Oceanic Anoxic Event 1a Research Articles

Paleoceanographic evolution and chronostratigraphy of the Aptian Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) in the Gorgo a Cerbara section (central Italy)

We performed a detailed study of the stratigraphic transition of Oceanic Anoxic Event 1a (OAE1a) to oceanic red bed 1 (ORB1) from the classic Gorgo a Cerbara section in the Umbria region of central Italy. We focused on a 25.5-m-thick stratigraphic succession, from which we analyzed 305 samples for total organic carbon (TOC), CaCO3, magnetic susceptibility, diffuse reflectance spectrophotometry and the stable carbon and oxygen isotopic composition of both bulk samples and organic matter. In the Gorgo a Cerbara section, the Selli Level of OAE1a (∼1.81 m thick) consists of laminated to bioturbated dark gray to black mudstones and shales with medium to dark gray radiolarian-rich silty to sandy layers and a maximum TOC content of 20.22%. The carbon isotopic values show a negative excursion (C3 stage, ∼0.14 m) at the base of the Selli Level, followed by a stepwise positive excursion (C4–C6 stages, ∼1.67 m) in the upper part of the Selli Level. The transition from OAE1a to ORB1 (∼3.19 m thick) is characterized by bioturbated greenish gray cherty limestones and marly limestones with subordinate marls, corresponding to stable carbon isotopic C7 stage and lasts for ∼0.75 Ma. The ORB1 interval (∼13.15 m) consists of reddish marly claystones, dark-red marlstones, red marly limestones and red calcareous shales which indicate a highly oxic environment. Our results reveal a stepwise transition from a predominantly mesotrophic and dysoxic to anoxic environment at the time that the OAE1a black shales were deposited to an oligotrophic and oxic environment during the transitional interval and finally to highly oxic conditions during the ORB1 interval. The nannoconid crisis occurs at the top of the C2 stage, just 0.34 m below the negative excursion in δ13C isotopic values. The massive CaCO3 dissolution phase occurs 0.25 m above the negative excursion; it persisted for 0.85 Ma and probably resulted from excess CO2, ocean acidification, and carbonate compensation depth (CCD) shoaling. Deposition of massive black shales occurs at the base of the C6 stage and lasted for 0.4 Ma.

The impact of OAE 1a on marine biota deciphered by size variations of coccoliths

The early Aptian Oceanic Anoxic Event 1a (OAE 1a; ∼120 Ma) was caused by a global perturbation of the Early Cretaceous climate. It supposedly affected the composition of the marine biosphere, including the primary producers. This study aims at using the size evolution of three species of coccolithophores (Biscutum constans, Zeugrhabdotus erectus and Watznaueria barnesiae) for better understanding the impact of the OAE 1a on primary producers. A total of 30 samples derived from three sites, which cover the upper Barremian–lower Aptian interval, have been analyzed from the North Sea and the Lower Saxony Basin. The sections expose near-shore and shallow marine sediments.The measured data of B. constans and Z. erectus are characterized by a size decrease synchronous to the negative carbon isotope excursion. This size reduction marks the early phase of the OAE 1a, more precisely the carbon isotope segment C3. Coccolith sizes recover to pre-OAE 1a values in the aftermath of this brief interval. The short termed size reduction is seen as a reaction of specific nannofossil taxa to an increase in humidity during the early phase of OAE 1a. Due to high weathering rates and a substantial run-off, the amount of detrital material transported into the marine system increased significantly. Consequently light availability diminished in the surface waters, causing habitat changes for the photoautotrophic primary producers. Light-sensitive species like B. constans and Z. erectus adjusted by forming smaller skeletons, thereby reducing their size. This strategy allowed for dwelling in shallower water depth and thus compensated for the decrease in sun light. The sizes of W. barnesiae in contrast remain stable throughout the entire OAE 1a interval. W. barnesiae was not affected by these environmental shifts and is thus interpreted as being robust with respect to changes of the sun light.

Gradual and sustained carbon dioxide release during Aptian Oceanic Anoxic Event 1a

A period of ocean anoxia about 120 million years ago coincided with high temperatures. A reconstruction of CO2 concentrations shows that volcanic outgassing from the Ontong Java Plateau caused CO2 levels to double during the anoxic event.

Barremian-Aptian-Albian carbon isotope segments as chronostratigraphic signals: Numerical age calibration and durations

Chemostratigraphic signals are becoming common tools in chronostratigraphic correlations. Specifically total organic and carbonate carbon percentages (TOCorg/carb) and carbon isotope curves are diagnostic in Cretaceous Aptian-Albian-Cenomanian strata. Cretaceous carbon isotope signals define oceanic anoxic events (OAEs) and have been subdivided into successive segments. These segments are used as correlation markers because they represent oceanographic and depositional conditions, and hence, incur time significance. Although the segments have been integrated with biovents in key sections, numerical ages of the segments have yet to be calibrated. Because direct correlation of these segments of carbon isotope curves with interbedded radiometrically dated beds is not possible currently, other methods must be applied to interpolate numerical ages. Graphic plots of five sections successfully integrate carbon isotope segments with established numerically dated chronostratigraphicmarkers. Carbon isotope segments were first defined in European Tethys Cismon and Rotter Sattel Barremian-Albian sections. The Aptian segments were subsequently identified in the Mexican Santa Rosa Canyon sectionwhere the lower Albian segments were defined. Each section haswell documented microfossils andmagnetochrons that relate the segments to biozones. These bioevents and chronozones enable the carbon segments to be integrated with a comprehensive Cretaceous time scale, the CRECSDB42. Ages assigned to the segments enable them to be used in sections where local fossil zones are recorded such as in the Chihuahua basin in northernMexico. The age calibration of carbon isotope segments measures the durations of the oceanic isotopic excursions. The durations fluctuate up to and after OAE 1a at irregular frequencies different from climatic cycles. Changing durations of carbon isotope segments may be clues to changes in processes or the effects of the process that altered the ocean carbon reservoir in addition to climatic changes.

The palynostratigraphy of the Upper Maiolica, Selli Level and the Lower Marne a Fucoidi units in the proposed Barremian/Aptian (Lower Cretaceous) GSSP stratotype at Gorgo a Cerbara, Umbria–Marche Basin, Italy

The section of Gorgo a Cerbara in Italy has been proposed as the Global Stratotype Section and Point (GSSP) for the Barremian/Aptian boundary (Lower Cretaceous). This section displays the transition between the white pelagic limestones of the Maiolica Formation (Tithonian–lowermost Aptian) and the polychrome marls of the Marne a Fucoidi Formation (Aptian–Albian). The Oceanic Anoxic Event 1a (OAE 1a) anoxic horizon (Selli Level) is located in the latter formation. Here, we report for the first time a palynostratigraphical assessment of this key section, with a very high resolution in the Selli Level.Palynomorph groups recorded include acritarchs, microforaminiferal test linings, 58 dinoflagellate cysts, 44 sporomorph taxa, and freshwater fungal and algal bodies. These include such biostratigraphical markers as Rhynchodiniopsis aptiana and Odontochitina operculata among the dinoflagellate cysts and Afropollis cf. jardinus among sporomorphs. Biostratigraphical events recognised allow comparisons with previously studied successions in other similar areas (e.g. Cismon apticore in the Southern Alps, Italy). The distribution of taxa recorded in the Gorgo a Cerbara section is generally consistent with published data, but several differences between the assemblages recorded in the Cismon apticore and in this study were noted.The best global markers for the Barremian/Aptian boundary are represented by the disappearance of R. aptiana and by the appearance of O. operculata, but in this section these occurred 0.65–1.35 Myr after the magnetostratigraphically defined boundary, within the Selli Level. The angiosperm pollen A. cf. jardinus is recorded only within the Selli Level (from 1.4 to 1.6 Myr after the Barremian/Aptian boundary), consistent with an Early Aptian age. A cluster of 10 extremely short-ranging dinoflagellate cyst excursion taxa appear in this section coincident with the Nannoconid Crisis event and its associated thermal peak, ca. 10–20 kyr before the onset of the OAE 1a. Several short-ranging Pteridophyta spore species are found only within the Selli Level itself.

Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)

An abrupt rise in temperature, forced by a massive input of CO2 into the atmosphere, is commonly invoked as the main trigger for Oceanic Anoxic Events (OAEs). Global warming initiated a cascade of palaeoenvironmental perturbations starting with increased continental weathering and an accelerated hydrological cycle that delivered higher loads of nutrients to coastal areas, stimulating biological productivity. The end-result was widespread anoxia and deposition of black shales: the hallmarks of OAEs. In order to assess the role of weathering as both an OAE initiator and terminator (via CO2 sequestration) during the Early Aptian OAE 1a (Selli Event, ∼120 Ma) the isotopic ratio of lithium isotopes was analysed in three sections of shallow-marine carbonates from the Pacific and Tethyan realms and one basinal pelagic section from the Tethyan domain. Because the isotopic composition of lithium in seawater is largely controlled by continental silicate weathering and high- and low-temperature alteration of basaltic material, a shift to lighter δ7Li values is expected to characterize OAEs. The studied sections illustrate this phenomenon: δ7Li values decrease to a minimum coincident with the negative carbon-isotope excursion that effectively records the onset of OAE 1a. A second negative δ7Li excursion occurs coeval with the minimum in strontium isotopes after the event. The striking similarity to the strontium-isotope record argues for a common driver. The formation and destruction (weathering) of an oceanic LIP could account for the parallel trend in both isotope systems. The double-spike in lithium isotopes is probably related to a change in weathering congruencies. Such a chemostratigraphy is consistent with the hypothesis that an increase in silicate weathering, in conjunction with organic-carbon burial, led to drawdown of atmospheric CO2 during the early Aptian OAE 1a.

Aptian giant explosive volcanic eruptions in the Songliao Basin and northeast Asia: A possible cause for global climate change and OAE-1a

Volcanism is a natural climate force that causes variations in temperatures. The Aptian Oceanic Anoxic Event 1a (OAE-1a) was preceded by a prominent negative C-isotope excursion attributed to major volcanism of the Ontong Java plateau (OJP), which presumably resulted in a pCO2 increase and a climatic change. However, the OJP alone may not adequately explain some important isotopic signatures such as the negative strontium-isotope excursion from 125 Ma to 113 Ma that is recorded in the corresponding marine deposits. We present an independent and hitherto undocumented case, the giant Aptian volcanism in the Songliao Basin and northeast Asia (SB-V) on the Cretaceous active continental margin between the Eurasian and the Pacific plates, which covered an area of ca. 2.3 × 106 km2, nearly matching the simultaneous case of the OJP. Intensive strong, explosive volcanic eruptions of the SB-V occurred at 121–109 Ma and introduced a large volume of fine-grained volcanic ash and degassing volatiles into the atmosphere. The Aptian isotopic ratios (87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) of marine carbonates from the Mid-Pacific shift in values between their Barremian pre-excursion high values and the negative magmatic values of the SB-V. The transient global cooling at the onset of the OAE-1a coincided with the beginning of the violent acidic eruption of the SB-V (119.9–120.2 Ma). We therefore infer that the SB-V must have played a role in the Aptian global climatic changes and OAE-1a through the heavy fall of volcanic dust and the outgassing of aerosol and greenhouse gases.

The Early Cretaceous Barents Sea Sill Complex: Distribution, 40Ar/39Ar geochronology, and implications for carbon gas formation

Mafic igneous rocks of Cretaceous age (80–130Ma) scattered around the Arctic Ocean are commonly referred to as the High Arctic Large Igneous Province (HALIP). We have mapped out the distribution of HALIP igneous rocks in the Barents Sea region over the past decade based on integrated seismic–gravity–magnetic interpretation, field work, review of publications, and analyses of new and vintage borehole and field samples. The mapping reveals abundant igneous rocks in the northern and eastern Barents Sea covering an area of ~900,000km2 with a conservative volume estimate of 100,000 to 200,000km3 of intrusions. The igneous province is dominated by sheet intrusions injected into Triassic and Permian sedimentary rocks. Hydrothermal vent complexes are rare, and only two potential vent complexes have been identified on seismic data in the eastern Barents Sea. We have further done extensive radiometric dating of the igneous samples in the Barents Sea region. New 40Ar/39Ar dating of thirteen samples from Svalbard reveal ages of crystallization and alteration. The large age span (60–140Ma for the raw ages) is likely due to partial or complete overprint of the K/Ar system in plagioclase, and the age of the magma emplacement is better represented by U/Pb TIMS ages. Only one of our 40Ar/39Ar analyses of plagioclase yielded a statistically valid age that is in line with the recently published U/Pb TIMS ages of 122–125Ma. The new data clearly document that relying on published data from the K/Ar system can lead to erroneous conclusions on the age of crystallization in this province without a careful use of additional 40Ar/39Ar degassing data (i.e., K/Ca). We propose that the magmatism on Svalbard and Franz Josef Land represents a distinct magmatic event near the Barremian/Aptian boundary (125Ma) in the Barents Sea. This Early Cretaceous Barents Sea magmatism resulted in the formation of the BSSC (Barents Sea Sill Complex). BSSC age rocks are also present in Arctic Canada (Sverdrup Basin) and on Bennett Island (New Siberia Islands). The massive injection of hot magma into potentially organic-rich sediments in the eastern and northern Barents Basin caused rapid organic matter maturation and formation of thermogenic gas and oil in contact aureoles. We estimate that up to 20,000 Gt of carbon were potentially mobilized, corresponding to 175 trillion barrels of oil equivalent. The production rates and fate of the carbon gases are uncertain. However, we speculate that rapid release of aureole greenhouse gases (methane) may have triggered the Oceanic Anoxic Event 1a (OAE1a) and the associated negative δ13C excursion in the Early Aptian. Some of the methane may also be trapped in the vast hydrocarbon gas accumulations found in the east Barents Basin.

The Cogollo Group and the oceanic anoxic events 1a and 1b, Maracaibo basin, Venezuela

ABSTRACTCarbonates of Cogollo Group (Apón, Lisure and Maraca formations) constitute the broader calcareous platform system originated during Aptian and Albian of Cretaceous in north-western South America, Maracaibo Basin, Venezuela. On the shallow shelf, a variety of calcareous sedimentary facies were deposited during marine transgressive and regressive cycles. Some of them developed porosity and constitute important hydrocarbon reservoirs. Due to some major marine transgressions, from early Aptian, the anoxic environment and characteristic facies of a pelagic environment moved from the outer slope and basin to the shallow shelf, during specific time intervals, favouring the sedimentation of organic matter-rich facies, which correspond to the oceanic anoxic events (OAEs) 1a and 1b. The source rock of Machiques Member (Apón Formation) was deposited during early Aptian OAE 1a (~ 120 Ma). The source rock of Piché Member, located at the top of the Apón Formation, was deposited during late Aptian OAE 1b (~ 113 Ma). Finally, La Luna Formation, from Cenomanian, that covers the OAE 2 (~ 93 Ma), represents the most important source rock in the Maracaibo Basin. In this way and based on sedimentological and organic geochemistry results from the determinations performed on 247 samples belonging to six cores in the Maracaibo Basin, we propose these two organic-rich levels, deposited on the shallow shelf of the Cogollo Group, as "effective source rocks", additional to La Luna Formation, with oil migration in relatively small distances to the porosity facies.

The influence of regional factors in the expression of oceanic anoxic event 1a (OAE1a) in the semi-restricted Organyà Basin, south-central Pyrenees, Spain

The global occurrence of oxygen-deprived conditions in the ocean characterizes the early Aptian; hence the hallmark of that time interval is the accumulation of organic-rich sediments during an episode of severe oxygen deficiency known as oceanic anoxic event 1a (OAE1a). As seen elsewhere in the Tethyan domain, an overall change in facies marked by increased input of terrigenous material distinguishes the lower Aptian in the study area, but the nature of the lithologies associated with the organic carbon-rich deposits remain dominated by carbonate sediments and are expressed differently with respect to other areas where the occurrence of black shales characterizes OAE1a. This comparison indicates that significant spatial heterogeneity occurred in sub-Tethyan basins coeval with the global oceanic event.The present work provides a high-resolution, multi-proxy study (petrography, total inorganic carbon (TIC), total organic carbon TOC, carbon isotope [δ13Corg], clay mineralogy and biomarkers) of the upper 155m of the El Pui section, Organyà Basin, including the record of OAE1a. The results reveal that oxygenic conditions fluctuated throughout the studied interval, with episodes of oxygen depletion that concurred with TOC values >1%. However, petrographic observations, molecular biomarkers, and redox sensitive trace elements (RSTEs) indicate that full anoxia was not achieved.The results also provide the first high-resolution δ13Corg profile for the El Pui section that replicates in greater detail the carbon isotopic signature reported for the Tethyan domain. The upper 155m of the El Pui section include expanded and well-defined trends of the carbon isotope segments C2 (upper part), C3, C4, C5, and C6 (partial), which correlate with other Tethyan sections and allow for the recognition of the lower Aptian OAE1a in the uppermost 40m of the section. Such improved high-resolution chemostratigraphic curve could contribute to a more precise chronostratigraphic correlation of Aptian events in a broader scale.

Morphological variability in response to palaeoenvironmental change – a case study on Cretaceous ammonites

Changes in ammonite morphology during the earliest Early Aptian (Cretaceous) of an epicontinental sea in northern Germany were investigated based on new and rich material of Deshayesites (Deshayesitidae). This is a globally distributed genus and one of the most important ammonites of the Cretaceous with respect to biostratigraphy and abundance. The purpose of our study was to describe changes in morphology over few hundred thousand years and to discuss their relationships to major palaeoenvironmental perturbations. Our material is derived from four different horizons, and the studied stratigraphical interval includes a sea-level change, a period of warming and an oxygen depletion event. We observe variable patterns in the occurrence of different morphological groups through time, probably indicating that morphotypes were individually impacted by environmental change. These morphological groups, unequivocally attributed to Deshayesites, cannot be fit into the existing species classification system. The greatest morphological disparity in Deshayesites is multi-causal. It may be the result of: (1) an invasion of two morphogroups new to the habitat, which thus immediately face considerably greater competition in an environment just recovered from low-oxygen conditions in parts of the water column during an early interval within Oceanic Anoxic Event 1a (OAE 1a); or, (2) it may be due to a rise in sea level with a simultaneous invasion of competitors of Tethyan origin. This sheds new light on the current concept for high morphological variability within ammonite species and poses challenges to our current ideas about ammonite diversity and the use of well-established index-species in supraregional correlations.

Geochemical and paleoenvironmental record of the early to early late Aptian major episodes of accelerated change: Evidence from Sierra del Rosario, Northeast Mexico

The lower to lower-upper Aptian succession of northern Mexico documents the drowning of the shallow-water Cupido/Cupidito carbonate platform system followed by deposition of the deep-water sediments of the La Peña Formation. Using δ13C stratigraphy, geochemical and mineralogical information coupled with previous microfacies, paleontological and total organic carbon (TOC) data from a stratigraphic section, which includes such lithological change, this study identifies distinctive episodes of accelerated global environmental change, and determines the paleoenvironmental conditions conducive to the deposition of TOC-rich intervals. Within the Cupidito unit, the Oceanic Anoxic Event 1a (OAE 1a) is recorded near the base of the section and the Intra-Furcata Negative Excursion in the topmost beds of the unit. The upper part of the section, within the La Peña Formation, is correlatable with the Noire level. Organic-carbon rich intervals occur in the lower and middle part of the OAE 1a, upper part of the Cupidito unit, base of the La Peña Formation, and in the Noire level equivalent. Reducing conditions within the sediment and oxic–dysoxic at the seafloor, locally controlled, persisted both before and during OAE 1a interval in the Cupidito lagoon. Oxygen-depleted conditions (dysoxic–anoxic) were more permanent and stronger during the deposition of the base of the la Peña Formation and the Noire equivalent level. It is proposed here that deposition of the lower-middle part of the OAE 1a and the base of the La Peña Formation was influenced by climate-controlled increases in detrital and accompanying nutrient influx that supplied especially biolimiting nutrients (Fe, P), fostering marine productivity and TOC burial. Upwelling of nutrient-rich deeper waters and minor arrival of nutrients from runoff, both account for the domination of radiolaria and organic-carbon burial during the Noire level equivalent. Record of the OAE 1a time-equivalent facies in the Cupidito lagoonal sediments confirms the diachronism of the Cupidito–La Peña flooding. Heterozoan carbonate production in the Cupidito ramp was uninterrupted during and after OAE 1a. Periodic arrival of detrital components and nutrients during warmer/humid periods accelerated platform drowning, which peaked during the diachronic deposition of the La Peña Formation.

Black shales – from coolhouse to greenhouse (early Aptian)

Approximately 120 Ma major volcanic outgassing on the Ontong Java Plateau resulted in perturbations of the global carbon cycle, in a change from cool to greenhouse climate conditions and in major changes in oceanography leading to widespread deposition of black shales during the early Aptian Oceanic Anoxic Event 1a (OAE1a). However, onset of black-shale deposition was diachronous and prior to OAE1a occurred under specific paleogeographic and paleoceanographic conditions. The goal of this study is to identify paleoceanographic constraints during coolhouse conditions that resulted in pre-OAE1a black-shale deposition and to investigate if and to what extent the short-term orbitally induced climate changes are also recorded in the sedimentary archive. We compared four lower Aptian pelagic sections from the former southern and northern continental margins of the Alpine Tethys Ocean and traced the evolution of carbon isotopes, carbonate and organic carbon content as well as palynofacies before and at the onset of OAE1a. Throughout the studied interval, the sections record frequent precession-controlled changes in carbonate content, which are reflected by limestone-marlstone alternations in the shallowest and most proximal Cismon core and by limestone-black-shale couplets in the deepest Pusiano section. Depth controlled sub-/anoxia is also suggested by the prominent OAE1a black shales, which occurred first in the deeper Pie del Dosso and Roter Sattel sections and only subsequently in the shallower Cismon core. However, contrary to expectations, the deepest Pusiano section exhibits – instead of an earliest onset of prominent OAE1a black shales – only a minor increase in TOC and a decrease in carbonate content. This suggests that the orbitally driven climate changes most strongly influenced water stratification and hence are most prominently expressed in the deepest sections. Conversely, the volcanically induced long-term climate changes seemed to more strongly affect organic matter production, the extension of the oxygen minimum zone and hence had the strongest impact on sections at intermediate depth.

Depositional mechanism of an Aptian petroleum source rock in the southern Arabian Gulf region: Is it related to Oceanic Anoxic Event 1a?

Depositional mechanism of an Aptian petroleum source rock in the southern Arabian Gulf region: Is it related to Oceanic Anoxic Event 1a?

Climate variability and ocean fertility during the Aptian Stage

Abstract. Several studies have been conducted to reconstruct temperature variations across the Aptian Stage, particularly during early Aptian Oceanic Anoxic Event (OAE) 1a. There is a general consensus that a major warming characterized OAE 1a, although some studies have provided evidence for transient "cold snaps" or cooler intervals during the event. The climatic conditions for the middle–late Aptian are less constrained, and a complete record through the Aptian is not available. Here we present a reconstruction of surface-water palaeotemperature and fertility based on calcareous nannofossil records from the Cismon and Piobbico cores (Tethys) and DSDP Site 463 (Pacific Ocean). The data, integrated with oxygen-isotope and TEX86 records, provide a detailed picture of climatic and ocean fertility changes during the Aptian Stage, which are discussed in relation to the direct/indirect role of volcanism. Warm temperatures characterized the pre-OAE 1a interval, followed by a maximum warming (of ~ 1.5–2 °C) during the early phase of anoxia under intense volcanic activity of the Ontong Java Plateau (OJP). A short-lived cooling episode interrupted the major warming, following a rapid increase in weathering rates. Nannofossils indicate that mesotrophic conditions were reached when temperatures were at their highest and OJP volcanism most intense, thus suggesting that continental runoff, together with increased input of hydrothermal metals, increased nutrient supply to the oceans. The latter part of OAE 1a was characterized by cooling events, probably promoted by CO2 sequestration during burial of organic matter. In this phase, high productivity was probably maintained by N2-fixing cyanobacteria, while nannofossil taxa indicating higher fertility were rare. The end of anoxia coincided with the cessation of volcanism and a pronounced cooling. The mid-Aptian was characterized by highest surface-water fertility and progressively decreasing temperatures, probably resulting from intense continental weathering drawing down pCO2. The lowest temperatures, combined with low fertility, were reached in the middle–late Aptian across the interval characterized by blooming of Nannoconus truittii. The prolonged cooling was followed by significant warming across the Aptian–Albian boundary. The data presented suggest that OJP activity played a direct role in inducing global warming during the early Aptian, whereas other mechanisms (weathering, deposition of organic matter) acted as feedback processes, favouring temporary cooler interludes.

The Barremian and Aptian stepwise development of the ‘Oceanic Anoxic Event 1a’ (OAE 1a) crisis: Integrated benthic and planktic high-resolution palaeoecology along the Gorgo a Cerbara stratotype section (Umbria–Marche Basin, Italy)

The Barremian and Aptian were times of global plate reconfiguration and profound changes in the ocean–climate system, culminating in the worldwide deposition of lower Aptian black shale layers (OAE 1a). Based on high-resolution lithostratigraphic and micropaleontological analyses, precursor conditions and timing of the anoxia are here reconstructed along a 33.06 m thick section in the proposed Barremian/Aptian boundary GSSP stratotype at Gorgo a Cerbara (Umbria–Marche Basin, Italy). A non-uniform history of benthic foraminiferal diversification is interrupted by the Selli Level (= OAE 1a) and by three turnover points for both benthic and planktic organisms, each highlighted by prominent breaks in lithology and outcrop morphology. The first two points, at ~ 1.1 Myr and ~ 20–50 kyr prior to the onset of the OAE 1a, correspond respectively to the nannoconid (= bathypelagic calcareous nannofossils) ‘decline’ and ‘crisis’ events, and separate: (a) pelagic limestone/chert-claystone lithological cycles of the uppermost Maiolica Formation, showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic Barremian benthic and planktic assemblage, dominated by k-selected nannoconids and Rhizammina; (b) greenish-grey cherty marls of the lowermost Marne a Fucoidi Formation, associated to declining sedimentation rates and eutrophic early Aptian assemblages dominated by radiolarians and lituolid benthic foraminifera; and (c) anoxic radiolarites and shales of the carbonate-free Selli Level. High magnitude and frequency assemblage fluctuations occur between the nannoconid crisis (~ 25 cm below the lowermost OAE 1a black shale layer) and the Selli Level base, with suggestions of bottom-water acidification. At ~ 800 kyr after the OAE 1a end, a third turnover point in the basal reddish member of the Marne a Fucoidi highlights ‘middle’ Aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera (Hedbergella spp.) and appearances of new benthic species, marking a shift towards better oxygenation. OAE 1a oxygen and pelagic carbonate factory crises are here suggested to have been step-wise and linked to global warming conditions.

Response and recovery of the Comanche carbonate platform surrounding multiple Cretaceous oceanic anoxic events, northern Gulf of Mexico

The ubiquity of carbonate platforms throughout the Cretaceous Period is recognized as a product of high eustatic sea-level and a distinct climatic optimum induced by rapid sea-floor spreading and elevated levels of atmospheric carbon-dioxide. Notably, a series of global oceanic anoxic events (OAEs) punctuate this time-interval and mark periods of significantly reduced free oxygen in the world's oceans. The best records of these events are often from one-dimensional shelf or basin sections where only abrupt shifts between oxygenated carbonates and anoxic shales are recorded. The Comanche Platform of central Texas provides a unique opportunity to study these events within a well-constrained stratigraphic framework in which their up-dip and down-dip sedimentologic effects can be observed and the recovery of the platform to equilibrium states can be timed and understood. Stable isotope data from whole cores in middle Hauterivian through lower Campanian mixed carbonate-siliciclastic strata are used to construct a 52-myr carbon isotope reference profile for the northern Gulf of Mexico. Correlation of this composite curve to numerous global reference profiles permits identification of several anoxic events and allows their impact on platform architecture and facies distribution to be documented. Oceanic anoxic events 1a, 1b, 1d, and 2 occurred immediately before, after, or during shale deposition in the Pine Island Member, Bexar Member, Del Rio Formation, and Eagle Ford Group, respectively. Oceanic anoxic event 3 corresponds to deposition of the Austin Chalk Group. Platform drowning on three occasions more closely coincided with globally recognized anoxic sub-events such as the Fallot, Albian-Cenomanian, and Mid-Cenomanian events. This illustrates that the specific anoxic event most affecting a given carbonate platform varied globally as a function of regional oceanographic circumstances.Using chemo- and sequence-stratigraphic observations, a four-stage model is proposed to describe the changing facies patterns, fauna, sedimentation accumulation rates, platform architectures, and relative sea-level trends of transgressive-regressive composite sequences that developed in response to global carbon-cycle perturbations. The four phases of platform evolution include the equilibrium, crisis, anoxic, and recovery stages. The equilibrium stage is characterized by progradational shelf geometries and coral-rudist phototrophic faunal assemblages. Similar phototrophic fauna typify the crisis stage; however, incipient biocalcification crises of this phase led to retrogradational shelf morphologies, transgressive facies patterns, and increased clay mineral proportions. Anoxic stages of the Comanche Platform were coincident with back-ground deposition of organic-rich shale on drowned shelves and heterotrophic fauna dominated by oysters or coccolithophorids. Eustatic peaks of this stage were of moderate amplitude (∼30 m), yet relative sea-level rises were greatly enhanced by reduced sedimentation rates. In the recovery stage, heterotrophic carbonate factories re-established at the shoreline as progradational ramp systems and sediment accumulation rates slowly increased as dysoxia diminished. Full recovery to equilibrium conditions may or may not have followed. Geochemical and stratigraphic trends present in the four stages are consistent with increased volcanism along mid-ocean ridges and in large-igneous provinces as primary drivers of Cretaceous OAEs and the resulting transgressive-regressive composite sequences.

Integrated biostratigraphy and geochemistry of the lower Cretaceous Radiolarian Flood Zone of the base of the Garau Formation, northwest of Zagros Mountains, Iran

The lower Garau Formation known as the Radiolaria Flood Zone (RFZ) is investigated for reconstructing palaeoceanography and controlling factors that resulted in the organic matter and radiolarian rich beds. A stratigraphic section named Tang-e-Haft located to the northwest of Zagros range with a thickness of 320 m mainly composed of argillaceous limestone, and limestone is used for this study. The RFZ is dated late Berriasian to earliest Aptian via using nannofossils, foraminifera, and palynology integrated. Semi-quantitative study of different microfossil groups and biofacies analaysis demonstrates deposition of this member in an intermediate depth under dys- to anaerobic marine condition with a shallowing upward trend. Richness of radiolarians with prevailed spumellaria assemblage and high organic matter content revealed presence of high palaeoproductivity during time of deposition. This productivity was induced by paleocurrents and upwelling systems. Values of total organic carbon (TOC), gamma ray, and amount of amorphous organic matter (AOM) in palynology slides in the lower parts of the section show a peak that indicates that palaeoproductivity was more intense in the lowermost part. Dominance of the radiolarians and consequently the RFZ unit ended up in the early Aptian. This time is coincided with Leupoldina cabri bloom and Oceanic Anoxic Event 1a (OAE1a). This event resulted in an extinction and significant reduction among radiolarians. Depopulation of radiolarians made new habitats for other organisms that planktonic foraminifera with elongated chambers were the first group to benefit the situation.

Climatic evolution across oceanic anoxic event 1a derived from terrestrial palynology and clay minerals (Maestrat Basin, Spain)

AbstractStudies dealing with the response of the continental biosphere to the environmental perturbations associated with Cretaceous oceanic anoxic events (OAEs) are comparatively rare. Here, a quantitative spore-pollen record combined with clay mineral data is presented, which covers the entire early Aptian OAE 1a interval (Forcall Formation, Maestrat basin, east Spain). The well-expressed OAE 1a carbon-isotope anomaly is paralleled by changes in the clay mineral assemblage and by a stepwise decline in the normalized frequency ofClassopollispollen (produced by xerophytic Cheirolepidiaceae) with lowest contents occurring during the positive δ13C shift. In contrast,AraucariacitesandInaperturopollenitespollen show a pronounced increase in relative abundance from low background values to become a significant component of the palynological assemblage during theClassopollisminimum. The observed changes in clay minerals and pollen distribution patterns are interpreted to reflect a major change in the composition of the hinterland vegetation of the Maestrat Basin, most probably due to short-lived but pronounced climatic cooling and changes in humidity. Temperature anomalies driven by organic carbon burial and associated CO2decline have been postulated for all major Mesozoic OAEs. The palynomorph record from the Iberian Maestrat basins indicates that the climax of this cooling episode was significantly delayed in comparison to the end of organic carbon-rich deposition in the world oceans.

Response of proto-North Atlantic carbonate-platform ecosystems to OAE1a-related stressors

Integrated biostratigraphic–chemostratigraphic studies provide evidence that the proto-North Atlantic realm witnessed major changes in carbonate platform production in the run-up of the Early Aptian oceanic anoxic event (OAE) 1a. Whereas pervasive growth of Lithocodium microencrusters represents an early harbinger of OAE1a-related environmental perturbation, the subsequent replacement of oligotrophic rudist-coral-nerineid by mesotrophic orbitolinid-oyster communities was clearly associated with the event itself. In order to test the supra-regional relevance of this major community replacement, two shallow-water sections in the southern Lusitanian Basin (Portugal) are investigated by means of geochemistry (carbon and oxygen isotopes), cement petrography and detailed sedimentological analysis. The focus is on a regional, prominent discontinuity surface (S4) at the transition between oligotrophic and mesotrophic carbonate platform production, which might indicate that the major biotic change could have been associated with a phase of non-sedimentation and possibly erosion. The studied sections (São Julião, Crismina) provide evidence that the major Early Aptian biotic turnover was preceded by numerous subordinate but significant changes in platform ecology, which mirrored a series of progressive short-term environmental changes in the course of OAE1. Several transient mass occurrences of orbitolinids indicate repeated phases of ecological stress arguably due to enhanced nutrient input and deepening. Small-scale sea-level changes at parasequence level below the major discontinuity surface are revealed by alternations of rudist assemblages dominated by clinger or recumbent forms as well as intercalated hardground and subaerial exposure stages. Expanded phases of subaerial exposure, however, can be largely ruled out following the geochemical and cement-petrographic data presented here. Enhanced continent-derived siliciclastic input characterising the lower orbitolinid-oyster dominated limestones is in support of a shift to more humid conditions during the middle Early Aptian. This is in line with palaeoclimatic data, which propose a southward movement of the mid-latitude arid climate belt during this time. The documented rapid replacement of oligotrophic assemblages by various environmental-stress adapted carbonate platform communities might be seen as explanation for ongoing Early Aptian proto-North Atlantic carbonate production during a time of widespread platform demise and drowning in the northern Tethyan realm.