Geology Reference
In-Depth Information
∼
°
C/km is commonly assumed), as a region
rises tectonically and the surface temperature
cools, temperature-sensitive plant species and the
animals dependent upon them should migrate off
the rising surface and move toward lower sites in
order to remain in the same temperature range.
Another approach to quantifying surface uplift,
therefore, begins with correlations between
specific assemblages of modern plants and the
ambient mean annual temperature. These analy-
ses rely on the observation that the shape or phys-
iognomy of leaves is sensitive to climatic conditions
(Wolfe, 1993). A smooth or serrated leaf margin,
the presence or absence of a drip tip (an elongate,
pointed tip that sheds moisture more readily), leaf
size, shape of the base, and spacing of the teeth
can each be statistically related to the mean annual
temperature at which the plant is growing.
For example, a well-documented positive
correlation exists between the percentage of
entire-margined (non-serrated) species in any
assemblage and the mean annual temperature
(Wolfe, 1971). Having established these correla-
tions, paleoassemblages of flora can be examined
and assigned a mean annual temperature (
T
int
)
based on their closest modern analog.
Subsequently, these temperatures can be com-
pared with those reconstructed for assemblages of
similar age that existed at sea level (
T
sl
). Based on
these estimates and an assumed terrestrial lapse
rate (TLR), a paleoaltitude (
z
) can be calculated:
6.5
Estimates of paleoaltitude
Determining the paleoaltitude of a surface or
rock sample represents one of the biggest chal-
lenges facing anyone attempting to document
uplift of surfaces in inland regions where
markers referenced to sea level cannot be readily
defined. For example, at an average modern
elevation of 5000 m, the Tibetan Plateau is the
highest region on Earth and has a major impact
on global climate, sediment fluxes, and ocean
chemistry. Despite its significance as a huge
orographic feature, the timing and magnitude of
uplift of Tibet are still highly controversial
(Molnar
et al.
, 1993; Murphy
et al.
, 1997; Rowley
et al.
, 2001), with estimates ranging from
Cretaceous to Late Pleistocene for when the
plateau nearly attained its present altitude.
Similar controversy surrounds the timing of
uplift or collapse of western North America and
the Puna-Altiplano plateau in the Andes.
How does one determine how high some
surface was in the past? One means to assess sur-
face uplift is to study the growth of mountain
ranges that impact local climatic conditions. The
rise of moist air masses as storms approach the
flanks of mountains causes orographic precipita-
tion to be focused on the windward side of the
range (Bookhagen and Burbank, 2010). In the lee
of the range, a rain shadow commonly develops.
If you could detect the development of a rain
shadow, you might infer that a mountain range
had grown in a windward position. For example,
the Miocene aridification of the western part of the
Basin and Range has been attributed to the rise of
the Sierra Nevada in California (Axelrod, 1957)
and its interception of moisture from Pacific storms
that used to penetrate farther into the continental
interior (Smith
et al.
, 1992). Although this interpre-
tation may be correct, it is difficult to place reliable
limits on how high the range would have to be in
order to produce the observed effect, nor can sub-
sidence of the Basin and Range, as opposed to
uplift of the Sierra Nevada, be ruled out as a cause
of the observed change. In addition, this approach
ignores the impacts of climate changes that occur
irrespective of the growth of a given range.
Because atmospheric temperatures generally
cool with increasing altitude (a lapse rate of
−
TT
=
sl
int
z
(7.11)
TLR
Although fairly straightforward, this approach
suffers from at least four limitations. First,
because many species have a fairly wide
temperature tolerance, their presence as fossils
may be only a loose indicator of former
temperatures. Second, relatively few fossils are
preserved in continental strata. In marine
studies, the problem resulting from the wide
temperature tolerance of any individual species
is overcome by examining many coexisting
species to determine a limited temperature
range for that assemblage. In terrestrial settings,
unfortunately, a paucity of fossil remains
