Oceanic Anoxic Event 1b Research Articles

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

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

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

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

Mid-Cretaceous paleoenvironmental changes in the western Tethys

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

Cyclostratigraphic and δ13C record of the Lower Cretaceous Adriatic Platform, Croatia: Assessment of Milankovitch-forcing

The 700-m-thick Hauterivian to Albian section from the interior of the large, Adriatic Platform is exposed on Mljet and Korčula islands, Croatia. The cyclic platform consists of meter-scale parasequences, bundled into parasequence sets, superbundles and then into breccia-bounded depositional sequences. Spectral analysis of time series (cycle number vs. cycle thickness) shows peaks in the short-term eccentricity (95 to 130 kyr) bands and 160 to 200 kyr bands.Isotope samples from micrite matrix of mudstone and wackestone have δ13C values generally from −1.5 to +2‰ VPDB, and δ18O from −1 to −3.5‰ VPDB. Adriatic Platform δ13C values are shifted ~2‰ VPDB more negative than marine pelagic values. A δ13C and δ18O meteoric signal is limited to just beneath sequence boundaries, suggesting that the isotopic shift is due mainly to marine phreatic‐ and synorogenic burial diagenesis. Major excursions on the 5-point smoothed δ13C profile of the Adriatic Platform, tied biostratigraphically to δ13C curves from pelagic sections, include the oceanic anoxic events OAE1a, 1b, and 1c.Time series constructed from δ13C values (unsmoothed data) vs. stratigraphic position show peaks in the Milankovitch band, especially short-term eccentricity. Most positive peaks are near parasequence set boundaries (50 to 60%) and regressive parts of parasequences (20%). Although diagenetically shifted, the δ13C record retains a highly clipped memory of astronomically-driven global δ13C fluctuations albeit modified by platform water restriction during cyclic deposition.

Mercury anomalies in upper Aptian-lower Albian sediments from the Tethys realm

The upper Aptian-lower Albian stratigraphic interval documents important perturbations in the carbon cycle associated with the Oceanic Anoxic Event 1b (OAE 1b) tentatively connected with pulses in volcanic activity and the formation of the southern Kerguelen Plateau. However, uncertainties in existing age models for this interval preclude a robust and definitely convincing cause-effect connection between these two events. A sedimentary record from the Poggio le Guaine section, Umbria-Marche Basin, shows anomalously high levels of mercury during the upper Aptian-lower Albian interval. The persistence of the anomalies of [Hg] after normalization against TOC, Al, and Fe suggests that chemical and physical processes related to organic-matter scavenging, adsorption onto clay minerals, hydrous iron oxides and sulphur only played a secondary role as mechanisms for Hg accumulation in the sediments. In agreement with previous studies in which anomalous enrichments of Hg in sedimentary records were interpreted as monitors of enhanced volcanic activity, we speculate that the Hg enrichments documented in the Poggio le Guaine section reflect a rapid increase in atmospheric Hg content associated with a massive release of volcanic Hg, and possibly suggest a multi-phase emplacement of the southern Kerguelen Plateau (SKP) as a realistic source. Nonetheless, comparison of Hg profiles and micropalaeontological data from the Poggio le Guaine section reasonably excludes the possibility that the global biotic turnover horizon documented in the upper Aptian could be directly attributed to the volcanic pulses of the SKP emplacement.

有機地球化学からみた白亜紀中期,海洋無酸素事変1aおよび1bにおける古生態・古環境変動

有機地球化学からみた白亜紀中期,海洋無酸素事変1aおよび1bにおける古生態・古環境変動

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.

Carbonate microfacies and chemostratigraphy of a late Aptian–early Albian marine distal section from the primitive South Atlantic (SE Brazilian continental margin): Record of global ocean-climate changes?

Late Aptian–early Albian limestones from the eastern Brazilian continental margin record the early evolution of the South Atlantic Ocean. In Tethyan and North Atlantic domains, a planktic foraminiferal turnover and organic-rich deposits related to Oceanic Anoxic Event 1b (OAE 1b) point to major ocean-climate changes through this interval. Coeval organic-rich deposits of the South Atlantic Ocean have been interpreted as the product of restricted circulation rather than attributed to a global event. However, previous investigations of the early marine phase of South Atlantic lack data from more distal facies, making correlations to global events difficult. Here, we present C, O, and Sr isotopes, elemental geochemistry, TOC and pyrolysis data, as well as a microfacies analysis of an upper Aptian–lower Albian distal section from the Campos Basin (southeastern Brazil). Our focus is on the paleoenvironmental characterization of and the possible association between organic-rich deposits and major perturbations related to Aptian–Albian transition. Five microfacies associations (MA) were identified in the informal units I and III, which were deposited in the neritic region on a carbonate ramp. Organic-rich deposits were described in unit III, composed of planktic-dominated wackestones interbedded with black shales, in a distal dysoxic to anoxic environment. The carbonates 87Sr/86Sr ratios showed a drastic increase (0.7072–0.7074), interpreted as enhanced chemical weathering, supported by the increase of continental input to the top of section. This trend was accompanied by a long-term δ13Ccarb negative excursion, which were assigned to the latest late Aptian–early Albian interval of the isotope reference curves, in accordance with the described occurrence of Colomiella recta. This scenario matches those proposed for the late Aptian–early Albian transition and OAE 1b set, as an enhanced greenhouse stage, pointing to the influence of the referred ocean-climate changes on the deposition of organic-rich deposits of the early South Atlantic Ocean. This investigation gives more evidences that these perturbations were a widespread event, as a product of broad-scale disturbances in the global carbon cycle which also controlled organic deposition and preservation on restricted settings.

Record of upper Aptian–lower Albian environmental perturbation in northeastern Mexico

Here we combine colomiellid biostratigraphy and strontium-isotope data of the upper Aptian–lower Albian, pelagic and organic-rich interval of the La Peña Formation in northeastern Mexico (western Gulf of Mexico basin). The studied sediments recorded a long-term negative carbon-isotope excursion during the Aptian–Albian transition recognizable worldwide and related to the Oceanic Anoxic Event (OAE) 1b set interval. Furthermore, punctuated and short-term negative spikes show values and positions similar to those associated with the Jacob, Kilian, Paquier and Leenhardt episodes. The highest total organic carbon (TOC) values and detrital mineral (quartz and phyllosilicate) contents occur during the 13C-depleted trend. An increase in the detrital index and the absence of feldspars suggest warm and humid conditions that may have led to intense biogeochemical weathering and runoff. An increased runoff may have been the cause of the δ13C decrease, resulting in density stratification of the basin that favored the preservation of organic-matter under oxygen-depleted bottom water conditions.

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.

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.

Paleoenvironmental change during the mid-Cretaceous Oceanic Anoxic Event 1b

Paleoenvironmental change during the mid-Cretaceous Oceanic Anoxic Event 1b

High-resolution chemostratigraphy of the late Aptian–early Albian oceanic anoxic event (OAE 1b) from the Poggio le Guaine section (Umbria–Marche Basin, central Italy)

Abstract An integrated study of multiple geochemical proxies (δ13Ccarb, δ13Corg, TOC, HI, CaCO3, trace elements/Al ratios) of the late Aptian–early Albian oceanic anoxic event (OAE) 1b has been performed on the pelagic sedimentary sequence of Poggio le Guaine (Umbria–Marche Basin, central Italy). A comparison of the newly collected stable isotope carbon curve with the records from the Vocontian Basin (SE France), DSDP Site 545 and Hole 1049C provided a reliable and precise identification of the four main prominent black shale levels (113/Jacob, Kilian, Urbino/Paquier and Leenhardt) that characterize the OAE 1b. The studied record shows an increase in the marine organic carbon accumulation rate, in particular in the 113/Jacob and Urbino/Paquier levels. In the other black shales, TOC values are Notably, the Kilian and Urbino/Paquier levels from the PLG section are characterized by the absence of correlative shifts in δ13Ccarb and δ13Corg. The increase in the δ13Corg values in these levels is explained by an increase in the relative contribution of 13C enriched marine planktonic archaeal biomass, while the concomitant negative excursions recorded in the δ13Ccarb could reflect a major contribution of isotopically light terrestrial carbonate ions from increased continental runoff during documented more humid conditions.

Expression of the late Aptian cold snaps and the OAE1b in a highly subsiding carbonate platform (Aralar, northern Spain)

Cretaceous climate records provide evidence that major volcanic pulses with duration of 103 to several 105yr triggered changes in climate and oceanography. Black shales of Oceanic Anoxic Event 1b (ca. 113–109Ma) are regarded as signatures of late Aptian greenhouse pulses associated with volcanic episodes. New TEX86 paleotemperature record indicates that warm Aptian climate was interrupted by repeated cold snaps alternating with described greenhouse pulses. An extraordinary thick shallow-water carbonate succession of the Aralar Platform from the southeastern Basque–Cantabrian Basin in Spain provides a unique opportunity to test the hypothesis that cold phases interrupted warm climate in the Aptian. Platform evolution is traced through time of the proposed Aptian cold snaps alternating with greenhouse episodes. New high-resolution C-isotope records established in the studied upper Aptian platform carbonates serve as chemostratigraphic tool providing a link between carbonate-platform evolution, basinal black-shale formation and associated changes in open ocean palaeotemperatures. The late Aptian cold snaps find their expression in a remarkably reduced neritic succession, punctuated by several emersion horizons formed during sea-level lowstands. The studied section bears evidence for several episodes of choked carbonate production related to increased runoff at times of sea-level rise, during which orbitolinids bloomed in a sediment-loaden lagoonal setting. These intervals coincide with the deposition time of organic-rich Kilian and Leenhardt levels belonging to OAE1b.

High-resolution analysis of the mid-Cretaceous Oceanic Anoxic Event 1b in the Vocontian Basin, Southeast France

High-resolution analysis of the mid-Cretaceous Oceanic Anoxic Event 1b in the Vocontian Basin, Southeast France

The neglected history of Oceanic Anoxic Event 1b: insights and new data from the Poggio le Guaine section (Umbria–Marche Basin)

The upper Aptian to lower Albian interval (~114–109 Ma) represents a crucial period during Earth’s history, with a major evolution in the nature of mid–Cretaceous tectonics, sea level, climate, and marine plankton communities. Interestingly, it also includes multiple prominent black shale horizons that are the sedimentary expression of oceanic anoxic event (OAE) 1b. However, due to a set of geological, stratigraphic, and taxonomic challenges, difficulties constraining this OAE may sometimes result in inaccurate correlation. At Poggio le Guaine (central Italy), a continuous and undisturbed section of central western Tethyan pelagic sediments deposited during the latest Aptian and earliest Albian, complete with black shale horizons that record the OAE1b carbon cycle perturbations, has provided a unique opportunity to address the shortcomings referred to above. High–resolution geochemical proxies (CaCO3, TOC, and δ13C), along with planktonic foraminifera and calcareous nannofossils, were used to establish an integrated and robust global stratigraphic framework in order to: (1) precisely correlate the prominent black shale horizons of OAE1b in different marine environments (the Vocontian Basin in southeast France, Deep Sea Drilling Project Site 545 and Ocean Drilling Program Hole 1049C in the eastern and western North Atlantic, respectively); and (2) provide an effective tool to reconstruct, in high resolution, the paleobiological, paleoceanographic and paleoclimatic changes across the upper Aptian to lower Albian interval. The Poggio le Guaine section stands out as a valuable reference section for OAE1b, its constituent subevents, and the δ13C record for the uppermost Aptian to lower Albian interval. The exceptional chemo- and biochronostratigraphic control allow direct comparison to the proposed candidate for the GSSP for the base of the Albian Stage at the Col de Pré-Guittard section in France.

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.

PLANKTIC FORAMINIFERAL SPECIES TURNOVER ACROSS DEEP-SEA APTIAN/ ALBIAN BOUNDARY SECTIONS

A major planktic foraminiferal species turnover accompanied by a dramatic reduction in shell size, a fundamental change in shell architecture, and a precipitous drop in the abundance of planktic relative to benthic species occurs across the Aptian/Albian boundary interval (AABI) at globally distributed deep-sea sections. Extinction of the large and distinctive planktic foraminifer Paraticinella eube-jaouaensis , used to denote a level at or near the Aptian/ Albian boundary, coincides with the extinctions of relatively long-ranging Aptian species of Hedbergella and Globiger-inelloides . At Deep Sea Drilling Project (DSDP) Site 511 (southern South Atlantic), which is the most complete and best preserved of the studied AABI sections, the Aptian assemblage is of low diversity and species are replaced by initially one and then two very small, smooth-surfaced, thin-walled species of Microhedbergella n. gen. The oldest species of this genus, Mi. miniglobularis n. sp., probably descended from Hedbergella praelippa n. sp. and is the nominate taxon for a new lowermost Albian interval zone and is considered ancestral to several small, gradually evolving microperforate species, including Mi. praeplanispira n. sp., Mi. pseudopla-nispira n.sp., and Mi. pseudodelrioensis n. sp., which range into the middle and upper Albian. The small hedbergellids that characterize the Mi. miniglobularis Interval Zone at Site 511 have also been identified from samples taken just above the Kilian black shale level in the Vocontian Basin of southeast France. The Albian record at Site 511 reveals a gradual increase in planktic foraminifera shell size and assemblage dominance, as well as the gradual evolution during the middle Albian of species characterized by a finely perforate, pustulose test. Taxa with this shell infrastructure are included in Muricohedbergella n. gen. A new “ Ticinella yezoana ” Partial Range Zone is erected at Site 511 for correlation of the middle-upper Albian at high latitudes. At Ocean Drilling Program (ODP) Site 1049 (western North Atlantic), which also yields well-preserved foraminifera across the AABI, Aptian species of Hedbergella , Globiger-inelloides , and Pseudoguembelitria blakenosensis n. gen., n. sp. are replaced by lower Albian assemblages composed only of two minute species of Microhedbergella . However, this boundary section is considered incomplete because of the absence of the Mi. miniglobularis Zone. A new upper lower Albian Ti. madecassiana Zone is defined at Site 1049 for the interval between the lowest occurrence (LO) of the nominate taxon and the LO of Ti. primula , the nominate species of the middle Albian Ti. primula Zone. The AABI at DSDP Site 545 (eastern North Atlantic) has a major unconformity spanning the uppermost Aptian through the upper Albian. A black shale sequence previously placed in the lower Albian and designated as Oceanic Anoxic Event 1b is now determined to be latest Aptian in age. The AABI at ODP Site 763 (southeast Indian Ocean) is also marked by an unconformity between the upper Aptian Pa. eubejouaensis Zone and the lower Albian Mi. rischi Zone. This site needs further study to resolve whether overlap of species from both zones is the result of downslope reworking or an exception to the pattern of abrupt species turnover observed at the other deep-sea sites. The dramatic changes in planktic foraminiferal assemblages across the AABI suggest major changes in carbonate chemistry, vertical stratification, or productivity in the surface mixed layer occurred during the last 1 myr of the Aptian. Understanding the cause or causes for these changes will require much further investigation.

The sedimentary expression of oceanic anoxic event 1b in the North Atlantic

AbstractThe Aptian/Albian oceanic anoxic event 1b contains the record of several perturbations in the global carbon cycle and multiple black shale levels, particularly in the Western Tethys. The local lithological expression of an oceanic anoxic event depends on palaeogeographical and depositional setting as well as on regional palaeoclimate. Marine sediments at a particular location may therefore be more or less organic‐rich (or not at all) and they may consist of different lithologies. In most studies, however, much of the lithological variability associated with oceanic anoxic events is left unaccounted for and, thus, the exact processes leading to the enrichment of organic matter in these marine sediments and their subsequent preservation in the geological record are unknown. This study focuses on the local sedimentary processes behind the deposition of organic‐rich sediments at Deep Sea Drilling Project Site 545 and Ocean Drilling Program Sites 1049 and 1276 in the North Atlantic during oceanic anoxic event 1b. Although specifically dealing with the sediments deposited during this particular event at these localities, it is expected that the same processes were responsible for determining the exact sedimentary products at localities in similar settings, as well as during other similar events in the Mesozoic. Here, it is shown that the deposition of organic‐rich sediments during oceanic anoxic event 1b was a consequence of the enhanced productivity favoured by upwelling and by riverine nutrient input, or aeolian fertilization of the euphotic zone depending on geographical location. Slope instability processes resulted in the transfer of part of these organic‐rich sediments from the shelf to deep sea depocentres as mud‐laden organic‐rich turbidites, especially in the northern North Atlantic. The so‐called ‘black shales’ are much more varied than their name implies. The end product of sedimentation during an oceanic anoxic event at a particular location is commonly the result of several equifinal processes acting on a local scale rather than the direct result of basinal or even global mechanisms.

Geochemistry of oceanic anoxic events

Oceanic anoxic events (OAEs) record profound changes in the climatic and paleoceanographic state of the planet and represent major disturbances in the global carbon cycle. OAEs that manifestly caused major chemical change in the Mesozoic Ocean include those of the early Toarcian (Posidonienschiefer event, T‐OAE, ∼183 Ma), early Aptian (Selli event, OAE 1a, ∼120 Ma), early Albian (Paquier event, OAE 1b, ∼111 Ma), and Cenomanian–Turonian (Bonarelli event, C/T OAE, OAE 2, ∼93 Ma). Currently available data suggest that the major forcing function behind OAEs was an abrupt rise in temperature, induced by rapid influx of CO2 into the atmosphere from volcanogenic and/or methanogenic sources. Global warming was accompanied by an accelerated hydrological cycle, increased continental weathering, enhanced nutrient discharge to oceans and lakes, intensified upwelling, and an increase in organic productivity. An increase in continental weathering is typically recorded by transient increases in the seawater values of 87Sr/86Sr and 187Os/188Os ratios acting against, in the case of the Cenomanian‐Turonian and early Aptian OAEs, a longer‐term trend to less radiogenic values. This latter trend indicates that hydrothermally and volcanically sourced nutrients may also have stimulated local increases in organic productivity. Increased flux of organic matter favored intense oxygen demand in the water column, as well as increased rates of marine and lacustrine carbon burial. Particularly in those restricted oceans and seaways where density stratification was favored by paleogeography and significant fluvial input, conditions could readily evolve from poorly oxygenated to anoxic and ultimately euxinic (i.e., sulfidic), this latter state being geochemically the most significant. The progressive evolution in redox conditions through phases of denitrification/anammox, through to sulfate reduction accompanied by water column precipitation of pyrite framboids, resulted in fractionation of many isotope systems (e.g., N, S, Fe, Mo, and U) and mobilization and incorporation of certain trace elements into carbonates (Mn), sulfides, and organic matter. Sequestration of CO2 in organic‐rich black shales and by reaction with silicate rocks exposed on continents would ultimately restore climatic equilibrium but at the expense of massive chemical change in the oceans and over time scales of tens to hundreds of thousands of years.