Abstract The study of past climate changes is pivotal for understanding the complex biogeochemical interactions through time between the geosphere, atmosphere, hydrosphere and biosphere, which are critical for predicting future global changes. The Toarcian Oceanic Anoxic Event, also known as the Jenkyns Event, was a hyperthermal episode that occurred during the early Toarcian ( c. 183 Ma; Early Jurassic) and resulted in numerous collateral effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia–dysoxia and biotic crisis. The IGCP-655 project of the IUGS–UNESCO has constituted an international network of researchers with different disciplinary skills who have collaborated and shared conceptual advances on uncovering drivers of the environmental changes and ecosystem responses. This volume, Carbon Cycle and Ecosystem Response to the Jenkyns Event in the Early Toarcian (Jurassic) , presents 16 works that investigate the early Toarcian environmental changes related to the global warming, sea-level rise, carbon cycle perturbation and second-order mass extinction through biostratigraphy, micropalaeontology, palaeontology, ichnology, palaeoecology, sedimentology, integrated stratigraphy, inorganic, organic and isotopic geochemistry, and cyclostratigraphy.

Abstract Micritic sediments containing dark, discrete, organic-rich burrows, situated in a light grey background carbonate mud, were deposited over a broad geographical area in deep-shelf, bathyal and basinal environments in the western margin of the Tethys Ocean during the Early and Middle Jurassic. These hemipelagic deposits represent a distinct depositional regime marked by low-energy, soft-bottom and only locally dysoxic environments. Still, it is unclear whether the trace-fossil assemblages occurring in these deposits pertain to a network of several community types – the ichnotaxa differing from basin to basin – or to a single community of environmentally broad-ranging, burrow-producing species. Lower Jurassic trace-fossil assemblages are found in the Western Carpathians and in the Subbetic, Betic Cordillera: that is, in basins separated by more than 2000 km in their original palaeogeographical areas. The stereotypical Chondrites and Zoophycos trace-fossil assemblages that occur in the analysed deposits share two ichnogenera of distinctive morphology ( Lamellaeichnus and Teichichnus ). Agglutinated foraminifera Bathysiphon occurs together with the described trace-fossil assemblage and determines the epibenthic palaeoenvironmental conditions. In the Western Carpathians, a Lamellaeichnus -dominated assemblage alternates with a Zoophycos -dominated assemblage in small, metre-scale cycles in the upper Pliensbachian, and the proportion of the Zoophycos assemblage increases stratigraphically upwards, probably owing to reduced basin ventilation during the early Toarcian. Within the southern Iberian palaeomargin, represented by the Betic Cordillera, Zoophycos is scarce in the facies.

Abstract The flooding of the Lower Jurassic shelf in the North Gondwana Palaeomargin during the early Toarcian occurred on a fragmented and irregular topography affected by differential subsidence – owing to the activity of listric faults along the North–South Axis of Tunisia – that favoured lateral changes in facies and thickness at a kilometric scale. The onset of Toarcian sedimentation (Polymorphum ammonite Zone, NJT5c nannofossil Subzone) in two adjacent sections was characterized by the deposition of limestones under high-energy conditions. The Châabet El Attaris section was located in a depressed sub-basin, and recorded restricted environmental conditions owing to water stagnation and an oxygen-depleted sea bottom. Therefore, dark mudstones developed, with increased total organic carbon contents and enhanced accumulation of redox-sensitive elements. The sedimentation of limestones bearing gutter cast structures is related to gravity flows probably linked to storm activities. These processes favoured the remobilization of sediments at the seafloor, as well as oxygen input to bottom waters, as shown by the record of trace fossils including Zoophycos , Ophiomorpha , and secondarily, Chondrites and Diplocraterion . The thinly interbedded dark mudstones are locally rich in thin-shelled bivalves that re-colonized the sea bottom after the sedimentation of these high-energy deposits, and subsequently underwent mass mortality related to the return of oxygen-depleted conditions. The Kef El Hassine section is located in the upper part of a tilted, less subsident block, as indicated by its reduced thickness compared with the Châabet El Attaris section; the absence of dark mudstones implies oxic conditions. The Polymorphum Zone consists of limestones showing evidence of sedimentation under high-energy conditions, along with hardgrounds. The occurrence of Zoophycos (deep-tiers) in the upper part of some limestone beds of the Polymorphum Zone is linked to minor erosive processes. The top of the high-energy sequence – below the deposits of a marly interval corresponding to the Levisoni Zone – is interpreted as a hardground given the high content of belemnites and Arenicolites , some of them boring on the eroded Zoophycos and Thalassinoides . This study shows that the sedimentary expression of the Jenkyns Event is not uniform across Tunisia, supporting the importance of local conditions in determining the development of anoxic conditions.

Abstract In this paper, we present a detailed review of upper Pliensbachian–lower Toarcian kerogen assemblages from the southern areas of the West Tethys shelf (between Morocco and northern Spain) and demonstrate the use of the Phytoclast Group as a tracer of palaeoenvironmental changes in the early Toarcian. The kerogen assemblages in the studied sections from the southern areas of the West Tethys shelf are dominated by the Phytoclast Group and terrestrial palynomorphs, although punctual increases in amorphous organic matter, freshwater ( Botryococcus ) and marine microplankton (dinoflagellate cysts, acritarchs and prasinophyte algae) were observed at specific stratigraphic intervals. The opaque/non-opaque phytoclasts ratio was used to trace changes in palaeoclimate and other palaeoenvironmental parameters and reflect climate gradients associated with water availability during early Toarcian. During the Pliensbachian–Toarcian and Jenkyns events, changes in kerogen assemblages in the southern areas of the West Tethys shelf correlated with changes in the northern Tethys and Panthalassa shelf. The acceleration of the hydrological cycle associated with the aforementioned events was less intense in the northern Gondwana, southern and western Iberian basins, a reflection of the palaeogeographic position of these basins within the semi-arid climate belt when compared with the northern Iberian region and other northern areas of the West Tethys and Panthalassa shelf, inserted in winter-wet and warm temperate climate belts. Amorphous organic matter enrichment associated with the Pliensbachian–Toarcian and Jenkyns events reflects an increase in primary productivity linked with increased continental weathering, fluvial runoff and riverine organic matter, and nutrient input into marine areas, inducing water column stratification and promoting the preservation of organic matter.

Abstract Detailed assessment of high-resolution elemental and isotopic geochemical datasets collected from the marl–limestone alternations cropping out at La Cerradura (Subbetic domain of the Betic Cordillera, Spain) and chrono- and chemostratigraphic correlation with the reference Mochras borehole (Cardigan Bay Basin, UK) unveiled valuable new insights into understanding of late Pliensbachian–early Toarcian palaeoenvironmental dynamics in a key geographical area between the northern European seaway and the Tethys Ocean. This study shows that deposition in the study area took place under dominantly oxic water column conditions, indicated, for example, by the generalized lack of enrichment in organic matter and redox metals typically associated with anoxia and euxinia. Carbon isotope stratigraphy (δ 13 C TOC ) allowed recognition of the spinatum (= emaciatum in the Submediterranean Province), Pliensbachian–Toarcian, and early Toarcian Oceanic Anoxic Event negative carbon isotopic excursions and the late Pliensbachian positive carbon isotopic excursion. It is here suggested that the observed periodic changes in lithology and sedimentary geochemistry occur at orbital frequencies (i.e. long and short eccentricity and, tentatively, precession), hinting at an astronomical control of the local–regional climate and environment during the Pliensbachian and Toarcian in the mid- to low-latitude South Iberian palaeomargin area.