Changes of the Deserts in Northern Central China during the last Glacial Cycle: Implications for Causes of Recent Desert-Forming Process Z.L. Ding, J.M. Sun, S.L. Yang and T.S. Liu (Institute of Geology and Geophysics, CAS, Beijing)

Desrtified Land : 3436 km 2 /year Causing tremendous environmental and social problems

Heavy Agriculture Overgrazing Causes for the desert-forming are not fully understood: Natural processes or human activities? Or both?

What measures should be taken to stop the desert-forming process? If geological records can provide any implication for it?

Western deserts: <150mm Eastern deserts: mm Helan Mts. Annual precipitation

Temporal and spatial changes of the deserts in the last glacial-interglacial period Paleo-records: Wind-blown loess deposits in the Loess Plateau Loess-sand deposits in the loess-deserts border Sand-soil deposits from within the deserts

Holocene Soil (11ka) LGM Loess (21-11ka) Loess Section

Relationship between Loess grain-size and the distance of the Loess Plateau to the deserts Desert expansion Desert retreat Coarse loess grain-size Fine loess grain-size Shorter distance Longer distance

Loess grain-size links to monsoon Northward move of monsoon rainfall belt Northward retreat of desert margin Increase in the distance of deserts to the LP Fine-sized eolian loess deposited in the Plateau

Sampling sites in the Loess Plateau

Map of the LP and deserts in northern China

10  1 27  2 48  4 55     7 75  9 Age(ka) Eolian sandLoessSoil Holocene Soil LGM Eolian Sand Loess Horizon Last Interglacial Soil Loess-Sand-Soil Section Along the Border Direct evidence for desert advance during the LGM ( S0) (L1-1,) (L1-2) (L1-3) (L1-4) (L-5) (S1)

Grain-size records of the loess sections along the transitional area of the Loess Plateau to the deserts LGM

Eolian section within deserts LGM Sand Holocene Soil

Rocky desert in the northern part

Dry grasslands in the northern part

sand-dunes in the middle part

Loess deposits in the southern part

Rocky desert Grassland Sand dunes Loess deposits Wind erosion areas Sand deposition Loess deposition North South Spatial differentiation of landscapes along a north-south transect in the last glacial

40 30 Present mountains desert Gobi desert Study sites Holocene Optimum(8-4ka) Sun, Ding, Liu (1998) Episodes

Holocene Soil LGM Sand Deposits Holocene soil eroded and LGM sand exposed

Influence of climate deterioration after the Holocene Optimum on the desert expansion?

温度 / ℃ 降水量 / % 时间 /KaBP Precipitation(%) Temperature Age (ka) Holocene climate change estimated from the pollen record of Qinghai lake (Shi, 1992)

A 14-ka pollen diagram at Daihai near desert ka, tree pollen over 20%, almost absent aftermath

Ancient lake level

Holocene LGM Loess Coarse-grained upper Holocene loess

WG HLD SGDL SY Desert Loess 50 Hunlun Buir Desert Hunshandake Desert (m) WG HLHD SGDL SY Aeolian sand Paleosol 0.04± ± ± ± ± ± ± ± ± ± ± ±1.3 (m) (kyr) OSL Dating Results

We do have geological evidence for late Holocene drying up of the land, which may have important impacts on the recent desert-forming processes.

Is the climate deterioration the most important cause for the current desert-forming?

Sand-dunes stabilized by planting trees for over a decade

Vegetation recovered by natural processes after the closure of the area for four years Closed Agriculturally-used

Major Conclusions 1. The recent desert-forming in the eastern deserts of northern China occurs mainly as the re-activation of the sand dunes formed in the LGM because of the erosion of the thin Holocene soil. 2. Although the post-Holocene Optimum climate deterioration may play a part in recent desert-forming, unreasonable human activities appear to be the most important cause for it. 3. The sand-dunes can be stabilized simply after the land is closed up for a couple of years.

Thank you!