Maria Rose Petrizzo

Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic Anoxic Event: correlation and implications based on three key localities

We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian–Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak δ13C values for all components are followed by intervals of high, near-constant δ13C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ13C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian–Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the δ13C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

Evidence for global cooling in the Late Cretaceous.

The Late Cretaceous ‘greenhouse’ world witnessed a transition from one of the warmest climates of the past 140 million years to cooler conditions, yet still without significant continental ice. Low-latitude sea surface temperature (SST) records are a vital piece of evidence required to unravel the cause of Late Cretaceous cooling, but high-quality data remain illusive. Here, using an organic geochemical palaeothermometer (TEX86), we present a record of SSTs for the Campanian–Maastrichtian interval (~83–66 Ma) from hemipelagic sediments deposited on the western North Atlantic shelf. Our record reveals that the North Atlantic at 35 °N was relatively warm in the earliest Campanian, with maximum SSTs of ~35 °C, but experienced significant cooling (~7 °C) after this to <~28 °C during the Maastrichtian. The overall stratigraphic trend is remarkably similar to records of high-latitude SSTs and bottom-water temperatures, suggesting that the cooling pattern was global rather than regional and, therefore, driven predominantly by declining atmospheric pCO2 levels.

Palaeoceanographic and palaeoclimatic inferences from Late Cretaceous planktonic foraminiferal assemblages from the Exmouth Plateau (ODP Sites 762 and 763, eastern Indian Ocean)

The evolution of planktonic foraminifera during the Late Cretaceous is marked in the Santonian by the disappearance of complex morphotypes (the marginotruncanids), and the contemporary increasing importance and diversification of another group of complex taxa, the globotruncanids. Upper Turonian to lower Campanian planktonic foraminiferal assemblages from Holes 762C and 763B (Ocean Drilling Program, Leg 122, Exmouth Plateau, 47°S palaeolatitude) were studied in detail to evaluate the compositional variations at the genus and species level based on the assumption that, in the Cretaceous oceans as in the modern, any faunal change was associated with changes in the characteristics and the degree of stability of the oceanic surface waters. Three major groups were recognised based on gross morphology, and following the assumption that Cretaceous planktonic foraminifera, although extinct, had life-history strategies comparable to those of modern planktonics: 1 – r-selected opportunists; 2 – k-selected specialists; 3 – r/k intermediate morphotypes which include all genera that display a range of trophic strategies in-between opportunist and specialist taxa. Although planktonic foraminiferal assemblages are characterised by a progressive appearance of complex taxa, this trend is discontinuous. Variation in number of species and specimens within genera has allowed recognition of five discrete intervals each of them reflecting different oceanic conditions based on fluctuations in diversity and abundance of the major morphotypes. Planktonic forms show cyclical fluctuations in diversity and abundance of cold (r-strategists) and warm taxa (k-strategists), perhaps representing alternating phases of unstable conditions (suggesting a weakly stratified upper water column in a mesotrophic environment), and well-stratified surface and near-surface waters (indicating a more oligotrophic environment). Interval 1, middle Turonian to early Coniacian in age, is dominated by the r/k intermediate morphotypes which alternate with r-strategists. These cyclical alternations are used to identify three additional sub-intervals. Interval 2, aged middle to late Coniacian, is characterised by the increasing number of species and relative abundance of k-strategists. After this maximum diversification the k-strategists show a progressive decrease reaching a minimum value in Interval 3 (early to late Santonian), which corresponds to the extinction of the genus Marginotruncana. In the Interval 4, latest Santonian in age, the k-strategists, represented mainly by the genera Globotruncana, increase again in diversity and abundance. The last Interval 5 (early Campanian) is dominated by juvenile globotruncanids and r-strategists which fluctuate in opposite phase. The positive peak (Interval 2) related to the maximum diversification of warm taxa (k-strategists) in the Coniacian seems to correspond to a warmer episode. It is followed by a marked decrease in the relative abundance of warm taxa (k-strategists crisis) with a minimum in the late Santonian (Interval 3), reflecting a decrease in temperature. Detailed analysis of faunal variations allows the Santonian faunal turnover to be ascribed to a cooling event strong enough to cause the extinction of the marginotruncanids.

Late Albian paleoceanography of the western subtropical North Atlantic

A late Albian–early Cenomanian record (~103.3 to 99.0 Ma), including organic-rich deposits and a ?13C increase associated with oceanic anoxic event 1d (OAE 1d), is described from Ocean Drilling Program sites 1050 and 1052 in the subtropical Atlantic. Foraminifera are well preserved at these sites. Paleotemperatures estimated from benthic ?18O values average ~14°C for middle bathyal Site 1050 and ~17°C for upper bathyal Site 1052, whereas surface temperatures are estimated to have ranged from 26°C to 31°C at both sites. Among planktonic foraminifera, there is a steady balance of speciation and extinction with no discrete time of major faunal turnover. OAE 1d is recognized on the basis of a 1.2‰ ?13C increase (~100.0–99.6 Ma), which is similar in age and magnitude to ?13C excursions documented in the North Atlantic and western Tethys. Organic-rich “black shales” are present throughout the studied interval at both sites. However, deposition of individual black shale beds was not synchronous between sites, and most of the black shale was deposited before the OAE 1d ?13C increase. A similar pattern is observed at the other sites where OAE 1d has been recognized indicating that the site(s) of excess organic carbon burial that could have caused the ?13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (?13C) rather than lithostratigraphically defined.

Organic-carbon deposition in the Cretaceous of the Ionian Basin, NW Greece: the Paquier Event (OAE 1b) revisited

We present new stable (C, O) isotopic, biostratigraphic and organic geochemical data for the Vigla Shale Member of the Ionian Zone in NW Greece, in order to characterize organic carbon-rich strata that potentially record the impact of Cretaceous Oceanic Anoxic Events (OAEs). In a section exposed near Gotzikas (NW Epirus), we sampled a number of decimetre-thick, organic carbon-rich units enclosed within marly, locally silicified, Vigla Limestone (Berriasian Turonian). All these units are characterized by largely comparable bulk geochemical characteristics, indicating a common marine origin and low thermal maturity. However, the stratigraphically highest of these black shales is further distinguished by its much higher total organic-carbon (TOC) content (28.9 wt%) and Hydrogen Index (HI) (529), and much enriched [delta]13Corg value ([minus sign]22.1% ). Planktonic foraminiferal and calcareous nannofossil biostratigraphy indicate a lower to middle Albian age for the strata immediately above, and a lower Aptian age for the strata below, the uppermost black shale. In terms of molecular organic geochemistry, the latter black shale is also relatively enriched in specific isoprenoidal compounds (especially monocyclic isoprenoids), whose isotopic values are as high as [minus sign]15 , indicating a substantial archaeal contribution to the organic matter. The striking similarities between the molecular signatures of the uppermost Vigla black shale and coeval organic-rich strata from SE France and the North Atlantic (ODP Site 1049C) indicate that this level constitutes a record of the Paquier Event (OAE 1b).

BIOSTRATIGRAPHY AND TAXONOMY OF LATE ALBIAN PLANKTONIC FORAMINIFERA FROM ODP LEG 171B (WESTERN NORTH ATLANTIC OCEAN)

A detailed taxonomic and biostratigraphic analysis of upper Albian planktonic foraminifera is presented for Ocean Drilling Program sites 1050 and 1052, which were deposited at ~23°N paleolatitude and ~1300 m and 300 m paleodepths, respectively, on the Blake Nose escarpment (subtropical western North Atlantic Ocean). The generally excellent preservation of the material, especially in the clay-rich layers, permits recognition of new bioevents within the uppermost Albian, in addition to most of the standard bioevents that have been previously defined in the Tethyan Realm. The taxonomic assignments and the stratigraphic range of some species that are often overlooked or misidentified in the stratigraphic record have also been clarified, and three species are formally described as new ( Hedbergella astrepta , H. praelibyca and H. blakensis ). Graphic correlation and age-depth curves constructed from integrated planktonic foraminifera and calcareous nannofossils datum events enable reliable estimation of the relative timing of species first and last occurrences and relative abundance variations, as well as determination of the timing and extent of an unconformity at Site 1050. While most of the species datum events are well correlated, several are found to be diachronous and/or unreliable probably as a result of different surface water conditions along the depth transect.

LATE CRETACEOUS PLANKTONIC FORAMINIFERAL BIOEVENTS IN THETETHYS AND IN THE SOUTHERN OCEAN RECORD: AN OVERVIEW

The Late Cretaceous planktonic foraminiferal distribution recorded at several drill sites (Deep Sea Drilling Project and Ocean Drilling Program) in the South Atlantic and south Indian Ocean and from sediment outcrops in the Tethyan region (Gubbio and El Kef) have been analyzed in order to investigate the presence of a similar sequence of bioevents occurring at low, middle, and high latitude. Comparative analysis highlights the co-occurrence of several bioevents; some of them are isochronous bioevents occurring in the Tethys and in the Southern Ocean record, whereas others are diachronous across latitudes but can be used for correlation at regional scale, as they show the same stratigraphic distribution in the South Atlantic and Indian Ocean record. Isochronous bioevents are the first and the last occurrence of Helvetoglobotruncana helvetica , and the first appearance of Falsotruncana maslakovae in the lower-middle Turonian. The first occurrence of Heterohelix papula in the Southern Ocean sites, correlated with the first occurrence of large heterohelicids in the Tethyan area, allows the Coniacian/Santonian boundary to be identified. The most reliable bioevents useful for correlation at a regional scale in the Southern Ocean record are the last occurrence of the marginotruncanids in the upper Santonian, the first and the last occurrence of Globigerinelloides impensus from the uppermost Santonian to upper Campanian, the first occurrence of Heterohelix rajagopalani in the middle-upper Campanian, and the appearance of Abathomphalus mayaroensis in the lower Maastrichtian.

Abrupt planktic foraminiferal turnover across the Niveau Kilian at Col de Pré-Guittard (Vocontian Basin, southeast France): criteria for defining the Aptian/Albian boundary

A detailed and quantitative study of foraminiferal assemblages across the Niveau Kilian in the Col de Pre-Guittard section (Vocontian Basin, southeast France) ably documents the planktic foraminiferal turnover across the Aptian - Albian boundary interval. The latest Aptian assemblage is dominated by few long-ranging Hedbergella and large-sized Paraticinella that completely disappear near the base of the Niveau Kilian organic-rich level. Planktic foraminiferal assemblages from across the Niveau Kilian to the top of the studied section are composed of minute, but very distinctive smooth-surfaced species of Microhedbergella miniglobularis and Mi. renilaevis. The appearance of Mi. renilaevis in the middle part of the Niveau Kilian represents a major step in the evolution and diversification of the Albian planktic fauna. The lowest occurrence of this taxon is recorded immediately above the extinction level of the Aptian hedbergellids and corresponds with a dramatic decrease in the number of planktic and benthic foraminifera specimens, with a 1 ‰ negative δ13C excursion in bulk carbonate, and evidence for regional rise in sea-surface temperature. Our careful study of the species composition permits refinement of the previous biozonation by using the appearance datum of Mi. renilaevis as a zonal boundary event. Changes include redefinition of the top of the Microhedbergella miniglobularis Zone and designation of a new Microhedbergella renilaevis Zone. The same sequence of events was reported from several deep-sea sites in the Atlantic and Indian Oceans. Therefore, documentation of the planktic foraminiferal turnover, combined with the carbon-isotope stratigraphy in the Col de Pre-Guittard section, provide new criteria for defining the Global Boundary Stratotype Section and Point for the base of the Albian Stage in a stratigraphically complete succession.

Remarks on the identification of the Albian/Cenomanian boundary and taxonomic clarification of the planktonic foraminifera index species globotruncanoides, brotzeni and tehamaensis

The planktonic foraminifera assemblage across the Global Boundary Stratotype Section and Point (GSSP) for the base of the Cenomanian Stage defined at Mont Risou (Haute-Alpes in France) is here restudied to clarify the identification and stratigraphic distribution of Thalmanninella globotruncanoides ( = Rotalipora globotruncanoides Sigal, 1948) and Pseudothalmanninella tehamaensis ( = Rotalipora tehamaensis Marianos & Zingula, 1966) whose appearance levels are primary and secondary criteria for placing the Albian/Cenomanian boundary. Since the ratification of the GSSP in 2002, the identification of the foraminifera index species across the Albian/Cenomanian boundary has been reported to be sometimes difficult either because of their rarity or uncertainty in the taxonomic identifications. We discuss the taxonomic status of Thalmanninella brotzeni Sigal 1948, a species regarded for a long time to be a junior synonym of Th. globotruncanoides , through images of Sigal9s type materials deposited at the Musee National d’Histoire Naturelle (Paris), and elucidate the taxonomically important characters that enable unequivocal identification of Th. brotzeni, Th. globotruncanoides and Ps. tehamaensis . Planktonic foraminifera marker species from Mont Risou are compared with well-preserved specimens from Blake Nose Plateau (ODP 171B, North Western Atlantic Ocean) to verify the reliability and stratigraphic distribution of these marker taxa outside the Mediterranean Tethyan area.

UPPER CRETACEOUS MERIDIONALLY COSTELLATE HEDBERGELLIDS: THE GENUS MERIDIONALLA EL-NAKHAL, 1982 VS. THE GENUS COSTELLAGERINA PETTERS, EL-NAKHAL AND CIFELLI, 1983

The two genera, Meridionalla El-Nakhal, 1982 and Costellagerina Petters, El-Nakhal and Cifelli, 1983, both erected to accommodate all the pre-Campanian meridionally costellate species, are discussed after comparison with the holotypes of the type species. Observations clearly indicate that Costellagerina is the appropriate genus, while the genus Meridionalla is inappropriate because its type species, Hedbergella murphyi Marianos and Zingula 1966 does not possess the textural ornamentation diagnostic for the genus.