PALEOCLIMATIC EFFECTS ON PLEISTOCENE FLORA AND FAUNA OF THE GREAT PLAINS INFERRED FROM STABLE ISOTOPE ANALYSIS

Temperature and precipitation control the modern distribution of C₃/C₄ grasses; however, changes in past atmospheric pCO₂ may have also affected C₄ distribution. The “quantum yield model” predicts that C₄ grasses, which typically thrive in warm climates, are able to outcompete C₃ plants under low atmospheric pCO₂. This model predicts that C₄ grasses should have expanded during glacial periods despite low mean annual temperature. However, recent evidence from Florida, Wyoming, and California shows a greater abundance of C₄ plants during interglacial periods. To investigate grassland composition in Nebraska in relation to temperature, we sampled horse (Equus sp.) teeth for stable carbon and oxygen isotopes from six Pleistocene (Irvingtonian) localities in northern Nebraska, and glacial and interglacial sites in southern Nebraska. We find that δ¹³C and δ¹⁸O are significantly correlated (n=33, r²=0.41, p<0.0001) in Equus sp. from northern Nebraska. The range of δ¹³C (-10.0‰ to -4.9‰) indicates that horses at some localities were consuming primarily C₃ plants (≤-9‰) while those at others had a mixed C₃/C₄ diet (>-8‰). Modern meteoric δ¹⁸O values are strongly correlated with temperature at mid latitudes. Thus, the correlation between δ¹³C and δ¹⁸O in northern Nebraska suggests that C₄ grasses expanded northward as temperature increased; this conclusion is further supported by the glacial and interglacial sites in southern Nebraska. Horses from the glacial site predominantly consumed C₃ plants (δ¹³C = -9.1‰) while horses from the interglacial site had a mixed C₃/C₄ diet (δ¹³C = -6.3‰). This suggests that temperature was a stronger controlling factor of C₄ distribution than pCO₂. To test if body size in horses was affected by climate change, we compared mean metacarpal length to mean δ¹³C values from each of the northern localities and found a strong, significant positive correlation (n=6, r²=0.83, p=0.01). This implies that body size increased with warming as C₄ grasses expanded northward and is the opposite of the relationship predicted from the modern geographic distribution of mammal body size and temperature (i.e., “Bergmann’s Rule”). We hypothesize that primary productivity increased during warm periods due to a longer growing season and/or increased precipitation, allowing horses to reach maximum body size.