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

摘要

Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with δ~(18)O and δ~(13)C_(DIC) (DIC: dissolved inorganic carbon) values of host water samples, provide a first data set that characterizes a wide range of modern aquatic environments in the Laguna Cari-Laufquen (41°S, 68-69°W) and the Lago Cardiel area (48-49°S, 70-71°W) 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) speings, 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 range between - 13 and -5‰ for oxygen, and between -15 and -3‰ for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichement. Limnocythere patagonica, Eucypris labyrinthica, Limnocythere sp. and Eucypris aff. 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 and range between -7 and +5‰ for oxygen, and -5 and +4‰ for carbon. 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, extending from -5 to +7‰ for oxygen, and from -5 to +6‰ for carbon. Living ostracodes show a positive offset from equilibrium values of up to 2‰ for oxygen. Carbon isotope values are up to 6‰ more negative than equilibrium values in highly productive pools. Comparison of ostracode and host water isotope signals permits assessment of the life span of the aquatic environments. Valves from dead ostracodes collected from ephemeral ponds and lakes show a wide scatter with each sample providing a snapshot of the seasonal history of the host water. The presence of the stream species I. ramirezi and a wide range of ostracode isotope values suggest that ephemeral ponds and lakes are fed by streams during spring run-off and seasonally dry. A temporary character of waters is also indicated by H. incongruens, a drought-resistant species that occupis most springs and seeps. In addition, L. rionegroensis has adjusted its reproduction strategies to its environment. Whereas only females were collected in fresh host waters, males were found in ephemeral ponds and lakes with higher solute content. Sexual reproduction seems to be the more successful reproduction strategy in waters with high and variable salinities and affected by seasonal droughts. The temporary character of the aquatic environments shows that the availability of meteoric water controls the life span of host waters and underlines the sensitivity of the area to changes in precipitation.