Assemblage diversity and Geochemical Signatures of modern ostracodes and host waters from Patagonia, Argentina

ANTJE SCHWALB; GABRIELA CUSMINSKY; PATRICIA PÉREZ; STEPHEN BURNS; ADRIAN GILK; DANIEL ARIZTEGUI; VERA MARKGRAF

San Fransisco, USA

Congreso; Fall Meeting EOS; 2001

American Geophysical Union

Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with isotope ratios and major trace element geochemistry of host water samples, provide a first data set that characterizes a wide range of modern aquatic environments in the Laguna Cari-Laufquen (41S, 68 - 69W) and the Lago Cardiel area (48 - 49S, 70 - 71W) in Patagonia, S. Argentina. Species assemblages, water chemistry and isotope values can be assigned to three groups; (1) springs, seeps and streams, (2) permanent ponds and lakes, and (3) ephemeral ponds and lakes. Springs, seeps and streams are characterized by Darwinula sp., Heterocypris incongruens, Eucypris fontana, Amphicypris nobilis and Ilyocypris ramirezi. Ostracode and water isotope values are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichment. {it Limnocythere patagonica}, {it Eucypris labyrinthica}, {it Limnocythere} sp. and {it Eucypris} aff. {it fontana} are typical species of permanent ponds and lakes. Isotope values indicate high degree of evaporation of lake waters relative to feeder springs and streams. {it Limnocythere rionegroensis} is the dominant species in ephemeral ponds and lakes. These systems display the most enriched isotope values in both ostracodes and host waters. Living ostracodes show a positive offset from equilibrium values of up to 2 per mil for oxygen. Carbon-isotope values are up to 6 per mil more negative than equilibrium values in highly productive pools. The proxies indicate that most host waters are of temporary existence. The availability of meteoric water controls their life span and the composition of ostracode species assemblages. Our results underline the sensitivity of the area to changes in precipitation.