Sediment structure, rock-magnetic and bio-geochemical records obtained from sapropel core samples in Lake Beloye, southern Western Siberia, indicate an abrupt ecosystem change at 3.4 calibrated ka BP. The lake changed from eutrophic to oligotrophic, its vegetation - from carbon-fixing macrophytic angiosperms to calcium-fixing macrophytic algae and angiosperms, and, as a result, the type of sedimentation shifted from peaty sapropel to carbonate-rich sapropel. Those changes could be induced by climate, which was warmer and wetter before 3.4 ka BP and cooler and drier after this. Nevertheless, our data show that those ecosystem changes occurred when the lake became shallower to get about 1 m depth and started to freeze to the bottom in winter time. The full freezing of the lake led to the deposition of carbonates, which resulted in: i) death of carbon-utilizing macrophytic angiosperms; and ii) winter precipitation of calcium carbonate. In summer, additional portions of Ca2+ and HCO3? ions come into the lake from the surrounding CaCO3 rich loess sediments with ground water.
