Early Toarcian Oceanic Anoxic Event Research Articles

New insights on Late Pliensbachian-Early Toarcian seawater chemistry based on belemnite rostra element content

Belemnites were a type of cephalopod abundant in the Jurassic and Cretaceous, whose fossils have been extensively used for paleo-oceanographic studies. For this study, we analyzed 69 elements by Inductively Coupled Plasma-Sector Field Mass Spectrometry (ICP-SFMS) in 15 belemnite rostra from the West Rodiles section in the Asturian Basin, Northern Spain. We aim to determine if belemnite rostra carbonate chemistry reflects changes in seawater chemistry during the Late Pliensbachian–Early Toarcian time interval and examine how belemnites can be used to trace known drivers of the Early Toarcian Oceanic Anoxic Event (T-OAE) and associated oceanographic processes.Discarding drastic intra-species effects and assuming a similar influence of growth rates, diet, and other physiological processes for all analyzed belemnite specimens and taking into consideration the sizeable analytical uncertainty, we assess if determined belemnite rostra element chemistry reflects broad and relative changes in paleo-seawater chemistry. Many determined elements are present in only ppb amounts, and their interpretation is uncertain. We found that Mg, Mn, and P increase in the interval chronocorrelative to the T-OAE. This is interpreted to have resulted from an increase in these elements' inventory in seawater due to an increase in continental weathering and fluvial runoff associated with this global event. Iron, K, and Na contents decrease upwards in the section, potentially indicating that these elements became limited and likely hampered oceanic productivity. Our study also found a correspondence between a change in the behaviour of several elements, warming, the T-OAE negative CIE, and a reduction in the diversity and size of both calcareous nannofossils and belemnites in the study area.

Lower Jurassic (Pliensbachian–Toarcian) marine paleoenvironment in Western Europe: sedimentology, geochemistry and organic petrology of the wells Mainzholzen and Wickensen, Hils Syncline, Lower Saxony Basin

AbstractOver the past few decades, Toarcian (Early Jurassic) black shale deposits of NW Europe have been extensively studied, and the possible global and regional mechanisms for their regional variation have been discussed. In this context, the black shales of the Northwest German Basin are still sparsely studied with regard to their palaeo-depositional history. This study aims to understand the connection between regional and global influences on the widespread Early Toarcian oceanic anoxic event by examining two wells covering the Upper Pliensbachian to Upper Toarcian sediments in the Northwestern German Basin. The core intervals were analysed using a multidisciplinary approach, including geochemistry, biostratigraphy and organic petrography. Marine palaeoenvironmental changes were reconstructed, and sediment sequences were stratigraphically classified to allow a supra-regional stratigraphic correlation. The results reveal complex interactions between sea level changes, climate warming, basin confinement, and Tethys–Arctic connectivity resulting in the Toarcian black shale deposition. Upper Pliensbachian sediments were deposited under terrigenous influence, shallow water depths, and predominantly oxic bottom water conditions. The deposition of black shale is characterized by algal organic material input and anoxic bottom water conditions. Strong correlations between water stratification, anoxia, and bioproductivity suggest that global warming and intensification of monsoonal rainfall, continental weathering, and increasing freshwater and nutrient inputs were the main factors controlling the formation of black shales. Prolonged deposition of OM-rich sediments in the NWGB may be related to intensified monsoonal precipitation in northern Europe and enhanced Tethys–Arctic connectivity at the serpentinum–bifrons transition. Graphical Abstract

A First Record of Lower Jurassic (Toarcian) Coleoid Jaws in Siberia

In the present paper, we describe several coleoid jaws discovered in the lower Toarcian black shales, cropping out along the Vilyui River (Yakutia, Russia). This is the first record of a Lower Jurassic coleoid jaw outside Europe and the first report of such a finding from the Mesozoic of Siberia. The described coleoid jaws demonstrate the same mode of preservation and morphology as the coeval jaws previously reported from Europe. Their preservation in Siberia became possible due to the widespread occurrence of black shale facies associated with the early Toarcian oceanic anoxic event (TOAE).

Glendonite-bearing concretions from the upper Pliensbachian (Lower Jurassic) of South Germany: indicators for a massive cooling in the European epicontinental sea

The Pliensbachian–Toarcian transition was characterised by a drastic turnover from a cool climate to a period of rapid global warming. While the warming associated with the Early Toarcian Oceanic Anoxic Event is rather well-studied, the cause, intensity and extent of the preceding cooling in the late Pliensbachian are still discussed. Occurrences of glendonite play an important role in this debate, since glendonite is a pseudomorph after the cryophilic carbonate mineral ikaite. This study describes the first glendonite-bearing carbonate concretions from South Germany (Buttenheim clay pit, northern Franconian Alb), which represent the southernmost glendonite occurrence in the late Pliensbachian documented so far. Based on petrographical and sedimentological investigations as well as stable isotope analyses it is concluded that a low temperature was the main factor for ikaite formation in the studied section, suggesting that the late Pliensbachian cooling had a more far-reaching impact on the temperature of the European epicontinental sea than previously assumed. To explain the low temperatures required for ikaite precipitation, a model for the sea-ice driven formation of cold bottom-water masses on the continental shelf is proposed. The occurrence of several layers containing reworked hiatus concretions in the studied outcrop is interpreted as the result of recurrent sea-level falls caused by multiple glacial pulses characterising the overall cool climate in the late Pliensbachian.

Early–Middle Jurassic brachiopods from Ponor Mountain, Western Balkan Mountains, Bulgaria; taxonomy, biostratigraphy and occurrence in the context of the early Toarcian Oceanic Anoxic Event

ABSTRACT An upper Pliensbachian–Aalenian brachiopod succession is reported from Ponor Mountain, Western Balkan Mountains, Bulgaria. This includes nine species (6 rhynchonellides and 3 terebratulides) from eight genera. A new genus Bulgariarhynchia and two new species, Bulgariarhynchia ponorensis and Capillirhynchia brezenensis, are formally described. A single species Homoeorhynchia ? aff. prona was found in the upper Pliensbachian. Most of the brachiopods derived from the Toarcian–Aalenian interval, and comprise: Pseudogibbirhynchia jurensis, Rhynchonelloidea angulata, Globirhynchia subobsoleta, Sphaeroidothyris uretae, Lobothyris? hispanica, and Telothyris jauberti. The brachiopod assemblages were dated using coeval ammonite biostratigraphy. Both ammonites and brachiopods display clear northwest European affinities. The new brachiopod taxa are considered autochthonous for the Balkan Mountains. Three biofacies (BF2, BF3 and BF5) were recognised and implied alternating suboxic to anoxic bottom water conditions. A few geochemical proxies are indicative of the early Toarcian Oceanic Anoxic Event (T-OAE) in the stratigraphic column. The main T-OAE pulse was fixed at the coeval positive δ13C and negative δ18O excursions, as well as increases in Hg/TOC ratios and redox-sensitive elements. The late Pliensbachian–earliest Toarcian time span was found to be the most hostile for the brachiopods. Above it, brachiopods lived in less stagnant environments.urn:lsid:zoobank.org:pub:22EF76B7-6A4B-4E54-A5BC-38C87125253E

Shallow- and deep-ocean Fe cycling and redox evolution across the Pliensbachian–Toarcian boundary and Toarcian Oceanic Anoxic Event in Panthalassa

The late Pliensbachian to early Toarcian was characterized by major climatic and environmental changes, encompassing the early Toarcian Oceanic Anoxic Event (T-OAE, or Jenkyns Event, ∼183 Ma) and the preceding Pliensbachian–Toarcian boundary event (Pl/To). Information on seawater redox conditions through this time interval has thus far come mainly from European sections deposited in hydrographically restricted basins, and hence our understanding of the redox evolution of the open ocean (and in particular Panthalassa – the largest ocean to have existed) is limited. Here, we present high-resolution Fe-speciation and redox-sensitive trace metal data from two Panthalassic Ocean sections across the Pl/To and the T-OAE intervals, one deposited in deep water (paleo-water depth >∼2.7 km) and the other on a shallow margin (paleo-water depth likely <∼50 m). Data from the deep-water open-ocean site indicate anoxic-ferruginous conditions from the late Pliensbachian to the onset of the T-OAE, with a rather fluctuating redox state alternating between oxic and anoxic/euxinic conditions across the Pl/To boundary. At least intermittent bottom-water euxinia characterized the T-OAE, followed by a subsequent transition toward more oxygenated conditions. By contrast, trace metal data from the shallow margin site indicate that oxygenated to possibly suboxic conditions prevailed. However, elevated highly reactive iron contents, dominated by Fe (oxyhydr)oxides, characterize this shallow-water site. These observations suggest that upwelling, driven in part by increased sea level and prevailing winds from the open ocean, brought anoxic-ferruginous waters onto the shelf, whereupon Fe2+ oxidation was initiated in oxic shallow waters.

Stratigraphy and facies of the Apennine Carbonate Platform (southern Italy): the record of Mesozoic OAEs and Miocene transgression

The Apennine Carbonate Platform of southern Italy witnesses nearly 150 Myr (Late Triassic to Late Cretaceous) of shallow-water carbonate sedimentation in the subtropical central Tethys. During this field trip, you will have a look at the stratigraphy and facies across some key intervals of global palaeoenvironmental perturbation and at some important stratigraphic discontinuities in the history of the platform.During the first day, you will visit three localities of the Matese Mts. At San Lorenzello you will look at Milankovitch cyclicity expressed in Lower Cretaceous (Valanginian-Hauterivian) peritidal carbonates. At Pietraroja you will have a look at the foramol facies of the Cusano formation, marking the Early Miocene transgression on top of the eroded Cretaceous substrate. Moving north, at the Regiapiana you will see the abandoned mines that exploited the mid-Cretaceous karst bauxites, which mark a prolonged subaerial exposure. Moreover, you will walk through the Middle Miocene synorogenic history of the platform, from its exposure and erosion in the forebulge to its incipient flexural subsidence, highlighted by the Lower Miocene carbonates of the Cusano formation, to its drowning below the photic zone, marked by a phosphatic hardground overlain by the Orbulina marls of the Longano formation.During the second day, you will head south toward Salerno. In a quarry at Mercato San Severino, you will look at the record of the early Toarcian oceanic anoxic event, marked by the abrupt demise of lithiotid bivalves and dasycladalean algae, the major carbonate producers of the Lower Jurassic carbonate platforms. At the base of Mt Tobenna, you will look at the Aptian Orbitolina level: a marker bed whose palaeoenvironmental meaning has been long debated. From Mt Tobenna you will move south toward Monteforte Cilento where you will look at the record of the Cenomanian-Turonian OAE2 in the Apennine Carbonate Platform.

Geochemistry of dolerite intrusions of the southeastern Main Karoo Basin, South Africa: Magma evolution, evidence for thermogenic gas sequestration, and potential implications for the early Toarcian Oceanic Anoxic Event

The igneous intrusions of the Karoo Large Igneous Province (LIP) provide critical information on the magma plumbing system, which served as source rock maturation agent for organic-rich sedimentary rocks of the Main Karoo Basin as well as pathway and barrier to subsurface fluid flow. Here, we present results from the research borehole KWV-01 drilled in the Eastern Cape Province of South Africa. The intersected succession, spanning the Dwyka to the lower Beaufort Group, provides representative samples of the Karoo LIP magma plumbing system.Using whole-rock and Sr-Nd isotope analyses, our data suggest that primary melts were derived from a sub-lithospheric mantle source and acquired a subduction signature through interaction with a metasomatised lithospheric mantle. Three magma types identified within this study show a geochemical overlap with previously reported data from the Golden Valley Sill Complex, ca. 220 km to the NW of the analysed borehole. The results of MELTS and EC-RAFC modelling on the three magma types reveal that their compositional differences are related to variable degrees of fractional crystallisation and contamination by lower crustal rocks and/or the sedimentary host rocks of the Main Karoo Basin.Our results indicate that at least one of the sills acted as fractured reservoir, thereby providing evidence that igneous intrusions are important for CO2 sequestration. We quantified the implications of this process for the Karoo LIP-related Toarcian Oceanic Anoxic Event using the thermogenic gas budget produced by state-of-the-art computer simulations and tested various end-member scenarios for partial thermogenic gas sequestration. In agreement with recent studies, we show that all the flat parts of the Karoo sills acted as sequestration reservoirs. Our results have general implications for LIP volcanism in organic-rich basins, suggesting that a good first-order approximation of thermogenic gas emission is half the amount mobilised in the contact aureoles of LIP plumbing systems.

Paleoenvironmental changes during the early Toarcian Oceanic Anoxic Event: Insights into organic carbon distribution and controlling mechanisms in the eastern Tethys

• Unlike N Europe, TOC contents are significantly varied in lower Toarcian sediments of the eastern Tethys . • Anoxic, but non-sulfidic conditions prevailing during the T-OAE. • Humid climates promoted moderate continental weathering during the T-OAE. • Marine productivity was commonly high during the T-OAE in the eastern Tethyssouthern Qiangtang Basin. • Organic carbon burial and oil shale accumulation sustained by increased primary productivity. The Toarcian Oceanic Anoxic Event (T-OAE; Early Jurassic, ca. 183 Ma) represents an intense Mesozoic organic carbon burial event that was associated with the formation of oxygen-deficient conditions at paleo-shelf seas, as well as in lacustrine settings. Global scale increase in organic carbon accumulation occurred in connection with profound changes in climate conditions and perturbations of biogeochemical cycles. This study presents geochemical data for lower Toarcian and Bajocian-Bathonian sediments exposed in the Bilong Co section, located in the southern Qiangtang Basin of central Tibet. During Early and Middle Jurassic times, the study site was located in the north-eastern marine of the Tethys Ocean. A total of 83 rock samples, spanning the Quse (lower Toarcian) and Sewa formations (upper Bajocian-Bathonian), were analyzed for their total organic carbon (TOC) content, carbon isotopes of bulk carbonate and kerogen, as well as major and trace elements. Data allowed to assess changes in the environmental conditions in the eastern Tethys Ocean and further allowed to reconstruct changes in paleo-weathering conditions, marine primary productivity, and redox regimes that occurred in association with the T-OAE. In addition, factors that controlled the accumulation of organic material in the sediment are discussed. Enrichment factors (EF) of redox-sensitive trace elements, their relationship with TOC, and C org /P total molar ratio attest to predominantly anoxic pore and bottom waters in southern Qiangtang Basin during the T-OAE. On the contrary, oxic conditions prevailed during pre- and post-T-OAE times. During the T-OAE, the development of anoxic conditions were most likely linked to an increase in marine primary productivity, as revealed by C org /P total molar ratio, as well as Al-normalized Ba, and Zn concentrations, and their relationship with TOC. Data further attest to a moderate to low marine primary productivity during pre- and post-TOAE times. During the T-OAE, an increase in the marine primary productivity was linked to intensified continental weathering, facilitated by semi-humid climate conditions. Compilation of TOC data from various depositional settings in the eastern Tethys Ocean revealed a high spatial variability, highlighting the importance of local intrabasinal processes on organic matter accumulation.

Weakening of the biological pump induced by a biocalcification crisis during the early Toarcian Oceanic Anoxic Event

Severe environmental changes during the early Toarcian (Early Jurassic; ∼182 Ma), comprising global warming, elevated pCO2, intensified continental weathering and a high amplitude sea level rise, led to the genesis of a Toarcian Oceanic Anoxic Event (T-OAE), expressed by the widespread occurrence of organic carbon (OC)-rich sediments (black shales) forming on anoxic epicontinental shelves. Here, we show that during the T-OAE black shales in some depocenters have formed under rather low organic carbon accumulation rates (OCAR). We explain this observation by a reduced export efficiency of OC to the sediment, caused by a drastic decline in the abundance of mineral ballast, due to a major biocalcification crisis and the demise of calcareous nannoplankton. The reduction of mineral ballast led to slowing of the sinking speed of particulate organic matter, which thus became more prone to biodegradation. A major sea level rise, accompanying the T-OAE, was associated with retrograding shorelines and shift of depocenters away from the basin centers to more coastal areas. This might have reduced nutrient fluxes to the distal depocenters and diminished surface productivity and export flux in the central basin. Moreover, during sea level rise reduced input of clastics from adjacent landmasses may have led to a decline in organic carbon dilution. A decline in both abundance and size is documented for faecal pellets in core FR-210-078 from the Lorraine Basin, substantiating the scenario of reduced marine primary productivity and organic matter transport efficiency. Accordingly, during the T-OAE low OCAR are here interpreted to reflect a weakening of the biological pump. These new data substantiate the role of organic matter preservation as major factor controlling the formation of black shales during the T-OAE. The Toarcian marine transgression led to an increase in the extent of shelf areas, accompanied by an expansion of oxygen-deficient conditions that favored the preservation of organic matter in shelf sediments. Consequently, despite low OCAR, net C-burial in shelf sea sediments increased during the T-OAE, which may have acted as negative feedback mechanism that caused global cooling in the aftermath of the event.

Early Toarcian (Jurassic) brachiopods from the Balearic Islands (Spain) and their paleobiogeographic context

The record of brachiopods in the Lower Toarcian of the Balearic Islands is described after a reassessment of previous works and new samplings in the Tramuntana Range of Mallorca. The recognized species have been assimilated to the assemblages described in the Iberian Range in the Tenuicostatum and Serpentinum biozones. Moreover, a detailed comparison with other Western Tethys basins and the study of the dispersion of the brachiopod faunas in relation with the Early Toarcian Oceanic Anoxic Event, allow to refine the paleobiogeographic relationship of the Balearic brachiopods. They belong to the Euro-Boreal Province of brachiopods in the Tenuicostatum Biochron and correspond to the”Spanish Fauna” that emerged after the deep paleobiogeographic disruption that took place at the base of Serpentinum Biochron. The occurrence of Prionorhynchia msougari Rousselle in the Serpentinum Zone of Mallorca indicates that, within this last assemblage, there would be a closer connection between the Balearic region and the southern margin of the Tethys (Betic and North African basins) than with the northern margin (Eastern Iberian Platform System and Eastern Pyrenees). This paleobiogeographic conclusion is consistent with the supposed position of the Balearic area in the Early Toarcian, on the southeastern margin of Iberia and near the Alboran and Kabylian margins.

Ecological adaptation of marine floras and faunas across the Early Jurassic Toarcian Oceanic Anoxic Event – A case study from northern Germany

We present new, high resolution bio- and chemostratigraphic data (calcareous nannofossils, ammonites, δ13Corg) of the lower Toarcian from an outcrop (Hondelage) in northern Germany. The three data sets, which cover lithostratigraphically the uppermost Amaltheenton-Formation (upper Pliensbachian) and the lower part of the Posidonienschiefer-Formation (lower Toarcian), allow a calibration of the different stratigraphic schemes in ultra high resolution. They further shed light on the ecologic conditions controlling the assemblage composition and evolutionary patterns of calcareous nannofossils, ammonites and gastropods.The distinctive negative δ13Corg excursion (negative CIE, Jenkyns Event), which is part of the early Toarcian Oceanic Anoxic Event (T-OAE), falls lithostratigraphically into the lower part of the laminated black shales of the Posidonienschiefer facies. The lower part of the Jenkyns Event is marked by the first occurrences of the calcareous nannofossil species Carinolithus superbus crassus and Diductius constans. These two events are biostratigraphically assigned to the Tiltoniceras antiquum ammonite Biohorizon (uppermost Dactylioceras tenuicostatum ammonite Zone). The top of the Jenkyns Event has been assigned to the Cleviceras exaratum ammonite Biohorizon (Harpoceras serpentinum ammonite Zone) of the lower Toarcian. The last occurrence of the nannofossil species Biscutum grande lies in the middle part of the Jenkyns Event.Calcareous nannofossils, ammonite faunas and gastropods are discussed with respect to the environmental and climatic changes, which occurred across the Pliensbachian/Toarcian boundary interval and throughout the Jenkyns Event. The extreme impoverished nannofossil and ammonite assemblages of the uppermost Pliensbachian are interpreted in the context of a cooling, primarily affecting the calcareous shelled primary producers. The onset of the Jenkyns Event in the lower Toarcian post-dates this cooling and is related to a subsequent warming. This climatic shift caused a bloom of diverse opportunistic nannofossil and ammonite taxa in the lower Toarcian. Nannofossil abundance patterns were controlled by an increased nutrient availability. The rapid evolution of the ammonite genera Tiltoniceras, Eleganticeras and Cleviceras (family Hildoceratidae), which started in the lower part of the Jenkyns Event, is potentially related to their adaptation to a shallow water dwelling habitat. The mass occurrence of phylogenetically first holoplanktonic gastropods in the lower part of the Jenkyns Event is interpreted in the context of impoverished ocean water oxygenation, which in turn triggered the evolution of a new ecological strategy.

Major sulfur cycle perturbations in the Panthalassic Ocean across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event

The early Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was characterized by marine deoxygenation and the burial of organic-rich sediments at numerous localities worldwide. However, the extent of marine anoxia and its impact on the sulfur cycle during the T-OAE is currently poorly understood. Here, stable sulfur isotopes of reduced metal-bound sulfur (δ 34 S pyrite ) and pyrite sulfur concentrations (S PY ) have been analyzed across the Pliensbachian-Toarcian boundary (Pl-To) and the T-OAE from the Sakahogi and Sakuraguchi-dani sections (Japan), which were deposited in the deep and shallow Panthalassic Ocean, respectively. Our data reveal marked positive δ 34 S pyrite excursions of >10‰ across both the Pl-To and the T-OAE at Sakahogi, coincident with increases in S PY , and a positive excursion of >20‰ at the onset of the T-OAE at Sakuraguchi-dani. Whilst the development of deep-water anoxic/euxinic conditions could have resulted in an enhanced burial of pyrite, and also partly contributed to the positive excursion of δ 34 S pyrite , variations in δ 34 S pyrite at Sakahogi were most likely controlled by elevated export production and/or preservation. On the shallow shelf generally low and highly variable S PY and the positive shift in δ 34 S pyrite were likely attributable mainly to elevated sedimentation rates, with redox playing only a minor role in controlling pyrite abundance. Our discovery of a positive δ 34 S pyrite excursion across the Pl-To at Sakahogi indicates a hitherto unrecognized perturbation to the deep-water sulfur cycle, potentially associated with increased seafloor organic matter flux and pyrite burial at this time, consistent with a transient interval of anoxia. • First pyrite S-isotope (δ 34 S pyrite ) data across the Toarcian OAE (T-OAE) • Positive pyrite S-isotope excursions across T-OAE from deep and shallow Panthalassa • Excursions mainly attributable to local depositional conditions. • Deep-water sulfur isotope excursion across Pliensbachian-Toarcian boundary

Deep-ocean anoxia across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event in the Panthalassic Ocean

The early Toarcian Oceanic Anoxic Event (T-OAE, or Jenkyns Event, ~183 million years ago) was a major hyperthermal and global carbon-cycle perturbation, likely associated with the release of a substantial volume of 12 C-enriched carbon to the Earth's surface. Seawater deoxygenation and the associated deposition of organic-rich facies during this event have been noted in many locations around the world, but evidence for pervasive and extreme oceanic anoxia has thus far been described mainly from European sections deposited in hydrographically restricted basins. Here we present new geochemical data on redox changes during the T-OAE, as well as the preceding Pliensbachian-Toarcian boundary (Pl/To) event, from a deep Panthalassic Ocean site at Inuyama, Japan. Redox-sensitive trace element data reveal an extended interval of seawater deoxygenation that began prior to the Pl/To and continued to the end of the T-OAE. A marked expansion of anoxic and possibly euxinic conditions to the seafloor occurred during both the Pl/To and T-OAE, accompanied by increased organic carbon burial. During these intervals of severe deep-ocean deoxygenation, our data suggest a global drawdown of trace elements such as Mo, U and As. Our findings highlight that: (1) the Panthalassic Ocean was likely an important locus of deoxygenation and organic carbon burial during the early Toarcian, and (2) the spread of anoxia during the T-OAE was a globally distributed phenomenon. • Anoxia in deep water Panthalassa from late Pliensbachian to early Toarcian. • A putative Oceanic Anoxic Event at the Pliensbachian-Toarcian boundary. • Global drawdown of redox-sensitive trace elements during Toarcian OAE. • Deep sea organic carbon burial at Pliensbachian-Toarcian and Toarcian OAE.

Molecular fossils and calcareous nannofossils reveal recurrent phytoplanktonic events in the early Toarcian

During the early Toarcian (Early Jurassic, ~183 Ma) global climate and environmental change profoundly impacted biogeochemical cycles and marine ecosystems culminating in the early Toarcian Oceanic Anoxic Event (T-OAE). The T-OAE exhibited a global temperature rise and carbon cycle perturbation that manifested in a stepwise negative Toarcian carbon isotope excursion. While the response of marine invertebrates towards early Toarcian environmental change is well documented, that of marine primary producers is only poorly constrained and is mainly inferred from changes in algae groups producing hard parts, such as calcareous nannoplankton and organic-walled dinoflagellates. Both biotic groups, however, represent only a small part of the marine phytoplankton, the majority of which lack parts with preservation potential in the geosphere. In order to obtain a holistic view on the response of marine primary producers towards the early Toarcian environmental perturbation, we here conducted a high-resolution study based on the combined application of molecular fossils (biomarker) and calcareous nannofossils. Upon the climax of the T-OAE our data indicate that the marine phytoplankton community structure was dominated by eukaryotic algae, which experienced at least five distinct paleoecological events. Each of these was characterized by a demise of red algae groups and shallow- and deep-dwelling calcareous nannoplankton, and a contemporaneous proliferation of opportunistic green algae groups. An increase of poorly developed coccoliths (resembling protococcoliths) during each phytoplankton event attests to periodically harsh environmental conditions in surface waters. Each phytoplankton event was followed by a recovery of pre-event algal communities, indicative of a cyclicity in unstable and fluctuating environmental conditions. During the onset of the T-OAE, phytoplankton events align with negative δ 13 C shifts, suggesting that changes in the global carbon cycle and the climate system impacted on marine primary producers. Phytoplankton events, particularly during the onset of the T-OAE, were accompanied by recurrent freshening of surface waters during warming periods, suggesting these were the most important ecological drivers. Cyclicity in phytoplankton events followed the short eccentricity (100 kyr) orbital frequency, emphasizing the role of orbital forcing in early Toarcian environmental change and ecosystem disturbances. Ecosystem change and extinction were not driven by a single event, but resulted from multiple disturbances that occurred superimposed onto long-term environmental change and culminated in the early Toarcian ecosystem crisis. • Biomarker and calcareous nannofossils reveal recurrent phytoplankton events. • Phytoplankton events record shift from red to green algae communities. • Phytoplankton events forced by high SST, surface water freshening and stratification. • Environmental change and phytoplankton events were paced by orbital cycles.

Rapid light carbon releases and increased aridity linked to Karoo\u2013Ferrar magmatism during the early Toarcian oceanic anoxic event

Large-scale release of isotopically light carbon is responsible for the carbon isotope excursion (CIE) of the Toarcian Oceanic Anoxic Event during the Lower Jurassic. Proposed sources include methane hydrate dissociation, volcanogenic outgassing of carbon dioxide and/or thermogenic methane release from the Karoo‐Ferrar magmatic province (southern Africa). Distinct small-scale shifts superimposed on the long-term CIE have been interpreted as rapid methane pulses linked to astronomically forced climate changes. In the Peniche reference section (Portugal), these small-scale shifts correspond to distinct brownish marly layers featuring markedly high mercury (Hg) and magnetic mineral concentration. Total organic carbon and Hg increase are uncorrelated, which suggests input of Hg into the atmosphere, possibly released after the intrusion of the Karoo-Ferrar sills into organic-rich sediments. Enhanced magnetic properties are associated with the presence of martite, washed-in oxidized magnetite, inferred to be due to increased aridity on the continental hinterland. This study provides strong evidence for a direct link between the Karoo-Ferrar magmatism, the carbon-isotope shifts and the resulting environmental changes.

The middle-late Aalenian event: A precursor of the Mesozoic Marine Revolution

The Aalenian was a time marked by profound environmental and carbon cycle changes. Still, the scarcity of detailed studies hinders a better understanding of the triggering mechanisms and the larger-scale context of Lower to Middle Jurassic environmental perturbations. This study provides an unprecedented high-resolution biostratigraphically well-constrained carbon isotope record (δ13Corg and δ13Ccarb) for the upper Toarcian–lower Bajocian interval of two expanded limestone-marl alternation successions in France (French Subalpine Basin) and Chile (Andean Basin). The comparison with available records from the Tethyan and Boreal domains highlights that medium-term carbon isotope fluctuations are reproducible across different palaeoceanographic settings from both hemispheres, providing for the first time compelling evidence for recurrent perturbations of the global carbon cycle during the Aalenian. Combined with a review of geological events, climate modes, abundance and diversity of major fossil groups, and trophic conditions inferred from the calcareous nannofossil record, our study fills the gap in our understanding of global environmental changes in a so-far poorly documented Middle Jurassic stage, intercalated between the early Toarcian Oceanic Anoxic Event and the early Bajocian carbonate crisis. Importantly, this compilation indicates that the Aalenian was a pivotal time interval of environmental perturbations, likely triggering the Mesozoic Marine Revolution.

First record of the early Toarcian Oceanic Anoxic Event in the Hebrides Basin (UK) and implications for redox and weathering changes

The early Toarcian (~183 Ma) was characterized by a prominent volcanism-induced warming event associated with a massive addition of 12 C-enriched carbon to the ocean-atmosphere system. This warming likely contributed to marked ocean deoxygenation during this time, giving the event its name: the early Toarcian Oceanic Anoxic Event (T-OAE). Although the T-OAE has been recognized globally, clear geographic differences in the character of the event and its environmental effects have been noted. Here we present new carbon isotope, element abundance and organic geochemical data from a lower Toarcian succession on the Isle of Raasay, Scotland (Hebrides Basin, Northwest European Shelf). These data provide the first evidence of the T-OAE in Scotland. The succession is generally enriched in organic matter through the T-OAE interval, though redox-sensitive trace element data indicate that oxic-suboxic bottom water conditions prevailed, potentially interspersed with ephemeral anoxic episodes. Our elemental data contrast with evidence for persistent anoxia/euxinia in nearby basins, and emphasize how deoxygenation was spatially variable and dependent on water depth and basin hydrography. Similarly, the data emphasize how anoxia was not a prerequisite for the deposition of organic-rich lithologies during the T-OAE. Sedimentological evidence, coupled with inorganic geochemical data, indicates increased coarse-grained detrital flux and enhanced chemical weathering during the T-OAE. Our findings support emerging evidence for a marked strengthening of hydrological cycling and increased storminess at tropical and subtropical latitudes globally in response to global warming during the T-OAE. • Toarcian Oceanic Anoxic Event described from Hebrides Basin (sub-Boreal realm) for the first time. • Negative organic carbon isotope excursion (~-2.7‰) during the early Toarcian • Oxic to suboxic conditions through the T-OAE, in contrast to strong anoxia in nearby basins • Significant strengthening of continental weathering through T-OAE at sub-tropical latitudes

An Early Jurassic (Sinemurian–Toarcian) stratigraphic framework for the occurrence of Organic Matter Preservation Intervals (OMPIs)

Lower Jurassic sedimentary successions in the Atlantic margin basins include several organic-rich intervals, some with source rock potential; time-equivalent units are also identified in on- and offshore areas worldwide. Despite decades of research, it is still unclear which mechanisms lead to the deposition of organic-rich sediments during the Early Jurassic. The objectives of this study are to construct a detailed temporal and geographical framework of Sinemurian–Toarcian organic matter preservation intervals (OMPIs; subdivided into local, regional, and superregional) and roughly constrain the relationship of OMPIs with the Lower Jurassic δ 13 C record. This survey combines an in-depth analysis of literature on the distribution of organic-rich facies in the Sinemurian–Toarcian with new geochemical studies [total organic carbon (TOC) and organic matter pyrolysis] from selected outcrop sections from Portugal, Spain, and Morocco. Strong local control on OMPIs during most of the Sinemurian is suggested. Regionally widespread organic-rich facies are associated with the most negative δ 13 C values of the broad Sinemurian–Pliensbachian negative carbon isotopic trend recorded in organic matter (including the Sinemurian–Pliensbachian Boundary Event). Pliensbachian OMPIs are expressive in the areas bordering the proto-Atlantic Ocean and are often linked with positive δ 13 C excursions and short-lived warm intervals, but OMPIs are also defined for the Late Pliensbachian cool interval. Early Toarcian superregional OMPIs are associated with some of the most pronounced δ 13 C excursions of the Mesozoic. Toarcian maximum TOC content occurs with the positive δ 13 C (recovery) trend following the δ 13 C negative shift typically linked with the Early Toarcian Oceanic Anoxic Event (T-OAE), supporting the notion that peak carbon sequestration/ocean anoxia post-dated the main phase of carbon input into the atmosphere, as also suggested by recent modelling efforts. However, additional superregional OMPIs predate and postdate the T-OAE, indicating that conditions favouring preservation of organic matter (increased productivity and/or enhanced preservation) during the Early Toarcian were not restricted to the T-OAE interval. The compilation of Sinemurian–Toarcian OMPIs presented in this paper demonstrates that organic-rich intervals of regional and superregional expression in the Lower Jurassic sedimentary record are ubiquitous and may even be more numerous than in the Cretaceous. Considering the association of some of the Sinemurian, Pliensbachian, and Toarcian (not taking into account the T-OAE related OMPIs) regional and superregional OMPIs with well-defined carbon isotopic excursions, it is here suggested that these hold the same relevance as the secondary OAEs of the Cretaceous, such as the Valanginian OAE (Weissert Event), Hauterivian OAE (Faraoni Event), and Late Aptian–Early Albian OAE (OAE 1b cluster).

Bottom- and pore-water oxygenation during the early Toarcian Oceanic Anoxic Event (T-OAE) in the Asturian Basin (N Spain): Ichnological information to improve facies analysis

Detailed bed-by-bed ichnological and sedimentological analysis of the early Toarcian Oceanic Anoxic Event (T-OAE) in two sections from the Asturian Basin – Rodiles and Lastres – reveals changes in the macrobenthic tracemaker community mainly induced by oxygen fluctuations. Ichnological analysis enabled us to document an abundant and diverse trace fossil assemblage, with ten ichnogenera, probably corresponding to alternations between the Zoophycos ichnofacies and the distal expression of the Glossifungites ichnofacies: Arenicolites, Chondrites, Diplocraterion, Halimedides, Palaeophycus, Planolites, Rhizocorallium, Thalassinoides, Trichichnus and Zoophycos. Six ichnofabrics were differentiated: laminated?, Diplocraterion–Thalassinoides, Chondrites-mottled, Chondrites–Palaeophycus–Planolites, Chondrites–Halimedides–Planolites and Chondrites-laminated. Ichnological features (including ichnodiversity, distribution of traces and cross-cutting relationships) allowed us to characterise a well-developed multi-tiered macrobenthic tracemaker community, as well as distinguish different tiering models associated with variations in bottom- and pore-water oxygenation. Anoxic conditions associated with the T-OAE corresponded mainly, but not exclusively, to black shale facies. Minor-order oxygen fluctuations from suboxic/anoxic to disoxic/oxic conditions developed during the middle part of the T-OAE. Laminated intra-T-OAE dark and light grey marls show lithological features similar to those of bioturbated pre- and post-event marls; thus, the ichnological information provides evidence that the T-OAE had a larger effect on biota than on depositional features.