Unusual biogenic calcite structures in two shallow lakes, James Ross Island,Antarctica

摘要

The floors of two shallow endorheic lakes, located on volcanic surfaces onJames Ross Island, are covered with calcareous organosedimentary structures.Their biological and chemical composition, lake water characteristics, andseasonal variability of the thermal regime are introduced. The lakes arefrozen down to the bottom for 8–9 months a year and their waterchemistry is characterised by low conductivity and neutral to slightlyalkaline pH. The photosynthetic microbial mat is composed of filamentouscyanobacteria and microalgae that are considered to be Antarctic endemicspecies. The mucilaginous black biofilm is covered by green spots formed bya green microalga and the macroscopic structures are packed together withfine material. Thin sections consist of rock substrate, soft biofilm,calcite spicules and mineral grains originating from different sources. Themorphology of the spicules is typical of calcium carbonate monocrystalshaving a layered structure and specific surface texture, which reflectgrowth and degradation processes. The spicules' chemical composition andstructure correspond to pure calcite. The lakes' age, altitude, morphometry,geomorphological and hydrological stability, including low sedimentationrates, together with thermal regime predispose the existence of thiscommunity. We hypothesise that the precipitation of calcite is connectedwith the photosynthetic activity of the green microalgae that were notrecorded in any other lake in the region. This study has shown that theunique community producing biogenic calcite spicules is quite different toany yet described.