Mollusk Successions of Holocene Shallow-Lake Deposits From the Southeastern Pampa Plain, Argentina

Abstract

We analyzed variations in taxonomic composition, abundance, and preservation of freshwater mollusks during the mid-to late Holocene in three shallow lakes of the southeastern Pampa plain. Mollusks were obtained from core samples (110-140 cm long) from the deepest part of the lakes, and sampled at 1 cm intervals. Six species were identified: Heleobia australis, Heleobia parchappii, Biomphalaria peregrina, Uncancylus concentricus, Succinea meridionalis, and Antillorbis nordestensis. Assemblages were dominated by H. parchappii, which displayed continuous and complete records in the three sections. With the exception of H. australis, fossil assemblages were represented by the same species that today inhabit Pampean lakes and preserved the same rank order of abundance as modern communities. A similar pattern of variation in mollusk assemblage composition was recognized in the three successions, which initiated prior to circa 6000 cal. year B.P, with assemblages composed of H. parchappii and S. meridionalis at low abundances. A broad expansion of H. parchappii occurred between circa 6000 and 5400 cal. year B.P., and the species rapidly dominated the whole malacofauna. At circa 5400 cal. year B.P., the freshwater B. peregrina was recorded for the first time, and its abundance gradually increased until circa 3600 cal. year B.P. At ,3700 cal. year B.P., the estuarine H. australis was recorded. The arrival of the freshwater U. concentricus occurred at ,1697 cal. year B.P. Finally, only H. parchappii and B. peregrina were represented in the topmost levels (younger than ,736 cal. year B.P.). Changes in mollusk composition were mostly related to changes in water salinity, which brings a historical perspective to understanding the modern dominance of the euryhaline H. parchappii in these lakes. Shell preservation became favored when water bodies developed brackish conditions.

Differential Preservation of Freshwater Diatoms and Mollusks in Late Holocene Sediments: Paleoenvironmental Implications

“…In fact, taphonomic changes agree well with environmental changes (see De Francesco et al 2013). For example, surface alteration was lower in levels where brackish to saline water conditions were inferred.…”

“…Previous studies of these records indicated the occurrence of wide paleoenvironmental changes, particularly related to fluctuations in salinity, water depth, and nutrient status (Stutz et al 2010(Stutz et al , 2012De Francesco et al 2013;Hassan 2013). These environmental changes may have affected diatom and mollusk preservation in different ways.…”

Taphonomic Signatures in Concentrations of Heleobia Stimpson, 1865 From Holocene Deposits of the Salado River Basin, Buenos Aires, Argentina: Their Utility in Paleoenvironmental Reconstructions

“…Both the modern and Quaternary mollusk communities in the province of Buenos Aires are represented by few families and genera (Tietze and De Francesco 2012;De Francesco et al 2013;Steffan et al 2014). Between the few species found, representatives of Helobia are most abundant.…”

“…Heleobia parchappii is the dominant species in the modern freshwater environments of the Pampean region (Tietze et al 2011) and shows relative abundances that reach 90%-100% of mollusk faunas in Quaternary sediments (De Francesco and Zárate 1999;De Francesco et al 2013). This is an opportunistic species with high reproductive rates (Tietze et al 2011) living in freshwater environments such as rivers, creeks and lakes, although it can tolerate higher salinities (Tietze and De Francesco 2010).…”

“…On the other hand, taphonomic studies conducted on terrestrial (e.g., Yanes et al 2008Yanes et al , 2011Wolverton et al 2010;Yanes 2012), freshwater (i.e., Cohen 1989;Briggs et al 1990;Cummins 1994;Kotzian and Simões 2006;Martello et al 2006;Wolverton et al 2010;Erthal et al 2011;Cristini and De Francesco 2012;Tietze and De Francesco 2012;De Francesco et al 2013) or estuarine environments ) are less common. This has largely prevented the comparison of fossilization processes among different environments as well as the development of a more integrated taphonomic approach.…”