『Abstract
Grain-size studies of eolian deposits (loess and Red Clay) on
the Chinese Loess Plateau have produced a detailed history of
the Asian winter monsoon covering the last 8 Ma. This paper extends
the grain-size record back to 13.1 Ma in the Linxia basin, at
the northeast edge of the Tibetan Plateau. By comparing the grain-size
distribution of the Linxia basin sediment with known eolian sediments
in the region we argue that the 10-70μm fraction is mostly eolian.
This fraction becomes increasingly dominant in the record after
7.4 Ma, with almost all sediments eolian in origin after 6.2 Ma.
Given that the 10-70μm fraction in the Linxia basin are well sorted
and have grain-size spectra that are very similar to sediments
transported by the Asian winter monsoon, we suggest that the continuous
presence of this fraction throughout our record indicates that
the Asian winter monsoon has been present in some form since 13.1
Ma. If the assumption that variation in grain-size spectra on
the million-year time scale is primarily controlled by the character
of eolian input to the basin is valid, the long-term pattern of
the eolian fraction reveals two abrupt intensifications of the
Asian winter monsoon at 7.4 Ma and 5.3 Ma. These are superimposed
on a gradual intensification of the Asian winter monsoon beginning
at 8.0-7.4 Ma. We note that the two abrupt intensifications may
be related to Arctic ice volume increase, and that the gradual
intensification of the Asian winter monsoon after 7.4 Ma is accompanied
by central Asia desertification and may therefore be related to
increase in the height or extent of the Tibetan Plateau.
Keywords: grain size; Asian winter monsoon; Linxia basin; Tibetan
Plateau』
1. Introduction
2. Present climate
3. Geological setting and sedimentary environment
4. Materials and methods
5. Results
6. Discussion
6.1. Eolian sediment in the Linxia basin
6.2. Grain-size implications for atmospheric circulation, 13.1-4.3
Ma
7. Conclusions
Acknowledgement
References