Holocene vegetation history of Hinojales pond, southeastern Buenos Aires Province, Argentina

Pollen data from Hinojales pond (37degrees34'S; 57degrees27'W) provide new information about the vegetational, hydrological and climatological history of the Pampa grasslands since the Middle Holocene. A freshwater pond developed since ca. 4,500 yr B.P. Fluctuations in the relative abundance and concentration of aquatic pollen types simultaneously with lithological changes suggest variation in local hydrological conditions during the entire period studied. Between ca. 4,500 and 3,300 C-14 yr B.P. the pond was situated in an environment dominated by a halophytic plant community that developed after the sea reached the pond during the marine transgressive maximum. Since ca. 3,300 C-14 yr B.P. a freshwater plant community and a broader open water began to develop. A paludal and submerged hydrophitic community without modern analogues prevailed between ca. 2,100 and 400 C-14 yr B.P. An increase in deep water elements (Myriophyllum and Pediastrum) suggests a larger and deeper water body. During this stage, the pond expanded at several episodes, related to increase in precipitation. After ca. 400 C-14 yr B.P. there was a reduction of the water body and a pond similar to present one began to develop within dryer conditions. The pollen record from the top of the sequence points out that present freshwater plant communities became installed recently. Vegetation history of the Hinojales pond indicates that it was active since Middle Holocene and that its origin is not related to modern processes.