Geology Reference
In-Depth Information
commonly precludes estimates with a narrow
temperature range. Third, by equating a change
in mean annual temperature with a change in
altitude of a surface, one ignores the effect of
global climate change. The onset of the Ice Ages
in the late Cenozoic has lowered mean global
temperatures, such that a floral grouping that
might have been well adapted at 3000 m in the
past is now confined to much lower altitudes.
This temperature-dependent response could
happen irrespective of surface uplift or subsid-
ence. Without independent evidence of the
amount of climate change, inferences of altitudi-
nal changes based on temperature-sensitive
assemblages alone are highly suspect. Fourth,
correlation of fossil assemblages with modern
analogs assumes little or no evolution of these
plants in response to environmental change.
Finally, the technique also assumes that a
terrestrial lapse rate can be reliably assigned to
a time typically tens of millions of years ago.
Floristic approaches to estimating paleoalti-
tudes have recently been improved. One new
method utilizes an increase in the stomatal
density in leaves that occurs in response to the
well-known decrease in the partial pressure
of CO
2
at higher altitudes (McElwain, 2004).
Analyses of the stomata of fossil leaves could
potentially reveal paleoaltitudes with a resolu-
tion of a few hundred meters. Leaf-margin anal-
yses have improved owing to two key advances.
First, application of multivariate analysis to the
physiognomy of leaves has led to a quantitative
calibration of modern meteorological environ-
ments with physiognomy (Wolfe, 1990). Second,
rather than assuming a lapse rate, the moist
static energy of an air mass is used to calculate
paleoaltitude (Wolfe
et al.
, 1998). Moist static
energy (
h
) is a thermodynamic parameter repre-
senting the total energy content of a parcel of
air, excluding a negligible amount of kinetic
energy. It comprises two components, enthalpy
(which consists of both latent heat and thermal
energy) and potential energy (due to altitude):
latent heat of vaporization,
q
is specific humid-
ity,
g
is acceleration due to gravity, and
Z
is
height. On the right-hand side,
H
is enthalpy
and equals
c
p
T
+
L
v
q
. Moist static energy is pre-
sumed to be conservative, which means that the
balance between potential energy, latent heat,
and thermal energy may change over time, but
their sum remains constant as an air mass moves
inland along a trajectory from a coastal site.
Multivariate analysis shows that enthalpy is
second only to mean annual temperature in
terms of its correlation with leaf characteristics.
If it is assumed that air masses move zonally
(within restricted latitudinal bands) and that the
preserved flora are truly isochronous at sites
being compared, then changes in enthalpy
between sites in the same latitudinal band can
be used to calculate changes in potential energy
and, therefore, altitude. Owing to likely depar-
tures from truly zonal circulation and to uncer-
tainties in the enthalpy calculation, recent
paleoaltitude calculations have uncertainties
estimated to be 750-900 m (Wolfe
et al.
, 1988).
Clearly, this approach is unsuitable for deter-
mining small variations of altitude at a given
site. Although the uncertainty on an individual
estimate is large, studies of multiple related sites
allow reduction of the overall uncertainty. The
validity of the assumptions (isochronous sites,
zonal transport) that underpin parts of this
methodology are difficult to evaluate in the past,
such that the actual uncertainty may depart
significantly from the formal uncertainty.
For many years, a lively controversy has
flourished about the paleoaltitude of western
North America. Many researchers concluded that
this region had risen to its present high elevation
during late Cenozoic times (Box 7.4). Application
of multivariate analyses on leaves (Gregory, 1994;
Gregory and Chase, 1992) and enthalpy calcula-
tions (Wolfe
et al.
, 1998) now suggest that many
of these areas have been high since early Cenozoic
times, and that some, such as the Basin and
Range, are considerably lower now than they
were 15 million years ago. Clearly, these recon-
structions change significantly the way one thinks
about the history of western North America!
As storms precipitate on the flanks of
mountains, the isotopic composition of their
h
=
c
p
T
+
L
v
q
+
gZ
=
H
+
gZ
(7.12)
where
c
p
is the specific heat capacity of moist air
at constant pressure,
T
is temperature,
L
v
is the
