ALBERT

Beschreibung: Ostracode species assemblages and stable oxygen and carbon isotope ratios of living and recent ostracodes, together with delta18O and delta13C_DIC 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) 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 range between –13 and –5 per mil for oxygen, and between –15 and –3 per mil for carbon. They are the most negative of the entire sample set, reflecting ground water input with little or no evaporative enrichment. 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 per mil for oxygen, and –5 and +4 per mil 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 per mil for oxygen, and from –5 to +6 per mil for carbon. Living ostracodes show a positive offset from equilibrium values of up to 2 per mil 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 Ilyocypris 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 is also indicated by Heterocypris incongruens, a drought-resistant species that occupies most springs and seeps. In addition, Limnocythere 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 high and variable salinities and 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.

Beschreibung: Site 716 is a continuous sequence (upper Miocene to Holocene) of periplatform oozes and chalks from the Maldives Ridge, Indian Ocean. Mineralogical and geochemical studies of these carbonate sediments indicate that submarine burial diagenesis has played an important role in the induration of sediments at this site. Metastable carbonates, high-magnesium calcite (HMC) and aragonite, convert to low-magnesium calcite (LMC) rapidly, within 1.1 and 6.0 Ma, respectively. Strontium concentrations in carbonate decrease with depth as the result of the burial diagenesis of calcium car- bonate, primarily aragonite, with excess strontium being expelled into pore waters. The formation of celestite at depth indicates that sufficient diagenesis of carbonate sediments has occurred to saturate or supersaturate pore waters with re- spect to this authigenic mineral. Sodium also decreases monotonically with depth as a result of the burial diagenesis of calcium carbonate. Magnesium and carbon and oxygen isotopic curves are remarkably similar. Carbon isotopic compositions record inputs of 13C-enriched components from shallow carbonate banks. Magnesium concentrations vary widely, recording enhanced episodes of cementation by LMC with slightly elevated magnesium contents. Positive shifts in oxygen isotopic composition also record episodes of cementation during burial diagenesis. Intervals with increased accumulation rates of metastable components have undergone more rapid diagenesis than intervals with predominately pelagic deposition.