Geoscience Reference
In-Depth Information
persisted during the Eocene and ended abruptly at the Eocene-Oligocene transition,
coincident with Cenozoic global cooling at 34-33 Ma, which was associated with the
inception of permanent Antarctic ice sheets. The timing of uplift in Tibet is poorly
constrained and probably time-transgressive, which prompted Dupont-Nivet et al.,
(
2007
) to conclude that the desiccation evident at the Eocene-Oligocene transition
in this part of Asia is more likely to have been a result of global cooling than of
regional tectonic events, although these events undoubtedly helped accentuate aridity.
[A major drop in temperature is also evident 34-33 Ma in the Great Plains of North
America (Zanazzi et al.,
2007
). We do not yet know with certainty when the Northern
Hemisphere ice caps began to grow (
Chapter 20
). Drop-stones from ice-rafted debris
laid down in the Norwegian-Greenland Sea between 38 and 30 Ma ago and apparently
derived fromEast Greenland suggest that northern high-latitude ice accumulation may
be far older than previously envisaged (Eldrett et al.,
2007
).]
The following year, Dupont-Nivet et al. (
2008
) presented the results of detailed
pollen analysis at two sites in the Xining Basin that spanned the climatic transition
from Eocene to Oligocene at 34 Ma. They noted the first appearance of the taxon
Picea
(
Piceapollenites
) in both sections, bracketed between 38.3 and 37.3 Ma, indic-
ating a shift to a cool, temperate climate. They concluded that the appearance of
coniferous taxa characteristic of high elevations at 38 Ma showed that there had been
significant regional uplift of the Tibetan Plateau at least 4 million years before the
Eocene-Oligocene transition, leading to enhanced silicate weathering and a concomit-
ant decrease in atmospheric carbon dioxide (CO
2
) concentration, as inferred by other
workers (Zachos and Kump,
2005
).
Late Cenozoic uplift elsewhere in the world, including the Rockies, the Andes
and the Ethiopian Highlands, with concomitant erosion and weathering, would have
accentuated the drawdown of atmospheric carbon dioxide. However, the sudden cool-
ing at the Eocene-Oligocene transition 34 Ma ago is unlikely to be solely a result of
the depletion in atmospheric CO
2
. The abrupt temperature decline suggests a sudden
change in boundary conditions, with the opening of Drake's Passage between South
America and Antarctica being the most likely cause, because it enabled Antarctica to
be girdled by the circum-Antarctic current without obstruction.
Another important factor also contributed to the long-term desiccation of Asia
during the past 35 million years or so. The gradual shrinking during the Oligocene
and Miocene of the warm and shallow Paratethys and Tethys seas that stretched
across Eurasia was followed by a change from evenly distributed rainfall to a more
seasonal rainfall regime. The Mediterranean is the last relic of the Tethys Sea and
still contributes moisture to the arid lands to the east but not on the same scale as the
former Tethys.
Quade et al. (
1989
) analysed the stable carbon isotopes preserved in fossil soils
and fossil herbivore teeth in the Potwar Plateau of Pakistan (see
Chapter 7
). They
found strong evidence of a major change in both flora and fauna between 7.3 and
