Biology Reference
In-Depth Information
rine dinofl agellate algae, in the Paraje Solo Formation near Coatzacoalcos,
Veracruz.
The Yucatán Platform of southern Mexico, Belize, and the Petén region
of northern Guatemala has an average elevation of about 200 m with a karst
topography, that is, a rough porous limestone without surface drainage,
with numerous underground streams and caves, and deep circular water-
fi lled openings called cenotes. The cenotes occur in a ring, and they are
now known to be collapsed structures surrounding the Chichulub Crater.
The Yucatán carbonate platform is important for several reasons, in addi-
tion to being the site of the K/T asteroid impact. One is that the organic
debris accumulating in the shallow waters has formed petroleum, and
the platform contains one of the richest deposits of hydrocarbons in Latin
America. As a result, the geology is known in great detail, including the age
and location of lignite deposits. Lignites are economically important as a
low-cost source of fuel, and they often contain abundant plant microfossils.
The Zona del Sur Division of Petróleos de Mexicanos (PEMEX) is located
at Coatzacoalcos, and when sites were being sought for our early studies on
tropical vegetation and environmental history, PEMEX provided informa-
tion about lignites in the middle Pliocene Paraje Solo Formation.
An important legacy of the cenotes resulting from the asteroid impact is
that they provided water, served as sites for religious ceremonies, and pre-
served relics of the great Lowland Maya civilization that reached its classic
period between 250 and 900 CE. Karst environments are marginal at best
for supporting an increasing population that in the Maya region at its height
may have reached 750 or more people per square mile. As noted by Jared
Diamond in chapter 5 of
Collapse: How Societies Choose to Fail or Succeed
,
this compares with modern population densities in Rwanda and Burundi of
750 and 540 people per square mile, respectively. For the New World the
number for El Salvador is 327 and for Haiti it is 307 (United Nations World
Populations Prospects Report for 2005). The Maya lowlands were a densely
populated region, and by about 750 CE, the landscape had been modifi ed
to the extent that obtaining fuel, water, and food was becoming diffi cult. As
is often the case with civilizations on the edge of sustainability, an environ-
mental event can be the tipping point that decides their fate. For the Maya,
this event was the Medieval Warm Period between about 800 and 1200 CE,
as revealed by limnological, vegetation, and archeological studies by Mark
Brenner, Gerald Haug, David Hodell, Barbara Leyden, and others. It can
be documented also by a variety of other innovative methods now being
applied to the study of Earth history. As noted previously, the amount of
titanium in different layers of marine sediments is an indication of rates of
