Towards an understanding of fossilization processes in freshwater environments: comparing mollusk life, death, and fossil assemblages in Pampean shallow lakes

DE FRANCESCO, CLAUDIO GERMÁN; TIETZE, ELEONOR; CRISTINI, PAULA ANDREA

Mendoza

Congreso; 4th International Paleontological Congress (IPC4); 2014

The development of taphonomic studies in freshwater environments has been scarce and restricted to certain regions of the globe (mainly the Northern Hemisphere), which has largely prevented the understanding of how the freshwater fossil record forms and how it differs when compared to other records that have been more extensively studied (marine, estuarine). We present here a detailed comparison of the composition, abundance, and preservation of live, dead, and fossil mollusk samples in freshwater shallow lakes of the southeastern Argentinean Pampas. Live-dead comparisons were conducted in four shallow lakes on a seasonal basis, during one year. Additionally, the taphonomic signature of death assemblages was analyzed. A sedimentary core, recovered from the deepest point of one of these lakes, was used to compare modern data with fossil assemblages. Results indicated that death assemblages reflected the richness, evenness, and relative abundance of living species, and exhibited variations due to local environmental conditions (salinity, hardness, productivity). The main process affecting shell surface was dissolution, as indicated by the granular texture and chalky appearance of shells. Slight differences in preservation were recorded between thin- and thick-shelled species, mainly attributed to the higher influence of dissolution on thinner shells. Fossil assemblages (since circa 6000 cal. years B.P.) preserved the rank-order of abundance of the dominant species, but exhibited live-fossil discordance in brackish species. Fragmentation and surface alteration were lower between 6000 and circa 700 cal. years B.P. in coincidence with the development of brackish-water conditions. After the modern-style freshwater environments were established (circa 700 cal. years B.P.), shells became increasingly affected by dissolution and fragmentation, reaching modern-level poor preservation values towards the top of the core. This strongly suggests that mollusk preservation has varied during the Holocene in relation to fluctuations in productivity and salinity, being favored under brackish-water low-productivity conditions. The increased biological activity and low carbonate concentration, together with the shallow nature and associated wind reworking characteristic of these lakes act against mollusk preservation by enhancing shell dissolution and fragmentation. Because water bodies developed before 700 years B.P. were characterized by significantly higher salinities and very low productivity regimes, their depositional conditions contrast with those of modern lakes, highlighting the need for expanding actualistic studies to cover a wider range of taphonomic conditions.