Aptian-Albian Boundary Research Articles

Revised stratigraphic position of a volcanic-ash-derived maximum depositional age in the Lower Cretaceous McMurray Formation

Abstract The stratigraphic framework developed for the McMurray Formation and Wabiskaw Member of the Clearwater Formation (Aptian, Lower Cretaceous) provides a consistent nomenclature and allows for correlation across the Athabasca Oil Sands Region. Event horizons (e.g. volcanic ash layers) within the stratigraphic framework provide crucial geochronologic ages that constrain the timing of deposition and improve upon biostratigraphic estimations. Here, we provide recognition criteria for genetic depositional units in the Firebag sub-basin with the intent of revising the stratigraphic position of a previously published volanic ash-derived maximum depositional age of 115.07 +/– 0.16 Ma. Results demonstrate that the previous placement of this event horizon at the top of the B1 parasequence set was not consistent with the accepted definition of the McMurray Formation–Wabiskaw Member boundary or application of the stratigraphic framework. Here, we establish that the ash-bearing coal horizon is at the top of the A2 parasequence set/channel belt of the McMurray Formation. The implications of this change include constraining the age of the A2 channel belt system to approximately 115 Ma which, under the previous study, would have been younger and unconstrained. We also discuss the implications for our understanding of the Aptian-Albian boundary in the context of allogenic drivers and global environmental change at that time.

Carbon isotope chemostratigraphy of the Yucca Formation from the Solitario, Big Bend Ranch State Park, Texas

Introduction: The Yucca Formation is a Lower Cretaceous sedimentary unit present in West Texas. Based on its relative stratigraphic position in the Cretaceous succession of West Texas, it is expected that the Yucca Formation is of Albian and/or Aptian age. It is also expected that the carbon isotope excursions associated with OAE 1a and OAE 1b should be identified in the Yucca Formation. The goals of this project are to 1. construct a carbon isotope chemostratigraphic record of the Yucca Formation, and 2. correlate the Yucca Formation with strata of similar age using chemostratigraphy.Methods: 163 samples were collected from Big Bend Ranch State Park (BBRSP) to determine the δ13C value of bulk sedimentary organic matter.Results: C-isotope values range from −27.02‰ to −18.42‰.Discussion: Carbon isotope excursions (CIEs) that are associated with the Aptian-Albian Boundary are identified as well as CIEs associated with Oceanic Anoxic Events (1a and 1b). This allows us to conclude that the Aptian-Albian boundary is recorded within the Yucca Formation strata at about 71 m above the base of the section exposed in the Lower Shutup of the Solitario in Big Bend Ranch State Park. Regional correlation of the Yucca Formation to other chemostratigraphic records from other Cretaceous strata suggests that the Yucca Formation in BBRSP is time equivalent to the Sligo, Pine Island, James, Bexar, and a portion of the lower Glen Rose Formation on the Comanche Platform and to a portion of the lower Glen Rose Formation in Big Bend National Park.

A first high-resolution carbon isotope stratigraphy from the Boreal (NW Germany) for the Berriasian to Coniacian interval—implications for the timing of the Aptian–Albian boundary

High-amplitude changes in sedimentary δ13C characterize the Cretaceous system and have been proven useful for supra-regional chemostratigraphic correlation. In the Cretaceous, these δ13C perturbations indicate large shifts between the global carbon reservoirs that are usually caused by volcanic activity of large igneous provinces, the widespread deposition of thick organic carbon-rich sequences and/or changes in orbital parameters. Here, we present an upper Berriasian to lower Coniacian (c. 142–88 Ma) composite carbon isotope record based on 14 drill cores, 2 outcrops, and almost 5,000 samples. The total record comprises a composite thickness of more than 1,500 m. All cores and successions are located in the larger Hanover area, which represents the depocenter of the North German Lower Saxony Basin in Early to mid-Cretaceous times. In Northern Germany, Boreal Lower Cretaceous sediments are predominantly represented by CaCO3-poor mud and siltstones of up to 2,000 m thickness, which become more carbonate-rich during the Albian–Cenomanian transition and even chalkier in the upper Cenomanian to Coniacian interval. The carbon isotope record reveals a number of global key events, including the Valanginian Weissert Event, the Oceanic Anoxic Events (OAEs) 1a and d, and the Kilian Event (Aptian–Albian boundary, part of OAE 1b). For the early Late Cretaceous, the Mid-Cenomanian Event, the OAE 2 (Cenomanian–Turonian Boundary Event), and the Navigation Event, among others, have been identified. The Kilian Event represents the Aptian–Albian boundary and has been identified herein for the first time in Northern Europe. Based on the evaluation of its relative position to the Vöhrum boundary tuff, we tentatively propose a slightly older age for the Aptian–Albian boundary of c. 113.65 Ma instead of 113.2 Ma. The observed chemostratigraphic events enable a detailed stratigraphic comparison with Tethyan and other Boreal records and associated paleoenvironmental data. Thus, this new detailed chemostratigraphy provides a unique opportunity to potentially overcome many still existing Boreal–Tethyan correlation issues. The presented record can be considered almost complete, albeit a 2-Myr gap during the early Albian is likely, and condensed intervals occur specifically during the lower Aptian.

High-resolution marine osmium and carbon isotopic record across the Aptian–Albian boundary in the southern South Atlantic: Evidence for enhanced continental weathering and ocean acidification

The Early Cretaceous Aptian–Albian boundary (AAB) interval (∼113 Ma) was marked by severe environmental perturbations, including Oceanic Anoxic Event (OAE) 1b, global warming, and a major species turnover of planktonic foraminifera. Most paleoenvironmental studies across the AAB have focused on Tethyan sedimentary sequences deposited at low latitudes, but, because of the scarcity of well-dated sedimentary sequences across the AAB at high latitudes, the global extent of these environmental perturbations remains uncertain. Deep Sea Drilling Project (DSDP) Site 511 was drilled on the Falkland Plateau, southern South Atlantic, and is one of the few sedimentary sequences at southern high latitudes to record an almost continuous AAB interval. Previous studies documented the planktonic foraminiferal changes across the AAB at DSDP Site 511, but chemostratigraphic information has been limited. We reconstructed an osmium and carbon isotopic stratigraphy to further constrain the position of the AAB. We also inferred the paleo-marine redox conditions at southern high latitudes by combining chemical and lithological information. We found an abrupt negative δ13Ccarb shift coincident with the first occurrence of Microhedbergella renilaevis, an event that marks the base of the Albian Stage. This negative δ13Ccarb shift has also been observed across the AAB interval in the Tethyan and Atlantic regions. The fact that the osmium isotopic ratios show a pronounced radiogenic shift after the AAB is consistent with osmium isotopic variations detected in the Tethys and Pacific regions. Because the Os shift corresponded to a warming event during OAE1b in the low-latitude Atlantic and Tethyan regions, we interpret this radiogenic osmium isotopic shift as a response to increased continental weathering caused by global warming. A large marcasite crystal in the AAB interval at DSDP Site 511 suggests that the southern Atlantic Ocean may have been oxygen-depleted with a low pH caused by contemporaneous global environmental perturbations.

Sedimentary record of the “Austrian” tectonic pulse around the Aptian–Albian boundary in SE France, and abroad

A tectonically-controlled forced regression occurred on the western margin of the French subalpine basin around the Aptian–Albian boundary. It nearly emptied the western part of the Vocontian Trough during the early and middle Albian. A compressional pulse associated with vertical movements along the Cevennes fault row and its satellites is inferred. This forced regression is correlated with an inverse transgressive trend, both in the Paris Basin and the northern subalpine chains. The black shales of the early Aptian OAE1a and Albian OAE1b of the Vocontian Trough succession occur within opposite regional sea level trends, transgressive for the former, regressive for the latter. The inferred tectonic pulse is also recorded on a broader scale from literature data. The overall picture also shows outphasings in relative sea level changes on a large scale, likely controlled by tectonics. Une régression forcée d’origine tectonique s’est produite sur la marge occidentale de la mer subalpine au passage Aptien–Albien. Elle a vidangé presque complètement la partie ouest de la fosse vocontienne pendant l’Albien inférieur et moyen. Une pulsation tectonique en régime compressif associée à des mouvements verticaux est suggérée le long du couloir de failles cévenol et satellites. La régression forcée est corrélée avec une tendance inverse transgressive dans le Bassin de Paris et dans les Chaines Subalpines nord. Les black shales vocontiens des événements anoxiques océaniques globaux OAE1a (Aptien) et OAE1b (Albien) s’inscrivent dans des tendances opposées en termes de variations du niveau marin relatif, largement transgressive pour le premier, fortement régressive pour le second. La pulsation tectonique déduite se manifeste à plus grande échelle d’après les données de la bibliographie. Elle est également responsable des déphasages constatés dans les variations locales du niveau marin à grande échelle.

Unattached carbonate platform evolution across the Aptian–Albian boundary: facies, magnetic susceptibility and sequence stratigraphy in the Constantine area of northeast Algeria

Unattached carbonate platform evolution across the Aptian–Albian boundary: facies, magnetic susceptibility and sequence stratigraphy in the Constantine area of northeast Algeria

Astronomical calibration of the latest Aptian to middle Albian in the South Atlantic Ocean

Carbon cycle perturbations dramatically affected the ocean-climate system during the Aptian–Albian transition. However, the chronology of this interval is still intensively debated. Here we present a comprehensive astronomical calibration of planktonic foraminiferal and calcareous nannofossil bioevents along a complete marine sedimentary succession, comprising the latest Aptian to middle Albian interval at drilling Core SER-03, located in the Sergipe-Alagoas Basin. The SER-03 astrochronology is based on the tuning of a high-resolution gamma ray (GR) record to the long eccentricity (405 kyr) metronome, and is integrated with high-resolution paleomagnetic records. Our results span the interval between ~114.7 and 109.5 Ma, within magnetochron C34n, and our age estimates for biostratigraphic markers agree within one long eccentricity cycle with previous estimates. We also present geochemical records, such as stable carbon isotopes (δ13C), organic carbon and carbonate contents, as well as redox-sensitive elemental ratios, measured on bulk sediments at Core SER-03. Most of these proxy records depict marked changes across the Aptian–Albian boundary, likely characterizing the local expression of Oceanic Anoxic Event 1b (OAE 1b), filling the gap for low-latitude records of this event.

The late Aptian–early Albian transgressions in the Chott area, southern Central Tunisia

The detailed sedimentological and biostratigraphic study of the upper Aptian–Albian series in the Chott area, southern Central Tunisia, leads to the identification of several, well-dated depositional sequences. The upper Aptian sequences are marked by very shallow marine, first partly evaporitic, then mainly clastic deposits. The last sequence yielded latest Aptian ammonites (H. jacobi zone). The Aptian–Albian boundary is assumed to be marked by a long-lasting hiatus, and by the abrupt change from clastic to carbonate deposits. The overlying “Knemiceras Beds” express the Albian transgression, and comprise four depositional sequences. New findings of ammonites allow to date these beds to the lower Albian (D. mammillatum superzone), and to correlate the four sequences with those defined in Central Tunisia. Five distinct morphologies of Knemiceras sp. are recognized in the “Knemiceras beds”. Because of their stratigraphic position, the overlying massive shelf carbonates (Radhouane Member) are ascribed to the uppermost lower Albian (T. camatteanum and L. pseudolyelli zones), and correlated with the Allam Limestone of Central Tunisia. This carbonate series is capped by a major discontinuity, correlated with the middle Albian discontinuity known in Central Tunisia. The ammonites and rudists fauna are presented and illustrated.

Upper Aptian–lower Albian of the southern-central Araripe Basin, Brazil: Microbiostratigraphic and paleoecological inferences

The aim of this study was to produce an integrative analysis of macroinvertebrates (decapods, gastropods, and bivalves) and microfossils collected in the Romualdo Formation, south-central Araripe Basin. Additionally, we provide the first report of calcareous nannofossils, a diverse and unique association of the marine ostracods Dicrorygma cf. minuta, Perissocytheridea spp., Dicrorygma (Orthorygma?) dimorpha, and Paracypris sp. and benthic and planktonic foraminifera. The Santo Antonio section contains assemblages of Tethyan origin that indicate an unstable paleoenvironment formed by hypersaline lagoons with two marine ingressions. Planktonic foraminifera assemblages comprised of Hedbergella aptiana and Pseudoguembelitria sp., suggest that the first marine ingression occurred in the Aptian (possibly late Aptian – planktonic foraminifera biozone Paraticinella rohri), while the second occurred during the early Albian (biozone CC8), as indicated by calcareous nannofossils (Hayesites albiensis). The fossil assemblages that characterize the Romualdo Formation, provide new biostratigraphic information and point towards the probable Aptian–Albian boundary in the Santo Antônio section.

Aptian–Albian clumped isotopes from northwest China: cool temperatures, variable atmospheric <i>p</i>CO<sub>2</sub> and regional shifts in the hydrologic cycle

Abstract. The Early Cretaceous is characterized by warm background temperatures (i.e., greenhouse climate) and carbon cycle perturbations that are often marked by ocean anoxic events (OAEs) and associated shifts in the hydrologic cycle. Higher-resolution records of terrestrial and marine δ13C and δ18O (both carbonates and organics) suggest climate shifts during the Aptian–Albian, including a warm period associated with OAE 1a in the early Aptian and a subsequent “cold snap” near the Aptian–Albian boundary prior to the Kilian and OAE 1b. Understanding the continental system is an important factor in determining the triggers and feedbacks to these events. Here, we present new paleosol carbonate stable isotopic (δ13C, δ18O and Δ47) and CALMAG weathering parameter results from the Xiagou and Zhonggou formations (part of the Xinminpu Group in the Yujingzi Basin of NW China) spanning the Aptian–Albian. Published mean annual air temperature (MAAT) records of the Barremian–Albian from Asia are relatively cool with respect to the Early Cretaceous. However, these records are largely based on coupled δ18O measurements of dinosaur apatite phosphate (δ18Op) and carbonate (δ18Ocarb) and therefore rely on estimates of meteoric water δ18O (δ18Omw) from δ18Op. Significant shifts in the hydrologic cycle likely influenced δ18Omw in the region, complicating these MAAT estimates. Thus, temperature records independent of δ18Omw (e.g., clumped isotopes or Δ47) are desirable and required to confirm temperatures estimated with δ18Op and δ18Oc and to reliably determine regional shifts in δ18Omw. Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and δ13C and δ18O subsampling. Our preliminary Δ47-based temperature reconstructions (record mean of 14.9 ∘C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average ∼ 15 ∘C) across the Aptian–Albian. This, supported by our estimated mean atmospheric paleo-pCO2 concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled δ18O and Δ47 records suggest shifts in the regional hydrologic cycle (i.e., ΔMAP, mean annual precipitation, and Δδ18Omw) that may track Aptian–Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).

The Hatillo Limestone, Pueblo Viejo district, Dominican Republic: Marginal reef or impermeable cap?

The Hatillo Limestone and the underlying Los Ranchos Formation are exposed over an east-west distance of 100 km in the eastern Dominican Republic. The lowermost portion of the Hatillo Limestone in the Pueblo Viejo district contains a Late Lower Albian fossil assemblage including corals and rudist bivalves indicative of a near-shore reef environment. Diamond drilling in the Pueblo Viejo district and exposures in the open pits show that the Hatillo Limestone conformably overlies the Early Cretaceous Los Ranchos Formation. Volcanogenic massive sulfide beds, exposed in the Moore pit, provide evidence for an Early Cretaceous, syn-mineralization paleosurface. Altered and mineralized clasts in the epiclastic, sedimentary host-rock section at the Pueblo Viejo mine indicate that the ore deposits were open to erosion during hydrothermal alteration and mineralization. The Hatillo Limestone did not overlie the ore deposits during the mineralizing event and, consequently, could not have acted as an impermeable cap to ascending hydrothermal fluids. Intra-oceanic island arc volcanism (Los Ranchos Formation) overlapped at the Aptian–Albian boundary (112 Ma) with a marginal fringing reef (basal Hatillo Limestone). The marginal reef gradually gave way to deeper-water facies as Hatillo Limestone deposition progressed through the middle Albian. Low-angle reverse faulting, penetrative deformation, and metamorphic recrystallization affected the Hatillo Limestone as well as the Los Ranchos and Maimón formations during the Late Cretaceous. Deformation intensity and metamorphic grade progressed from incipient metamorphism in the Pueblo Viejo district to schists in the Maimón Formation to amphibolite near a faulted contact with the Loma Caribe peridotite.

Synsedimentary tectonics vs paleoclimatic changes across the Aptian-Albian boundary along the Southern Tethyan margin: The panormide carbonate platform case history (NW Sicily)

The Aptian-Albian boundary is a deeply investigated time interval on the basis of the deep oceanographic, environmental and climatic changes, globally recorded along the sedimentary rock successions.A regional latest Aptian extensional pulse in the Southern Tethyan margin is here envisaged as being responsible for different events of sedimentation and erosion in the Panormide carbonate platform as highlighted by the outcropping sections studied from the NW Sicilian fold and thrust belt.Integrating facies and stratal pattern analysis with backstripping methods has permitted to define the interaction between tectonics and sedimentary factors of the Lower Cretaceous shallow-water carbonates.Based on geometric configuration, lateral facies relationships, and thicknesses variations of the Lower Cretaceous Requienid limestone, is here envisaged a paleophysiography characterised by banks separated by deep channels. The margins of the isolated carbonate platform were oriented towards both the present-day west, where originally the Cala Rossa intraplatform basin developed driven by transtensional movements, and to the present-day east, where the continental slope was spreading.Including subsidence analysis and significance of the top-unconformity of the Requienid limestone a tectonic event at the end of the Aptian was detected as responsible for the fragmentation of the platform in various rotating fault-blocks and reworking of shallow-water carbonates towards the slope. The variable subsidence rates correspond to the different responses, both emersion and drowning, of the various platform-blocks to the extensional tectonics.Paleoceanographic changes, detected around the world during the Aptian-Albian, are well recorded also from the Sicilian sections, where continental clays – as the product of weathering of crystalline rocks – metallic crusts (hardgrounds) and condensed phosphatized pelagic deposits – as the product of sediment starvation, condensation and drowning – cover the Lower Cretaceous shallow-water carbonates respectively in the different faulted blocks of the segmented carbonate platform. Chemical and mineralogical analyses confirm for these deposits an influence in their formation caused by variations in oceanographic and environmental conditions, as increasing of metal ions in the surficial sea-water and high rates of groundwater precipitation. Paleoclimate evaluation reveals changes from the greenhouse arid conditions, developed during the lower Cretaceous and permitting the sedimentation of the shallow-water carbonates, to warm-humid climate condition during the latest Aptian-lower Albian boundary.

Lithology and planktic foraminifera biostratigraphy of Aptian–Albian boundary as encountered in Jebel Sidi Salem (northeastern Tunisia)

The Aptian–Albian boundary has long been difficult to identify along the foreland basin of the Tunisian Atlas, although ammonites occur in the Fahden Formation. The revision of Aptian–Albian series from Jebel Sidi Salem, in the northeastern part of the Tunisian Ridge, provides new data for the recognition of this boundary. The Jebel Sidi Salem section contains a rich succession of planktic foraminifers’ index species which document a complete marine record of Aptian–Albian boundary with no sign of redeposition. Six zones of planktic foraminifera have been identified through the late Aptian to middle Albian; including Globigerinelloides algerianus Zone, Hedbergella trocoidea Zone, Planomalina cheniourensis Zone, Paraticinella eubejaouensis/Ticinella roberti Zone, Hedbergella planispira Zone and Ticinella primula Zone.

Marine Os isotopic evidence for multiple volcanic episodes during Cretaceous Oceanic Anoxic Event 1b

The Aptian–Albian boundary is marked by one of the major oceanic perturbations during the Cretaceous, called Oceanic Anoxic Event (OAE) 1b. Extensive volcanic episodes at the Southern Kerguelen Plateau has been suggested as the trigger of OAE1b, but compelling evidence remains lacking. Here, we reconstructed the temporal variations of marine Os isotopic ratios across the Aptian–Albian boundary in the Tethyan and Pacific pelagic sedimentary records to elucidate the causal links between OAE1b, the biotic turnover, and volcanic episodes. Our new Os isotopic records show two negative spikes that correlate with a period of planktonic foraminiferal turnover across the Aptian–Albian boundary during OAE1b and suggest multiple submarine volcanic events. By comparing our Os isotopic profile with carbon isotopic compositions of carbonate, CaCO3 content, and the relative abundances of agglutinated foraminifera, we conclude that ocean acidification caused by the massive release of CO2 through extensive volcanic episodes could have promoted the major planktonic foraminiferal turnover during OAE1b.

Ammonite stratigraphy and the belemnite genus Hibolithes in the higher Serdj Formation (Aptian-Albian boundary interval) in north central Tunisia

Ammonite stratigraphy and the belemnite genus Hibolithes in the higher Serdj Formation (Aptian-Albian boundary interval) in north central Tunisia

Volcanic Rocks and Granitoids from Cape Svyatoy Nos (Eastern Arctic): Their Age, Composition, and Paleotectonic Reconstructions

For the first time, geochemical and geochronological (U–Pb SIMS, zircon) data are obtained for volcanic rocks and tuffs of the volcanic–sedimentary sequence and for granitoids intruding them in the Cape Svyatoy Nos area (eastern Arctic). The concordant U–Pb age of basalts is 146 ± 2. The ages of granitoids are 114 ± 1, 112 ± 1, and 114 ± 2 Ma. These ages correspond to the Late Jurassic (Tithonian) and the Aptian–Albian boundary. The section structure and the geochemical characteristics of the volcanic rocks and tuffs, as well as their similarities with the Mariana arc volcanics, indicate their formation in the settings of an intraoceanic subduction zone. The data obtained allow us to suggest that the volcanogenic–sedimentary rocks of Cape Svyatoy Nos represent a continuation of the Kul’pol’nei oceanic arc, which marked the northeastern convergent boundary between the Siberian continent and the South Anyui Basin in the Late Jurassic. In terms of their petrographic composition and petrogeochemical characteristics, the Aptian–Albian granitoids correspond to type I granites. The time of their intrusion coincides with the extension after the ending of the collision between the Siberian continent and the Chukotka–Arctic Alaska microplate as a result of the South Anyui Basin consumption.

Precession-driven monsoonal activity controlled the development of the early Albian Paquier oceanic anoxic event (OAE1b): Evidence from the Vocontian Basin, SE France

In the Vocontian Basin in southeastern France, the Aptian-Albian boundary is marked by the occurrence of four laminated black shales (Jacob, Kilian, Paquier, Leenhardt) corresponding to Oceanic Anoxic Event 1b (OAE1b). Equivalents of one or more of these intervals are also known from different regions in the Tethys, Atlantic, and Pacific, and correspond in time to major climatic and evolutionary perturbations. Here, we performed an integrated multi-proxy high-resolution study on the section at l'Arboudeysse in the northwestern part of the Vocontian Basin including mineralogical (whole rock and clay minerals) and geochemical (carbon and oxygen isotopes, organic matter, major and trace elements, mercury contents) analyses. The aim of this study is to reconstruct the level of productivity and related state of oxygenation, in addition to the weathering coditions on the adjacent continent during the Paquier event, Modest enrichments in redox-sensitive trace elements (RSTE; Mo, As, V, U, Ni) associated with moderate total organic carbon contents (between 0.57 and 5.52 wt %) indicate the development of suboxic/anoxic conditions during deposition of the Paquier level. The strong enrichment of Mo (10–229) relative to other RSTE (<5) may be explained by the importance of a particulate Fe–Mn oxyhydroxide shuttle. The high phosphorus (P) contents are related to the presence of fish remains. The weathering and detrital indices estimated from clay and whole-rock mineralogy suggests temporal fluctuations in weathering conditions and detrital input, likely resulting from variable humid conditions. These variations display a cyclic pattern linked to Milankovitch precession cycles varying from humid, high weathering conditions, causing oxygen-depletion in the bottom water to dryer, low weathering conditions associated with less oxygen depleted bottom waters. This oscillation was probably controlled by changes in evaporation/precipitation patterns related to variably intense monsoonal activity. A lack of significant mercury enrichments is not supportive of a direct link between volcanic activity and environmental change during the Paquier event.

Morphometric response of late Aptian planktonic foraminiferal communities to environmental changes: A case study of Paraticinella rohri at Poggio le Guaine (central Italy)

The Aptian-Albian boundary interval represents a pivotal time of pronounced climate/ocean changes associated with Oceanic Anoxic Event 1b (OAE 1b) that seriously influenced marine pelagic ecosystems. The sedimentary successions record the first and most significant species turnover in the evolutionary history of planktonic foraminifera, consisting of the selective extinction/disappearance of large Aptian species and resulting in inter-specific size reduction (the post-extinction dwarfing as in the Lilliput effect) in the earliest Albian assemblages. In this study we investigate how the planktonic foraminifera respond to crucial environmental stress and if any intra-specific decrease in test size predate the extinction (pre-extinction dwarfing). Paraticinella rohri was chosen as the most representative of the Aptian planktonic foraminiferal taxa and its test-size evolution was compared with the high-resolution record of redox and productivity geochemical proxies through the uppermost Aptian - lowermost Albian pelagic succession of Poggio le Guaine (central Italy). Our geochemical data document long-term fluctuations characteristic of a system with transitions among different states (oligotrophic-mesotrophic-eutrophic and oxic-dysoxic-anoxic). The strong correlation of the large test-size changes of Pa. rohri during its total range with higher productivity (increased Baexcess concentrations) suggests that the trophic regime was the key factor controlling robust abundance and size variability in this taxon. The progressive increase in surface water fertility during the late Aptian offered optimum ecological conditions for planktonic foraminifera, that increased in abundance, size, and specific diversity. After the transient anoxic 113/Jacob sub-event, smaller to dwarfed specimens of Pa. rohri and depauperate planktonic foraminifera assemblages faced high selective pressures due to more oligotrophic conditions. Transition to a more eutrophic ocean in the latest Aptian irreversibly eroded their resilience, and Aptian dwarfed k-strategist taxa became extinct. The planktonic foraminifera turnover has been therefore the result of an irreversible and drastic regime shift toward a more eutrophic ecosystem that promoted the thriving of small-sized opportunistic/disaster planktonic foraminiferal species and the siliceous radiolaria.

Mid-Cretaceous paleoenvironmental changes in the western Tethys

Abstract. We present a continuous record of surface water temperature and fertility variations through the latest Barremian–Cenomanian interval (ca. 27 Myr) based on calcareous nannofossil abundances from the western Tethys. The nannofossil temperature index, calibrated with TEX86 sea surface temperatures, suggests that warmest (34–36 ∘C) conditions were reached during oceanic anoxic event (OAE) 1a onset, the Aptian–Albian boundary interval hyperthermals (113, Kilian level and Urbino level OAE 1b) and during a ca. 4 Myr long phase in the middle Albian. Coolest temperatures (29 ∘C) correspond instead to the late Aptian. Generally warm conditions characterized the Albian followed by a progressive cooling trend that started in the latest Albian (at the Marne a Fucoidi–Scaglia Bianca Formation transition). Temperate conditions occurred in the Cenomanian with frequent short-term variations highlighted by abundance peaks of the cold-water nannofossil species E. floralis and R. parvidentatum. Mid-Cretaceous surface water fertility was rather fluctuating and mostly independent from climatic conditions as well as from black shales intervals. Intense warming and fertility spikes were systematically associated only with black shales of OAE 1a and of the Aptian–Albian boundary hyperthermals. The Albian–Cenomanian rhythmic black shales are, in fact, associated with varying long-term climatic and fertility conditions. The similarity of western Tethys climatic and fertility fluctuations during OAE 1a, OAE 1b, the middle Albian and OAE 1d with nannofossil-based records from other basins indicated that these paleoenvironmental conditions were affecting the oceans at supra-regional to global scale.

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.