Geochemical and sedimentological evidence of productivity cycles recorded in Toarcian black shales from the Belluno basin, southern Alps, northern Italy

Organic carbon rich black shales cyclically alternating with bioturbated limestones and gray marlstones, which crop out in the Belluno Trough (Southern Alps, Italy), are expression of the early Toarcian anoxic event in the Tethys basin. Sedimentological and geochemical features such as well developed lamination, common very small (< 10 mu m) pyrite framboids, and high TOC and V, coupled with abundant radiolarians and fetal pellets and relatively high Ba in the presence of very common dinoflagellates, suggest formation of the shales under conditions of low to very low dissolved oxygen at the seafloor associated with high productivity in surface waters. Chemical and mineralogical characteristics of the bioturbated, organic-poor limestones indicate that in this facies a high carbonate flux: was paralleled by well-oxygenated bottom waters. Compared with the shales and limestones, the marlstones, which include abundant Mn-rich carbonates, formed under intermediate conditions in terms of both bottom-water oxygen concentrations and supply of organic carbon to the seafloor. A negative delta(13)C excursion at the middle part of the studied interval is correlative to maxima in TOC, V/Rb, and Ba/Rb values, identifying, within the Toarcian anoxic event, a stage of more intense bottom water anoxia coupled with high surface fertility.