Evidence of salinity changes due to marine influence during the last 3,000 years in Laguna Matanzas (Central Chile) from sedimentological, geochemical and chironomid analyses.

JENNY, B., J. MASSAFERRO, B. VALERO-GARCÉS, M. GEYH & A. RIZZO

Kingston, Ontario. Canada

Simposio; 8th International Symposium on Paleolimnology; 2000

In this paper, we present the results of a paleolimnological study about Laguna Matanzas, Central Chile  (33° 45‘S, water depth: 3m, surface area: 1.5 km2). This small, shallow lake is one of a system of  lagoons located near the shore. It is separated from the ocean mainly by a dune belt. Data from sedimentology,  geochemistry and subfossil chironomid analyses on a 8 m long sediment core covering the last 3000 years were compared in order to provide information about  salinity changes during the late  Holocene. A major change was detected at about 3 m (around 1420-1650 yr. AD). The system most probably changed from a marine lagoon to a hypersaline lake. This interpretation is strongly supported by the change in sedimentology, the drop of the chloride content as well as the increase in magnetic susceptibility. The dramatic change observed on chironomids at about 3 m depth also indicates an abrupt change in the lake system. Chironomids, because of their narrow ecological tolerances and habitat preferences, are a valuable tool for reconstructing fluctuations in climate –sensitive environments like Laguna Matanzas. Chironomus spp., with a broad salinity range, is the only taxa represented in the lake until the environmental change took place. The appearance of new species (Dicrotendipes sp., Ablabesmya sp., Macropelopia sp., Pseudotanytarsus sp. and Tanytarsus spp.) in the system soon after 395 ± 120 14C yr. (1420-1650 AD) may indicate better conditions with lower salinity for the living midges. The reason for the closing of the lagoon is still unclear. Aeolian or tectonic activity may play a major role.