Diplodon shells from Northwest Patagonia as continental proxy archives

SOLDATI, ANALIA L.; JACOB, DORRIT E.

Grindelwald

Workshop; 8th International NCCR Climate Summer School: "Climate variability, forcings, feedbacks and responses: the long-term perspective"; 2009

NCCR and IGBP-PAGES

Freshwater mussels of the genus Diplodon (Bivalvia, Hyriidae) are the most abundant bivalve (today and in the past) in freshwater bodies at both sides of the South-Andean Cordillera. However, despite their wide geographic distribution, Diplodon species have only rarely been used as climate archives in the southern hemisphere. Kaandorp et al. (2005) demonstrated for Diplodon longulus (Conrad 1874) from the Peruvian Amazonas that oxygen isotopic patterns in the shells could be used in order to reconstruct the precipitation regime and dry/wet seasonal of the monsoonal system in Amazonia (1). Although this study demonstrated the potential of Diplodon in climatological and ecological reconstructions in the southern hemisphere, as of yet, no systematic study of Diplodon as a multi-proxy archive has been undertaken for the Patagonian region. In this work we present d18Oshell analyses supported by sclerochronological methods in recent mussel of Diplodon chilensis patagonicus (D'Orbigny, 1835) collected at Laguna El Trébol (42ºS-71ºW, Patagonia Argentina), one of the best studied water bodies in the region for paleoclimate analysis. Additionally, water temperature was measured every six hours for one year using a thermometer (Starmon mini®) placed at 5m depth in the lake, close to a mussel bank and d18Owater was monitored monthly; air temperature and precipitation records were obtained from the National Meteorological Service (Figure 1). D. ch. patagonicus mussels exhibit annually very well developed growth lines, which allow calibrating a precise temporal scale with calendar years in the shell (2). Extremely wide increments, related to distinct periods (so-called benchmark increments) are present in all specimens and can be used to anchor the growth curves in order to create a master curve for the lake. d18Oshell in D. ch. patagonicus from Lake El Trébol (Figure 2) varies seasonally, presenting minima during the warm season (November/March) and maxima in the austral autumn/winter (May/October), according to the temperature behavior in the region (Figure 1). The resolution of the d18Oshell measurement varies for each year: samples obtained from larger annual increments (>1 mm, generally mussels younger than 10 years old) allow a resolution of ca. 2-3 months (4-6 samples per year), and sometimes even give a 4 weeks resolution, while thinner annual increments (between 1 and 0.1 mm, generally older than 10 years) allow only a 4-6 month resolution (2-3 points per sampled year). Three individual d18Oshell curves calibrated for each specimen and plotted in a common time axis (Figure 2) show the same low frequency behavior, with small differences in the high frequency signal due to the difference in milling resolution from shell to shell, possible due to different growth rates and/or to a time-averaging milling effect. Thus, these curves can be used to calculate a “master d18Oshell curve” extending the record of the individual samples.