Paleoenvironmental evolution and vegetation changes during the Holocene at the NE coastal plain of Buenos Aires, Argentina

MARIEL LUENGO; ISABEL VILANOVA; ENRIQUE FUCKS

Dublin

Workshop; Using ecological and chronological data to improve proxy-based paleo sea level reconstructions (2019).; 2019

INQUA

This study integrates paleontological (pollen, non- pollen palynomorphs, shells) proxydata from the central zone of Bahia Samborombón, with radiocarbon dating fromsedimentary organic matter and shell material to reconstruct the saltmarsh vegetationhistory and environmental changes at millennial to centennial time scale. There wereselected 2 sedimentary sequences PM1 (35°58´14.8?S/57°24´49.72??O) y PM2(35°36´8.23"S/57°15´32.52"O). The PM2 analysis shows the paleoenvironmentalevolution in response to the sea-level increase during the Mid-Holocene. During theinterval ~ 7000-6670 cal yr BP, halophytic communities prevailed in a middle-high marshin an environment of extensive tidal plains and open sea, which was located at ~ 10 kmfrom the present coastline. Between ~ 6670 and 6500 cal yr BP the middle-high marshvegetation was replaced by low marsh vegetation communities; which is related withthe sea-level rise during this period and evidenced by the gradual increase of thedinocysts Operculodinium spp. y Spiniferites spp. during the transgression phase. From6500 to 6400 cal yr BP, continues the predominance of marsh vegetation in coexistencewith freshwater vegetation under increasing marine influence. From ~ 6400 cal yr BP,the palynological record is scarce; nevertheless autochthon mollusk assemblages ofTagelus plebeius indicate a stabilized period of the sea-level. During the last 3000 cal yrsBP, the multi-proxy analysis of PM1 showed the paleoenvironmental evolution from asubtidal-intertidal environment under significant tidal influence, to a supratidalenvironment with brackish-freshwater influence related to the sea-level decrease duringthis time. During the period ~ 2700-1660 cal yr BP, halophytic vegetation from a marshdeveloped in the vicinity of a subtidal-intertidal depositional environment at ~ 5.7 kmfrom the present coastline. Within this period, at ~ 2050 cal yr. BP a gradual and partialaerial exposition occurred in relation to a lower tidal influence due to the sea-levelsteady fall; along with the development of middle-high marsh vegetation, similar to thepresent vegetation of this transitional zone, evidenced by the presence of Limoniumbrasiliense and the decrease of dinocysts abundance. Between 1660 and 950 cal yr. BP astable mature saltmarsh established in environments that change from those ofintertidal conditions to supratidal conditions with shallow water bodies developed asreflected by gradual increase of Azolla filiculoides and Ricciaceae. This change ofenvironments is linked to the coastal progradation. The last period, from ~ 950 cal yr BPto the present, the predominance of marsh vegetation continues althoughaccompanied by freshwater vegetation developed in small water bodies with rare ornull tidal influence. The climatic signal was possible to infer once the marine influenceleft to be superimposed related to the sea level fall; which was characterized by agreater precipitation regime as suggested by the high increase of the freshwatercomponents.