Date of Award

5-17-2007

Document Type

Thesis

Abstract

Analysis of pollen, charcoal, and stable carbon and nitrogen isotopes ([delta]¹³C and [delta]¹⁵N) 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. The low C:N of the sediments indicates that organic matter is primarily composed of autochthonous material. 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 Artemisia + Chenopodiaceae by Poaceae pollen, is used to identify changes in moisture availability. High aridity indices and spikes of charcoal influx record relatively humid conditions at the site between 8.0 and 5.5 k years BP. Low charcoal influx rates and peak values of the aridity index between 4.5 and 4.0 k years BP 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 k years BP combined with progressive increase in [delta]¹⁵N indicates increasing aridification from the mid-Holocene to nearly the present. Intervals of humidification at Lake Terhiyn-Tsagaan are thus synchronous with the waxing and waning of the Asian monsoon and out of phase with humid intervals recorded at Lake Telmen, approximately 250 km to the northwest. It is 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.

Handle

http://hdl.handle.net/11122/5846

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