Assemblage diversity and isotopic signals of modern ostracodes and host waters from Patagonia, Argentina

SCHWALB, A,; S. BURNS; G. CUSMINSKY; K. KELTS; V. MARGRAF

PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY

Elsevier

Lugar: Amsterdam; Año: 2002 p. 323 - 339

0031-0182

Ostracodes species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracods together with delta 18 O and delta 13 C dic (DIC: dissolivel inorganic carbon) values of hots water samples, provide a first daa set that characterizes a wide range  of modern aquatic environments in the Laguna Cari-Laufquen (41º, 68-69ºW) and the Lago Cardiel area (48-49ºS, 70-71ºW) in Patagonia, Argentina. This data set will ultimately be used to interpret and calibrate data acquired from lake sediment cores with the goal of reconstructing past climate. Species assemblages and isotope values can be assigned to three groups : (1)  springs, seeps ans streams, (2) permantne ponds and lakes and (3 ephemeral ponds and lakes. Springs, seeps and streams are characterized by Darwinula sp. Heterocypris incongruens, Eucypris fontana, Amphycypris nobilis and Ilyocypris ramirezi. Ostracode and water isotope values range between  -13 and -5 0/00 for oxygen, and between  -15 and -3 0/00 for carbon. They are the most negative of the entirse samples set, reflecting groung water input with little or no evaporative enrichment. Limnocythere patagonica, Eucypris labyrinthica, Limnocythere sp and Eucypris aff E. fontana are typical species of permantne ponds and lakes. Isotope values indicate high degree of evaporation of lake waters relative to feeder springs and streams and ranges between -7 and + 5 0/00 for oxygen, and -5 and +4 0/00 for carbon. Limnocythere rionegroensis is the dominant species in ephemeral  ponds and lakes. These systems display the most enriched isotope values in both ostracods and host waters, extending from -5 to +7 0/00 for oxygen , and from -5 to +6 0/00 for carbon. Living ostracods shows a positive offset from equilibrium values of up to 2 0/0 for oxygen. Carbon isotpoe values are up to 6 0/00 more negative than equilibrium values in highly productive pools. Comparision of ostracods and host water isotope signals permits assessmente of the life span of the aquatic environments. Valves from dead ostracods collected from ephemeral ponds and lakes shows a wide scatter with each samples providing a snapshot of the seasonal history of the host water. The presence of the stream species I. ramirezi and wide range of ostracode isotope values suggeste theat ephemeral ponds and lakes are fed by streams during spring run-off and seasonlally dry. A. temporary character of waters is also indicated by H. incongruens, a drough-resistants species that occupies monst springs and seeps. In addition, L. rionegroensis has adjusted its reproduction strategies ot its environment. Whereas only female were colleted in fresh host waters, male were found in ephemeral ponds and lakes with higher solute content. Sexual reproduction seems to be the more succesful reproduction strategy in waters with hight and variable salinities and affected by seasonal droughts. The temporary character of the aquatic environments shows that the availabilith of meteoric water controls the like span tha host waters and underlines the sensitivity of the area to changes in precipitations.