Triassic–Jurassic boundary events: Problems, progress, possibilities

As for most geological period boundaries, the Triassic–Jurassic (T–J) transition, ∼200 million years ago, was a critical juncture in Earth history during which profound biotic and environmental changes took place. Early comparisons with the end-Cretaceous extinction and the involvement of extraterrestrial impact have now largely, although not entirely, given way to more Earthbound explanations of events. At the T–J boundary the supercontinent Pangaea, which had dominated the palaeogeographic face of the Earth for the previous ∼100 million years, began a fragmentation that has lasted through to the present day. The most obvious manifestation of this process was the production of an estimated two and a half million cubic kilometres of magma with a focus at the centre of Pangaea, and now known as the Central Atlantic Magmatic Province, or CAMP. At more-or-less the same time profound changes took place in the key elements of the biosphere, most notably and obviously in the marine carbonate producing organisms, including those upon which we rely for precise stratigraphic correlation such as ammonites. The case for a dominant volcanic deus ex machina now looks incontestable, even if the origin of the volcanism and the precise mechanisms by which environmental changes were driven require much further explanation. Details of timing are crucial for understanding cause and effect relationships in Earth history, and the lack of a reliable and widely applicable biostratigraphic framework has greatly hampered our understanding of T–J events. It is also plainly the case that in order to reconstruct past events, a physical record of their passing is essential. Here again the Triassic–Jurassic boundary has proved problematic because complete marine sedimentary successions are both few and not very far apart, an observation that has strongly suggested unusually low global sea levels. The relative lack of good marine successions has also delayed the definition of the boundary and the selection of a global stratotype section and point (GSSP); at the time of compilation of this collection of papers decisions had not been made. In order to facilitate advances in these major issues, IGCP Project 458 was set up in 2001 under the leadership of the editors of this special issue. The project was conceived as multi-disciplinary with the aim of integrating palaeontological, stratigraphical, sedimentological, geochemical, geochronological, palaeomagnetic and mineralogical data from T–J boundary sections globally. Amongst the principal activities we anticipated were: field studies directed towards previously known localities as well as recently or newly discovered ones; compilation of global databases with improved and revised taxonomy, biochronology and palaeobiogeography of major fossil groups, and analysis of patterns of the end-Triassic extinction and Early Jurassic recovery; new radiometric ages and high resolution biostratigraphic correlation to establish a reliable temporal framework; assessment of environmental perturbations and their role in different extinction scenarios using geochemical proxy methods; further studies of the Central Atlantic Magmatic Province and the search for a hypothetical end-Triassic impact to provide clues to the trigger of global environmental change. The overarching view was that reconstruction of the end-Triassic events would use an Earth systems approach to integrate all new findings into the most plausible models. The papers collected in the present volume individually touch upon many of the areas of study anticipated for IGCP project 458. For convenience we have grouped the papers into four main thematic sections, whilst recognizing that many of them span several of these topics. Some of the most important results in terms of relative timing of events around at boundary are summarized in Fig. 1. Palaeogeography, Palaeoclimatology, Palaeoecology 244 (2007) 1–10 www.elsevier.com/locate/palaeo