Central Africa's Tropical Congo Basin Was Arid, Treeless In Late Jurassic

 An ancient soil crack, called a clastic dike, from
alternate wetting and drying cycles
of seasonal rainfall.
(Credit: Image courtesy
of Southern Methodist University)

ScienceDaily   — The Congo Basin -- with its
massive, lush tropical rain forest -- was
far different 150 million to 200 million years ago.
At that time Africa and South America
were part of the single continent Gondwana.
The Congo Basin was arid, with
 a small amount of seasonal rainfall,
and few bushes or trees populated
the landscape, according to
a new geochemical analysis of rare ancient soils.

The geochemical analysis provides
new data for the Jurassic period, when
very little is known about
Central Africa's paleoclimate, says
Timothy S. Myers, a paleontology doctoral
student in the Roy M. Huffington Department
of Earth Sciences at
Southern Methodist University in Dallas.

"There aren't a whole lot of terrestrial deposits
from that time period preserved
in Central Africa," Myers says. "Scientists
have been looking at Africa's paleoclimate
for some time, but data from
this time period is unique."

There are several reasons for the
scarcity of deposits: Ongoing armed conflict
makes it difficult and challenging
to retrieve them; and the thick vegetation,
a humid climate and continual erosion
prevent the preservation of ancient deposits,
which would safeguard clues
to Africa's paleoclimate.

Myers' research is based on
a core sample drilled by a syndicate
interested in the oil and mineral deposits
in the Congo Basin. Myers accessed
the sample -- drilled from a depth
of more than 2 kilometers -- from
the Royal Museum for Central Africa
in Tervuren, Belgium, where it is housed.
With the permission of the museum,
he analyzed pieces of the core
at the SMU Huffington Department
of Earth Sciences Isotope Laboratory.

"I would love to look at an outcrop
in the Congo," Myers says, "but I
was happy to be able to do this."

The Samba borehole, as it's known,
was drilled near the center
of the Congo Basin. The Congo Basin
today is a closed canopy
tropical forest -- the world's second
largest after the Amazon. It's home
to elephants, great apes, many species
of birds and mammals, as well
as the Congo River. Myers' results are
consistent with data from
other low paleolatitude, continental,
Upper Jurassic deposits in Africa
and with regional projections
of paleoclimate generated by
general circulation models, he says.

"It provides a good context for
the vertebrate fossils found in Central Africa,"
 Myers says. "At times, any indications
of the paleoclimate are listed
as an afterthought, because climate
is more abstract. But it's important
because it yields data about
the ecological conditions. Climate determines
the plant communities, and not just
how many, but also the diversity of plants."

While there was no evidence of
terrestrial vertebrates in the deposits
that Myers studied, dinosaurs were present
in Africa at the same time.
Their fossils appear in places that were
once closer to the coast, he says,
and probably wetter and more hospitable.

The Belgium samples yielded
good evidence of the paleoclimate.
Myers found minerals indicative of
an extremely arid climate typical
of a marshy, saline environment.
With the Congo Basin at the center
of Gondwana, humid marine air from
the coasts would have lost much
of its moisture content by the time
it reached the interior of the massive continent.

"There probably wouldn't have been
a whole lot of trees; more
scrubby kinds of plants," Myers says.

The clay minerals that form
in soils have an isotopic composition
related to that of the local rainfall
and shallow groundwater.
The difference in isotopic composition
between these waters and the clay minerals
is a function of surface temperature,
he says. By measuring the oxygen and
hydrogen isotopic values of the clays
in the soils, researchers can estimate
the temperature at which the clays formed.
For more information
see www.smuresearch.com.

Myers presented his research, "Late Jurassic
Paleoclimate of Central Africa,"
at a scientific session of
the 2009 annual meeting of The Geological
Society of America in Portland, Ore., Oct. 18-21.

The research was funded by
the Roy M. Huffington Department
of Earth Sciences at SMU,
and the Institute for the Study
of Earth and Man at SMU.

Link here
--
J-L K
Sent from Kigali, Rwanda