Multiproxy reconstruction of hydrological changes during the late Holocene in Chaltel Lake (Southern Patagonia, Argentina).

MAIDANA, N., ; LAPRIDA, C.; RAMÓN MERCAU, J.; FEY, M.; MASSAFERRO, J.; SALSA SCIENCE TEAM

CABA

Workshop; 1er Workshop Argentino de PASADO y Primer reunión internodos Proyecto Interdisciplinario Patagonia Austral; 2010

Lakes are excellent sponsors of environmental change, including climatic change. Diatoms and ostracods are among the most useful biological indicators for paleoenvironmental reconstruction found in lake sediments, due to their sensitiveness to abiotic variables (Lotter et al., 1999; De Deckker & Forester, 1988).However, the use of these microfosils in quaternary studies for Southern Patagonia is still limited and poorly understood because of the paucity of regional taxonomic and paleoenvironmental knowledge. The multiproxy study of lacustrine sediments from Chaltel presented here aims to partially fill this gap.Chaltel (49° 57? S- 71° 06? W) is a crater lake located in the semiarid steppe of Patagonia (Fig. 1). Several cores obtained in the framework of the South Argentinean Lake Sediment Archives and Modelling (SALSA) international research project were analyzed using a multi-proxy approach to reconstruct the paleoenvironmental history of the lake. Changes in diatom, ostracod, and chironomid assemblages and detailed geochemical analysis provided information about past lake water level fluctuations and hydrological changes in the watershed. For diatom analysis, a subsample of each sample was processed following estándar procedures (Battarbee, 1986) and permanent slides were mounted with Naphrax®. Identifications were based on Rumrich et al. (2000) and others. For ostracods studies, samples were washes and dried. Specimens were picked onto special slides. To explore the relations between species and environmental variables, a Canonic Correspondence Analysis (CCA) was performed considering the most abundant species frequencies (>3%) (CANOCO program; ter Braak, 1988) The significance ofthe canonical axes was tested using Monte Carlo?s permutation of samples. Ostracods were present in both studied cores CHA04/4 (60 cm, center of the lake) and CHA04/5 (100 cm, more litoral) while diatoms were present only in the uppermost 30 cm of CHA04/4 but they were found all along CHA04/5. The results of the combination of the information coming from both cores show several interesting changes. Coniss diagram (Tilia program, Grimm 1999) allowed us to recognize four zones in the sedimentary record of core CHA04/4 (Fig. 2).Zone A (4500 - 3150 cal BP). Relatively moist conditions can be inferred fromostracods. Notably, diatoms were not present at this zone, probably due to diageneticdissolution. The ostracod Limnocythere rionegroensis was by far the dominant form.Other species were present but in lower numbers, and only in the subzone A1-older than 4000 cal BP. In subzone A2, geochemical data point to allochthonous input of 40 organic matter to the lake while ostracods assemblages composition suggest a shift to saline conditions probably higher than 2-3 mg/l and Na-dominated waters being L. rionegroensis the only species recovered. Both biotic and abiotic proxies indicate low water level and increased littoral erosion by that time. Zone B (3150 - 2400 cal. BP). Conditions became moister with a concurrent lakelevel rise. Moderate salinities and the presence of abundant marginal vegetation are inferred from ostracods (mainly the contribution of L. patagonica) and the increased abundance of planktonic and epiphytic diatoms (Thalassiosira patagonica and Cocconeis placentula, respectively). Zone C (2400 - 1300 cal. BP). Changes in diatom and ostracod assemblages suggests lower salinities, and probably higher productivity. The progressive decrease in the abundance of L. rionegroensis agrees with that of the heavily silicified ?Hyalodiscus? sp A noticeable increase in diatom abundances may have been enhanced by expansion of littoral habitats. These less saline conditions could be related to an increase in the lake level. Zone D (younger than 1300 cal. BP). A new shift to moister conditions took place at around 1400 cal. BP, when modern lake conditions were established. The cooccurrence of L. patagonica and Kapcypridopsis megapodus allow us to infer salinities lower than ca. 2 ? and Ca-dominated waters. The absolute dominance of a true planktonic diatom species support the idea of an increased water level. The present work, which was partially financed by the Southern Patagonia Interdisciplinary Project (PIPA, PICT/R 2006 ? 2338) and includes neolimnological information obtained by its members, enhances the understanding of spatial patterns of past hydrological changes in Southern Patagonia steppe, indicating further hydrological variations during the late Holocene.