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Changes in diatom assemblages in a sediment core taken from a tundra lake near the Lena River Delta, northeastern Siberia, reflect climatic and environmental changes throughout the Holocene. Significant limnological changes occurred at the same time as trees advanced into the region c. 9100 cal. yr BP (8500 radiocarbon yr BP) and subsequently retreated c. 3800 cal. yr BP (3500 radiocarbon yr BP). Prior to this tree-line shift, diatom assemblages were dominated by small benthic Fragilaria species, and diatom-inferred alka linity values were high, suggesting that climatic conditions were cool and relatively dry, and that lake pro ductivity was probably low. Coincident with tree-line advance into the region c. 9100 cal. yr BP, inferred alkalinity decreased and remained low throughout the forested period, reflecting soil …