HOLOCENE PALEOENVIRONMENTAL CHANGES IN THE FOREST-STEPPE ECOTONE OF NORTHERN MONGOLIA BASED ON PALYNOLOGICAL AND SEDIMENTOLOGICAL ANALYSES OF CORES FROM LAKE KHARGAL

Palynological, macrofossil, and geochemical analyses of cores from Lake Khargal (49.9°N, 102.77°E) allow reconstruction of vegetation and climate changes in northern Mongolia throughout the Holocene. Khargal is a closed-basin lake located near the northern border of the forest-steppe ecotone. Dated archeological sites provide evidence of human habitation in the local area beginning ~3450 cal yr BP. Livingstone piston cores up to 4.4 m long were collected from a fluvial delta submerged beneath ~9 m of water. AMS radiocarbon dates of plant macrofossils indicate a basal age of 10300 cal yrs BP at 430 cm. Eight additional dates from wood and charcoal were used to construct a chronology in Bacon. A binary geochemical plot of Al₂O₃ vs. TiO₂ indicates basaltic to granitic source rocks, and an A-CN-K ternary plot shows that the sediment is derived from source rocks enriched in plagioclase, gabbro, tonalite and granodiorite throughout the record. The chemical index of alteration (CIA) and the chemical index of weathering (CIW) indicate a low degree of weathering in the source area, consistent with a generally arid climate. Mean grain size fluctuates between fine silt and clay to fine sand, with sandy horizons indicating increased runoff at ~4000 and ~4400 cal yr BP. Total organic carbon (TOC) varies between 2.6% and 32.5%; relatively high values between ~7800-7600, 7430-7300, 6170-6070, and 4200-4100 cal yr BP are interpreted as intervals of increased temperature or precipitation. Plant macrofossil analysis indicates birch woodland with herbaceous taxa during the mid-Holocene (~7000 cal yr BP). The presence of weedy taxa suggests disturbed ground. Subsequently, birch became less common and is virtually absent from the macrofossil record by ~4000 cal yr BP, after which rare conifer needles indicate the presence of larch. Continued presence of herbaceous macrofossils and an abundance of mycorrhizal fungal spores (Glomus) in coeval palynological assemblages indicate disturbed ground and active soil erosion at this time. Whereas pollen of Artemisia and Poaceae dominate assemblages at ~4000 cal yr BP, birch is the most abundant woody taxon, indicating persistence in the watershed. By ~1500 cal yr BP, abundant conifer needles record growth of larch near the lake, possibly due to increased moisture availability and southward expansion of the taiga during the late Holocene.