CAMBIOS DE LA VEGETACIÓN DURANTE EL HOLOCENO EN EL SE DE LA PROVINCIA DE BUENOS AIRES: ANÁLISIS POLÍNICO DEL ARROYO LA BALLENERA

Resumen

With the aim to study the vegetational changes related to the variations of sea level during the Holocene in the SE of Buenos Aires province, Argentina, a palynological study was performed at an exposed profile at La Ballenera creek (38º 19'S, 57º 57'W). The 230 cm long sediment profile dated in 6790 yr B.P. near the base and in 4120 yr B.P. at the top with an intermedia te date of 6120 yr B.P. produced a pollen and stratigraphic record. Five zones are distinguished in the profile. The lowermost zone, before 6790 yr B.P., represents regional vegetation dorninated by Poaceae and without marine influence. The sediments consist of massive muds, rich in organic matter and very fine sands. After 6790 yr B.P. began the marine influence suggested by the first appearance of marine elements (acritarchs) and the increasing values of Chenopodiineae. The sediments become rich in medium and coarse sands and contain fossiliferous layers (Littoridina sp.). Then a fresh water input is suggested by the high values of Cyperaceae. Between ca. 6200 and ca. 5500 yr B.P. the marine elements continue increasing, suggesting a great marine influence. A halophilous community dominated by Chenopodiineae and the aquatics Ruppia and Botryococcus indicate brackish conditions. Towards ca. 5000 yr B.P. the Chenopodiineae reach the highest values indicating a salt marsh vegetation. The greatest marine influence occurs between ca. 5000 and 4100 yr B.P., characterized by the highest values of marine elements. At the end of the profile the increasing of Chenopodiineae suggests again the establishment of the salt marsh community.

KEY WORDS. Pollen. Vegetation. Sea level change. Holocene. Argentina.