Unusual biogenic calcite structures in two shallow lakes, James Ross Island, Antarctica
- 1Centre for Polar Ecology, Faculty of Science, University of South Bohemia, Na Zlaté Stoce 3, 37005 České Budějovice, Czech Republic
- 2Institute of Botany, Academy of Sciences of the Czech Republic, Dukelská 135, 37982 Třeboň, Czech Republic
- 3Department of Ecology, Faculty of Science, Charles University in Prague, Albertov 6, 12843 Prague, Czech Republic
- 4Czech Geological Survey, Klárov 3, 11821 Prague, Czech Republic
- 5Department of Geography, Faculty of Science, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic
Abstract. The floors of two shallow endorheic lakes, located on volcanic surfaces on James Ross Island, are covered with calcareous organosedimentary structures. Their biological and chemical composition, lake water characteristics, and seasonal variability of the thermal regime are introduced. The lakes are frozen down to the bottom for 8–9 months a year and their water chemistry is characterised by low conductivity and neutral to slightly alkaline pH. The photosynthetic microbial mat is composed of filamentous cyanobacteria and microalgae that are considered to be Antarctic endemic species. The mucilaginous black biofilm is covered by green spots formed by a green microalga and the macroscopic structures are packed together with fine material. Thin sections consist of rock substrate, soft biofilm, calcite spicules and mineral grains originating from different sources. The morphology of the spicules is typical of calcium carbonate monocrystals having a layered structure and specific surface texture, which reflect growth and degradation processes. The spicules' chemical composition and structure correspond to pure calcite. The lakes' age, altitude, morphometry, geomorphological and hydrological stability, including low sedimentation rates, together with thermal regime predispose the existence of this community. We hypothesise that the precipitation of calcite is connected with the photosynthetic activity of the green microalgae that were not recorded in any other lake in the region. This study has shown that the unique community producing biogenic calcite spicules is quite different to any yet described.