Biology Reference
In-Depth Information
1990), dune formation (Murillo de Nava et al. 1999), and diatom, spore,
and pollen diagrams (Metcalfe 2006; Metcalfe et al. 1997; Minckley and
Jackson 2008) from northern Mexico. The estimated change during glacial
times was a MAT lower by about 5°C-6°C and about twice the winter rain
(Rhode 2002). Through interglacial times, the vegetation sequence was
from a juniper-piñon pine woodland at 18-12 kyr (
Juniperus scopulorum
,
J. coahuilensis
,
Pinus edulis
), to a drier oak-juniper woodland around 9 kyr,
and ultimately to the modern Chihuahuan Desert circa 4 kyr (
Larrea tri-
dentata
,
Fouquieria splendens
). The moist glacial intervals in the region are
also refl ected in the faunal record (Shaw and McDonald 1987). A metacar-
pal of the giant anteater (
Myrcemophaga tridactyla
) was discovered in early
Pleistocene deposits from Sonora, Mexico. Its present northern limit is
3000 km to the south in the more moist climates of Central and South
America. The dynamic nature of the Quaternary environment in the Basin
of Mexico is shown by evidence for the Younger Dryas, a mid-Holocene dry
period, and the Little Ice Age (Lozano-García and Ortega-Guerrero 1998;
Metcalfe et al. 2007). The paleobotanical and other records show once
again that in relatively recent time seemingly moderate changes in climate
produced profound changes in the established ecosystems.
In the porous limestone karst topography of the Yucatán Peninsula of
Mexico, Belize, and northern Guatemala, moisture is a limiting factor. The
average elevation today is around 200 m; during the glacial maxima, sea
levels were lower by about 120 m, and water tables dropped by 26-40 m.
Lakes and cenotes in the lowland Maya region have been studied by spore
and pollen analysis, sediment chemistry, stable isotope records (Leyden
et al. 1998; Hodell et al. 2005), and by carbon and oxygen stable isotope
records from stalagmites (Webster et al. 2007). There is evidence for
(1) Milankovitch-paced forcing of the climate, (2) lower water tables and
dry conditions at the LGM, and (3) a particularly dry period between 1785
and 930 BP corresponding to the collapse of the lowland Maya civilization
(Haug et al. 2003).
The Antilles
In the vicinity of the Greater Antilles, an interesting discovery from the
Sargasso Sea is that sea surface temperatures (SSTs) were warmer by about
1°C during the Medieval Warm Period, and cooler by about 1°C during the
Little Ice Age (Keigwin 1996). In another study, ratios of Mg/Ca and coral
18
O isotopes off southwestern Puerto Rico at about 18°N show that SSTs
were 2°C cooler during the Little Ice Age (Watanabe et al. 2001). Pollen
