The Qi values range from 0.73 to 2.31. The Mn/Fe ratio values are below 0.40. Qi > 1 for Al, Fe, Mn, Mg, K, Na, P, Ni, Co, Zn, Pb, Cd, Cu, Ba, and V, and Qi < 1 for Si; Qi = 1 for Ca. The hierarchical cluster analysis shows two groups: the first one includes the samples from the central and western parts, while the second one includes those from the eastern and southern parts of the lake. The water column is subject to oxic conditions, while the sediments are anoxic. The rapid consumption of oxygen is due to organic mineralization, which is the main diagenesis observed in the lake. This phenomenon is more accentuated in the western part of the lake.

Reconstruction of the Paleo‐Ocean Environment Using Mineralogical and Geochemical Analyses of Mixed‐Type Ferromanganese Nodules From the Tabletop of Western Pacific Magellan Seamount

AbstractVariations in the geochemistry and mineralogical compositions of ferromanganese (Fe‐Mn) nodules are closely related to variations in environmental parameters. Therefore, analysis of Fe‐Mn nodules can reconstruct the paleo‐ocean environment. Here, three differently shaped Fe‐Mn nodules were collected from the open seamount 9‐1 of the western Pacific Magellan Seamount. Geochemical and mineralogical analyses were conducted using micro X‐ray fluorescence (μ‐XRF) and Synchrotron X‐ray powder diffraction to reconstruct the paleo‐ocean environment. We found that the shape of the three nodules was different; however, the variations in their geochemical and mineralogical properties were similar. For all three nodules, the μ‐XRF elemental distribution revealed a distinct alternation between the Mn‐rich and Fe‐rich layers. The Mn‐rich and Fe‐rich layers surrounded the nuclei and appeared as concentric circles. Fe‐rich and Mn‐rich layers exhibited low Mn/Fe ratios (&lt;2.5) and were dominated by vernadite, whereas layers with high Mn/Fe ratios (&gt;2.5) were dominated by todorokite, respectively. These findings suggest an early diagenesis process at tabletop of the Magellan Seamount. The high contents of Ca and P and the existence of carbonate fluorapatite in the center of Fe‐Mn nodules indicate that the formation of these nodules began during the Miocene phosphatization event (19–16 Ma). In the early stages of nodule formation, a diagenetic process drove Fe‐Mn nodule formation. Because of global cooling from 9 Ma, early diagenesis was weakened, after which the formation of hydrogenetic Fe‐Mn nodules became dominant. As a result, variations in oxygen minimum zones and oxic‐suboxic fronts were recorded in Fe‐Mn nodules from tabletop of the seamount.

Identification of the main factors of sedimentogenesis and early diagenesis in bottom sediments of the Kara Sea based on methods of mathematical statistics

In the article, methods of mathematical statistics were used to study the processes of sedimentogenesis and early diagenesis of the surface sediments of the Kara Sea. Correlation analysis established two groups of oxides associated with certain granulometric types of deposits. As a result of applying one-way analysis of variance, the regularity of the distribution of sand, silt and pelite was established in accordance with the conditions of sedimentation based on differences between their average values. Based on these data, maps of the distribution of sand, silt, and pelite were constructed within the studied part of the Kara Sea. On the basis of statistical data, some features of the main factors of sedimentogenesis for this region were revealed. As a result of the established relationship between the Mn/Al, Mn/Fe, MnO/SiO2 ratios and depth, using a one-dimensional regression analysis, the intensity of early oxidative diagenesis within the shelf zone of the Kara Sea was studied.

Geochemistry of Early Diagenesis in Sediments of Russian Arctic Glacial Lakes (Norilo–Pyasinskaya Water System)

The Russian Arctic region is lacking in studies on geochemical changes reflecting early sediment diagenesis in lake environments. The paper presents new data on the compositions of bottom sediments and sediment pore water from two lakes of the Norilo–Pyasinskaya water system in Arctic Siberia. Lakes Pyasino and Melkoye occupy basins left by glaciers that originated from the Putorana Plateau during the Last Glacial Maximum (LGM). Clayey sediments were continuously deposited in the lakes, and the depositional environment has changed only slightly for the last ca. 20 ka. Two sediment cores with lengths of 4.0 and 3.2 m were collected in Lakes Pyasino and Melkoye, respectively, with a Livingstone-type piston corer providing undisturbed, stratigraphically consistent sedimentary sequences. Their analyses revealed a change from oxidized to reduced conditions at a depth of ~10 cm. The concentrations of Ca2+, Mg2+, Na+, and K+, as well as the HCO3−/Ca2+ ratio in pore water, showed a depthward increase indicating the progressive degradation of organic matter. Another trend was the gradual decrease in SO42− alongside increasing HCO3−/SO42− caused by bacterial sulfate reduction, although this was rather weak, judging by the low concentrations of S (II) bound to Fe-sulfides, H2S, etc. Additionally, the microbial digestion of organic matter caused a release of its mobile components, which led to the enrichment of the water in NO3−, PO43−, and DOC. Most of the analyzed elements (Al, B, Ba, Co, Cu, Mo, Ni, Si, Sr, V, and Zn) reach higher concentrations in the pore water than in the lake water above the water-sediment boundary, which is evidence of diagenetic processes. As a result of redox change, immobile Fe (III) and Mn (IV) natural oxides were reduced to mobile Fe (II) and Mn (II) species and migrated from the solid phase to the pore water, and eventually precipitated as authigenic Fe sulfides and Mn carbonates. The results are useful for better understanding the early diagenesis processes in different geographical settings over the huge Eurasian continent.

Reliability assessment and paleo-oceanographic signals of geochemical and isotopic indicators in the North Pacific Subtropical Gyre

As the largest marine ecosystem on our planet, the North Pacific Subtropical Gyre (NPSG) plays an important role in global environmental and climate change. However, little is known about its environmental changes in an orbital scale. In this study, a constant flux model with excess 230Th was used to establish a time frame for our sediment core. Several geochemical and isotopic indicators were evaluated for their reliability in reconstructing paleoenvironment, including grain size distribution (GSD), Ti, Cu, Zn, Ba, total organic carbon (TOC), total nitrogen (TN), TOC/TN ratio, organic carbon and nitrogen isotopic composition (δ13C, δ15N). In addition, appropriate indicators were used to reveal changes in atmospheric dust deposition and biological productivity during the glacial and interglacial periods in the NPSG. Our results show that in such a low sedimentary environment, TOC, TN, TOC/TN ratio, δ13C, δ15N, Cu and Zn has been altered with the degradation of organic matter in the early diagenesis process. In contrast, GSD and Ti-derived eolian dust flux (Feolian) are suitable indicators for atmospheric dust deposition, and the mass accumulation rate of biogenic silica (MARBSi) and the deposition flux of excess Ba after correction for diffusion (Fexess Ba) are suitable for the reconstruction of biological productivity. The productivity during the glacial periods of Marine Isotope Stage (MIS) 2–4 and 6 is higher than the interglacial periods of MIS 1, 5, which corresponds well to the coarser dust particles and higher eolian dust supply. Therefore, we propose that different with former works in the subtropical Pacific, the fluctuations in productivity in the NPSG is mainly controlled by atmospheric dust deposition on a millennial scale.

Atypical Diagenesis and Geochemistry of Redox-Sensitive Elements in Hydrothermal Sediments of the Southern Okinawa Trough

Early diagenesis processes and the geochemistry of redox-sensitive elements (RSEs) in four sediment cores in an active hydrothermal field in the southern Okinawa Trough (OT) was investigated. Dissolved Fe, Mn, ΣHS–, and several other RSEs (Mo, U, and V) in pore water were measured. We also studied metal distribution in sediments using sequential extraction methods. Very high dissolved Fe concentrations (∼140 μmol L–1) but insignificant dissolved Mn were observed in surficial pore waters in the station adjacent to the hydrothermal vent, where highly reactive Fe, Mo, U in the sediments were also measured. Such an atypical diagenetic sequence found in those cores could be driven by the overwhelmingly high reactive Fe fraction (mostly Fe oxides) delivered from the vents. Consequently, significant upward benthic fluxes of Fe and Mo were estimated for the studied stations. In addition, we performed a principal component analysis (PCA), together with relative ratios of carbonate-related elements (Sr, Ca, Mg), to identify particles’ origins in the hydrothermal field; two endmembers being the hydrothermal source and hydrogenous processes. This comprehensive study on a unique set of samples collected by advanced technology provided valuable data to demonstrate distinctive geochemical features that occur in hydrothermal sediments.

Mechanism and Influencing Factors of REY Enrichment in Deep-Sea Sediments

Deep-sea sediments with high contents of rare-earth elements and yttrium (REY) are expected to serve as a potential resource for REY, which have recently been proved to be mainly contributed by phosphate component. Studies have shown that the carriers of REY in deep-sea sediments include aluminosilicate, Fe-Mn oxyhydroxides, and phosphate components. The ∑REY of the phosphate component is 1–2 orders of magnitude higher than those of the other two carriers, expressed as ∑REY = 0.001 × [Al2O3] − 0.002 × [MnO] + 0.056 × [P2O5] − 32. The sediment P2O5 content of 1.5% explains 89.1% of the total variance of the sediment ∑REY content. According to global data, P has a stronger positive correlation with ∑REY compared with Mn, Fe, Al, etc.; 45.5% of samples have a P2O5 content of less than 0.25%, and ∑REY of not higher than 400 ppm. The ∑REY of the phosphate component reaches n × 104 ppm, much higher than that of marine phosphorites and lower than that of REY-phosphate minerals, which are called REY-rich phosphates in this study. The results of microscopic observation and separation by grain size indicate that the REY-rich phosphate component is mainly composed of bioapatite. When ∑REY &gt; 2000 ppm, the average CaO/P2O5 ratio of the samples is 1.55, indicating that the phosphate composition is between carbonate fluoroapatite and hydroxyfluorapatite. According to a knowledge map of sediment elements, the phosphate component is mainly composed of P, Ca, Sr, REY, Sc, U, and Th, and its chemical composition is relatively stable. The phosphate component has a negative Ce anomaly and positive Y anomaly, and a REY pattern similar to that of marine phosphorites and seawater. After the early diagenesis process (biogeochemistry, adsorption, desorption, transformation, and migration), the REY enrichment in the phosphate component is completed near the seawater/sediment interface. In the process of REY enrichment, the precipitation and enrichment of P is critical. According to current research progress, the REY enrichment is the result of comprehensive factors, including low sedimentation rate, high ∑REY of the bottom seawater, a non-carbonate depositional environment, oxidation conditions, and certain bottom current conditions.

Studies of Sediment Systems of the European Arctic during Cruise 75 of the R/V Akademik Mstislav Keldysh

Abstract—The paper presents the results of systemic oceanological studies of the European part of the Arctic during cruise 75 of the R/V Akademik Mstislav Keldysh. The expedition covered the waters of the Norwegian, Greenland, and Barents Seas. The main goal was a conjugate study of dispersed sedimentary matter (atmospheric aerosols, water suspended particulate matter), and bottom sediments with an emphasis on sedimentogenesis and early diagenesis processes in combination with paleoceanological reconstructions. Earlier studies [1] on the role of endogenic material in modern sedimentary processes in the ocean based on the example of a subpolar section (Mohns Ridge) of the Mid-Atlantic Ridge are continued.

Early diagenesis of redox-sensitive trace metals in the northern Okinawa Trough

The early diagenesis processes of several redox-sensitive trace metals (RSMs) (Mo, U and V) were studied with several short sediment cores (~25 cm) collected in the northern Okinawa Trough (OT). Pore water vertical profiles indicated that the sedimentary environments in all cores were between oxic and suboxic, not yet reaching anoxic sulfidic conditions. The recycling process of Mo in sediments was clearly associated with Mn and yielded little authigenic accumulation, while U showed a downcore increase in sediment and its authigenic mass accumulation rate (MAR) was estimated to be ≈23% of the Changjiang (Yangtze) and Huanghe (Yellow) riverine flux. Benthic diffusive fluxes and MAR were calculated and the comparison of them showed that U and V fluxes matched relatively well both in direction and in magnitude, implying that diffusion processes at the sediment-water interface is the dominant process controlling the remobilization or burial of V and U in northern OT. This work provided a systematic study (both in pore water and solid phase) on the RSMs geochemical behaviors during early diagenesis process, yielding a quantitative assessment of the remobilization or burial fluxes of the RSMs in northern OT. Such studies are in general lacking in the coastal margin of Northwest Pacific Ocean.

Quantification of diagenetic transformation of continental margin sediments at the Holocene time scale

Early diagenesis processes in the Bay of Biscay have been studied previously on short interface cores. In order to complete the data base and to characterize anoxic biogeochemical processes that affect Quaternary deposits, 1–3 m long gravity cores were collected at 150, 550, 1000 and 2000 m water depth on the continental slope of the south-eastern part of the Bay of Biscay. The aim of this work was to determine how a recorded signal could be changed in the several thousand-year situation of the Holocene period. For that, we determined radiocarbon and 210Pb dating, sedimentological characteristics, and the vertical distribution of particulate and pore water biogeochemical compounds. The sediment consists of a muddy facies deposited continuously during the Holocene. Results show that the distribution of redox sensitive compounds follows the well-established depth sequence of diagenetic reactions governed by the preferential use of the electron acceptor for the bacterially mediated oxidation of organic matter. Mineralization of organic matter occurs down to the bottom of the cores studied, indicating that Holocene sediment of the Bay of Biscay are in a transient state and they keep on being diagenetically transformed today. Effects of anoxic processes on buried sediments were estimated from fluxes induced by chemical gradients in pore waters. Anoxic processes consume up to 3.6 mg particulate organic carbon (POC) per g of dry sediment. They yield also precipitation of secondary calcium carbonate, which represent up to 5% of total buried carbonates. Early diagenetic reactions, which affect proxies commonly used in paleo-oceanography, such as POC, carbonates, phosphorus, or manganese, should be quantified in order to distinguish the part of a sedimentary record being the result of past environmental conditions from that due to post-depositional diagenetic processes. Diagenesis of Holocene deposits also contributes to a low, but significant fraction of N and P flux to the Bay of Biscay sea water.

Effects of Algal Blooms and Their Degradation on the Sediment-water Micro-interface

Effects of algal blooms and their degradation on the sediment-water micro-interface under conditions of disturbance and darkness were investigated. The concentration of chlorophyll a in water was determined via the acetone method, profiles of oxygen near the interface were measured using a microelectrode, and profiles of nutrients and metals in sediments were measured at the millimeter level. The results showed that algal bloom degradation at the sediment-water micro-interface decreased the concentration of oxygen and affected nutrient and metal profiles, at depths over approximately 3 mm. Disturbance played an important role in algal bloom degradation and early diagenesis processes of nutrients, such as C and P, and metals, such as Fe, Ca, Mg, Al, and K. Darkness sped up algal bloom degradation and reduced the influence of disturbance. Under control conditions of no disturbance nor algae, and supplied with enough light, a layer of benthic algae appeared at the sediment surface, whose photosynthesis produced oxygen and greatly changed the microenvironment. This brought about significant differences in the concentrations of nearly all nutrients and metals in the surficial sediment compared with those of other treatments. Correlation analyses showed that the vertical variability of sediment P concentration was most strongly related to sediment Mn, followed by sediment C. The results of this high-resolution research showed that algal bloom degradation had an effect on the sediment-water micro-interface at the millimeter level.

Seasonal variation of early diagenesis and greenhouse gas production in coastal sediments of Cadiz Bay: Influence of anthropogenic activities

Greenhouse gas production in coastal sediments is closely associated with the early diagenesis processes of organic matter and nutrients. Discharges from anthropogenic activities, particularly agriculture, fish farming and waste-water treatment plants supply large amounts of organic matter and inorganic nutrients that affect mineralization processes. Three coastal systems of Cadiz Bay (SW Spain) (Guadalete River, Rio San Pedro Creek and Sancti Petri Channel) were chosen to determine the seasonal variation of organic matter mineralization. Two sampling stations were selected in each system; one in the outer part, close to the bay, and another more inland, close to a discharge point of effluent related to anthropogenic activities. Seasonal variation revealed that metabolic reactions were driven by the annual change of temperature in the outer station of the systems. In contrast, these reactions depended on the amount of organic matter reaching the sediments in the outermost part of the systems, which was higher during winter. Oxygen is consumed in the first 0.5 cm indicating that suboxic and anoxic processes, such as denitrification, sulfate reduction and methanogenesis are important in these sediments. Sulfate reduction seems to account for most of the mineralization of organic matter at the marine stations, while methanogenesis is the main pathway at the sole freshwater station of this study, located inside the estuary of the Guadalete River, because of the lack of sulfate as electron acceptor. Results point to denitrification being the principal process of N2O formation. Diffusive fluxes varied between 2.6 and 160 mmol m−2 d−1 for dissolved inorganic carbon (DIC); 0.9 and 164.3 mmol m−2 d−1 for TA; 0.8 and 17.4 μmol m−2 d−1 for N2O; and 0.1 μmol and 13.1 mmol m−2 d−1 for CH4, indicating that these sediments act as a source of greenhouse gases to the water column.

Numerical modeling of early diagenetic processes in Haiyang 4 Area in the northern slope of the South China Sea

Early diagenesis affects the distribution of solutes and minerals in unconsolidated sediments. The investigation of diagenesis is critical to understanding the geochemical transformation and benthic fluxes of elements. During the cruise mission SO-177 in 2004, gravity coring samples were recovered in the Haiyang 4 Area of the northern slope of the South China Sea (SCS). The geochemical concentrations in interstitial water were determined onboard. The 1D C.CANDI reactive transport software was used to model the early diagenesis processes at four sites: 56-GC-3, 70-GC-9, 94-GC-11, and 118-GC-13. All of the simulations reproduced concentration profiles that matched the measurements with the implemented geochemical reactions. The degradation of organic carbon and anaerobic oxidation of methane (AOM) primarily determine the distribution of solutes in the working area. The degradation is active in the top 150 cm, and AOM is vigorous at depths below 200 cm. The local advective flux, sediment rate, and kinetic reaction constants of organic matter, methane and sulfate were calibrated based on the existing concentrations of pore water solutes. Geochemical reactions in this area occur in considerably deeper layers compared to depths cited in the literature. The model results provide evidence for the existence of deep hydrocarbon reservoirs that provide methane to the upper sediments.

Early diagenesis and authigenic mineral formation in mobile muds of the Changjiang Estuary and adjacent shelf

Abstract Large-river delta-front estuaries (LDEs) and their adjacent shelf margins are sites of dynamic diagenetic processes that play a significant role in coastal biogeochemical cycling. In this study, we used dissolved inorganic carbon (DIC), redox sensitive elements (Fe 2 + and Mn 2 + ), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 − , and NO 2 − ), major cations and anions (K + , Ca 2 + , Mg 2 + , SO 4 2 − , and Cl − ) in bottom-water and sediment pore-waters, to investigate the early chemical diagenesis and authigenic mineral formation in mobile-mud deposits of the Changjiang Estuary and adjacent inner shelf of the East China Sea (ECS). Vertical profiles of DIC and NH 4 + in pore-waters had similar trends at most sites, showing a significant increase with depth near the Changjiang Estuary and being relatively constant at offshore sites. Higher pore-water DIC and NH 4 + concentrations were observed in nearshore sites in winter, which were likely attributed to exposure of deeper deposits by winter coastal erosion. Nitrification was observed at most sites, and AOB (ammonia-oxidizing bacteria) played a leading role in ammonia oxidation in the study areas. The nitrification-denitrification was likely important in contributing to the loss of DIN in offshore sites during summer. Large inputs of organic carbon (OC) and terrestrial materials from Changjiang River resulted in intense sulfate reduction and Fe and Mn reduction in nearshore sites. Lower C/N and C/S ratios coupled with an apparent decrease in pore-water Ca 2 + and Mg 2+ concentrations with depth near the Changjiang Estuary, which indicated that authigenic carbonate formation occurs in these sediments. Decreases in K + and Mg 2 + with depth reflected that reverse weathering was an important process of authigenic mineral formation in these sediments. We conclude that adsorption process, seasonal erosion-redeposition, and summer hypoxic conditions of bottom-waters may play an important role in early diagenesis processes and remineralization of SOC in the Changjiang LDE.

Pore formation and occurrence in the organic-rich shales of the Triassic Chang-7 Member, Yanchang Formation, Ordos Basin, China

Shale-reservoir appraisal depends greatly on its pore characteristics (e.g., diameter, geometry, connectivity). Using a new pore-classification scheme based on the matrix type and occurrence state, four types of pores are identified in the organic-rich shales of the Triassic Chang-7 Member: intergranular, intragranular, organic pore, and microfracture. The intergranular pores are subdivided into primary pores between clastic grains, clay-mineral aggregates, and secondary dissolution pores between clastic grains or clay-mineral aggregates based on their origins, respectively. The intragranular pores are subdivided into secondary dissolved pores in feldspars, intra-clay-mineral aggregates and inter-pyrite. Organic pores include primarily microfractures in the organic matter and isolated organic pores. Microfracture is mainly developed along sandy and muddy laminations. Analysis by integration of data from pore imaging, low-temperature liquid nitrogen absorption, relationships between pore geometry and mineral components and between TOC and maturity of organic matter indicates that depositional environment, diagenesis, and thermal evolution of organic matter controlled the formation and preservation of pores. Organic-rich shales deposited in a deep and semi-deep lake environment contains thinly bedded turbidite sandstones, which are characterized by high content of clastic particles and thus favor the development of primary intergranular and intragranular pores, as well as microfractures along sandy laminations. During the early diagenesis process, precipitation of pyrite favors the development of inter-pyrite pores. However, compaction reduced the diameter and bulk pore volume. Organic pore has been greatly reduced under compaction. Dissolution led to formation of both inter and intra-feldspar pores, which has improved reservoir quality to some extent. Organic pore started to develop after shale maturity reaches a threshold (RO = 0.75%). Consequently, low organic pores could have been caused by the combination effect of low maturity and compaction. In addition, hydrocarbon is adsorbed on the surface of the organic matter, which could have caused the expansion of kerogen and then the reduction of organic pore.

Biosignatures and microbial fossils in endolithic microbial communities colonizing Ca-sulfate crusts in the Atacama Desert

Since the description of microbial communities colonizing Ca-sulfate crusts in the Atacama Desert, there has been much interest in the mechanisms that could lead to the formation and preservation of biosignatures or microbial fossils of these communities. A key to understanding physico-chemical processes of taphonomy and early diagenesis is to examine microfossils in their natural environment. In this study, we characterize organomineral traces and microbial fossils found around microbial communities present in these Ca-sulfate crusts. Through scanning electron microscopy, microanalytical (EDS) and Raman spectroscopy techniques, calcium carbonate precipitates were detected around remnants of cryptoendolithic algae beneath the crust surface. As what seems to be the final step in the organomineralization of these cryptoendolithic communities, we also observed alga cell remains permineralized by Mg-Si-rich minerals inside gypsum crystals. Additionally, Mg-Si bearing minerals formed a web-like structure within the hypoendolithic cyanobacterial habitat via permineralization of extracellular polymeric substances. Our observations indicate that despite the extremely hyperarid environment, microenvironmental conditions may be appropriate for the formation of biosignatures and microbial fossils of extinct endolithic microbial communities. A model of the possible organomineralization processes involved is presented.

Influence of early diagenesis on the vertical distribution of metal forms in sediments of Bohai Bay, China

The influence of early diagenesis on the vertical distribution of metal forms in the sediments of Bohai Bay was discussed in this paper. The results showed that the concentrations were: Al>Fe≈Ca>Mn>Cr>Zn>Cu>Pb>Cd. In vertical distribution, the forms of Cr and Pb were stable from the top to the bottom. However, the exchangeable forms and acid-extracted forms of Cd, Cu and Zn presented an obvious declining trend. The metals would be transformed to more stable forms during the early-diagenesis process. Further analysis found that early diagenesis can change the sedimentary environment, affecting pH, oxidation–reduction potential (ORP), total dissolved solid (TDS) and the structure of organic matter (OM), all main factors influencing metal forms in the sediments of Bohai Bay.

A neuro fuzzy approach to recognize rock magnetic and lithological patterns in a stratigraphic well from the Llanos Foreland Basin (Colombia)

The Neuro Fuzzy System (NFS) is a hybrid algorithm that combines fuzzy logic with neural networks. Since it can be used as a pattern recognition technique, we explore its potential to characterize the major lithological units encompassed by the first 512 m of the Colombian stratigraphic well Saltarin 1A (Guayabo and Leon Formations). Thus, we employ the NFS to infer the magnetic remanence S-ratio using bulk magnetic susceptibility (κ), κ-normalized saturation isothermal magnetization (SIRM κ) and/or volume of shale (Vsh) obtained from a gamma-ray log. The best results in terms of their corresponding Root Mean-Square Error (RMSE) values, throughout most of the upper Guayabo Formation, where magnetite seems to be an important magnetic phase, are attained with logκ and SIRM κ as input variables. Beyond 350 m downcore, the quality of the inference decreases over the Leon Formation, characterized by a significant presence of pyrrhotite. However, the extra input variable Vsh adjusts the inferred S-ratio to their experimental counterparts throughout this formation suggesting that the early diagenesis process that led to the formation of dispersed clay in these samples was also responsible for the formation of pyrrhotite. Hence the inclusion of manifold input data increases the ability of the net to predict S-ratio in complex geological settings with a sequence of changing lithologies, varying amounts and types of magnetic minerals, and different distributions of mineral grain sizes. In case these variables do not properly infer the actual S-ratio data, the extent of the different lithostratigraphic units would be still identifiable in some cases by the uneven quality of the correlation observed between inferred and experimental values.