The maar lake Laguna Cháltel (southern Patagonia, Argentina) - first results of a multi-proxy sediment study .

FEY, M.; CORBELLA, H.; HABERZETTL, T.; JANSSEN, S.; KÜCK, B.; LÜCKE, A.; MAIDANA, N. I.; MAYR, C.; OHLENDORF, C.; SCHÄBITZ, F.; SCHLESER, G.; WILLE, M.; ZOLITSCHKA, B.

POSTDAM

Otro; 19th Colloquium on Latin American Geosciences; 2005

The maar lake Laguna Cháltel is located in a semiarid steppe environment in southern Patagonia, Argentina (49°57.640’S; 71°06.500’W). Situated at about 800 m a.s.l. on the Cenozoic Pampa Alta volcanic plateau in the province of Santa Cruz, Laguna Cháltel is a typical maar lake with steep ca. 40 m high subaerial crater walls, a mean lake diameter of 2.6 km and a maximum water depth of 41 m. There are several apparently permanent inflows fed by springs on the crater walls entering the lake via three deeply incised canyons. During a field campaign in 2004 eight short sediment cores were taken from different coring positions. Here we present first results of a multi-proxy study of a 58 cm long sediment core recovered from the lake center. With the light coring equipment used for this pilot study a deeper penetration was impeded by a conspicuous horizon of finely cristallized calcite at about 52 cm depth. We hypothesize that this layer was precipitated autochthonously from the water column during a lake level low stand. Values of total organic carbon, total nitrogen and d13C of bulk organic matter point to a higher primary productivity below that calcite horizon as well as above ca. 15 cm sediment depth. In the uppermost part higher productivity presumably can be related to a marked higher lake level giving rise to the dominance of the planktonic diatom species Cyclostephanos sp. while diatom assemblages in the rest of the core are dominated by epiphytic or benthic species indicating larger littoral habitats of shallower water and partly pointing to higher salinity. In the lowermost part of the core diatom preservation is very poor, probably due to early diagenetic dissolution of the diatom valves. Besides being associated with an increased productivity, the higher lake level inferred from the diatom record in the upper part of the core is also consistent with higher total pollen concentrations and an enhanced clastic input, both indicating more precipitation which results in better growing conditions for plants and more fluvial transport to the lake. The latter is reflected e.g. by higher values of dry density and  magnetic susceptibility. The preliminary age model is based on three AMS 14C dates showing ages between 4030 and 2830 shcal. BP. This age model suggests that there either is a reservoir effect of about 2000 years, for which there is, however, no evidence detectable by AMS 14C dating of living aquatic mosses (113.2 ±  0.3 pMC), or there are one or more macroscopically invisible hiatuses above 19 cm sediment depth. Additional 14C dates are in progress to resolve that question.