Phytoplankton response and geochemical evidence of the lower Toarcian relative sea level rise in the Umbria-Marche basin (Central Italy)

Quantitative and qualitative analyses have been carried out on the lower Toarcian organic rich lithologies of the Umbria-Marche domain (Marne di Monte Serrone formation) with the aim to emphasize the potential of organic and geochemical parameters as palaeoenvironmental indices. The organic matter and phytoplankton data suggest that the palaeoenvironmental evolution of this area was controlled by a third order cyclicity, induced by climatic-eustatic fluctuations. During the early Toarcian sea level rise, two different phases have been recognized. The phase of facies retrogradation, referable to the Transgressive System Tract, lying in the early and middle part of the D. tenuicostatum Zone. This phase was characterized by a relative increase of basinal CaCO3, a better fertility within the photic zone and a decrease of sea bottom oxygenation and of the micronutrients in surface waters. The maximum flooding event, at the top of the retrogradation phase, corresponds with the maximum of palaeoenvironmental distality, sedimentary starvation and total organic carbon (TOC) values. The phase of facies progradation, corresponding to the High Stand System Tract, occurred in the uppermost portion of the D. tenuicostatum Zone. This phase induced a more efficient water circulation, producing a re-oxygenation at the sea bottom, a better nutrient recycling and a reflourishing of nannoplakton. The CaCO3 in the basin was mainly extrabasinal because the carbonate platforms started producing and exporting the carbonate mud basinwards again. This approach confirms that eustatic fluctuations strongly influenced the variations of phytoplankton total abundance and species diversity and the distribution of organic matter in the sediments. It also regards a significant tool for the investigation of lithologies where the lack of significant sedimentary structures does not give detailed palaeoenvironmental information.