LATE PLEISTOCENE/HOLOCENE ENVIRONMENTAL HISTORY OF THE SOUTHERN BRAZILIAN PANTANAL WETLANDS

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This study presents paleolimnological records from two lakes denominated locally as Lagoa Negra and Lagoa Castelo (Pantanal Brazilian Wetland), using sediment cores. The sediment core was dated using 14C geochronologies and samples were analyzed for bulk density, grain size, organic matter, carbonate, phosphorus, carbon and nitrogen elemental and isotopic composition (δ13C and δ15N), sedimentary pigments and sponge spicules. Pollen and diatoms remains were analyzed only in Lagoa Negra because they were scanty and poorly preserved in Lagoa Castelo core. Paleolimnological and palynological records were used with the objective to gain a better understanding of the environmental history of the Pantanal during the last 21,000 cal yrs BP (calibrated years Befor e Present). The different phases in the development of the lakes were based on the palynological analysis of Lagoa Negra core (CONISS analysis) used as a proxy key. The Lagoa Negra core indicates than the late Pleistocene (~21,000 to 12,200 cal yrs BP) was marked by strong river-lake connectivity (~21,000-19,000 cal yrs BP and ~13,000-12,200 cal yrs BP) and with regression of the Paraguay River between ~19,000-13,000 cal yrs BP. The emergence of the gallery forest in the early to mid-Holocene (~12,200 to 8500 cal yrs BP) indicates wetter conditions, similar to the present. In such conditions began the development of Lagoa Negra, from ~11,500 cal yrs BP, with the deposition of fine organic-rich sediments. In the mid to late Holocene (8500 cal yrs BP to present) the connectivity with the river was reduced or lost completely, between ~8200 and 6500 cal yrs BP (Lagoa Negra). The semi-closed Lagoa Castelo reduced connectivity with the Paraguay River only about 6500 cal yrs BP, when abruptly occurs the transition to deposition of organic sediments. A reactivation of the river-lakes interaction was detected after this period, although not as intense as in the early Holocene.