Taphonomic processes affecting late Quaternary molluscs along the coastal area of Buenos Aires Province (Argentina, Southwestern Atlantic)

Abstract

Molluscan concentrations were abundantly deposited along the coastal area of Buenos Aires Province (Argentina, Southwestern Atlantic) by different sea-level episodes during the late Quaternary. Most of them represent Holocene ridges (highly energetic depositional environments) subparallel to the modern shore where the main associations preserved are parautochthonous, and others (mainly autochthonous associations) occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them (low-energetic). Analysis of three physical taphonomic signatures (disarticulation, fragmentation and abrasion) of the molluscan skeletons of four bivalve species ( Tagelus plebeius, Mactra isabelleana, Pitar rostratus, Brachidontes rodriguezi), which exhibit different shell shape and thickness conditions, habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study, support the distinction of both palaeoenvironments, characterized by low and high taphonomic grades, respectively. The taphonomic alteration (breakage and abrasion) of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone, more than a consequence of transport from a distant original habitat. Abrasion signatures are the only probably correlated with after death time span, shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance. The infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal ( T. plebeius, M. isabelleana) are better preserved (less fragmented and abraded) than shells of the epibyssate B. rodriguezi, which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure. Although these nearshore concentrations may represent a time-averaging of 1000 yr B.P., taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations, they still represent valid indicators of past marginal marine conditions in the area, but are only reliable to establish long-term palaeoenvironmental variations.