Past environmental changes during the mid-Holocene sea-level rise (7- 5.5 Ka) at the NE coastal plain of Buenos Aires, Argentina.

MARIEL LUENGO; ISABEL VILANOVA; ENRIQUE FUCKS

Dublin

Congreso; 20th Congress of the International Union for Quaternary Research (INQUA); 2019

International Union for Quaternary Research

Coastal plains contain important archives of the environmental changes, in particular, salt marsh sediments has been an important tool for the understanding of sea level fluctuations during the Holocene. The NE coastal plains located adjacent to Río de la Plata estuary, specifically from Bahía Samborombón area, developed over several sea-level transgressive periods during the Quaternary. The sedimentary sequences along this plain have a stratigraphic continuity and abundant fossil record characterizing different depositional environments. In this regard, these sedimentary sequences are considered as key archives for reconstructing the geomorphological history as well as past environmental conditions related to the Holocene sea-level changes. This study integrates geomorphological and paleontological (pollen, non- pollen palynomorphs, shells) proxy data from the central zone of Bahía Samborombón (~35.6°S), to reconstruct the salt marsh vegetation history and environmental changes at millennial to centennial time scale, and to develop a geomorphological evolution model during the mid-Holocene sea-level transgressive phase corresponding to MIS 1. The dominance of Chenopodiaceae and Poaceae suggest that between ~7000-6670 cal yr BP halophytic salt marsh vegetation predominates over an extensive tidal flat environment under significant marine influence. It is consistent with geomorphological evidences like relict tidal channels. Pollen assemblages registered during this period present similarity with the middle-high salt marsh zone developed in the current coastal strip of Bahía Samborombón. This environment developed at ~10 km from the present shoreline. During this period relative sea-level rose, reaching ~+3 meters at 6800 cal yr BP. Between 6670-6535 cal yr BP the replacement of a middle-high salt marsh vegetation by another of low salt marsh zone occurred as a result of continuous sea level rise. The gradual increase of dinocysts Operculodinium spp. and Spiniferites spp. indicates the greatest tidal influence in response to sea-level rise that continues in this period. Between 6535-6453 cal yr BP continues the predominance of marsh vegetation under significant and increasing marine influence. This halophyte vegetation coexists with freshwater communities. After ~6450 cal yr BP, the palynological record is limited to dinocysts and scarce pollen content. However, the record of autochthonous mollusk assemblages of Tagelus plebeius with articulated valves and in life position could be indicating environmental stabilization. This is coherent with the maximum transgressive of the sea-level registered for the zone, which is followed by a period of stability of the relative sea-level at +4 m during ~6000-5500 cal yr BP. After 5500 cal yr BP the formation of sand bars and beach ridges towards the east (seaward) occurs, isolating the environments that developed behind, and protected from the action of the marine tidal influence.Eolian and pedogenetic processes took place on the ancient tidal plain at ~10 km from thecurrent coast.