Lake Chungará (18º15' S, 69º09’ W, 4520 m a.s.l. 22.5 Km2 and 40 m of water depth) is a hydrologically closed lake located on the Andean Altiplano. The lake is polymictic, meso to eutrophic and currently primary productivity is mainly governed by diatoms and chlorophyceans. Three laminated intervals were selected for detailed petrographical studies as well as a high resolution diatom oxygen isotope analysis. These laminated sediments are made up of bands of white and dark mm-thick laminae and thin layers of diatomaceous ooze with variable carbonates and amorphous organic matter. Assuming no significant changes in the isotopic compositions of the water sources and according to the diatom microstratigraphy of the laminae, two alternating environmental scenarios can be described. Lower-level water conditions are interpreted during the intervals of white laminae deposition (high values of delta18O), since those conditions are more favourable for the massive short-term deposition of monospecific large centric diatom blooms and delta18O enrichment. These white laminae are probably the result of exceptional periods of mixing of the shallow water column during lowstands, which recycle nutrients from the hypolimnion. Lower delta18O values, and therefore deeper water conditions, are more favourable for the development of dark laminae (normal annual cycle of the lake with alternating phases of stratification and mixing). These conditions would lead to the development of a complex diatom community, among other algal groups.

1 página.-- Awards Ceremony Speeches and Abstract of the 17th Annual V. M. Goldschmidt Conference Cologne, Germany August 2007, 17th Annual V.M. Goldschmidt Conference Cologne, Germany August 2007.

Peer reviewed