Differential preservation of freshwater diatoms and mollusks in late Holocene sediments: paleoenvironmental implications

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

Mendoza

Congreso; 4th International Paleontological Congress. The history of life: A view from the Southern Hemisphere; 2014

International Paleontological Association

One of the limitations ofpaleoenvironmental reconstructions based on multiple bioindicators is the lackof knowledge on the differential preservation of hard parts, which may lead tobiases in the interpretations. This is particularly important when biologicalproxies differ in their intrinsic properties, as skeletal mineralogy or size.We explored and compared the preservational patterns of siliceous (diatoms) andcarbonatic (mollusks) indicators during the late Holocene (last ca. 4000 cal.years BP) in two lacustrine sedimentary successions from Argentina (Nahuel Rucáand Hinojales San Leoncio). Fragmentation and fine-scale surface alterationindices were calculated on two target species: the diatom Cyclotellameneghiniana and the snail Heleobia parchappii. The taphonomiccurves 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 thedominant 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 SanLeoncio), which decreased towards the top of both successions. On the otherhand, trends in surface preservation were opposite (r=-0.34, p<0.001 forNahuel Rucá; r =-0.42, p<0.01 for Hinojales San Leoncio): while diatomsexhibited higher alteration in the oldest sedimentary levels, mollusk surfacewas more altered at the topmost levels. These differential preservationalpatterns were related to changes in salinity and productivity. Overall, diatompreservation was worse under saline conditions as consequence of higher silicasolubility. However, salinity would have promoted higher carbonateconcentrations and lower bioerosional activity, thus, favoring molluskpreservation. Under freshwater and more productive conditions increasedbioerosional activity prevented mollusk preservation. Hence, the accuracy ofthe paleoenvironmental information provided by both indicators under thesecontrasting conditions is strongly affected by the taphonomic biases suffered,which highlight the relevance of including taphonomic traits in Quaternarypaleoenvironmental or paleoclimatic studies.