Continental molluscs as indicators of hydrological changes during the Late Quaternary in plain rivers

PISANO MARÍA F.; LUENGO MARIEL

Dublin

Congreso; 20th Congress of the International Union for Quaternary Research; 2019

International Union for Quaternary Research

Molluscs have conquered successfully different types of continental environments, both terrestrial and aquatic, have an important record in Quaternary sediments and are usefulness as paleoenvironmental and paleoclimatic indicators. Salado River in Buenos Aires province, Argentina, like all river of plains is a system highly sensitive to environmental changes, both during periodic episodes of intense rains, as it happens today, as dry conditions that causing great economic loss in one of the most important agricultural areas of Argentina.However, alternation of dry and wet periods is not recent and analysis of the mollusk assemblages allow to know these type of event also during Late Quaternary in the region. Mollusks assemblages have been subjected to an integral analysis, taking into account composition and dominance species changes and their taphonomic characteristics in more than 15 localities throughout of the principal course where sediments have been accumulated during last 13,000 years BP.Knowing the composition of the communities and habits that species present today, associations show three types of main variations. First, Heleobia parchappii plays is the dominant specie in all studied assemblages, analyzing the co-dominant species can berecognized two principal groups: one composed of aquatic gastropods suchas Biomphalaria peregrina, Uncancylus concentricus or Pomacea canaliculata, andanother with species of hydrophilic or terrestrial habits as Succineameridionalis, Miradiscops brasiliensis or Gastrocopta nodosaria. These compositional changes, especially the presence of associations of first type from 6000 years BP, allow reconoced an improve in the availability and development of bodies of freshwater.When analyzing the number of recovered specimens, the density of the individualsrecovered it varies from 0 to 10000 individuals, and the alteration of the external surface of the shells has allowed to recognize differents events. Accumulations with high densities of individuals, mainly of H. parchappii, and low values of taphonomic alteration (fragmentation scarce, good preservation of color and ornamentation original) would allow to recognize assemblages that have not been transported outside their original habitat and that have been accumulated during an extraordinary flood event.Conversely, the presence of poorly preserved associations conserved, with evident signs of dissolution and loss of original ornamentation represent shells that have remained exposed more time, probably during drought periods, favoring a greater alteration of the surface in the shells.Therefore, these changes could be related to the residence time of the remains near the water-sediment interface and the differences in the sedimentation rate of the different exposure cycles suffered by them.Thereby, integral analysis of the mollusk assemblages from different perspectives (ecological, taphonomic, actalist) has allowed to recognize different events of flood and drought that have occurred in the region in a cyclical way, not only at present but throughout the Late Quaternary.