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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]

Oceanic Anoxic Event 2 (OAE 2), which took place around the Cenomanian–Turonian boundary (∼94 Ma), is associated with extreme perturbations to the global carbon cycle, affected ocean basins worldwide and was associated with significant biological turnover. Although this event has been well studied in the northern hemisphere, the evolution and character of OAE 2, particularly in terms of the vertical and lateral extent of anoxia, is poorly constrained in the palaeo-Pacific Ocean. Furthermore, the precise timing, duration and character of this event, and the exact mechanisms driving OAE 2 environmental changes, are still being debated. Here, we present the first high-resolution records of carbon isotopes, total organic carbon and magnetic susceptibility from the southern palaeo-Pacific Ocean during OAE 2, sampled at two sections in New Zealand. The carbon isotope records from both localities reveal a ∼2‰ positive excursion that represents the global change in the carbon cycle associated with OAE 2. When combined with a cyclostratigraphic age model, these new records constrain the duration of the OAE 2 carbon isotope excursion to at least 930 ± 25 ky and indicate a minimum duration of 200 ± 25 ky for the 'Plenus Cold Event' that took place during OAE 2. The lithologies and low organic-carbon contents of the New Zealand sections imply that oxic conditions prevailed along, at least parts of, the margins of the palaeo-Pacific Ocean at mid- to high southern latitudes during OAE 2 while, contemporaneously, conditions were locally anoxic in the mid-water column of the equatorial Pacific Ocean. Despite these apparently oxic conditions in the New Zealand region, there was a partial collapse of benthic ecosystems leading up to, and during, OAE 2, suggesting environmental deterioration caused by intermittent oxygen deprivation, or other chemical or biological disturbances in the South Pacific region that remain to be elucidated. • First high-resolution C-isotope stratigraphy for OAE 2 from a high southern latitude. • Duration of the OAE 2 CIE was ≥930 ± 25 ky. • Duration of the 'Plenus Cold Event' within the OAE 2 CIE was ≥200 ± 25 ky. • Oceanic conditions were mostly oxic in the Southwest (SW) Pacific during OAE 2. • Intermittent ocean anoxia likely degraded the SW Pacific environment during OAE 2. [ABSTRACT FROM AUTHOR]

The 420-m thick stratigraphic section of the Mural Formation that is exposed in the Cerro Pimas area of northern Sonora, Mexico, is composed of limestone lithofacies ranging from bioclastic wackestone to boundstone, whose biota is characterized by low diversity. Prominent age-diagnostic fossils are benthic foraminifera and long-ranging calcareous algae that indicate the Aptian/Albian boundary is close to the base of the Los Coyotes Member. The carbonates of this formation have negative to positive δ 13 C values (−4.63 to +2.6‰) and highly depleted δ 18 O values that range from −12.74 to −8.34‰. The absence of correlation between δ 13 C and δ 18 O values supports a primary marine origin for the δ 13 C values of these limestones. The carbon-isotopic curve of the Cerro Pimas stratigraphic section has well-defined δ 13 C segments (C8 – C15) that compare with published curves of similar age. In the lower part of the early Albian Los Coyotes Member, the presence of OAE 1b is indicated by an increase followed by a decrease in δ 13 C values, suggesting correlation with the Kilian Event. The middle part of the Los Coyotes Member has a significant negative carbon-isotope excursion correlated with the globally recognizable early Albian Paquier event. Moreover, another significant negative carbon-isotope shift is observed in the upper part of the Los Coyotes Member, which can be correlated with the Leenhardt Event. The occurrence of the Kilian, Paquier and Leenhardt Events (OAE 1b cluster) in the Cerro Pimas stratigraphy confirms the global nature of these early Albian disturbances of the carbon cycle. [ABSTRACT FROM AUTHOR]

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]