Geoscience Reference
In-Depth Information
well as in patterns of migration and interspecies competition. To provide a detailed
account of these topics is beyond the scope of the present work, so only a few of the
more salient topics will be considered here.
The Palaeocene-Eocene thermal maximum (around 55.8 Ma), recorded in the
Arctic and many other global marine records, is also evident on land, but sites with
fossils of this age are quite rare. Two sites from the northern Rockies in Wyoming
have yielded plant macrofossils (see
Chapter 16
) from alluvial deposits spanning the
Palaeocene-Eocene boundary (Wing et al.,
2005
). These unique floras are a mixture
of native North American species from the south and east and immigrant species
from Europe. The
13
C composition of the organic matter in soils developed on fine-
grained alluvium indicates that temperatures rose by about 5
C. Leaf area analysis
suggests that mean annual rainfall dropped by approximately 40 per cent at the start
of this interval and then increased late in the interval, when the warm, wet climate
allowed the formation of thick paleosols (Wing et al.,
2005
). The response of the
flora to these climatic fluctuations was species specific and relatively rapid, taking
place within about 10,000 years, which is comparable to the postglacial changes in
the North American flora described in
Section 20.7.5
.
As noted in
Section 20.5
, there was a sudden drop in temperature at the Eocene-
Oligocene transition around 33.5Ma ago, both in the Arctic and in the oceans through-
out the world. Less well-known is the degree and impact of this cooling on land.
Zanazzi et al. (
2007
) investigated the
°
18
O composition of fossil tooth enamel and
fossil bone spanning this transition at six localities scattered across Wyoming, South
Dakota and Nebraska. They found a drop in mean annual temperature of 8.2
±
C
over a period of about 400,000 years and a possible increase in temperature season-
ality, but no perceptible evidence of any change in aridity at that time, from which
they concluded that the drop in temperature was caused by a significant decrease in
the atmospheric concentration of carbon dioxide.
The expansion of grasslands across the world took place in the late Cenozoic and
appears to be related to changes in temperature, precipitation and, perhaps, atmo-
spheric carbon dioxide concentration. In the Great Plains, a series of reasonably
well-dated fossil soils allows us to follow the change from forests to grasslands over
the past 23 Ma. Fox and Koch (
2003
) analysed the stable carbon isotopes in the soil
carbon and soil carbonate in a series of paleosols in the Great Plains extending from
Nebraska and Kansas through NewMexico and Oklahoma to Texas. Plants that follow
the C
3
photosynthetic pathway (
Chapter 7
) (henceforth, C
3
plants) have present-day
mean
3.1
°
13
Cvaluesof
−
27
‰
with a range of
−
35
‰
to
−
22
‰
.C
4
grasses have mean
13
Cvaluesof
, and comprise 50-100 per
cent of the modern biomass in this region. Fox and Koch (
2003
) found that the C
4
biomass amounted to 12-34 per cent during the Miocene, increased between 6.4 and
4.0 Ma and attained present levels by 2.5 Ma. They also noted that the Miocene
ecosystems of the Great Plains have no modern analogue and concluded that the
−
13
‰
, with a range of
−
14
‰
to
−
10
‰
