Biogeochemical evidence for freshwater periods during the Last Glacial Maximum recorded in lake sediments from Nam Co, south-central Tibetan Plateau

Roman Witt,Tandong Yao,Franziska Günther,Gerd Gleixner,Roland Mäusbacher,Thomas Kasper,Stefan Lauterbach

doi:10.1007/s10933-015-9863-1

Abstract

Improved knowledge of deglaciation pro- cesses during the termination of the Last Glacial Maximum on the Tibetan Plateau can provide impor- tant information for understanding deglaciations in climate-sensitive high-altitude ecosystems. Little, however, is known about this time interval because most lacustrine sediment records from the Tibetan Plateau are younger than 19,000 years. This study focused on a lake sediment record from Nam Co, south-central Tibetan Plateau, covering the interval from *23.7 to 20.9 cal ka BP. We analysed the distribution and compound-specific hydrogen isotope composition (dD) of sedimentary n-alkanes, as well as the bulk sediment TOC, TN, d 13 Corg and d 15 N composition, to infer lake system development. Pro- nounced changes in environmental conditions between *21.6 and 21.1 cal ka BP, as well as between 23.1 and 22.5 cal ka BP (Greenland Inter- stadial 2), were inferred from increased aquatic n- alkane amounts and decreased dDn-C23 values within these time intervals, respectively. Freshwater inputs, which most likely resulted from enhanced glacier melting, causedthese changes. Our results suggestthat mountain glacier retreat on the Tibetan Plateau started earlier than previously assumed. The required energy for thawing was probably provided by temperature changes caused by reorganization of atmospheric circulation,whichhasalsobeenrecorded inGreenland ice records.

Highlights

Global climate change and associated temperature increase have strong effects on highaltitude freshwater resources, through glacier melting, increased evaporation and loss of water-storage capacity
The resulting dominance of colder and dryer air masses may have caused a decrease in annual precipitation, from 1000 to 200 mm, which forced the expansion of continental glaciers under extreme cold during the Last Glacial Maximum (LGM), resulting in glacier extent on the Tibetan Plateau 15 times larger than today (Shi 2002)
The onset of mountain glacier retreat on the Tibetan Plateau is generally believed to have occurred only between 18 and 16 cal ka BP, our results indicate that first retreat events likely occurred by 23 cal ka BP in the Nam Co region

Summary

Introduction

Global climate change and associated temperature increase have strong effects on highaltitude freshwater resources, through glacier melting, increased evaporation and loss of water-storage capacity. In this regard, the Tibetan Plateau, with the world’s third largest amount of glaciers and numerous lakes, represents a key region of interest, especially because it is the main freshwater source area for millions of people in Asia. Investigating past, large-scale climate changes in the Nam Co region, e.g. the Last Glacial Maximum (LGM), can contribute to the understanding of developmental stages during deglaciation, and shed light on environmental changes that the region is experiencing today and under future warming. The precise timing of deglaciation in this area, as well as local environmental changes during this period, are still a matter of debate (Clark et al 2009; Hou et al 2012)

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