Recurring flooding events during he Holocene in the South American extratropics Laguna Mar Chiquita, Argentina

PIOVANO, E.L., D. ARIZTEGUI, S BERNASCONI E I. HAJDAS,

Miramar

Simposio; Holocene Environmental catastrophes in South America: From the Lowland to the Andres; 2005

Recent weather patterns combined with results of high resolution paleostudies indicate that the role of tropical and subtropical areas may have been underestimated as forcing factors of changes within the global climate system. Thus, the study of well-constrained paleoenvironmental records in these regions became critical. The sedimentary and isotopic record of Laguna Mar Chiquita (30°54´S-62°51´W) provides a unique archive of the alternating low and highstands during the 20th century that have been instrumentally and historically documented reflecting regional rainfall variability. This sequence of variations in the hydrological balance of Laguna Mar Chiquita is synchronous and in phase with other hydrological changes observed in southeastern South American rivers pointing towards the regional scale of the reconstructed paleohydrology. The associated flooding events have had a catastrophic impact on the lake catchment and are seriously compromising the sustainable development of this mostly agricultural region. Additionally, substantial changes in biodiversity and population dynamics have accompanied these lake level changes. A first well-calibrated sedimentary record covered only the last 300 years and thus is insufficient to formulate predictive models of future catastrophic events. A series of long cores were recovered allowing the reconstruction of lake-level variations prior to 20th century, covering the hydrological changes that occurred in this area since the Last Glacial Maximum (LGM). This quantitative estimation of paleolake levels shows a recurrent pattern of highstands since the late Pleistocene holding often a magnitude equivalent to present-day conditions. A wet phase followed the negative water balance during the LGM that ended at ca. 13,700 BP. A subsequent abrupt negative hydrological balance was followed by highstands conditions and thus positive P-E ratios since the Early Holocene until ca. 4,200 BP when started a new hydrological reversal. Dry conditions were mostly dominant from the Middle Holocene until the last quarter of the 20th century when an outstanding humid phase started. The integration of these paleodata with a refined chronology may provide a critical tool to further elaborate realistic models of sustainable development in a region of ascertain environmental vulnerability.