Suboxic Conditions Research Articles

Turbidite‐induced re‐oxygenation episodes of the sediment‐water interface in a diverticulum of the Tethys Ocean during the Oceanic Anoxic Event 1a: The French Vocontian Basin

AbstractWidespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi‐enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain‐size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal‐distal transect. This study shows that the GL was perturbed by turbidites. In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon >3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water‐column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface‐water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water‐column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re‐oxidized the sediment‐water interface as well as underlying sediments. Such episodes of benthic re‐oxygenation could have altered the long‐term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

Biosurfactant Production in Sub-Oxic Conditions Detected in Hydrocarbon-Degrading Isolates from Marine and Estuarine Sediments.

Hydrocarbon bioremediation in anoxic sediment layers is still challenging not only because it involves metabolic pathways with lower energy yields but also because the production of biosurfactants that contribute to the dispersion of the pollutant is limited by oxygen availability. This work aims at screening populations of culturable hydrocarbonoclastic and biosurfactant (BSF) producing bacteria from deep sub-seafloor sediments (mud volcanos from Gulf of Cadiz) and estuarine sub-surface sediments (Ria de Aveiro) for strains with potential to operate in sub-oxic conditions. Isolates were retrieved from anaerobic selective cultures in which crude oil was provided as sole carbon source and different supplements were provided as electron acceptors. Twelve representative isolates were obtained from selective cultures with deep-sea and estuary sediments, six from each. These were identified by sequencing of 16S rRNA gene fragments belonging to Pseudomonas, Bacillus, Ochrobactrum, Brevundimonas, Psychrobacter, Staphylococcus, Marinobacter and Curtobacterium genera. BSF production by the isolates was tested by atomized oil assay, surface tension measurement and determination of the emulsification index. All isolates were able to produce BSFs under aerobic and anaerobic conditions, except for isolate DS27 which only produced BSF under aerobic conditions. These isolates presented potential to be applied in bioremediation or microbial enhanced oil recovery strategies under conditions of oxygen limitation. For the first time, members of Ochrobactrum, Brevundimonas, Psychrobacter, Staphylococcus, Marinobacter and Curtobacterium genera are described as anaerobic producers of BSFs.

The linkage of nitrogen isotopic composition and depositional environment of black mudstones in the Upper Triassic Yanchang Formation, Ordos Basin, northern China

In this study, we report new analyses of stable nitrogen isotopic compositions (δ15N) of the black mudstones from the Chang-7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, China, to examine the possible linkage of such isotopic compositions with the depositional environment. These mudstones were deposited mainly under suboxic bottom water column conditions. The core descriptions and trace element concentrations differ between samples obtained from the Chang-73 and Chang-71,2 members, situated at the bottom and top of the Chang-7 Member, respectively, such that the depositional condition of the former implies a more reduced water regime. The bulk rock nitrogen isotope (δ15Nbulk) values of the samples obtained from the Chang-73 Member are higher than those of the Chang-71,2 Member. No significant differences were identified in the organic carbon isotopes (δ13Corg), total organic carbon (TOC), or total nitrogen (TN) values between the two units. The results of δ15Nbulk cross-plotting with the TOC, TN, and carbon to nitrogen atomic ratio (C/N) show an absence of significant modification of the primary nitrogen isotopic compositions from post-depositional processes. Given the depositional history of these units, we suggest that the sedimentary redox environment can have a significant influence on δ15Nbulk. Denitrification, which occurs mainly in suboxic conditions, plays an important role in such an environment, resulting in 15N enrichment during the original deposition. Thus, owing to its sensitivity to sedimentary redox environments, the δ15Nbulk index can be an effective tool for evaluating depositional redox conditions, particularly for further subdivision of suboxic environments.

Microbial and animal evolution in relation to redox fluctuations in a deep-water setting of South China during the Ediacaran-Cambrian transition (ca. 551–523 Ma)

The Ediacaran-Cambrian (E-C) transition witnessed significant ecosystem perturbations of the oceanic environment (e.g., oxygen concentrations of seawater) and biological evolution. However, in spite of abundant Ediacara-type megafossils, trace fossils, small shelly fossils (SSFs), macroscopic algae and metazoans recovered from shallow-water E-C transition strata throughout South China, the manifestation of geological events in deep-water settings remains elusive. Here, we present new paleontological and geochemical evidence from the ~38-m-thick deep-water Liuchapo Formation (ca. 551–523 Ma) of the Yangtou section, northeastern Guizhou Province, China, which consists mainly of chert through the E-C transition. The Liuchapo Formation can be divided into two members based on biostratigraphy and redox proxies. The lower member (0–32 m) is characterized by low Mo/Al (0.15–1.82 ppm/%) and U/Al (0.47–2.51 ppm/%) values that suggest deposition under suboxic conditions. These deposits are dominated by Horodyskia minor, Paleopascichnus jiumenensis, spherical cyanobacteria (?Myxococcoides) and random acritarchs. The upper member (32–38 m) records a transition to anoxic (non-sulfidic) conditions as suggested by marked increases in Mo/Al (5–47 ppm/%) and U/Al (8–810 ppm/%). The upper member contains SSFs, sponge spicules, filamentous cyanobacteria, multicellular algae and sheet-like fossils. The survival of diverse organisms under strongly reducing conditions reflected in the upper member suggests that, marine redox fluctuations may limit the growth of Late Ediacaran organisms and provide the living space and elevated nutrients for early Cambrian biota.

Formation of Siderite in Microbial Microcosms Derived from a Marine Sediment

Exceptionally well-preserved fossils are frequently encased by carbonate concretions. The initial steps of their formation in marine and freshwater sediments are induced by microbial activity. The role of the involved microbial communities, however, is not well understood. In this study, siderite (FeCO3) formation in microbial microcosms is observed, with various fatty acyl compounds (lipids, surfactants) as substrates and Wadden Sea sediment samples as inocula. In actively growing microcosms, sulfate-reducing bacteria (the genus Desulfofrigus in particular) dominate the microbial community and submicroscopic siderite precipitates on bacterial cell surfaces were identified. We suggest that these biologically induced mineralization processes may, in the natural environment, initiate the formation of large concretions under suboxic conditions in coastal sediments.

Suboxic-dominated conditions in the Ediacaran shallow ocean: Geochemistry of pelitic siltstones in the Doushantuo Formation, South China

The Ediacaran witnessed a significant oxygenation of the Earth's surface in line with the emergence and diversification of early animals, but the redox conditions of the shallow waters during this period are still uncertain. Here, we present geochemical profiles from neritic pelitic siltstones of the Doushantuo Formation at the village of Xiaoyang on the northwestern margin of the Yangtze Platform in South China. Most sediments show moderate to low FeHR/FeT ratios (≤0.38), low FePy/FeHR ratios (<0.7), and low enrichment factors (EF ≤ 1) for most redox-sensitive elements, suggesting predominately oxygenated shallow waters. However, we also find moderately enriched Re (EF ~10–25) combined with depleted U and Mo (EF ≤ 1), indicating suboxic conditions. Furthermore, a few samples show FeHR/FeT > 0.38, suggesting episodic anoxia in the shallow ocean. Overall, the Xiaoyang section represents a suboxic shallow ocean with occasional episodes of anoxia during the Doushantuo stage, implying that the oxygenation level in the early-mid Ediacaran ocean was still limited, even under some shallow conditions. It also supports the high redox heterogeneity in the early-mid Ediacaran ocean, which could have restricted the evolution and distribution of early metazoans.

Biogenesis of the Neoproterozoic kremydilite manganese ores from Urucum (Brazil) – A new manganese ore type

The Urucum district in Mato Grosso do Sul (Brazil), hosts the youngest and largest sedimentary Mn ore of Neoproterozoic age; units Mn-1, Mn-2, and Mn-3 are found in jaspilites and ironstones, and represent approximately 600 Mt of extractable rock with 27–44% Mn and 12–30% Fe. High-resolution optical- and cathodoluminescence microscopy, as well as Raman and FTIR spectroscopy show that the lower Mn-1 is ferruginous, while the upper Mn-1 consists mainly of 30–75 vol% braunite, <0.5% aegirine, 3–15% quartz, 5–10% feldspar, and 1–5% clay minerals, including apatite, chlorite, and organic matter. Here, we model the control of this ore mineralogy by homogeneous oxidation and microbial processes. Layers Mn-2 and Mn-3 contain kremydilite, as a characteristic ore structure, with 77–95 vol% cryptomelane, 0–23% hollandite, 9–19% braunite, 7–21% hematite, and 0–5% pores filled with clay minerals and organic matter. These are present within a micro-nodule matrix composed of cryptomelane and hematite in varying proportions. The first syngenetic products of microbial enzymatic oxidation were, on the Fe side, ferrihydrite and lepidocrocite, and on the Mn side, vernadite, todorokite, birnessite, and manganite. These formed under obligatory oxic (Mn) and suboxic (Fe) conditions and close to neutral pH. We describe the genesis of Urucum via complex diagenetic processes, which include the decomposition and mineralization of cellular- and extracellular-polymeric substances from Fe and Mn bacteria and cyanobacteria. The kremydilite forms in successive stages of oxidation of organic matter mediated by microbes, which generate pores and produce methane and CO2/H2 bubbles. They are a unique type of diagenetic structure formed by heterotrophic cell colonies randomly activated in the microbialite milieu following burial in suboxic neutral/alkaline conditions, side-by-side with the lithification and stabilization of the mineral assemblages. Significance statementThe Neoproterozoic Urucum manganese deposit (Brazil) is a ~600 Mt microbially-mediated sedimentary Mn ore. Proto-ore formation via sedimentation and diagenesis occurred under suboxic-oxic and semi-neutral pH conditions in the Ediacaran ocean, wherein microbial Mn(II) oxidation ensued from the fine-grained accumulation of Mn oxides and organic matter. Oxic conditions that facilitated enzymatic Mn oxidation and overwhelmed microbial Fe oxidation appears as a sharp contact between manganese and iron beds. The Urucum deposit arose from a complex suite of diagenetic processes, including decomposition and mineralization of microbially-derived organic matter involving extracellular polymeric substances. Kremydilite – a new type of diagenetic concentric Mn mineral structure – formed by randomly activated heterotrophic cell colonies that generated pores in the microbialite sediment after burial, coincident with lithification.

Labile Fe(III) from sorbed Fe(II) oxidation is the key intermediate in Fe(II)-catalyzed ferrihydrite transformation

Ferrihydrite (Fh) is a major Fe(III)-(oxyhydr)oxide nanomineral distinguished by its poor crystallinity and thermodynamic metastability. While it is well known that in suboxic conditions aqueous Fe(II) rapidly catalyzes Fh transformation to more stable crystalline Fe(III) phases such as lepidocrocite (Lp) and goethite (Gt), because of the low solubility of Fe(III) the mass transfer pathways enabling these rapid transformations have remained unclear for decades. Here, using a selective extractant, we isolated and quantified a critical labile Fe(III) species, one that is more reactive than Fe(III) in Fh, formed by the oxidation of aqueous Fe(II) on the Fh surface. Experiments that compared time-dependent concentrations of solid-associated Fe(II) and this labile Fe(III) against the kinetics of phase transformation showed that its accumulation is directly related to Lp/Gt formation in a manner consistent with the classical nucleation theory. 57Fe isotope tracer experiments confirm the oxidized Fe(II) origin of labile Fe(III). The transformation pathway as well as the accelerating effect of Fe(II) can now all be explained on a unified basis of the kinetics of Fe(III) olation and oxolation reactions necessary to nucleate and sustain growth of Lp/Gt products, rates of which are greatly accelerated by labile Fe(III).

Source Rock Characterization of Silurian Tanezzuft and Devonian Awaynat Wanin Formations the Northern Edge of the Murzuq Basin, South West Libya

Drill cutting samples (n = 92) from the Devonian Awaynat Wanin Formation and Silurian Tanezzuft Formation, sampled from three wells F1, G1 and H1, locate in the northern edge of the Murzuq basin (approximately 700 kilometers south of Tripoli). The studied samples were analyzed in the objective of their organic geochemical assessment such as the type of organic matter, depositional conditions and thermal maturity level. A bulk geochemical parameters and precise biomarkers were estimated, using chromatography-mass spectrometry (GC-MS) to reveal a diversity of their geochemical characterizations. The rock formations are having varied organic matter contents, ranged from fair to excellent. The total organic carbon (TOC) reached about 9.1 wt%, ranging from 0.6 to 2.93 wt% (Awaynat Wanin), 0.5 to 2.54 wt% (Tanezzuft) and 0.52 to 9.1 wt% (Hot Shale). The cutting samples are ranged oil-prone organic matter (OM) of hydrogen index (HI) ranged between 98 –396 mg HC/g TOC, related kerogen types are type II and II/III, with oxygen index (OI): 6 - 190 with one sample have value of 366 mg CO2/g. Thermal maturity of these source rocks is different, ranging from immature to mature and oil window in the most of Tanezzuft Formation and Hot Shale samples, as reflected from the production index data (PI: 0.08 - 034). Tmax and vitrinite reflectance Ro% data (Tmax: 435 – 454 &amp; Ro%: 0.46 - 1.38) for the Awaynat Wanin. Biomarker ratios of specific hydrocarbons extracted from represented samples (n = 9), were moreover used to study thermal maturity level and depositional environments. Pristine/Phytane (Pr/Ph) ratios of 1.65 - 2.23 indicated anoxic to suboxic conditions of depositional marine shale and lacustrine source rock.

Origin and depositional environments of source rocks and crude oils from Niger Delta Basin: Carbon isotopic evidence

ABSTRACT Thirty-nine crude oils and twenty-one rock samples from Niger Delta Basin, Nigeria have been characterized based on their isotope compositions by elemental analysis-isotope ratio mass spectrometry and gas chromatography-isotope ratio mass spectrometry. The bulk carbon isotopic values of the whole rock extracts, saturate and aromatic fractions range from –28.7‰ to –26.8‰, –29.2‰ to –27.2 ‰ and –28.5 ‰ to –26.7 ‰, respectively while the bulk carbon isotopic values of the whole oils, saturate and aromatic fractions range from –25.4 ‰ to –27.8 ‰, –25.9 ‰ to –28.4 ‰ and –23.5 ‰ to –26.9 ‰, respectively. The average carbon isotopic compositions of individual alkanes (nC12-nC33) in the rock samples range from –34.9‰ to –28.2‰ whereas the average isotopic values of individual n-alkanes in the oils range from –31.1‰ to –23.8‰. The δ13C isotope ratios of pristane and phytane in the rock samples range from –29.2 ‰ to –28.2 ‰ and –30.2 ‰ to –27.4 ‰ respectively while the pristane and phytane isotopic values range from –32.1‰ to –21.9‰ and –30.5‰ to –26.9‰, respectively. The isotopic values recorded for the samples indicated that the crude oils were formed from the mixed input of terrigenous and marine organic matter and deposited under oxic to sub-oxic condition in lacustrine-fluvial/deltaic environments. The stable carbon isotopic compositions were found to be effective in assessing the origin and depositional environments of crude oils in the Niger Delta Basin.

Geochemical Characterization of Kazhdumi Source Rock in the Khaviz Oil Field, Southwest of Iran, Using Rock-Eval Pyrolysis

The Khaviz oil field located in Dezful embayment is one of Iran’s southwest oil fields. In this study, a total of 28 cutting samples from Kazhdumi formation (well No. KZ1, Khaviz oil field) were subject to geochemical investigation using Rock-Eval pyrolysis for the first time. The results of pyrolysis indicated that Kazhdumi source rock has significant hydrocarbon production potentiality and already entered the oil generation window. As inferred from the diagram of OI versus HI, Kazhdumi source rock contains organic matter type II kerogen deposited in paralic environment with anoxic to suboxic conditions. Using the diagram of S2 versus TOC, the absorbed carbon content, neutral carbon, and active carbon were calculated to be 0.42, 0.39, and 2.43 wt.% respectively.

PROVENANCE AND DEPOSITIONAL ENVIRONMENT OF THE CARBONATES FROM BALUTI FORMATION (LATE TRIASSIC), KURDISTAN REGION, IRAQ

The carbonate of Late Triassic Baluti Formation, Kurdistan Region, Iraq was geochemically investigated to interpret provenance, depositional environments and sources of the rare earth elements (REEs) based on major, trace, and rare earth elements geochemistry. The predominance of Ca and the low percentage of Mg suggest that mineral phase is calcite at Sararu; in contrast, the dominance of MgO is in Sarki indicates the carbonate is subject to intensive dolomitization. P and Sr are dominated over the other elements, especially for Sararu carbonate. The positive correlation of ΣREE with Ti, Al, K, Sc, V, Rb, Y, Zr, and Nb and negative correlation with Ca in addition to slight enrichment of LREE suggest an input of siliciclastic sediments from the continent. The low values of the Y/Ho ratio (29.83 to 33.85) relative to marine water and approach of Ce anomaly to 1.0 refer to increasing the clastic flux from the adjacent continent and freshwater intrusion to seawater during the precipitation of Baluti carbonates. The low abundance of Cr, Ni, Co, and the Th/Sc, La/Sc, Cr/Th ratios indicate the felsic source rocks; this is confirmed by the slightly LREE enriched and flat HREE patterns and a slightly high average of (La/Yb)sn. Low authigenic U contents, U/Th ratios and slightly negative Ce anomalies suggest the Baluti carbonates were precipitated under oxic conditions and shale at Sarki section was influenced by suboxic conditions. The high Mg/Ca and low Sr/Ca ratios reflect shallow marine environments for Baluti carbonates with diagenetic effect especially in Sarki section. The Mn/Sr ratio implies the Sararu samples have higher degrees of preservation relative to Sarki. Most of REE contents in Sararu is attributed to the terrigenous origin, and the weak non-significant positive correlation of Si, Ti, and Al with REE content suggest little contribution in the Sarki and the biogenic carbonate from the seawater may play a role in the REE concentration. These carbonates possess non-seawater-like REE patterns with slightly LREE enrichment and HREE depletion, and characterized by –ve Ce*.

Hydrocarbon potential and depositional paleoenvironment of a Middle Jurassic succession in the Falak-21 well, Shushan Basin, Egypt: Integrated palynological, geochemical and organic petrographic approach

The topmost Lower and Middle Jurassic formations of organic-rich clastics with minor carbonate deposits were recognized as very significant source/reservoir rocks in the North Western Desert of Egypt. Although several studies are devoted to their source rock characteristics and petroleum potential, the depositional paleoenvironment and paleoredox conditions are still poorly understood. In the current study, these sediments were assessed in terms of their organic matter richness, hydrocarbon generative potential and thermal maturity, total sulfur (TS) contents and organic petrographic characteristics. Depositional paleoenvironment was investigated using dinoflagellate cysts (dinocysts) and the composition of particulate organic matter along with organic petrographic characteristics. Redox conditions were assessed based on TS- total organic carbon (TOC) relationship and type of kerogen that was derived mainly from palynofacies analysis.TOC contents show fair to excellent generative potential of hydrocarbons mainly of kerogen type III (gas-prone). The thermal maturity, based on Tmax and Rr, indicates mature stage of oil window.Palynological analysis yielded poor to rich intervals of moderately-preserved, biostratigraphically useful dinocysts in the upper part of the Khatatba Formation. An assemblage of 81 species belonging to 54 genera of moderately diverse dinocysts, spores and pollen grains was identified. Freshwater algae, acritarchs and microforaminiferal test linings are sporadically documented within the studied interval. Based on the first downhole appearance (FDA) of the recovered marker dinocyts, the upper part of the Khatatba Formation was palynologically dated as late Bathonian-Callovian.The depositional environment was interpreted and three environments were deduced. The lower interval representing the topmost Ras Qattara and Yakout Red Shale Member was deposited in a fluvial-lacustrine environment, where oxic conditions prevailed as deduced from the color of reddish-brown sandstone and red shale. The middle interval spans Unit IIB, Kabrit and Unit III, which accumulated in a deltaic to shallow marine environment under dysoxic-suboxic settings. The upper interval covers Unit I and Unit IIA that have been deposited in a fluvio-deltaic to shallow marine with high paleoproductivity under suboxic conditions. One sample in the upper interval in Unit IIA reflected anoxic setting.

Microbially Mediated Ore-Forming Processes and Cell Mineralization.

Sedimentary black shale-hosted manganese carbonate and oxide ores were studied by high-resolution in situ detailed optical and cathodoluminescence microscopy, Raman spectroscopy, and FTIR spectroscopy to determine microbial contribution in metallogenesis. This study of the Urucum Mn deposit in Brazil is included as a case study for microbially mediated ore-forming processes. The results were compared and interpreted in a comparative way, and the data were elaborated by a complex, structural hierarchical method. The first syngenetic products of microbial enzymatic oxidation were ferrihydrite and lepidocrocite on the Fe side, and vernadite, todorokite, birnessite, and manganite on the Mn side, formed under obligatory oxic (Mn) and suboxic (Fe) conditions and close to neutral pH. Fe- and Mn-oxidizing bacteria played a basic role in metallogenesis based on microtextural features, bioindicator minerals, and embedded variable organic matter. Trace element content is determined by source of elements and microbial activity. The present Urucum (Brazil), Datangpo (China), and Úrkút (Hungary) deposits are the result of complex diagenetic processes, which include the decomposition and mineralization of cell and extracellular polymeric substance (EPS) of Fe and Mn bacteria and cyanobacteria. Heterotrophic cell colonies activated randomly in the microbialite sediment after burial in suboxic neutral/alkaline conditions, forming Mn carbonates and variable cation-bearing oxides side by side with lithification and stabilization of minerals. Deposits of variable geological ages and geographical occurrences show strong similarities and indicate two-step microbial metallogenesis: a primary chemolithoautotrophic, and a diagenetic heterotrophic microbial cycle, influenced strongly by mineralization of cells and EPSs. These processes perform a basic role in controlling major and trace element distribution in sedimentary environments on a global level and place biogeochemical constraints on the element content of natural waters, precipitation of minerals, and water contaminants.

Larval Fish Habitats and Deoxygenation in the Northern Limit of the Oxygen Minimum Zone off Mexico

AbstractThe present state of deoxygenation in the northern limits of the shallow oxygen minimum zone off Mexico is examined in order to detect its effects on larval fish habitats and consider the sensitivity of fish larvae to decreased dissolved oxygen. A series of cruises between 2000 and 2017 indicated a significant vertical expansion of low oxygen waters. The upper limit of suboxic conditions (&lt;4.4 μmol/kg) has risen ~100 m at 19.5°N off Cabo Corrientes and ~50 m at 25°N in the mouth of the Gulf of California. The larval habitat distribution was related to the geographic variability of dissolved oxygen and water masses between these two latitudes. One recurrent larval habitat, with Bregmaceros bathymaster larvae as the indicator species, extended throughout the water column off Cabo Corrientes from Subtropical Subsurface Water (suboxic conditions) to the surface (220 μmol/kg). The second recurrent habitat was located between the oxycline (&gt;44 μmol/kg) and the surface in association with the Gulf of California Water, with Benthosema panamense as the indicator species. During the warm El Niño event of 2015–2016, a tropical larval fish habitat (Auxis spp.) associated with Tropical Surface Water appeared to change the larval habitat distributions. These results indicate that some species are resilient to changes of dissolved oxygen and temperature generated by El Niño events and by continuing deoxygenation, although other species with more limited environmental windows could be affected by deoxygenation, probably leading to a change of the pelagic ecosystem over time.

Elemental Composition and Organic Petrology of a Lower Carboniferous-Age Freshwater Oil Shale in Nova Scotia, Canada.

A 59 m-thick section of a freshwater oil shale interbedded with marlstone of Lower Carboniferous (Tournaisian) age from the Big Marsh area in Antigonish Basin, Nova Scotia, Canada, was examined using reflected light microscopy, Rock-Eval pyrolysis, X-ray diffractometry analysis, inductively coupled plasma-mass spectrometry for elemental analysis, and prompt γ for boron concentration. The oil shale was deposited in a lacustrine environment based on geology, sedimentology, variation in organic matter, and boron content (28–54 ppm). Organic petrology classified the oil shale into three broadly distinct types. Type A oil shale is a coastal facies shale containing terrestrially derived macerals, such as vitrinite and inertinite, sporinite, with some lamalginite, and amorphous bituminous matrix. Type B oil shale was deposited in a shallow-water facies and contains mostly lamalginite and some vitrinite and sporinite. Type C oil shale is a relatively deep-water facies, associated with open-water Torbanite-type oil shale and contains mostly Botryococcus colonial telalginite. The oil shale is thermally mature (Tmax is 441–443 °C). Total organic carbon (TOC) varies from 5.8 to 7.3 wt %, and the hydrogen index is between 507 and 557 mg HC/g TOC. The rate of sedimentation as determined by the Th/U ratio indicates possibility of three sedimentation periods: an irregular but mostly slow rate of sedimentation from the base of the section up to 68 m, followed by a regular and slow rate between 68 and 53 m, and a regular and fast rate between 53 m and the top of the section. The higher Th/U ratio during deposition of the shallow-water facies was due to the input of allochthonous U. The redox conditions, as reflected in the variation of Cr to Mo, U, and Ni + V, indicate that the oil shale was deposited under suboxic–dysoxic conditions. The high organic productivity by phytoplankton and bacteria is characterized by a low Cr and high V/Cr ratio and suboxic conditions. In contrast, the well-oxygenated and uniform, warm-temperature upper water level supports a dysoxic environment. Variation of Sr/Ca vs Mn/Ca ratios indicates that most samples have low values, a characteristic of colder water and high terrigenous influx. The post-Archean Australian shale (PAAS)-normalized rare earth elements (REEs) follow three trends. Type A oil shale has the highest concentration of total REEs (648 ppm) and light REEs (LREEs, 605 ppm) as compared with type C (269 and 233 ppm), which are less than half of type A. Type B oil shale has the lowest total REEs (184 ppm) and LREEs (152 ppm). The concentration of heavy REEs decreased from 43 ppm in type A oil shale to 36 ppm in type C oil shale. Comparison of PAAS-normalized REEs for the three oil shale types indicates a reduction of the negative Eu anomaly with depth, which is possibly related to sedimentary sorting as a result of accumulation of fine sediments in the deeper water zone of the lake. The concentration of most elements of environmental concern is similar to and/or lower than the world shale. However, there are instances of higher concentrations of hazardous elements (e.g., As, Cd, Mo, and Se).

Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian-Famennian biotic crisis

The lowermost Famennian deposits of the Kowala quarry (Holy Cross Mountains, Poland) are becoming famous for their rich fossil content such as their abundant phosphatized arthropod remains (mostly thylacocephalans). Here, for the first time, palaeontological and geochemical data were integrated to document abundance and diversity patterns in the context of palaeoenvironmental changes. During deposition, the generally oxic to suboxic conditions were interrupted at least twice by the onset of photic zone euxinia (PZE). Previously, PZE was considered as essential in preserving phosphatised fossils from, e.g., the famous Gogo Formation, Australia. Here, we show, however, that during PZE, the abundance of arthropods drastically dropped. The phosphorous content during PZE was also very low in comparison to that from oxic-suboxic intervals where arthropods are the most abundant. As phosphorous is essential for phosphatisation but also tends to flux off the sediment during bottom water anoxia, we propose that the PZE in such a case does not promote the fossilisation of the arthropods but instead leads to their impoverishment and non-preservation. Thus, the PZE conditions with anoxic bottom waters cannot be presumed as universal for exceptional fossil preservation by phosphatisation, and caution must be paid when interpreting the fossil abundance on the background of redox conditions.

Paleoenvironmental evolution and organic matter accumulation in an oxygen-enriched lacustrine basin: A case study from the Laizhou Bay Sag, southern Bohai Sea (China)

Depositional environments play important roles in the formation of lacustrine organic-rich sediments. A series of experiments (total organic carbon, Rock-Eval pyrolysis, major and trace element experiments) were performed on samples from Es3-Es4 in the Laizhou Bay Sag to elucidate the paleoenvironmental evolution. The results show that the source rocks from Es3M-L and the top of Es4U are of excellent quality characterized by Type II kerogen, while other sections are in poor to moderate quality characterized by Type III kerogen. The samples from the Laizhou Bay Sag are enriched in CaO and specific trace elements (e.g., Sr, Mo, Ba, and U). Many inorganic proxies were employed to analyze the climate, productivity, redox condition, sedimentation rate and detrital input independently. The Mo–U covariation pattern and Corg:P index reveal the ubiquitous oxygen-rich bottom water in the Laizhou Bay Sag during the deposition of Es4 and Es3. The paleoenvironmental evolution in the present study can be divided into the following three stages: (1) Es4L to the bottom of Es4U: sediments in this stage were deposited under an arid and cool climate. The paleolake was characterized by modest productivity in the surface water and oxidizing (oxic to suboxic) conditions in the bottom water, with a relatively high detrital flux. (2) The top of Es4U to Es3M-L: sediments formed under a warm and humid climate, which resulted in the high productivity in surface water. The bottom water was under suboxic condition, and the detrital flux was relatively low. (3) Es3U: paleoclimate during this stage was warm and humid but exhibited a trend towards an arid and cool climate, leading to a decrease in productivity. The sediments in Es3U were deposited in oxic bottom water with high detrital inputs. The mechanism for organic matter accumulation in the Laizhou Bay Sag is also discussed. Primary productivity and detrital flux are considered as the major factors controlling organic matter enrichments. Paleoclimate has an indirect impact on organic matter accumulation.

Assessment of sedimentary environment from PAHs and aliphatic biomarkers: The case study of Fahdene black shales in northern Tunisia

Polyaromatic and aliphatic biomarkers extracted from the Albian-Early Cenomanian source rock (Fahdene Formation) were evaluated by Gas Chromatography – Mass Spectrometry (GC-MS) and are compared to classic geochemical and mineralogical data. In addition to terpanes, hopanes, diasteranes and steranes, thirty-seven aromatic hydrocarbon elements were identified.The absence of the angiosperm-derived biomarker (18α(H)-Oleanane) and the low abundance of trimethylnaphthalenes isomers (1,2,5-TMN and 1,2,7-TMN) suggest that no land-plant traces were founded in the studied organic matter. In this fact, the Kerogene III identified by Rock Eval is not detected, and we can attribute these indices to the algae effect. The check of maturity by Tmax and the biomarker parameters (C27 18α-22,29,30-trinorneohopane (Ts), sterane isomerization ratio ββ/(αα+ββ), C32 22S/(22S + 22R) HH and the methylphenanthrene index (MPI-1)) indicates that these organic rich sediments are reached the early stage of oil window. In the other hand, aliphatic and polyaromatic biomarkers confirm classic data of X-ray diffraction and calcimetry and suggest a carbonate facies with some clayey intercalations in the middle part. The fracturing analysis of these carbonates prove the presence of high density of fractures with various sizes and directions. About filling, the fibrous calcite is the major element of mineralized veins, with the presence of bitumen traces. These fractures have two main orgin; the fluid pressure and the regional tectonic stress regime.Therefore, the Albian-Early Cenomanian fractured carbonates in the Mahjouba area were deposited, under suboxic conditions, in a marine environment affected by the eutrophication phenomenon, and have just reached the equilibrium stage of maturity.

The Gondwanan Green Alga Tasmanites sp. in the Permian Lacustrine Deposits of West Timor

DOI: 10.17014/ijog.6.3.255-266This paper discusses a palynological research on the Permian lacustrine sediments of West Timor. Ten outcrop samples were collected from a 5 m high cliff. Lithologically, the outcrop mostly consists of noncalcareous black shale showing papery structures and forms part of Bisane Formation. Palynomorphs extracted from the studied samples are rich, but of a low diversity indicating a non-marine environment. More than 80% of pollen assemblages are of the green alga Tasmanites sp., whilst the rest are mostly represented by striate and non-striate bisaccate pollen and trilete spores which indicate a Permian age. The chromatograms of GC and GCMS (saturate) indicate that the studied samples were deposited in a lacustrine environment of an anoxic to suboxic condition. Furthermore, in term of petroleum exploration, abundant Tasmanites sp. is believed to be the biological source for tricyclic terpanes which are well known as the primary source of hydrocarbons. This discovery offers an excellent opportunity for establishing a new petroleum system within the Paleozoic sequences of Timor Island.