Geoscience Reference
In-Depth Information
7.0 Ma. Before 7.3 Ma, the dominant vegetation was forest and woodland. After
7.0 Ma, tropical grassland expanded rapidly as the forest cover dwindled. They sug-
gested that these changes were consistent with the strengthening (or possibly even
the start) of the Indian summer monsoon. The global increase in plants following the
C
4
photosynthetic pathway (see
Chapter 7
) between about 8 and 6 Ma ago, and the
corresponding decrease in C
3
plants are in accord with a decrease in the concentration
of atmospheric CO
2
(Quade et al.,
1989
). C
4
grasses were already present in the
Tugen Hills of northern Kenya by 15.3 Ma but did not become a major component
of herbivore diet in Kenya and Pakistan until around 7 Ma ago (Morgan et al.,
1994
).
The initial change from C
3
to C
4
grasses began first in the lowland tropics, because
the threshold for C
3
photosynthesis is higher in warmer latitudes.
Dettman et al. (
2001
) built on the pioneering work of Quade et al. (
1989
)and
analysed the stable oxygen isotopic composition of Neogene freshwater bivalve shells
fromNepal, mammal teeth of similar age fromPakistan and soil carbonates fromNepal
and Pakistan. The full record covered the last 11 Ma. They found evidence of a strong
dry season signal from 10.7 Ma onwards, with little change in seasonal variability
after 9.5 Ma. They also discovered evidence of significantly higher wet season rainfall
before 7.5 Ma and drier conditions thereafter, which is consistent with the vegetation
history and fossil soil evidence. They concluded that the Tibetan Plateau was already
sufficiently high by at least 10.7 Ma to generate a strong summer monsoon.
19.5 Quaternary environmental fluctuations
19.5.1 Desert dust and loess
Very few terrestrial records span the entire duration of the Pleistocene. One out-
standing exception is the Chinese loess record, which shows an alternation of loess
accumulation in central China during cold, dry and windy climatic interludes and soil
development (Kemp,
2001
) under a re-established cover of moderately dense veget-
ation during the warmer, wetter intervals when the summer monsoon had become
stronger once more. The dry intervals were coeval with glacial or stadial climatic
phases, and the wet intervals corresponded to interglacial or interstadial phases (Liu,
1985
;Liu,
1987
; Kukla,
1987
;Liu,
1991
). The Loess Plateau of China (see
Chapter 9
,
Section 9.6
,
Figure 9.4
) occupies an area of about 440,000 km
2
. The loess deposits
have a mean thickness of about 100 m but locally attain 350 m and form some of
the richest agricultural land in the world. The loess overlying the Pliocene eolian
Red Clay in central China is Quaternary in age, but dust deposition has been active
in central Asia from 24-22 Ma onwards (Guo et al.,
2002
; Sun et al.,
2010
), with
significant dust accumulation in China by at least 8 Ma (An et al.,
2001
; Porter and
An,
2005
). The alternation of loess and well-developed soils with fossil pollen and
mollusca indicative of woodland or forest (Kukla,
1987
; Liu and Ding,
1998
; Kohfeld
