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The early Toarcian Oceanic Anoxic Event (Jenkyns Event) was associated with major world-wide climatic changes with profound effects on the global carbon cycle. This review revisits the available literature covering the Jenkyns Event applying an updated common stratigraphic definition, allowing illustration of the development and evolution of anoxia in the Alpine-Mediterranean Tethys, north African margin, and North European epicontinental basins within a high-resolution temporal framework. The survey combines geographic and stratigraphic distribution of black shale, organic-matter properties (total organic carbon content and composition), variations in benthic fauna, distribution of euhedral and framboidal pyrite, and redox conditions reconstructed on the basis of both inorganic and organic geochemical data. The compilation demonstrates that bottom waters were generally well oxygenated prior to the negative carbon-isotope excursion of the Toarcian Oceanic Anoxic Event whose onset was marked by the synchronous deterioration in bottom-water oxygen conditions at supra-regional scale. Persistent euxinia was dominantly confined to the north European epicontinental basins and sub-basins, paralleled by a supraregional decline in oxygen content at the seafloor also in the Alpine-mediterranean Tethys area. In the interval of time represented by the core of the negative carbon- isotope excursion the most extreme redox conditions were reached along with intense euxinia extending periodically into the photic zone accompanied by deposition of black shales whose organic-matter content reached maximum values. Recovery to better oxygenated conditions was a diachronous process that started, in most places, at a time immediately following the end of the negative carbon-isotope excursion. The Alpine-Mediterranean Tethys became well oxygenated, while north European epicontinental areas experienced anoxia with less intense and intermittent sulphidic conditions interspersed with brief periods of oxygenation. Δ18O variations reflect a progressive increase in fresh-water input to the northern European epicontinental basins and sub-basins that reached its acme in correspondence with the lowest values of the negative carbon-isotope anomaly. In these areas, the proximity to sources of fresh-water input and the local physiography or geographic restriction limited water exchange with the Tethys Ocean, favouring the onset of anoxia/euxinia and organic-matter preservation. These basins and sub-basins, due to their relatively closed physiography and redox conditions, acted as pools of dissolved divalent manganese associated with accumulation of iron sulphides. Part of the soluble manganese spilled out of these basins in oxygen minimum zones, being deposited/precipitated at the edge of the more oxygenated Tethys Ocean, and thereby leading to the formation of local manganese-rich carbonates deposited during the Jenkyns Event. • Widespread anoxia/euxinia started to develop at the onset of the negative CIE. • Anoxia-euxinia mainly developed in north European epicontinental basins. • Most extreme conditions correlate with lowest values of the δ13C anomaly. • Recovery from anoxia-euxinia occurred diachronously. • Unstable anoxia-euxinia persisted in north European areas after the negative CIE. [ABSTRACT FROM AUTHOR]

Abstract: The Early Jurassic was characterized by a global disturbance of the carbon cycle known as the Toarcian Oceanic Anoxic Event (T-OAE). This event is recorded worldwide by a negative excursion in marine and terrestrial carbon-isotope ratios, typically interrupting an overarching positive trend attributed to large-scale burial of marine organic matter under oxygen-depleted conditions. The negative excursion is attributed to introduction of isotopically light carbon into the ocean–atmosphere system. Three sections from the Ionian Zone in Greece have been analysed in terms of biostratigraphy, Total Organic Carbon (TOC), CaCO3, δ13Ccarb, δ18Ocarb and δ13Corg. On the basis of bio- and chemostratigraphy, the age of Pliensbachian–Toarcian formations from the Ionian Zone in Greece has been refined and the geochemical signature of the T-OAE recognized. All sections illustrate the characteristic negative excursion in carbon isotopes from both carbonates and organic matter and, in only one locality, a positive excursion has also been recorded. The recognition of the T-OAE in this part of the Tethyan continental margin offers additional information on the global impact and amplitude of this important Jurassic palaeoceanographic event. [Copyright &y& Elsevier]

The Triassic–Jurassic transition is characterized by the end-Triassic mass extinction approximately synchronous with the onset of emplacement of the Central Atlantic Magmatic Province (CAMP), and associated with a major negative carbon-isotope excursion (CIE) affecting the ocean–atmosphere system. Here, we present new data (total organic carbon, pyrolysis analysis, carbon-isotopes from bulk organic matter, elemental mercury, and other elemental contents) from a southern-hemisphere Triassic–Jurassic boundary succession in the Neuquén Basin, Argentina. The end-Triassic mass extinction there coincides with a relatively small (2–3‰) negative CIE in bulk organic matter, and we present a model that suggests that extreme aridity across the western Pangaean landmass may have resulted in rather limited terrestrial organic-matter flux to the sedimentary realm in eastern Panthalassic marine basins, hypothetically reducing the magnitude of the observed negative CIE in δ13C TOC. Increased deposition of sedimentary Hg (and Hg/TOC and Hg/Zr) in the marine Neuquén Basin began stratigraphically before the negative CIE associated with the end-Triassic mass extinction, and thus before the commencement, in North America and Africa, of CAMP-related basaltic volcanism, but possibly coinciding with the early emplacement of CAMP-associated intrusives (dykes and sills). This relative chronology suggests thermal alteration of intruded country rocks and/or intrusive magmatic degassing of Hg as potential major sources of elevated Hg fluxes to the atmosphere at this time. The Neuquén Basin experienced the development of dysoxic–anoxic marine conditions across the Triassic–Jurassic transition, enabling increased preservation of organic matter. Simple mass-balance calculations show that enhanced carbon burial rates can explain the inferred evolution of the global exogenic carbon cycle across this time-interval. [ABSTRACT FROM AUTHOR]