Resumo

ZANOR, Gabriela A. et al. Paleohydrologic reconstruction of the Ambargasta lake (Argentina) during the last 45,000 years by means of stable isotope geochemistry. Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.3, pp.505-527. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2017v69n3a1.

Salina de Ambargasta is a playa system located at the subtropical region in South America (29°S), and influenced by the South American Monsoon System. The present work analyzed the isotopic records of δ13C and δ18O in carbonates throughout two sedimentary cores, previously dated by 14C AMS, in order to identify hydrological changes from ca. 45000 years cal. BP to the present. Additionally, the Pearson correlation coefficients were inspected among the stable isotopes, and also between each isotope and the Loss on Ignition (LOI and LOI ) values in the sediments. Following a dry phase at ca. 45000 years cal. BP, the most negative isotopic values of δ13Ccarb (average: -5.10‰) reflected a more humid period between ca. 39600 and 23600 years cal. BP (MIS 3) that favored water inflow into perennial carbonate and ephemeral sulfate lakes. Values of δ18O carb were more positive (average: 2.76 and maximum:4.86‰) suggesting periods influenced by high evaporation rates. From ca. 23600 years cal. BP to the present (MIS 2 and MIS 1), average compositions of δ13C carb were relatively more positive (-4.50‰), and δ Ocarb average values were comparatively more negative (-4.35‰), indicating less water input to the system and lower evaporation, respectively. From these records, the beginning of the Last Glacial Maximum (LGM) in Ambargasta is dated between ca. 25000 and 18000 years cal. BP in southern South America. In the sequence of the carbonate perennial lake, the existence of a hydrologically closed lacustrine system (ca. 39600 - 26700 years cal. BP) was revealed by a correlation between δ13Ccarb and δ18Ocarb (r = 0.62). Positive correlations between PPI550 and PPI1000 (carbonate lake; r = 0.88; ca. 39600 - 26,700 years cal. BP) and negative (sulfate lakes; average r = -0.81; ca. 39600 - 23600 years cal. BP) indicated high productivities associated with precipitation of evaporites during evaporative concentration stages (isotope positive excursions). This research allows inferring that between ca. 39600 - 23600 years cal. BP, a more humid and warmer stage occurred with respect to the period between ca. 23600 years cal. BP and the present in the Ambargasta region (central Argentina). This work documents important changes in the Precipitation/Evaporation ratio throughout the Late Pleistocene and Holocene, and provides vital information about time windows practically unexplored in southern South America. Taken together, these allow defining regional patterns of paleoclimatic variability during the Quaternary.

Palavras-chave : salina; stable isotopes; carbonates; Paleoclimatic variability; Late Pleistocene; Last Glacial Maximum.

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