The palaeoecological record of delta dynamics: testing the faunal response of benthic foraminifers and ostracods

The mid-late Holocene subsurface succession of the Po River plain records complex delta outbuilding under a predominantly autogenic control during the sea-level high stand phase. Chronologically constrained phases of the Po Delta evolution represent a solid base to test the response of microfossils to short time-scale delta dynamics. Benthic foraminifers and ostracods from the 21m-thick shallow-marine succession of core EM13 were quantitatively analysed and statistically compared to modern North Adriatic meiofaunal assemblages, in order to assess the main palaeoecological factors that controlled the composition of assemblages, and compare the palaeoenvironmental resolution provided by these two benthic groups. Benthic foraminifers from the lowermost shallow-marine succession indicate the transition from inner shelf to coastal conditions, marked by the gradual increase in Ammonia tepida and Ammonia parkinsoniana. Ostracods from the same stratigraphic interval are comparable to those nowadays recorded in shallow-marine settings affected by relatively low river flows. Chronologically in accordance with the development of cuspate deltas during the Roman Age, benthic foraminifers record the approach to coastal environments, subject to low fluvial inputs as evidenced by ostracods. The overlying deposits with high frequencies (ca. 50%) of A. tepida and A. parkinsoniana suggest the transition to a proximal prodelta, as confirmed by the similarity with modern assemblages at shallow depths, few km far from the river mouths. Upwards, the overwhelming dominance of the same taxa is indicative of extreme vicinity to river mouths. Within the same stratigraphic portion, ostracods indicate conditions similar to those nowadays present along the mid-Adriatic Italian coast, where the dominant action of longshore current distributes river-supplied material enriched in organic matter. Following one of the main avulsion events in the Po Delta history (XII century), the transition from wave- to fluvially dominated deltaic systems is marked by a great change in ostracod assemblages. Therefore, distinct palaeoecological information are provided by each fossil group: benthic foraminifers give indications about the distance from river mouths, whereas the dominant depositional regime is suggested by ostracods. Benthic foraminifers and ostracods are excellent palaeoenvironmental indicators in river-influenced shelves, but refined sub-environmental characterization can be achieved with their combined application only.