Holocene freshwater microbialites of Lake Sevan (Armenia)

Combined onshore and offshore investigations reveal microbialites in Lake Sevan ranging from 1915 m a.s.l. (i.e., 15 m above modern lake level, 1–30 cm thick) to submerged structures (2–5 m wide, 0.4–1 m high) at 45 m water depth. These carbonate deposits exhibit diverse macrostructures such as domical, spherical, tabular, elliptical, planar, and pendant/lobate crusts and oncolites. Thin section and SEM analyses reveal thrombolite microstructures with clots and shrubs, stromatolites (columnar and planar), leiolites, and striated mesostructures. Various types of spherulites, peloids, filamentous structures, microborings, and well-preserved remains of cyanobacteria provide evidence of biological involvement in precipitation. Radiocarbon ages of microbialites range from 9060 ⁺¹⁹⁵/_-70 to 190 ⁺¹²⁵/_-190 cal BP, based on 10 samples from different locations. Some of these samples indicate periods of natural lake level fluctuations during their formation. Radiated aggregates of fibrous crystals, formed exclusively in the lake’s deep, low-temperature environments, were dated to 1510 ⁺¹⁶⁰/_-115 cal BP. Geochemical and EDX analyses reveal low-magnesium calcite as the primary precipitate, with secondary rhodochrosite (MnCO₃) on the surface of submerged samples. Microalgae communities in sand, silt, and water are dominated by Cyanophyta, Bacillariophyta, and Chlorophyta. Cyanobacteria such as Oscillatoria sp. and Anabaena sp. were detected both within microbialites and in the surrounding environments, demonstrating their long-standing presence in the lake. At 4.3 m water depth in Qanagegh, reduced carbonate content in sands is linked to local gas seepage. The abundant presence of cyanobacteria, particularly Oscillatoria limnetica, suggests a relationship between microbial growth and gas-driven geochemical conditions.