The early Toarcian Oceanic Anoxic Event (Jenkyns Event) in the Alpine-Mediterranean Tethys, north African margin, and north European epicontinental seaway

Abstract

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.