Changes of organic matter sources in sediment cores from a high-altitude lake (Pumoyum Co, southeastern Tibetan plateau) over the last 19,000 years

Lake Pumoyum Co is on the southeastern Tibetan plateau (altitude, ∼5030 m asl; lake surface area, 281 km2; maximum water depth, 65 m). We obtained 14C dates on total organic carbon (TOC) for three sediment cores from Lake Pumoyum Co, covering the period from the last glacial to the Holocene, and compared them with 14C ages on plant residue concentrates (PRC fraction, mainly terrestrial plant residues) and aquatic plant fragments in the cores. The calibrated ages of TOC were older than those of the PRC fraction in the sediment cores by ca. 0.6-11.3 kyr. The calculated dead carbon fraction (fdeadcarbon) values for TOC increased during 9.5-5.5 cal ka BP, and fluctuations of fdeadcarbon paralleled variations in Asian monsoon intensity [Y. Wang et al., Nature 451 (2008) 1090]. One possible explanation for these results is that large amounts of terrestrial organic materials containing old carbon were supplied to the lake ('old carbon effects' from lake terrace, paleosol and/or stratum with dead 14C) when the lake level rose rapidly and its catchment area expanded during the Holocene climate optimum (9.5-5.5 cal ka BP).