The Evidence from Flora
The assemblage of plant fossils has been used as a marker
for paleoelevation for almost half a century. This method is
based on the relationship between the vegetation type and
the average temperature at a given location. It is broken
down into several variants. The first method relies on finding
an assemblage of modern plant taxa equivalent to that of the
fossil site (or at least sharing the highest possible number of
taxa with it), but it assumes that there was no adaptation by
these taxa to climatic variations in the past (Su et al. 2019).
This method is particularly well adapted to the last 10 million years. A second method is based on the physiognomy of
the leaves of plants (size, shape, thickness, type of leaf
margin), synonymous with the adaptation of the plant to a
given climatic context. A relationship between the physiognomy of current plant leaves and the mean annual temperature has been established. This method assumes,
however, that the response of the leaf physiognomy to the
climate has been constant over time. This tool is calibrated
on dicots and does not take into account all biogeographic
provinces (e.g., Australia) or all taxa (such as conifers).
Moreover, it is only applicable as far back as the Upper
Cretaceous, since flowering plants only appeared during the
Cretaceous. However, the main advantage of this approach
is that it avoids any systematic recognition of taxa because
only the morphological characteristics count (which implies
optimal fossilization conditions). To determine altitude, it is
necessary to know the average temperature at sea level of a
site of the same age. The difference in temperature between
these two sites divided by the vertical gradient of the temperature indicates the paleo-altitude. However, the temperature lapse rate varies from 4 to 10 °C/km. This depends on
the latitude, the humidity of the air mass, the continentality
of the site and the topography itself. The choice of this
parameter is therefore decisive. Uncertainty remains high
(Peppe et al. 2010), around 700–1000 m for paleo-altitudes
estimated at 3–4 km.
To overcome the problem of the vertical gradient, a
method based on the preservation of moist static energy h in
the atmosphere has been developed (Forest et al. 1995). The
parameter h is the sum of a thermodynamic parameter, the
enthalpy H of humidity and the potential energy gZ and has
the advantage of a distribution that is relatively zonal,
especially in the mid-latitudes of the northern hemisphere.
This method requires an atmospheric circulation where
horizontal movements predominate over vertical movements, to ensure the conservation of moist static energy. The
humid enthalpy H was calibrated on current leaf indices
(similar to the mean temperature in the previous method). To
determine the paleo-altitude of a site, its humid enthalpy
H and the humid enthalpy H 0 of a contemporary reference
site located at sea level must be known. The uncertainty on
paleoaltitude is only slightly lower than for the preceding
method.
The plant-based methods are also subject to uncertainties
because of the impact of vegetation on the climate that are
ignored. The vegetation cover affects the radiative balance of
the Earth through its albedo and the water balance through
evapotranspiration (e.g. Otto-Bliesner and Upchurch 1997),
thereby could introduce a bias and cause paleo-altitudes to
be mis-estimated.
The Evidence from Erosion Sediment
By restoring the mass of deposited sediments in a basin to
the original relief, it is possible to calculate paleo-altitudes.
This presumes that the geometry of the sediment source is
known and that this has not changed over time, that there are
no sediment losses due to subduction or to incorporation
during more recent orogenic events and finally it assumes
that the relief has always been in isostatic equilibrium. These
considerations considerably limit the use of this method to a
few endorheic basins that do not have subduction zones
(intracontinental), such as the Tien Shan range in Asia
during the Miocene. Its application to older orogeny is
complex.
Stable Isotope Paleoaltimetry
Water oxygen is made of different stable isotopes (
16 O,
17
O,
18 O, e.g. Chaps. 11, 14, 15, 16, 21). The ratio of heavy to
light isotopes, when compared to a global reference value
(namely the Vienna standard mean ocean water, VSMOW),
is noted d
18 O. It can be measured in various surface waters
(ice, ocean, lakes, rivers), carbonates from pedogenic or
lacustrine sediments, or biogenic archives. At the global
scale, is has been observed that d
18 O in rainfall decreases
from low to high latitudes, and from coastal to inland areas.
On continents, d
18 O measured directly in precipitation or in
rivers along different elevation transects also has been shown
to scale with altitude, with d
18 O decreasing as elevation
increases. These observations have been explained by the
Rayleigh-type distillation process that occurs theoretically in
a cooling air parcel ascending along a mountain range: As
air rises and cools, water vapor condenses then precipitates.
During these steps, heavy isotopes are more favorably
removed from the air parcel, progressively depleting water
vapour along the way. Ultimately, rainfall is more and more
depleted in heavy isotopes with elevation, i.e. d
18 O decreases (Fig. 2.3). This theoretical framework, together with
regional measurements of d
18 O have led to determine
30
F. Fluteau and P. Sepulchre
The assemblage of plant fossils has been used as a marker
for paleoelevation for almost half a century. This method is
based on the relationship between the vegetation type and
the average temperature at a given location. It is broken
down into several variants. The first method relies on finding
an assemblage of modern plant taxa equivalent to that of the
fossil site (or at least sharing the highest possible number of
taxa with it), but it assumes that there was no adaptation by
these taxa to climatic variations in the past (Su et al. 2019).
This method is particularly well adapted to the last 10 million years. A second method is based on the physiognomy of
the leaves of plants (size, shape, thickness, type of leaf
margin), synonymous with the adaptation of the plant to a
given climatic context. A relationship between the physiognomy of current plant leaves and the mean annual temperature has been established. This method assumes,
however, that the response of the leaf physiognomy to the
climate has been constant over time. This tool is calibrated
on dicots and does not take into account all biogeographic
provinces (e.g., Australia) or all taxa (such as conifers).
Moreover, it is only applicable as far back as the Upper
Cretaceous, since flowering plants only appeared during the
Cretaceous. However, the main advantage of this approach
is that it avoids any systematic recognition of taxa because
only the morphological characteristics count (which implies
optimal fossilization conditions). To determine altitude, it is
necessary to know the average temperature at sea level of a
site of the same age. The difference in temperature between
these two sites divided by the vertical gradient of the temperature indicates the paleo-altitude. However, the temperature lapse rate varies from 4 to 10 °C/km. This depends on
the latitude, the humidity of the air mass, the continentality
of the site and the topography itself. The choice of this
parameter is therefore decisive. Uncertainty remains high
(Peppe et al. 2010), around 700–1000 m for paleo-altitudes
estimated at 3–4 km.
To overcome the problem of the vertical gradient, a
method based on the preservation of moist static energy h in
the atmosphere has been developed (Forest et al. 1995). The
parameter h is the sum of a thermodynamic parameter, the
enthalpy H of humidity and the potential energy gZ and has
the advantage of a distribution that is relatively zonal,
especially in the mid-latitudes of the northern hemisphere.
This method requires an atmospheric circulation where
horizontal movements predominate over vertical movements, to ensure the conservation of moist static energy. The
humid enthalpy H was calibrated on current leaf indices
(similar to the mean temperature in the previous method). To
determine the paleo-altitude of a site, its humid enthalpy
H and the humid enthalpy H 0 of a contemporary reference
site located at sea level must be known. The uncertainty on
paleoaltitude is only slightly lower than for the preceding
method.
The plant-based methods are also subject to uncertainties
because of the impact of vegetation on the climate that are
ignored. The vegetation cover affects the radiative balance of
the Earth through its albedo and the water balance through
evapotranspiration (e.g. Otto-Bliesner and Upchurch 1997),
thereby could introduce a bias and cause paleo-altitudes to
be mis-estimated.
The Evidence from Erosion Sediment
By restoring the mass of deposited sediments in a basin to
the original relief, it is possible to calculate paleo-altitudes.
This presumes that the geometry of the sediment source is
known and that this has not changed over time, that there are
no sediment losses due to subduction or to incorporation
during more recent orogenic events and finally it assumes
that the relief has always been in isostatic equilibrium. These
considerations considerably limit the use of this method to a
few endorheic basins that do not have subduction zones
(intracontinental), such as the Tien Shan range in Asia
during the Miocene. Its application to older orogeny is
complex.
Stable Isotope Paleoaltimetry
Water oxygen is made of different stable isotopes (
16 O,
17
O,
18 O, e.g. Chaps. 11, 14, 15, 16, 21). The ratio of heavy to
light isotopes, when compared to a global reference value
(namely the Vienna standard mean ocean water, VSMOW),
is noted d
18 O. It can be measured in various surface waters
(ice, ocean, lakes, rivers), carbonates from pedogenic or
lacustrine sediments, or biogenic archives. At the global
scale, is has been observed that d
18 O in rainfall decreases
from low to high latitudes, and from coastal to inland areas.
On continents, d
18 O measured directly in precipitation or in
rivers along different elevation transects also has been shown
to scale with altitude, with d
18 O decreasing as elevation
increases. These observations have been explained by the
Rayleigh-type distillation process that occurs theoretically in
a cooling air parcel ascending along a mountain range: As
air rises and cools, water vapor condenses then precipitates.
During these steps, heavy isotopes are more favorably
removed from the air parcel, progressively depleting water
vapour along the way. Ultimately, rainfall is more and more
depleted in heavy isotopes with elevation, i.e. d
18 O decreases (Fig. 2.3). This theoretical framework, together with
regional measurements of d
18 O have led to determine
30
F. Fluteau and P. Sepulchre