Suboxic Conditions Research Articles

Middle Permian palaeoclimatic-palaeoceanographic evolution and its controls on organic matter accumulation in the Lower Yangtze upwelling region

For more than 10 million years (∼272–260 Ma), the Middle Permian underwent periodic episodes of vigorous upwelling and massive organic matter burial along the west coasts of continents, as testified by the abundant black chert-mudstone records. However, less attention has been paid to the palaeoclimatic-palaeoceanographic evolution in this upwelling area and its relationship with the mechanisms of coeval organic matter accumulation. To better understand this scenario, multiple geochemical analyses were performed on organic-rich deposits of the Gufeng Formation of the Lower Yangtze region. Our results showed that the Middle Permian climatic conditions gradually shifted from warm-humid to hot-humid. Comprehensive geochemical analyses using multiple proxies suggested that the study area was characterised by a hybrid palaeoceanographic system influenced by both upwelling and restriction. The coupled climatic-oceanic evolution went through four stages. The Roadian stage was characterised by gradually enhanced upwelling but relatively oxic conditions and low productivity; the Wordian-early Capitanian stage was marked by the alternating development of perennial and seasonal upwelling with higher productivity and anoxic-euxinic conditions; the early-middle Capitanian stage was distinguished by an intermittent seasonal upwelling with relatively decreased productivity and weak anoxic conditions; the middle Capitanian stage immediately after the deposition of the Gufeng Formation transitioned from a transient-restricted condition to a weak upwelling setting and was dominated by extremely low productivity and suboxic conditions. Further studies on these organic-rich sediments show that the integration of biological productivity, redox conditions, detrital inputs, and hydrographic restrictions under coupled climatic-oceanic forcing could have had a complex influence on their formation. In this upwelling setting, organic matter accumulation is dominated by changes in both upwelling-driven primary productivity and oxygen minimum zone-related reducing conditions, but to a lesser extent by the dilution of climate-influenced detrital inputs.

Microbially-mediated Mn redox cycling and Mn carbonate precipitation in the Marinoan glacial aftermath, South China

Carbonate-type Mn deposits developed at the top of the Ediacaran Doushantuo Formation (Marinoan glacial aftermath) along the northern margin of the Yangtze Block, South China, provide an exceptional opportunity to study Mn redox cycling in deep time, which remains a controversial topic of Mn biogeochemistry. To address this issue, here we conduct integrated petrological and geochemical work. Results show that Mn oxidizing microorganisms may fix CO2 into organic matter by oxidizing Mn(II). This process provides energy for metabolism of the microorganisms under obligatory oxidation conditions in natural geological environments. At the same time, Mn(II) is oxidized to unstable todorokite, with accompanying Co and Ce oxidation and enrichment. Under suboxic conditions, heterotrophic Mn-reducing bacteria use organic matter as an electron donor and Mn bio-oxide as a terminal electron acceptor to generate Mn(II) and 12C-rich dissolved inorganic carbon (DIC; carbonate and/or bicarbonate). Furthermore, microorganisms and macroalgae also provide nucleation sites for the precipitation of Mn carbonate minerals with structures consistent with microbially precipitated carbonates. Spherical and ellipsoidal Ca-rich rhodochrosite was formed in the syngenetic stage and cemented by Mg-rich kutnohorite in the early diagenetic stage. Microorganisms thus play an important role in Mn redox cycling and precipitation of Mn carbonate minerals, in a possibly universal phenomenon in geological evolution. The bloom of microorganisms and associated formation of mineral deposits may be a general phenomenon in the aftermath of a glaciation.

Metagenomics-resolved genomics provides novel insights into chitin turnover, metabolic specialization, and niche partitioning in the octocoral microbiome

BackgroundThe role of bacterial symbionts that populate octocorals (Cnidaria, Octocorallia) is still poorly understood. To shed light on their metabolic capacities, we examined 66 high-quality metagenome-assembled genomes (MAGs) spanning 30 prokaryotic species, retrieved from microbial metagenomes of three octocoral species and seawater.ResultsSymbionts of healthy octocorals were affiliated with the taxa Endozoicomonadaceae, Candidatus Thioglobaceae, Metamycoplasmataceae, unclassified Pseudomonadales, Rhodobacteraceae, unclassified Alphaproteobacteria and Ca. Rhabdochlamydiaceae. Phylogenomics inference revealed that the Endozoicomonadaceae symbionts uncovered here represent two species of a novel genus unique to temperate octocorals, here denoted Ca. Gorgonimonaseunicellae and Ca. Gorgonimonasleptogorgiae. Their genomes revealed metabolic capacities to thrive under suboxic conditions and high gene copy numbers of serine-threonine protein kinases, type 3-secretion system, type-4 pili, and ankyrin-repeat proteins, suggesting excellent capabilities to colonize, aggregate, and persist inside their host. Contrarily, MAGs obtained from seawater frequently lacked symbiosis-related genes. All Endozoicomonadaceae symbionts harbored endo-chitinase and chitin-binging protein-encoding genes, indicating that they can hydrolyze the most abundant polysaccharide in the oceans. Other symbionts, including Metamycoplasmataceae and Ca. Thioglobaceae, may assimilate the smaller chitin oligosaccharides resulting from chitin breakdown and engage in chitin deacetylation, respectively, suggesting possibilities for substrate cross-feeding and a role for the coral microbiome in overall chitin turnover. We also observed sharp differences in secondary metabolite production potential between symbiotic lineages. Specific Proteobacteria taxa may specialize in chemical defense and guard other symbionts, including Endozoicomonadaceae, which lack such capacity.ConclusionThis is the first study to recover MAGs from dominant symbionts of octocorals, including those of so-far unculturable Endozoicomonadaceae, Ca. Thioglobaceae and Metamycoplasmataceae symbionts. We identify a thus-far unanticipated, global role for Endozoicomonadaceae symbionts of corals in the processing of chitin, the most abundant natural polysaccharide in the oceans and major component of the natural zoo- and phytoplankton feed of octocorals. We conclude that niche partitioning, metabolic specialization, and adaptation to low oxygen conditions among prokaryotic symbionts likely contribute to the plasticity and adaptability of the octocoral holobiont in changing marine environments. These findings bear implications not only for our understanding of symbiotic relationships in the marine realm but also for the functioning of benthic ecosystems at large.ExJEFy3aqoijs3T6N7B8ZcVideo

Nitrogen cycling during the peak Cambrian explosion

Previous studies on early-Cambrian nitrogen (N) cycle suggested a modern-type nitrogen reservoir with elevated nitrate (NO3−) availability in a globally oxygenated ocean for the Cambrian explosion. However, recent marine redox studies on the Chengjiang Biota revealed a stratified ocean with anoxic deeper waters for the peak Cambrian explosion, asking for further works on the N cycling during this critical period. Chengjiang and Qingjiang biotas are the two top-rank Burgess Shale-type fossil Lagerstätten that recorded peaked metazoan diversification of the Cambrian explosion at different paleoenvironmental regimes during the Cambrian Age 3. In this study, we conducted a detailed geochemical investigation of the early-Cambrian Shuijingtuo Formation from two drill cores containing the Qingjiang Biota in Hubei Province, South China. The results suggest progressive oxygenation of the depositional environment from euxinic to fluctuating, suboxic conditions with a low chemocline in the water column, accompanied by an increase of δ15N values from <−4‰ to between −2‰ and +1‰. Especially, both event and background beds of the Qingjiang Biota are characterized by δ15N values ranging from −1‰ to +1‰. Comparative analysis reveals consistent variations of δ15N signatures and marine redox states at the Qingjiang and Chengjiang areas during the Cambrian Age 3, with the latter exhibiting systematically higher δ15N values ranging mainly from +1‰ to +2‰ at the biota interval. Taken together, the δ15N signatures of the Qingjiang and Chengjiang biotas can be most parsimoniously explained by prevailed aerobic marine nitrogen cycling featured by active biological N2 fixation and a limited nitrate reservoir in the shallow ocean. The nitrogen isotope gradient between the Qingjiang and Chengjiang areas may be reflective of varied NO3− availability in seawater. These findings contribute to a clearer biogeochemical context that supported prosperous marine ecosystems during the peak Cambrian explosion, and provide new insights on spatiotemporal evolution of the marine nitrogen cycle during this critical period.

Assessment of Mineralogical and Geochemical Composition of Oligocene/Eocene Black Shale Deposits in Beni Suef Area, Egypt

Black shale deposits are widespread in the Paleogene successions in the Beni Suef area, Egypt. In this work, the mineralogical composition, total organic carbon (TOC), and total sulfur (TS) contents of black shales were determined. The organic matter (OM), thermal maturation, and kerogen type can be determined using Rock-Eval pyrolysis. The results show that smectite and kaolinite are the dominant clay minerals in the analyzed samples. Al2O3 and SiO2 as major elements and Zr as a trace element are dominant constituents of terrigenous origin of the Dabaa and El Fashn formations. The total organic carbon content reaches 2.27 wt% at the El Fashn Formation in the Ghaida Al-Sharqia quarry but 0.36 wt% in the Oligocene Dabaa Formation in the Maydoum quarry. However, the total sulfur content reaches 0.58 wt% in the Dabaa Formation but 1.52 wt% in the El Fashn Formation. These results confirm that (1) the Dabaa Formation was formed in oxidizing environments and oxidized the high volume of OM and (2) the El Fashn Formation accumulated in reducing environments of mostly suboxic conditions. The Rock-Eval pyrolysis results of the black shale deposits reflected a kerogen type III of terrestrial origin of the El Fashn Formation due to the high OM content preservation in suboxic shallow marine environments. The Tmax and production index parameters of Rock-Eval pyrolysis reflected that the black shale deposits in the El Fashn Formation are immature, but with surface retorting, it can generate natural gas.

Recent expansion and intensification of hypoxia in the Arabian Gulf and its drivers

The Arabian Gulf (also known as Persian Gulf, hereafter Gulf) is a shallow semi-enclosed subtropical sea known for its extreme physical environment. Recent observations suggest a decline in oxygen concentrations in the Gulf over the past few decades accompanied by an expansion of seasonal near-bottom hypoxia. Here, we reconstruct the evolution of dissolved oxygen in the Gulf from 1982 through 2010 and explore its controlling factors. To this end, we use an eddy-resolving hindcast simulation forced with winds and heat and freshwater fluxes from an atmospheric reanalysis. We show that seasonal near-bottom hypoxia (O2&amp;lt; 60 mmol m-3) emerges in the deeper part of the Gulf over summer and peaks in autumn in response to enhanced vertical stratification inhibiting mixing and O2 replenishment at depth. We also find a significant deoxygenation in the Gulf over the study period, with the Gulf O2 content dropping by nearly 1% per decade and near-bottom O2 decreasing by between 10 and 30 mmol m-3 in the deeper part of the Gulf between the early 1980s and the late 2000s. These changes result in the horizontal expansion of seasonal bottom hypoxia with the hypoxia-prone seafloor area increasing from less than 20,000 km2 in the 1980s to around 30,000 km2 in the 2000s. The expansion of hypoxia is also accompanied by a lengthening of the hypoxic season with hypoxia emerging locally 1 to 2 months earlier in the late 2000s relative to the early 1980s. Furthermore, declining near-bottom O2 levels result in the expansion of suboxic conditions (O2&amp;lt; 4 mmol m-3) and the emergence and amplification of denitrification there. An analysis of the Gulf oxygen budget demonstrates that deoxygenation is essentially caused by reduced oxygen solubility near the surface and enhanced respiration near the bottom. While reduced solubility results from the warming of the Gulf waters, enhanced respiration is mostly driven by an increased supply of nutrients imported from the Arabian Sea due to the weakening of winter Shamal winds over the study period. Our findings suggest that recent changes in local climate are not only altering the Gulf physical environment but are also having a strong impact on the Gulf biogeochemistry with profound potential implications for the ecosystems and the fisheries of the region.

Controls of the paleoenvironment on differential organic matter enrichment of lacustrine fine-grained rocks: A case study of the Jurassic Da’anzhai Member, central Sichuan Basin, SW China

• The majority of siliceous shales in the Da’anzhai member have high TOC contents. • The Da’anzhai member was deposited under oxic-suboxic conditions. • OM accumulation was controlled by clastic input and sedimentation rate . • The differential OM enrichment is explained by high and low clastic input models. Organic matter (OM) is not only a source of oil and gas but also the dominant host for hydrocarbon storage in unconventional reservoirs; thus, understanding the controls on OM accumulation are of great significance for unconventional hydrocarbon exploration. Lacustrine fine-grained rocks of the Jurassic Da’anzhai Member have variable total organic carbon (TOC) contents ranging from 0.11 to 3.11 wt%. However, it is still debatable regarding mechanisms responsible for such variable enrichment. Here, we present petrological, organic, and elemental geochemical data of three wells in the Jurassic Da’anzhai Member in the central Sichuan Basin, South China. The low enrichment of redox-sensitive trace elements (Mo EF , U EF , and V EF ) in lacustrine fine-grained rocks indicates oxic to suboxic conditions during deposition of the Da’anzhai Member, which are not conducive to OM preservation. The chemical index of alteration (CIA) values vary between 75 and 100, suggesting a warm/hot and humid paleoclimate. Relatively high productivity coincides with high CIA, suggesting that weathering fluxes may have played important role in the regulating primary productivity. Furthermore, the relationship between TOC and (La/Yb) N implies the influence of specific sedimentation rates on OM enrichment. Our results suggest that the OM accumulation in the Jurassic Da’anzhai Member is mainly controlled by clastic input and sedimentation rate. We propose that under conditions of variable clastic input, the appropriate sedimentation rate may have played a key role in the accumulation of OM. Finally, our study suggests that the siliceous shale may be a potential lithology for unconventional hydrocarbon production in the Jurassic Da’anzhai Member.

Organic Geochemistry and Oil-Source Correlation of Oil Seepages and Potential Source Rocks in Qiangtang Basin, China.

Different viewpoints on the oil-source correlation of the Angdarco paleo-reservoir in the Qiangtang Basin, northern Tibet, China, have seriously affected oil and gas exploration in this area. In this work, based on the results of biomarker analysis on six oil seepage samples and 11 potential source rock samples, the oil source of the oil seepages in the Angdarco area was examined by correlating the composition characteristics of n-alkanes, multiple factors of biomarkers, correlograms of biomarker parameters, and δ13C values of individual n-alkanes between oil seepages and potential source rocks. In addition, the sedimentary environment and thermal maturity characteristics of the potential source rocks were also discussed. The biomarkers indicated that the potential source rocks of the Lower Jurassic Quse Formation (J1q) presented characteristics of littoral sediments, deposited under suboxic conditions, while the potential source rocks of the Middle Jurassic Buqu Formation (J2b) were deposited in a marine sedimentary environment and presented characteristics of a low mature to mature stage. The potential source rocks of the Middle Jurassic Xiali Formation (J2x) were deposited in a low-salinity environment and presented characteristics of low-mature source rocks. Finally, the oil seepages of the Buqu Formation were deposited in a suboxic littoral environment and presented characteristics of a low mature to mature stage. It is concluded that the oil seepages in the Angdarco paleo-reservoir were mainly sourced from the Lower-Jurassic Quse oil shales. This work provides a scientific basis for the exploration of paleo-reservoirs in the southern Qiangtang Basin.

The deposition and significance of an Ediacaran non-glacial iron formation.

Most Neoproterozoic iron formations (NIF) are closely associated with global or near-global "Snowball Earth" glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non-glacial NIF genesis, including the paleo-environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non-glacial NIF within a Neoproterozoic volcano-sedimentary sequence in North Qilian, northwestern China. New U-Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and εNd (t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ56 Fe=-0.04‰-1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)-rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals.

Paleoceanographic Conditions of Neo-Tethys Deposits in Northeast Iraq (Shiranish Formation) by Geochemical Proxies

Geochemical proxies provide detailed information on depositional environment and diagenetic processes. The main objectives of the present study are the evaluation of the paleoenvironment and diagenetic conditions of selected three sections of the Shiranish Formation (Late Campanian-Maastrichtian) in Dokan-Sulaymaniyah, Kurdistan Region, northeast of Iraq. The major and some trace elements were analysed by X-Ray Fluoresces. These geochemical results showed a positive correlation of SiO2, Al2O3, Fe2O3, MgO and TiO2 between each other due to the influence of detrital influx from the active oceanic margins and thrust belts of the northeast Arabian Plate. The terrigenous supply of silicates (K-feldspar) and clay minerals (Illite) caused a positive correlation between K2O values and Al2O3. In general, there is a low abundance of SiO2 and Al2O3 contents, referring to the deepening of the Neo-Tythus ocean basin in the formation's lower part. The Mn* factor of the lower and middle part of the formation exhibited anoxic conditions while oxic- suboxic conditions at the upper part. The Mn/Sr ratios were less than one, which means that Shiranish marly limestone was well preserved out of diagenetic processes. The paleoredox proxies {Th/U, V/Cr &amp;V/(V+Ni)} confirmed that the lower part of the Formation was deposited in a relatively anoxic deep-outer shelf environment with a rising in paleoproductivity. In contrast, the upper part was characterised by dyoxic-oxic redox. Due to the inner shelf environment, this redox fluctuation reflects a regression phase of sea level, which is compatible with the closure events of the Neo-Tethys in the Eocene epoch.

Miocene organic-rich layers of the Saouef Formation (Oriental Tunisia); insights into temporal and spatial variability of environmental conditions during deposition

Miocene organic-rich sediments (coal and carbonaceous clays) from the La Falaise section of the Saouef Formation have been investigated by organic petrographic, palynofacies and organic geochemical approaches. The results are compared with data obtained during a previous study of the nearby Saouef section. The sedimentary succession is built up by repetitive sedimentary sequences (i.e., parasequences) deposited during progradation and retrogradation, respectively. The coal, or carbonaceous layers, developed during the establishment of clay-dominated sedimentation.The organic-rich layers within the samples, containing high amounts of mineral matter, are dark brown, with a stratified texture and are classified as humic coal. Microscopically, the huminite group dominates the maceral composition of the carbonaceous clays and lignite (av. 75% mmf). The variable degrees of the tissue preservation index (TPI) and highly fluctuating vegetation index (VI) values provide evidence for different contributions of vascular plants vs. herbaceous vegetation and macrophytes, to peat formation. Higher contributions of macrophytes (e.g., reed grasses) to peat accumulation at La Falaise are concluded, based on n-alkane distributions. High mineral matter contents and changing gelification index (GI), and ground water index (GWI) values, found in both sections, provide evidence for rheotrophic hydrogeological conditions, and peat formation within wet forest swamp to inundated marsh environment. The low sulphur contents attest to peat formation within a freshwater environment and the relative proportions of ββ to (ββ + αβ)-C31 hopanes argue for slightly acidic conditions. In comparison to the results of the Saouef section (average pH of 5.6), a lower pH (4.2) is revealed from the average hopane isomerization ratio of the La Falaise samples.The palynofacies investigation of the La Falaise deposits indicates the dominance of phytoclasts over amorphous organic matter (AOM) in most of the samples and suggests good preservation of the higher plant remains. In contrast, increased degradation of plant tissues under oxygen-deficient conditions is concluded from the enhanced proportions of AOM found in several of the previously investigated Saouef samples. The molecular analysis of hydrocarbons from all the samples indicated the dominance of n-alkanes and hopanoids. The obtained pristane/phytane ratios argue for sub-oxic conditions during peatification at La Falaise, whereas data from our previous study of the Saouef section provided evidence for a reducing environment. Terpenoid hydrocarbon compositions reveal highly variable contributions of conifers and angiosperms to peat formation, consistent with previous findings at the Saouef sampling site. The presence of polycyclic aromatic hydrocarbons (PAHs), as well as fusinite, provides evidence for mire or forest fires.The comparison of the results from lignite of both sections of the Saouef Formation indicate comparable, highly variable conditions during peat accumulation, probably governed by eustatic sea level changes. The peatlands most likely developed due to the establishment of flood basins situated between alluvial ridges. The obtained regional differences are suggested to reflect slightly different hydrological conditions and associated vegetation cover in the peatlands.

Palaeo-environmental evolution and organic matter enrichment of Eopalaeozoic shales, northwestern Tarim Basin, China: Integrated organic and inorganic geochemistry approach

The Eopalaeozoic was a pivotal transitional period following the Neoproterozoic glaciation that witnessed dramatic changes in global ocean environments. The black rock series were extensively sedimented, preserving the original seawater signature. However, due to the scarcity of geological records, the palaeo-environment and organic matter (OM) enrichment in the Tarim Basin, China, are poorly constrained. Herein, Eopalaeozoic shales from typical outcrops in the Kalpin block, northwestern Tarim Basin, have been subjected to integrated organic and inorganic geochemistry research. In the study area, Eopalaeozoic black rock series can be found in the Yuertusi Formation (YUF), Saergan Formation (SAF) and Yingan Formation (YIF) from bottom to top. Although the maturity is moderate (~1.2%), the OM abundance varies greatly vertically. Microscopic observations and biomarkers revealed that the OM source was dominated by diverse planktonic algae. The chemical alteration index indicates a transition from cold and dry climates with low chemical weathering to warm and humid climates with moderate chemical weathering, and subsequently back to cold and dry climates. The stratified water column and euxinic–suboxic conditions are revealed by redox proxies (Mo, enrichment factors, Th/U, MoU covariation pattern, pristane/phytane and gammacerane index). The YUF was deposited in a euxinic environment. Productivity proxies (biogenic Ba, Cu/Al, Ni/Al and carbon isotopes) show that primary productivity was superelevated in the lower YUF and moderate in the other formations. Primary productivity and preservation conditions were heavily influenced by complementary hydrothermal activities and upwelling currents. The hydrothermal activities and upwelling currents provide abundant nutrients and highly reductive environments, creating enormous possibilities for the formation of excellent source rocks with considerable hydrocarbon geological significance.

Degraded mangroves as sources of trace elements to aquatic environments

Mangrove forests have been reported as sinks for metals because of the immobilization of these elements in their soils. However, climate change may alter the functioning of these ecosystems. We aimed to assess the geochemical dynamics of Mn, Cu, and Zn in the soils of a mangrove forest dead by an extreme weather event in southeastern Brazil. Soil samples were collected from dead and live mangroves adjacent to each other. The physicochemical parameters (total organic carbon, redox potential, and pH), total metal content, particle size, and metal partitioning were determined. Distinct changes in the soil geochemical environment (establishment of suboxic conditions) and a considerable loss of fine particles was caused by the death of the mangroves. Our results also showed a loss of up to 93 % of metals from soil. This study highlights the paradoxical role of mangroves as potential metal sources in the face of climate change.

Hydrocarbon Potential and Biomarker Studies of EE-1 Well, Offshore Eastern Dahomey Basin, SW Nigeria

Abstract The hydrocarbon potential, organic source input, and paleodepositional environment of subsurface sediments from EE-1 well, offshore Eastern Dahomey Basin, were assessed using Rock-Eval pyrolysis and biomarker geochemistry. The total organic carbon (TOC) and soluble organic matter (SOM) in the sediments ranged from 0.96wt% to 8.92wt% and 676.12 ppm to 2883.85 ppm, respectively, indicating adequate to excellent organic richness. The pseudo-Van Krevelen plot classified the sediments as types II and III kerogen, which have the potential to generate both oil and gas. The Tmax and production index (PI) ranged from 422°C to 431°C (average, 426°C) and 0.03 to 0.24, respectively, suggesting low thermal maturity. The presence of C27–C29 steranes, oleanane, and hopane/sterane ratio (1.53/16.11), indicated organic matter from mixed sources with more terrigenous input. Cross plots of Pr/nC17 against Ph/nC18, C35/C31 – C35 homohopane index (0.05 – 0.17) and other related biomarker ratios such as C21/C23 trycyclic terpane ratio (0.33 – 0.82) signified that the sediments were deposited in mixed marine/terrigenous environment under oxic – suboxic conditions. This study has demonstrated that sediments had adequate organic matter with the potential to generate both oil and gas at appropriate thermal maturity.

Lithostratigraphy and Depositional Environments of the Bazhenovo Formation in the Koltogorsk-Nyurol’ka trench (Western Siberia)

The paper presents the results of a study of the structure and stages of accumulation of the Bazhenovo Formation in the area of the Koltogorsk-Nyurol’ka trench and adjacent positive structures of the Southeastern region of Western Siberia. Based on the use of unified lithological criteria, taking into account geophysical and paleontological-stratigraphic data, the lower four out of six members identified within the Bazhenovo Formation in the Central Region of Western Siberia were traced in the studied area. In the Southeastern region, members 1-4 constitute the entire Bazhenovo Formation, which corresponds here to most of the Volgian-lower part of the Ryazanian. At the end of the Middle Volgian time, an increase in the supply of clay material to the study area is noted due to the proximity of this area to sediment sources and some decrease in global sea level. The depositional environments of the Bazhenovo Formation in the Southeastern region were suboxic during the Middle Volgian and changed to anoxic in the Late Volgian-first half of the Ryazanian. The development of suboxic conditions in the second half of the Ryazanian led to the cessation of the accumulation of Bazhenovo deposits in the studied area.

Potential role of submerged macrophytes for oxic methane production in aquatic ecosystems

AbstractMethane (CH4) from aquatic ecosystems contributes to about half of total global CH4emissions to the atmosphere. Until recently, aquatic biogenic CH4production was exclusively attributed to methanogenic archaea living under anoxic or suboxic conditions in sediments, bottom waters, and wetlands. However, evidence for oxic CH4production (OMP) in freshwater, brackish, and marine habitats is increasing. Possible sources were found to be driven by various planktonic organisms supporting different OMP mechanisms. Surprisingly, submerged macrophytes have been fully ignored in studies on OMP, yet they are key components of littoral zones of ponds, lakes, and coastal systems. High CH4concentrations in these zones have been attributed to organic substrate production promoting classic methanogenesis in the absence of oxygen. Here, we review existing studies and argue that, similar to terrestrial plants and phytoplankton, macroalgae and submerged macrophytes may directly or indirectly contribute to CH4formation in oxic waters. We propose several potential direct and indirect mechanisms: (1) direct production of CH4; (2) production of CH4precursors and facilitation of their bacterial breakdown or chemical conversion; (3) facilitation of classic methanogenesis; and (4) facilitation of CH4ebullition. As submerged macrophytes occur in many freshwater and marine habitats, they are important in global carbon budgets and can strongly vary in their abundance due to seasonal and boom‐bust dynamics. Knowledge on their contribution to OMP is therefore essential to gain a better understanding of spatial and temporal dynamics of CH4emissions and thus to substantially reduce current uncertainties when estimating global CH4emissions from aquatic ecosystems.

Geochemical, mineralogical, and petrological characteristics of the Cretaceous coal from the middle Benue Trough Basin, Nigeria: Implication for coal depositional environments

The Benue Trough Basin (BTB) of Nigeria is geologically and geo-morphologically subdivided into upper, middle, and lower segments. The BTB is the subject of geological research due to its rich coal deposits that have the potential for oil and gas. The purpose of the present study is to examine the origins, depositional environments, and thermal history of the selected coals and the processes that influence their quality. Coal samples from different open cast coal mines in the middle BTB were examined using proximate and ultimate, x-ray diffraction (XRD), x-ray fluorescence (XRF), laser ablation induced coupled plasma (LA-ICPMS), and petrographical analyses. The coal samples contained mainly quartz, kaolinite, and organic carbon. The XRD spectra peaks revealed the triclinic and monoclinic structure of kaolin. The SiO2/Al2O3 ratios confirmed the dominance of quartz and kaolinite. The Ni/Co versus V/Cr, Cu/Zn, and V/Mo values in the studied coals suggest oxic depositional environments, whereas the V/(Ni + V) and V/(V + Cr) values indicate oxic to suboxic conditions. The Ce/Ce∗ values are slightly below 1, which indicates a suboxic depositional environment. Maceral texture is indicative of transformations from sub-bituminous to high volatile C or B bituminous coal. The maceral components and mineral matter (≥10%) of the studied coals imply deposition in the planar margin mire, and a river system of planar margin mire environments, respectively. The high gelification index (GI) and tissue preservation index (TPI) values indicate peat accumulation developed within a wet forest swamp. The ternary diagram of the maceral component suggests deposition in a wet moor environment with intermittent moderate to high flooding episodes. The studied coals are sub-hydrous vitrinite inferring hydrogen-poor and thermally immature characteristics. The low V and Ni contents, low H/C and high O/C, indicate Type Ⅲ terrestrial organic matter with the potential to generate gaseous hydrocarbons. The investigated coals were deposited by the river within telmatic, limnic, and limno-telmatic zones in the planar margin mire depositional environments. Overall, the integrated petrologic and geochemical data used in this study provides a reliable approach for the assessment of coal depositional environments.

Upper Cretaceous-Paleocene Unconventional Oil Shales in Egypt and Jordan: Insights into Organic Geochemistry

The present paper investigates the unconventional hydrocarbon potential and characteristics of Campanian–Paleocene oil shales at five outcrop sections in central Egypt and Jordan. One-hundred and seven outcrop samples were analyzed for their content of total organic carbon and total sulfur. Twenty-four samples were selected for further geochemical treatment, including Rock-Eval pyrolysis, and six samples were subjected to molecular analyses using gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS). The distribution of aliphatic hydrocarbon biomarkers indicated that most of the organic matter in the studied samples had a marine origin under anoxic to suboxic conditions. The results indicated that the upper Campanian Duwi Formation in the Quseir area of central Egypt and the Maastrichtian-Paleocene Muwaqqar Chalk Marl (MCM) Formation in the Lajjun area of central Jordan had a relatively high hydrocarbon generating potential with a TOC content of up to 20%wt of immature Type I/II kerogen. Biomarker studies of the investigated formations revealed that the origin of the organic matter is marine algae with a minor contribution of bacterial biomass. The terrigenous/aquatic ratio (TAR) indicated the marine origin of the Jordanian organic matter (MCM and Al-Hisa Phosphorite Formation formations). By contrast, the Duwi Formation in Egypt and the Amman silicified limestone Formation in the Umm Qais section of Jordan showed that these localities were affected by a more terrigenous influx to the principal marine organic matter compared with other rock units. In addition, in the Abu Tartur section the Duwi Formation showed a significant microbial reworking of organic matter reducing the actual amount of organic matter and its hydrocarbon generating potentiality. Clear correlations between nature and the preservation of organic matter is noticed, where the high organic matter mainly occurs in regional paleo-upwelling areas, strongly related to an increase in marine phytoplanktonic/algal and bacterial productivity.

Response and Potential Indication to Hypoxia in the Changjiang River Estuary and its Adjacent Waters: Insight From Redox-Sensitive Trace Elements in Sediment Core

The migration and enrichment of redox-sensitive trace elements (RSEs) from seawater to sediments are controlled not only by their geochemical properties but also by marine redox conditions. Therefore, RSEs, such as Mo, U, and V, are extensively used to indicate redox states in marine environments. To retrieve the historical redox state in the hypoxic zone near the Changjiang River Estuary and its adjacent waters, the distribution and enrichment degree of Mo, U, and V in a sediment core collected from the recurrently hypoxic region were investigated. The correlation analysis shows that the authigenic enrichment of Mo and U in sediment core 3,050–2 is primarily controlled by redox conditions, rather than sediment grain size, adsorption on organic matters, Fe–Mn (hydr)oxides, and terrigenous detritus input. Mo-U covariation is selected to analyze the evolution of redox conditions in the seasonally hypoxic zone. The MoEF/UEF values in this sediment core are almost between 0.1 and 0.3×Mo/Umodern seawater value and show a general decreasing trend with depth, indicating an overall progressive transition from oxic to suboxic conditions in this area since 2005. The close MoEF/UEF values and similar chronological Mo-U variations by comparing our results with previous studies in adjacent areas offer common evidence for the gradually intensified bottom water–oxygen stress, suggesting the possible application of MoEF/UEF in retrieving redox development in this coastal hypoxic zone.

New REE-Constrained Terrigenous Depositional Changes in the Central Okinawa Trough and their Responses to Climate since 92 Millennia

Abstract The continental margin is one of the major depocenters for terrigenous sediments in the world ocean. Complex interactions between land and sea control the sedimentary processes along the continental margin. The Okinawa Trough (OT) receives huge amounts of terrigenous materials transported by rivers from the adjacent landmass, while the Kuroshio Current (KC) and the North Pacific Intermediate Water jointly play major roles in oceanic climate and regional deposition patterns of terrigenous sediments. Various studies have investigated the changes in terrigenous sediment provenance in the central OT only since the last glacial. Still, few studies focus on more long-term variations in terrigenous deposition over the last few glacial-interglacial cycles. Here, we report a new high-resolution sediment geochemistry record of a well-dated core MD01-2404 taken from the central OT covering a timespan of 92 kyr. Our results show dynamic glacial-interglacial changes in sediment compositions and terrigenous deposition patterns. Bulk sediments in MD01-2404 comprise relatively high biogenic components during interglacials, while higher terrigenous fractions during the last deglacial and glacial periods. We suggest that the provenance of terrigenous sediments is mainly derived from Taiwanese rivers, Yangtze River, and Yellow River with hydrothermal inputs. During the time interval of 37 to 46 kyr, a new finding of “Negative Ce Anomaly Event” in the central OT firstly reported here is attributed to a combination of suboxic conditions in deep and pore waters and the addition of depleted Ce-bearing sediments. Temporal changes of deposition patterns in the central OT are attributed to complex interactions among dynamic glacial-interglacial changes in eustatic sea level, the East Asian summer monsoon, and the KC.