Oceanic Anoxic Events

In the Jurassic period, the Early Toarcian oceanic anoxic event (about 183 million years ago) is associated with exceptionally high rates of organic-carbon burial, high palaeotemperatures and significant mass extinction. Heavy carbon-isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon, which is isotopically light. In contrast, examples of light carbon-isotope values from marine organic matter of Early Toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12C versus 13C. Here, however, we report carbon-isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic, biospheric and atmospheric carbon reservoirs, and that this occurred despite the enhanced burial of organic carbon. We propose that—as has been suggested for the Late Palaeocene thermal maximum, some 55 million years ago—the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental-margin sediments.

1] Sedimentary molybdenum, [Mo] s , has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic-rich marine facies deposited under oxygen-depleted conditions. A detailed analysis of [Mo] s -total organic carbon (TOC) covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that (1) [Mo] s , while useful in distinguishing oxic from anoxic facies, is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and (2) patterns of [Mo] s -TOC covariation can provide information about paleohydrographic conditions, especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal. These inferences are based on data from four anoxic silled basins (the Black Sea, Framvaren Fjord, Cariaco Basin, and Saanich Inlet) and one upwelling zone (the Namibian Shelf), representing a spectrum of aqueous chemical conditions related to water mass restriction. In the silled-basin environments, increasing restriction is correlated with a systematic decrease in [Mo] s /TOC ratios, from $45 ± 5 for Saanich Inlet to $4.5 ± 1 for the Black Sea. This variation reflects control of [Mo] s by [Mo] aq , which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal (the ''basin reservoir effect''). The temporal dynamics of this process are revealed by high-resolution chemostratigraphic data from Framvaren Fjord and Cariaco Basin sediment cores, which exhibit long-term trends toward lower [Mo] s /TOC ratios following development of water column stratification and deepwater anoxia. Mo burial fluxes peak in weakly sulfidic environments such as Saanich Inlet (owing to a combination of greater [Mo] aq availability and enhanced Mo transport to the sediment-water interface via Fe-Mn redox cycling) and are lower in strongly sulfidic environments such as the Black Sea and Framvaren Fjord. These observations demonstrate that, at timescales associated with deepwater renewal in anoxic silled basins, decreased sedimentary Mo concentrations and burial fluxes are associated with lower benthic redox potentials (i.e., more sulfidic conditions). These conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction (e.g., silled basins) and are not valid for low-oxygen facies in open marine settings such as continent-margin upwelling systems.

Current chemostratigraphical studies of the Jurassic System primarily involve the use of one sedimentary component (marine organic carbon), one divalent transition metal substituted in carbonate (manganese), and two isotopic tracers: strontium-isotope ratios (87Sr/86Sr) and carbon-isotope ratios (δ13Ccarb and δ13Corg) in carbonate and in organic matter. Other parameters such as Mg/Ca and Sr/Ca ratios in calcite, oxygen-isotope ratios (ä18O) in carbonate, sulphur-isotope ratios (δ34S) in carbonate-hosted sulphate, nitrogen- isotope ratios (δ15Norg) in organic matter, osmium-isotope ratios (187Os/188Os) in black shales and neodymium-isotope ratios (143Nd/144Nd) in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors. Stratigraphical patterns in total organic carbon (TOC) can be of either local or regional significance, depending on the lateral extent of the former nutrient-rich and productive water mass. Divalent manganese follows a similar pattern, being concentrated, most probably as a very early diagenetic phase, only in oxygen-depleted waters that typically underlie zones of elevated organic productivity. Shifts in Mg/Ca and Sr/Ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes. Strontium-isotope ratios from pristine skeletal calcite provide a global signal; δ13C values from carbonates with minimal diagenetic overprint potentially do the same, although small spatial differences in palaeo-water-mass composition may have been locally significant. Oxygen-isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful, because they reflect palaeotemperature, the evaporation–precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase. Nitrogen-isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification, and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis. Neodymium- isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems. Preliminary data on sulphur-isotope ratios in carbonates and osmium-isotope ratios in organic-rich shales, both potentially offering global correlation, indicate that these tracers may be valuable, although the records at present are not sufficiently well established to allow high-resolution regional correlation. In all cases, biostratigraphically well-dated reference sections, against which the relevant geochemical data have been calibrated, are required in the first instance. To date, studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern (Boreal) and southern (Tethyan) Europe; carbon- isotope stratigraphy has been undertaken primarily, but not exclusively, on bulk pelagic sediments from the Alpine–Mediterranean or Tethyan domain; and strontium-isotope stratigraphy has been undertaken largely on calcitic skeletal material (belemnites and oysters) from northern and southern Europe. In many sections, including those containing ammonites, multi-parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means. Strontium-isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record.

The C org :P ratios of organic-rich facies (TOC N 1%) exhibit considerable variation through the Phanerozoic, from b 10:1 in the Permian Phosphoria Formation to N 1000:1 in some Devonian black shales. Relative to the composition of phytoplankton (C:P ∼106:1), which provide the bulk of organic C and P to organic-rich marine sediments, the range of Phanerozoic sedimentary C org :P ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of C and P remobilization. Enhanced preservation of organic C at lower redox potentials combined with enhanced sedimentary retention of remineralized organic P at higher redox potentials results in a strong relationship between benthic redox conditions and sedimentary C org :P ratios. The strong secular coherence of the Phanerozoic C org :P record (i.e., limited variation within narrow time slices) suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale, such as atmospheric pO 2 . Inversion and scaling of the Phanerozoic C org :P record yields a new atmospheric pO 2 model that has strong similarities to existing models based on elemental and isotopic mass balances, yet differs in some potentially significant respects. In particular, the new model suggests that atmospheric pO 2 was markedly lower during the Early to Middle Paleozoic than previously inferred. During most of the Devonian, atmospheric O 2 levels may have been sufficiently low (b 13%) as to prevent the sustained combustion of plant material. This feature of the model may account for an anomalous gap in the fossil charcoal record during the Devonian. The redox-dependent burial of P in the ocean results in positive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales. Temporal variations in the burial of P relative to C suggest that the strength of these feedback mechanisms varied over the Phanerozoic.

The Cenomanian-Turonian boundary (90.4 Ma) represents a major period of worldwide environmental disturbance. The physical manifestations of this are: elevated atmospheric and oceanic temperatures; a significant sea-level transgression; and a period of widespread anoxia, leading to the formation of oceanic black shales, and the extinction of 26% of all genera. Elevated 13 C values and enrichment of trace elements in Cenomanian-Turonian boundary sediments, combined with a reduction in 87 Sr/ 86 Sr, also imply a severe environmental perturbation. At this time oceanic crustal production rates reached their highest level of the last 100 million years. This was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere, and the development of vast areas (up to 1 10 6 km 2 ) of thickened oceanic crust in the Pacific and Indian Oceans. The anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c. 90 Ma. These eruptions would also have resulted in the emission of large quantities of CO 2 into the atmosphere, leading to global warming. Additionally, the emission of SO 2 , H 2 S, CO 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate, and further release of CO 2 . This run-away greenhouse effect was probably put into reverse, by the decline of the anomalous volcanic activity, and by increased (CO 2 -driven) productivity in oceanic surface waters, leading to increased organic carbon burial, black shale deposition, anoxia and mass extinction in the ocean basins.

The carbon-isotope composition of fossil wood fragments, collected through a biostratigraphically well-constrained Aptian (Lower Cretaceous) shallow-marine siliciclastic succession on the Isle of Wight, southern Britain, shows distinct variations with time. The results indicate that the stratigraphic signature of δ13Cwood through the Aptian was influenced primarily by fluctuations in the isotopic composition of CO2 in the global ocean-atmosphere system, as registered in marine carbonates elsewhere, and was not governed by local paleoenvironmental and/or paleoecological factors. Negative and positive excursions in δ13Cwood through the lower Aptian occur in phase with inferred transgressions and regressions, respectively—a pattern that contrasts with that observed in many previous studies for different time intervals. The relationship between δ13C variations and relative sea-level change is tentatively interpreted as a response to various climatic and eustatic factors, relating to rapid sea-floor spreading, thermal uplift of ocean floor, emplacement of plateaus, volcanic CO2 emissions, weathering, and sedimentary rate.

The latest Permian mass extinction (LPE), just prior to the Permian-Triassic boundary at~252 Ma, resulted in the disappearance of~90% of skeletonized marine taxa and the replacement of the Paleozoic Fauna by the Modern Fauna. In the immediate aftermath of the extinction, shallow-marine ecosystems were dominated by microbial communities and diminutive disaster taxa ("Lilliput faunas"), and the post-crisis period in general was characterized by high-abundance, low-diversity marine communities occupying a reduced ecological space. The recovery of marine ecosystems was a protracted process, lasting throughout the~5-Myr-long Early Triassic. Some clades failed to recover and subsequently went extinct ("dead clades walking"), while others recovered only to suffer secondary crises during the Early Triassic. Profound environmental changes accompanied these biotic developments. Deep oceans experienced sustained hypoxia or anoxia for at least several million years following the boundary event. Anoxia was also widespread in shallow-marine areas, especially at the time of the LPE, but it was more episodic and abated more rapidly than in the deep ocean, making it an unlikely factor in the delayed recovery of Early Triassic marine ecosystems. The development of anoxia was due at least in part to a sharp temperature rise following the LPE but may have been stimulated by changes in marine nutrient inventories and productivity rates as well. These conditions may have been the result of a concurrent increase in subaerial weathering rates that was sustained into the Early Triassic. In addition to increasing nutrient fluxes to marine areas, rapid weathering caused a massive influx of sediment to shallow-marine systems, subjecting marine biotas to siltation stress. This scenario implies close teleconnections between terrestrial and marine environments during the Permian-Triassic boundary crisis, with perturbations of the terrestrial realm contributing to the marine crisis.

Two Permian/Triassic boundary sections in central Japan provide a rare window into environmental conditions within the Panthalassic Ocean, which encompassed more than half the Earth's surface at 252 Ma. Integration of petrographic, geochemical, and time series data provides new insights regarding the fluxes of major and trace components to the sediment as well as environmental conditions in both the deep and intermediate water masses at each study site. The Ubara section was located in a high-productivity periequatorial location, whereas the Gujo-Hachiman section was located in a moderate-productivity location at some distance from the paleoequator. An upward transition from gray organic-poor cherts to black siliceous mudstones at both sites occurred in conjunction with increased primary productivity, intensified euxinia within the oxygen-minimum zone (OMZ), and decimation of the radiolarian zooplankton community. Euxinia in the OMZ of the equatorial Panthalassic Ocean developed episodically for a~200-250 kyr interval during the Late Permian, followed by an abrupt intensification and lateral expansion of the OMZ around the Permian-Triassic boundary. Throughout the study interval, bottom waters at both sites remained mostly suboxic, a finding that counters hypotheses of development of a "superanoxic" Permo-Triassic deep ocean as a consequence of stagnation of oceanic overturning circulation.

The~90% biodiversity loss among marine invertebrate species during the end-Permian mass extinction, the most severe Phanerozoic biocrisis, has been attributed to widespread oceanic anoxia ("superanoxia"). Recent studies of the lithofacies, inorganic geochemistry, and biomarkers of Permian-Triassic boundary (PTB) sections have yielded an inferred pattern of oceanic redox changes that is complex and sometimes inconsistent. In this contribution, we review the ecology of radiolarians, an important microzooplankton group during the Permo-Triassic transition, and consider its implications for contemporaneous changes in paleoceanographic conditions. Our analysis shows that (1) Late Permian radiolarian faunas were cosmopolitan with different orders having specific depth preferences, i.e., albaillellarians favoring deeper waters and spherical forms (entactinarians and spumellarians) shallower waters; (2) deep-water taxa declined earlier than shallow-water taxa; (3) many Paleozoic-type taxa survived into the Early Triassic but went extinct during a second crisis in the Dienerian ("dead clade walking"), followed by the appearance or expansion of Mesozoic-type taxa during the Olenekian; and (4) the survival of Paleozoic-type taxa during the Early Triassic was greater at higher paleolatitudes than in the tropics. These observations are consistent with a first-order redox control on radiolarian faunal distributions, specifically a shallowing and poleward expansion of the oceanic oxygen-minimum zone during the PTB crisis. The consequent adaptation of some deepwater taxa to shallower environments resulted in Early Triassic radiolarian faunas in continental basins becoming more similar to those in pelagic settings. Enhanced survival of Paleozoic-type taxa in high-latitude regions indicates that the main stressor on Permo-Triassic radiolarian communities was reduced oceanic oxygen levels (stronger in the tropics) rather than climatic warming (stronger at the poles).

Oceanic Anoxic Event 2 (OAE2), spanning the Cenomanian‐Turonian boundary (CTB), represents one of the largest perturbations in the global carbon cycle in the last 100 Myr. The δ13Ccarb, δ13Corg, and δ18O chemostratigraphy of a black shale–bearing CTB succession in the Vocontian Basin of France is described and correlated at high resolution to the European CTB reference section at Eastbourne, England, and to successions in Germany, the equatorial and midlatitude proto‐North Atlantic, and the U.S. Western Interior Seaway (WIS). Δ13C (offset between δ13Ccarb and δ13Corg) is shown to be a good pCO2 proxy that is consistent with pCO2 records obtained using biomarker δ13C data from Atlantic black shales and leaf stomata data from WIS sections. Boreal chalk δ18O records show sea surface temperature (SST) changes that closely follow the Δ13C pCO2 proxy and confirm TEX86 results from deep ocean sites. Rising pCO2 and SST during the Late Cenomanian is attributed to volcanic degassing; pCO2 and SST maxima occurred at the onset of black shale deposition, followed by falling pCO2 and cooling due to carbon sequestration by marine organic productivity and preservation, and increased silicate weathering. A marked pCO2 minimum (∼25% fall) occurred with a SST minimum (Plenus Cold Event) showing >4°C of cooling in ∼40 kyr. Renewed increases in pCO2, SST, and δ13C during latest Cenomanian black shale deposition suggest that a continuing volcanogenic CO2 flux overrode further drawdown effects. Maximum pCO2 and SST followed the end of OAE2, associated with a falling nutrient supply during the Early Turonian eustatic highstand.

Bulk sedimentary nitrogen isotope (d15Ntot) data have been generated from Lower Jurassic black, carbon-rich shales in the British Isles and northern Italy deposited during the early Toarcian oceanic anoxic event. A pronounced positive d15Ntot excursion through the exaratum Subzone of the falciferum Zone (defined by characteristic ammonites in the British Isles) broadly correlates with a relative maximum in weight percent total organic carbon and, in some sections, with a negative d13Corg excursion. Upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of d15Ntot, and parallels may be drawn with Quaternary sediments of the Arabian Sea, Gulf of California, and northwest Mexican margin. The development of Early Toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity. Approximately coincident phenomena include the following: a relative climatic optimum, realignment of major oceanic current systems, and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event.

During these last years, considerable attention has been given to unconventional oil and gas shale in northern Africa where the most productive Paleozoic basins are located (e.g. Berkine, Illizi, Kufra, Murzuk, Tindouf, Ahnet, Oued Mya, Mouydir, etc). In most petroleum systems, which characterize these basins, the Silurian played the main role in hydrocarbon generation with two main ‘hot’ shale levels distributed in different locations (basins) and their deposition was restricted to the Rhuddanian (Lllandovery: early Silurian) and the Ludlow-Pridoli (late Silurian). A third major hot shale level had been identified in the Frasnian (Upper Devonian). Southern Tunisia is characterized by three main Paleozoic sedimentary basins, which are from North to South, the southern Chotts, Jeffara and Berkine Basin. They are separated by a major roughly E-W trending lower Paleozoic structural high, which encompass the Mehrez-Oued Hamous uplift to the West (Algeria) and the Nefusa uplift to the East (Libya), passing by the Touggourt-Talemzane-PGA-Bou Namcha (TTPB) structure close to southern Tunisia. The forementioned major source rocks in southern Tunisia are defined by hot shales with elevated Gamma ray values often exceeding 1400 API (in Hayatt-1 well), deposited in deep water environments during short lived (c. 2 Ma) periods of anoxia. In the course of this review, thickness, distribution and maturity maps have been established for each hot shale level using data for more than 70 wells located in both Tunisia and Algeria. Mineralogical modeling was achievd using Spectral Gamma Ray data (U, Th, K), SopectroLith logs (to acquire data for Fe, Si and Ti) and Elemental capture Spectroscopy (ECS). The latter technique provided data for quartz, pyrite, carbonate, clay and Sulfur. In addition to this, the Gamma Ray (GR), Neutron Porosity (ΦN), deep Resistivity (Rt) and Bulk Density (ρb) logs were used to model bulk mineralogy and lithology. Biostratigraphic and complete geochemical review has been undertaken from published papers and unpublished internal reports to better assess these important source intervals.

Recovery from the end-Permian mass extinction is frequently described as delayed, with complex ecological communities typically not found in the fossil record until the Middle Triassic epoch. However, the taxonomic diversity of a number of marine groups, ranging from ammonoids to benthic foraminifera, peaked rapidly in the Early Triassic. These variations in biodiversity occur amidst pronounced excursions in the carbon isotope record, which are compatible with episodes of massive CO2 outgassing from the Siberian Large Igneous Province. Here we present a high-resolution Early Triassic temperature record based on the oxygen isotope composition of pristine apatite from fossil conodonts. Our reconstruction shows that the beginning of the Smithian substage of the Early Triassicwas marked by a cooler climate, followed by an interval of warmth lasting until the Spathian substage boundary. Cooler conditions resumed in the Spathian. We find the greatest increases in taxonomic diversity during the cooler phases of the early Smithian and early Spathian. In contrast, a period of extreme warmth in the middle and late Smithian was associated with floral ecological change and high faunal taxonomic turnover in the ocean. We suggest that climate upheaval and carboncycle perturbations due to volcanic outgassing were important drivers of Early Triassic biotic recovery.

The magnitude and timing of a major rapid negative carbon-isotope excursion recorded in marine and terrestrial matter through the Early Toarcian (Early Jurassic) and Early Aptian (Early Cretaceous) oceanic anoxic events (OAEs) have been proposed to be the result of large methane gas-hydrate dissociation events. Here, we develop and evaluate a global carbon-isotope mass-balance approach for determining the responses of each component of the exogenic carbon cycle (terrestrial biosphere, atmosphere and ocean). The approach includes a dynamic response of the terrestrial carbon cycle to methane-related CO2 increases and climatic warming. Our analyses support the idea that both the Early Toarcian and Early Aptian isotopic curves were indicative of large episodic methane releases (~5000 and ~3000 Gt respectively) promoting warm ‘greenhouse’ conditions in the Mesozoic. These events are calculated to have increased the atmospheric CO2 concentration by ~900 and ~600 ppmv respectively and land surface temperatures by 2.5° to 3.0°C. However, we show that much of the methane released from oceanic sediments is rapidly sequestered by terrestrial and marine components in the global carbon cycle, and this effect strongly attenuated the potential for ancient methane gas-hydrate dissociation events to act as major amplifiers in global warming. An increase in oceanic carbon sequestration is consistent with the deposition of globally distributed black shales during these OAEs. Our analyses point to the urgent need for high-resolution marine and terrestrial carbon-isotope records to better characterize the nature of the Toarcian and Aptian events and improve our interpretation of their consequences for the global carbon cycle.

The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic-zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.

Considerable attention has been given to the Carnian (Late Triassic) Pluvial and Reingraben events associated with organic rich shale accumulation in the Germanic basin, Alps, southern Appenines as well as in northwestern Tethyan margins. Less
interest has been shown to the southern Tethyan portion represented by the northern margin of Africa, including Tunisia.
Tunisian basins represented by the Tellian domain, Tunisian trough, the ‘Dorsale’, and the North–South Axis (NOSA)
belong to the southern Tethyan margin, where northern and central areas record the early Carnian anoxic event within an
extensive carbonate platform. This short-lived (~2 million year) period of anoxia ranges within the Aonoides to Austriacum
ammonite zones, and corresponds in Tunisia generally to intermediate to shallow water environments marked by organic rich black limestone, dolomite, and shale. Interestingly, toward the south, the dysaerobic conditions in the Jeffara–southern Dahar basin appear to have prevailed locally also in the early Carnian. Here we review evidence of early Carnian anoxia in Tunisia based on the analysis of more than 17 Triassic sections and wireline logs from several petroleum exploration wells penetrating the black dolomites, limestones, and shales. In addition, biostratigraphic and complete geochemical reviews have been undertaken from published papers and unpublished internal reports to better assess this important and promising hydrocarbon source interval.

During the Pliensbachian–Toarcian interval of the Early Jurassic, there is a well-known second order marine extinction that is observable at the species and genus levels. Ammonite diversity data from successions throughout Europe and parts of the Arctic suggest that this extinction may have been multi-phased with di- versity declining over six separate intervals. The main-phase of decline begins at the Pliensbachian–Toarcian boundary and extends into the Lower Toarcian, to a level that is correlative with the Tenuicostatum/ Serpentinum Zone boundary. To date, only the main-phase of extinction has been demonstrated as being global in extent and affecting multiple taxonomic groups. This multi-phased extinction has been attributed to regional and global controlling mechanisms that are associated with the Volcanic Greenhouse Scenario which links the eruption of the Karoo–Ferrar large igneous province (LIP) to global warming and mass extinction.
We compare stratigraphic ranges of ammonite and foraminiferal species in Pliensbachian–Toarcian successions of western North America to the record in Europe and parts of the Arctic in order to test the geographic extent of the multiple phases of extinction. Our results show six intervals of species level decline that correlate with those recognized in Europe: 1) middle of the Lower Pliensbachian (middle Whiteavesi–middle Freboldi Zones), 2) middle of the Upper Pliensbachian (upper Kunae–lower Carlottense Zone), 3) Pliensbachian/Toarcian boundary into the Lower Toarcian (upper Carlottense–middle Kanense Zones), 4) Middle Toarcian (upper Planulata–lower Crassicosta Zones), 5) upper Middle–lower Upper Toarcian (middle Crassicosta–Hillebrandti Zones) and 6) Upper Toarcian (lower Yakounensis Zone).
Recognition of this multi-phased event in three separate ocean basins (paleo Pacific, paleo Arctic, and Tethys Oceans), in at least two taxonomic groups, greatly expands the known geographic extent of this multi-phased event and argues for a controlling mechanism that is global in its reach. In relation to the Volcanic Greenhouse Scenario, our study shows that four of the six pulses of extinction occur within the main-phase of Karoo magmatism. The decline in the Early Pliensbachian, previously thought to be separate from this event, occurs within error range of the onset of Karoo magmatism and the decline in the Late Toarcian coincides with the later stages of magmatism. These observations extend the known duration of this multi-phased extinction event to the Early Pliensbachian and support the Volcanic Greenhouse Scenario, specifically the eruption of the Karoo–Ferrar LIP, as a preeminent factor driving the multi-phased extinction of the Pliensbachian–Toarcian.

Our understanding of the ancient ocean-atmosphere system has focused on oceanic proxies. However, the study of terrestrial proxies is equally necessary to constrain our understanding of ancient climates and linkages between the terrestrial and oceanic carbon reservoirs. We have analyzed carbon–isotope ratios from fossil plant material through the Valanginian and Lower Hauterivian from a shallow-marine, ammonite-constrained succession in the Crimean Peninsula of the southern Ukraine in order to determine if the Upper Valanginian positive carbon–isotope excursion is expressed in the atmosphere. δ13Cplant values fluctuate around –23‰ to –22‰ for the Valanginian–Hauterivian, except during the Upper Valanginian where δ13Cplant values record a positive excursion to ~–18‰. Based upon ammonite biostratigraphy from Crimea, and in conjunction with a composite Tethyan marine δ13Ccarb curve, several conclusions can be drawn: (1) the δ13Cplant record indicates that the atmospheric carbon reservoir was affected; (2) the defined ammonite correlations between Europe and Crimea are synchronous; and (3) a change in photosynthetic carbon–isotope fractionation, caused by a decrease in atmospheric pCO2, occurred during the Upper Valanginian positive δ13C excursion. Our new data, combined with other paleoenvironmental and paleoclimatic information, indicate that the Upper Valanginian was a cool period (icehouse) and highlights that the Cretaceous period was interrupted by periods of cooling and was not an equable climate as previously thought.

Oued Bahloul is a reference section for the Cenomanian-Turonian that was located on the North African palaeomargin, where Oceanic Anoxic Event 2 (OAE2) is represented by black limestones and grey marls. Biostratigraphic study allowed the identification of calcareous nannofossil UC3 to UC6 zones. Quantitative analyses of calcareous nannofossil assemblages indicate that high-fertility surface-water conditions predominated throughout most of the OAE2 interval. The most eutrophic conditions probably developed during deposition of the lowermost part of the Whiteinella archaeocretacea Zone (UC3 to UC4 zones), where nannofossil-derived fertility indices reach high values and the small Zeugrhabdotus gr. predominates over Biscutum spp. The temperature index based on calcareous nannofossil assemblages indicates a warming at the beginning of the OAE2, followed by progressive cooling (the same trend indicated by δ 18 O bulk values). The significant proportion of holococcoliths suggests high-stress conditions in surface waters during most of the OAE2 interval. The enrichment in redoxsensitive elements, together with the abundance of low-oxygen tolerant benthic foraminifera, indicates dysoxic conditions both in the deep-water column and sea-bottom at the beginning of the OAE2 (W. archaeocretacea Zone). Geochemical detrital proxies indicate an enhanced rate of terrigenous supply during the OAE2 interval resulting from increased riverine influx related to a more humid climate. Enhanced fluvial run-off increased nutrient input to marine surface waters contributing to eutrophication (recorded by calcareous nannoplankton and planktic foraminifera). Plankton consumption of surface water nutrients increased organic matter production, whose decay contributed to the decrease in dissolved oxygen, generating increasingly dysoxic/anoxic conditions and favoring preservation of organic matter. Although low-latitude nannofossil assemblages such as those of Oued Bahloul reflect an increase in nutrient availability during the OAE2 interval, assemblages from mid-latitude areas record a decreased nutrient content.

Open marine sediments deposited during the CenomanianeTuronian transition are well exposed in the Spanish Ba~ nos de la Hedionda section (Betic Cordillera, South Iberian Palaeomargin). Analysis of fora-miniferal assemblages and geochemical proxies allow inferences on the impact of the Oceanic Anoxic Event 2 (OAE2) in this area of the western Tethys. Three main intervals have been identified corresponding to different lithological units and biozones. (1) The top of the Capas Blancas Member (Rotali-pora cushmani Biozone) represents the pre-extinction phase with diverse foraminiferal assemblages and well developed water-column tiering, well-oxygenated, oligotrophic deep-waters and oxygenated to poorly oxygenated, mesotrophic surface-waters. Foraminiferal opportunist species point to a minor event with dysoxic conditions preceding the OAE2. (2) The black radiolaritic shales (Whiteinella archaeocretacea Biozone) consist of a foraminiferal-barren interval, except for the lowermost centimetres where planktic surface-dweller opportunists are common. Redox sensitive elements (Cr/Al, V/Al, U/Th, Mo EF , Mo aut , U EF and U aut) and increased TOC values reflect oxygen depleted conditions related to the OAE2. The increase in P/Ti values at the base of this stratigraphic interval indicates an abrupt increase in productivity. High concentrations of radiolarians are congruent with high surface productivity probably related to changes in oceanic circulation and enhanced upwelling currents, as well as subsequent shallowing of the oxygen-minimum zone. The increase in Mo EF and Mo aut towards the top of the black radiolaritic shales indicates temporal euxinic conditions. (3) A slow, bottom-up recovery of foraminiferal assemblages is inferred at the base of the Boquer on Member (Helvetoglobotruncana helvetica Biozone), with seafloor recolonization by benthic foraminifera being recorded previous to the water column colonization by planktic forms, mainly by intermediate-dwellers typical of mesotrophic waters. The subsequent proliferation of surface-dweller opportunists and deep-dweller opportunists adapted to mesotrophic to eutrophic conditions, and the decrease in planktic foraminiferal diversity, may indicate the persistence of poorly oxygenated conditions in the water column towards the lower-middle part of the H. helvetica Biozone.

The Hushpuckney Shale Member of the Swope Formation (Missourian Stage, eastern Kansas) is the core shale of a Kansastype cyclothem, formed during the late transgressive to early regressive phases of a Late Pennsylvanian glacio-eustatic cycle. Coeval high-frequency climato-environmental dynamics of the Midcontinent Seaway are preserved in the KGS Orville Edmonds No. 1A study core as centimeter-scale variation in major components (organic carbon, authigenic sulfides and phosphate, detrital siliciclastics), organic macerals, trace-element redox proxies (Mo, U, V, Zn), and ichnofabric features. Benthic O 2 levels declined sharply from the base of the black shale (0 cm), went sulfidic at f4 cm, and reached a redox minimum at f21 cm; above this level, redox potential gradually rose, fluctuating between sulfidic and nonsulfidic conditions from f35 cm to the black shale/gray shale contact at 52 cm. Onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft-bodied organisms comprised of deep-tiered tracemakers tolerant of low-O 2 conditions (Trichichnus), and shallow-tiered tracemakers favoring ''soupground'' (Helminthopsis) or ''firmground'' substrates (the Zoophycos -Phycosiphon -Schaubcylindrichnus -Planolites association). Most geochemical records exhibit a ''low-order'' cycle spanning the full 52 cm thickness of the black shale submember, reflecting a dominant glacio-eustatic control. The base and top of the black shale submember record lateral migration of the pycnocline across the Midcontinent Shelf during the transgressive and regressive phases, respectively. The maximum flooding surface (MFS) is at f17 -23 cm, an interval containing the euxinic peak and characterized by high concentrations of illite (representing a cratonic siliciclastic flux) and terrestrial organic macerals, the product of transient increases in humidity, weathering rates, and the export of coal-swamp vegetation associated with the interglacial highstand of the Swope cyclothem. Although a transgressive ''surface of maximum starvation'' (SMS) cannot be confidently identified, a phosphate-rich ''regressive condensation surface'' at f28 -34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth; a correlative shift in dominance from terrestrial-to marine-derived organic macerals reflects increased upwelling of nutrient-rich deepwaters and enhanced primary productivity. Penecontemporaneous paleogeographic and -climatic factors (e.g., semirestricted circulation, monsoonal precipitation) predisposed the Midcontinent Seaway toward sensitivity to high-frequency climato-0009-2541/$ -see front matter D (J.C. Hower).

Carbon sources and sinks are key components of the climate feedback system, yet their response to external forcing remains poorly constrained, particularly for past greenhouse climates. Carbon-isotope data indicate systematic, million-year scale transfers of carbon between surface reservoirs during and immediately after the Late Cretaceous thermal maximum (peaking in the Cenomanian – Turonian, c. 97 – 91 million years, Myr, ago). Here we calibrate Albian to Campanian (108 – 72 Myr ago) high-resolution carbon isotope records with a refined chronology and demonstrate how net transfers between reservoirs are plausibly controlled by ~1-Myr changes in the amplitude of axial obliquity. The amplitude modulating terms are absent from the frequency-domain representation of insolation series and require a non-linear, cumulative mechanism to become expressed in power spectra of isotope time series. Mass-balance modeling suggests that the residence time of carbon in the ocean-atmosphere system is – by itself – insufficient to explain the Myr-scale variability. It is proposed that the astronomical control was imparted by a transient storage of organic matter or methane in quasi-stable reservoirs (wetlands, soils, marginal zones of marine euxinic strata, and potentially permafrost) that responded non-linearly to obliquity-driven changes in high-latitude insolation and/or meridional insolation gradients. While these reservoirs are probably underrepresented in the geological record due to their quasi-stable character, they might have provided an important control on the dynamics and stability of the greenhouse climate.

The use of molybdenum isotope data (δ 98 Mo) from organic-rich shales to draw inferences concerning marine paleoredox conditions at a global scale is predicated upon the assumptions of (1) a residence time of Mo in seawater much greater than the ocean mixing time, and (2) quantitative removal of Mo from a strongly euxinic ([H 2 S] aq > 11 μM) water column to the sediment, thus preserving the seawater δ 98 Mo signature. In this study we analyze Mo isotopic variation in the Hushpuckney Shale, a 73-cm-thick unit representing the late transgressive to early regressive stages of a glacio-eustatic cyclothem (Swope Formation) deposited in the Late Pennsylvanian Midcontinent Sea (LPMS) of North America. The Hushpuckney can be subdivided into four stratigraphic zones of distinctive geochemical character. Zones I and III, which accumulated under weakly euxinic conditions, acquired relatively high δ 98 Mo values (+0.9 to +1.1‰), whereas Zone II, which accumulated under intensely euxinic conditions, acquired lower δ 98 Mo values (~+0.6‰). Zone IV, which accumulated under suboxic conditions in the water column, acquired the heaviest δ 98 Mo values (+1.1 to + 1.8‰). These results contrast with the pattern of redox -δ 98 Mo covariation in modern marine environments, in which the heaviest δ 98 Mo values are found in the most intensely euxinic facies. We evaluate three different hypotheses to account for the Mo isotopic patterns of the Hushpuckney Shale. One hypothesis, seawater-freshwater mixing, is rejected owing to isotopic mass balance considerations. A second hypothesis is a local control on δ 98 Mo by water-column redox cycling of Mn, with particulate Mnoxyhydroxides adsorbing isotopically light Mo and transferring it to the sediment, a process that was most active during deposition of Zone II. The significance of this scenario is that euxinic black shales may not reliably record global seawater δ 98 Mo in areas where a Mn-particulate shuttle is operative. A third hypothesis is based on rapid secular variation of the Mo isotope composition of Late Pennsylvanian global seawater. In order to account for δ 98 Mo trends within the Hushpuckney Shale, seawater δ 98 Mo must have varied by~1.2‰ at a~100-kyr timescale, which would have been possible only if the residence time of Mo in Late Pennsylvanian seawater was b 100 kyr. Although both the second and third hypotheses are viable based on the present limited δ 98 Mo dataset, we discuss how each model might be tested through additional Mo isotope data.

Global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans, leading to increases in organic production, the degradation of which causes a further decrease in dissolved oxygen. In extreme cases in the geological past, this trajectory has led to catastrophic marine oxygen depletion during the so-called oceanic anoxic events (OAEs). How the water column oscillated between generally oxic conditions and local/global anoxia remains a challenging question, exacerbated by a lack of sensitive redox proxies, especially for the suboxic window. To address this problem, we use bulk carbonate I/Ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the Cretaceous OAE 2. In general, I/Ca ratios were relatively low preceding and during the OAE interval, indicating deep suboxic or anoxic waters exchanging directly with near-surface waters. However, individual sites display a wide range of initial values and excursions in I/Ca through the OAE interval, reflecting the importance of local controls and suggesting a high spatial variability in redox state. Both I/Ca and an Earth System Model suggest that the northeast proto-Atlantic had notably higher oxygen levels in the upper water column than the rest of the North Atlantic, indicating that anoxia was not global during OAE 2 and that important regional differences in redox conditions existed. A lack of correlation with calcium, lithium, and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the I/Ca proxy during OAE 2.

During the Early Toarcian there was a significant disruption in the short-term active carbon reservoir as revealed by carbon-isotope records, which show a broad positive shift that is interrupted by a large 5–7‰ negative excursion (δ13Corg). Carbon-isotope excursion co-occurs with the deposition of organic-rich shales in many areas. This perturbation in carbon isotopes is thought to be indicative of severe climate change and marine anoxia. The two leading hypotheses as to the cause of this event invoke either global or regional controls. Here we present carbon-isotope data from Haida Gwaii, British Columbia, Canada showing a significant perturbation within a temporally constrained Early Toarcian succession that was deposited in the northeastern paleo-Pacific Ocean. These data reinforce the concept that the short-term active carbon reservoir was affected globally, and assist with the correlation of ammonite zonal schemes between western North America and Europe. The δ13Corg data show a broad positive shift that is interrupted by a sharp and pronounced negative excursion of 7‰ (8.5‰ in δ13Cwood) in the Early Toarcian Kanense Zone. This negative excursion also coincides with increasing total organic carbon (TOC) from ~0.4% to ~1.2%. These data suggest that the Early Toarcian carbon-isotope perturbation was indeed global and imprinted itself on all active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).

Profound biotic changes accompanied the late Cenomanian d 13 C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid d 13 C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding d 13 C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80-90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.

Major advances in our understanding of paleoclimate change derive from a precise reconstruction of the periods, amplitudes and phases of the ‘Milankovitch cycles’ of precession, obliquity and eccentricity.  While numerous quantiative approaches exist for the identification of these astronomical cycles in stratigraphic data, limitations in radioisotopic dating, and instability of the theoretical astronomical solutions beyond ~50 Myr ago, can challenge identification of the phase relationships needed to constrain climate response and anchor floating astrochronologies. Here we demonstrate that interference patterns accompanying frequency modulation (FM) of short eccentricity provide a robust basis for identifying the phase of long eccentricity forcing in stratigraphic data. One- and two-dimensional models of sedimentary distortion of the astronomical signal are used to evaluate the veracity of the FM method, and indicate that pristine eccentricity FM can be readily distingushed in paleo-records. Apart from paleoclimatic implications, the FM approach provides a quantitative technique for testing and calibrating theoretical astronomical solutions, and for refining chronologies for the deep past.

We present two case studies that use the FM approach to evaluate major carbon-cycle perturbations of the Eocene and Late Cretaceous. Interference patterns in the short-eccentricity band reveal that Eocene hyperthermals ETM2 (‘Elmo‘), H2, I1 and ETM3 (X; ~52–54 Myr ago) were associated with maxima in the 405-kyr cycle of orbital eccentricity. The same eccentricity configuration favored regional anoxic episodes in the Mediterranean during the Middle and Late Cenomanian (~94.5–97 Myr ago).  The initial phase of the global Oceanic Anoxic Event II (OAE II; ~93.9–94.5 Myr ago) coincides with maximum and falling 405-kyr eccentricity, and the recovery phase occurs during minimum and rising 405-kyr eccentricity. On a Myr scale, the event overlaps with a node in eccentricity amplitudes. Both studies underscore the importance of seasonality in pacing major climatic perturbations during greenhouse times.

A new Nd-isotope record from the NW European shelf sea (Eastbourne) across OAE 2.Negative and positive εNd excursions are found in the English Chalk during OAE 2.Changes in ocean circulation associated with a climatic cooling during OAE 2.An input of radiogenic Nd from LIP volcanism during OAE 2.Possible enhanced latitudinal seawater exchange during transient cooling (OAE 2).Nd isotopes of fish debris collected from the English Chalk at Eastbourne (Sussex, UK) are used to reconstruct the history of ocean circulation in the NW European shelf sea during Oceanic Anoxic Event 2 (OAE 2, Cenomanian–Turonian). The Eastbourne εNd record exhibits a 1-unit negative excursion (decreasing from ∼−9 to ∼−10), immediately followed by a 3-unit positive excursion reaching ∼−7. The onset of the negative εNd excursion lags the global δ13C rise characteristic of OAE 2, suggesting stable patterns of ocean circulation in the NW European shelf sea at this time. Both negative and positive Nd-isotope excursions took place during a transient cooling episode within OAE 2. The negative εNd excursion is interpreted as due to a change in ocean circulation with northerly sourced water masses becoming the dominant bottom waters at Eastbourne. The positive excursion is best explained by the transport of radiogenic Nd derived from a volcanic source, possibly the High Arctic or Caribbean large igneous province (LIP). An input of volcanic Nd may reconcile the Eastbourne record with coeval εNd records on Demerara Rise in the western tropical Atlantic. The broad synchroneity of high εNd values (∼−7) registered at both sites suggests a possible period with efficient oceanic mixing between the tropical Atlantic and the NW European shelf sea during the cooling episode. The Eastbourne εNd record of OAE 2, together with coeval temperature reconstructions, provides evidence for the coincidence of changes in ocean circulation and transient climatic cooling, implying a tight coupling between the two phenomena during this interval.

The TEX 86 is a widely used paleotemperature proxy based on isoprenoid glycerol dibiphytanyl 16 glycerol tetraethers (GDGTs) produced by Thaumarchaeota. Archaeal membranes are composed of 17 GDGTs with polar head groups (IPL-GDGTs), most of which are expected to be degraded completely 18 or transformed into more recalcitrant core lipid (CL)-GDGTs upon cell lysis. Here, we examined the 19 differences in concentration and distribution of core lipid (CL)-and intact polar lipid (IPL)-GDGTs in 20 surface sediments at different deposition depths, and different oxygen bottom water concentrations (<3 21 to 83 mol · L -1 ). Surface sediments were sampled from 900 to 3000 m depth on a seamount (Murray 22

Geochemical analyses of extraordinarily well preserved late Aptian–early Albian foraminifera from Blake Nose (Ocean Drilling Program Site 1049) reveal rapid shifts of δ18O, δ13C, and 87Sr/88Sr in the subtropical North Atlantic that may be linked to a major planktic foraminifer extinction event across the Aptian/Albian boundary. The abruptness of the observed geochemical shifts and their coincidence with a sharp lithologic contact is explained as an artifact of a previously undetected hiatus of 0.8–1.4 million years at the boundary contact, but the values before and after the hiatus indicate that major oceanographic changes occurred at this time. 87Sr/88Sr increase by ~0.000 200, δ13C values decrease by 1.5‰ to 2.2‰, and δ18O values decrease by ~1.0‰ (planktics) to 0.5‰ (benthics) across the hiatus. Further, both 87Sr/88Sr ratios and δ18O values during the Albian are anomalously high. The 87Sr/88Sr values deviate from known patterns to such a degree that an explanation requires either the presence of inter-basin differences in seawater 87Sr/88Sr during the Albian or revision of the seawater curve. For δ18O, planktic values in some Aptian samples likely reflect a diagenetic overprint, but preservation is excellent in the rest of the section. In well preserved material, benthic foraminiferal values are largely between 0.5 and 0.0‰ and planktic samples are largely between 0.0‰ to –1.0‰, with a brief excursion to –2.0‰ during OAE 1b. Using standard assumptions for Cretaceous isotopic paleotemperature calculations, the δ18O values suggest bottom water temperatures (at ~1000–1500 m) of 8–10°C and surface temperatures of 10–14°C, which are 4–6°C and 10–16°C cooler, respectively, than present day conditions at the same latitude. The cool subtropical sea surface temperature estimates are especially problematic because other paleoclimate proxy data for the mid-Cretaceous and climate model predictions suggest that subtropical sea surface temperatures should have been the same as or warmer than at present. Because of their exquisite preservation, whole scale alteration of the analyzed foraminifera is an untenable explanation. Our proposed solution is a high evaporative fractionation factor in the early Albian North Atlantic that resulted in surface waters with higher δ18O values at elevated salinities than commonly cited in Cretaceous studies. A high fractionation factor is consistent with high rates of vapor export and a vigorous hydrological cycle and, like the Sr isotopes, implies limited connectivity among the individual basins of the Early Cretaceous proto-Atlantic ocean.

Ocean anoxia has been widely implicated in the Permian-Triassic extinction. However, the duration and distribution of the ocean anoxia remains controversial. In this study, the detailed redox changes across the Permian-Triassic boundary (PTB) in the shallow platform interior at Great Bank of Guizhou (GBG) has been reconstructed based on the high-resolution microfossil composition and multiple paleo-redox proxies. The shallow platform is characterized by low sulfur (total sulfur (TS) and pyrite sulfur (S py )) concentrations, low S py /TOC ratios, and low DOP values before the mass extinction, representing oxic conditions well. Following the mass extinction, the shift of multiple geochemical proxies, including high S py /TOC ratios and DOP values, indicates dysoxic-anoxic conditions in shallow ocean. Furthermore, we reconstruct the transition of the redox conditions of Nanpanjiang Basin: the intense volcanic eruptions, which release huge CO 2 and SO 2 before the mass extinction, provoke the temperature rising and the collapse of terrestrial ecosystem. As a result, the increased weathering influx causes the carbon isotopic negative excursion and the expansion of the ocean oxygen minimum zone (OMZ). When the OMZ expanded into the photic zone, the episodic H 2 S release events enhance the pyrite burial at Dajiang section. Thus, intense volcanic eruptions, temperature increase, and oceanic hypoxia together lead to the PTB extinction. Recent studies show high temperature might be the key mechanism of the PTB extinction. In addition, this study confirms that the microbialites were formed in the dysoxicanoxic shallow water. redox condition, mass extinction, Permian-Triassic boundary, volcanic eruption, shallow platform Citation: Song H Y, Tong J N, Tian L, et al. 2014. Paleo-redox conditions across the Permian-Triassic boundary in shallow carbonate platform of the Nanpan-

Gulf of Mexico estuaries contain an abundance of habitat-forming submerged vegetation that provide various ecosystem services. However, these estuaries now harbor numerous invasive macrophytes, such as Eurasian milfoil Myriophyllum spicatum. Previously, we showed that milfoil gained a foothold in the Mobile-Tensaw Delta, Alabama, USA, in protected waters north of a manmade causeway that significantly reduces wave action. Here, we collected associated organisms using a suction sampler and compared the composition and abundances of fauna residing in M. spicatum and wild celery Vallisneria americana, the most abundant native macrophyte, during day and night. North of the causeway, where water flow is limited, a 5-fold reduction in the abundance of organisms was documented in M. spicatum at night, while abundance in V. americana varied little. We found that this nocturnal decrease was accompanied by changes in community structure in M. spicatum north of the causeway, driven by reductions in invertebrates such as Gammarus amphipods, Neritina snails, and Callinectes sapidus crustaceans. In contrast, communities in V. americana, although distinct from M. spicatum, exhibited little spatial or temporal difference. Subsequent analyses indicated that reduced dissolved oxygen in M. spicatum north of the causeway at night drives assemblage patterns. These results suggest that hydrological alterations exacerbate M. spicatum’s negative effects through the creation of hypoxic zones and that daily migrations into these habitats may be necessary for organisms to garner the benefits of this vegetation. Finally, this work highlights that our understanding of the concomitant impacts of altered hydrology and invasive macrophytes is incomplete without considering diel variability.

The data presented here comprise Ryazanian–Valanginian carbon isotope ratios analyzed from fossil wood and belemnites from the shallow marine Boyarka River succession in Siberia. Additional belemnite carbon isotope ratios from the Izhma River succession (also Ryazanian–Valanginian) in Russia are also presented. The wood-derived and belemnite-derived isotope ratios are considered to primarily reflct changes in the terrestrial and marine carbon isotope reservoirs respectively. The δ13Ccarb and δ13Cwood records reveal a distinct mid-Valanginian positive carbon isotope excursion, with the initiation occurring near the Boreal Russian michalskii–polyptychus zone boundary, which is broadly time-equivalent Tethyan campylotoxus–verrucosum boundary. The Ryazanian–Valanginian δ13Ccarb values fluctuate between c. –1 and +1.5‰ but reach a maximum of +4.1‰ in the Late Valanginian, whilst the δ13Cwood values fluctuate between c.–27 and –23.5‰ and reach a Late Valanginian maximum of –21.2‰. The excursion maximum in the Boreal Russian bidichotomus zones corresponds with the peak of the Tethyan marine carbonate excursion in the verrucosum–eperegrinus zones, the peak of a marine carbonate excursion recorded in the Argentinean atherstoni Zone and also with the peak of a terrestrial organic carbon isotope excursion in the Crimean trinodosum–ecallidiscus ammonite zones. The synchroneity of the positive carbon isotope event between the marine and terrestrial records and between the northern and southern hemispheres and Tethys, clearly indicates a strong coupling of the ocean-atmosphere system at this time and also confirms that this was a global event, which would have affected the total exchangeable carbon reservoir.

Variations in the seawater 87Sr/86Sr curve through time can be caused by fluctuations in the strontium flux or variations in the isotopic ratio from at least six different sources and sinks. Thus, 12 or more parameters control each single measurement although widely accepted assumptions allow this to be reduced to typically six unknowns. Interpreting the causes of time-variation in the seawater 87Sr/86Sr curve is therefore hampered by inherent non-uniqueness. However, this problem is under-constrained rather than unconstrained. As a result, whilst there are an infinite number of possible interpretations, these all come from a few families of very similar solutions. Using this insight, it is possible to find solutions having the smallest possible variations in source flux or source 87Sr/86Sr ratio. Thus, lower- bounds can be placed upon the source variations responsible for the observed fluctuations in the seawater 87Sr/86Sr curve. When applied to the evolution of the Early Jurassic 87Sr/86Sr seawater curve, this approach demonstrates that a short-lived Toarcian event is genuine since it is present in all models, regardless of the values chosen for the unknown source fluxes and unknown source isotope ratios. However, the variations in strontium flux or isotopic ratio necessary to explain the Toarcian event may be significantly smaller than would be predicted assuming modern values for the unknown parameters.

The Cretaceous was punctuated by several episodes of accelerated global change, defined as Oceanic Anoxic Events (OAEs), that reflect abrupt changes in global carbon cycling. The Aptian Oceanic Anoxic Event (OAE1a; 120 Ma) represents an excellent example, recorded in all major ocean basins, and associated with massive burial of organic matter in marine sediments. The OAE1a is concomitant with the " nannoconid crisis " , which is characterized by a major biotic turnover, and a widespread demise of carbonate platforms. Many studies have been published over the last decades on OAE1a's from different sections in the world, and provide a detailed C-isotope stratigraphy for the event. Nevertheless, new high-resolution studies across the event are essential to shed light on the precise timing and rates of the multiple environmental and biotic changes that occurred during this critical period of Earth history. Here we present a new drill core recovering an Aptian section spanning the OAE1a in southern Spain. The so-called Cau section was drilled in the last quarter of 2015. The Cau section is located in the easternmost part of the Prebetic Zone (Betic Cordillera), which represents platform deposits of the southern Iberian palaeomargin. The lower Aptian deposits of the Cau section belong to a hemipelagic unit (Almadich Formation), deposited in a highly subsident sector of the distal parts of the Prebetic Platform. Previous work on the early Aptian of the Cau succession has focused on stratigraphy, bioevents, C-isotope stratigraphy, and organic and elemental geochemistry. A more recent study based on biomarkers has presented a detailed record of the pCO 2 evolution across the OAE1a (Naafs et al., 2016). All these studies reveal that the Cau section represents an excellent site to further investigate the OAE1a, based on its unusually high sedimentation rate and stratigraphic continuity, the quality and preservation of fossils, and the well-expressed geochemical signatures.

Although ocean circulation plays a vital role in the climate system, its response to major carbon-cycle perturbations during the mid-Cretaceous, including mid-Cenomanian event I (MCE I) and the Cenomanian-Turonian oceanic anoxic event (OAE 2), is poorly constrained. Here we present Nd isotope evidence for episodic increases in the influence of boreal seawater in the European epicontinental sea during MCE I. The start of this circulation reorganization lagged the onset of the δ13C positive excursion defining MCE I. This sequence of change is similar to that observed during OAE 2 in the same area, showing a consistent response of regional circulation to changes in the global carbon cycle. Brief intervals of invasion of boreal fauna to mid-latitude seas, two during MCE I and one during OAE 2 (Plenus cold event), all started after the influence of boreal seawater was enhanced, implying a slower biological response to climate cooling rather than passive transport of fauna by boreal waters. The lack of an Nd isotope positive excursion in our record across MCE I supports a volcanic origin for prominent increases in seawater Nd isotope values found in the European epicontinental sea and the tropical Atlantic during OAE 2. The observed tight circulation-carbon cycle coupling may help the upper ocean replenish nutrients from deep waters and/or volcanic sources, providing a critical feedback allowing continuation of MCE I and OAE 2 over long durations.

Ichthyoliths (fossil fish microremains) from geological core samples are commonly recorded and where present allow dating and contribute to the knowledge of the evolution of fish faunas through time (see, for instance, Sibert et al., 2014; Sibert and Norris, 2015, for the Ocean Drilling Program).

However, reports of articulated fish specimens in cores are much rarer due to the limited chance of a core drill returning a small-diameter cylinder of rock encompassing the fossil to the surface.

Examples of articulated fishes described from cores include specimens of Permo-Carboniferous actinopterygians near the city of Zurich, Switzerland (B€urgin, 1990), as well as a new taxon of a Cenomanian ellimmichthyiform from Alberta, Canada (Hay et al., 2007), and two new Eocene cypriniform fishes from Guangdong and Henan, China (Liu, 1957; Zhou, 1990; Chang and Chen, 2008).

In this paper, we report the discovery of an ichthyodectiform fish fossil collected during re-evaluation of a core from DSDP Leg 41 Site 367 at the IODP Bremen Core Repository, Germany (Fig. 1A). The core was recovered from a well drilled in the Cape Verde Basin, ca. 400km offshore from the West African Atlantic Margin, to test the early basin evolution of the Central Atlantic region (Lancelot et al., 1977). The fossil
fish was discovered in core 21 section 6 at 699.9 m measured depth, preserved on the bedding surface of a bituminous marl.

Reports from the DSDP expedition had previously recorded fish vertebrae in the same subunit, suggesting abundance and/or high preservation rate of fish remains in Central Atlantic waters during the Albian to Cenomanian period (Lancelot et al., 1977:fig. 6). Associated calcareous nannofossils indicate an earliest Cenomanian age for the cored interval containing this specimen (Fig. 1B).

1] Profiles of Mo/total organic carbon (TOC) through the Lower Toarcian black shales of the Cleveland Basin, Yorkshire, United Kingdom, and the Posidonia shale of Germany and Switzerland reveal water mass restriction during the interval from late tenuicostatum Zone times to early bifrons Zone times, times which include that of the putative Early Toarcian oceanic anoxic event. The degree of restriction is revealed by crossplots of Mo and TOC concentrations for the Cleveland Basin, which define two linear arrays with regression slopes (ppm/%) of 0.5 and 17. The slope of 0.5 applies to sediment from the upper semicelatum and exaratum Subzones. This value, which is one tenth of that for modern sediments from the Black Sea (Mo/TOC regression slope 4.5), reveals that water mass restriction during this interval was around 10 times more severe than in the modern Black Sea; the renewal frequency of the water mass was between 4 and 40 ka. The Mo/TOC regression slope of 17 applies to the overlying falciferum and commune subzones: the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years. The more restricted of the two intervals has been termed the Early Toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to NW Europe. Crossplots of Re, Os, and Mo against TOC show similar trends of increasing element concentration with increase in TOC but with differing slopes. Together with modeling of 187 Os/ 188 Os and d 98 Mo, the element/TOC trends show that drawdown of Re, Os, and Mo was essentially complete during upper semicelatum and exaratum Subzone times (Mo/TOC regression slope of 0.5). Drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of Re, Os, and Mo, via a low-salinity surface layer, created isotopic excursions of up to 1.3% in d 98 Mo and up to 0.6% for 187 Os/ 188 Os. Restriction thereby compromises attempts to date Toarcian black shales, and possibly all black shales, using Re-Os chronology and introduces a confounding influence in the attempts to use d 98 Mo and initial 187 Os/ 188 Os for palaeo-oceanographic interpretation.

Foraminifera (single-celled protists) have a long geological record. They are both numerous and taxonomically diverse in assemblages throughout the Mesozoic and Cenozoic and have been used extensively in the oil industry since Józef Gryzbowski studied the stratigraphy of the Potok Oilfield (southern Poland) in the late 19th century. Both planktic and benthic foraminifera are used for biostratigraphy in various locations throughout the world, based on both processed residues and—where appropriate—thin-sections. In the Cretaceous successions of the Gulf of Mexico and the passive margins of the Atlantic Ocean, foraminifera provide vital evidence of changing paleoecology, paleoceanography, and major extinction events. This is particularly significant in the interpretation of oceanic anoxic events in the mid-Upper Cretaceous and the extinction events associated with the Cenomanian/Turonian and Cretaceous/Paleogene boundaries. By using planktic:benthic ratios and various forms of morphotype analysis—coupled with an understanding of modern ecology—the distribution of foraminifera are often important indicators of sea-level changes, ocean acidification events, and the low-oxygen environments, which often create hydrocarbon source rocks. Shelf successions below modern-day oxygen-minimum zones, while supporting different species from those from fully oxygenated environments, do not normally generate anoxic sediments and restricted assemblages: there must be other factors, such as basin isolation, involved. Models developed for the offshore basins of Brazil have been applied in many areas of the world, with the neritic areas of the Gulf Coast and North Atlantic margins providing many examples of how such models can be applied elsewhere.

During these last years, considerable attention has been given to Jurassic oil and gas shale in the middle and high latitudes of the northern hemisphere such as the west Europe and Russian platform where the most attractive Jurassic basins are located (e.g. Cleaveland basin, UK; Boulonnais, northern France; Pechora Basin, Moscow Basin, etc). In most petroleum systems which characterize these basins, the Jurassic (mainly the early Toarcian) played the major role in hydrocarbon generation which constitutes 25% of the global reserve of hydrocarbon. Tunisian basins represented by the northern Tunisian trough, the Dorsale and the North South Axis (NOSA) belong to the southern Tethyan margin where northern and central area have recorded the early Toarcian oceanic anoxic event (T-OAE). This short lived (c. 2 Ma) period of anoxia ranges within the whole Harpoceras serpentinum Zone which corresponds to deep water environments marked by black shale accumulation. Interestingly, towards the south, the dysaerobic conditions in the Chott basin appear to have begun largely later where the oxygen depletion is assumed to have prevailed during the Callovian, whereas it was limited to the early Toarcian in the Central and Northern Tunisia only a few hundreds of kilometers away. In addition, biostratigraphic and complete geochemical review has been undertaken from published papers and unpublished internal reports to better assess these important source intervals.

In this paper, we describe a new method for exploration procedure based on the knowledge of the Oceanic Anoxic Events (OAE’s) distribution maps in order to delineate the main source rocks existence and thus contribute to improve new petroleum systems. We describe especially the main Mesozoic OAE’s which have occurred during short times through the Toarcian-Turonian interval (Jurassic to Mid-Cretaceous), their main thickness/facies distribution, geochemical
characteristics (Total Organic Carbon and Maturity) and chemostratigraphic characters (mainly carbon isotopic signatures). The case study concerns the Gulf of Tunis basin which exposes the panoply of complex geological structures (thrusting, grabens, salt tectonics, etc.) in which exploration has ended since the 1990’s after
drilling six failed wells during more than twenty years. In order to improve and optimize exploration procedure within this basin (offshore) and in addition to the OAE’s concept (source rocks distribution), we describe a basin modeling study based on the calibration, burial histories (of drilled wells), generation and expulsion of hydrocarbon from a chosen OAE (Faraoni event of the late Hauterivian level) and hydrocarbon pathway migration (migration line hydrocarbon volumes) from the chosen source rock. The basin modeling generated
new prospects which seem to be near or off-structures from the positions of the past drilled wells. However, it is assumed that the basin modeling does not take into account the rapid change of facies and geological complexity such the case of the Gulf of Tunis. This is the reason why a Multiple-Point Geostatistics (MPS) method
is recommended for the integration of the basin modeling study in order to more improve the model. This method is applied in basin modeling to improve the efficiency of the regional exploration procedure in addition to the use of OAE’s concept.

May, A. (1997g): Ein Modell zur Erklärung der Bio-Events und der biogeographischen Entwicklung im Devon durch den Meeresspiegel. - Dortmunder Beiträge zur Landeskunde, Naturwissenschaftliche Mitteilungen, 31: 137-174, 5 figs.; Dortmund.

Abstract: The attempt is made, to show and to explain connections between the changing sea-Ievel, marine biogeography, and bio-events within the Devonian. The Devonian development of marine faunal provincialism is the result of changes in global sea-Ievel. Within the Emsian sea-Ievel was low and marine faunal provincialism was strong, while within the Frasnian sea-Ievel was high and marine faunal provincialism was weak. Some particular transgressive intervals made themselves felt as bio-events, conducive to fast decreases in the intensity of provincialism. Starting with the Daleje Event of the Emsian and ending with the Kellwasser Event of the Frasnian, the bio-events are discussed in detail. In this context new data concerning the Middle Devonian of the Eastern Rhenish Massif are presented. The best explanation for most Devonian bio-events is the occurrence of oceanic anoxic events. The model presented here uses fast transgressions caused by rapid sea-Ievel rises (see fig. 1). Fast transgression increased primary biomass production (caused by an increased influx of nutrients and an increased area of shallow sea) and decreased oxygen supply in the oceans (caused by global temperature increase resulting from albedo decrease). Both factors together resulted in the oceanic anoxic event conducive to the bio-event. The fact, that bio-event intensity increased from the Emsian to the Frasnian is explained by the decrease of marine faunal provincialism within this time. Both the development of provincialism and the occurrence and intensity of bio-events within the Devonian can be deduced directly or indirectly from the changes in global sea-Ievel. This comprehensive attempt at explanation has the advantage of needing only a few influencing factors and renouncing incalculable influences (e. g. meteorite impacts). Possibly, the model of oceanic anoxic events presented is also transferable to other bio-events (e. g.: bio-events of the Famennian, Cenomanian/Turonian Event). Kurzfassung: Der Artikel versucht, Zusammenhänge zwischen den Veränderungen des Meeresspiegels, der Entwicklung der marinen Biogeographie und den Bio-Events im Devon herauszuarbeiten und zu erklären. Die Entwicklung des Faunen-Provinzialismus im Devon ist ein Ergebnis der Entwicklung des globalen Meeresspiegels. Im Emsium war der Meeresspiegel niedrig und der Provinzialismus der Meeresfauna groß, im Frasnium war der Meeresspiegel hoch und der Provinzialismus sehr gering. Insbesondere einige Trangressions-Schübe, die sich auch als Bio-Events bemerkbar machten, führten zu schnellen Abnahmen der Größe des Provinzialismus. Die Bio-Events vom Daleje-Event des Emsiums bis zum Kellwasser-Event des Frasniums werden detailliert besprochen. In diesem Zusammenhang werden auch neue Daten zum Mitteldevon des Rechtsrheinischen Schiefergebirges gegeben. Die beste Erklärung für die meisten devonischen Bio-Events ist das Auftreten von ozeanischen anoxischen Events. Das hier vorgestellte Modell setzt bei schnellen Transgressionen an, die durch schnelle Meeresspiegel-Anstiege verursacht wurden (siehe Abb. 1). Die Konsequenzen einer schnellen Transgression waren gesteigerte Primärproduktion organischer Substanz und geringere Sauerstoff-Versorgung der Ozeane, was zusammen zum ozeanischen anoxischen Event und zum Bio-Event führte. Der Umstand, daß die Intensität der Bio-Events vom Emsium zum Frasnium zunahm, wird durch die Abnahme des Provinzialismus der Meeresfauna in dieser Zeit erklärt. Sowohl die Entwicklung des Provinzialismus der marinen Fauna als auch das Auftreten und die Intensität der Bio-Events im Devon lassen sich direkt oder mittelbar aus der Entwicklung des globalen Meeresspiegels ableiten. Dieser sehr umfassende Erklärungsansatz hat den Vorteil, mit sehr wenigen Einflußfaktoren auszukommen und auf unkalkulierbare Einwirkungen (wie z. B. Meteoriten-Einschläge) völlig verzichten zu können. Vermutlich ist das hier entwickelte Konzept eines ozeanischen anoxischen Events auch auf andere Bio-Events übertragbar.

May, A. (1996a): Relationship among sea-level fluctuation, biogeography, and bioevents of the Devonian: an attempt to approach a powerful, but simple model for complex long-range control of biotic crises. - Geolines, 3: 38-49, 2 figs.; Praha.

ABSTRACT. Connections among changing sea-level, development of marine biogeography and appearance of bioevents in Devonian times are emphasised. Decreasing Devonian provincialism of the marine fauna reflects rising sea-level of global ocean. Whereas the Emsian sea-level lowstand was accompanied by strong provincialism of the marine communities, the Frasnian highstands were linked to an unusual decrease of provincialism. As generally documented, the partial but intensive transgressional events (often related to bioevents) were usually associated with a decrease of the provincialism. The best explanation for the majority of Devonian bioevents is the effect of oceanic anoxia. A model suggested herein operates with a fast transgression related to fast ocean sea-level rise. A direct consequence of this fast transgression was the increased primary biomass production (caused by both increased infiux of nutrients and increased total surface of shallow seas). Decreased oxygen supply of oceans continued this process being caused by the global temperature increase which resulted from the albedo decrease. These consequences of these combined factors produced an oceanic anoxic event which started the extinction processes of the bioevent. The well-documented pattern of intensified biological crises during the Devonian, time-slice by time-slice (from Emsian to Frasnian), is explained as a direct result of decreased provincialism of the marine fauna. Both decrease of the provincialism and increased intensity of the Devonian bioevents can be deduced directly or indirectly from changes of global sea-level. This simply constructed but very complex explanation yields two advantages: first, it needs only very few infiuencing factors, second, it can basically renounce incalculable influences (e.g. meteorite impacts). The suggested concept of oceanic anoxic events can explain also other major bioevents of the Earth history.

KURZFASSUNG: Der Artikel versucht, Zusammenhänge zwischen den Veränderungen des Meeresspiegels, der Entwicklung der marinen Biogeographie und den Bioevents im Devon herauszuarbeiten und zu erklären. Die Entwicklung des Faunen-Provinzialismus im Devon ist ein Ergebnis der Entwicklung des globalen Meeresspiegels. Im Emsium war der Meeresspiegel niedrig und der Provinzialismus der Meeresfauna gross, im Frasnium war der Meeresspiegel hoch und der Provinzialismus sehr gering. Insbesondere einige Trangressions-Schübe, die sich auch als Bioevents bemerkbar machten, führten zu schnellen Abnahmen der Grösse des Provinzialismus. Die beste Erklärung für die meisten devonischen Bioevents ist das Auftreten von ozeanischen anoxischen Events. Das hier vorgestellte Modell setzt bei schnellen Transgressionen an, die durch schnelle Meeresspiegel-Anstiege verursacht wurden. Die Konsequenzen einer schnellen Transgression waren gesteigerte Primär produktion organischer Substanz und geringere Sauerstoff- Versorgung der Ozeane, was zusammen zum ozeanischen anoxischen Event und zum Bioevent führte. Der Umstand, dass die Intensität der Bioevents vom Emsium zum Frasnium zunahm, wird durch die Abnahme des Provinzialismus der Meeresfauna in dieser Zeit erklärt. Sowohl die Entwicklung des Provinzialismus der marinen Fauna als auch das Auftreten und die Intensität der Bioevents im Devon lassen sich direkt oder mittelbar aus der Entwicklung des globalen Meeresspiegels ableiten. Dieser sehr umfassende Erklärungsansatz hat den Vorteil, mit sehr wenigen Einfiussfaktoren auszukommen und auf unkalkulierbare Einwirkungen (wie z.B. Meteoriten-Einschläge) völlig verzichten zu können. Vermutlich ist das hier entwickelte Konzept eines ozeanischen anoxischen Events auch auf andere Bioevents übertragbar.

KEYWORDS: Devonian, Kellwasser Event, ecosystems, biogeography, sea level fiuctuation.