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, Tag elus plebeius, Corbula patagonica and Erodona mactroides) recorded fro m 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 alon g 400 km in the coastal area of Buenos Aires Province (Argentina) has been undertaken. This has yielded important palaeoecological and palae obiogeographical data with respect to the palaeonvironmental reconstru ction of the area in relation to sea-level changes, especially since c .7,000 ybp. These data provide new evidence of the strong influence th at sea-level fluctuations and climatic change have exerted on the moll uscan fauna. This fauna varied in composition and distribution primari ly due to progressively decreasing salinity (due to falling sea-level trend after c.7,000-6,500 ybp) and declining temperature (caused by po st-hypsithermal cooling after c.4,000 ybp) affecting the bonaerensian coastal area (Rio de La Plata and Argentine Sea in the western South A tlantic) during the Holocene. In the Destacamento Rio Salado Formation (below or at present day m.s.l., muddy and silty sediments, coastal l agoon environment Pleistocene-Holocene boundary) the absence of typica l euhaline and estuarine species, together with very low faunal divers ity, could be a consequence of the cooler 'Younger Dryas' event, recog nized in other areas of the Southern Hemisphere c.10,800-10,000 ybp. O n the basis of the abundance of M. isabelleana and the absence or scar city 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 inters tadial but not the last Pleistocene transgression (Sangamon) as origin ally proposed. The data presented here support similar hypotheses by o ther authors for the area of study and for other localities in the Sou thern Hemisphere (southern Africa, Australia, New Zealand).