The Mactracea, Tellinacea and Myacea (Bivalvia) in the marine Holocene of northeastern Buenos Aires Province (Argentina, South America): Indicators of environmental change

A systematic revision of the Mactracea, Tellinacea and Myacea (Mactra isabelleana, Raeta plicatella, Macoma uruguayensis, Abra aequalis, Tagelus plebeius, Corbula patagonica and Erodona mactroides) recorded from marine taphocoenoses (Las Escobas Formation; +4.5-2.5 m above m.s.l., mid-Holocene transgression, c.7,000-2,500 C-14 ybp) distributed along 400 km in the coastal area of Buenos Aires Province (Argentina) has been undertaken. This has yielded important palaeoecological and palaeobiogeographical data with respect to the palaeonvironmental reconstruction of the area in relation to sea-level changes, especially since c.7,000 ybp. These data provide new evidence of the strong influence that sea-level fluctuations and climatic change have exerted on the molluscan fauna. This fauna varied in composition and distribution primarily due to progressively decreasing salinity (due to falling sea-level trend after c.7,000-6,500 ybp) and declining temperature (caused by post-hypsithermal cooling after c.4,000 ybp) affecting the bonaerensian coastal area (Rio de La Plata and Argentine Sea in the western South Atlantic) during the Holocene. In the Destacamento Rio Salado Formation (below or at present day m.s.l., muddy and silty sediments, coastal lagoon environment Pleistocene-Holocene boundary) the absence of typical euhaline and estuarine species, together with very low faunal diversity, could be a consequence of the cooler 'Younger Dryas' event, recognized in other areas of the Southern Hemisphere c.10,800-10,000 ybp. On the basis of the abundance of M. isabelleana and the absence or scarcity among the remaining taxa of euhaline warm water molluscan species, the Pascua Formation (+6-10 m above m.s.l., marine sediments; Upper Pleistocene, >35,000 C-14 ybp) may represent a late Pleistocene interstadial but not the last Pleistocene transgression (Sangamon) as originally proposed. The data presented here support similar hypotheses by other authors for the area of study and for other localities in the Southern Hemisphere (southern Africa, Australia, New Zealand).