Carbon-isotope records of the Early Jurassic (Toarcian) oceanicanoxic event from the Valdorbia (Umbria–Marche Apennines)and Monte Mangart (Julian Alps) sections: palaeoceanographicand stratigraphic implications

The Toarcian oceanic anoxic event (ca 183 Ma) coincides with a globalperturbation marked by enhanced organic carbon burial and a general decreasein calcium carbonate production, probably triggered by changes in thecomposition of marine plankton and elevated carbon dioxide levels in theatmosphere. This study is based on high-resolution sampling of two stratigraphicsuccessions, located in Valdorbia (Umbria–Marche Apennines) and MonteMangart (Julian Alps), Italy, which represent expressions of the Toarcianoceanic anoxic event in deep-water pelagic sediments. These successions arecharacterized by the occurrence of black shales showing relatively low totalorganic carbon concentrations (compared with coeval strata in Northern Europe),generally < 2%, and low hydrogen indices. On this basis, they are similar to otherToarcian black shales described from the Tethyan region. The positive andnegative carbon-isotope records from the two localities permit a high-resolutioncorrelation such that ammonite biostratigraphy information fromValdorbia can betransferred to those parts of the Monte Mangart section that lack these fossils.Spectral analyses of d13Corg values and of CaCO3 percentages from the sedimentaryrecords of both the Valdorbia and Monte Mangart sections reveal a strong cyclicpattern, best interpretedas aneccentricity signalwhichhence implies adurationofca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotopecurves obtained, the high-resolution correlation between the Italian successionsand a section in Yorkshire (Northern Europe) confirms the supposition that theapparentmismatch between the dating of the Toarcian oceanic anoxic event in theBoreal and Tethyan realms is an artefact of biostratigraphy.