Anoxic Shelf Research Articles

Contourite-drift archive links Late Devonian bioevents with periodic anoxic shelf water cascading

Analysis of a Devonian contourite depositional system in the eastern Anti-Atlas of Morocco reveals the formation of widespread erosional hiatuses and organic-rich bioclastic contourites (ORCs) coinciding with the expansion of an anoxic water mass during Frasnian bioevents, ultimately culminating in the Kellwasser crisis (Frasnian-Famennian extinction event). The identified contourite terrace formed on the uppermost slope of the northern passive margin of Gondwana. Its inner part was bounded by an along-slope contourite channel and a small mounded drift at its downslope margin. Facies- and drift-scale contourite features evidence northwest-directed bottom currents driven by repeated overflows of dense, highly saline, anoxic water originating from the northern Gondwana Epicontinental Sea. These periodic overflows were channeled through the Ougarta trough, then deflected westward over the Tafilalt contourite terrace by the Coriolis force and cascaded downslope until reaching a density equilibrium level, probably forming an intermediate water mass. The cascading of dense, anoxic shelf water supports the photic-zone eutrophication (top-down) model proposed for the Kellwasser crisis and related Devonian anoxic events. We propose a direct link between the anoxic overflows and the Devonian evolutionary events.

Paleoenvironment, Geochemistry, and Pore Characteristics of the Postmature to Overmature Organic-Rich Devonian Shales in Guizhong Depression, Southwestern China

Investigating shale pore characteristics has deepened our understanding of shale reservoir, while that of postmature-overmature shales is yet to be revealed, which is especially critical for shale gas evaluation in southern China. Ten Middle-Upper Devonian organic-rich shale samples were collected from well GY-1 in the Guizhong Depression, and the paleoenvironment, geochemistry, and pore system were analyzed with a series of experiments, including trace element analysis, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), low-pressure N2 adsorption, and source rock geochemistry. Results show that the Middle-Upper Devonian shales in the Guizhong Depression are organic-rich mudstones with TOC ranging from 0.14% to 6.21%, which is highest in the Nabiao Formation ( D 2 n ) and Lower Luofu Formation ( D 2 l ) that were deposited in the anoxic and weak hydrodynamic deep-water shelf. They are thermally postmature to overmature with equivalent vitrinite reflectance ( EqV R o ) of 3.40%~3.76% and type I kerogen. The lithofacies in D 2 n and D 2 l are primarily siliceous/argillaceous mixed shale as well as a few siliceous argillaceous shales and argillaceous siliceous shales as well. Organic matter- (OM-) hosted pores within bitumen are primary storage volume, rather than inorganic pores (interparticle and intraparticle) which are rare. The total helium porosity of samples varies between 1.20% and 4.49%, while total surface area and pore volume are 2.39-14.22 m2/g and 0.0036-0.0171 ml/g, respectively. Porosity, pore surface area, and pore volume are in accordance with increasing TOC, R o , and siliceous mineral contents. Considerable OM-macropores are found in shales with R o > 3.6 % in our study which demonstrates that the porosity at postmature to overmature stage ( R o = 3.5 − 4.0 % ) does not change fundamentally. The high level of maturity is not considered the main controlling factor that affects shale gas content, and more attention should be paid to preservation conditions in this area.

Chromium reduction and associated stable isotope fractionation restricted to anoxic shelf waters in the Peruvian Oxygen Minimum Zone

The marine chromium (Cr) cycle is still insufficiently understood, in particular the mechanisms modulating the spatial distribution of dissolved stable Cr isotopes in seawater. Redox transformations between its main oxidation states, Cr(VI) and Cr(III), have been held accountable for the observed tight inverse logarithmic relationship between the dissolved Cr concentration [Cr] and its isotopic composition (δ53Cr), whereby isotopically light Cr(III) is removed in surface waters and oxygen minimum zones (OMZs), and subsequently released to deeper waters from remineralized particles or sediments.Seawater [Cr] and δ53Cr were investigated in a series of depth profiles across the Peruvian margin OMZ, covering a wide spectrum of dissolved oxygen concentrations ranging from 2 to 242 µmol/kg. We found [Cr] ranging from 1.5 to 5.5 nmol/kg, associated with δ53Cr variations between +1.59 and +0.72‰, but no systematic relationship to dissolved oxygen concentrations. However, distinctly different seawater profiles were observed above the suboxic/anoxic shelf compared to those located further offshore, with substantial Cr removal restricted to suboxic or anoxic environments on the shelf. This suggests that suboxic conditions ([O2] < 5 µmol/kg) alone are not sufficient to account for substantial Cr removal. Given that environmental conditions under which Cr can be reduced remain restricted spatially, the role of this sink in the marine Cr cycle may therefore be small. Additionally, some observations corroborate the assumption that Cr reduction is not necessarily accompanied by immediate adsorption of the formed Cr(III) onto particles, leading to its removal from the dissolved phase. Instead, partial removal of Cr(III) via particles, leaving a residual dissolved Cr(III) pool, may be more widespread in suboxic waters.

Controlling factors on the formation and distribution of “sweet-spot areas” of marine gas shales in South China and a preliminary discussion on unconventional petroleum sedimentology

Shale gas resources in China are abundant with the marine shales accounting for the largest proportion, i.e. 13 × 1012 m3. The Wufeng-Longmaxi Shale system deposited across the Late Ordovician and Early Silurian transition in South China is currently the target for shale gas exploration and development in China. Its annual output in 2018 reached 10.8 × 109 m3, making China the largest shale gas production region outside North America. Within this shale system the sweet-spot intervals of shale gas formed vertically and sweet-spot areas of shale gas formed regionally. The sweet-spot intervals occur mainly at the bottom of the Wufeng-Longmaxi Shale and are 10–40 m in thickness. The sweet-spot interval is characterized by high total organic carbon (TOC) and gas contents, relatively high porosity, high brittle mineral content, and abundant lamination fissures and fractures. The formation of shale gas sweet-spot intervals is mainly controlled by four factors in South China. They are anoxic shelf environment, abundant nano-pores in organic matter, sealed roof and floor strata, and abundant lamination and high silica content in shale, respectively. As the regional extension of the sweet-spot intervals, the formation of the sweet-spot area requires all the controlling factors for the sweet-spot interval. It was “born” in regional anoxic shelf with stable tectonics and “preserved” in relatively stable tectonic zones. Three sweet-spot areas are identified for the Wufeng-Longmaxi Shale in South China, i.e. Fuling, Weiyuan-Fushun-Yongchuan and Changning-Zhaotong areas. The total area is about 2 × 104 km2 with technically recoverable resources of 3.3 × 1012 m3.In addition, with the rapid progresses of exploration and development of unconventional petroleum resources such as shale gas, the petroleum industry around the world extends to unconventional petroleum resources, gradually accomplishing an important change of petroleum geology research focus from conventional to unconventional. Unconventional petroleum geology research focuses on continuously or semi-continuously distributed sweet-spot areas, for which the formation would be the result of coupling sedimentology from several geological events such as global or regional tectonic activity, volcanic eruption, climate change, anoxic water, biotic mass extinction and radiation, and gravity current. It is necessary to innovatively construct new insights of unconventional petroleum sedimentology in order to enrich and improve the petroleum sedimentology research and its application. Therefore, the concept of unconventional petroleum sedimentology is proposed, and its research task, method, aim and importance are preliminarily discussed.

Shelf-to-basin iron shuttle in the Guaymas Basin, Gulf of California

Enrichments of highly reactive iron (Fe) (sum of Fe (oxyhydr)oxide, carbonate and sulfide minerals) in marine sediments and sedimentary rocks are commonly interpreted as an indication of anoxic conditions in the bottom water at the time of deposition. The model system for this proxy rationale is the semi-restricted Black Sea, where sediments underneath the anoxic and sulfidic (i.e., euxinic) deep-water are enriched in reactive Fe, which was mobilized from the surrounding shelf areas. To test whether such a shelf-to-basin Fe shuttle can operate in semi-restricted basins without euxinic deep water, we investigated sedimentary Fe speciation and Fe isotope compositions in sediments of the Guaymas Basin, Gulf of California. Sediments on the slope underneath the eastern equatorial Pacific oxygen minimum zone and sediments within the oxic deep basin are both enriched in reactive Fe, with reactive Fe making up 45 ± 11% of the total Fe pool. The following mechanisms may contribute to these Fe enrichments: (1) Release of dissolved Fe from anoxic shelf and slope sediments followed by lateral transport of dissolved and/or particulate Fe in the water column; (2) preferential transport of fine-grained, terrigenous particles with a high reactive Fe content into the basin; (3) microbially mediated conversion of non-reactive silicate minerals to reactive Fe minerals during transport; (4) hydrothermal venting and lateral Fe transport within the deep water. The first process can explain reactive Fe enrichments in slope sediments, whereas all processes may contribute to sedimentary Fe enrichments in the deeper basin. The δ 56 Fe value of sediments increases from shelf to slope and decreases from the slope into the basin. This lateral pattern of δ 56 Fe, as well as the pattern of Fe enrichment, is similar to that observed in other marine systems with a Fe shuttle. However, the size of the Fe enrichment, and the range in δ 56 Fe (−0.06 to +0.16‰) is smaller. This difference is due to higher terrigenous sedimentation rates in the Guaymas Basin and, therefore, more intense dilution of shuttle-derived reactive Fe. We argue that, depending on the extent of bathymetric restriction and terrigenous background sedimentation, reactive Fe enrichments can form under a broad range of redox conditions and in diverse sedimentary environments. The concepts applied in this study can be used to identify those circumstances in the paleo-record.

Palynological and organic geochemical studies of the Upper Jurassic-Lower Cretaceous successions, Western Desert, Egypt: Implications for paleoenvironment and hydrocarbon source rock potential

The Upper Jurassic-Lower Cretaceous successions in the north Western Desert of Egypt are one of the most prolific hydrocarbon plays in North Africa. However, the source rock characteristics and depositional environments are still poorly understood. The current work assesses these sedimentary sections utilizing an integrated palynofacies and organic geochemical approach for a comprehensive source rock characterization. The investigation was carried out on 70 samples from the OBA. 3-1/1A (interval: 2540 to 3650 m) and OBA. S-C (interval: 2705 to 3782 m) wells. The results obtained are sufficient to categorically establish two types of organofacies linked to depositional environments and kerogen types, namely palynofacies PF I and PF II. The first facies (PF I) is characterized by kerogen type II and III and occurs in the Alamein and Alam El Bueib members of the Lower Cretaceous Burg El Arab Formation. This is further confirmed by pyrolysis results that revealed fair organic richness and gas generation potential in the Alamein Member with TOC ranges from 1.0 to 2.5 wt% and HI from 64 to 112 mg HC/gTOC). The Alam El Bueib Member illustrated better organic richness and quality with TOC ranges from 1.6 to 3.1 wt% and HI from 121 to 318 mg HC/gTOC. The thermal maturity assessment indicates that the Alamein Member is immature and the Alam El Bueib Member is early to oil-mature. PF II occurs in the Jurassic Masajid Formation, which is a transitional mature to overmature, fair to good source rock with mixed kerogen type II and III (TOC 1.3 to 2.4 wt% and HI 148 to 238 mg HC/gTOC). The paleoenvironmental conditions were deduced mainly from the APP ternary plot. The Alamein and Alam El Bueib members were deposited in a suboxic to anoxic basin, whereas the Masajid Formation was deposited under distal, dysoxic to anoxic basinal or proximal, suboxic to anoxic shelf conditions. In summary, it is concluded that the Late Jurassic and Early Cretaceous witnessed deposition of gas-prone source rocks similar to those found in the southern onshore Nile Delta Basin. The results have significant implications for the understanding of the source rock types in the northern onshore and offshore Egyptian basins and the future of the gas exploration.

Shoreface facies model of Cretaceous Jessu Formation, Yola Sub-basin, Northern Benue Trough, northeast Nigeria: New insights from facies analysis and molecular geochemistry

The Yola Sub-basin in the Northern Benue Trough is a frontier exploration basin in northeastern Nigeria and to date, there is no subsurface data available. The oldest sedimentary unit in the Yola Sub-basin is the Bima Formation, followed by Yolde, Dukul, Jessu, Sekuliye, Numanha and Lamja Formations, ranging in age from Berremian-Aptian-Albian to Santonian. The sediments of Jessu Formation were studied based on facies analysis (lithofacies and palynofacies) and geochemistry (biomarker distribution) with an objective of analysing and interpreting the source input/origin, paleodepositional conditions, paleodepositional environment and to reconstruct depositional and facis model for the studied successions. The study revealed that the sediments were deposited in a distal oxic-suboxic to relatively anoxic shelf (shallow marine) environment with both aquatic,-marine and terrestrially derived source inputs. Depositional framework indicate a progradational wave and storm-dominated shoreface-offshore facies succession, displaying a coarsening upward trend. This represent a regressive shoreface-offshore depositional system, consistent with earlier interpretations that the Cretaceous sediments of Jessu Formation were deposited during a brief regressive phase in the Northern Benue Trough.

Palynology of the Cenomanian Raha Formation, Gulf of Suez, Egypt: Biostratigraphical, palaeoenvironmental and palaeobiogeographical implications

Abstract The current study presents a fully qualitative palynological investigation carried out on the Raha Formation encountered from three wells in the Bakr Oil Field of the Gulf of Suez, Egypt. Around 30 species of pteridophytic spores, 26 species of angiosperm pollen, 24 species of gymnosperm pollen and 27 species of dinoflagellate cysts have been recorded. However, achritarchs, microforaminiferal test linings and freshwater algae are impoverished and sparsely documented throughout the Raha Formation. Two palynozones have been identified based on some stratigraphically significant pollen and spores, arranged from youngest to oldest: (1) Palynozone I (Classopollis brasiliensis–Tricolpites sagax Assemblage Zone) of late Cenomanian age; (2) Palynozone II (Afropollis jardinus–Crybelosporites pannuceus Assemblage Zone) of early-middle Cenomanian age. The distribution and ecological affiliation of specific palynomorph species, as well as various palynofacies parameters, are interpreted. A shallow marine environment from supratidal to distal inner neritic under proximal suboxic–anoxic to dysoxic–anoxic shelf conditions is reconstructed. Palaeobiogeographically, the absence of elaters from the recovered taxa is interpreted in terms of minor floral variation. This may be attributed to climatic and/or an environment-controlled niche establishment, which possibly was shaped by the existence of a physical barrier hindering the distribution of such type of elaterate parent plants.

H2S events in the Peruvian oxygen minimum zone facilitate enhanced dissolved Fe concentrations

Dissolved iron (DFe) concentrations in oxygen minimum zones (OMZs) of Eastern Boundary Upwelling Systems are enhanced as a result of high supply rates from anoxic sediments. However, pronounced variations in DFe concentrations in anoxic coastal waters of the Peruvian OMZ indicate that there are factors in addition to dissolved oxygen concentrations (O2) that control Fe cycling. Our study demonstrates that sediment-derived reduced Fe (Fe(II)) forms the main DFe fraction in the anoxic/euxinic water column off Peru, which is responsible for DFe accumulations of up to 200 nmol L−1. Lowest DFe values were observed in anoxic shelf waters in the presence of nitrate and nitrite. This reflects oxidation of sediment-sourced Fe(II) associated with nitrate/nitrite reduction and subsequent removal as particulate Fe(III) oxyhydroxides. Unexpectedly, the highest DFe levels were observed in waters with elevated concentrations of hydrogen sulfide (up to 4 µmol L−1) and correspondingly depleted nitrate/nitrite concentrations (<0.18 µmol L−1). Under these conditions, Fe removal was reduced through stabilization of Fe(II) as aqueous iron sulfide (FeSaqu) which comprises complexes (e.g., FeSH+) and clusters (e.g., Fe2S2|4H2O). Sulfidic events on the Peruvian shelf consequently enhance Fe availability, and may increase in frequency in future due to projected expansion and intensification of OMZs.

Palynostratigraphic, palynofacies, organic geochemical and palaeoenvironmental analysis of the Silurian Tanezzuft Formation in the Ghadames Basin of north-west Libya

Two palynofacies associations are documented from the Silurian Tanezzuft Formation in the Ghadames Basin. These are characteristic of the basal ‘Hot Shale’ and the overlying deposits, referred to here as the Cold Shale. The former reflects deposition in distal suboxic anoxic conditions and is dominated by highly oil-prone amorphous organic matter (AOM) typical of deposition in generally anoxic, restricted marine basins. Only a few acritarchs, prasinophyte algae and chitinozoans occur in association: virtually no spores or cryptospores were recorded. Thick-walled prasinophytes are most numerous in this part of the Tanezzuft Formation in both the Ghadames and Murzuq basins, suggesting enhanced surface water productivity. Deposition took place after the melting of the Late Ordovician ice sheets, which led to a major marine transgression. The palynofacies recorded from the overlying ‘Cold Shale’ deposits indicate deposition in distal shelf and basin conditions that were also relatively anoxic. They contain more palynomorphs, especially acritarchs, and generally less AOM. The phytoplankton assemblages are dominated by simple and thin-walled prasinophyte algae (leiospheres), suggesting dysoxic–anoxic conditions. Overall the middle and the upper parts of the Tanezzuft Formation are regarded as being deposited in distal dysoxic–anoxic shelf, distal dysoxic–oxic shelf and distal suboxic–anoxic basin respectively. Because chitinozoans are very rare, age determinations of the samples investigated are based mainly on acritarchs. The Hot Shale is dated as early-mid Rhuddanian (early Llandovery) whereas the rest of the formation is considered to late Rhuddanian–Telychian in age. As documented previously from other samples of the Tanezzuft Formation in both the Ghadames and Murzuq basins, the Hot Shale has a very high TOC content and excellent source potential for liquid hydrocarbons, whereas rest of the formation is less rich in organic matter with larger terrestrial and oxidized components and hence reduced potential for sourcing hydrocarbons (both oil and gas).

Factors Affecting Shale Gas Accumulation in Overmature Shales Case Study from Lower Cambrian Shale in Western Sichuan Basin, South China

A series of comprehensive and systematic experimental data and log data of lower Cambrian Qiongzhusi Formation, from the first successful Well Jin-1 in western Sichuan Basin, were analyzed to investigate the properties of lower Cambrian organic-rich shale and to illustrate its shale gas potential, so as to investigate the factors affecting shale gas potential in overmature and ultratight shale. Trace element ratios (V/Cr and Ni/Co) and the concentrations of Cu + Zn imply the lower Cambrian shale deposited under suboxic–anoxic bottom water with high primary productivity. The organic matter is enriched in Qiongzhusi Formation as sapropelic (type I) mainly, and Ro (2.65–2.97%) values indicate the organic matter is in the overmature stage. Pores are dominated by organic pores in lower organic-rich shale and by inorganic intragranular and intergranular pores in upper organic-rich shale. Generally, organic-rich shale was deposited in anoxic shelf (high ratios of V/Cr and Ni/Co) with high productivity (high conc...

Palynofacies and Source Rock Potential of the ST-7H Well, Offshore Tano Basin, Western Region, Ghana

Samples from ST-7H well offshore Tano basin in western region of Ghana have been subjected to palynofacies, palynological and geochemical analysis. Five palynofacies associations (I – V) have been identified based on the percentage relative abundances of the sedimentary organic matter (SOM). Palynofacies type I and type IV reflects deposition in a distal dysoxic-anoxic shelf (nearshore) environment, palynofacies type II suggest distal dysoxic to anoxic shelf to deep basin environment with abundant AOM, palynofacies type III is indicative of distal dysoxic to oxic shelf (fluvio-deltaic) environment of deposition and palynofacies type V, a mud –dominated oxic distal shelf (open marine) environment. Based on marker palynomorphs, an Aptian to Maastrichtian age, have been assigned to the sediments in the ST 7H well, with an unconformity between the Cenomanian and Campanian sediments. Geochemical data indicate that the samples from ST 7H well have fair to very good petroleum potential. Most of the samples, however, fall out of the hydrocarbon generating zone because of the low (< 0.10) Production Index (PI). Kerogen types show type II, II/III and III which are oil prone, oil-gas prone and gas prone respectively. Thermal maturity from within the well indicates immature to early mature hydrocarbons.

Palynofacies characterization for evaluation of hydrocarbon source rock potential of Lower Paleogene (Thanetian-Ypresian) sub-surface sediments of Barmer Basin, western Rajasthan, India

This paper deals with the hydrocarbon source rock evaluation and depositional environment of Lower Paleogene sub-surface sediments from borehole MMK-26 of Barmer Basin, Rajasthan. Hydrocarbon potential of these sediments are estimated on the basis of plant derived Sedimentary Organic Matter (SOM) analysis. The palynofacies analysis indicate that the sediments are rich in organic matter with amorphous organic matter constituting the majority of the SOM in the lower part of the borehole where as the phytoclasts dominate in the upper part. The dominance of sapropelic facies (Amorphous Organic Matter) at a depth of 152 m and below suggests that these sediments are a good source for hydrocarbon generation. Further the categorized palynofacies components are plotted on Tyson's Amorphous Organic Matter-Phytoclast-Palynomorph diagram which reveal that the studied sediments are characterized by type-II (oil generating) and type – III kerogen deposited in a dysoxic – anoxic shelf. Thermal Alteration Index (TAI) values were determined to asses the organic maturity of the sediments and a maturation value of 2.80–3.0 for the lower part of the bore core suggest that the sediments have attained an optimum maturation level to produce hydrocarbons. The increasing frequency of marine palynomorphs in association with continental components indicates a transgressive phase of sea level during the deposition of the studied sediments.

Palynology, palynofacies and petroleum potential of the Upper Cretaceous–Eocene Matulla, Brown Limestone and Thebes formations, Belayim oilfields, central Gulf of Suez, Egypt

Palynological, palynofacies and organic geochemical results of 46 samples retrieved from the Upper Cretaceous – Eocene Matulla, Brown Limestone and Thebes formations, Belayim oilfields, central Gulf of Suez, Egypt are presented. The two latter formations are not dated palynologically as their lithology is not promising for palynological yield. However the Matulla Formation is dated as Turonian–Santonian age, based on the combined evidence of pollen and dinocysts.Palynofacies analysis carried out under both transmitted and fluorescent microscopy indicated that both the Thebes and Brown Limestone formations are deposited under a distal suboxic–anoxic environment. On the other hand, the Turonian–Santonian Matulla Formation supported the existence of a marginal marine deposition under dysoxic–anoxic basin to proximal suboxic–anoxic shelf environments. Rock-Eval pyrolysis and TOC results indicated that most of the studied formations are thermally immature to marginally mature and have a good petroleum potential. They are organically-rich in both oil- and gas-prone kerogen Type-II and II/III, deposited under marine reducing conditions favorable for hydrocarbon generation and expulsion.

Spatiotemporal variability of ocean chemistry in the early Cambrian, South China

Following the Ediacaran metazoan radiation, the “Cambrian Explosion” set up the major framework of todays’ animal phyla as well as modern marine ecosystem. Here, we present a preliminary investigation on the temporal and spatial (from shallow to deep waters) variations of the early Cambrian ocean chemistry in South China through analyzing a Fe-S-C systematic dataset integrated from literature. Our investigation indicates that the early Cambrian deep ocean in South China was still anoxic and Fe2+-enriched (i.e., ferruginous) although its surface was oxic, and in between a metastable euxinic (anoxic and sulfidic) water zone may have dynamically developed in anoxic shelf waters with an increasing weathering sulfate supply. Furthermore, accompanying marine transgression and regression cycles in the early Cambrian, such a “sandwich” structure in ocean redox chemistry demonstrates five evolutional stages, which can be well correlated to the spatiotemporal patterns of fossil records in South China. The good correlation between metazoan fossil occurrences and water chemistry in South China suggests that early animals possibly possessed ability to inhabit anoxic but generally not euxinic environments as free H2S was fatal to most eukaryotes. This view can well explain why those small shell fauna and sponges disappeared from shelf to slope areas where sulfidic Ni-Mo-rich shales were widely deposited. Thus, we conclude that the spatiotemporal variations of ocean chemistry and its biological effects probably played a key role in the phased animal radiations and “extinctions” in the early Cambrian.

Quantitative palynostratigraphy and palaeoecology of Tethyan Paleocene–Eocene red beds in north of Zagros sedimentary basin, Iran

Biostratigraphic and quantitative characteristics of dinocyst assemblages and palynofacies have been used to interpret the paleoecology and sea level changes of Paleocene–Eocene red beds in north of Zagros sedimentary basin, Iran. One hundred forty-eight species and 69 genera of dinoflagellate cysts are recorded from 32 rock samples collected from the 153-m-thick outcrop section. Three dinocyst zones comparable to North Hemisphere mid-latitude zonation are established and described. Three palynofacies differentiated—VI: proximal suboxic–anoxic shelf, VIII: distal dysoxic–anoxic shelf, and IX: distal suboxic–anoxic basin—suggest a suboxic–anoxic proximal–distal shelf and distal basin for the strata under investigation. The sequence stratigraphy events have been documented on the basis of palynological asset recorded such as palynofacies, SOM/MP ratio, dinocyst abundance and diversity, peridinioid/gonyaulacoid (P/G) ratio, proximochorate and cavate cysts ratio, and specific inner neritic, outer neritic, and oceanic taxa trends). The record of sea level changes for the Paleocene–Eocene red beds shows a third-order sequence with one sequence boundary (type II). The P/G ratio shows increasing upward trend of paleoproductivity in this sequence.

Consumption of Methane and CO 2 by Methanotrophic Microbial Mats from Gas Seeps of the Anoxic Black Sea

The deep anoxic shelf of the northwestern Black Sea has numerous gas seeps, which are populated by methanotrophic microbial mats in and above the seafloor. Above the seafloor, the mats can form tall reef-like structures composed of porous carbonate and microbial biomass. Here, we investigated the spatial patterns of CH(4) and CO(2) assimilation in relation to the distribution of ANME groups and their associated bacteria in mat samples obtained from the surface of a large reef structure. A combination of different methods, including radiotracer incubation, beta microimaging, secondary ion mass spectrometry, and catalyzed reporter deposition fluorescence in situ hybridization, was applied to sections of mat obtained from the large reef structure to locate hot spots of methanotrophy and to identify the responsible microbial consortia. In addition, CO(2) reduction to methane was investigated in the presence or absence of methane, sulfate, and hydrogen. The mat had an average delta(13)C carbon isotopic signature of -67.1 per thousand, indicating that methane was the main carbon source. Regions dominated by ANME-1 had isotope signatures that were significantly heavier (-66.4 per thousand +/- 3.9 per thousand [mean +/- standard deviation; n = 7]) than those of the more central regions dominated by ANME-2 (-72.9 per thousand +/- 2.2 per thousand; n = 7). Incorporation of (14)C from radiolabeled CH(4) or CO(2) revealed one hot spot for methanotrophy and CO(2) fixation close to the surface of the mat and a low assimilation efficiency (1 to 2% of methane oxidized). Replicate incubations of the mat with (14)CH(4) or (14)CO(2) revealed that there was interconversion of CH(4) and CO(2.) The level of CO(2) reduction was about 10% of the level of anaerobic oxidation of methane. However, since considerable methane formation was observed only in the presence of methane and sulfate, the process appeared to be a rereaction of anaerobic oxidation of methane rather than net methanogenesis.

Periodic anoxic shelf in the Early-Middle Ordovician transition: ichnosedimentologic evidence from west-central Utah, USA

Ichnosedimentologic evidence of periodic anoxic shelf in the Early-Middle Ordovician transition includes lower ichnodiversity, shallower bioturbation and burrowing depth (＜4 cm ) , rare domichnia, tiny Chondrites occupying shallower or shallowest tiering, widely distributed nodules of limonite pseudomorphs after pyrite, occurrence of trace fossils being closely associated with the storm event layers, and stratigraphic successions with orbital cyclostratigraphic architecture. It is suggested that lower atmospheric oxygen level during the Early Paleozoic, the Ordovician radiation, dramatic transgression and warmer temperatures would result in the periodic anoxia in the Early-Middle Ordovician transition. This episode began at the later Early Ordovician and lasted about 3.4 Ma on the basis of orbital cyclostratigraphy.

Vertical mercury distributions in the oceans

The vertical distribution of mercury (Hg) was determined at coastal and open ocean sites in the northwest Atlantic and Pacific Oceans. Reliable and diagnostic Hg distributions were obtained, permitting major processes governing the marine biogeochemistry of Hg to be identified. The northwest Atlantic near Bermuda showed surface water Hg concentrations near 4 pM, a maximum of 10 pM within the main thermocline, and concentrations less than or equal to surface water values below the depth of the maximum. The maximum appears to result from lateral transport of Hg enriched waters from higher latitudes. In the central North Pacific, surface waters (to 940 m) were slightly elevated (1.9 ± 0.7 pM) compared to deeper waters (1.4 ± 0.4 pM), but no thermocline Hg maximum was observed. At similar depths, Hg concentrations near Bermuda were elevated compared to the central North Pacific Ocean. This inter-ocean pattern results from a combination of enhanced supply of Hg to the northwest Atlantic by rainfall and scavenging removal along deep water circulation pathways. An upper water column Hg maximum was also observed at a Slope Water station in the northwest Atlantic and in the Tasman Sea. We hypothesize that the source of this Hg comes from diagenetic reactions in oxic margin sediments, releasing dissolved Hg to overlying water. In contrast, a broad minimum (1.5 ± 0.2 pM) in the vertical distribution was observed in the coastal waters off Peru which coincided with the intense oxygen minimum zone in the water column. Removal of dissolved Hg from solution during contact with highly anoxic shelf and slope sediments is proposed as the mechanism for producing this unique profile. Geochemical steady-state box modeling arguments predict a relatively short (~350 years) mean residence time for Hg in the oceans, demonstrating the reactive nature of Hg in seawater and precluding significant involvement in “nutrient-type” recycling. Mercury's distributional features and reactive nature suggest that interaction of Hg with settling particulate matter and margin sediments play important roles in regulating oceanic Hg concentrations. Oceanic Hg distributions are governed by an “external cycling process”, in which water column distributions reflect a rapid competition between the magnitude of the input source and the intensity of the (water column) removal process. In this regard, Hg shares many distributional features with other elements, such as Pb and Bi, which have very short oceanic residence times.

Rare earth elements in ichthyoliths: Variations with redox conditions and depositional environment

A study has been made of the rare earth element (REE) geochemistry of ichthyoliths from six oceanic regions (Peru/Chile Shelf, Tyrrhenian Sea, Namibian Shelf, Cook Islands, Pacific Ocean and the Cape Basin), including both oxic and anoxic sedimentary depositional environments. The REE contents of vertebrae and scales of modern fish are extremely low (e.g. 0.01–0.06 μg g −1 Nd), but REE are significantly enriched in ichthyoliths recovered from sediments (e.g. 0.78–1460 μg g −1 Nd) Ichthyoliths from anoxic shelf sediments have low REE contents and positive or weakly negative Ce anomalies. Ichthyoliths from oxic deep-sea sediments have high REE contents and strongly negative Ce anomalies. The REE patterns reflect the heavy REE enrichment of sea water, but do not record the inter-oceanic differences shown by sea water. The Ce anomalies of deep-sea ichthyoliths both resemble sea waters and generally reflect inter-oceanic differences (more negative Ce anomalies in the Pacific than in the Atlantic ocean). The REE uptake by ichthyoliths appears to occur during very early diagenesis. The REE patterns are useful indicators of the redox status of the sedimentary depositional environments, and, for deep-sea ichthyoliths, provide an approximate record of the redox condition of contemporaneous sea water.