RECORDS OF HOLOCENE HUMIDITY FROM MONGOLIAN LAKE TERHIYN-TSAGAAN: IN SYNCH WITH THE WAXING AND WANING OF THE MONSOON

Analysis of pollen, charcoal, and stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) from a sediment core of Mongolian Lake Terhiyn-Tsagaan show increases in moisture availability coincident with mid- to late Holocene expansion of the Asian monsoon. The lake is freshwater and located in an intermontane depression on the flanks of the Hangai Range of north-central Mongolia. The site lies within the forest-steppe biome ~400 m below tree line.

Artemisia and Chenopodiaceae pollen are indicators of dry steppe and semi-desert vegetation, whereas Poaceae pollen is more abundant in humid meadow-steppe or forest-steppe assemblages. An aridity index calculated by dividing the percentage of Artemisia plus Chenopodiaceae by the percentage of Poaceae pollen is used to identify changes in moisture availability. Low aridity indices and spikes of charcoal influx record relatively humid conditions at the site between 8.2 ka and 5.5 ka. Low charcoal influx rates and peak values of the aridity index between 4.5 and 4.0 ka correspond to a documented interval of drought in southern Asia and northern Africa attributed to a weak Asian monsoon. A decrease in charcoal influx since 7.5 ka combined with progressive increase in δ¹⁵N indicates increasing aridification from the mid-Holocene to the present.

Intervals of humidification at Lake Terhiyn-Tsagaan are in synch with the waxing and waning of the Asian monsoon and out of phase with humid intervals recorded at lakes Telmen, approximately 250 km to the northwest, and Lake Dood, which lies within the Baikal watershed. It is thus possible that the Terhiyn-Tsagaan drainage lies at the northern and/or western edge of the region that received precipitation from an expanded Holocene summer monsoon.