OAE2 Research Articles

Geochemical Events at the Cenomanian–Turonian Boundary: Relation of Anoxia with Volcanism, Climate, and Ocean Circulation

Abstract —The paper presents a detailed review of chemostratigraphic data on the Bonarelli oceanic anoxic event (OAE2) at the Cenomanian–Turonian boundary from different regions worldwide, with a focus on the respective redox conditions in the basins, metal anomalies, and δ13C excursions. Marine sediments with OAE2 signatures in Eastern Kamchatka are compared with their counterparts from submarine volcanic rises in the northwestern Pacific. The available isotope ages of lavas from some large igneous provinces are correlated with the age of OAE2. The Bonarelli event was triggered and governed by several interrelated factors: volcanism, climate, and ocean circulation. The supply of nutrients into oceans as a result of enhanced magmatism, hydrothermal activity, and weathering on continents played an important role in maintaining anoxia. The spatial patterns of black-shale deposition were controlled by ocean circulation. Geochemical and biochemical processes associated with high productivity ultimately led to its reduction and to the recovery of oxic conditions in oceans.

Quantifying the missing sink for global organic carbon burial during a Cretaceous oceanic anoxic event

The Cretaceous experienced numerous global and local climatic perturbations to the ocean–atmosphere system, especially during periods of known widespread organic-carbon burial termed oceanic anoxic events (OAEs). The Cenomanian–Turonian boundary event (∼93.9 Ma), or OAE-2, is the best documented and widespread organic carbon (OC) burial event in Earth history—with more than 170 sections published. Despite the substantial number of locations, the majority is found within the proto-Atlantic Ocean, Tethys Ocean and epicontinental seaways. It has been hypothesized that the pervasive burial of OC during OAE-2 caused the observed positive carbon isotope excursion (2 to 7‰, average ∼3‰). The isotope excursion can help constrain the global burial of OC, even for unstudied portions of the global ocean. This approach can solve for ‘missing’ OC sinks by comparing model estimates with the known distribution of OAE-2 sediments and their OC contents. Specifically, mapping the known spatial extent of OC burial in terms of mass accumulation rates (MARs), and comparing those results with the prediction using a forward box model to derive the amount of OC burial to reproduce the globally observed positive carbon isotope excursion. The available OC data from outcrop and drill core, with reasonable extrapolation to analogous settings without data, quantifies ∼13% of the total seafloor, mostly from marginal marine and epicontinental/epeiric settings. However, this extrapolation for OC burial, plus using most appropriate MARs to unknown portions of the seafloor, fail to account for the amount of OC burial predicted for a 3‰ positive carbon isotope excursion. This discrepancy remains even when considering additional sinks of organic carbon burial such as coal, lacustrine environments, authigenic carbonate, and the loss of OC associated with hydrocarbon reservoirs. This outcome points to a large reservoir of OC that is not currently constrained, such as highly productive margins and/or equatorial regions, or a small but significant increase deep ocean OC burial. Another possibility is that the carbon fluxes are less than those used in the model which would require less OC burial to explain a ∼3‰ carbon isotope excursion.

The Albian to Turonian carbon isotope record from the Shilaif Basin (United Arab Emirates) and its regional and intercontinental correlation

AbstractThe sedimentary record of the Arabian Shelf offers a unique opportunity to study the Cretaceous (Albian–Turonian) greenhouse climate from a palaeoequatorial perspective. In particular, hemipelagic to pelagic carbonate successions from the extensive Shilaif intra‐shelf basin have the potential to produce an excellent record of carbon cycle perturbations during this interval. This study presents a 269 m thick chemostratigraphic (carbonate δ13C and δ18O) record from the Middle Albian to Early Turonian of central Abu Dhabi (United Arab Emirates), representing over 14 Myr of uninterrupted carbonate sedimentation. The Mauddud to Shilaif formations represent outer ramp to basinal intra‐shelf carbonates with variations from laminated organic‐rich to clean bioturbated intervals. Isotopic evidence of the latest Albian Anoxic Event (Oceanic Anoxic Event 1d), Middle Cenomanian Event I and the Cenomanian–Turonian Anoxic Event (Oceanic Anoxic Event 2) are confirmed and biostratigraphically calibrated by means of calcareous nannofossils. The carbon isotope record allows correlation with other regional records and well‐calibrated records across the Tethyan Ocean and represents a significant improvement of the chronostratigraphic framework of the United Arab Emirates (Shilaif) and Oman (Natih) intra‐shelf basins. The study further confirms that low carbon isotope values corresponding to the two source rock intervals in the Shilaif Formation clearly precede the isotopic expressions of Oceanic Anoxic Event 1d and Oceanic Anoxic Event 2.

The ‘Black Band’: local expression of a global event

The organic-rich, black mudstones that were initially described as the Black Band in Lincolnshire, Humberside and Yorkshire are known to be a local representation of the Cenomanian–Turonian Boundary Event (CTBE). This world-wide event is known as Oceanic Anoxic Event ll (OAEll) and it marks a distinctive extinction event within the Cretaceous biota. Since some of the original work on the benthic foraminifera that are found in both the Black Band and coeval sedimentary rocks, there has been a significant increase in the understanding of the biology of foraminifera, and their response to both modern and fossil low-O 2 environments. While the overall event is clearly global, the local response appears to be a function of both geological setting and water depth with the occurrence of organic-rich sediments as a combination of this setting, plankton productivity and preservation.

The Cenomanian-Turonian Boundary Event (CTBE) in northern Lebanon as compared to regional data – Another set of evidences supporting a short-lived tectonic pulse coincidental with the event?

Constrained by both foraminifera and isotope data, the Zeitoun and Nahr Ibrahim sections in northern Lebanon record a steady flooding of the Cenomanian carbonate platform during the uppermost Cenomanian-lower Turonian interval. The flooding began just before the inception of the CTBE δ13C positive shift. It is marked by the deposition of finely bedded to laminated, fine-grained limestone overlying platformal carbonates. This scenario is entirely different to what has been published in northern Israel, where repeated emergences are instead recorded during the event. A survey of published data from nearby Middle East countries to North Africa suggests that the CTBE is accompanied on a larger scale, especially along the Syrian Arc, by tectonic disturbances responsible for sequence stratigraphic heterogeneity and palaeogeographic changes. Detailed analysis of available data makes it possible to extract from the sedimentary record four sequence stratigraphic scenarios implying two phases of heterogeneity bounding the event, and responsible for the outphasing of relative sea level changes in some sensitive areas. These disturb a stepped early Cenomanian to early Turonian overall transgressive trend recorded in some large stable areas, often used to support the concept of Mesozoic eustasy. The discussion focuses on the global processes that are possibly involved.The hypothesized tectonic pulse does not explain the CTBE. It is just another set of data to be taken into account in the search for causes.

Patterns of local and global redox variability during the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) recorded in carbonates and shales from central Italy

AbstractCareful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic carbon deposition. These comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events. Well‐documented examples are known from the Mesozoic Era, which is characterized by episodes of widespread organic carbon deposition known as Oceanic Anoxic Events. This organic carbon burial typically leads to coeval positive carbon‐isotope excursions. Geochemical data are presented here for several palaeoredox proxies (Cr/Ti, V, Mo, Zn, Mn, Fe speciation, I/Ca and sulphur isotopes) from a section exposed at Furlo in the Marche–Umbrian Apennines of Italy that spans the Cenomanian–Turonian boundary. Here, Oceanic Anoxic Event 2 is represented by a ca 1 m thick radiolarian‐rich millimetre‐laminated organic‐rich shale known locally as the Bonarelli Level. Iron speciation data for thin organic‐rich intervals observed below the Bonarelli Level imply a local redox shift going into the Oceanic Anoxic Event, with ferruginous conditions (i.e. anoxic with dissolved ferrous iron) transiently developed prior to the event and euxinia (i.e. anoxic and sulphidic bottom waters) throughout the event itself. Pre‐Oceanic Anoxic Event enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event. However, the greater global expanse of dysoxic to euxinic conditions during the Oceanic Anoxic Event greatly reduced redox‐sensitive trace‐metal concentrations in seawater. Pyrite sulphur isotopes document a positive excursion during the Oceanic Anoxic Event. Carbonate I/Ca ratios were generally low, suggesting locally reduced bottom‐water oxygen conditions preceding the event and relatively increased oxygen concentrations post‐event. Combined, the Furlo geochemical data suggest a redox‐stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event, globally widespread euxinic bottom waters during the Oceanic Anoxic Event, followed by chemocline shallowing but sustained local redox stratification following the event.

Depositional environment and source rock potential of Cenomanian and Turonian sedimentary rocks of the Tarfaya Basin, Southwest Morocco

Detailed organic and inorganic geochemical analyses were used to assess the depositional environment and source rock potential of the Cenomanian and Turonian oil shale deposits in the Tarfaya Basin. This study is based on core samples from the Tarfaya Sondage-4 well that penetrated over 300m of Mid Cretaceous organic matter-rich deposits. A total of 242 samples were analyzed for total organic and inorganic carbon and selected samples for total sulfur and major elements as well as for organic petrology, Rock-Eval pyrolysis, Curie-Point-pyrolysis-gaschromatography-Mass-Spectrometry and molecular geochemistry of solvent extracts. Based on major elements the lower Cenomanian differs from the other intervals by higher silicate and lower carbonate contents. Moreover, the molecular geochemistry suggests anoxic bottom marine water conditions during the Cenomanian-Turonian Boundary Event (CTBE; Oceanic Anoxic Event 2: OAE2). As a proxy for the Sorg/Corg ratio, the ratio total thiophenes/total benzenes compounds was calculated from pyrolysate compositions. The results suggest that Sorg/ Corg is low in the lower Cenomanian, moderate in the upper Cenomanian, very high in the CTBE (CenomanianTuronian Boundary Event) and high in the Turonian samples. Rock-Eval data reveal that the lower Cenomanian is a moderately organic carbon-rich source rock with good potential to generate oil and gas upon thermal maturation. On the other hand, the samples from the upper Cenomanian to Turonian exhibit higher organic carbon content and can be classified as oil-prone source rocks. Based on Tmax data, all rocks are thermally immature. The microscopic investigations suggest dominance of submicroscopic organic matter in all samples and different contents of bituminite and alginite. The lower Cenomanian samples have little visible organic matter and no bituminite. The upper Cenomanian and CTBE samples are poor in bituminite and have rare visible organic matter, whereas the Turonian samples change from bituminite-fair to bituminite-rich and to higher percentages of visible organic matter towards the younger interval. These differences in the organic matter type are attributed to i) early diagenetic kerogen sulfurization and ii) the upwelling depositional environment. Moreover, kerogen sulfurization was controlled by the relationship between carbonate, iron and sulfur as well as the organic matter. Thus, the organic carbon-rich deposits can be grouped into: i) low Sorg and moderately organic matter-rich oil prone source rocks, ii) moderate Sorg and organic-carbon-rich oil prone source rocks, iii) high Sorg and organic carbon-rich oil prone source rocks and iv) very high Sorg and organic carbon-rich oil prone source rocks, the latter representing the CTBE interval. Types 2 to 4 will generate sulfur-rich petroleum upon maturation or artificial oil shale retorting. This integrated organic and inorganic approach sheds light on the various processes leading to the development of the world-class oil shales deposited through the Cenomanian to Turonian. In addition, this study shows how the changes in the depositional environment might have controlled kerogen sulfurization and organic matter preservation and structure. This detailed approach provides a better understanding on source rock development during the Cenomanian to Turonian in a global context, as many of the geochemical features were identified worldwide for deposits related to OAE2.

The Cenomanian–Turonian Boundary Event (CTBE) on the southern slope of the Subalpine Basin (SE France) and its bearing on a probable tectonic pulse on a larger scale

The Cenomanian–Turonian Boundary Event (CTBE) event is not associated with a transgression on the southern margin of the Subalpine Basin, but with a steady shallowing-up trend beginning in the lower half of the δ13C positive shift. The SW–NE Rouaine Fault had a complex role, first in isolating a black shale basin to the west and a large, deep submarine plateau devoid of black shale to the east, then by a strike-slip movement that induced a forced progradation to the north of the southern platform in the eastern compartment. This compressive tectonic reactivation of the southern margin began around the deposition of the local equivalent of the Plenus bed of boreal basins, as shown by correlation supported by both isotope and palaeontological data. Other local data are pieced together to suggest that the whole of SE France underwent a short-lived transpressive tectonic pulse around the Cenomanian–Turonian boundary, probably connected with the early compressive movement of Africa vs. Europe. On a larger scale, other published data suggest that this pulse could be a global one. It is coeval with renewed thrust loading, volcanism and transgression in the North-American Western Interior, local emergences during the event along the eastern Atlantic margin, suggesting a slight tendency to inversion of the margin, and a tilting to the east of the North-Africa plate that could explain the large transgression recorded from Morocco to Tunisia on the Saharan Craton.New isotope and palaeontological (coiling ratio of Muricohedbergella delrioensis) data from SE France suggest that two coolings of suprabasinal importance occurred just before and during the build-up of the d13C shift, including the boreal “Plenus Marls“, especially its middle limestone bed and its SE France equivalent.Regarding the extinction of the genus Thalmaninella and Rotalipora and during the event, neither anoxia nor climate changes can fully explain the palaeontological crisis, given that Rotalipora cushmani crosses the first phase of anoxia without harm, as well as the two coolings, not only in SE France but on a large scale, as shown by the correlation of the published data. This extinction needs alternative explanations as we challenge both anoxia and climate as major causes.

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle

Understanding the global redox state of the oceans and its cause-and-effect relationship with periods of widespread organic-carbon deposition is vital to interpretations of Earth's climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency. Here, we present a compilation of new and published data from an organic-rich locality within the proto-North Atlantic Ocean during the Cenomanian–Turonian boundary event that shows a dramatic drawdown of redox-sensitive trace elements. Iron geochemistry independently suggests euxinic deposition (i.e., anoxic and sulfidic bottom waters) for the entire section, thus confirming its potential as an archive of global marine metal inventories. In particular, depleted molybdenum (Mo) and vanadium (V) concentrations effectively record the global expansion of euxinic and oxygen-deficient but non-sulfidic waters, respectively. The V drawdown precedes the OAE, fingerprinting an expansion of oxygen deficiency prior to an expansion of euxinia. Molybdenum drawdown, in contrast, is delayed with respect to V and coincides with the onset of OAE2. Parallel lipid biomarker analyses provide evidence for significant and progressive reorganization of marine microbial ecology during the OAE in this region of the proto-North Atlantic, with the smallest relative eukaryotic contributions to total primary production occurring during metal-depleted intervals. This relationship may be related to decreasing supplies of enzymatically important trace elements. Similarly, box modeling suggests that oceanic drawdown of Mo may have approached levels capable of affecting marine nitrogen fixation. Predictions of possible nitrogen stress on eukaryotic production, locally and globally, are consistent with the low observed levels of Mo and a rise in 2-methylhopane index values during the peak of the OAE. At the same time, the environmental challenge presented by low dissolved oxygen and euxinia coincides with increased turnover rates of radiolarian clades, calcareous nanofossils, and foraminifera, suggesting that the temporal patterns of anoxia/euxinia and associated nutrient limitation may have contributed to the fabric of OAE2-related turnover.

Bioevents and redox conditions around the Cenomanian–Turonian anoxic event in Central Mexico

The Xilitla section of central Mexico (western margin of the proto-North Atlantic) is characterized by pelagic sediments enriched in marine organic matter. Using biostratigraphic and radiometric data, it was dated at the latest Cenomanian–earliest Turonian transition. We identified an interval coeval with the faunal turnover associated with the Oceanic Anoxic Event 2 (OAE 2), recording the Heterohelix shift and the “filament event” for the first time in Mexico. An integral analysis of sedimentary facies, pyrite and geochemical proxies reveals vertically variable redox conditions, with prevailing anoxic to dysoxic bottom waters. Along with phosphorous and manganese depletion, the highest content of total organic carbon and of certain redox-sensitive trace elements (RSTEs) is found during part of the anoxic event, confirming more uniform and constant oxygen-depleted conditions. This interval is also characterized by a significant enrichment in biogenic barium and elevated TOC/NTOT ratios, suggesting a link between productivity and anoxia. Sulfur isotope fractionation has a maximum value within the anoxic event, favored by the increase in the flux of organic matter and intensified through sulfur recycling. Highly bioturbated beds representing short-lived episodes of oxic conditions are intermittent within the OAE 2 and become more frequent in the early Turonian. This study proposes a model similar to that of modern upwelling regions. High marine productivity controlled organic matter burial and oxygenation at the seafloor, varying between anoxic (laminated facies with small pyrite framboids) and dysoxic conditions (bedding-parallel bioturbated facies with inoceramid bivalves and large pyrite framboids), interrupted by short-term well-oxygenated episodes (thoroughly bioturbated facies with common benthic foraminifera). General low-oxygen conditions led to the formation of glauconite and pyrite (bacterially mediated); the enrichment of redox-sensitive trace elements in sediments (Cd, Zn, V and Cr scavenged by organic matter and Ni, Mo, Pb, Co and Re by pyrite) and resulted in Mn and P depletion.

Pseudorhapydionina bilottei sp. nov., an endemic foraminifera from the post-Cenomanian/Turonian boundary (Pyrenees, NE Spain)

A new species of the foraminiferal genus Pseudorhapydionina is reported from the Santonian shallow-water carbonate and mixed deposits of La Cova Unit, in the Montsec and Pedraforca thrust sheets (Southern Pyrenees, NE Spain). Pseudorhapydionina bilottei sp. nov. differs from the Cenomanian species of the genus in its larger test size and the number of chambers in its early planispiral-involute stage. The discovery of P. bilottei sp. nov in the Santonian indicates that the genus Pseudorhapydionina escaped extinction during the Cenomanian-Turonian boundary (CTB) event in the Pyrenean bioprovince. A new subfamily Pseudorhapydionininae is described.

Recognition of vegetation change associated with negative d13C excursion at the bottom of the Cenomanian-Turonian boundary event: A molecular paleontological approach

Recognition of vegetation change associated with negative d13C excursion at the bottom of the Cenomanian-Turonian boundary event: A molecular paleontological approach

Stable-isotope stratigraphy of the Cenomanian–Turonian (Upper Cretaceous) boundary event (CTBE) in Wadi Qena, Eastern Desert, Egypt

A high-resolution δ13C isotope record from Cenomanian–Turonian boundary interval of shallow marine successions in Egypt is presented. The δ13C curves show the typical features of the globally documented Cenomanian–Turonian positive excursion, including three of the main positive isotope peaks defining the Cenomanian–Turonian Boundary Event (CTBE). Based on high-resolution ammonite biostratigraphy, the CTBE started in the study area above the Late Cenomanian Neolobites vibrayeanus Zone within the Galala Formation, directly above the global sequence boundary Cenomanian 5 (SB Ce 5). A stratigraphic gap at that level cuts out the lower a-peak of the CTBE. The Cenomanian–Turonian boundary is located within the upper part of the positive excursion between carbon excursion peaks c and d, coinciding with the boundary between the Late Cenomanian Vascoceras cauvini and the Early Turonian Vascoceratid zones. The CTBE ended up-section of peak d, at the base of the Choffaticeras spp. Zone. The amplitude of the positive δ13C excursion in Egypt is very high (reaching 6.5‰ vs. V-PDB) and largely matches curves of European standard sections and others localities from different basins. Furthermore, the Lower Turonian Holywell Isotope Event, an important marker within the lowermost Turonian, has tentatively been recognized. The positive carbon stable isotope curves presented herein represent the outreach of the oceanic anoxic event (OAE) 2 in shallow-water nearshore sequences.

Paleoenvironmental and paleoceanographic changes across the Cenomanian–Turonian Boundary Event (Oceanic Anoxic Event 2) as indicated by foraminiferal assemblages from the eastern margin of the Cretaceous Western Interior Sea

Two sites near the eastern margin of the Cretaceous Western Interior Sea (WIS) were investigated. The section at Cuba, Kansas (CK) is ~630km east of the GSSP for the Cenomanian/Turonian boundary at Rock Canyon, Colorado (RC), and the section near Sioux City, Iowa (SCI) is ~315km northeast of the Cuba site. Surprisingly, planktic foraminifera dominate all the studied samples despite the relative proximity of the sites to the paleo-shoreline and presumed neritic water depths. Such dominance suggests inhospitable benthic environments, and benthic foraminiferal assemblages indicate that seafloor oxygen in the WIS decreased eastward. Therefore, the hypothesis of an influx of freshwater into the WIS from the western margin forming a fresh water cap that diminished eastward is not supported. Benthic foraminifera are scarce in most studied samples and species diversity is low. In the CK section, diversity and abundance increase abruptly in the uppermost Cenomanian Benthonic Zone at the initiation of the δ13Corg positive excursion marking the onset of Oceanic Anoxic Event (OAE 2). In the SCI section, however, the Benthonic Zone is recognized only by presence of Nodosaria bighornensis that has a stratigraphic range restricted to this interval; other benthic taxa are very rare or absent. The favorable conditions of the Benthonic Zone were short-lived. While relatively diverse planktic foraminiferal assemblages, including keeled species, characterize the initial δ13C excursion at the CK site, dwarfed specimens of Heterohelix and Hedbergella dominate the assemblages at the more proximal Sioux City site. Furthermore, most of the biostratigraphic events recognized in the basin center RC section can be traced into the CK section, but not into the SCI section, including the Rotalipora spp. and Globigerinelloides bentonensis extinctions, the Heterohelix shift event, and the brief benthic recovery event. Correlation between eastern WIS sites shows that the SCI section is thicker, suggesting a major sediment source to the east. This may explain the inhospitable benthic conditions and development of a ‘dead zone’ (seasonal hypoxia) analogous to the modern shelf of the northern Gulf of Mexico proximal to active discharge of the Mississippi River. Development of estuarine circulation in the WIS provided the eastern part of the basin with input of calcareous plankton from the south including planktic foraminifera. Foraminiferal assemblages, total organic carbon (TOC), and δ13C data of the studied sections suggest a two-fold history of Greenhorn transgression and OAE 2 development. The initial phase is a global signal characterized by a positive δ13C excursion, low TOC values, and high foraminiferal species diversity. The second phase is characterized by overprinting by local conditions in the WIS, including high fluvial input, relatively high TOC values, and low foraminiferal species diversity. These findings suggest that the eastern margin records unique depositional and biotic environments further revealing the dynamic and complex nature of the WIS and its sedimentary cycles.

Carbon isotope stratigraphy, magnetostratigraphy, and 40Ar/39Ar age of the Cretaceous South Atlantic coast, Namibe Basin, Angola

We present the δ13C and paleomagnetic stratigraphy for marine strata at the coast of southern Angola, anchored by an intercalated basalt with a whole rock 40Ar/39Ar radiometric age of 84.6±1.5Ma, being consistent with both invertebrate and vertebrate biostratigraphy. This is the first African stable carbon isotope record correlated to significant events in the global carbon record spanning the Late Cenomanian to Early Maastrichtian. A positive ∼3‰ excursion seen in bivalve shells below the basalt indicates the Cenomanian–Turonian Boundary Event at 93.9Ma, during Oceanic Anoxic Event 2. Additional excursions above the basalt are correlated to patterns globally, including a negative ∼3‰ excursion near the top of the section interpreted as part of the Campanian–Maastrichtian Boundary Events. The age of the basalt ties the studied Bentiaba section to a pulse of Late Cretaceous magmatic activity around the South Atlantic and significant tectonic activity, including rotation, of the African continent.

Mexican archives for the major Cretaceous Oceanic Anoxic Events

Oceanic Anoxic Events (OAEs) are interpreted as brief episodes of oxygen-depleted conditions in the global ocean that resulted from profound perturbations in the carbon cycle. These events favored widespread deposition of organic carbon-rich sediments and the subsequent formation of hydrocarbon source rocks. The most important of these events for the Cretaceous period are the globally recognized OAE 1a (early Aptian, Selli event), the OAE 2 (Cenomanian/Turonian boundary, Bonarelli event), and the Atlantic-restricted OAE 3 (Coniacian/Santonian boundary). In Mexico, several sedimentary successions of these ages are proved hydrocarbon source rocks and potential targets for oil and gas shale exploration; however, in most cases, it is unknown how these global events influenced redox conditions under which they were deposited. In general, there is little research to document and characterize properly these events. This work exposes and analyzes the current state of the study of these events in Mexico, and proposes new stratigraphic units to prospect and methodologies for further studies. The OAE 1a has been isotopically constrained in the northeastern part of the country within sediments with high organic carbon content in the lowermost part of the La Pena Formation. However, recent research suggests that the base of the La Pena Formation seems isochronous and younger than the OAE 1a. Accordingly, this event must be recorded in the underlying sediments of the Cupido/Lower Tamaulipas formations. Because of its age and lithostratigraphic features, the Agua Salada Formation of the Lampazos Platform also seems to be linked to this event. The OAE 2 has been documented in northeastern Mexico in the Agua Nueva and Indidura formations, and in southern Mexico in the uppermost part of the Morelos Formation. Trace metal enrichment in these rocks indicates that the emplacement of the Caribbean plateau probably played an important role in the record of this anoxic event across Mexico. Poorly oxygenated conditions during the Cenomanian/Turonian in northeastern Mexico lasted until the early Coniacian. Other stratigraphic units that probably record this event are the Agua Nueva, Eagle Ford, Soyatal, and Maltrata formations. The record of the OAE 3 remains unknown. We hypothesize that Coniacian/Santonian Mexican paleogeography and sedimentary pattern could trigger at least intermittent anoxic/dysoxic conditions favorable for organic carbon burial, and suggest searching for these conditions in the San Felipe, Indidura or Austin formations.

Depositional environments and sequence architecture of the Raha and Abu Qada formations (Cenomanian–Turonian), west central Sinai, Egypt

Cenomanian–Turonian deposits are important reservoirs for many oil fields in the Western Desert and the Gulf of Suez region of Egypt. Study of the Raha and Abu Qada formations (Cenomanian–Turonian), from five dip-oriented outcrop locations in west central Sinai; indicates deposition of a mixed siliciclastic-carbonate system on a ramp setting. The inner ramp facies (bivalve and benthonic foraminiferal wackestone) grades northward to the mid ramp facies (echinoderm calcisphere packstone, and oyster floatstone), and outer ramp facies (planktonic foraminiferal wackestone and calcisphere wackestone).The two studied formations comprise one second-order depositional sequence (duration of approximately 10 Million years). This large scale sequence includes four third-order depositional sequences, three of which are observed in the Raha Formation, with the other one recorded in the Abu Qada Formation. Because west central Sinai was tectonically stable during the Cenomanian and Turonian, the main factor controlling the lateral and vertical distribution of facies tracts is likely due to changes in the relative sea level. The Cenomanian–Turonian boundary event is known as the largest oceanic anoxic event during the Cretaceous. This global event has been documented in three of the studied sections. The recorded δ13C excursions range from+3.04‰ to+5.24‰. These high positive excursions in δ13C are associated with highly negative values of δ18O (values range from −6.01‰ to −1.38‰).

Soft–part preservation in heteromorph ammonites from the Cenomanian–Turonian Boundary Event (OAE 2) in north–west Germany

Abstract: From thinly laminated marlstones of the Hesseltal Formation, representing the Late Cenomanian Oceanic Anoxic Event (OAE) 2, at Lengerich in the Teutoburger Wald (Westfalen, north‐west Germany), 17 sediment‐compacted baculitid ammonites with carbonised and partially phosphatised soft parts are recorded. Some preserve remains of the buccal mass, including jaws (occasionally articulated) and radulae, as well as of the cephalic cartilage, such as eye capsules. Such have not yet been recorded previously for the order Ammonoidea. In addition, originally unmineralised parts found preserved in these specimens include extensive portions of the digestive tract, the siphonal tube, false colour patterns (megastriae), as well as traces of what would appear to be the oviduct. At the same levels, patches with numerous isolated horny upper and rarer lower jaws as well as radulae occur; these may represent regurgitates or faeces of larger predators. The cephalopod remains described were deposited in an epicontinental setting, possibly at palaeodepths between 200 and 600 m. In this particular Late Cretaceous fossil Lagerstätte, upper jaws and anaptychi of ammonites rank among the commonest fossils.

Geochemical and palaeoecological record of the Cenomanian–Turonian Anoxic Event in the carbonate platform of the Preafrican Trough, Morocco

Geochemical analyses (major and trace elements) and palaeontological studies were carried out based on rock samples and thin sections collected from three outcrop sections (Goulmima, Ziz and Tazzouguert) in the upper Cenomanian–lower Turonian in the carbonate platform of the Preafrican Trough (South Morocco), in order to determine if oxygen-depleted conditions affected the platform, and to highlight the possible impact of oxygen variation on carbonate production, particularly during the Oceanic Anoxic Event 2 (OAE2), which took place in the late Cenomanian.During this interval, the carbonate platform was composed of two contrasting environments. A shallow-water environment (peritidal to mid-ramp) was developed in the East (Ziz and Tazzouguert sections), characterized by a high diversity and abundance of benthic organisms, and in some cases, by a high diversity of planktonic organisms, associated with low concentrations of authigenic markers (U, Mo, V and Cr: redox proxies), indicating oxic conditions. In the West (Goulmima section), the environment was deeper (mid ramp/outer ramp) and redox proxies record a high enrichment, an indicator of reducing conditions, associated with a low diversity and abundance of organisms and the presence of Buliminidae. Bottom waters were dysoxic and the palaeoproductivity was low at the sea surface. Thus, during the late Cenomanian, oxygenation conditions were relatively good over the area of the Preafrican Trough, even during the OAE2, except in the deeper environments toward the West, where bottom waters were depleted in oxygen. During the Cenomanian transgression, and more precisely during the OAE2, this dysoxia was linked to the influx of poorly-oxygenated waters from the deeper anoxic basins of the Atlantic and/or Tethys oceans.Geochemical, palaeontological and sedimentological disturbances are recorded in the lower Turonian everywhere in the outer-ramp environments of the platform. Palaeontological proxies show a huge decrease in group diversity, with the disappearance of benthic foraminifera – except for the Buliminidae which proliferated – and only shallow-water planktonic foraminifera survived. Geochemical proxies of redox conditions and palaeoproductivity show a marked enrichment in all three outcrop sections, with higher values in the West. This dysoxia occurred in response to a climatic change, from warm with contrasting seasons to hot and wet, which induced an increase of chemical weathering in the Anti-Atlas to the southwest of the platform. This substantial input of nutrients increased the palaeoproductivity, which finally enhanced the consumption of O2 owing to the decay of organic matter. Moreover, the late Cenomanian transgression led to the supply of additional nutrients to the platform, due to the leaching of soils.

The Cenomanian/Turonian Boundary Event (CTBE) at Tarfaya, Morocco, northwest Africa: Eccentricity controlled water column stratification as major factor for total organic carbon (TOC) accumulation: Evidence from marine palynology

From the exploration core S75 that covers the Cenomanian/Turonian Boundary Event (CTBE) and lower Turonian at Tarfaya, Morocco, northwest Africa, 96 samples from a section 24.5 m thick was analysed palynologically. The palynological data are discussed with various data on isotope- and bio-geochemistry as well as on time series analyses published by Kolonic et al. (2005, Paleoceanography 20, PA 1006) and Kuhnt et al. (2005, Int. J. Earth Sci. 94, 147–159). The dinocyst assemblage is of low to moderate diversity and strongly dominated by peridinioids throughout most of the section. It is suggested, that potentially eccentricity controlled cycles of increased total organic carbon (TOC) accumulation are closely related to increases of the ratio in proportions of peridinioid to gonaulacoid dinocysts (p/g ratio) and proportions of Botryocccus as well as of Bosedinia spp., a peridinioid cyst of probable strongly reduced-salinity water. At higher resolution, however, episodic increases in the p/g ratio are more punctuated in distribution and largely inversely correlated to abundances of Bosedinia spp. It is suggested that warm-humid climatic boundary conditions prevailed in the Tafaya region, episodically superimposed by upwelling events, reflected by increased proportions of Palaeohystrichophora spp. and Subtilisphaera spp. Accordingly, enhanced preservation, supported by strong episodic production pulses, is proposed for the TOC accumulation in the Tarfaya Basin. The p/g ratio peaks decrease both in prominence and frequency when pre-excursion values of δ13C isotopes are re-established after the positive anomaly. This may reflect a moderate shift in control from production to preservation of TOC following the CTBE proper. With respect to δ18O isotopes, episodes of pre-excursion values, possibly reflecting moderate cooling intervals and eustatic sea-level fall, are characterised by the appearance of dinocyst taxa diagnostic of neritic waters. This may reflect the gradual change from pelagic sedimentation characteristic of the plateau- and early recovery-period of δ13C isotopes towards neritic conditions within the late recovery and post-excursion period. Biomarkers suggestive of photic zone oxygen deficiency show only a limited relationship to the distribution of the different algal groups. However, according to the quantitative distribution of prasinophytes, considered to be critically controlled by ammonium availability, denitrification was more prominent during the late Cenomanian interval of the section and decreased significantly with the onset of the Turonian.