Diplodon shells from Northwest Patagonia as continental proxy archives: Oxygen isotopic results and sclerochronological analyses

SOLDATI, ANALIA L.; BEIERLEIN, LARS; JACOB, DORRIT E.

Vienna

Conferencia; European Geological Union General Assembly (EGU2009); 2009

European Geological Union

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. There are about 25 different Diplodon genera in Argentina and Chile that could be assigned almost completely to the species Diplodon chilensis (Gray, 1828) and two subspecies: D. ch. chilensis and D. ch. patagonicus; this latter species is found in Argentina between Mendoza (32° 52’ S; 68° 51’ W) and Chubut (45° 51’ S; 67° 28’ W), including the lakes and rivers of the target area, the Nahuel Huapi National Park (Castellanos, 1960). 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) collected 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. 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 sclerochronological analyses supported by _18Oshell 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. Water temperature was measured every six hours for one year using a temperature sensor (Starmon mini®) placed at 5m depth in the lake, close to a mussel bank. Additionally, _18Owater was measured monthly for the same time range.g_18Oshell values obtained by micromilling at high spatial resolution in the growth increments of three Diplodon shells were compared to these records,  and to air temperature and precipitation records in the region (from the NOAA database, station: airport of S.C. de Bariloche: 41ºS-71ºW). D. ch. patagonicus exhibit very well developed annual growth lines, which allow calibrating a precise temporal scale with calendar years in the shell (Soldati et al., 2008). Extremely wide increments (e.g. years 2000-2001 and 1978-1979), 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. _18Oshell varies seasonally, presenting minima during the warm season (November/March) and maxima in the austral autumn/winter (April/October), reproducing the temperature fluctuations in the region. The resolution of the _18Oshell 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 months (5-7 samples per year), and sometimes even give a 5 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). Because of their long live span of ca. 100 years, Diplodon shells are useful to construct an accurate climatological archive for Patagonia with time windows of around a century, resolving the environmental signal annually and even seasonally. References: CASTELLANOS Z.A. (1960). Almejas nacaríferas de la República Argentina. Género Diplodon. Secretaría de Agricultura, Publicación Miscelánea, 421: 1–40. KAANDORP R.J.G., VONHOF H.B., WESSELINGH F.P., PITTMAN L.R., KROON D. & HINTE J.E.V. (2005). Seasonal Amazonian rainfall variation in the Miocene Climate Optimum. Palaeogeography, Palaeoclimatology, Palaeoecology, 221: 1-6. SOLDATI A.L., JACOB D.E., SCHÖNE B.R., BIANCHI M.M., HAJDUK A. (2008). Seasonal periodicity of growth and composition in valves of Diplodon chilensis patagonicus (D’Orbigny, 1835). Journal of Molluscan Studies, doi:10.1093/mollus/eyn044.