Geoscience Reference
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from 4.2 to 2.7 Ma ago, after which it became weaker again and more variable. They
also found high concentrations of
44
Ca in the core sediments during the interval of
intense monsoon, and concluded that high rates of chemical weathering and rapid
denudation at that time caused a reduction in the atmospheric concentration of carbon
dioxide (pCO
2
) and triggered the onset of late Pliocene major Northern Hemisphere
glaciation around 2.7 Ma ago.
Tian et al. (
2002
) offered a somewhat different interpretation of monsoon strength
based on their analysis of the
18
O composition of benthic foraminifera from marine
sediments collected from ODP Site 1143 in the South China Sea. The core spanned
the last 5 Ma. They concluded that the strengthening of the East Asian monsoon
took place after 2.5 Ma, probably as a consequence of the increase in continental ice
volume in the Northern Hemisphere.
One way to reconcile the apparently conflicting conclusions of Tian et al. (
2002
)
and Zhang et al. (
2009
) is to propose that mid-Pliocene accelerated weathering and
erosion in the headwaters of the large tropical Asian rivers may have been only one
of several causes of the drop in pCO
2
at that time, which contributed to the start of
major ice accumulation in the Northern Hemisphere, and that accumulation may in
turn have been one of several factors influencing the East Asian monsoon.
19.6 Conclusion
The collision of Greater India and Asia some 45 million years ago caused uplift of
the Tibetan Plateau and led to major changes in atmospheric circulation and climate
across Asia. Evidence of early cooling associated with this uplift dates back to 38 Ma
in China, with marked cooling at the transition from Eocene to Oligocene 34 Ma
ago, when permanent ice became established on Antarctica. Shrinking of the Tethys
Sea during the Oligocene and Miocene accentuated the growing aridity in Asia, with
desert dust deposition by at least 24-22Ma. The monsoon system developed in stages,
with strong rainfall seasonality by 10.7 Ma. The unique Chinese loess record spans
the entire Quaternary and consists of alternating couplets of unweathered loess and
fossil soils. The soils developed during warm, wet interglacial and interstadial phases
when the Asian summer monsoon was more active, and the loess accumulated during
cold, dry glacial and stadial phases when the summer monsoon was weak and the
winter monsoon was strong. The fragmentary record from dunes, lakes and rivers is in
general consistent with the loess record, although in the Thar Desert of India (and the
Wahiba Sands of Arabia), dune accretion was greatest at 15-14 ka, when the summer
monsoon was once more becoming strong. In this case, wind velocity outweighed
aridity as a cause of dune mobility. Elsewhere, the LGM was cold, dry and windy,
with dune movement and dust mobilisation. Speleothem samples obtained from caves
in China provide a high-resolution record of monsoon history for the past 200 ka. It
appears that when the Greenland ice cores indicate a warmer regional climate, the
