Differential preservation of freshwater diatoms and mollusks in Late Holocene sediments: Paleoenvironmental implicatios

HASSAN, GABRIELA SUSANA; TIETZE, ELEONOR; CRISTINI, PAULA ANDREA; DE FRANCESCO, CLAUDIO GERMÁN

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

One of the limitations of paleoenvironmental reconstructions based on multiple bioindicators is the lack of knowledge on the differential preservation of hard parts, which may lead to biases in the interpretations. This is particularly important when biological proxies differ in their intrinsic properties, as skeletal mineralogy or size. We explored and compared the preservational patterns of siliceous (diatoms) and carbonatic (mollusks) indicators during the late Holocene (last ca. 4000 cal. years BP) in two lacustrine sedimentary successions from Argentina (Nahuel Rucá and Hinojales San Leoncio). Fragmentation and fine-scale surface alteration indices were calculated on two target species: the diatom Cyclotella meneghiniana and the snail Heleobia parchappii. The taphonomic curves obtained were smoothed along depth with a locally weighted regression (LOESS), and statistically compared using Spearman correlations. Additionally, past environmental conditions were inferred from the autoecology of the dominant taxa. Diatoms and mollusks displayed similar tendencies in fragmentation (r=0.31, p<0.001 for Nahuel Rucá; r =0.87, p<0.001 for Hinojales San Leoncio), which decreased towards the top of both successions. On the other hand, trends in surface preservation were opposite (r=-0.34, p<0.001 for Nahuel Rucá; r =-0.42, p<0.01 for Hinojales San Leoncio): while diatoms exhibited higher alteration in the oldest sedimentary levels, mollusk surface was more altered at the topmost levels. These differential preservational patterns were related to changes in salinity and productivity. Overall, diatom preservation was worse under saline conditions as consequence of higher silica solubility. However, salinity would have promoted higher carbonate concentrations and lower bioerosional activity, thus, favoring mollusk preservation. Under freshwater and more productive conditions increased bioerosional activity prevented mollusk preservation. Hence, the accuracy of the paleoenvironmental information provided by both indicators under these contrasting conditions is strongly affected by the taphonomic biases suffered, which highlight the relevance of including taphonomic traits in Quaternary paleoenvironmental or paleoclimatic studies.