Anoxic Event Research Articles

Carbon and sulfur cycling across the Silurian-Devonian boundary in the Qujing Basin, South China

During the late Silurian – Early Devonian there were significant changes in the biosphere and environment. However, there are few integrated studies of carbon and sulfur cycling in the Qujing Basin, in South China, which has extremely well-preserved sarcopterygian fossils. Here we report C-isotopic data of organic carbon (δ13Corg) and multiple S-isotopes of pyrite (δ34S and Δ33S) from two sections in the Qujing Basin. Our δ13Corg data show an increase of ~4‰ near the Silurian-Devonian boundary which can be correlated globally. We suggest that this rise in δ13Corg near the Silurian-Devonian boundary may have resulted from the weathering of the 13C-enriched carbonate platform, rather than from oceanic anoxia. This interpretation may explain why there is little evidence of a mass extinction near the Silurian-Devonian boundary in the Qujing Basin. Our S-isotopic data show positive δ34S values with either positive or negative Δ33S values, and most of the δ34S and Δ33S data can be explained by microbial sulfate reduction and Rayleigh distillation processes. The S-isotopic data from both sections suggest that seawater sulfate concentrations were very low in the Qujing Basin during the late Silurian – Early Devonian.

Residual Complex I activity and amphidirectional Complex II operation support glutamate catabolism through mtSLP in anoxia

Anoxia halts oxidative phosphorylation (OXPHOS) causing an accumulation of reduced compounds in the mitochondrial matrix which impedes dehydrogenases. By simultaneously measuring oxygen concentration, NADH autofluorescence, mitochondrial membrane potential and ubiquinone reduction extent in isolated mitochondria in real-time, we demonstrate that Complex I utilized endogenous quinones to oxidize NADH under acute anoxia. 13C metabolic tracing or untargeted analysis of metabolites extracted during anoxia in the presence or absence of site-specific inhibitors of the electron transfer system showed that NAD+ regenerated by Complex I is reduced by the 2-oxoglutarate dehydrogenase Complex yielding succinyl-CoA supporting mitochondrial substrate-level phosphorylation (mtSLP), releasing succinate. Complex II operated amphidirectionally during the anoxic event, providing quinones to Complex I and reducing fumarate to succinate. Our results highlight the importance of quinone provision to Complex I oxidizing NADH maintaining glutamate catabolism and mtSLP in the absence of OXPHOS.

Integrated cyclostratigraphy of the Cau core (SE Spain) - A timescale for climate change during the early Aptian Anoxic Event (OAE 1a) and the late Aptian

We report a cyclostratigraphic study performed on the Cau core (Spain), which is considered an Aptian stratigraphic reference for global correlation and paleoenvironmental reconstruction. This investigation presents an astronomical timescale for the Aptian from the Ap2a to Ap14 carbon-isotope stages. Based on the evaluation of a multiproxy dataset from the Cau core, we recalibrate the age and duration of different biozones, bioevents, chemostratigraphic substages and horizons from the early and late Aptian, with special focus on the Selli Event, providing a new astronomical framework for Aptian climate. From the recognition of 14 long-eccentricity cycles, we propose a time span of 5.67 Ma from C-isotope segments Ap2a to the top of Ap14, and ages of 120.82 Ma for the onset of the nannoconid crisis, and 120.20 Ma for the onset of oceanic anoxic event (OAE) 1a. Calculations yield a duration of 1.47 Ma for OAE 1a. We estimate the age for the onset of the main non-radiogenic phase of the Os isotopes at 120.08 Ma, 120 ka after the onset of OAE 1a. The high-resolution data from the Cau core provide further insights in the temporal constraints of the OAE 1a and other Aptian paleoclimatic events. The onset of the main non-radiogenic excursion in Os isotopes occurring 120 ka after the onset of OAE 1a reinforces the theory of rapid destabilization of methane hydrates as the trigger of the anoxic event, that preceded the onset of large-scale volcanism.

Sedimentary environment and benthic oxygenation history of the Upper Cretaceous Austin Chalk Group, south Texas: An integrated ichnological, sedimentological and geochemical approach

ABSTRACTOxygen concentration in the ocean is vital for sustaining marine ecosystems. While the potential impacts of deoxygenation on modern oceans are hard to predict, lessons can be learned from better characterizing past geological intervals formed under a greenhouse climate. The greenhouse Cretaceous containing several oceanic anoxic events characterized by widespread oxygen‐deficient water is ideal in this regard. The Austin Chalk Group in south Texas (USA) shows organic‐rich intervals that can be linked to oxygen depletion in the ocean, but the exact bottom water oxygenation conditions have not been estimated. This study aims to reconstruct both sediment interstitial and bottom water oxygenation history during Austin Chalk Group deposition by integrating detailed ichnological, sedimentological and geochemical (X‐ray fluorescence and X‐ray diffraction) analyses, thereby providing a consistent model that may be applicable across a range of marine shelf settings. The 141.12 m Gise #1 core contains a continuous record of the Austin Chalk Group, providing an opportunity for unravelling oxygenation and deoxygenation events. Whereas the anaerobic–exaerobic deposits are essentially nonbioturbated, four oxygen‐related ichnocoenoses are defined, further refining the transition of aerobic to dysaerobic conditions in the sediment interstitial water. Omission surfaces and glauconitic grains, products of current‐induced scouring and condensation, suggest sporadic high‐energy events in the Austin Chalk Group ramp that drove elevated terrestrial inputs. Geochemical data further help to identify anoxic bottom water conditions within the anaerobic facies. Additionally, the lowermost part of the Austin Chalk Group illustrates redox cycles, whereas dilution events characterized by elevated terrestrial input are identified throughout the rest of the Austin Chalk Group. The evolution of oxygenation levels in sediment interstitial water and bottom water disputes the existence of a long‐lasting oxygen‐deficient sea in south Texas. The refined depositional model may be applicable to coeval shelfal settings. Moreover, the results provide insights into variable, evolving palaeoclimatic and palaeoceanographic conditions of the greenhouse Late Cretaceous.

Chronostratigraphy of the Upper Cretaceous succession in the Middle Magdalena Valley of Colombia (Northern South America)

The Upper Cretaceous succession in northern South America is significant because the related rock deposits are among the most prolific oil sources in the region. Although several geological studies have focussed on this period, we still have poor knowledge of the exact deposition time of the lithostratigraphic units. In this study, we constructed a chronostratigraphic framework based on U–Pb ages and time-calibrated carbon isotopic events to constrain the chronostratigraphy of the Upper Cretaceous formations. The carbon isotope curve of the La Luna-1 core allowed us to identify several oceanic anoxic events also identified in the stratotypes of the Upper Cretaceous formations. The La Luna-1 also preserves a complete record of ash beds, from which zircons were extracted for U–Pb dating. The stratigraphic distribution of the ash beds indicates a crucial influence of volcanic activity during the Late Cretaceous. Sedimentological descriptions and mineralogical analysis were used to establish the primary lithological differences between the units. The Upper Cretaceous formations can be categorised as mixed rocks comprising variable percentages of carbonates, quartz and clay minerals. They also show silicification and, to a lesser extent, dolomitisation and recrystallisation. The Salada, Pujamana, and Galembo formations range in age between the Cenomanian and Coniacian, whereas the overlying La Renta Formation spans from the Santonian to at least the middle Campanian. The top of the La Renta Formation corresponds to a regional disconformity that contacted the middle Campanian with the Maastrichtian.

The fossil insect assemblage associated with the Toarcian (Lower Jurassic) oceanic anoxic event from Alderton Hill, Gloucestershire, UK.

Extreme global warming and environmental changes associated with the Toarcian (Lower Jurassic) Oceanic Anoxic Event (T-OAE, ~183 Mya) profoundly impacted marine organisms and terrestrial plants. Despite the exceptionally elevated abundances of fossil insects from strata of this age, only assemblages from Germany and Luxembourg have been studied in detail. Here, we focus on the insect assemblage found in strata recording the T-OAE at Alderton Hill, Gloucestershire, UK, where <15% of specimens have previously been described. We located all known fossil insects (n = 370) from Alderton Hill, and used these to create the first comprehensive taxonomic and taphonomic analysis of the entire assemblage. We show that a diverse palaeoentomofaunal assemblage is preserved, comprising 12 orders, 21 families, 23 genera and 21 species. Fossil disarticulation is consistent with insect decay studies. The number of orders is comparable with present-day assemblages from similar latitudes (30°-40°N), including the Azores, and suggests that the palaeoentomofauna reflects a life assemblage. At Alderton, Hemiptera, Coleoptera and Orthoptera are the commonest (56.1%) orders. The high abundance of Hemiptera (22.1%) and Orthoptera (13.4%) indicates well-vegetated islands, while floral changes related to the T-OAE may be responsible for hemipteran diversification. Predatory insects are relatively abundant (~10% of the total assemblage) and we hypothesise that the co-occurrence of fish and insects within the T-OAE represents a jubilee-like event. The marginally higher proportion of sclerotised taxa compared to present-day insect assemblages possibly indicates adaptation to environmental conditions or taphonomic bias. The coeval palaeoentomofauna from Strawberry Bank, Somerset is less diverse (9 orders, 12 families, 6 genera, 3 species) and is taphonomically biased. The Alderton Hill palaeoentomofauna is interpreted to be the best-preserved and most representative insect assemblage from Toarcian strata in the UK. This study provides an essential first step towards understanding the likely influence of the T-OAE on insects.

Morphometric analyses of Eprolithus floralis from Oceanic Anoxic Event 2 in the Eastbourne section.

Morphometric analyses of Eprolithus floralis from Oceanic Anoxic Event 2 in the Eastbourne section.

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Abstract The globally recognized Cretaceous Oceanic Anoxic Event 2 was recorded at Integrated Ocean Drilling Program Site U1407 in the North Atlantic in sediments that were deposited under varying oxygen concentrations, conditions often associated with decreased foraminiferal test size. A &amp;gt;63-µm size fraction is typically used for collecting foraminifera that live in low-oxygen and deep-sea settings, particularly for paleoceanographic reconstructions where key environmental indicator species may be under-represented or absent in coarser sizes. Using 31 sediment samples from the site, we test the best size fraction to utilize when exploring biotic responses to an oceanic anoxic event. Counts of benthic and biserial (heterohelicid) planktic foraminifera from the standard &amp;gt;63-µm and finer 38–63-µm sieve sizes are compared to determine if critical information is lost if the smaller size fraction is not used. Results show similar trends in relative abundances for both size fractions. There are higher abundances in 38–63-µm residues, but species identification and counts are overly time-consuming for obtaining the general trends. We recommend the use of &amp;gt;63-µm sieves for foraminifera collected from low-oxygen sediments but encourage examination of the 38–63-µm fraction to verify intervals recognized as barren are truly absent of foraminifera.

Early Aptian Oceanic Anoxic Event 1a (OAE1a): Evidence based on calcareous nannofossils in the Qaleh Zoo section, central Kopet Dagh, Iran.

Early Aptian Oceanic Anoxic Event 1a (OAE1a): Evidence based on calcareous nannofossils in the Qaleh Zoo section, central Kopet Dagh, Iran.

Unusual calcareous nannofossil assemblages from the Cenomanian–Turonian (93.9Ma) of North America: Implications for nannofloral response to Oceanic Anoxic Event 2.

Unusual calcareous nannofossil assemblages from the Cenomanian–Turonian (93.9Ma) of North America: Implications for nannofloral response to Oceanic Anoxic Event 2.

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Effect of Size Fractions of Foraminifera on Reconstructing the Paleoceanography of Cretaceous Oceanic Anoxic Event 2

Paleoceanographic importance of tri- and di-unsaturated alkenones through the early phase of Cretaceous Oceanic Anoxic Event 2 from southern high latitudes of the proto-Indian Ocean

Alkenones are biomarkers derived exclusively from species of haptophyte algae. The relative abundance of di- to tri-unsaturated C37 alkenones expressed as UK'37 is widely applied as a sea surface paleotemperature proxy for Cenozoic marine sediments. However, the absence of alkatrienones prior to the Eocene has precluded application of the UK'37 proxy for assessment of Cretaceous paleoclimates. Herein, we report a C40 alkatrienone (tetraconta-9E, 16E, 23E-trien-3-one; C40:3 Et) in deep-sea sediments from southern high latitudes (International Ocean Discovery Program: IODP site U1516). This discovery extends the geologic record of alkatrienones to the late Cenomanian, ∼70 million years earlier than previous reports. The parallel occurrence of higher abundances of a C40 alkadienone (tetraconta-16E, 23E-dien-3-one; C40:2 Et) allowed calculation of the UK’40 unsaturation index, comparable to UK’37. Stratigraphic variations in the δ13C of C40:2 Et revealed an elevated (∼1.5 ‰) positive carbon isotope excursion (CIE) relative to those observed in carbonate from other OAE2 sequences likely reflecting a decrease in global pCO2. The UK’40 profile suggests a concurrent drop in sea surface temperature associated with the decline in pCO2 during the early phase of OAE2. The timing of these environmental perturbations in the southern high latitude of the proto-Indian Ocean suggests they were triggered by volcanism associated with large igneous province (LIP) formation.

Metallogeny and temporal-spatial distribution of sedimentary phosphorite mineralization in Iran, relation to tethyans oceans evaluation and implications for future exploration

There are several deposits and mineral occurrences of sedimentary phosphorite (SP) mineralization in Iran. The main tectonic-structural zones of Iran that host SP deposits are: (1) Alborz Magmatic Belt (AMB), (2) the Central Iran and (3) Zagros Folded Thrust Belt (ZFTB). These deposits were formed during separate time: (1) Neoproterozoic-Early Cambrian (Soltanieh Formation), (2) Ordovician-(Silurian) (Shirgesht, Mila and Seyahou Formations), (3) Upper Devonian (Jeirud Formation), (4) Permian to Jurassic (Abkhory, Taleqan and Hosseinabad area) and (5) Cretaceous to Paleogene (Gurpi and Pabdeh Formations). The stratigraphic sequences of these deposits are commonly formed from sedimentary rocks, which are similar to other SP districts in the Middle East and North Africa. The main period of SP mineralization in Iran was from the Neoproterozoic-Early Cambrian, Upper Devonian and Late Cretaceous to Paleogene. The Jeirud Formation of the Upper Devonian and the Pabdeh Formation of the Paleocene-Eocene contained the largest sedimentary phosphorites in Iran. The formation and distribution of sedimentary phosphorite deposits in the different structural zones can be explained by divergent and convergent events related to the Proto, Paleo, and Neo-Tethyan oceans. The SP deposits hosted by the Neoproterozoic-Early Cambrian Soltanieh Formation are related to the Proto-Tethys Ocean. Mineralization in the Ordovician Mila and Shirgesht formations and Upper Devonian Jeirud Formation are age related to the Paleo-Tethys Ocean, and finally, the SP deposits formed in the Cretaceous- Paleogene (Gurpi and Pabdeh formations) sequence were formed the evolution of the Neo-Tethys Ocean cycle. Passive margins are the principal plate tectonic settings for Iranian SP deposits. Iranian SP deposits are stratigraphically controlled in which phosphorite mineralization occurs as laminae, disseminated grains, and replacement. SP mineralization is commonly closely related to framboidal pyrite and biologic events. The AMB and ZFTB are the main tectonic zones for Iranian sedimentary phosphorite deposits. These zones are the most favorable metallogenic provinces for the exploration of sedimentary phosphorite deposits, representing in excess of 80% of the as of now known SP deposits and occurrences in Iran. High sea level, low oceanic anoxic events (OAE's), warm climates, latitude and longitude close to the equator and shallow slope environments are among the most important factors controlling Iranian SP deposits.

Cretaceous climate change evidenced in the Senegalese rock record, NW Africa

Climate change directly impacts the source, mode and volume of sediment generation which can be observed in the rock record. To accurately model source to sink systems, in addition to hinterland geology, tectonics and transport distance, a thorough comprehension of the climate is essential. In this study we evaluate the role of climate on Cretaceous sediment delivery into the Senegal Basin, NW Africa, using data recorded from extensive sampling of basinal sediments. This is achieved through the mineralogical characterisation by X-ray diffraction and 146Nd/144Nd and 86Sr/88Sr isotopic analyses, which are correlated against existing, climate, tectonic and oceanographic models.Examples of climatic indicators include the change from predominantly smectitic deep marine basinal-clays recorded from the Cretaceous in DSDP wells 367 and 368 to clays with increased illite and kaolinite content, observed during the Albian and Cenomanian-Turonian, interpreted to be representative of higher humidity following the kaolinisation of hinterland source-rocks. Another climate indicator is the observation of palygorskite in deep-marine sediments, noted to be indicative of ocean anoxia related to the authigenesis of marine-smectite, a product of warm saline bottom waters and increased abundancy of silicon. The increase in salinity is interpreted to be a biproduct of elevated temperatures throughout the Cenomanian and increased denudation of the North Atlantic circumjacent continental evaporite-belts. Increase in silicon (biogenic) is related to a result of ocean-wide mass extinction of foraminifera during OAE2 triggered by the eruption of the Caribbean large igneous province.The results suggest that Cretaceous climate evolution of Senegal can be divided into four stages: 1. Berriasian-Barremian; an arid-period with monsoonal weather producing modest fluvial systems restricted to coastal regions. 2. Aptian-Albian; the establishment of a paleo-Intertropical Convergence Zone began to increase global temperature and humidity as recognised by the increase in kaolinite content. 3. Cenomanian-Turonian; the Cretaceous Thermal Maximum hothouse period incurring exceptional temperatures and humidity. This is represented as an antithetical shift in clay mineralogy from chlorite-illite to smectite-kaolinite throughout most of the onshore and nearshore basinal sediments. 4. Coniacian-Maastrichtian; transitional from tropical-to-tropical swamp-like conditions evidenced by increased onshore basin sediment capture and a shift in vegetation to aquatic-fern species.The impact of climate change throughout the Cretaceous produced dynamic shifts in both river size and source-catchment, witnessing exception rates of denudation during the hotter and more humid periods, which climaxed during the Cenomanian and Turonian as a result of the Cretaceous Thermal Maximum. This eroded sediment was deposited in both the onshore and offshore basins during the mid-late Cretaceous but became increasingly restricted to the onshore segment of the basin during the Late Cretaceous.

Mercury sequestration pathways under varying depositional conditions during Early Jurassic (Pliensbachian and Toarcian) Karoo-Ferrar volcanism

Major extinction events appear coincident with the emplacement of large igneous provinces (LIPs) throughout Earth's past, but the cause-and-effect relationship is often poorly understood due to challenges in the stratigraphical correlation between LIP occurrence and sedimentary archives. Large igneous provinces are thought to have released large amounts of greenhouse gasses and toxic compounds (including mercury) into the global ocean-atmosphere system, leading to major climatic and environmental perturbations and ecosystem deterioration. In recent years, the analyses of bulk sedimentary mercury (Hg) concentrations have increasingly been used to assess the temporal occurrence and geographic reach of LIPs, as observed through elevated Hg concentrations in sedimentary archives which otherwise show no direct evidence of volcanism at those times.Mercury can be sequestered into sediments in different ways, but dominant Hg sequestration pathways can vary through space and time depending on the depositional environment and changes therein, which may disguise changes in global Hg fluxes into Earth's surficial environments, significantly complicating the interpretation of sedimentary Hg-records. Here, we statistically analyse sedimentary geochemical records from three distinctly different depositional environments that occurred during the Toarcian Oceanic Anoxic Event (T-OAE, at ∼183 Ma, and coincident with Karoo-Ferrar LIP volcanism), and assess the temporal and spatial variability in dominant Hg sequestration pathways during LIP volcanism.The geochemical data show significantly different affinities of mercury with assessed sedimentary geochemical parameters in each of the studied successions. Principal component analyses (PCA) of sedimentary geochemical data (including Hg), from the restricted, anoxic-euxinic Cleveland Basin (Yorkshire, UK) show that Hg covaries mostly with organic-sulphide-associated elements and to a lesser extent with the redox-sensitive elements, as opposed to the semi-restricted, sub-oxic Cardigan Bay Basin (Wales, UK) and the well-connected, oxygenated Subbetic Basin (South Iberian Palaeomargin, SE Spain), where Hg covaries dominantly with the redox-sensitive elements and secondarily with detrital elements. These findings show that mineral phases and materials other than organic matter need to be considered as dominant Hg carriers in non-anoxic depositional settings. Furthermore, observed differences between the studied sites regarding the dominant Hg-sequestration processes, suggest temporal and spatial variance in Hg drawdown fluxes, and possibly local marine residence times.

Intrashelf basin record of redox and productivity changes along the Arabian margin of Neo-Tethys during Oceanic Anoxic Event 1a

The biotic, environmental, climatic, oceanic, and sea-level perturbations during the Early Aptian Oceanic Anoxic Event (OAE) 1a have been extensively documented from both deep- and shallow-marine deposits worldwide. However, there has been relatively little comparative assessment of the simultaneous interplay among organic carbon burial, redox conditions, terrigenous output, and productivity, leading to a lack of precise constraints on these relationships. Here, we use analyses of stable carbon isotopes (δ13Corg, δ13Ccarb, and Δ13C), total organic carbon (TOC), detrital proxies (Al, Si, Ti, K), redox-sensitive (RSTE: U, V, Mo) and productive-sensitive (PSTE: P, Cu, Ni) trace elements from a continuous, predominantly carbonate succession of the Kazhdumi Intrashelf Basin to evaluate the culprits for the OAE1a-associated changes in bottom-water oxygenation, organic-rich layer formation, and biotic shifts along the Arabian margin of the Neo-Tethys. Concentrations of Al-normalized RSTE and TOC values indicate that the bottom water conditions ranged from oxic prior to and at the onset of the OAE 1a (carbon-isotope segments C2 to basalmost C4 sensu Menegatti et al., 1998), to anoxic-suboxic but not euxinic (Mo < 25 ppm) during the lower C4 through C5 + C6 segments, and then returned to oxic-suboxic in the remaining C5 + C6 segment. The increase in Al-normalized PSTE coupled with TOC concentrations in the basal C4 is coeval with a change from predominantly orbitolinid-ostreid to planktic foraminifera-radiolarian biota. The periodically high productivity, driven both by the surface-water productivity as well as by phosphorus recycling from the sediments, continued through the C5 + C6 segments as evidenced by matching Si/Al and PSTE peaks (Cu/Al and Ni/Al). The study sheds new light on the causes of variations in bottom-water deoxygenation, organic content, nutrient availability, and biotic shifts in semi-restricted, relatively deep (>100 m), continental-margin basins during major oceanic perturbations.

Depositional characteristics of fine-grained sedimentary rocks and the links to OAE-3 and PETM of the Upper Cretaceous-Paleogene Madingo Formation, lower Congo Basin, West Africa

Fine-grained sedimentary rocks of the Madingo Formation in the Lower Congo Basin (LCB), are significant targets for Upper Cretaceous–Paleogene depositional system analysis in West Africa. Six types of fine-grained sedimentary rocks of the Madingo Formation were identified based on integrated analyses of petrographic characteristics, geochemical element analysis and electron back-scattered diffraction of thin sections, including (1) siliceous mudstone, (2) claystone, (3) marlstone, (4) radiolarian-rich mudstone, (5) foraminifera-rich mudstone, and (6) phosphatic mudstone. Geochemical indexes (U/Th, V/(V + Ni), V/Cr, and Ni/Co) indicate that the Madingo Formation on the deep-water slope was characterized by typical oxygen-poor and even anaerobic environment, preserving organic matter and thus preserving sediments with relatively high TOC, abundant marine fossils and framboidal pyrite. The LCB experienced two different marginal marine environments during the deposition of the Madingo Formation. The lower Madingo Formation was characterized by a radiolarian-rich mudstone (the first stage of upwelling) deposited in a marginal semi-restricted sea bay environment (correlated with the Coniacian–Santonian Oceanic Anoxic Event 3; OAE-3), with up to 4.0% TOC, and types II2 and II1 organic matter. The upper Madingo Formation was characterized by foraminifera-rich and phosphatic mudstone (the second stage of upwelling) deposited in a marginal open marine environment (correlated with the Paleocene-Eocene Thermal Maximum; PETM), with up to 4.83% TOC and 5.0 mg/g phosphate that is rich in types I and II1 organic matter. The whole Madingo deposition is estimated to have spanned a 52-Myr period, and the low sediment accumulation rate of approximately 9 μm/yr can lead to a high TOC content. The three identified condensed sections include radiolarian-rich, foraminifera-rich or phosphatic mudstones that deposited in the deep-water slope, were probably marine source rocks for post-salt exploration in West Africa.

Fire activities and their impacts on local ecosystems in the southern Ordos Basin during the Middle Jurassic: Evidence from pyrogenic PAHs and petrography of inertinite-rich coal

The high frequency of fires during the formation of some mire systems in the Jurassic is thought to have influenced how terrestrial ecosystems evolved. Ancient mires, now present as coal seams, contain high-resolution paleoenvironmental and paleoclimatic records. Thus, the analysis of coal beds provides valuable, insightful, and practical knowledge on the changes in paleoclimate and, in this case, fire prevalence. Using petrographic, carbon isotopic, mercury content, and polycyclic aromatic hydrocarbon (PAH) determinations, the Middle Jurassic No. 4 coal from the Binchang region in the Ordos Basin was examined in order to understand the nature of fire during the formation of the mires. The results show that the No. 4 coal exhibits a high content of inertinite (average content 70%). Notably, the correlation coefficient (r = 0.79) between the concentration of PAHs (with >3 rings) and the proportion of inertinite indicates that the high content of inertinite was predominantly the result of fire. The presence of natural char and coke in the coals further indicates the frequent incidence of fires during the Middle Jurassic in the southern Ordos Basin. High frequency of fires can be attributed to the availability of fuel (i.e., the buildup of organic material), high atmospheric oxygen levels, and intermittent dry climatic conditions. The random reflectance of inertinite (specifically, fusinite and semifusinite) in the No. 4 coal bed ranges from 1.25% to 2.45%, which, with the presence of pyrogenic PAHs, suggests that their formation may have been predominantly affected by low- and moderate-temperature fires. The high frequency of fires is thought to have significant impacts on terrestrial and hydrological ecosystems. These effects may have included the circulation of atmospheric mercury, as well as the possible occurrence of anoxic events in the lakes.

An overview of the Cretaceous oceanic anoxic events in Egypt, southern Tethys

Abstract The Cretaceous Period ( c. 143–66 Ma) attested to successive phases of regional tectonic activities and large-scale submarine volcanism resulting in increased atmospheric CO 2 concentrations, enhanced greenhouse climate and global warming, surface water bioproductivity and well-developed pore and bottom water anoxia, especially during the mid-Cretaceous (Aptian to Turonian). These conditions triggered widespread deposition of organic carbon (OC)-rich black shales during episodes of severe oxygen exhaustion, commonly known as oceanic anoxic events (OAEs). Oceanic anoxic events had many important consequences, including the widespread mass extinction of marine and terrestrial organisms. Several intervals of enhanced oxic ventilation and deposition of OC-poor pelagic–hemipelagic red carbonates and claystone interrupted Cretaceous OAEs, which are commonly known as Cretaceous oceanic red beds (CORBs). The mid-Cretaceous black shales from many parts of the world and the corresponding global carbon cycle changes indicated that OAE1 (Selli Level) and OAE2 (Bonarelli Event) are the most widespread and well-defined OAEs compared with the more regional OAE3 and Valanginian–Hauterivian black shales (Weissert Event). These appear to have been more restricted to the Atlantic and adjacent areas, a result of more regional conditions rather than being forced by global environmental perturbations. In the course of this review, the Egyptian territory in the southern Tethys shelf is taken as a case study area for the development, evolution and regional expression of mid-Cretaceous OAEs. The region is characterized by a thick Cretaceous sedimentary succession of OC-rich black shale and OC-poor carbonates and mudstone interbeds. The late Albian OAE1d was reported in the North Western Desert (upper Kharita Formation) based on a positive carbon isotope excursion and moderate total sulfur, total organic carbon and redox-sensitive elements. The end-Cenomanian OAE2 was reported in several regions of Egypt based on positive δ 13 C excursions. However, differential deposition between OC-rich and OC-poor facies took place, especially in marginal to shallow marine settings where black shales are absent, such as in the Eastern Desert and Sinai. During the Coniacian–Santonian, OC-poor limestone, calcareous shale and red claystone were deposited under enhanced water column respiration in Egypt. This indicates that enhanced oxygenated ocean circulation controlled organic matter decomposition during weak continental weathering and enhanced carbonate production, all of which led to the deposition of OC-poor CORBs in this region.

Prologue: the founding of the Cretaceous System

Abstract Following the publication of country-wide geological maps by William Smith and George Bellas Greenhough, Jean Baptiste Julien d'Omalius d'Halloy embarked on the geological mapping of France and the adjacent areas of the Low Countries and Northern Italy, publishing his map in 1822. In the legend of that map was the rock unit identified as ‘Terrain Crétacé’ and that was the birth of the Cretaceous System that we still use today. During the remainder of the nineteenth century, many geoscientists in Europe, and elsewhere, worked on the Cretaceous strata and – over time – generated our present understanding of the longest period in the Phanerozoic. This includes luminaries such as Alcide d'Orbigny, Charles Darwin, Edmond Hébert, Charles Barrois, Alfred Jukes-Browne and Arthur Rowe. Gradually, the appreciation for Cretaceous climates, sea levels, oceanic anoxia, palaeontology and biogeography expanded, and many of these topics will be covered in contributions to this Special Publication that celebrates the 200th anniversary of the founding of the Cretaceous System.