Taphonomic field experiment in a freshwater shallow lake: alteration of gastropod shells below the sediment?water interface

CRISTINI, PAULA ANDREA; DE FRANCESCO, CLAUDIO GERMÁN

JOURNAL OF MOLLUSCAN STUDIES

OXFORD UNIV PRESS

Lugar: Oxford; Año: 2019

0260-1230

We conducteda taphonomic field experiment to evaluate the effects of (1) depth below the sediment-water interface and (2) time of exposure on the accrual of damage (particularly through dissolution) to empty mollusc shells in freshwater environments. The experiment, which lasted 30 months, was carried out in the littoral area of Lake Nahuel Rucá, a freshwater shallow lake in the Pampa plainof Argentina. Bags (0.5-cm-mesh) containing empty, cleaned shells of the freshwatergastropods Heleobia parchappii, Biomphalaria peregrina and Pomacea canaliculata were buried atthree different depths (5, 20 and 35 cm) below the sediment-water interface (SWI). Damage was assessed every 3 months. All experimental shells exhibited fine-scale surface alteration, but the extent of this damage was relatively low, even in shells recovered after 30 months of exposure. Most of the damage consisted of minor pitting. For H. parchappii and B. peregrina, shell surface alteration varied significantly with depth, but not with time; in both species,alteration occurring mainly at the beginning of the experiment. For Pomacea canaliculata, shell surface alteration varied significantly only with respect to time, and this was thecase for all three burial depths. All shells of this species exhibited a lower levelof damage than what was observed for H.parchappii and B. peregrina. These differences may be related to the fact that P. canaliculata is characterized by a larger and thicker shell than the other two species. The shell attributes of larger size and greater thickness are known to confer a greater resistance to shell dissolution. In addition, the reactive surface area available for dissolution varies with shell size. Larger-sized shells have a lower potential for dissolution than smaller shells due to their lower surface area:volume ratio. In contrast, species such as H. parchappii and B.peregrina, which have smaller and thinner shells, are likely to be more rapidly destroyed, because they are more vulnerable to dissolution and have less preservation potential. Our results demonstrate that dissolution is a significant taphonomic process affecting shells even during burial, and that the influence of dissolution on shell alteration might be significant in cases of long persistence within the taphonomically active zone. Consequently, we suggest that when working on taphofacies in the context of aquatic environments, assessments of taphonomic alteration should includechanges at and below the sediment-water interface.