CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-12-1635-2016Reconstructing geographical boundary conditions for palaeoclimate modelling during the CenozoicBaatsenMichielm.l.j.baatsen@uu.nlvan HinsbergenDouwe J. J.von der HeydtAnna S.https://orcid.org/0000-0002-5557-3282DijkstraHenk A.SluijsAppyhttps://orcid.org/0000-0003-2382-0215AbelsHemmo A.BijlPeter K.IMAU, Utrecht University, Princetonplein 5, 3584CC Utrecht, the NetherlandsDepartment of Earth Sciences, Utrecht University, Heidelberglaan 8, 3584 CS Utrecht, the NetherlandsDepartment of Geosciences and Engineering, Delft University of Technology, Stevinweg 1, 2628 CN Delft, the NetherlandsMichiel Baatsen (m.l.j.baatsen@uu.nl)11August2016128163516443September201521October201520June201623June2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/12/1635/2016/cp-12-1635-2016.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/12/1635/2016/cp-12-1635-2016.pdf
Studies on the palaeoclimate and palaeoceanography using numerical model
simulations may be considerably dependent on the implemented geographical
reconstruction. Because building the palaeogeographic datasets for these
models is often a time-consuming and elaborate exercise, palaeoclimate models
frequently use reconstructions in which the latest state-of-the-art plate
tectonic reconstructions, palaeotopography and -bathymetry, or vegetation
have not yet been incorporated. In this paper, we therefore provide a new
method to efficiently generate a global geographical reconstruction for the
middle-late Eocene. The generalised procedure is also reusable to create
reconstructions for other time slices within the Cenozoic, suitable for
palaeoclimate modelling. We use a plate-tectonic model to make global masks
containing the distribution of land, continental shelves, shallow basins and
deep ocean. The use of depth-age relationships for oceanic crust together
with adjusted present-day topography gives a first estimate of the global
geography at a chosen time frame. This estimate subsequently needs manual
editing of areas where existing geological data indicate that the altimetry
has changed significantly over time. Certain generic changes (e.g. lowering
mountain ranges) can be made relatively easily by defining a set of masks
while other features may require a more specific treatment. Since the
discussion regarding many of these regions is still ongoing, it is crucial to
make it easy for changes to be incorporated without having to redo the entire
procedure. In this manner, a complete reconstruction can be made that
suffices as a boundary condition for numerical models with a limited effort.
This facilitates the interaction between experts in geology and palaeoclimate
modelling, keeping reconstructions up to date and improving the consistency
between different studies. Moreover, it facilitates model inter-comparison
studies and sensitivity tests regarding certain geographical features as
newly generated boundary conditions can more easily be incorporated in
different model simulations. The workflow is presented covering a middle-late
Eocene reconstruction (38 Ma), using a MatLab script and a complete
set of source files that are provided in the supplementary material.
Introduction
As the interest grows in assessing how the climate may change under
increasing anthropogenic forcing, the field of climate modelling is steadily
growing and improving. These models are mainly tested by making hindcasts of
the present-day and pre-industrial climate. Anthropogenic forcing may lead to
future climate states that are beyond the range for which models can be
validated with observations. Reconstructing the palaeoclimate may help to
increase the confidence in predictions by simulating episodes in Earth's
history with climatic conditions that were drastically different to that of
today. While the number of measurements is generally much more limited than
for the present day, a considerable amount of proxies is available to
constrain the conditions of several such periods, like the Paleocene–Eocene
thermal maximum, Eocene–Oligocene transition and Eemian. The equable climate
problem is one good example showing that models still lack
some essential dynamics to explain how the meridional atmospheric (or sea
surface; ) temperature gradient can become so flat in a
greenhouse climate. It is therefore essential to try and simulate the past
climate with a similar amount of detail as is done for the present to better
understand the climate system when it is forced far beyond its present state.
Simulating deep time palaeoclimate is challenging, since the applied boundary
conditions can be poorly constrained. Between reconstructions, differences
in,
e.g. lat./long. coordinates, heights, coastlines and basin depths are often
considerable. Next to the atmospheric composition, solar insolation and land
surface properties, the global geography can be of great importance
. The breakup and collision of continents likely drove
major climate transitions in the past. For example, the India–Eurasia
collision caused a large scale uplift that resulted in regional climate
changes but also affected the global circulation and hydrological cycle
.
Furthermore, the opening of ocean gateways such as the Tasmanian Gateway
impacted ocean circulation and thus the redistribution of heat, resulting in
both changed surface and deep ocean temperatures .
Many of these themes are the subject of ongoing studies and continuous
acquisition of new data, either of palaeo-altimetry/bathymetry or from
tectonic reconstructions. It is therefore crucial that new relevant insights
can be incorporated easily and swiftly into boundary conditions used for
palaeoclimate modelling.
Implementing different geographies into climate models is not without issues;
the increasing but limited model resolution obscures certain potentially
critical features (such as narrow ridges, mountain ranges and straits).
Additionally, plate-tectonic reconstructions have their own uncertainties and
are continuously being expanded and improved. This causes large discrepancies
between model simulations because different studies use reconstructions of
varying age and approach. A simple but striking example is given by
, who used the output of several coupled global
atmosphere-ocean models to force an ice sheet model simulating the Antarctic
glaciation during the Eocene-Oligocene transition (∼ 34 Ma). Their
main conclusion states that it is nearly impossible to determine the CO2
threshold for the inception of ice sheets since the models use such different
topographies. Geographical reconstructions used in palaeoclimate modelling
are often based on older reconstructions (such as ) that are adjusted for a specific case. These reconstructions may
thus be based partially on information that is outdated and are usually only
made for a unique set of time frames, which greatly limits their application
in other studies. Since small-scale features can be crucial in the climate
system, they always need to be considered and may require additional manual
changes. Finally, developments in coding and computer hardware allow for
model simulations with a significantly higher resolution that in turn require
more detailed boundary conditions.
This calls for a new approach where the latest geological reconstructions can
directly be incorporated together with improving constraints on
palaeoaltimetry (e.g. ) and translated into a gridded
topography and bathymetry map. In this paper, a method is presented that
allows for the creation of geographical reconstructions based on the most
recent geological insights with relative ease. Since there is a relative
scarcity in middle and late Eocene reconstructions, the composition of a
38 Ma palaeogeography will be presented and serve as an example of the
general workflow. The set of files (and downloads) required to reproduce this
procedure for 38 Ma, as well as other time frames, is provided in the
supplementary material along with a Readme for guidelines. Although somewhat
similar workflows have been presented in the past , the innovative aspects here are the approach of directly
incorporating a plate-tectonic reconstruction, the freedom of choosing a
different plate-tectonic model framework and the ability to consider multiple
time frames.
Plate tectonic reconstruction
The backbone of our method is a plate-tectonic model that provides a
first-order, 200 Ma to present-day reconstruction of palaeogography
. This model is provided in the free plate reconstruction
software GPlates (www.gplates.org, ), which is widely
used in tectonic research. The reconstruction of serves as a
basis, and can be updated when new reconstructions become available. Because
we aim to study palaeoclimate, we place our reconstruction in a
palaeomagnetic reference frame . Another
reference frame can be used by simply changing the rotation file of the
plate-tectonic model, only the ocean depth grid needs to be consistent with
the latter. Starting from the present-day topography, the outlines of land,
continental shelves and shallower regions such as intra-oceanic ridges and
plateaus are defined. Using the reconstruction model, these features are then
shifted towards their positions at a chosen time frame (here: 38Ma)
and exported as Shapefiles (.shp). The latter are then transformed
into numerical grids and stored as a NetCDF file using a simple Matlab script
for easy and unambiguous interpretation by most computational programs. An
example showing the gridded masks for the middle-late Eocene is shown in
Fig. .
Earth reconstruction from the plate tectonic model at 38 Ma
showing shifted present-day land surface (white), continental shelves (blue)
and relatively shallow ocean regions (black). Remaining in grey are then the
deep ocean and zones that are yet unidentified in the plate-tectonic
reconstruction model.
For its application as a climate model boundary condition, the
final geographical reconstruction should be stored as a gridded
(latitude-longitude) topography-bathymetry dataset. To start with, ocean
bathymetry is estimated using the present-day ocean floor age grid provided
by in combination with the plate tectonic model. Using the
latter, the age grid is shifted into its position at the considered time frame
(38 Ma here) to be compatible with the rest of the reconstruction. The
position and age (corrected for the plate motions that have occurred since
then) of oceanic plates in the model is then translated into a bathymetry map
using the general depth–age relationship for oceanic crust. Here, we use an
average of the relations given for the main ocean basins (Atlantic, Pacific
and Indian Ocean) treated by :
ZA<90Ma=2620+330A,ZA>90Ma=5750,
where Z is the basin depth in metres and A the ocean floor age in
millions of years.
First stage of the reconstruction: global geography at 38 Ma
using the depth-age relationship, the reconstruction,
shallow basins, continental shelves and the present-day topography. The areas
shaded in light grey are gaps which still have to be addressed.
The method above can only be applied to regions where the ocean floor is
still preserved today, leaving large gaps where oceanic crust has subducted
since the reconstruction time (e.g. the western and southeastern Pacific
Ocean). An estimated palaeo-age grid was already translated into a rastered
geo-referenced depth to basement dataset by . The latter
also provides a reconstructed bathymetry, including sediments, but this makes
it incompatible with other areas where only the age–depth relationship is
used. Furthermore, sediments are only significant in a few locations where
the ocean floor is still very deep (generally below 5000 m), resulting
usually only in small changes. This is even more so when the bathymetry grid
is translated in depth layer masks that are used in numerical models, as they
rarely go beyond 5–5.5 km. One exception to this is the Neotethyan
Ocean where up to 1500 m of sediment is added in , but
this region is treated specifically here resulting in a similar final depth
(4.5 km). Finally, the age–depth relationship used here
is very similar to the one used to reconstruct the depth
to basement grids as differences are generally around
50 m or less.
The grids are provided in a hotspot reference frame and
thus need to be converted to the palaeomagnetic reference frame used here in
the scope of climate modelling . We applied
this conversion by fixing the entire bathymetry grid to Africa which is the
central pole in both of the reconstruction frameworks. First, the movement of
Africa in the past 38 Ma is reversed by using an adapted version of the
rotation file such that it moves back to its present
position starting from where it was at 38 Ma instead of the opposite.
The bathymetry map (defined at 38 Ma) is then shifted to the
configuration it would have at the present day, disregarding the individual
plate movements relative to Africa. Finally, the same is done using the
(unaltered) rotation file of the palaeomagnetic
framework, moving the bathymetry grid back to its configuration at
38 Ma as if it were made using this reconstruction model. After doing
so, the adapted depth grid fits within the continental
margins of the plate-tectonic reconstruction. Differences between the
rotational frameworks used may lead to discrepancies in the reconstruction,
but since oceanic plateaus and ridges are treated separately in our approach
this does not result in any problems over at least the Cenozoic. For earlier
periods, more specific adjustments may be necessary to cope with possible
inconsistencies. In addition to the deep ocean, shallow basins and plateaus
are adjusted to their present-day depth at the according 38Ma
positions, using the same plate movements of which the resulting boundary
masks are shown in Fig. . Finally, all of the defined
continental shelves are put to a fixed depth (200 m is taken here,
values between 50 and 500 m are found in literature) as they mark the
outer margins of continental crust, that are assumed to be fairly robust over
time. It is important to consider here that the shape and depth of
continental shelves may be subject to change, depending on the considered
time frame . Since the middle-late Eocene had a greenhouse
climate, a flat and relatively shallow shelf is implemented here but this can
easily be done differently for other periods.
Second stage of the reconstruction: topography and bathymetry with
adjustments to polar regions and a lowering of several mountain ranges.
Changes can be noted mainly in Greenland, Antarctica, the Neotethyan Ocean,
the Andes and the Himalayas.
The next, more challenging step is creating a realistic land topography for a
chosen time frame. Good estimates of altimetry are only available for a
limited number of locations and time intervals, making the reconstruction of
palaeotopography a difficult task that requires a lot of manual input. For
example, the global Eocene topography by is still used
and referred to regularly in palaeoclimate modelling. Our approach here is to
first consider the present-day topography (using the ETOPO1 dataset,
http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1, ) and adjust it to the
38 Ma configuration using the plate tectonic model and the land mask
shown in Fig. . Later on, specific topographical features
can be corrected manually, depending on the specific demands of the
application and the amount of information that is available. Since the
topography is coupled to the plates and shifted within GPlates, there are no
issues of area preservation or distortion. The resulting first stage of the
reconstruction is given in Fig. .
Masks used for manipulation of the reconstruction; white represents
land, cyan shallow water, blue sea and black ocean. In southern Asia, green,
yellow, orange and red indicate areas that are lowered manually by different
ratios.
Generic adjustments
The geography obtained at this stage (Fig. ) is not yet
accurate and contains a number of gaps that remain to be filled: mainly
collision zones and transition regions between continental and oceanic crust.
Especially the land topography needs to be adjusted so that it better matches
palaeoaltimetry estimates for the considered time frame (38 Ma).
Although glaciers possibly existed on Antarctica and
Greenland , the presence of continental-scale ice sheets
in the Eocene is unlikely . Therefore, to correct for the
effect of isostasy the present-day bedrock topography is raised by 1/3 of the
current ice thickness. The changes mainly affect Greenland and Antarctica,
although for the time window chosen here a highly detailed palaeotopography
of Antarctica was derived by , considering both isostatic
rebound and sediment restoration with respect to the present-day topography.
The latter, however, assumes the Antarctic continent to be at its present-day
location which does not fit into this reconstruction. Similar to how the
topography was treated above, the reconstructed Antarctic palaeotopography is
fixed to the plate polygons defined in the plate-tectonic model and adjusted
towards its 38 Ma configuration. In practice, only the area above sea
level is incorporated in the reconstruction and combined with the previously
determined bathymetry. This is done to minimise possible discrepancies
between regions of the ocean floor that are reconstructed using different
methods.
Although most of the ocean floor has been reconstructed, a large gap is still
present in the Neotethyan region between Africa, Eurasia, India and Indochina
at 38 Ma. While in the circum-Pacific area the crust lost to subduction
was entirely oceanic, the former was characterised by multiple continental
collision and subduction episodes . The Neotethyan Ocean
is introduced manually, based on the incorporation of GPlates reconstructions
covering the Mediterranean region by , the Arabia-Eurasia collision zone by
, the Arabia-India plate boundary zone by
and the India-Asia collision zone by
. A deep ocean basin with a depth
of 4500 m is assumed, which is close to the value used in
, representing the Neotethyan Ocean at 38 Ma.
Reconstructed topography and bathymetry with specific adjustments
indicated by shaded areas, these include mainly Antarctica, southern Panama,
the Amazon and several other river basins, the Hudson Bay and Great Lakes,
Neotethyan Ocean and Paratethys Seaway, the Ninety-East Ridge and the Tasman
Rise.
Next, we adjust the elevation of several mountain ranges towards values that
are more representative for the considered time frame. Since most of the
current elevated regions underwent a considerable part of their uplift after
the late Eocene (such as the Andes, Himalayas and East African Rift) they
will have to be lowered, while eroded regions may require some uplifting (for
instance large river catchments). More specific changes for the example
considered here will be discussed in more detail in Sect. ,
the result of the generic changes is first shown Fig. . As this
is clearly the most tedious part of the procedure, it will consume a large
part of the time required to acquire a palaeogeography for another time
frame. However, time-specific adjustment masks are created as polygons within
GPlates and fixed to the plate movements. Consequently, only their extents
and effects will need to be adjusted and some masks may need to be removed or
added, which facilitates this step significantly. When available, the
incorporation of a digital palaeo-elevation model, such as the one presented
by can replace the present-day topography so that no
further adjustments are needed.
Specific adjustments for the middle-late Eocene
The example shown thus far is largely representative for the middle-late
Eocene (38 Ma) geography, but now a number of time-frame specific
adjustments are made that have not yet been treated in the previous sections.
Several major tectonic events took place prior to and during the
Eocene-Oligocene transition that had an impact on the global climate. A brief
overview of these events and how their potential impacts are implemented
specifically is given below and a map of the masks used to apply the various
changes is provided in Fig. . The combined effect of these
adjustments is indicated by the highlighted areas in Fig. ,
note that the effects of the land mask shown (in white) in Fig. have not yet been implemented. The latter is used to correct the
land boundaries after the manipulations discussed in Sect. .
Final smoothed global topography and bathymetry reconstruction for
the middle-late Eocene at 38 Ma.
Flowchart of the procedure; the plate-tectonic reconstruction is
made with GPlates, using a set of reconstructed plate trajectories
, a reference rotational frame
and a set of boundary masks delimiting present-day
coastlines, continental shelf breaks, shallow ocean regions and some regions
that need manual adjustments (see Figs. and ).
In addition to the reconstruction, present-day (PD) topography and bathymetry
base files are manipulated using the same rotational frame and adjusted to
the configuration at the aimed time frame. The resulting output files as well
as some external input are then fed into a MATLAB script which builds up a
global geography dataset through several consecutive steps and stores it as a
NetCDF file.
The collision between Greater India and Eurasia is causing profound changes
to the surrounding geography . The Neotethyan Ocean
narrows due to the combined northward propagation of Africa and India,
leaving the Mediterranean Sea and the northern Indian Ocean. Further north, a
major intra-continental seaway known as the Paratethys covered much of
central Europe and a considerable part of western Asia. This was mainly a
shallow basin except for the South Caspian and Black Sea areas, as it was
mostly underlain by unextended continental crust. The extent of the
Paratethys as well as the distribution of the surrounding continents is
determined using the GPlates reconstructions mentioned in Sect. . The Neotethyan subduction-collision systems also create a
large-scale uplift in southern Asia with primarily the formation of the
Himalaya mountain range and the Tibetan Plateau. To the north and west, this
uplift forces the westward retreat of the relatively shallow Paratethys
leaving several isolated basins . In
addition, the formation of a rift between Antarctica and South America causes
the opening of the Drake Passage with the first formation of oceanic crust at
around 35 Ma. A similar process is taking place
south of Australia with the formation of the Tasmanian Gateway. While its
opening already occurs around 50 Ma, the Tasmanian Gateway initially
remains shallow. Coincident with the transition from a slip-boundary towards
North-South spreading between Australia and East Antarctica ,
a profound deepening phase occurs from 35.5 Ma onwards. The deepening
and widening of the Tasmanian gateway is further complicated by a northward
extension of the Antarctic shelf and an elevated Tasman Rise which both sank
over time . Dependent on which absolute plate
reconstruction frame is used, the opening of the Tasmanian Gateway can occur
either to the north or south of 60∘ S palaeolatitude. This can
have important consequences for the direction of a possible initial
throughflow and again stresses the importance of correct palaeogeographies in
paleoclimate modelling. The related northward motion of Australia combined
with the rotation and rifting taking place around Indochina also causes a
gradual blocking of the Indonesian Throughflow during the Oligocene
.
As was pointed out before, some smaller geographical features have also been
treated individually. Mountain ranges that are believed to have experienced
major uplifts since the Eocene are mainly the Andes, Himalayas, Sierra Nevada
(USA), East African Rift and Middle East, which are all lowered
significantly. The southern Tibetan Plateau was lowered more than the
Himalayas since a narrow yet high ridge was already present while the
adjacent plateau was still much lower . The northern
part of the Tibetan Plateau is changed into lowland that surrounds the
retreating Paratethys in the Tarim Basin . Related to the changes in Eurasia mentioned above,
the narrow Tethys is drawn as a deep ocean while the Paratethys consists
mainly of shallow seas with the Turgai Strait almost closed
. The latter forms the connection between the Arctic Ocean
and the Paratethys and is an important contributor to the water budget in
Central Asia. During the early Eocene, the Arctic Ocean may have been
geographically isolated as indicated by freshening reported in
. No such events have yet been documented for the middle
and late Eocene so here we assumed several relatively narrow and shallow
channels to be open. The western part of the Amazon basin was still flooded
during the middle Eocene . A kinematic reconstruction of the
Caribbean region by is adopted to treat the Panama
Seaway. This gateway connecting the tropical Pacific with the Atlantic Ocean
is still open but only with shallow waters (similar to the Indonesian
Throughflow today), as it consists of an intra-oceanic volcanic arc since the
late Cretaceous . The Ninety-East Ridge
was also enhanced (made wider and higher) as it is a
pronounced yet narrow feature in the eastern Indian Ocean that would
otherwise be obscured by smoothing later on. Regarding the areas discussed
above, three masks are applied defining ocean (deep oceanic), sea (deep
continental) and shallow basins (shallow continental) implying a depth of
4500, 1000 and 100 m, respectively.
Final modifications and smoothing
All major features of the geological reconstruction are now present but
before it can be used as a numerical grid for a climate model simulation,
some additional modifications still need to be made. Firstly, to estimate the
remaining gaps where the altimetry or bathymetry is still unknown, an
interpolation is performed using the inpaintn algorithm developed by
and . An additional land mask was made
(shown in white in Fig. ) containing regions that should be
above sea level to avoid creating artificial basins with this technique. The
land mask is based on several global reconstructions such as those made by
R. Blakey (https://www2.nau.edu/rcb7/globaltext2.html) and C. R. Scotese (http://www.scotese.com/earth.htm), assuming that large
alluvial plains were not entirely flooded and glacial basins had not yet
formed.
Secondly, during most of the past climate periods sea level was different
from today due to changes in ocean water volume, as influenced by ice sheet
volume and steric effects, as well as varying global ocean basin volume,
related to the average ocean floor age, shelf depth and continental
distribution. In the middle-late Eocene, mean sea level was about 100 m
above that of today , so we decrease the elevation (and
similarly increase the depth) of all data points by 100 m to correct
for this. The depths that were implemented before, using the masks shown in
Fig. , were initially corrected so they end up being the actual
values stated above after changing the global sea level (such that an area
assigned to be, e.g. 1000 m deep does not end up being 1100 m
instead). Again, the land mask is used to correct the elevation of certain
inundated regions back to sea level where land is believed to be present at
the considered time. For instance, river basins such as the Amazon, Rio de la
Plata, Congo and Mississippi were just carved out deeper after the sea level
dropped and other depressions were only created during glacial periods (e.g.
the Great Lakes).
Finally, the application of the geographical reconstruction will usually be
at a lower resolution than the one used until now (0.1∘) and the
high level of detail in the present-day topography may be misleading with
respect to how precise palaeotopography is actually known. The last step is
then to apply a squared smoothing mask, performing a 2-dimensional running
mean with a 1∘ width for the example, of which the result is shown
in Fig. . The mask size can be chosen differently and after
smoothing, the global geography data are extrapolated onto the desired output
grid (e.g., longitude-latitude) to be stored as a NetCDF file.
Conclusions
We have presented a new method to create global geographical reconstructions
that can be used as boundary conditions in palaeoclimate model simulations.
Starting from a plate-tectonic reconstruction, the depth of the ocean floor
was estimated and combined with a reconfigured present-day topography. An
overview of the different steps making up the procedure is given in
Fig. . Reconstructed plate trajectories, a background
rotational frame and a set of boundary lines are fed into GPlates to make a
global plate-tectonic reconstruction for a particular time frame. Within the
same setup, present-day topography and ocean floor data are manipulated and
shifted to fit the same geographical configuration as the rest of the output.
The latter is then used in a MatLab script together with some external input
(mostly from literature) to create a global gridded altimetry-bathymetry
dataset that is stored as a NetCDF file. This results in a highly detailed
dataset that needs some time-specific changes, after which it is smoothed and
extrapolated onto the desired computational grid.
Even though we performed a number of manual adjustments, interfacing GPlates
with our MatLab script allows for the development of global geographical
reconstructions with limited effort. The primary focus was to use a plate
tectonics model not originally designed to create numerical grids as output
(but it can use and manipulate them), that can be altered afterwards without
having to run through the entire process again (only some of the masks may
need to be redefined). Creating a new grid for a different point in time or a
new configuration (e.g. using a different rotation framework) should allow
the user to make multiple reconstructions in a few days' time. In case of a
time slice experiment, one can also create appropriate boundary conditions
for each time frame instead of making some minor adjustments to an average
geography, to really try and catch the effect of gradual but global changes.
The amount of manual handling depends on the demands of a specific
application and can consequently be very limited, but the procedure does not
inhibit additional adjustments for further detail and correctness. In the
example treated here, the output grid is defined as having a 0.1∘
resolution but can easily be extrapolated onto a coarser grid, which may
require some more elaborate smoothing.
As the procedure starts from a plate tectonic model, most of the features
important for modelling applications are quite realistic. The width and depth
of ocean basins, presence of oceanic ridges and plateaus, position of
continents and opening or closing of gateways are all directly incorporated
in the final result. The set of boundaries shown in Fig.
are coupled to individual plates and can easily be linked to a different
reconstruction framework. When comparing the result (Fig. ) to
other recent reconstructions used in palaeoclimate modelling, several
differences quickly become evident. For instance, the global geography used
in is coarser and contains less detail in both land
topography and ocean bathymetry. The Southern Ocean gateways are also much
wider, which is probably a result of both the large grid spacing used for the
climate model and the difference in time frame considered as this
reconstruction is based on an Oligocene geography .
have manually added a lot of detail to both the land
surface and ocean floor in their reconstruction but here the problem remains
that there can be considerable differences in continental positions as the
geography is bound to a specific time frame. Details within the contours of
an older reconstruction can still be added but changing the relative
positions of various land masses is not straightforward.
In summary, the reconstruction method presented here can provide detailed
palaeogeographical datasets with relative ease for climate model studies and
permits users to customise the result to their specific needs. This should
facilitate the communication between geologists and climate modellers in
their efforts to improve the quality and consistency of future palaeoclimate
simulations.
The Supplement related to this article is available online at doi:10.5194/cp-12-1635-2016-supplement.
Acknowledgements
This work was carried out under the program of the Netherlands Earth System
Science Centre (NESSC). The authors would like to thank Nadine McQuarrie,
Alexis Licht, Guillaume Dupont-Nivet, Christopher R. Scotese,
Dietmar Müller, Mathew Huber and Robert M. DeConto for their
advice and help to make the reconstruction presented here. Peter K. Bijl
acknowledges funding through NWO-ALW VENI grant 863.13.002.
Edited by: A. Paul
Reviewed by: M. Huber and R. D. Müller
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