Late Holocene environmental and vegetation changes in the NE coastal plains, Buenos Aires province, Argentina

MARIEL LUENGO; ISABEL VILANOVA; ENRIQUE FUCKS

Salvador

Congreso; XIV International Palynological Congress X International Organisation of Palaeobotany Conference; 2016

IFPS ? International Federation of Palynological Societies, IOS ? International Organisation of Palaeobotany and ALPP ? Asociación Latinoamericana de Palaeobotánica y Palinología.

The northeastern coastal plains located between 35° and 36° S in Buenos Aires province, Argentina, are characterized bya semi-circular bay of 140-km long (Bahía Samborombón) in which important wetlands (RAMSAR site) extend along the coastal zone. These wetlands encompass aquatic and terrestrial ecosystems under the influence of mixing waters from Río de la Plata and the Atlantic Ocean. Consequently, there are particular ecological conditions that support high biodiversity of the wetlands. The sedimentary sequences from this area have a stratigraphic continuity and high fossiliferous wealth from which it has been established the development of different depositional environments during Late Quaternary such as coastal barriers, beach ridges, tidal flats and lagoons. These sedimentary sequences represent key archives for reconstructing the geomorphological history, past vegetation and environments, particularly related to the Holocenesea-level changes and climatic conditions, which provide information to understand the resilience of plant  communities and the environmental dynamic over time in this area, which is currently underth e risk of inundations and erosion processes due to sea-level rise and globalwarming. Thus, the aim of this study is to reconstruct the vegetation and environmental changes during the Late Holocene related to the sea-level regressive phase, based on palynological content, lithology and stratigraphy from an outcropping sedimentary sequence from Bahía Samborombón(35°58´14.86´´S; 57°24´49.72´´W). The results show that after circa 2350 cal years BP saltmarsh vegetation predominated,with some variations over time regarding plant distribution and environmental conditions that can be described in several stages: -Firstly, the dominance of Chenopodiaceae and Poaceae suggest a halophytic saline-to brackish marsh, under frequent marine influence from an intertidal flat environment reflected by high proportions of Spinieritessp. and Operculodinium sp dinoflagellate-cysts and Mactra isabelleana valves. Then, the increase of Chenopodiaceae and Limonium brasiliense record suggest a saltmarsh with higher salinity in an environment with less marine tidal frequency indicated by lower dinoflagellate-cyst records; all of which is associated with sea-level fall; Toward the present, mixed halophytic- hydrophytic community developed, revealed by an increase in Cyperaceae and Ambrosia types, the decrease of Chenopodiaceae and high proportions of spores from the freshwater fern Azolla filiculoides. These environmental conditions can be related to the isolation of this area from the bay shoreline by beach ridge progradation and to increasing fluvial-freshwater influence. In the uppermost part of the sequence a soil developed supporting introduced herbs and trees, with the bay shoreline located 5 km to the east.