THE RESPONSE OF MOLLUSK ASSEMBLAGES TO SEA-LEVEL CHANGES ASSESSED USING COMPUTER-INTENSIVE METHODS: A CASE STUDY BASED ON THE LAST 150KY CORE RECORDS FROM PO PLAIN, NORTH ITALY

Whereas the impact of sequence stratigraphy on various aspects of sedimentary and biotic records has been well established in previous research, there are comparatively few studies that explore changes in mollusk associations in response to climate-driven sea-level changes using rigorous quantitative data. The present research investigates sample-level diversity, diversity turnover, ecological structure, and taxonomic patterns in mollusk assemblages preserved in Pleistocene-Holocene sequences, which include continental, coastal and shallow-marine deposits. Two transgressive-regressive sequences, deposited in the Po Plain (Northern Italy) over the present and last interglacial cycle, were sampled from four cores. Facies analysis and detailed stratigraphic correlations both suggest that these cycles correspond to 4th-order depositional sequences, representing a time span of ~100ky. Continental deposits of the lowstand systems tract were scarcely fossiliferous (therefore not sampled), whereas shell enrichment was found in marine deposits of the retrogradational transgressive systems tracts and the overlying progradational highstand systems tract. To evaluate mollusk assemblages in terms of their responses to climate-driven sea-level changes, several computer-based strategies (rarefaction, multivariate ordination methods [CA, DCA], and randomization techniques) have been applied to 99 samples recovered from the four cored boreholes. Associations recovered by multivariate analysis are arranged along a bathymetric gradient. In the modern Po delta environment, associations similar to those recognized in this study lay on an ecocline ranging from 0 to 30 m depth. Temporal patterns in mollusk distribution suggest a link with environmental shifts that took place over the duration of these sequences (the species appear to track their favored environment). Diversity patterns track closely the sequence stratigraphic architecture, reflecting a combination of climate-driven environmental changes and sequence stratigraphic artifacts postulated by the Holland’s model.