Hostile climate during the Last Glacial Maximum caused sparse vegetation on the north-eastern Tibetan Plateau

Abstract

Due to insufficient palaeo-environmental records, the vegetation history and hydrology of the Tibetan Plateau during the Last Glacial Maximum (LGM) are uncertain. In this study, a 316-cm-long sediment core covering the last 33.8 thousand years recovered from Koucha Lake on the north-eastern Tibetan Plateau was analysed for pollen, along with total organic carbon (TOC) and total nitrogen (TN), to explore past vegetation and hydrology changes. Alpine desert was widely distributed around the lake with sparse vegetation cover between 33.8 and 16.8 cal ka BP, particularly in the LGM, as inferred from the extremely low pollen concentrations together with high proportions of desert pollen taxa and exogenous arboreal taxa. Alpine steppe developed around the lake thereafter and was replaced by alpine meadow at ca. 7 cal. ka BP, which reflects the increasing effective moisture during the mid and late Holocene. The ancient Koucha Lake existed from 33 to 29 cal ka BP as a shallow lake and probably dried out in the LGM with a sediment hiatus (presence of coarse gravel layer), as suggested by the high and low content of Pediastrum. The current Koucha Lake formed at ca. 20 cal. ka BP with an increase in water level at ca. 16.8 cal ka BP as revealed by faint stratification of the sediments along with higher pollen concentration and presence of Pediastrum and Botryococcus. Our research suggests that pollen concentration should be paid more attention when investigating past vegetation, and that pollen percentages are not comparable among samples with large differences in pollen concentrations.