Study of the paleoenvironemental changes using Calcareous Nanofossils in Lower Jurassic (Toarcian Stage)

Quantitative methods is used to reconstruct the paloenvironnemental conditions in Toarcian stage by using the ecological indicators of calcareous Nano fossils in three sections (Toka, Petousi, Chionistra). These sections are situated inThe Ionian Zone of northwestern Greece (Epirus region). A statistic program (PAST) is used to analysis the data of nanoplankton species. Three Unitary Associations are determinate in the three sections. The biostratigraphic correlation indicates the presence of a first Unitary Association (UA1) in all studied sections corresponding to the Lower Toarcian, but the second UA2 is noticed in Toka section and it’s absent in Petousi section. This absence is related to the presence of sedimentary lacuna in this section. The curve of biodiversity increases from Chionistra to Petousi and becomes maximum in Toka. The high biodiversity of calcareous nanofossils correspond to the high nutriment fluctuation in pelagic section related to upwelling responsible to supply the nutriment from the profound marine zone. Principal Component Analysis(PCA) was applied to calcareous nanofossils assemblages in order to interpret palaeoenvironmental changes. Two factors were extracted by PCA. The first factor PCA1 is interpreted by the increase of nutrient fluctuation and the domination of high trophic (meso- to eutrophic) conditions within the oceanic photic zone. The second factor PCA2 present the increase in marine level corresponding to the domination of high temperature and eutrophic conditions in Toarcian stage.