Late Pleistocene C4 plant Dominance
and Summer Rainfall in the
Southwestern United States from
Isotopic Study of Herbivore Teeth
Connin S.L., Betancourt, J., Quade, J., 1998,
Quaternary Research v.50, p. 179-193, article #
QR981986
Difference between C3 and C4 plants
Temperate Environment
Arid Environment
Plant types are an environmental
indicator
• C4 plants: desert grasses, some herbs and
shrubs
– dominate region after 18,000 BP
• C3 plants: dicotelydenous plants and
temperate grasses
• dominate the region during the late Pleistocene
Current distribution of C4 plants
Carbon Isotopes in Plants
• Different plants incorporate different types
of carbon in their structure
• Megaherbivores eat the plants, and
incorporate the carbon in their structure
C and O Isotopes in tooth Enamel
• C 13 values in tooth enamel correlate to bulk
diet
• C 13 values differ for C4 and C3 plants
• C4 ( 12%,) C3 ( -26%)
• O 18 in enamel relates to drinking water and
plant water
Location Map
Megaherbivores
Musk Ox and Bison
Megaherbivores
Equus
Megaherbivores
Woolly Mammoth
Megaherbivores
Giant Ground Sloth
Assumptions
• 1) Physiology of modern and glacial C3 and C4
grasses is similar
• 2) C4 diets largely reflect the consumption of
grasses
• 3) Enamel C13 values provide an accurate record
of C4 plants on local landscape
Validity of assumptions
• 1) Difficult to test but likely sound
• 2) Supported by spatial variations of grasses but is
best seen in Mammuthus spp. and Bison spp.
• 3) Overlooks possibility of migratory behavior
Abundance of C4 and C3
grasses and shrubs
Isolines indicate annual
proportion of rainfall in
summer months
Mostly C3
Mostly C4
Comparison of past and present conditions
Climatic Implications
• Proximity of glaciers caused a shift in the jet
stream
• This shift changed rainfall patterns, favoring the
growth of more C4 plants
• Most of the rainfall in summer
• A tropical or sub-tropical source of Summer
precipitation
Climatic Comparison: Pleistocene vs. Present
Comparison of Isotope studies
Conclusions
• C13 values increased with decreasing latitude
• More work needed, as this is the first study of this
kind
• A weak sub-tropical high produced low cutoff
pressure cells that steered rain patterns over the
Southwest
• Tropical or sub tropical rainfall patterns in the
Southwest during the Pleistocene