Cenomanian-Turonian Boundary Interval Research Articles

Late Cretaceous (Cenomanian-Turonian) temperature evolution and biotic response in the Adriatic Carbonate Platform region of Friuli, northeast Italy

Climate modelling and palaeotemperature reconstructions derived from several proxies (TEX86, δ18O) suggest that the Late Cretaceous was one of the warmest intervals in Earth history. The greenhouse climate reached its acme near the Cenomanian-Turonian boundary (CTB) interval with sea surface temperatures over 35 °C in low and mid- palaeolatitudes, coinciding with the highest sea level stands of the Phanerozoic. Following this warming phase, a general cooling trend punctuated by short-term hot snaps characterized the rest of the Cretaceous. Palaeotemperature and palaeoenvironmental changes are well documented in deep-water carbonate deposits based on variations in geochemical proxies and distribution patterns of macro- and microfauna. However, such climatic and environmental fluctuations severely impacted carbonate platforms, sedimentary environments that are particularly sensitive to external perturbations. Palaeontological data show major changes within the main carbonate platform producers (rudists and benthic foraminifera) during the early Late Cretaceous. Both groups experienced major taxonomic turnovers at the CTB, followed by a further re-radiation in the middle/late Turonian. The lack of detailed palaeoclimatic reconstructions in shallow water carbonate succession hampers the possibility to investigate a possible causal relationship between temperature changes and faunal evolution. In this paper, we report integrated geochemical, sedimentological, and palaeontological data from the Adriatic Carbonate Platform (Friuli, north-east Italy). Benthic foraminifera biostratigraphy and isotope stratigraphy (δ13C and 87Sr/86Sr) were used to precisely constrain the stratigraphy of the late Cenomanian-middle Turonian interval. δ18O analysis on both well-preserved rudist shells and bulk rock samples enabled the construction of high-resolution palaeotemperature curves. The resulting temperature trends are comparable with those from the deep-water record across the late Cenomanian-Turonian interval. The warmest phase was in the Early Turonian with sea surface palaeotemperatures as high as 41–45 °C recorded in rudist shells. Data show temperature fluctuation from the late Cenomanian to the Early Turonian with a magnitude >7 °C. Benthic foraminifera and rudist evolution responded to these palaeotemperature fluctuations, and their decline in the late Cenomanian and full recovery in the Turonian suggest a strong link with temperature changes.

Benthic foraminiferal response to the Oceanic Anoxic Event 2 (Late Cretaceous) and evidence of bottom water re‐oxygenation during the Plenus Cold Event at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian-Turonian boundary interval is characterized by environmental perturbations related to the Oceanic Anoxic Event 2 (OAE 2) that severely affected the marine biota, including benthic and planktonic foraminifera. We present a continuous high-resolution benthic foraminiferal record in combination with published planktonic foraminiferal and geochemical data across the Cenomanian-Turonian boundary interval in the Clot Chevalier section (Vocontian Basin, SE France) with the aim to interpret paleoceanographic changes in bottom waters. Benthic foraminiferal assemblages are characterized by low diversity and indicate an outer shelf-upper bathyal environment. Changes in composition and abundance of the benthic assemblages throughout the stratigraphic section allow identification of seven distinct environments. In the middle Cenomanian, below the OAE 2, benthic foraminifera show high species diversity and abundance of infaunal and epifaunal taxa, suggesting an environment characterized by oxic conditions at the seafloor. The microfossil assemblages in the upper Cenomanian within the OAE 2 interval are dominated by radiolaria (> 50%) followed by benthic calcareous taxa (Gavelinella sp., Gyroidinoides sp., Praebulimina elata, Tappanina laciniosa). Benthic agglutinated taxa (Ammobaculites sp., Ammodiscus cretaceus, Gaudryina sp., Textulariopsis bettenstaedti) show high abundance in the lowest part of the OAE 2 and disappear close to the top of the interval. The assemblage composition and the highest values of Total Organic Carbon (TOC) registered in this interval suggest eutrophication and suboxic conditions at the seafloor. However, at ∼1 m above the onset of the OAE 2, the repopulation by inferred oxic benthic foraminiferal taxa (i.e., Frondicularia sp. and Ramulina aculeata) suggests ventilation of bottom water masses, recording the Plenus Cold Event (PCE). In the lowermost Turonian, the assemblages exhibit high species diversity and abundance with the re-appearance of calcareous taxa, indicating oxic conditions, and of agglutinated taxa associated to moderate oxygen concentrations. Results show that the distribution of benthic foraminifera follows fluctuations in oxygenation and carbon export flux at the seafloor interpreted as the environmental changes associated to the onset of the OAE 2 and of the PCE across the Cenomanian-Turonian boundary interval.

Chemostratigraphy of the lower Danubian Cretaceous Group (Cenomanian–lower Turonian, Bavaria, SE Germany)—A new carbon isotope reference curve and inter-basinal correlation

Core section KB 06-2020, drilled north of Regensburg (SE Germany), covering the lower Danubian Cretaceous Group, has been logged in detail and sampled for chemostratigraphic investigation throughout the Cenomanian–lower Turonian. Resting unconformably on Upper Jurassic carbonates, the succession includes the Regensburg (lower Cenomanian–middle upper Cenomanian), the Eibrunn (Cenomanian–Turonian boundary interval), the Winzerberg (lower Turonian; Reinhausen and Knollensand members) and the Kagerhöh formations (middle Turonian, Eisbuckel and Pulverturm members only). The correlation of carbon stable-isotope trends from core section KB 06-2020 with the expanded Dover section (southern England) and the Bad Abbach section (south of Regensburg) provides evidence for a diachronous, NE-directed onlap of the Cretaceous strata onto the Bohemian Massif. Several Cenomanian carbon isotope events have been recognised (Mid-dixoni, P/B Break, Monument as well as the b1 and b2 peaks) while the selective absence of events above conspicuous sedimentary unconformities provides evidence for stratigraphic gaps associated with sea-level falls. A significant positive carbon isotope excursion in the Cenomanian–Turonian boundary interval, Oceanic Anoxic Event 2 (OAE 2), has been recorded in the Eibrunn Formation. Correlation with the European reference section from Eastbourne (southern England), Pont d'Issole (southern France), and the regional Grub section provides evidence of a rather complete development of OAE 2 in the Danubian Cretaceous Basin. The δ13C curve obtained from core section KB 06-2020 is proposed as a regional reference curve. Additionally, the Plenus Cold Event (PCE) is recognisable in the oxygen stable-isotope data.

Biostratigraphic and chemostratigraphic record of the Cenomanian/Turonian boundary interval of the Tellian Atlas (Algeria)

A multi-proxy approach integrating data on planktonic foraminifera, calcareous nannofossils, δ13Ccarb, δ18Ocarb and the coiling direction of Muricohedbergella delrioensis was carried out for two sections in the Tellian Atlas (Algeria) both to establish a precise stratigraphic scheme and to precise the climatic events.The Sour el Ghozlane section offers a good record of the Oceanic Anoxic Event 2 (OAE2) in the southern margin of the Tethys, with organic-rich sediments and the continuous presence of planktonic foraminifera, nannofossils and other microfossils. The Cenomanian/Turonian boundary (CTB) in the Sour el Ghozlane section lies in the interval limited at the base by the last occurrence of Helenea chiastia and the first occurrence of Quadrum gartneri respectively located immediately below and above the last peak (C) of the carbon isotope excursion. However, in the Sidi Nadji section, the CTB interval is extremely thin; the last occurrences of Rotalipora cushmani and Thalmanninella greenhornensis coincide with the last occurrences of Corollithion kennedyi, Helenea chiastia, Axopodorhabdus albianus and Rhagodiscus asper and occur in the same level, therefore a hiatus is suspected.Sour el Ghozlane and Sidi Nadji are so far the southernmost sections where the percentage of sinistral M. delrioensis has been investigated. High proportion of left-coiled morphotypes among M. delrioensis indicate transient cooling episodes, including the Plenus Cold Event at the onset of OAE2 and before the warm conditions at the CTB characterized by the dominance of dextral forms.

Exploring the paleoceanographic changes registered by planktonic foraminifera across the Cenomanian-Turonian boundary interval and Oceanic Anoxic Event 2 at southern high latitudes in the Mentelle Basin (SE Indian Ocean)

Planktonic foraminiferal population dynamics and benthic foraminiferal and radiolaria distributions combined with δ13C and δ18O measurements of both bulk carbonate and foraminifera provide clues concerning the paleoceanographic changes across the Cenomanian-Turonian boundary interval and the Oceanic Anoxic Event 2 (OAE 2) at southern high latitudes. Samples analyzed are from Integrated Ocean Discovery Program (IODP) Expedition 369 Site U1516 in the Mentelle Basin (eastern flank of the Naturaliste Plateau, Indian Ocean, SW Australia). Site U1516 was located at 60°–62°S paleolatitude during the mid-Cretaceous, and it is the first high latitude locality in the Southern Hemisphere where planktonic foraminifera are consistently recorded across the OAE 2 interval and its associated positive δ13C excursion. The sedimentary record at Site U1516 consists of a sequence of alternating black, dark greenish gray, and light greenish gray claystone in the Cenomanian that grade to white and light gray calcareous chalk interbedded with chert in the Turonian. The correlation between the δ13C and δ18O profiles at Site U1516 and the European reference section at Eastbourne (England) coupled with the integrated calcareous plankton biostratigraphy and stable isotopic data at Site U1516, indicate that a complete record of OAE 2 at Site U1516 was recovered. Below and in the lower part of OAE 2, the planktonic foraminiferal assemblages are dominated by small-sized (125–38 μm) opportunistic species of Microhedbergella and radiolaria indicating a dominantly eutrophic regime. Above the onset of OAE 2, a trough in the δ13C profile (Plenus Carbon Isotope Event: P-CIE) coinciding with a δ18O increase may correspond to the Plenus Cold Event as observed at low latitudes, although no evidence of cooling is registered in the microfossil assemblages. At Site U1516, the middle part of OAE 2 at the initiation of the plateau phase of the δ13C profile is masked by absence of carbonate, by the highest TOC values, and high biogenic silica (dominance of radiolaria) indicating this interval corresponded to a time of highly stressed eutrophic conditions with possible shoaling of the Carbonate Compensation Depth (CCD). Above this interval, bulk isotopic results yield lower δ13C values, and the CaCO3 increases are associated with the presence of even smaller-sized Microhedbergella showing cyclic fluctuations in absolute abundances with benthic foraminifera indicating dominantly eutrophic conditions likely affected by upwelling of nutrient-rich and δ13C-depleted intermediate water masses. Toward the top of OAE 2 and across the Cenomanian-Turonian boundary interval, the planktonic foraminiferal assemblages show changes in composition (e.g., Microhedbergella is replaced by Muricohedbergella), species occupying relatively deep ecological niches appear and an overall increase in diversity is observed. These features coupled with the foraminiferal species-specific δ13C and δ18O patterns reveal that Site U1516 occupied a paleoceanographic setting still affected by eutrophy likely related to enhanced input of nutrients but with episodes of stability with ecological/thermal separation in the surface waters. This interval also records the highest sea surface water paleotemperatures values estimated as 20°–23°C based on δ18O values of foraminiferal test and assuming seawater δ18O values of −1‰V-SMOW. Mesotrophic to oligotrophic conditions persisted after the OAE 2 and throughout the Turonian as evidenced by a diverse planktonic foraminiferal assemblage with different species occupying separate ecological niches in the mixed layer and thermocline.

The Cenomanian–Turonian boundary event in Egypt: Foraminiferal turnover and carbon isotopic change

This study deals with the integration between geochemical data (carbon isotope and, total organic carbon) and foraminiferal turnover to track the Oceanic Anoxic Event 2 (OAE2) from the OCT-A8st1 well, which located in October field, central part of Gulf of Suez, Egypt. The studied well is subdivided into two formations, namely the Galala (Early–Late Cenomanian) and Abu Qada (Late Cenomanian–Early Turonian). Concerning the foraminiferal assemblage, agglutinated foraminifera dominate the Galala Formation, while planktonic taxa only occur within a few levels in the Abu Qada Formation. Due to the absence of keeled planktonic forms, the biostratigraphic framework is established on the integration between benthonic and planktonic foraminifera. Three biozones are recognized, including Thomasinella aegyptia–Merlingina cretacea Interval Zone (Early–Middle Cenomanian), Thomasinella fragmentaria–Praebulimina aspera Interval Zone (Middle–Late Cenomanian), and Whiteinella archaeocretacea Taxon Range Zone (Late Cenomanian–Early Turonian). Therefore, the Cenomanian–Turonian boundary interval (CTBI) is placed within the lower part of the Abu Qada Formation, which is characterized by the presence of black shales and low richness of both foraminifera and ostracods. A remarkable increase in TOC values is observed close to the CTBI. Also, the carbon isotope curve shows a distinctive positive δ13C anomaly at the same level of the maximum TOC. The comparison between the recorded δ13C data and those obtained from other sections worldwide, indicates that the base of the Turonian is placed directly above the δ13C excursion. Furthermore, the recorded foraminiferal community shows evidence for the deepening of the environment and extension of the oxygen minimum zone during the CTBI. This conclusion is based on a sequence of bioevents starting with the disappearance of some agglutinated benthic foraminifera, followed by the dominance of opportunistic groups including buliminids, whiteinellids and heterohelicids. Consequently, the studied well may be considered as a local reference section for OAE2.

Regional chronostratigraphic synthesis of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) interval, Western Interior Basin (USA): New Re-Os chemostratigraphy and 40Ar/39Ar geochronology

AbstractFluctuations in depositional conditions during the onset of severe climate events in Earth history predispose stratigraphic archives to hiatuses, often hindering complete reconstructions of paleoclimate events and their triggers. Several studies have proposed that a hiatus of unknown duration exists at the base of Oceanic Anoxic Event 2 (OAE2) in the North American Western Interior Basin at the base Turonian global boundary stratotype section and point (GSSP) in Pueblo, Colorado, which potentially influences integrated radioisotopic, biostratigraphic, and astrochronologic age models of the Cenomanian-Turonian boundary interval. To quantify the duration of this hiatus, refine the chronology of OAE2, and assess marine geochemical perturbations associated with the onset of the event, we present new 40Ar/39Ar dates from regional bentonites along with a new proximal-distal chemostratigraphic transect of the epeiric Western Interior Basin (WIB), including initial osmium isotope (Osi) and stable carbon isotope (δ13C) data. The new 40Ar/39Ar age determinations confirm and further constrain previous estimates of Cenomanian-Turonian boundary timing. Further, the regional chemostratigraphic synthesis demonstrates the conformity of the OAE2 successions correlated to Pueblo, shows that the duration of the lag between the onset of the Osi and δ13C excursions is ∼60 k.y., and thus constrains the magnitude of the pre-OAE2 hiatus in Pueblo to less than this value. The new astronomically tuned, conformable Osi record across the onset of OAE2 captures a geologically rapid onset of large igneous province volcanism, consistent with other records, such that the addition of CO2 to the ocean-atmosphere system may have driven changes in marine carbonate chemistry. Additionally, the refined chronostratigraphy of OAE2 and the Cenomanian-Turonian boundary in the central WIB improves correlation with other records, such as those in the Eagle Ford Group, Texas. The correlations highlight that discrepancies among OAE2 age models from globally distributed sections commonly stem from differing definitions of the event and uncertainties associated with astronomical tuning, in addition to stratigraphic preservation.

Cretaceous (Albian-Turonian) calcareous nannofossil biostratigraphy of the onshore Cauvery Basin, southeastern India

A suite of outcrop samples from the Cauvery Basin belonging to the mudrock-claystone dominated Karai Formation were analysed for nannofossil biostratigraphy in two newly measured sections at Karai and Garudamangalam. The age of the Karai Section is interpreted as early Albian to early Turonian, whereas the Garudamangalam Section is interpreted as late Albian to late Cenomanian. The Albian ‘BC’ zones of Bown et al. (1998) are applicable in both sections, whereas the Cenomanian and Turonian ‘UC’ zones of Burnett (1998), are only partially applicable, due to some problematic primary and secondary markers. The Albian-Cenomanian boundary appears to be continuous and is approximated in both sections using the FO of lower Cenomanian ammonites of the M. mantelli Zone. The Cenomanian-Turonian boundary interval is incomplete in the two sections, with a hiatus of ~0.66 myr, indicated by the absence of the upper Cenomanian Nannofossil Zone UC5. The nannoplankton assemblages are composed of broadly cosmopolitan taxa, despite the relatively high-latitude setting of SE India during the Albian (~45°S), which is reflected in the common occurrence of biogeographically bipolar taxa such as Repagulum parvidentatum and Seribiscutum primitivum. The palaeobiogeographic affinity of the nannoplankton, however, does not bear a distinct Austral stamp, as typical Austral taxa, such as Sollasites falklandensis and Zeugrhabdotus kerguelenensis are very rare in the studied sections. The early appearance of Crucibiscutum hayi in the lower Albian, and Gartnerago segmentatum in the upper Albian in the Cauvery Basin suggests that these two species may have originated in southern high latitudes before migrating to the northern Boreal regions. Four new calcareous nannofossil species, Calculites karaiensis, Loxolithus bicyclus, Manivitella fibrosa and Tranolithus simplex are described.

An Albian–Turonian shallow-marine carbonate succession of the Bey Dağları (Western Taurides, Turkey): biostratigraphy and a new benthic foraminifera Fleuryana gediki sp. nov.

The studied Cretaceous succession is exposed at the Toçak Mountain in the southeastern part of the Bey Dağları Carbonate Platform (BDCP). The Alakır outcrop section presents seemingly continuous shallow-marine carbonate sedimentation during the Albian–Turonian times which is known from very few parts of the peri-Mediterranean platforms. Approximately 550 m thick platform carbonate succession is unconformably overlain by carbonate breccia/conglomerate and pelagic limestones of Campanian age. A benthic foraminiferal biostratigraphic zonation scheme is presented. The biozones are tentatively placed into the stages, without chronostratigraphic calibration, based on stratigraphic distribution of common benthic foraminifera in the peri-Mediterranean platforms. Protochrysalidina elongata–Cuneolina pavonia assemblage zone and Coskinolinella bariensis taxon range subzone are assigned to the upper Albian. In the conformably overlying limestones, Sellialveolina gr. viallii taxon range zone (lower–middle Cenomanian), Pseudorhipidionina casertana assemblage zone (upper Cenomanian), Pseudorhapydionina dubia–Pseudolituonella reicheli assemblage zone (uppermost Cenomanian) and Pseudocyclammina sphaeroidea assemblage zone (Turonian) have been distinguished. Two-step pattern of extinction of benthic foraminifera across the Cenomanian–Turonian boundary interval, which was first recorded from the Apennine Carbonate Platform, has been documented also from the BDCP. The tentative Cenomanian–Turonian Boundary is constrained into an interval of one meter between the last occurrence of the Cenomanian larger foraminifera and the first occurrence of the Turonian benthic foraminiferal assemblage comprising morphologically-simple and less-known taxa and other taxa left in open nomenclature. Fleuryana gediki sp. nov. is described from the Turonian.

Ichnological analysis of the Cenomanian–Turonian boundary interval in a collapsing slope setting: A case from the Rio Fardes section, southern Spain

The Cenomanian–Turonian Boundary interval (CTB) at the Rio Fardes section (Betic Cordillera, southern Spain) is characterized by pelagic and hemipelagic deposition mixed with mass-flow conglomerates and sandstones. The Rio Fardes section lends itself to high-resolution ichnological analysis of the late Cenomanian Oceanic Anoxic Event (OAE-2) on a collapsing slope affected by gravity-flow sedimentation. The trace fossils include Alcyonidiopsis isp. A., Alcyonidiopsis isp. B., Chondrites intricatus, C. stellaris, C. targionii, Planolites isp., ?Pilichnus isp., Thalassinoides isp., and Trichichnus isp. This ichnoassemblage differs from most CTB sections given the presence of small Alcyonidiopsis isp., Chondrites stellaris, and ?Pilichnus isp., and the absence of Zoophycos. The particular depositional setting, affected by coarse clastic gravity flow sedimentation, may be responsible for these differences by affecting palaeoenvironmental conditions. The lower part of the Rio Fardes section shows a more or less continuous record of diverse and abundant trace fossils, generally produced under aerobic/low dysaerobic conditions. The middle part is characterized by a diminution in trace fossil diversity and abundance, related to moderately/highly? dysaerobic conditions disrupted by short intervals of anoxia. Both the lower and middle parts represent pre-Bonarelli event conditions. The upper part reflects the most significant change in ichnological characteristics. The absence of discernible trace fossils caused by decreasing oxygenation corresponds to the Bonarelli Level. In comparison with previously studied CTB sections, the lower number and thickness of anoxic layers in the Rio Fardes section point to generally better oxygenation on the sea-floor. A variable influence of factors limiting the macrobenthic trace maker community during the OAE-2 can be related to depositional or palaeogeographic contexts. Oxygenated water in the Rio Fardes section was delivered from shallower areas along with gravity-flow deposits. The usually bioturbated background pelagic or hemipelagic deposits above the coarse gravity-flow deposits suggest rapid re-establishment of the macrobenthic trace maker communities following each mass depositional event.

A new marine palynomorph from the Turonian (Upper Cretaceous) in the USA

This paper documents a previously undescribed marine palynomorph, Doricodinium obscurum gen. et sp. nov., from Upper Cretaceous sections in the central and southern Western Interior of the USA. Although an archaeopyle has not been confirmed, a case is made here for a probable dinoflagellate cyst, possible peridinioid affinity. The type locality of D. obscurum is the Global boundary Stratotype Section and Point (GSSP) for the base of Turonian Stage, at Pueblo, Colorado, where it consistently occurs in intra-Lower to middle Turonian deposits. The taxon also occurs consistently in intra-Lower Turonian strata from the Iona-1 core, SW Texas, where astronomical age modelling gives an age range of 93.92–93.25 ± 0.12 Ma in that section. Outside the USA, it has until now only been recorded from the Cenomanian–Turonian boundary interval in Pakistan.

The expression of early Aptian to latest Cenomanian oceanic anoxic events in the sedimentary record of the Briançonnais domain

Abstract The Brianconnais domain is an important paleogeographic unit in the Alps, which represents a structural high within the former Alpine Tethys Ocean. Expressions of the most important Cretaceous oceanic anoxic events (OAE1a-d and OAE2) are exceptionally well preserved in the Subbrianconnais unit at Roter Sattel in the Fribourg Prealps, Switzerland. The main objective of this study was to reconstruct oxygen contents and trophic levels in the water column for each OAE. To accomplish this we used an integrated multi-proxy approach, including total organic carbon (TOC), phosphorus (P), and redox and productivity-sensitive trace-element contents (RSTE, PSTE). RSTE enrichments associated with the highest TOC values (up to 5.4 and 4.8 wt%) and with the lowest Mn contents in sediments equivalent to the Selli and Paquier OAEs (1a and 1b) indicate the presence of intermittent anoxic to euxinic conditions during these events. Sediments equivalent to Bonarelli OAE2 near the Cenomanian-Turonian (CT) boundary are also characterized by significant trace-element enrichments in comparison to average shale values. Mo and U enrichment factors (EFs) in the lower Aptian and lower Albian intervals are characteristic of an unrestricted marine environment, and in the Cenomanian-Turonian boundary interval they indicate a relationship with a particulate Fe-Mn-oxyhydroxide shuttle within the water column. In the OAE1a interval the elevated total P content associated with higher Corg/Ptot ratios and maxima in TOC values suggests that a part of the remobilized P remained trapped in the sediments. By contrast, the lower Ptot values associated with RSTE and Corg/Ptot enrichments in the OAE1b and OAE2 intervals indicate that a significant part of P was remobilized and escaped to the bottom water. Our results highlight the combined roles of regional and global parameters in the development of oxygen-deficient conditions that may lead to anoxia and increase in organic matter preservation during the OAEs. The differentiated topography of the Brianconnais domain associated with global climate and sea-level change and its influence on weathering in general, and more specifically on the mobilization of sediments on the adjacent ridge, modulated nutrient availability in different proportions during the different “mid”-Cretaceous OAEs.

Ammonites and the mid-Cretaceous saga

Ever since the introduction of the Cretaceous System, nearly two centuries ago, the terms ‘mid-Cretaceous’ and ‘middle Cretaceous’ keep appearing in the literature, implying a three-fold division rather than the more generally accepted two-fold division of the system/period. The frequent and persistent use of these informal terms proves that they fulfil a need in stratigraphic communication; consequently, formalisation of a middle series/epoch of the Cretaceous System/Period is justified and desirable and will put an end to the present terminological confusion with respect to division of the Cretaceous. According to long-established practice, biotic turnover events (major extinctions and/or radiation events) of biostratigraphically significant fossil groups provide convenient, correlatable horizons for positioning chronostratigraphic boundaries. As a first step towards establishing a solid database for a formal proposal for a three-fold division of the Cretaceous, taxonomic turnover events at the level of family- and genus-group taxa (families/subfamilies and genera/subgenera) for the historically most important fossil group, the ammonites, have been analysed for the broad ‘mid-Cretaceous’ interval, ranging from base Barremian to base Santonian. The results provide little support for a series boundary at the traditional Lower–Upper Cretaceous boundary at the base of the Cenomanian. Instead, the most significant ammonite turnover event occurred at the Aptian–Albian boundary interval, which then emerges as a potential lower boundary of a middle Cretaceous series. Higher in the Cretaceous, no similarly distinctive turnover event is recorded for the ammonites, although the turnover at the Cenomanian–Turonian boundary interval is slightly more pronounced. For a formal proposal for a three-fold division of the Cretaceous, the ammonite data need to be integrated with data on other chronostratigraphically significant fossil groups, such as inoceramid bivalves, foraminifers, calcareous nannofossils and dinoflagellates, and with magnetostratigraphy, chemostratigraphy and cyclostratigraphy.

A Southern Hemisphere record of global trace‐metal drawdown and orbital modulation of organic‐matter burial across the Cenomanian–Turonian boundary (Ocean Drilling Program Site 1138, Kerguelen Plateau)

AbstractDespite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian–Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed biostratigraphy and chemostratigraphy indicate that the record of Oceanic Anoxic Event 2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of Oceanic Anoxic Event 2 marks the end of the hiatus interval, which can be explained by a relative sea‐level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic‐matter burial during Oceanic Anoxic Event 2 at Site 1138 compared with other Oceanic Anoxic Event 2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of Oceanic Anoxic Event 2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea‐floor oxygenation suggests that the cycles were mainly productivity‐driven phenomena. Available age‐control points constrain the periodicity of the coupled changes in sedimentary parameters to ca 20 to 70 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid‐latitudes both during, and after, Oceanic Anoxic Event 2: fluctuations that were superimposed on the impact of global‐scale processes. Finally, trace‐metal data from the black‐shale unit at Site 1138 provide the first evidence from outside of the proto‐North Atlantic region for a global drawdown of seawater trace‐metal (Mo) inventories during Oceanic Anoxic Event 2.

FORAMINIFERAL RESPONSE TO ECOLOGICAL PERTURBATIONS ALONG THE EASTERN MARGIN OF THE CANADIAN WESTERN INTERIOR SEAWAY, CENOMANIAN-TURONIAN INTERVAL

Ecological perturbations during the Cenomanian-Turonian boundary interval along the eastern margin of the Canadian Western Interior Seaway (WIS) were investigated in a subsurface core from eastern Saskatchewan (distal) and outcrop at the Manitoba Escarpment (proximal). Paleoecological controls on biota included transgressive/regressive cyclicity, the Oceanic Anoxic Event 2, frequent ashfalls, and a stratified water column. Changes in foraminiferal assemblage composition, species richness, and abundance can be correlated between the core and outcrop. The lower Turonian biofacies at both sites is made up exclusively of planktic species, reflecting the persistent bottom water anoxia in the Canadian WIS. The low-diversity assemblage is nearly entirely composed of opportunistic, surface-dwelling species of the genera Muricohedbergella and Heterohelix , responding to water turbidity, reduced salinity, and shallow water depth. Abundances of planktic species are significantly higher in the distal core section. During the early Turonian, diversity slightly increased, with the presence of Whiteinella aprica reflecting periodic improvement of the water column conditions at both sites. The appearance of the clavate species, Clavihedbergella simplex , reflects an expanded oxygen minimum zone, also confirmed by biomarkers, particularly in outcrop. Dwarfing of foraminiferal tests is interpreted as a response to frequent ashfalls that affected the water column. Lowering of sea level during the middle Turonian allowed for seafloor winnowing and caused a brief disappearance of planktic foraminifera. The north-south temperature gradient in the WIS resulted in a lower planktic foraminiferal diversity in what is now Canada compared to the waters that covered what is now the United States and the absence of benthic and deeper-dwelling, keeled planktic species can be attributed to an increasingly well-developed oxygen minimum zone or increasingly stratified water column. The appearance datum of planktic foraminifera is distinctly diachronous along a south to north transect.

BENTHIC FORAMINIFERA IN THE AFTERMATH OF THE CENOMANIAN-TURONIAN BOUNDARY EXTINCTION EVENT IN THE CARBONATE PLATFORM FACIES OF THE SOUTHERN APENNINES (ITALY)

After the two-step extinction event that eliminated all the complex larger benthic foraminifera in the latest Cenomanian, the early Turonian benthic foraminiferal assemblages consisted of small, morphologically simple taxa; these were found in the few carbonate platforms that survived drowning during the Cenomanian-Turonian boundary interval. In this paper, we provide detailed morphologic descriptions of the most typical species of the Turonian benthic foraminiferal assemblages from the Apennine carbonate platform of southern Italy, along with some remarks on their suprageneric systematic position. Their chronostratigraphic range is firmly constrained by integrating carbon- and strontium-isotope stratigraphy.

A new proxy for Cretaceous paleoceanographic and paleoclimatic reconstructions: Coiling direction changes in the planktonic foraminifera Muricohedbergella delrioensis

Among some modern and recent fossil species of planktonic foraminifera, the proportion of left- to right-coiled shells in a population appears to be temperature-dependent; the relative abundance of each morphotype reflecting ecological preferences. A similar relationship is identified among Muricohedbergella delrioensis (Carsey, 1926) at the Cenomanian–Turonian stage boundary in mid-latitude sites of the Western Interior Seaway, including the Pueblo type section. The increase of sinistral M. delrioensis in the assemblage is related to higher δ18O carb values and decrease in inner porosity which suggests that changes in the coiling direction in this morphospecies could represent a new proxy for constraining Sea Surface Temperature (SST) variations. As surface dwellers, muricohedbergellids were not affected by Oceanic Anoxic Event (OAE2) and provide a continuous paleoclimatic signal throughout the Cenomanian–Turonian boundary interval. Furthermore, genetic evidence obtained from extant foraminifera indicates that shifts of coiling ratios in planktonic foraminifera species can express the signature of distinct genetic types, which are revealed through their opposite coiling directions. Coiling direction could be a genetic trait, implying that cryptic species may occur in the Mesozoic.

Planktonic foraminiferal biostratigraphy and assemblage composition across the Cenomanian–Turonian boundary interval at Clot Chevalier (Vocontian Basin, SE France)

The Cenomanian–Turonian boundary interval is generally considered a critical time for planktonic foraminifera due to the environmental perturbations associated with Oceanic Anoxic Event 2. However, only the rotaliporids became extinct at the onset of the event, whilst several lineages evolved and/or diversified. This remarkable morphologic plasticity is often overlooked in the literature, partly because a number of stratigraphic sections have only been studied in thin-section due to the degree of lithification of the samples. Improved documentation of the morphological variability of planktonic foraminifera and better defined species concepts are required in order to improve biostratigraphy, particularly as Helvetoglobotruncana helvetica is an unreliable marker for the base of the Turonian. At the same time, detailed study of the planktonic foraminiferal response to OAE 2 demands a more profound knowledge of the assemblage composition.We present new biostratigraphic, taxonomic, and quantitative data for planktonic foraminiferal species from the Clot Chevalier section (Vocontian Basin, SE France), with the aim of (1) providing a detailed biostratigraphic analysis of the section, (2) documenting the morphological plasticity of specimens in this time interval and stabilizing species concepts, and (3) identifying promising markers to improve the resolution of the present biozonation and allow regional correlation. Samples were processed with acetic acid to extract isolated planktonic foraminifera. Assemblages were assigned to the upper Cenomanian Rotalipora cushmani Zone and to the uppermost Cenomanian–lowermost Turonian Whiteinella archaeocretacea Zone. Planktonic foraminiferal bioevents and assemblage composition identified at Clot Chevalier are compared with the well-studied Pont d'Issole section located ca. 15 km to the NE, highlighting similarities and differences in the species occurrences that may complicate the stratigraphic correlation between the two sections.The results of our study support the validity and common occurrence of species that have been misidentified and/or overlooked in the literature (i.e., Dicarinella roddai, Praeglobotruncana oraviensis, Marginotruncana caronae) and indicate that primitive marginotruncanids evolved before the onset of OAE 2, although species diversification occurred only after the event. Moreover, we believe that the first appearance of P. oraviensis might represent a promising bioevent for approximating the Cenomanian/Turonian boundary, after calibration with bio- and chemostratigraphically well-constrained sections. Finally, we describe three new trochospiral species, named “Pseudoclavihedbergella” chevaliensis, Praeglobotruncana pseudoalgeriana and Praeglobotruncana clotensis.

Sequence stratigraphic architecture of marine to fluvial deposits across a passive margin (Cenomanian, Atlantic margin, Morocco, Agadir transect)

Seven sections, covering the upper Albian to lowermost Turonian, have been correlated from full-marine to continental-dominated deposits across a passive margin, along a transect 425 km long, from the present-day Atlantic coast to the ``Pre-African Trough'' between the Anti-Atlas and the High-Atlas. The thickness of the Cenomanian succession changes from around 500 metres in the fully marine sections to 250 metres in mostly continental facies in the western High-Atlas, about 150 km updip, to a few tens of metres in the Bou Tazoult area. The strata thicken again eastwards into the Pre-African Trough where they can be traced without major facies changes to the Kem Kem embayment and to the Bechar area in Algeria. Over all this eastern area, continental facies are overlain by the fully-marine shallow-water deposits of the Cenomanian-Turonian boundary interval. A first major conclusion is that fluvial aggradation in high-frequency transgressive-regressive sequences is coeval with the seaward-shift of the shoreline, in accordance with the genetic sequence stratigraphic model of GALLOWAY (1989). Both the flatness of the depositional profile and the corresponding very low energy of the marine environment during the transgressions account for the blanket of red continental clays on top of marine facies in updip depositional sequences, which is then preserved under the marine transgressive surface of the next sequence. A second major conclusion is that the high-frequency transgressive-regressive (T-R) sequences do not look like classical parasequences bounded by transgression surfaces. They usually exhibit a surface created by a sea-level fall within the regressive half-cycle. This is interpreted in the following way: regressions did not operate through a regular seaward-shift of the shoreline, but through stepped sea-level falls. The very low slope of the depositional ramp is thought to have enhanced the sequence stratigraphic record of such stepped regressions. Short-term, high-frequency sequences are organized into medium-frequency T-R sequences (seven in the Cenomanian) which show an overall aggrading and slowly retrograding pattern along the whole transect. Comparisons with other basins show that medium-frequency sequences do not fit the third-order depositional sequences described elsewhere, casting doubts about a eustatic mechanism for their deposition.

Pacific 187Os/188Os isotope chemistry and U–Pb geochronology: Synchroneity of global Os isotope change across OAE 2

Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy (187Os/188Os–Osi) from two proto-Pacific sites that span the Cenomanian–Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187Os/188Os seawater chemistry of the proto-Pacific. Additionally we combine new 206Pb/238U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age–depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI.The YG shows an almost identical Osi profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008; Du Vivier et al., 2014). The characteristics of the Osi profile are radiogenic and heterogeneous (∼0.55–0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Osi abruptly become unradiogenic and remain relatively homogeneous (∼0.20–0.30) before showing a gradual return to more radiogenic Osi(∼0.70) throughout the middle to late OAE 2. A206Pb/238U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Osi value constrains the age of the Osi inflection at 94.44±0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Osi profile (94.28±0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the Osi profile of the YG is correlative, within uncertainty, with a similar trend in the WIS based on the U–Pb age–depth model. The interpolated ages, 94.78±0.12 Ma and 94.66±0.25 Ma for this Osi inflection in the YG and WIS, respectively, indicate that palaeocirculation was sufficient to simultaneously influence transbasinal seawater chemistry. In contrast, the pre-OAE 2 Osi profile for the GVS is disparate to that of the YG and those of the proto-Atlantic and European pelagic shelf locations. We interpret the pre OAE 2 heterogeneous Osi values (0.30–0.95) to record a palaeobasin that was regionally influenced interchangeably by both unradiogenic (hydrothermal flux) and radiogenic (continental flux) Os.We conclude that the Osi profiles from the proto-Pacific sections record both trends that are consistent globally (OAE 2 onset, syn and post OAE 2), but also show regional differences (pre OAE 2) between OAE 2 sections worldwide. As such the Osi profiles coupled with U–Pb geochronology facilitate the correlation of OAE 2 stratigraphy, and demonstrate both regional and global ocean dynamics.