MICRO-SCALE TAPHONOMY OF MOLLUSK ASSEMBLAGES FROM THE LATE QUATERNARY OF THE PO PLAIN, ITALY

Upper Quaternary sequences of the Po Plain (Italy) include two, well developed transgressive-regressive cycles that record the two most recent interglacial cycles. The two cycles are nearly identical in terms of facies architecture and associated mollusk-dominated faunal assemblages. This study focuses on the upper cycle made up of middle-Pleistocene to Holocene spatially complex successions that exhibit alternating terrestrial, brackish-to-lagoonal, and offshore marine depositional environments. These various facies yield mollusk accumulations with varying degrees of shell degradation (i.e. bioerosion, color staining, fragmentation, and abrasion). The purpose of this project is to conduct a comprehensive investigation of the time-averaged mollusk assemblages, focusing on physical, biological, and bio/chemical factors that could alter shell composition and structure using SEM techniques. The investigation compares shells of different levels of taphonomic degradation through time and across system tracks to better understand variations in shell durability.

As part of a larger study on the stratigraphic paleobiology of the Po Plain, a total of 465 samples were acquired from 11 cores. The samples have yielded a total of 71,346 mollusk specimens representing 235 species from 127 genera. For taphonomic analyses reported here specimens of the corbulid bivalve Lentidium mediterraneum from several cores was used. Individual shells were analyzed using taphonomic scores from Kidwell (2001) and environmental scanning electron microscopy (ESEM) at high vacuum, with 20.00 Kv accelerating voltage, 5.0 spot size, at 10-12 mm optimal working distance. Both secondary electron (SE) and backscattered electron (BSE) microscopy were used and evaluation of relative shell composition was done by using energy dispersive x-ray spectroscopy (EDS) to determine relative composition of the shells. With the combination of SEM techniques and classical taphonomic ranks, this study hopes to reveal micro-scale changes in shell composition and structure across system tracks and through time. This technique can also be applied to studies focusing on heavy metal contamination within shells in modern polluted systems.