EXPERIMENTAL TAPHONOMY OF FRESHWATER DIATOMS: DISCRIMINATING BETWEEN CHEMICAL AND PHYSICAL CAUSES OF FRUSTULE FRAGMENTATION

Abstract

ABSTRACT Frustule fragmentation is one of the major taphonomic factors affecting diatom preservation in Pampean shallow lakes. Although this has been demonstrated by taphonomic studies conducted on extant and fossil material, little is known about the causes of frustule breakage and its environmental significance. Field studies along modern environmental gradients showed a low but significant correlation between dissolution and fragmentation. Reworking of sediments by wind action has also been suggested as a plausible explanation for the fragmentation patterns observed in the field. To evaluate the relative importance of chemical and physical causes of diatom fragmentation, three laboratory experiments were conducted to test for (1) the effect of physical agitation; (2) the effect of chemical dissolution; and (3) the joint effect of both variables on diatom fragmentation. Diatom samples were subjected to physical agitation with a reciprocating shaker at 250 rpm for 20 days. Chemical dissolution was tested by mixing diatom assemblages with solutions of different concentrations of NaCl and NaHCO3, and pH 10 for 20 days. The joint effect of both processes was tested by dissolving diatom assemblages in salt solutions and subjecting them simultaneously to agitation for 20 days at 250 rpm. In all cases, aliquots of each assemblage were sampled at initial and final experimental times and their preservation analyzed via the application of fragmentation and dissolution indices. Significant and similar differences in fragmentation indices were observed in both dissolution alone and combined experiments, but no differences were found in samples subjected to agitation alone. In all cases, dissolution and fragmentation indices were correlated, suggesting a causal relationship between them. Overall, our results suggest that the debilitation of the diatom frustule by chemical dissolution may be the main cause of the fragmentation patterns observed in surface sediments of Pampean shallow lakes.