Past environmental and climatic changes related to tree-line shifts inferred from fossil diatoms from a lake near the Lena River Delta, Siberia

Changes in diatom assemblages in a sediment core taken from a tundra lake n ear the Lena River Delta, northeastern Siberia, reflect climatic and enviro nmental changes throughout the Holocene. Significant limnological changes o ccurred 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 treeline shift, diatom assemblages w ere 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 productivity was probably low. Coincident wit h tree-line advance into the region c. 9100 cal. yr BP, inferred alkalinity decreased and remained low throughout the forested period, reflecting soil development and increased organic runoff associated with the shift in catc hment vegetation. The establishment of a larch forest in the catchment corr esponded with an initial period of limnological change between 9100 and 770 0 cal. yr BP, suggesting that soil and forest development rook place over a pproximately 1400 years. Between c. 7700 cal. yr BP (c. 7000 radiocarbon yr BP) and 3800 cal. yr BP, a stable diatom assemblage dominated by Achnanthe s species and low diatom-inferred alkalinity indicate the influence of orga nic runoff associated with a fully developed forest. A return to cooler con ditions and reversion to shrub tundra c. 3800 cal. yr BP coincided with a s hift back to a Fragilaria-dominated diatom assemblage and an increase in di atom-inferred alkalinity. Recent conditions have been marked by declining a lkalinity estimates and a minor change in diatom assemblages, most likely r eflecting an influx of humic substances from catchment pears. The close cor relation between trends in reconstructed alkalinity and catchment vegetatio n suggests that diatom-inferred alkalinity may provide a sensitive record o f soil and forest development, in unpolluted circumneutral or alkaline tree -line lakes.