Oceanic Anoxic Event 1d Research Articles

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.

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

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

Biostratigraphy, carbon isotopes and cyclostratigraphy of the Albian-Cenomanian transition and Oceanic Anoxic Event 1d in southern Tibet

During the Albian and Cenomanian, the Earth underwent profound climatic and oceanographic changes that were recorded in sedimentary successions on a global scale. Carbon isotope records spanning this time interval have been established in the western Tethys, eastern Pacific and North Atlantic Oceans, but not yet in the eastern Tethys Ocean. In this paper, we present biostratigraphic, chemostratigraphic and cyclostratigraphic characteristics of the uppermost Albian–lowermost Cenomanian in an eastern Tethyan section (Youxia, southern Tibet). Based on calcareous nannofossil biozones and the bulk rock δ13C curve, the Albian-Cenomanian boundary interval (ACBI) was identified and correlated to the western Tethys and Atlantic Oceans. In the Youxia section, δ13C values range from approximately 0‰ to +1.3‰ (−0.03‰ to +1.31‰). Four subevents (a, b, c and d) were distinguished in the ACBI carbon isotope curve via correlation with other sections. Based on a spectral analysis of the carbonate content, we recognized Milankovitch short eccentricity (~100 kyr) and precession (22.2 kyr) cycles, suggesting that orbital variations modulated depositional processes. The duration of the ACBI was estimated at ~311 kyr, while OAE 1d lasted for ~233 kyr in the eastern Tethys Ocean, consistent with the duration calculated from Atlantic Ocean records.

The timing of oceanic anoxic events in the Cretaceous succession of Cauvery Basin: Constraints from 40Ar/39Ar ages of glauconite in the Karai Shale Formation

This study presents 40Ar/39Ar ages of autochthonous glauconites from the lower segment of the onshore Karai Shale Formation of the Cauvery Basin that constrain its age from 100.3 ± 0.7 to 92.6 ± 0.6 Ma. The 40Ar/39Ar plateau ages of the 3 glauconite samples are consistent with the existing biostratigraphic age of Late Albian to Middle Turonian of the Karai Shale Formation. These ages are significant because of the occurrences of oceanic anoxic event (OAE)1d and oceanic anoxic event (OAE)2 in contemporaneous offshore deposits, the Sattapadi Shale and Bhuvanagiri Formation, respectively. Although in the onshore deposits, the glauconite ages mark condensed deposits of the transgressive Karai Shale. As onshore glauconitic deposits may be transitional to organic‐rich black shale deposits offshore in modern and ancient sedimentary settings, the 40Ar/39Ar ages precisely constrains the timing of Cretaceous oceanic anoxic events reported in the Cauvery Basin.

Late Albian calcareous dinocysts and calcitarchs record linked to environmental changes during the final phase of OAE 1d – a case study from the Tatra Mountains, Central Western Carpathians

Calcareous dinocysts and calcitarchs have been investigated for the first time within the Upper Albian limestone and marl succession of the Zabijak Formation from the High-Tatric Unit in the Tatra Mountains (Central Western Carpathians), related to the Oceanic Anoxic Event 1d (OAE 1d). Four groups of morphotaxa of calcareous dinocysts have been distinguished. They totally dominate the assemblages, and belong to the pithonellids. They are represented by Pithonella sphaerica (Kaufmann in Heer) and P. ovalis (Kaufmann in Heer), which dominate, as well as P. trejoi Bonet and P. lamellata Keupp in Keupp and Kienel, which are less abundant. Two other morphotaxa, Colomisphaera gigantea (Borza) and Cadosina oraviensis Borza, occur sporadically in the assemblages. Both forms represent the calcitarch group, which assembled calcispheres of unknown taxonomic affinity. The calcareous dinocyst and calcitarch diversity is low to moderate, compared to the general species richness known from Late Albian assemblages in other Western Tethyan sections. This is interpreted as a result of nutrient input fluctuations due to changes in the circulation pattern of surface and intermediate waters. The changes in the P. sphaerica / P. ovalis ratio along the Upper Albian section are here correlated with short-term (third-order) sea level fluctuations including transgressive and regressive events and a highstand. Pelletization processes might have influenced cyst abundance on the sea floor, especially during periods with oligotrophic surface waters

Response of western South American epeiric-neritic ecosystem to middle Cretaceous Oceanic Anoxic Events

Little is known about the impact of the mid-Cretaceous Oceanic Anoxic Events (OAEs) on the neritic carbonate systems in South America. In order to fill this knowledge gap, the present paper reports on the record of environmental changes in the Albian–Turonian neritic carbonates from the western South American domain in Peru. Owing to the very expanded and well-exposed sections in the Oyon region of central Peru, the OAE 1d and 2 intervals were sampled at high temporal resolution for both bulk micrite and bulk organic matter carbon isotopes, allowing us to compare the fingerprint of these two events between the northern and central Peruvian regions. This suggests the installation of two marked depositional modes: 1) the Albian–Turonian formation of a regional facies belt constituted by oyster-rich mixed siliciclastic-carbonate deposition along the western South America platform; 2) a restricted oligotrophic environment, characterized by the mass occurrence of Perouvianella peruviana and associated miliolids in central Peru during the late Cenomanian–Turonian. These observations advocate for the following scenario: Global warming during the late Albian–early Turonian resulted in humid climate on the western platform. This in turn caused enhanced chemical weathering rates on the Brazilian Shield, resulting in high runoff of nutrients onto the western platform. Nutrient runoff promoted the diversification of benthic oyster communities. Due to the uplift of the Marañon Massif and the installation of the Huarmey Trough, central Peru was isolated from the Pacific and from eastern deltaic influx of the Brazilian continental basement, allowing the local development of oligotrophic conditions during OAE 2. Furthermore, an increased influx of argillaceous sediment and reduced carbonate production is recorded in northern Peru at the onset of OAE 2, marked by a prominent negative shift in δ13C. This negative carbon-isotope excursion has also been identified in other sections in the Pacific domain and can be linked to an increase in isotopically light pCO2 induced by the formation of the Caribbean large igneous province.

Marine primary producer community during the mid-Cretaceous oceanic anoxic events (OAEs) 1a, 1b and 1d in the Vocontian Basin (SE France) evaluated from triaromatic steroids in sediments

Desmethyl and A-ring methyl triaromatic (TA) steroids, including dinosteroids, were analyzed in the sediments deposited during the mid-Cretaceous oceanic anoxic events (OAEs) 1a (Goguel), 1b (Jacob, Kilian and Paquier) and 1d (Breistroffer) from the Vocontian Basin, SE France. These TA steroids were identified by GC–MS using non-polar (DB5-HT) and mid-polar (VF-200ms) columns; their peaks are more clearly separated by the mid-polar column. The triaromatic dinosteroid indices (TADS and TADS′) are defined as alternative indicators for reconstructing dinoflagellate production. In these relationships, the TADS′ values tend to increase in the samples that have higher abundances of the co-eluted 3-methyl TA steroids. The TADS values are lower in the Goguel, Jacob and Kilian samples but increase in the Paquier and Breistroffer samples, and are especially consistently higher in the Paquier. These results suggest that dinoflagellates flourished under the stratified and eutrophic conditions in surface waters during the Paquier deposition (OAE1b). Moreover, an inverse correlation between the TADS and C27 TA steroid (TAS) values is noticeable in the samples from the Kilian and Paquier deposits. Thus, the C27 TA steroids in these sediments, which originated from C28 sterols, may be primarily derived from an ecological competitor of dinoflagellates. We propose that their competitors are possibly coccolithophorids and/or prasinophyte algae. During the mid-Cretaceous hothouse period, coccolithophorids/prasinophytes had the potential to flourish under more oligotrophic conditions, whereas dinoflagellates were dominant as primary producers under eutrophic conditions.

The Albian–Cenomanian transition and Oceanic Anoxic Event 1d – an example from the Boreal Realm

AbstractThe upper Albian to lower Cenomanian interval in the Hanover area (Germany) is characterized by a well‐developed lithological transition from upper Albian monotonous (marly) claystones of several hundred metres thickness, which become enriched in biosilica in the topmost Albian, to increasingly CaCO3‐rich marls and limestones in the lower and middle Cenomanian. This study presents a new 165 m thick composite record spanning the latest Albian to mid‐early Cenomanian (ca 103·0 to 97·5 Ma) based on two core drillings at Anderten in the central Lower Saxony Basin (east of Hanover), which is a sub‐basin of the North German Basin. Due to the lack or delayed occurrences of Tethyan biostratigraphic markers for the late Albian and early Cenomanian in the Boreal Realm, the definition of the Albian–Cenomanian boundary, as based on the lowest occurrence of Thalmanninella globotruncanoides, is not applicable. In order to overcome this issue, an integrated biostratigraphic–chemostratigraphic approach has been employed. In addition to biostratigraphy (calcareous nannofossils, ostracods, planktonic foraminifera and inoceramids), the Anderten succession has been studied using sedimentary geochemistry (CaCO3, total organic carbon, δ13C and δ18O), γ‐ray logging and X‐ray fluorescence core scanning. This approach enabled successful correlation of the composite Anderten record to European reference sections in England, south‐east France and Italy, as well as to further low‐resolution records from the Lower Saxony Basin (Wunstorf and Konrad 101 core). This work has made it possible to identify the δ13Ccarb expression of: (i) the Oceanic Anoxic Event 1d; (ii) the Albian–Cenomanian boundary interval; and (iii) potentially the Lower Cenomanian Event(s). Chemostratigraphic age assignments are supported by biostratigraphic results. Calcareous nannofossils indicate an extended Upper Cretaceous 0 (UC0) zone (upper Albian) up to Upper Cretaceous 2 (UC2; upper lower Cenomanian), and the occurrence of the ostracod Physocythere steghausi supports an early Cenomanian age for most of the upper part of the composite record. Notably, influxes of the planktonic foraminifer Thalmanninella aff. reicheli have previously been described from this area and the first appearance of this taxon is usually used to define the base of the middle Cenomanian Substage. However, findings from the Anderten cores and elsewhere suggest that this taxon has an apparently earlier first appearance, questioning previous age assignments from the Boreal Realm solely based on planktonic foraminifera. Due to both moderately well‐preserved microfossils and high sedimentation rates, the drilled succession of Anderten is considered as a potential Boreal reference section for the Albian–Cenomanian transition.

Onset and demise of Cretaceous oceanic anoxic events: The coupling of surface and bottom oceanic processes in two pelagic basins of the western Tethys

AbstractThe upper Albian–lower Turonian pelagic successions of the Tethys record processes acting during the onset, core, and recovery from perturbed conditions across oceanic anoxic event (OAE) 1d, OAE 2, and the mid‐Cenomanian event I (MCE I) relative to intervening intervals. Five sections from Umbria‐Marche and Belluno Basins (Italy) were analyzed at high resolution to assess processes in surface and deep waters. Recurrent facies stacking patterns (SP) and their associations record periods of bottom current activity coupled with surface changes in trophic level. Climate changes appear to have been influential on deep circulation dynamics. Under greenhouse conditions, vigorous bottom currents were arguably induced by warm and dense saline deep waters originated on tropical shelves in the Tethys and/or proto‐Atlantic Ocean. Tractive facies postdating intermittent anoxia during OAE 1d and in the interval bracketed by MCE I and OAE 2 are indicative of feeble bottom currents, though capable of disrupting stratification and replenish deep water with oxygen. The major warming at the onset of OAE 2 might have enhanced the formation of warm salty waters, possibly producing local hiatuses at the base of the Bonarelli Level and winnowing at the seafloor. Hiatuses detected at the top of the Bonarelli Level possibly resulted from most effective bottom currents during the early Turonian thermal maximum. Times of minimal sediment displacement correlate with cooler climatic conditions and testify a different mechanism of deep water formation, as further suggested by a color change to reddish lithologies of the post‐OAE 1d and post‐OAE 2 intervals.

Organic matter in upper Albian marine sediments in the High-Tatric units, central western Carpathians related to Oceanic Anoxic Event 1d—Geochemistry, microfacies and palynology

A multidisciplinary study of upper Albian hemipelagic sediments related to the Oceanic Anoxic Event 1d (OAE1d) from the High-Tatric units (central western Carpathians, Polish part of the Tatra Mountains) was undertaken to characterize organic matter (OM) and to investigate its origin. Microfacies and palynological records combined with mineralogical and geochemical data from these sediments show the occurrence of both marine and land-derived OM in the limestone and marl successions whose deposition corresponds to the Parathalmanninella apenninica Zone in the late Albian. Both types of OM were strongly degraded under the oxic conditions that prevailed during the entire time of their deposition. The sedimentation of marls with low contents of total organic carbon (<0.24%), dominated by land-derived OM, occurred during the final phase of OAE1d. It was probably related to a sudden change in the circulation pattern at the margins of the Tatric–Zliechov sedimentary basin connected with a sea-level rise. The coastal upwelling along the Tatric Ridge that prevailed during the early late Albian gradually weakened or disappeared and was replaced by a downwelling current flowing from the shallow parts of the basin that transported land-derived OM from nearshore coastal plains.

Carbon- and oxygen-isotope records of mid-Cretaceous Tethyan pelagic sequences from the Umbria – Marche and Belluno Basins (Italy)

The late Albian – early Turonian time interval was marked by major environmental changes at regional to global scales. Oceanic Anoxic Events (OAE) 1d and OAE 2 and the mid-Cenomanian Event (MCE I) are all associated with ?13C anomalies that indicate perturbations of the global carbon cycle. In this study, new carbon- and oxygen-isotope records are presented from four Italian Tethyan sections: Furlo, Le Brecce and Monte Petrano (Umbria – Marche Basin) and Cismon (Belluno Basin), deposited in pelagic settings characterized by the alternation of nannofossil-planktonic foraminiferal limestones, radiolarian-rich levels and shales. The ?13C records exhibit a ? 1 ‰ positive excursion in ?13Ccarb corresponding with OAE 1d and a much larger positive anomaly (? 3–4 ‰) in ?13Corg corresponding with OAE 2 (Bonarelli Level). Between the strata that record these two events a double-spiked minor (? 0.7 ‰) positive excursion in ?13Ccarb marks the MCE I. Locally, after deposition of the MCE I but before formation of the Bonarelli Level, sedimentation shifted to alternate dysoxic/anoxic and well-oxygenated conditions as represented by lithological rhythms of black shales/black chert bands and whitish limestones. ?18O data indicate a progressive decrease in values from the upper Albian up to the Bonarelli Level (uppermost Cenomanian), suggesting an overall warming trend. Carbon-isotope stratigraphy, calibrated against biostratigraphy, provides a robust framework for dating and correlation, and allows identification of hiatuses in the studied sections. In particular, the high-resolution ?13C profiles through the Bonarelli Level highlight the presence of hiatuses of variable extent that systematically affect the middle to upper part of the characteristic carbon-isotope excursion and part of the following interval in all of the studied sections. Locally, a hiatus may also be present at the base of the Bonarelli Level. Correlations with carbon-isotope reference curves from other localities allow the duration of the missing intervals to be estimated as between 160 and 510 kyr in the middle to latter part of OAE 2 and the succeeding interval, showing that the Bonarelli Level extends no higher than the uppermost Cenomanian, even though the isotopic definition of the oceanic anoxic event indicates continuation into the early Turonian. The origin of these stratigraphic gaps can be attributed to physical and/or chemical processes operating at the sea floor. Complete recovery from the peculiar physico-chemical conditions that characterized the deeper parts of the Tethys Ocean during OAE 2 took at least 1 million years.

Oceanographic and eustatic control of carbonate platform evolution and sequence stratigraphy on the Cretaceous (Valanginian–Campanian) passive margin, northern Gulf of Mexico

AbstractAn integrated sequence stratigraphic study based on outcrop, core and wireline log data documents the combined impact of Cretaceous eustacy and oceanic anoxic events on carbonate shelf morphology and facies distributions in the northern Gulf of Mexico. The diverse facies and abundant data of the Comanche platform serve as a nearly complete global reference section and provide a sensitive record of external processes affecting Cretaceous platform development. Regional cross‐sections across the shoreline to shelf‐margin profile provide a detailed record of mixed carbonate–siliciclastic strata for the Hauterivian to lower Campanian stages (ca 136 to 80 Ma). The study window on the slowly subsiding passive margin allows the stratigraphic response to external forcing mechanisms to be isolated from regional structural processes. Three second‐order supersequences comprised of eight composite sequences are recognized in the Valanginian–Barremian, the Aptian–Albian and the Cenomanian–Campanian. The Valanginian–Barremian supersequence transitioned from a siliciclastic ramp to carbonate rimmed shelf and is a product of glacial ice accumulation and melting, as well as variable rates of mid‐ocean ridge volcanism. The Aptian–Albian supersequence chronicles the drowning and recovery of the platform surrounding oceanic anoxic events 1a and 1b. The Cenomanian–Campanian supersequence similarly documents shelf drowning following oceanic anoxic event 1d, after which the platform evolved to a deep‐subtidal system consisting of anoxic/dysoxic shale and chalk in the time surrounding oceanic anoxic event 2. Each period of oceanic anoxia is associated with composite sequence maximum flooding, termination of carbonate shelf sedimentation and deposition of condensed shale units in distally steepened ramp profiles. Composite sequences unaffected by oceanic anoxic events consist of aggradational to progradational shelves with an abundance of grain‐dominated facies and shallow‐subtidal to intertidal environments. Because they are products of eustacy and global oceanographic processes, the three supersequences and most composite sequences defined in the south Texas passive margin are recognizable in other carbonate platforms and published eustatic sea‐level curves.

Albian to Santonian carbon isotope excursions and faunal extinctions in the Canadian Western Interior Sea: Recognition of eustatic sea-level controls on a forebulge setting

The forebulge region of the Cretaceous Canadian Western Interior Sea (CWIS) was susceptible to subaerial exposure and marine erosion during sea level lowstands. The middle Albian to Santonian record as cored at Cold Lake, east-central Alberta, Canada documents numerous disconformities that are expressed in bioclastic concentration horizons and faunal extinctions and turnovers. Detailed comparison between a newly established δ13Corg. record measured on bulk sediment at Cold Lake and a combined δ13Ccarb. reference curve based on the Cretaceous English chalk and SE France hemipelagic marlstones highlights missing positive and negative δ13C excursions at the CWIS forebulge and thus missing sections that precisely corroborate with sequence boundaries. Disconformable boundaries correlate closely with global sea-level lowstands as established for the Cretaceous North Atlantic suggesting a pronounced eustatic influence on the CWIS forebulge setting. Sequence boundaries occur in the uppermost Middle Albian, lowermost Upper Albian, Albian/Cenomanian boundary, Cenomanian/Turonian boundary, middle Turonian to lower Coniacian and uppermost Middle Santonian, each followed by a positive δ13C excursion. Oceanic anoxic events 1d, 2 and 3 are recognized and linked to major faunal and floral assemblage changes. Of these the Albian/Cenomanian biotic turnover is the most severe in the CWIS marked by the total loss of Albian benthic foraminifera species. Causes of this benthic extinction might be linked to a period of anoxia (OAE 1d) during the latest Albian followed by sea-level controlled basin restriction.

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 δ13C 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 δ18O 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‰ δ13C increase (∼100.0–99.6 Ma), which is similar in age and magnitude to δ13C 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 δ13C 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 δ13C increase has (have) yet to be found. Our findings add weight to the view that OAEs should be chemostratigraphically (δ13C) rather than lithostratigraphically defined.

Reconstruction of short-term palaeoceanographic changes during the formation of the Late Albian ‘Niveau Breistroffer’ black shales (Oceanic Anoxic Event 1d, SE France)

The Niveau Breistroffer black shale succession in the Vocontian Basin (SE France) is the regional equivalent of the widely distributed Late Albian Oceanic Anoxic Event 1d. The studied black shale-rich interval at the Col de Palluel section is 6.28 m thick and comprises four black shale units with up to 2.5 wt% total organic carbon (TOC) intercalated with marlstones. Calcareous nannofossil, palynomorph, planktic Foraminifera and stable isotopic data from the Niveau Breistroffer succession suggest that short-term climate changes influenced its deposition, with relatively warm and humid climate during black shale formation in comparison with relatively cool and dry climatic conditions during marlstone deposition. An increase in the terrigenous/marine ratio of palynomorphs indicates enhanced humidity and higher runoff during black shale formation. A nutrient index based on calcareous nannofossils and the abundance pattern of small (63–125 μm) hedbergellid Foraminifera show short-term changes in the productivity of the surface water. Surface-water productivity was reduced during black shale formation and increased during marlstone deposition. A calcareous nannofossil temperature index and bulk-rock oxygen isotope data indicate relative temperature changes, with warmer surface waters for black shale samples. At these times, warm–humid climate and reduced surface-water productivity were accompanied by greater abundances of ‘subsurface’-dwelling calcareous nannofossils (nannoconids) and planktic Foraminifera (rotaliporids). These taxa presumably indicate more stratified surface-water conditions. We suggest that the formation of the Niveau Breistroffer black shales occurred during orbitally induced increase in monsoonal activity that led to increasing humidity during periods of black shale formation. This, in turn, caused a decrease in low-latitude deep-water formation and probably an increase in surface-water stratification. The combination of these two mechanisms caused depleted O 2 concentrations in the bottom water that increased the preservation potential of organic matter.

Calcareous nannoplankton response to late Albian oceanic anoxic event 1d in the western North Atlantic

Well‐preserved nannofossil and stable isotope records from the mid‐Cretaceous of Ocean Drilling Project Leg 171B (western North Atlantic) indicate cyclical, productivity‐based variations in surface water characteristics, suggesting orbitally paced changes in upwelling intensity and the strength of deep mixing associated with oceanic anoxic event (OAE)1d. Paleontologic and isotopic evidence suggest that collapse of upper water column stratification associated with OAE1d was preceded by approximately 1 m.y. of progressively decreasing water column stability and increasing surface water fertility. Thirteen species went extinct during a short (ca. 200 k.y.) interval associated with OAE1d. Nine of these have morphological characters suggesting adaptation to specific depths in the photic zone. Simultaneous extinction of these depth‐zoned species suggests that stratification collapse disrupted the habitat space for these specialized forms, resulting in extinction. A cyclostratigraphic model provides age estimates of several important nannofossil datums including the first appearances of Eiffellithusturriseiffelii (100.95 Ma) and Corollithionkennedyi (99.55 Ma).

Ammonoid Abundance Variations Related to Changes in Trophic Conditions Across the Oceanic Anoxic Event 1d (Latest Albian, SE France)

Abstract Macrofauna abundance has been quantified across the Breistroffer interval (OAE 1d, Latest Albian) in the Vocontian basin (southeast France). This interval is characterized by a high abundance of ammonoids showing significant morphologic disparity. Twenty ammonoid genera are recognized and assigned to seven morphostructural groups. The aims of this study are to: (1) interpret abundance variations of ammonoid taxa in terms of taphonomic processes, sedimentary dilution, and paleoenvironmental factors; and (2) discuss the habitat and mode of life of some Late Albian ammonoids with respect to their morphology and abundance variations. Most variations in ammonoid absolute abundance are not the result of post-mortem shell transport. High abundance within the Breistroffer interval is due partly to a low accumulation rate, which is related to a maximum-flooding stage. The different trends observed among ammonoid taxa can be interpreted partly in terms of response to changes in trophic conditions inferred ...

Albian high-resolution biostratigraphy and isotope stratigraphy: The Coppa della Nuvola pelagic succession of the Gargano Promontory (Southern Italy)

High-resolution δ13C and δ18O curves, calibrated against planktonic foraminiferal and calcareous nannofossil biostratigraphy, are provided for the upper Aptian–lower Cenomanian pelagic succession of the Gargano Promontory (Coppa della Nuvola section, southern Italy). The succession consists of two superimposed formations: the Marne a Fucoidi and the Scaglia (lower portion only). According to our integrated biostratigraphy, the entire succession spans the latest Aptian (planktonic foraminiferal T. bejaouaensis and calcareous nannofossil R. angustus Zones) and early Cenomanian stages (R. cushmani Zone CC9c). The Marne a Fucoidi–Scaglia transition falls in the late Albian (R. ticinensis Zone; CC9a+b Subzones). The high-resolution δ13C curve from the Coppa della Nuvola section can be sub-divided into characteristic segments. Four negative shifts of δ13C are recorded, followed by increasing values in, respectively, the early Albian (C11, C12), the early late Abian (C14, C15), the late Albian (C16, C17), and the early Cenomanian (C22). The late Albian carbon-isotope event, corresponding to the Oceanic Anoxic Event (OAE) 1d or Breistroffer Event, is possibly missing in the Coppa della Nuvola section as a result of condensation or erosion (C19–C20?). Even if, in the section studied, carbon-rich levels are not recorded (though some stratigraphic intervals are covered by Quaternary deposits in the lower portion), it is likely that the lower Albian (T. primula; P. columnata Zones) and the upper Albian (T. praeticinensis Subzone–R. subticinensis Zone; R. achlyostaurion Zone) positive δ13C peaks succeeding negative trends in δ13C (C11–C12; C16–17) record the pattern of global carbon burial, documented in other areas of the Gargano Promontory and elsewhere and connected to the OAE1b and OAE1c. Some or all of the negative δ13C shifts may record the introduction of isotopically light carbon into the ocean–atmosphere system from the dissociation of gas hydrates. The δ18O curve of the Coppa della Nuvola section shows a similar trend to that of the δ13C curve. Although partly of diagenetic origin, the negative shifts in the early Albian, late Albian and early Cenomanian may be interpreted as records of warming events resulting from the introduction of methane and its oxidation product carbon dioxide into the atmosphere. The palaeoceanographic conditions inferred by the biotic and isotopic changes suggest fluctuating meso-eutrophic conditions through the late-Aptian–early-middle Albian and increasingly stable oligotrophic situation starting from the late Albian interval. Two main possible episodes of increased eutrophy, suggested by pulses in radiolarian abundance and marked drop in foraminiferal diversity, occurred during the Albian and correlate with the above-mentioned positive carbon-isotope shifts and regional and supraregional accumulation of organic matter (OAE1b and OAE1c).

Minor changes in surface-water fertility across the oceanic anoxic event 1d (latest Albian, SE France) evidenced by calcareous nannofossils

The latest Albian sediments deposited in the Vocontian basin in south-eastern France record the oceanic anoxic event (OAE) 1d, locally named the Breistroffer interval. They are represented by argillaceous marls and marls, containing organic matter (total organic carbon varying from 0.8 to 1%). Changes in calcareous nannofossil abundance and assemblage composition as well as macrofauna abundance, ichnofossil assemblage, and bioturbation intensity were analysed in order to evaluate the main palaeoenvironmental factors controlling the deposition of the OAE 1d sediments in this area. The pelagic carbonate production is limited and the carbonate fraction is predominantly of nektonic/benthic, and of allochthonous origin from carbonate platforms. The enrichment in organic carbon within the Breistroffer interval is weak and not associated with high surface-water productivity. The organic matter is mainly terrigenous. Its record is due to (1) a good preservation under dysoxic conditions within the sediments, and (2) a weak input of allochthonous carbonates. Eustatic fluctuations strongly influenced the variations of nannofossil and macrofauna abundances. Distinctive patterns in nannofossil assemblages and macrofauna abundances within the Breistroffer interval are also recognized, reflecting changes from mesotrophic to more oligotrophic conditions which are probably controlled by climate.

Deposition of sedimentary organic matter in black shale facies indicated by the geochemistry and petrography of high-resolution samples, Blake Nose, western North Atlantic

Abstract A transect of three holes drilled across the Blake Nose, western North Atlantic Ocean, retrieved cores of black shale facies related to the Albian Oceanic Anoxic Events (OAE) 1b and 1d. Sedimentary organic matter (SOM) recovered from Ocean Drilling Program Hole 1049A from the eastern end of the transect showed that before black shale facies deposition organic matter preservation was a Type III–IV SOM. Petrography reveals that this SOM is composed mostly of degraded algal debris, amorphous SOM and a minor component of Type III–IV terrestrial SOM, mostly detroinertinite. When black shale facies deposition commenced, the geochemical character of the SOM changed from a relatively oxygen-rich Type III–IV to relatively hydrogen-rich Type II. Petrography, biomarker and organic carbon isotopic data indicate marine and terrestrial SOM sources that do not appear to change during the transition from light-grey calcareous ooze to the black shale facies. Black shale subfacies layers alternate from laminated to homogeneous. Some of the laminated and the poorly laminated to homogeneous layers are organic carbon and hydrogen rich as well, suggesting that at least two SOM depositional processes are influencing the black shale facies. The laminated beds reflect deposition in a low sedimentation rate (6m Ma −1 ) environment with SOM derived mostly from gravity settling from the overlying water into sometimes dysoxic bottom water. The source of this high hydrogen content SOM is problematic because before black shale deposition, the marine SOM supplied to the site is geochemically a Type III–IV. A clue to the source of the H-rich SOM may be the interlayering of relatively homogeneous ooze layers that have a widely variable SOM content and quality. These relatively thick, sometimes subtly graded, sediment layers are thought to be deposited from a Type II SOM-enriched sediment suspension generated by turbidites or direct turbidite deposition.