A 4 MILLION YEAR RECORD OF PALEO-EROSION RATES FROM THE QILIAN SHAN, CHINA

Recent work at Tianshan demonstrates a 9-Ma record of paleo-erosion rates reconstructed from cosmogenic ¹⁰Be from folded Neogene sediments [1]. A remarkable doubled erosion rates near 2 Ma has been proposed to be response to the onset of glaciation, consistent with increased sediment accumulation rates in central Asia at 2-4 Ma [2]. However, the ocean dissolved ¹⁰Be/⁹Be isotope record spanning the past 12 Myr indicates stable weathering fluxes during the late Cenozoic era and no clear evidence for increased erosion nor a pulse in weathered material to the ocean [3]. To test the hypothesis that this is a regional climatic signal, we constructed a 4 Ma record of paleo-erosion rates from the Qilian Shan, which forms the northeastern topographic boundary of the Tibetan Plateau.

We analyzed 22 samples for ¹⁰Be to determine paleo-erosion rates from the Laojunmiao (LJM) section exposed in an anticline near the city of Yumen. A subset were also analyzed for ²⁶Al. The LJM section has previously been dated by magnetostratigraphy [4]. After accounting for changes in sedimentation rates, we observe no clear change in inherent ¹⁰Be concentration (source area erosion rates) from 4 Ma to 1.2 Ma; however ¹⁰Be concentration decreased abruptly and erosion rates increase by an order of magnitude during a discrete interval extending from 1.2-0.8 Ma, subsequently returning to near the background rate. The interval of rapid erosion is found immediately between two unconformities in the section.

Our erosion rate record indicates that climate change across the Plio-Pleistocene boundary did not strongly influence erosion in the sparsely glaciated Qilian Shan. Although we cannot exclude that climate change near the Middle Pleistocene transition may have driven the rapid erosion we observe, we prefer the explanation that rapid erosion is a transient response to a tectonic uplift and fault activity near the range front, as evidenced by the regional unconformity.

[1] Charreau, J., P. H. Blard, et al. (2011). Earth and Planetary Science Letters 304(1-2): 85-92.

[2] Zhang Peizhen, P. M. W. R. D. (2001). Nature 410: 891-896.

[3] Willenbring, J. K. and F. von Blanckenburg (2010). Nature 465(7295): 211-214.

[4] Fang, X. (2005). Science in China Series D 48(7): 1040.