Taphonomic damage of molluscan shells in the Nile Delta under natural and anthropogenic sources of environmental variability

Environmental reconstruction can benefit from the interpretation of the postmortem history of shell remains. The
molluscan death assemblages obtained from grab samples at forty-nine sites in the Manzala lagoon, the largest
lagoon in the Nile Delta, were analyzed to infer the taphonomic alteration. The preservation of shells at each site
has been semi-quantitatively ranked based on the taphonomic features formed by encrustation, bioerosion,
abrasion, dissolution, precipitation, fragmentation, discoloration, and loss of sheen (low, moderate, high). A
significant difference has been observed between sites of low and high environmental stress. Water depth and
substrate type (hydrodynamics) correlate significantly to the percent number of altered shells. The southern parts
of the lagoon, which are characterized by lower water energy and higher rates of sedimentation, are highly
polluted by heavy metals (Cd, Pb, Zn) and organic pollutants. The southern parts receive much sediment from the
agricultural drains, which increases burial of shelly material and decreases their time on the TAZ. The northern
parts of the lagoon have slower rates of sedimentation, so that the molluscan shells remain longer at the
sediment-water interface (Taphonomic Active zone; TAZ), where fragmentation is enhanced by winnowing
under higher water-energy. The long residence in the TAZ also favored both bioerosion (Trypanites and Oichnus)
and encrustation by bryozoan colonies, polychaetes, and oysters. Therefore, the taphonomic processes have the
potential to cause severe damage and taphonomic loss of shell materials. Chemical dissolution and precipitation
of shell remains may be related to acidic pore water. Moreover, high alkalinities in the southern polluted parts
may also reduce the dissolution in contrast to the acidic water in the northern parts of the lagoon.