The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 118

36.6 mya (just after the Eocene-Oligocene boundary) to about 760 ppmv. Following
this there was a rebound to just over 11 000 ppm over the next 100 000 years, and
then a decline again. The modelled threshold for Antarctic glaciation is between 700
and 850 ppm. What exactly caused this temporary dip is not clear: there are a number
of possibilities and combinations thereof. Irrespective of our lack of knowledge,
after this the Earth continued to cool and Antarctica's ice sheets grew. As we shall
shortly see, the expansion of grasses and C 4 plants, ocean circulation changes due
to continental shift and orogenesis (building of mountains such as the Himalayas;
hence silicate weathering to form carbonate; see section 3.3.3) are all implicated in
this long-term decline in carbon dioxide over many millions of years.
4.2 TheendMiocene(9-5.3mya)
As mentioned in section 3.3.11 the new C 4 photosynthetic plants were better able to
take advantage of the atmosphere's lower carbon dioxide levels compared to plants
that evolved earlier, in the Tertiary (prior to 20 mya), and marginally help with
carbon dioxide drawdown. This ability to thrive in the then carbon dioxide-poorer
atmosphere gave them an advantage over C 3 plants where other conditions (such
as soil, temperature and moisture) were favourable. As noted in the last chapter,
C 4 plants originated well before they became globally established: global expansion
takes time. Indeed, it is thought that atmospheric carbon dioxide had already declined
to the equivalent of Quaternary levels (of the past 2 million years or so) by 15 mya, and
if anything atmospheric carbon dioxide is thought to have been on a minor increase
between 15 and 5 mya. So, although the lowering of atmospheric carbon dioxide
over much of the Tertiary facilitated speciation and the rise of C 4 plants, lowering
atmospheric carbon dioxide by itself cannot explain their global spread at the end
of the Miocene. It may be that tectonic activity, changes in seasonal precipitation
and some other factor(s) or combinations thereof facilitated matters (Pagani et al.,
1999). Indeed, as mentioned in the previous section there was the opening of the
high-latitude southern hemisphere oceanic passages and, of course, uplift, forming
the Tibetan Plateau. By 10 mya the east-Antarctic ice sheet had grown (but was still
smaller than today) from its beginnings 35 mya, and 10 mya one had already formed
on Greenland and was itself slowly growing. Superimposed on this overall trend of
a cooling Earth and increasing ice was Milankovitch-driven ice-sheet waxing and
waning. Climate change was taking place.
As noted in the last chapter, the spread of C 4 plants during this time has been charted
in a number of ways. The US geophysicist Thure Cerling and colleagues published in
1993 the results of isotope analysis of soil and tooth remains: metabolic pathways in
C 3 and C 4 plants have different preferences for (or fractionation of) the 13 C and 12 C
carbon isotopes. Initially there was some discussion that this interpretation might be
undermined if animals migrated from where they fed to where they died. However, in
1997 Cerling and colleagues published the results of carbon analysis of tooth remains
from more than 500 hypsodont equids from Asia, Africa, Europe and North and South
America. In addition, they analysed the teeth of fossil proboscideans (elephants and