Paleoenviromental variability in the Pampas region, southeastern South America during the last millennium based on palynological analyses

ALDO R. PRIETO; HORACIO FRAZER; EDUARDO A. GÓMEZ; C. MARCELA BOREL

Salvador

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

Asociación Latinoamericana de Paleobotánica y Palinología

Palynological analysis from Laguna Chasicó (LCh) (38° 37? S; 63° 05? W) has provided information on the paleoenvironmental history of the last 1000 years of the southwestern semi-arid Pampas region. LCh (20m b.s.l.), the lowest water body of South America, is located in the Pampa grasslands-xerophytic woodland ecotone. During the last ~110 years the area has experienced several flood and drought events which are reflected in highly variable lake water depth and size, and salinity as a consequence of changes in precipitation related to climatic variability. Continuous three cores were recovered from the deepest part of the lake (~14 m) for past environmental studies. The deepest core (143 cm) was selected for high-resolution palynological study. Forty bottom surface pollen samples from both LCh and its only tributary Arroyo Chasicó provided information for the understanding and interpretation of the fossil palynological record. The age model covers a period from ~AD 1100 to the present. It is based on 14C dating (at 43 and 125 cm depth) and the first appearances and changes of exotic pollen trees introduced in the area since the early 20th century which are related to historical data. Changes in grassland/xerophytic woodland pollen ratios and shifts of the local halophytic taxa suggest variations in lake size and salinity. The reconstruction provided evidence of alternating wet and dry phases at century-scale and a lake size similar to the present and low salinity from AD 1100 to near the early 20th century. Subsequent abrupt increase in halophytic and xerophytic taxa from AD 1920 to AD 1977 indicates the driest period and extremely low lake level when LCh reduced its area to ~31 km2 and salt content was ~100 g/L in the 1960s. The uppermost recent decrease in halophytic taxa and fluctuating increases in grassland taxa represent a wet period, high water level and a decrease in salinity after AD 1977, coeval with a general increase in precipitation across the Pampas which led to a fast water level rise in 1980 when LCh reached 100 km2 and salt content decreased up to ~20 g/L. The inundation peak in AD 1983 coincided with one of the strongest ?El Niño? events of the 20th century. The main human impact on vegetation is indicated by the increase in weeds and exotic trees from ~AD 1970 onwards. This paleoenvironmental reconstruction provides a new high-resolution record of the climatic variability in the Pampas during the last millennium.