Paleoenvironmental evolution and sea-level fluctuations on the southeastern Pampa grasslands coast of Argentina during the Holocene

VILANOBA, I., PRIETO, A.R. AND ESPINOSA, M.E.

Puerto Madryn

Simposio; Reunión Anual de Comunicaciones y Simposio del 50º Aniversario de la APA; 2005

APA

Vegetation history and paleoenvironmental evolution have been studied based on pollen and diatom analyses, along with complementary information from sedimentary stratigraphy, mollusc diversity and abundance and also from isotopes records at southeastern Pampa grassland coast. Late Pleistocene pollen records indicate a herbaceous vegetation within a continental environment. For the Holocene, analyses of pollen and diatoms data have provided a paleosalinity record, which is related to sea level variations. Both, pollen and diatom records, show the same basic trends about salinity conditions and marine influence. The most conspicuous change occur at ca 3900 ± 80 14C yr BP. Between ca 6700 and  3900 14C yr BP is characterised by higher marine influence which is related to the sea level transgression phase. High salinity conditions are suggested by  halophytic vegetation. Maximum sea level transgression is evidenced by mollusc deposits, mostly gastropods and bivalves from supratidal and intertidal environments. The paleoenvironmental evolution show between ca 6200 and 5700 14C yr BP a development of estuarine shallow waters with tidal influence, surrounded by halophytic vegetation, and diatom assemblage dominated by the brackish/epiphyte taxa. Salinity conditions were high, with some variations. These conditions evolved between ca 5700 and 3900 14C yr BP, into a salt marsh vegetation within an environment with brackish waters as ditches or small lagoon under tidal influence, which is also evidenced by estuarine mollusc, wavy bedding structures and isotopes records. Paleosalinity conditions were very high during this period, without variations. Between 3900 and 2000 14C yr BP is characterised by less saline condition with high salinity variations. Marine/tidal influence became sporadic and freshwater input is inferred. These conditions are related to the regression phase of sea level. Vegetation changes indicate a transition to a less saline  marsh under freshwater influence and periodic desiccation processes toward, ca 3000 14C yr BP. Marine / brackish diatoms decrease while brackish / freshwater taxa increase. Freshwater mollusc started to be registered while isotopes record showed after ca 3000 14C yr BP a freshwater influence. However halophytic vegetation increase toward ca 200014C yr BP indicate  saline conditions, possibly as a consequence of desiccation processes but also to extraordinarily high tides, also evidenced by marine shell fragments. Modern vegetation in the area and surface pollen samples show that no halophytic vegetation exists in the area,  characterised by freshwater taxa on creek shores and psammophitic taxa on sand dunes.