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
Woolly Mammoth
Megaherbivores
Giant Ground Sloth
Megaherbivores
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 C4
Mostly C3
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
