Climate, sea-level and anthropogenic processes controlling the environmental evolution of shallow lakes in the southeastern Pampa plain (South East South America) during the last 12 ka

The evolution of both Lake Hinojales-San Leoncio (37 degrees 23'S; 57 degrees 23'W) and Tobares (37 degrees 30'S; 57 degrees 28'W) was reconstructed using pollen, non-pollen palynomorphs, plant macrofossil remains and associated fauna, diatoms and mollusks. This is the first paleolimnological reconstruction covering the hydroclimatic variability over the last 12 ka for the southeastern Pampa plain. At the onset of the Holocene (12-11 ka), shallow clear brackish-to-hypersaline water bodies subject to episodic expansion/retraction cycles developed in the area under wet climatic conditions. During the Early Holocene, unstable environments were inferred due to intercalated episodes of total desiccation, associated with a drop in rainfall that promoted drier conditions than those previously inferred. The Middle Holocene (only represented in Lake Hinojales-San Leoncio) was characterized by the development of a brackish-saline lake with a deeper water column, under drier climatic conditions than those recorded previously. The temporal persistence of this saline paleolake might have been influenced by the marine transgressive-regressive cycle dated between 6000 and ca. 2000 yr BP in the coastal area close to the lake. From nearly 2500 to ca. 600 cal yr BP there was a gradual change from brackish to freshwater-brackish water conditions, as a consequence of a regional increase in rainfall together with the decrease in sea level that promoted the establishment of modern humid to sub-humid climatic conditions prevailing today in the Pampa plain. The last 35 cal yr BP represented the onset of modern eutrophic conditions within the lake, which are concomitant with the agriculture intensification in the region. The regional changes in the different biological proxies analyzed here, support the hypothesis of a predominance of external climatic triggers over intrinsic limnological processes as main causes of paleoenvironmental changes modulating the evolution of lakes. The main environmental forces affecting these lakes seem to be indirectly related to precipitation during the Early Holocene, sea level changes during the Middle Holocene and the recent (Anthropocene) human impacts related to the intensification of farming activities.