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.     

Carbon isotope records of the early Albian oceanic anoxic event (OAE) 1b from eastern Tethys (southern Tibet,China)

The early Albian Oceanic Anoxic Event (OAE), i.e., OAE1b, is well documented in western Tethys and in the primary North Atlantic Ocean, but has not yet been reported from eastern Tethys. In this paper, we present bulk carbon isotope data of hemipelagites to examine if it was recorded in eastern Tethys. Samples were taken from the upper Chuangdepu Member (nannofossil zone CC8) of the lower Gyabula (former Shadui) Formation at the Bangbu section, Qonggyai, southern Tibet of China. The δ¹³C values mainly range from −0.6‰ to 1.8‰ with a maximum of 1.87‰ and a minimum of −0.69‰. Three stages of carbon isotope evolution were distinguished with three boundaries. By the constraint of the stratigraphic sequence and nannofossil biostratigraphic zone CC8, the rapid δ¹³C change and correlation with western Tethys and Atlantic Ocean together suggest that these three boundaries of the carbon isotope evolution probably correspond to three subevents of the early Albian OAE1b, and the subevent levels of upper Kilian, Paquier, and Leenhardt are recorded in eastern Tethys (southern Tibet). The fact that the amount of δ¹³C shift is less by ∼1.5–2.0‰ in eastern Tethys than in western Tethys and Atlantic Ocean is interpreted as a result of possible cool sea surface (∼14–16 °C) of the southeastern Tethys (northern Indian passive margin of Greater India), which was probably located in a medium–high latitude during the Albian, leading to low primary productivity. The recognition of OAE-1b from Tethys Himalaya can improve our understanding of the Tethys and global paleoclimatic and paleoceanographic changes during the mid-Cretaceous.     

Organic matter characterisation of the Selli Level (Umbria-Marche Basin, central Italy)

The Selli Level is a marker-bed in the Umbria-Marche Apennines that represents the regional sedimentary expression of the Lower Aptain Oceanic Anoxic Event 1a. This one to three-metre-thick interval shows a remarkable uniformity on a regional scale, with a green to grey marly lower part overlain by black shales. Bulk geochemical studies and an examination of palynofacies were carried out on three sections in order to characterise the distribution and nature of the organic matter in the Selli Level which has been poorly known hitherto. The organic content is medium (0.5 to 2.9% TOC) in the basal part and higher (4.5% on average, and up to 18% TOC) in the black shales. Pyrolysis and palynological data indicate that the organic matter is mainly of marine origin. The geochemical characteristics of the Selli Level are compared to other, more or less organic-rich marker-beds in the Cretaceous succession of the Umbria-Marche Apennines.

Abstract

Le Niveau Selli est un niveau-repère des Apennins d'Ombrie-Marches qui correspond à l'enregistrement régional de l'événement anoxique océanique 1a. Ce niveau, épais de un à trois mètres, présente une remarquable uniformité à l'échelle régionale avec une partie inférieure argileuse, verdâtre à grise, surmontée par des black shales. Des analyses géochimiques et l'examen des palynofacies ont été entreprises sur trois coupes afin de caractériser le contenu en matière organique du niveau Selli, jusqu'à présent mal connu. Les teneurs en carbone organique sont moyennes (0,5 à 2,9% COT) dans la partie basale et élevées (4,5% en moyenne et jusque 18% COT) dans les black shales. Les résultats de pyrolyse et des observations palynologiques indiquent que la matière organique est essentiellement d'origine marine. Les caractéristiques géochimiques du niveau Selli sont comparées aux autres niveaux repères, plus ou moins riches en carbone organique, qui émaillent la série du Crétacé du basin d'Ombrie-Marches.     

Micropaleontological evidence for redox changes in the OAE3 interval of the US Western Interior: Global vs. local processes

The Coniacian-Santonian interval has been proposed as the youngest of the Cretaceous ocean anoxic events (OAE3), but this designation has long been debated. OAE3 is associated with a long-lasting (∼3 myr) succession of black shales from the central and South Atlantic, Caribbean region, and the North American Western Interior; in the Western Interior it is characterized by an abrupt increase in total organic carbon (TOC) and corresponding trace metal indicators for anoxia. However, the modern concept of OAEs is predicated on detection of global carbon cycle perturbations as recorded by substantial carbon isotope excursions (CIE), and the protracted Coniacian-Santonian black shale interval does not have a large CIE. A more conservative definition of OAE3 might limit the event to the modest positive carbon isotope excursion restricted to the upper Coniacian Scaphites depressus Ammonite Zone. Trace metal proxies suggest that oxygen levels abruptly declined prior to the onset of this CIE in the Western Interior Sea (WIS), but it is unknown whether regional anoxic conditions were confined to sediments/pore waters, or how anoxia may have affected the biota. In an effort to characterize the oxygenation history of the WIS and to better understand the nature of the hypothesized OAE3, we present micropaleontological evidence of declining oxygen in bottom waters prior to the event using benthic foraminifera, which are sensitive to dissolved oxygen. Changes in benthic foraminiferal abundances suggest a decline in oxygen at least 1-myr prior to the CIE (including a nadir immediately below the start of the excursion), improving bottom water oxygen during the CIE, and re-establishment of persistent anoxia following the isotope excursion. Anoxia endured for nearly 3 myr in the central seaway, showing some signs of recovery toward the top of the Niobrara Formation. Our findings suggest that declining oxygen concentrations in the seaway eventually reached a tipping point, after which dissolved oxygen quickly dropped to zero.The late Coniacian CIE is an exception to the trend of declining oxygen in the WIS, and part of a larger pattern in the oxygenation history of the Niobrara Formation which suggests that it does not adhere to standard black shale models. Transgressive periods, including the Fort Hays Limestone and the lower limestone unit of the Smoky Hill Shale (which corresponds to the CIE) are relatively oxic, while periods of highstand (i.e., most of the Smoky Hill) correspond to deteriorating oxygen conditions. This contrasts with the standard black shale model for sea level and oxygen, where transgressions typically correlate with maximum TOC enrichment, interpreted to result from both sediment condensation and oxygen deficiency. The association of global carbon burial/anoxia (as indicated by carbon isotopes) with a regional increase in oxygen and decrease in organic matter preservation is reminiscent of the Cenomanian-Turonian Greenhorn Limestone, which contains OAE2. In both cases, the facies are not typical black shales, but instead have appreciable carbonate content. Western Interior redox trends support the rejection of the original concept of a protracted Coniacian-Santonian OAE3 because it is not a distinct “event.” Increasing local oxygen during the late Coniacian CIE also argues against a narrower OAE designation for this event, because the excursion can't be tied to anoxia here or anywhere else it has been described. Nevertheless, the Late Coniacian Event (as we prefer to call this CIE) still represents an important perturbation of the global carbon cycle. This is emblematic of the shift away from widespread, discrete anoxic events during the ongoing paleoceanographic reorganization of the Late Cretaceous, even as large carbon cycle perturbations continued.     

A new sediment core from the Bedoulian (Lower Aptian) stratotype at Roquefort-La Bédoule,SE France

New drill cores from the Lower Aptian historical stratotype at Roquefort-La Bédoule (SE France) provide continuous high-resolution geochemical and isotope records which closely track the onset of OAE 1a in a subtropical intra-shelf basin (South Provençal Basin). The drilling operation recovered a total of 180 m of undisturbed sediments in three holes. The lowermost 67 m correspond to the Bedoulian (core LB1) and are here analyzed in high-resolution using geochemical proxies (stable carbon isotopes, stable oxygen isotopes, and carbonate content) and foraminiferal biostratigraphy. Pervasive bioturbation through core LB1 suggests mostly oxygenated bottom water conditions with transient dysoxic episodes, as shown by higher pyrite and glauconite concentrations within the marlstones. Unprecedented resolution over the negative δ¹³C excursion preceding OAE 1a (segment C3) reveals a characteristic double trough extending over ∼5.5 m in core LB1. This long-lasting negative excursion was possibly linked to multiple pulses of enhanced CO₂ release to the atmosphere. Estimated sedimentation rates of 1.6–2 cm/kyr indicate that the negative δ¹³C excursion lasted >200 kyr, while the main positive carbon isotope shift (segment C4) had a duration of >300 kyr. Fluctuations in δ¹⁸O suggest transient episodes of climate warming and cooling at the northern margin of the Tethys or even on a more global scale prior to the onset of OAE 1a.     

Oceanic events and biotic effects of the Cenomanian-Turonian anoxic event, Tarfaya Basin, Morocco

Profound biotic changes accompanied the late Cenomanian δ¹³C excursion and OAE2 in planktic foraminifera in the Tarfaya Basin of Morocco. Planktic foraminifera experienced a severe turnover, though no mass extinction, beginning with the rapid δ¹³C excursion and accelerating with the influx of oxic bottom waters during the first peak and trough of the excursion. Species extinctions equaled the number of evolving species, though only the disaster opportunists Guembelitria and Hedbergella thrived along with a low oxygen tolerant benthic assemblage. The succeeding δ¹³C plateau and organic-rich black shale deposition marks the anoxic event and maximum biotic stress accompanied by a prolonged drop in diversity to just two species, the dominant (80–90%) low oxygen tolerant Heterohelix moremani and surface dweller Hedbergella planispira. After the anoxic event other species returned, but remained rare and sporadically present well into the lower Turonian, whereas Heterohelix moremani remained the single dominant species. The OAE2 biotic turnover suggests that the stress to calcareous plankton was related to changes in the watermass stratification, intensity of upwelling, nutrient flux and oxic levels in the water column driven by changes in climate and oceanic circulation. Results presented here demonstrate a 4-stage pattern of biotic response to the onset, duration, and recovery of OAE2 that is observed widely across the Tethys and its bordering epicontinental seas.     

The carbon isotopic response of algae, (cyano)bacteria, archaea and higher plants to the late Cenomanian perturbation of the global carbon cycle: Insights from biomarkers in black shales from the Cape Verde Basin (DSDP Site 367)

The stable carbon isotopic compositions of free and sulfur (S)-bound biomarkers derived from algae, (cyano)bacteria, archaea and higher plants and total organic carbon (TOC) during the first phase of the late Cenomanian/Turonian oceanic anoxic event (OAE) were measured in black shales deposited in the southern proto-Atlantic Ocean in the Cape Verde basin (DSDP Site 367) to determine the response of these organisms to this major perturbation of the global carbon cycle resulting from widespread burial of marine organic matter. The average positive isotope excursions of TOC and biomarkers varied from 5.1‰ to 8.3‰. The δ¹³C values were cross correlated to infer potential common sources of biomarkers. This revealed common sources for C₃₁ and C₃₂ hopanes but no 1:1 relationship for pristane and phytane. The correlation of δ¹³C_TOC with the δ¹³C value of sulfur (S)-bound phytane is the strongest. This is because S-bound phytane is derived from phytol that originates from all marine primary producers (algae and cyanobacteria) and thus represents a weighted average of their carbon isotopic compositions. The δ¹³C values of S-bound phytane and C₃₅ hopane were also used to estimate pCO₂ levels. Before the OAE burial event, pCO₂ levels are estimated to be ca. 1300 ppmv using both biomarkers and the independent maximum Rubisco fractionation factors. At times of maximum organic carbon burial rates during the OAE, reconstructed pCO₂ levels are estimated to be ca. 700 ppmv. However, compared to other C/T OAE sections the positive isotope excursion of S-bound phytane is also affected by an increased production during the OAE. When we compensate for this, we arrive at pCO₂ levels around 1000 ppmv, a reduction of ca. 25%. This indicates that burial of organic matter can have a large effect on atmospheric CO₂ levels.     

白垩纪至早第三纪的极端气候事件

地球科学界正在将预测未来气候变化的研究重点放到地球过去突然发生的气候变暖事件。白垩纪至早第三纪发生的极端气候事件被认为是最接近于现今的地球系统,对其研究有利于理解现今地球系统过程在碳循环快速搅动时的响应。这些气候事件主要包括：古新世-始新世最热事件（PETM,～5 5 MaBP）、早阿普第晚期和森诺曼-土仑界线的大洋缺氧事件（OAE1a,～120 Ma;OAE2,～93．5 MaBP）。PETM事件是中白垩世以来一次突然变暖事件,在10 ka年以内深海温度增加～5 ℃,表层海水温度增加 4～8 ℃,而δ¹³C至少发生 3．0‰的负偏移。目前普遍认为PETM事件是由于海洋气水化合物（CH₄）的巨量释放造成的。大洋缺氧事件（OAEs）记录了海洋环境下有机质的大量埋藏,代表了碳循环和海洋生物系统的重大搅动事件。综合大洋钻探计划（IODP）将极端气候确定为优先研究领域,将采取特定的钻探策略,在世界大洋范围内获取最低限度蚀变的新生代至白垩纪沉积物,研究精度要求达到米兰柯维奇的天文调谐时间尺度,其最终目标是定量描述过去全球气候变化,并为未来气候变化预测提供依据。     

Regional record of a global oceanic anoxic event: OAE1a on the Apulia Platform margin, Gargano Promontory, southern Italy

The Gargano Promontory of southern Italy, located on the eastern margin of the Apulia Platform, represents a peculiar Tethyan area where the transition between carbonate platform and adjacent basins is exposed on land. The Aptian stratigraphic record, represented in shallow-water, slope and deep-water deposits, provides a good opportunity to investigate the regional response to the worldwide documented climatic, biotic and palaeoceanographic changes related to Oceanic Anoxic Event 1a (OAE1a). A synthesis of data previously published (Ischitella and Coppitella sections), together with original data (Val Carbonara and Coppa della Guardia sections), from four stratigraphic sections from different depositional settings (proximal to distal) is provided, using an integrated, high-resolution micropalaeontological (planktonic foraminifera and calcareous nannofossils) and, for one section, geochemical (stable carbon and oxygen isotopes) approach. Organic matter preservation is confined to the more distal areas and consists of two thin intervals of black shales in the Aptian portion of the Marne a Fucoidi Formation. Biostratigraphic data assign the older black shale (5 cm thick) to the Selli Level equivalent (OAE1a, Lower Aptian); this carbon-rich interval is immediately followed by another black shale (7–10 cm thick) of early Late Aptian age.OAE1a is generally interpreted as a high-productivity event during a warming interval, followed by a cooling trend. In the Gargano Promontory, although the oxygen isotope curve indicates the above-mentioned climatic evolution, the micropalaeontological data do not support high fertility of the surface water, whereas micropalaeontological and geochemical data for the younger black shale do record high productivity (radiolarian increase in overall abundance, low nannofossil and foraminiferal species richness, increase in abudance of nannofossil fertility indices) associated with a cooling trend. The carbon and oxygen isotope record is in line with evidence from the curves documented elsewhere, whereas, among the biotic events, only the “nannoconid crisis” preceding OAE1a is revealed to be globally correlable.Environmental models for the two episodes of organic matter preservation are proposed, taking into account both global and local controlling factors.     

Recycled oceanic crust as a source for tonalite intrusions in the mantle section of the Khor Fakkan block,Semail ophiolite (UAE)

Several types of felsic granitoid rocks have been recognized, intrusive in both the mantle and the crustal sequence of the Semail ophiolite. Several models have been proposed for the source of this suite of tonalites, granodiorites, trondhjemites intrusions, however their genesis is still not clearly understood. The sampled Dadnah tonalites that intruded in the mantle section of the Semail ophiolite display arc-type geochemical characteristics, are high siliceous, low-potassic, metaluminous to weakly peraluminous, enriched in LILE, show positive peaks for Ba, Pb, Eu, negative troughs for U, Ti and occur with low δ¹⁸O_H2O, moderate ε_Sr and negative ε_Nd values. They have crystallized at temperatures that range from ∼550 °C to ∼720 °C and pressure ranging from 4.4 kbar to 6.5 kbar. The isotopic ages from our tonalite samples range between 98.6 Ma and 94.9 Ma, slightly older and overlapping with the age of the metamorphic sole. Our field observations, mineralogical, petrological, geochemical, isotopic and melt inclusion data suggest that the Dadnah tonalites formed by partial melting (∼10%–15% continuous or ∼12% batch partial melting), accumulation of plagioclase, fractional crystallization (∼55%–57%), and interaction with their host harzburgites. These tonalites were the end result of partial melting and subsequent contamination and mixing of ∼4% oceanic sediments with ∼96% oceanic lithosphere from the subducted slab. This MORB-type slab melt composed from ∼97% recycled oceanic crust and ∼3% of the overlying mantle.We suggest that a possible protolith for these tonalites was the basaltic lavas from the subducted oceanic slab that melted during the initial stages of the supra-subduction zone (SSZ), which was forming synchronously to the spreading ridge axis. The tonalite melts mildly modified due to low degree of mixing and interaction with the overlying lithospheric mantle. Subsequently, the Dadnah tonalites emplaced at the upper part of the mantle sequence of the Semail ophiolite and are geochemically distinct from the other mantle intrusive felsic granitoids to the south.     

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.     

Cyclostratigraphic dating in the Lower Badenian (Middle Miocene) of the Vienna Basin (Austria): the Baden-Sooss core

The scientific borehole Baden-Sooss penetrates a succession of Badenian (Langhian, Middle Miocene) sediments at the type locality of the Badenian, the old brickyard Baden-Sooss in the Vienna Basin. The sedimentary succession of the 102-m-cored interval consists of more than 95% bioturbated, medium-to-dark gray marly shales with carbonate contents between 11 and 25% and organic carbon contents between 0.35 and 0.65%. Biostratigraphic investigations on foraminifera (mainly lower part of Upper Lagenid Zone) and calcareous nannoplankton (standard zone NN5) indicate an early Badenian (Langhian) age. Cycles in carbonate content, organic carbon content, and magnetic susceptibility have been identified by power spectra analysis. Correlations between the three variables are extremely significant. Using cross-correlation, periods around 40 m correlate significantly with the 100 kyr⁻¹ eccentricity cycle, the ∼20 m periods with the obliquity cycle, and the 15 to 11-m periods with both precession cycles. Wavelet transformation and decomposition of composite periodic functions were used to obtain the position of the cycle peaks in the profile. Cross-correlation with orbital cycles (La2004) dates the Baden-Sooss core between −14.379 ± 1 and −14.142 my ± 9 kyr.     

Aptian and Albian atmospheric CO2 changes during oceanic anoxic events: Evidence from fossil Ginkgo cuticles in Jilin Province,Northeast China

The Early Cretaceous was a time with super-greenhouse conditions and episodic global oceanic anoxic events. However, relative timing of atmospheric CO₂ emissions and oceanic anoxic events, and their causal relationships remain matters of debate. Using the stomatal index approach, well-preserved fossil cuticles of Ginkgo from the Lower Cretaceous Changcai Formation, eastern Jilin, and from the Lower Cretaceous Yingcheng Formation, central Jilin, Northeast China, were investigated to reconstruct atmospheric CO₂ concentrations during the Aptian and earliest Albian (Early Cretaceous). The results indicate that the CO₂ concentrations reached 1098–1142 ppmv (Carboniferous standardization) or 970–1305 ppmv (regression function) during the Aptian and earliest Albian. Our estimates of palaeoatmospheric CO₂ concentrations during the earliest Albian (OAE 1b) are slightly higher than the data between the early Aptian Selli (OAE 1a) and the middle Aptian Fallot OAEs; this may indicate the absence of any great emissions of CO₂ during the latest Aptian and earliest Albian.     

Sedimentology,biostratigraphy and event stratigraphy of the Early Aptian Oceanic Anoxic Event (OAE1A) in the Apulia Carbonate Platform Margin – Ionian Basin System (Gargano Promontory,southern Italy)

The stratigraphic and biotic signatures of severe environmental changes across the late Early Aptian Oceanic Anoxic Event (OAE1a) in central Tethyan settings have been investigated in the almost undeformed Apulia Carbonate Platform Margin-Ionian Basin system (ACPM-IB) exposed in the Gargano Promontory (southeastern Italy). The nature of the observed Lower Aptian biofacies shifts within the investigated carbonate system are analyzed using an integrated biostratigraphic approach, based on ammonite-calibrated orbitolinids and caprinids coupled with the published δ¹³C and δ¹⁸O record of coeval pelagic sections of the Ionian basin (Coppitella, eastern Gargano, and Paliambela, northwestern Greece). Detailed field analysis of facies and tracing out of key biosedimentary and chemostratigraphic markers of the Lower Aptian have elucidated the evolution of the platform margin and allowed integrated correlations across the Gargano margin to basin transition.It is shown here that the global-scale environmental changes leading to the OAE1a event (i.e., fluctuations of surface-ocean temperature, available trophic resources and, above all, pCO₂) are matched consistently by significant shifts of the biotic associations inhabiting the ACPM. The onset of greenhouse, mesotrophic conditions in the surface ocean undergoing an increasing acidification by CO₂ excess favored the rapid spread of calcite shelled, filter feeding, eurytopic opportunist organisms (chondrodontids, ostreids, chaetetids sponges, along with minor bryozoans) as well as echinoids and orbitolinids (foramol productivity mode); this change is recorded by the deposition of a 4–6 m Crisis Interval (CI) that sharply overlies the “Urgonian”-type, rudist-rich platform margin complex (Montagna degli Angeli Limestones) formed from predominantly aragonite shelled, stenotopic organisms (mostly caprinid rudists, with minor corals and Dasycladales and Bryopsidales green algae) (chlorozoan productivity mode). The CI heralded the incipient drowning of the ACPM which occurred immediately after the deposition of a hothouse “out of balance” brachiopod (Orbirhynchia nadiae)-cyanobacteria association (microbial productivity mode). The biostratigraphically constrained CI and the related drowning of the ACPM have been physically correlated with specific chemostratigraphic segments of the δ¹³C_carb curve that is available for the facing, proximal Ionian Basin. The early (earliest ?) Late Bedoulian, greenhouse CI (early D. deshayesi – early P. cormyi zones) was deposited during the late C2 interval, whereas the early Late Bedoulian, hothouse “out-of-balance” guild and the ACPM drowning (early deshayesi zone) accompanied the subsequent negative excursion and culmination, respectively, at the very base of the Chemostratigraphic Selli Level (CSL) (latest C2 and C3 interval). These correlations permit regional to global interpretations of historical patterns and explanatory paleoeanographic and paleoecologic hypotheses; furthermore, they suggest that the Apulia shallow-water ecosystem reacted to the environmental disruptions linked with the OAE1a sooner than that of the facing open ocean. The onset of the drowning event was synchronous with the hothouse-induced bloom of the “out of balance” brachiopod-cyanobacteria association that occurred simultaneously with the pronounced negative excursion of the δ¹³C_carb curve. This suggests a cause-and-effect relationship with the sudden environmental perturbations linked with the injection into the atmosphere-hydrosphere system of ¹³C-depleted CO₂. The numerical age model of the major biotic and stratigraphic events at the Lower Aptian ACPM, along with their inferred genetic processes call for different timings and causal mechanisms associated with platform demise in northern and central Tethyan settings.     

Benthic microbial mats: a possible major component of organic matter accumulation in the Lower Aptian oceanic anoxic event

Oceanic anoxic events (OAEs) throughout the Cretaceous were periods of high organic carbon burial leading to drawdown of atmospheric carbon dioxide and lowering of bottom-water oxygen concentration, thereby enhancing the preservation of organic matter (OM). Two dynamic depositional models have been proposed for these events in the Tethyan domain: one is based on strong thermohaline stratification and low surface productivity, the other on high surface productivity with intensified deep-water circulation. Here, we propose another explanation for the concentration of OM, derived essentially from microscopical observations (scanning electron microscope and transmission electron microscope) in the organic-richest interval of an Early Aptian OAE in central Italy (OAE1a or Selli level, 116 Ma). This high-resolution microscopical study of OM highlights benthic microbial mats as the possible source of organic-rich samples where amorphous OM is the major organic constituent. These mats could be more common in OAE black shales than previously thought.     

Microbial-foraminiferal episodes in the Early Aptian of the southern Tethyan margin: ecological significance and possible relation to oceanic anoxic event 1a

Two regionally significant microbial-foraminiferal episodes (∼150 kyr each) occur within the Early Aptian shallow marine platform in Oman and throughout eastern Arabia. The stratigraphically lower of these two intervals is characterized by isolated or coalescent domes that share similarities with modern, open-marine stromatolites from the Exuma Cays, Bahamas. The upper interval is predominantly built by a problematic Lithocodium/Bacinella consortium in buildup and massive boundstone facies. Based on high-resolution chemostratigraphy, these shoalwater intervals are coeval with oceanic anoxic event 1a (OAE1a; Livello Selli). Field evidence demonstrates that the buildup episodes alternate with stratigraphic intervals dominated by rudist bivalves. This biotic pattern is also recognized in other coeval Tethyan sections and is perhaps a characteristic shoalwater expression of the OAE1a. The short-lived regional expansions of this microbial-foraminiferal out-of-balance facies cannot be explained by local environmental factors (salinity and oxygen level) alone and the buildup consortia do not occupy stressed refugia in the absence of grazing metazoans. Judging from recent analogues, the main fossil groups, i.e. microbial assemblages, macroalgae, larger sessile foraminifera, and rudist bivalves, all favoured elevated trophic levels but with different tolerance limits. The implication of this is that the influence of palaeofertility events, possibly related to OAE1a, on carbonate platform community structures must be investigated. The observations made in these coastal sections are a significant first step for the improved understanding of the Early Aptian period of biotic, oceanic and climatic change.     

Palaeoecology and biostratigraphy of early Cretaceous (Aptian) calcareous nannofossils and the δCcarb isotope record from NE Iran

Upper Barremian-Lower Aptian sediments of the Sarcheshmeh and Sanganeh formations in the Kopet Dagh area, northeast Iran were studied with regard to their calcareous nannofossil content and their δ¹³C_carb signal. The sediments are composed mainly of marlstones, argillaceous limestones and limestones. Based on the occurrence of biostratigraphic index taxa, the calcareous nannofossil zones NC5, NC6 and the NC7A Subzone were recognised. The calcareous nannofossils and the δ¹³C_carb data enable recognition of the early Aptian Oceanic Anoxic Event 1a (OAE 1a). The deposits of the OAE 1a interval are characterised by the rarity of nannoconids and a sharp negative δ¹³C_carb excursion (1.36‰), followed by an abrupt positive δ¹³C_carb excursion of 4-5‰; both events have been recognised elsewhere in OAE 1a deposits in the Tethys. In the OAE 1a interval, the relative abundance of Watznaueria barnesiae/Watznaueria fossacincta is higher (more than 40%) than that of Biscutum spp., Discorhabdus spp. and Zeugrhabdotus spp., which indicates dissolution. In the upper part of the section, the higher relative abundance of mesotrophic and oligotrophic taxa (Watznaueria spp. and nannoconids respectively) and the enhanced relative abundance of eutrophic taxa (Biscutum spp., Discorhabdus spp., Zeugrhabdotus spp.) is indicative of an environment with slightly increased nutrient content. The presence of warm water taxa (Rhagodiscus asper and nannoconids) and the absence of cool water taxa (Repagulum and Crucibiscutum) suggest warm surface-water conditions.     

Carbon-isotope and microfaunal stratigraphy bounding the Lower Aptian Oceanic Anoxic Event 1a in northeastern Tunisia

Using an integrated multidisciplinary approach the upper part of the Lower Cretaceous northeastern Tunisian Jebel Ammar sedimentary succession was examined in detail. The method applied included lithologic and microfacies analyses, micropalaeontology, sedimentology, variations in organic matter (OM) content and carbonate carbon stable isotope (δ¹³C) record. A major result was the identification in this sector of Tunisia of the Early Aptian (Bedoulian) OAE1a event in a biostratigraphically well-calibrated context, its location keyed to planktonic foraminiferal zones and isotopic stages.The most striking feature to the Jebel Ammar Aptian sequence is the presence of a 25 m interval of black limestones and marly limestones, of which the microfacies shows that these darker beds consist of wackestones with the presence of abundant radiolarians, a fair number of diversified planktonic and relatively rare benthic foraminifers, together an indication of a pelagic palaeoenvironment. The foraminiferal marker Schackoina (Leupoldina) cabri (still very rare at the beginning of its range) first occurs about six metres above the base of this interval, but becomes much more abundant in its uppermost part, together with the radially- elongate-chambered praehedbergellids. This “acme” of S. (L.) cabri is nearly contemporary with a radiolarian bloom. Rock Eval analyses show TOC values up to 4.59% and a Tmax ranging between 441 and 513 °C, which indicates an overmature OM. The δ¹³C isotope curve shows an evolution similar to those recognized worldwide. The lower part of the darker beds includes a marked shift in isotope values from −2.40 to +3.02 ‰/PDB. This increase is assumed to be equivalent to the isotopic C4 stage of Menegatti et al. (1998, Paleoceanography, 13, 530–545). The signature of the middle and upper part of the isotopic curve is tentatively interpreted as comprising the C5–C7 stages. The first occurrence of S. (L.) cabri is located in the upper part of C4 and its acme as well as the radiolarian bloom is situated within the span of the C7 stage. These results are fairly consistent, though with minor discrepancies, with what has been published from several parts of the North Tethyan margin and more particularly the Lower Aptian type area of southeastern France, where S. (L.) cabri first occurs at the C3/C4 stage boundary with an acme that begins in the lower part of C7.     

Benthic foraminiferal assemblages of late Aptian-early Albian black shale intervals in the Vocontian Basin, SE France

Benthic foraminiferal assemblages were analyzed from three black shale intervals in the upper Aptain to lower Albian of the Vocontian Basin, SE France based on Q-mode principal component analyses. Variations in the distribution patterns of benthic foraminifera around these events suggest differences in the origin of the black shales. Differences between faunas of bioturbated marly and laminated black shale facies have been observed in the Niveau Paquier, Oceanic Anoxic Event (OAE) 1b and Niveau Leenhardt. Here, the faunal composition and plankton/benthos ratios suggest eutrophic conditions during the deposition of organic-rich sediments leading to black shales. No major variations have been observed in black shales of the upper Aptain Niveau Jacob. Benthic assemblages and low plankton/benthos ratios indicate mesotrophic conditions. Third order sea-level changes are believed to control mainly the origin of the investigated black shale levels.     

North Atlantic climatic events recorded in Aptian Naskapi Member cores,Scotian Basin

Early Aptian black laminated organic mudstones, including the Selli Level, are recognized over large areas of Tethys and western European basins. This interval was investigated in a 75 m-thick continuously cored section of varicoloured shale in the lower Naskapi Member from the Panuke B-90 well in the Scotian Basin, offshore eastern Canada. This study complements the palaeogeographic range of correlatable Selli black shales and provides information on their relationship to sea-level change and palaeoclimate. Total organic carbon (TOC) was measured on 127 discrete samples, chemical environmental proxies (Th/K, Mn/Ti, K/Ti, Th/Ti, V/Ti) were measured with a portable X-ray fluorescence (pXRF) spectrometer, and colour parameters L*a*b* were measured by spectrophotometer. Several black shale levels are recognized and correlated with similar shales in Europe between the Barremian–Aptian boundary and the Selli Level. The Th/K ratio proxy for hinterland humid or arid climate conditions shows no systematic variation with black shale levels. Several sea-level lowstands are inferred from condensed sandy intervals with some brackish water biota and tidal sedimentary structures. Black shale intervals are found in highstand intervals, with no systematic relationship to inferred transgressions. The formation of black shales is related to palaeoceanographic changes that may be driven by conditions remote from the Scotian Basin.