Climate Conditions on the Tibetan Plateau During the Last Glacial Maximum and Implications for the Survival of Paleolithic Foragers

Abstract

Environmental conditions on the Tibetan Plateau (TP) during the last glacial maximum (LGM) are poorly known. Existing studies of environmental proxies and climate model simulations are contradictory, with interpretations varying between cold-dry and cold-wet environmental conditions which differentially influenced lake volumes, loess deposition and vegetation communities across the TP. Genetic and archaeological studies suggest anatomically modern paleolithic foragers initially occupied the TP between 60 and 30 ka, and may have seasonally occupied the TP during the LGM. Hence, a better understanding for LGM environmental conditions is needed in order to estimate whether paleolithic foragers could have survived on the TP during the extreme LGM cold stage. Here we report the investigation of lacustrine sediments and beach deposits within two paleoshorelines around Dagze Co on the southern TP, ∼22 and ∼42 m higher than the present lake level. Optical age estimates suggest the sediments were deposited during the LGM and mid-Holocene, respectively. TraCE-21 climate model simulation results suggest that net annual LGM precipitation in the Dagze Co basin was lower than the mid-Holocene, but about the same as that of the past 1,000 years. Combining the optical age estimates with TraCE-21 and CAM4 climate model simulation results, we deduce that increased summer precipitation and glacier meltwater supply, combined with decreased lake surface evaporation, produced LGM lake levels ∼22 m higher than present. We also synthesized paleoenvironmental records reported across the TP spanning the LGM. This synthesis suggests that the LGM climate in the northern TP was cold and dry, but that some of the southern TP was cold and wet. These relatively wetter LGM conditions in the southern TP may have favored the growth of cold-resistant plants which, in turn, may have supported larger herbivore populations, and provided food for paleolithic foragers. We conclude that seasonal or short-term human occupation of the TP during the LGM was thus more likely in the southern TP than in the north.

The Driving Forces Underlying Spatiotemporal Lake Extent Changes in the Inner Tibetan Plateau During the Holocene

“…We previously reported OSL ages of ∼22.7 and 6.1-5.5 ka for the late glacial and Holocene highstands of Dagze Co, respectively (Liu et al, 2020). Here, we add to these other lake records by presenting a newly reconstructed Holocene lake level fluctuation curve for Dagze Co, based on OSL dating of paleoshorelines (Supplementary Figure S2).…”

“…Dagze Co, with a modern lake surface elevation of 4,480 m a.s.l., is located in the southern ITP (31.82°-31.98°N, 87.36°-87.65°E) (see Liu et al (2020) for setting and modern climate descriptions). We identified and mapped 15 paleoshorelines surrounding the now brackish lake.…”

“…The highest paleoshoreline we identified, S15, is ∼41.6 m above the modern lake level, and was formed during the mid-Holocene (Figure 4). We obtained OSL age estimates for eight other paleoshorlines which appear to have formed during brief stillstands as the lake regressed from this Holocene high (see Liu et al (2020) for field and laboratory methods). Dagze Co would have overflowed to the west into lower basins when it reached the highest level at S15 as evidenced by a remnant channel connecting Dagze Co with the western basins (Liu et al, 2020).…”

“…We obtained OSL age estimates for eight other paleoshorlines which appear to have formed during brief stillstands as the lake regressed from this Holocene high (see Liu et al (2020) for field and laboratory methods). Dagze Co would have overflowed to the west into lower basins when it reached the highest level at S15 as evidenced by a remnant channel connecting Dagze Co with the western basins (Liu et al, 2020). The dividing crest between Dagze Co and the western basins is approximately the same elevation as paleoshoreline S15.…”

“…The dividing crest between Dagze Co and the western basins is approximately the same elevation as paleoshoreline S15. Hence, the lake level would maintain approximately the same height as paleoshoreline S15 or slightly higher during the early Holocene ISM intensification stage when several other lakes in ITP experienced their highest Holocene lake levels (Liu et al, 2016b;Liu et al, 2020). The Holocene lake level variation curve for Dagze Co was then reconstructed based on the OSL ages of these nine paleoshorelines (Figure 4).…”